UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO PROGRAMA DE POSGRADO EN CIENCIAS DE LA TIERRA CENTRO DE GEOCIENCIAS RELACIÓN ENTRE EPISODIOS TECTONO-MAGMÁTICO CRETÁCICO TARDÍO - OLIGOCENO TEMPRANO Y EL DESARROLLO DE MULTIPLES EVENTOS MINERALIZANTES DE PLATA – ORO EN EL DISTRITO MINERO SAN DIMAS, SIERRA MADRE OCCIDENTAL, MÉXICO TESIS QUE PARA OPTAR POR EL GRADO DE: DOCTORADO EN CIENCIAS DE LA TIERRA PRESENTA: MSc PAULA ANDREA MONTOYA LOPERA TUTORES DR. LUCA FERRARI DR. GILLES LEVRESSE CENTRO DE GEOCIENCIAS MIEMBRO DEL COMITÉ TUTOR DR. ÁNGEL NIETO CENTRO DE GEOCIENCIAS CDMX, AGOSTO 2020 UNAM – Dirección General de Bibliotecas Tesis Digitales Restricciones de uso DERECHOS RESERVADOS © PROHIBIDA SU REPRODUCCIÓN TOTAL O PARCIAL Todo el material contenido en esta tesis esta protegido por la Ley Federal del Derecho de Autor (LFDA) de los Estados Unidos Mexicanos (México). El uso de imágenes, fragmentos de videos, y demás material que sea objeto de protección de los derechos de autor, será exclusivamente para fines educativos e informativos y deberá citar la fuente donde la obtuvo mencionando el autor o autores. Cualquier uso distinto como el lucro, reproducción, edición o modificación, será perseguido y sancionado por el respectivo titular de los Derechos de Autor. HOJA EN BLANCO iii Declaración de originalidad "Declaro conocer el Código de Ética de la Universidad Nacional Autónoma de México, plasmado en la Legislación Universitaria. Con base en las definiciones de integridad y honestidad ahí́ especificadas, aseguro mediante mi firma al calce que el presente trabajo es original y enteramente de mi autoría. Todas las citas de, o referencias a, la obra de otros autores aparece debida y adecuadamente señaladas, así́ como acreditadas mediante los recursos editoriales convencionales." iv Agradecimientos Este estudio de investigación fue posible gracias al patrocinio de los proyectos CONACYT - CB 237745-T otorgado a L. Ferrari y DGAPA-PAPIIT – IN106017 otorgado a G. Levresse, y fue desarrollado en Centro de Geociencias de la Universidad Autónoma de México (UNAM), campus Juriquilla. Me gustaría también dar sinceros agradecimientos a First Majestic Silver Corp., que apoyó proporcionando información y soporte logístico para el desarrollo de este estudio. Expreso también, grandes agradacimientos a mis tutores Dr. Luca Ferrari y Dr. Gilles Levresse ya que sin su ayuda y asistencia el proyecto no hubiera sido posible. Especiales agradecimientos al Dr. Ángel Nieto, Dra. Teresa Orozco, Dra. Eliza Fitz, Dr. Aldo Ramos, Dr. Martin Valencia por su asistencia y guía durante diferentes etapas del proyecto doctoral. A los Ingenieros Luis Mata, Nicolas Landón y Miguel Pérez por compartir su basto conocimiento y soporte logístico durante las salidas de campo. Agradezco también a los diversos laboratorios dentro y fuera de UNAM, no solo por la posibilidad de usar sus equipos, sino también por el apoyo intelectual para el desarrollo exitoso de este doctorado: Dr. Luigui Solari, Dr. Carlos Ortega, Dr. Fanis Abdullin, por su asistencia con técnicas U/Pb y trazas de fisión; Dra. Gabriela Hernandez, Dra. Margarita López por su asistencia con las técnicas Ar/Ar; Dra. Marina Vega y Ing. Carlos Linares López por asistencia con microanálisis químicos de minerales con microsonda electrónica ; Dr. Pedro Morales, M.Sc. Edith Cienfugos Alvarado, MSc Francisco Javier Otero Trujano, por su asistencia en isotopía estable. Juan Tomás Vazquez y Manuel por la elaboración de láminas delgadas y asistencia en trituración y separación de minerales. M.Sc. “Conchis” por su asistencia en recubrimiento de láminas, Dr. Andrea Luca Rizzo, Maria Grazia y Mariano Tantillo de la Universidad de Palermo (INGV) en Italia por su ayuda en la preparación de muestras, análisis de gases nobles y sus valiosas discusiones y aportes en la escritura del documento. Asimismo, agradezco a Seequent Limited por otorgar una licencia académica del software Leapfrog GEO y EAGE. Por último, pero no los menos importantes a todos mis colegas de doctorado los cuales de una manera positiva aportaron de una u otra forma al desarrollo de la investigación, Dra. v Tatiana Lobato, Dr. Kurt Wogau, Dra. Maria Isabel Sierra, Dr. Rodrigo León, M.Sc. Vania Ferrer, Dr. Roberto Molina, Dra. Laura Culí, M.Sc. Maria Clara Madrigal, M.Sc. Sebastian Giraldo, M.Sc. Santiago Urquiza, Dr. Mattia Parolari, Dr. Fernando Corbo, Dr. Adolfo Pacheco, M.Sc. Gabriela Contreras y Dra. Paola Botero. vi Resumen El Distrito San Dimas es un clásico referente a nivel mundial de yacimientos epitermales de Ag/Au de baja sulfuración (tipo cuarzo + adularia + sericita) localizado en la Sierra Madre Occidental (SMO) al oeste de México. A pesar de ser un depósito de clase mundial que ha tenido producción por hace más de un siglo, la génesis de sus sistemas vetiformes de variable contenido Ag/Au continua sin ser bien entendido. En el modelo actual, basado principalmente en el Bloque Tayoltita, la evolución del sistema de vetas ha sido dividida en tres fases: (a) temprana, (b) mineralizante y (c) tardía. La fase mineralizante presenta un rango de edades entre ~41 a 32.7 Ma en adularia, y una edad de ~31.9 Ma en sericita. Basado en estas edades, el sistema de vetas del distrito San Dimas fue considerado como el resultado de un solo evento hidrotermal desarrollado en un periodo de ~10 Ma. Éste a su vez se asoció genéticamente al emplazamiento de cuerpos intrusivos de afinidad intermedia con edades K- Ar en el Eoceno tardío. Debido a la fuerte alteración hidrotermal en estas rocas, las edades reportadas probablemente no representan la edad de cristalización de los cuerpos sino una edad de reseteo parcial de los minerales. Por otro lado, el modelo metalogenético tradicional de un solo evento no concuerda con la presencia de vetas con dos orientaciones distintas y relación Au/Ag variable. Nuevas edades U-Pb en zircón y trazas de fisión en apatito reportadas en este estudio corresponden a tres pulsos magmáticos: uno entre ~77 y 64 Ma asociados al arco laramídico, otro entre ~49 y 45 Ma durante la formación del batolito Piaxtla y el otro entre ~32 y 27 Ma que se relaciona al primer pulso ignimbrítico de la SMO. En este contexto, las edades K-Ar previas de la mineralización estarían ubicadas durante un periodo sin actividad magmática desarrollada entre el segundo y tercer pulso, lo que indica que el periodo continuo de la mineralización de ~10 Ma sugerido en trabajos previos puede deberse al reseteo parcial de algunos minerales. Los resultados presentados en esta investigación demuestran que el distrito San Dimas exhibe múltiples eventos mineralizantes durante diferentes episodios magmáticos y tectónicos desde el Cretácico Tardío al Oligoceno temprano. Un primer evento está asociado a pórfidos de cobre emplazados en el edad Cretácico Tardío y asociados al arco laramídico. vii En el Eoceno, un segundo evento mineralizante desarrolló vetas mesotermales ricas en Ag en sistemas de orientación ~E-W, principalmente de tipo cuarzo-adularia. La formación de estas vetas se estima a temperaturas mayores de 300ºC y profundidades de aproximadamente 3 km, asociadas a etapas finales de formación del batolito Piaxtla. Un tercer evento se caracteriza por vetas epitermales tipo sericita-clorita de baja sulfuración ricas en Au con edades en el Oligoceno temprano, emplazadas en sistemas de orientación NNW-SSE. Para estas vetas se estimaron temperaturas de formación promedio de 250ºC y profundidades someras (< 1km). La geoquímica de los circones indica una asociación con pulsos magmáticos reducidos y fértiles desarrollados al final del primer pulso ignimbrítico de la SMO. Palabras claves Sierra Madre Occidental, fluidos hidrotermales, epitermal, mesotermal, telescopeo, plata, oro, San Dimas. viii Abstract The San Dimas (SD) district is a world reference Ag/Au epithermal low sulfidation (quartz + adularia + sericite type) deposit located in the Sierra Madre Occidental (SMO) of western Mexico. Despite being a world class deposit under production for over a century, the genesis of its vein system with variable Ag/Au content is still not well understood. In the current model the vein system has been divided in a traditional early, ore, and late phases at Tayoltita Block. The ore phase ages ranging from ~41 to 32.7 Ma in adularia, and a single age of ~31.9 Ma in sericite. On the basis of these ages, the San Dimas vein system was considered the result of a single hydrothermal event developed over a ~10 Ma period, genetically associated to the emplacement of intermediate intrusive bodies of Eocene age. Given the widespread alteration of these rocks, the K-Ar ages are doubtful and were probably the result of partial resetting of the dated minerals. In addition, the traditional metallogenetic model not only includes genetical ambiguities, but also it was constructed based on the studies of a limited portion of the district. Our new U-Pb and FT geochronologic study of the district, revealed three main magmatic pulses at ~77-64 Ma (Laramide arc), ~48-45 Ma (Piaxtla batholith) and ~32-27 Ma (first SMO ignimbrite flare up). In this frame previous K-Ar ages would place the mineralization episode during a magmatic lull between the second and third magmatic pulse, confirming our previous suggestion of partial resetting of minerals dated by K-Ar method. Our revision of mineralized veins and the origin and evolution of the ore-forming fluids and genesis demonstrate that SD exhibits multiple mineralization events during different magmatic and tectonic episodes from Late Cretaceous to early Oligocene. The results suggest that the SD deposit developed through two different mineralization episodes: 1) Ag- dominant mesothermal Eocene veins that formed at temperatures > 350ºC developed at ca. 3 km depth, associated to the final stages of intrusion of the Piaxtla batholith, and 2) epithermal low sulfidation Au-dominant Oligocene veins formed at 250ºC, at shallower depths (<1km), associated to the faults feeding rhyolitic domes developed at the end of the main ignimbrite flare up of the SMO. ix Our results highlight the importance of a multidisciplinary approach, such as field observations, geochronological and geochemical studies, to better understand the complexity of the hydrothermal magmatic processes involved in the formation of many Mexican ore deposits and their proper classification Keywords Sierra Madre Occidental, hydrothermal fluids, epithermal, mesothermal, telescoping, silver, gold, San Dimas 10 Tabla de Contenido Declaración de originalidad .................................................................................................. iii Agradecimientos ......................................................................................................................... iv Resumen ........................................................................................................................................ vi Abstract ...................................................................................................................................... viii Tabla de Contenido .................................................................................................................. 10 Capítulo 1: Introducción ........................................................................................................ 12 1.1 Introducción .......................................................................................................................... 12 1.2 Objetivos ................................................................................................................................. 15 1.3 Procedimientos analíticos ................................................................................................ 15 Capítulo 2: New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico ........................................................................ 22 Capítulo 3: New geological, geochronological and geochemical characterization of the San Dimas mineral system: evidence for a telescoped Eocene-Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico ............................................... 44 Capítulo 4: Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes. ......................................... 60 Capítulo 5: Conclusiones ........................................................................................................ 75 Capítulo 6: Referencias ........................................................................................................... 79 Capítulo 7: Anexos .................................................................................................................... 86 11 Anexo 1. Material suplementario del artículo: Montoya-Lopera, P., Ferrari, L., Levresse, G., Abdullin, F., Mata, L. (2019). New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico. Ore Geology Reviews. V. 105, p. 273-294 …………………………………………………………………...…………………...…….86 Anexo 2. Material suplementario del artículo: Montoya-Lopera, P., Levresse, G., Ferrari, L., Orozco-Esquivel, T., Hernán-Quevedo, G., Abdullin, F., Mata, L. (2019). New geological, geochronological and geochemical characterization of the San Dimas mineral system: Evidence for a telescoped Eocene-Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico. Ore Geology Reviews. V. 118, p. 1-15 ……………………………………………86 Anexo 3. Material suplementario del artículo: Montoya-Lopera, P., Levresse, G., Ferrari, L., Rizzo, A.L., Urquiza, S., Mata, L. (2020). Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes. Ore Geology Reviews. V. 120, p. 1-14 ..……………………………………………………………………………………….86 12 Capítulo 1: Introducción 1.1 Introducción La Sierra Madre Occidental (SMO) es el resultado de diferentes episodios magmáticos y tectónicos ocurridos entre el Cretácico y el Cenozoico, asociados a la subducción de la placa Farallón debajo de la placa norteaméricana y a la apertura del Golfo de California (Ferrari et al., 2005). Estos episodios magmáticos se pueden agrupar en dos grandes eventos: 1) el desarrollo del Arco Larámide (~90-45 Ma) con el emplazamiento de los batolitos de Sonora, Sinaloa y Jalisco y su cobertura volcánica (Fm. Tarahumara y sus equivalentes, Wilson y Rocha, 1949; Ferrari y Rosas-Elguera, 2000; McDowell et al., 2001; Valencia et al., 2013), y 2) un episodio de extensión litosférica que se desarrolla a partir del final del Eoceno favoreciendo la fusión parcial de la corteza con producción de magmas diferenciados que dan origen a la llamada gran provincia silícea de la SMO, caracterizada por dos grandes pulsos ignimbríticos entre ~34-28 Ma y ~24-18 Ma (Ferrari et al., 2014 y 2018). Estos procesos geodinámicos mayores han generado grandes distritos mineros en la SMO, reconocidos a nivel mundial por su importancia económica en la producción de Ag, Au, Cu (Albinson et al., 2001; Camprubí et al., 2001; Camprubí et al., 2003; Levresse et al., 2017; Zamora-Vega et al., 2018). Ferrari et al., (2005) describen de manera general el ambiente geológico, estructural en la SMO y muestran, en la parte sur de la SMO, la complejidad y la multiplicidad de los procesos de mineralización y sus relaciones con los eventos magmáticos a escala regional. El distrito minero de San Dimas está catalogado como un clásico depósito epitermal de baja sulfuración de Ag/Au de talla mundial (Henshaw, 1953; Clarke, 1986). Éste se encuentra localizado en el flanco occidental de la parte central de la SMO cerca del límite entre los estados de Durango y Sinaloa en México. San Dimas es uno de los distritos mineros mas importantes de México en cuanto a producción de metales preciosos (11 Moz Au, 745 13 Moz Ag) siendo explotado desde el año 1757 hasta el presente (First Majestic, 2018). El distrito en mención se encuentra en una región que ha sufrido diferentes eventos magmáticos acompañados de eventos de extensión cortical desde el Cretácico Tardío. El conocimiento geológico del distrito se remonta a los trabajos históricos realizados por Henshaw, (1953), Smith y Hall, (1974), Nemeth, (1976), Enríquez y Rivera, (2001b) y Henry et al., (2003), entre otros. Dichos autores definen la geología, cronoestratigrafía y la mineralización del distrito, mencionando varios eventos magmáticos. El primero está constituido por el Batolito Piaxtla de edad eocénica (~52-43 Ma, K/Ar en hornblenda y biotita, Enríquez y Rivera, 2001b; Henry et al., 2003; ~47 Ma, U/Pb en circón, Henry et al., 2003), que representa el basamento de toda la columna estratigráfica. Este cuerpo intrusiona una secuencia volcánica y subvolcánica de composición riolítico–andesítica del Complejo Volcánico Inferior (CVI), dividida en unidades conocidas en el área como Riolita Socavón, Andesita Buelna, Riolita Portal, Andesita Productiva, Andesita Intrusiva, Diorita Arana, estas últimas fechadas por el método K/Ar, reportando edades entre ~36.6 y 40 Ma (Enríquez y Rivera, 2001b), y las formaciones sedimentarias de carácter continental como la Formación Camichín y Formación Las Palmas. Sobreyaciendo el CVI y en discordancia angular, se describen paquetes lávicos andesíticos con edades de ~24 Ma (K/Ar en plagioclasa, Enríquez y Rivera, 2001b), los cuales son nombrados Andesita Guarisamey. Finalizando la secuencia estratigráfica, se reporta un paquete llamado “riolita capping”, nombrado así por no tener conexión aparente con la mineralización, con una edad de ~20 Ma (K/Ar en plagioclasa, Enríquez y Rivera, 2001b). Las unidades anteriores fueron cortadas por diques porfídicos andesíticos (diques Bolaños), y por diques de basalto con piroxeno (diques San Luis), los cuales cortan desde la base hasta el “capping”, así como por un dique porfídico dacítico (dique Santa Rita), que intruye desde la base hasta la secuencia sedimentaria de la Formación Las Palmas (Enríquez y Rivera, 2001b). Ninguno de los diques había sido fechado hasta el momento. En cuanto al conocimiento estructural, el distrito minero San Dimas se encuentra subdividido estructuralmente en 4 bloques por estructuras NNW-SSE: (a) Oeste, (b) Graben Sinaloa, (c) Central y (d) Tayoltita. Horner y Enríquez (1999) muestran la relación entre 14 estructuras extensionales y transtensionales y las trampas para la generación de depósitos minerales, describiendo 3 eventos principales: 1) fuerzas compresivas E-W a NW-SE con desarrollo de estructuras de E-W y ENE-WSW subverticales relacionados con las partes finales de la Orogenia Larámide, las cuales hospedan los metales (eg. vetas Cedral, Culebra, Candelaria y Castellana). En general son sistemas de vetas emplazadas en todo el CVI y la formación sedimentaria Las Palmas; 2) de fallamiento lateral derecho con componente normal desarrollando estructuras transtensionales NNW-SSE, produciendo los sistemas de fallas Arana y la Peña, sirviendo como canales para el desarrollo los sistemas de vetas Arana, La Patricia y 3) de extensión regional desarrollada durante el Oligoceno y el Mioceno provocando el desarrollo de sistema de fallas de dirección NNW-SSE. Ambos sistemas de vetas se definieron como tipo cuarzo-clorita-adularia a cuarzo-adularia formándose en tres fases generales, llamadas como, temprana, mineralizante y tardía con base al estudio en el Bloque Tayoltita. Para la fase mineralizante, o evento de alta ley, se consideraba un rango de edades entre ~41.0 a 31.9 Ma, en adularia, y en sericita (Henry, 1957; Enríquez y Rivera, 2001b; Enríquez et al., 2018). Basado en estas edades, los autores interpretaban el yacimiento como el resultado de un solo evento hidrotermal desarrollado en un periodo de ~10 Ma, y asociado genéticamente al emplazamiento de cuerpos intrusivos de afinidad intermedia (Andesita Productiva, Andesita Intrusiva, Diorita Arana) debido a que comparten el mismo rango de edades (Enríquez y Rivera, 2001b; Enríquez et al., 2018). Estudios previos de inclusiones fluidas fueron enfocados solamente en un nivel llamado “nivel de bonanza” para Ag/Au en las estructuras NNW-SSE del BloqueTayoltita (Clarke y Title, 1988; Smith et al.,1982). Los anteriores autores definen este nivel basados en la correlación entre los radios Ag/Au y la salinidad de las inclusiones fluidas, indicando una profundidad entre 400 a 1000 metros desde la paleosuperficie. En general las temperaturas de homogenización de la fase mineralizante en el nivel de bonanza presentan un rango entre 250ºC a 310ºC, con promedio 260ºC (Smith et al., 1982; Clarke and Title, 1988; Conrad et al., 1992; Enríquez and Rivera, 2001; Albinson et al., 2001; Churchill, 1980). Se reportaron puntos de enfriamiento con un rango de variación entre -0.11ºC a -1.5ºC (Smith et al., 1982; Clarke y Title, 1988; Conrad et al., 1992; Enríquez and Rivera, 2001b). Los valores de δ18Oqtz de las fases mineralizantes se reportaron en un rango de 3.9 a 9.5 ‰ y para δ18OH2O recalculado en un rango -2.9 a 3.7 ‰, lo cual indica un sistema hidrotermal dominado 15 por aguas meteóricas (Smith et al., 1982; Conrad y Chamberlain, 1992). Los valores reportados de espectrometría de gases indican fases bifásicas en las cuales el agua es superior al gas, a su vez el gas predominante es CO2 en comparación con CO, y trazas de H2, CH4, N2, C2H6, H2S, C3H8, SO2 and NO (Smith et al., 1982). La contribución científica principal de este estudio doctoral se enfoca en el entendimiento de la evolución geológica y tectónica durante el desarrollo del arco laramídico hasta los inicios de la apertura del Golfo de California y su asociación genética y fisicoquímica para el desarrollo de múltiples eventos mineralizantes sobreimpuestos en la generación de yacimientos de clase mundial en la Sierra Madre Occidental. 1.2 Objetivos El objetivo principal de este estudio doctoral es la actualización del modelo geológico del distrito minero de San Dimas (SD). Esto implica determinar las condiciones geológicas, temporalidad y geoquímicas necesarias para la formación de un yacimiento de clase mundial dentro de la evolución geodinámica regional de la SMO. Para lograr el objetivo se combinaron los siguientes procedimientos. 1) caracterización petrológica, petrográfica y geocronológica detallada de la columna estratigráfica de San Dimas; 2) caracterización petrológica y petrográficamente de los diferentes sistemas de vetas del distrito minero San Dimas; 3) se obtuvieron nuevos datos geocronológicos y se reinterpretaron las edades existentes de los eventos mineralizantes del área de estudio; 4) se desarrolló una caracterización fisicoquímica de los diferentes fluidos hidrotermales y sus relaciones con la roca encajonante; 5) se propuso un nuevo modelo metalogenético del área. 1.3 Procedimientos analíticos Con el objeto de obtener datos analíticos que permitan evaluar los objetivos propuestos, este estudio doctoral integra diferentes técnicas analíticas que permiten generar nuevas 16 interpretaciones sobre los eventos que llevaron al desarrollo de múltiples mineralizaciones en el distrito de SD en diferentes contextos tectónicos y magmáticos. Petrografía La petrografía de roca caja, composición mineral y texturas fue documentada a través de análisis petrográfico y microscopía electrónica. Las láminas delgadas pulidas se examinaron con el microscopio óptico Olympus® BX-50. Las fases minerales fueron identificadas usando el microscopio electrónico Hitachi TM-1000 con microscopía de microanálisis EDS en el Laboratorio de Geofluidos del Centro de Geociencias de la UNAM, Campus Juriquilla, Querétaro, México (CGEO). Química mineral La composición química de los minerales (Au, Ag, sulfosales) fue determinada con el Microscopio Electrónico con Analizador (EMPA) JOEL JXA 8900R en el Laboratorio Universitario de Petrología de la UNAM, Campus Ciudad de México, usando estándares sintéticos y naturales. Los metales asociados a la mineralización fueron analizados con un haz de corriente de 20 nA y una aceleración de voltaje de 20 keV. Todos los análisis fueron llevados a cabo por medio de “targeting” de búsqueda basados en las relaciones texturales con otros minerales. Geocronología U/Pb Treinta y dos muestras de roca con control estratigráfico fueron seleccionadas para fechamientos U/Pb. Las muestras fueron trituradas y tamizadas (Mallas 200-50). Los minerales pesados fueron concentrados usando técnicas convencionales (batea y magnetómetro isodinámico Frantz). Los cristales de circón fueron seleccionados manualmente (hand-picked) con el uso de un microscopio binocular y posteriormente montados en un EpoFix® con anillo de plástico de diámetro 2.5 cm, posteriormente fueron pulidos. Los puntos de ablación fueron seleccionados basados en las imágenes de 17 catodoluminiscencia con el fin de identificar los centros y bordes de crecimiento del circón. Cada ablación se desarrollo con el sistema de ablación laser Resonetics RESOlution® LPXPro (193 nm, ArF excimer), acoplado a Thermo® Scientific iCAP® Qc quadrupole ICP- MS en el Laboratorio de Estudios Isotópicos (LEI) en el CGEO. Para cada circón, la ablación consistió en la adquisición de 15 s señales del fondo (gas blanco), 30 s de ablación, y 15 s de estabilización dejando así que la señal alcance la línea base de nuevo. El diámetro de cada spot fue de 33 µm, usando una velocidad de 6 J/cm2 con una repetición de 5 Hz. Se registraron todos los isótopos requeridos para obtener la edad U/Pb (206Pb, 207Pb, 208Pb, 232Th y 238U), adicional este método (LA-ICP-MS) permite detectar otros elementos mayores, trazas y tierras raras (REE) simultáneamente. Estos datos fueron relevantes para identificar los diferentes pulsos magmáticos en términos de la hidratación y fertilidad. La calibración de ICP-MS se realizó siguiendo los procedimientos de Solari et al., (2010) y Ortega-Obregón et al., (2014). La corrección de los radios isotópicos y el error por edad fueron calculados con Iolite (Paton et al., 2011) usando la data de reducción VizualAge (Petrus y Kamber, 2012). La composición química de cada circón fue obtenida basado en los estándares de vidrio NIST. Se uso el circón 91500 (Wiedenbeck et al, 1995) como referencia principal para los análisis de U/Pb. Para cada muestra se analizaron ~35 circones, y las edades fueron reportados en base a la edad promedio de cristalización utilizando la metodología propuesta por Ludwing (2008). Los análisis por fuera de 2s fueron descartados. Porcentajes con discordancias mayores al 20 % no fueron considerados. Para rocas volcánicas, la edad reportada fue la edad de la población de circones más jóvenes. En rocas ígneas los circones más antiguos se interpretaron como herencias del basamento. Para rocas sedimentarias se analizaron alrededor de ~100 circones. La edad máxima de sedimentación fue asociada a los circones más jóvenes. Para esto se analizaron diferentes poblaciones de circones y se reportaron haciendo uso de histogramas y diagramas de densidad de probabilidad. Termocronología trazas de fisión en apatitos y titanitas El fechamiento de trazas de fisión (TF) en apatitos fue desarrollada en el Laboratorio de Estudios Isotópicos (LEI) del CGEO usando la técnica LA-ICP-MS (Hasebe et al., 2004; Donelick et al., 2005). Los detalles de la metodología se describen en Abdullin et al., (2018). 18 Se usaron los cristales de Durango F-apatito con una edad de 31.4 ± 0.5 Ma (Hurford, 2019) como equivalente de calibración (Hasebe et al., 2004; Donelick et al., 2005; Vermeesch, 2017), así como también las medidas de Cl en un apatito desconocido (0.43 ± 0.03wt% de Cl in Durango, Goldoff et al., 2012). Las edades de TFA para cada apatito con 1s fueron calculadas usando IsoplotR (Verneesch, 2017, 2018). La edad promedio de TFA y los diferentes picos de edad fueron obtenidas usando RadialPlotter (Verneesch, 2009). En el caso de las titanitas, se separaron 200 cristales que se montaron en un EpoFix® con anillo de plástico de diámetro 2.5 cm. Los EpoFix fueron pulidos con papel-arena SiC P1500 y P2500 hasta exponer las superficies internas de los cristales (4π), posterior fueron pulidas con 3, 1, 0.3 y 0.1µm de aluminio en suspensión. Las titanitas pulidas fueron grabadas con 1HF:2HNO3: 3HCL:6H2O a temperatura ambiente (24ºC) por 16 minutos (Kohn et al., 2019). El estándar de titanita Fish Canyon Tuff (FCT) fue también grabado por 22 minutos. El conteo de trazas de fisión fue desarrollado usando el microscopio Carl Zeiss AxioScopeA1 acoplado con cámara digital y objetivos “secos”. Se uso la técnica LA-ICP-MS con un haz de rayo laser 60 µm, se ablacionó exactamente en las mismas áreas que fueron usadas para determinar los valores de ρs (densidad de trazas). El estándar de titanita (FCT) de edad 28.4 ± 0.1 Ma (Schmitz y Bowring, 2001) fue usado para la calibración equivalente (Hasbe et al., 2004; Donelick et al., 2005; Vermeesch, 2017). Los resultados crudos fueron reducidos con Iolite 3.4 (Paton et al., 2011). Se uso NIST612 para los resultados de medidas isotópicas (Pearce et al., 1997) y fueron normalizados usando 29Si como estándar interno, tomando un promedio de SiO2 de 30 ± 1 wt% para todas las titanitas analizadas. Las edades de TF en titanitas fueron calculados con IsoplotR (Vermeesch, 2017, 2018). Se usaron los protocolos internos de LA-CIP-MS del LEI. Geocronología 40Ar /39Ar Seis muestras de adularia con control geológico asociadas a diferentes eventos mineralizantes en SD fueron seleccionadas para ser fechadas por 40Ar /39Ar en base a calentamiento por pasos utilizando un láser. La preparación de las muestras se desarrollo en el CGEO. Las muestras fueron trituradas y tamizadas a un tamaño de 420-840µm y posteriormente lavadas 19 con agua desionizada, acetona al 98%, y secadas en horno durante una noche a 50ºC. Los cristales de adularia fueron seleccionados manualmente en un microscopio binocular. Las muestras fueron irradiadas en dos diferentes paquetes (JUR02, JUR03) en la posición 8C en el reactor de investigación U-enriquecido de la Universidad de McMaster en Hamilton, Ontario, Canadá. Durante la irradiación, las muestras y el monitor de flujo de neutrones fueron cubiertos con un haz de Cd para bloquear neutrones térmicos. Se usó el estándar de sanidino, Fish Canyon tuff (FCT-2 (28.198 ± 0.044 Ma, Kuiper et al., 2008) como monitor. Las muestras fueron analizadas en el Laboratorio Interinstitucional de Geocronología de Argón (LIGAr), instalado en el CGEO, usando el láser para extracción de gases Coherent Innova 200-20 Ar-ion, una línea de gas automatizado para limpieza, dos captadores SAES GP-50, y un espectrómetro multicolector de masas para gases nobles. El haz de iones de isótopos de Ar fue simultáneamente medido por cuatro colectores Faraday con 1012 Ω amplificadores (m/z 36 a 39), y un colector con 1011 Ω amplificadores (m/z 40). Cada corrida consistió en 20 ciclos de 10s, con una integración de tiempo de 1s, precedido de 30 medidas base con integración de tiempo de 1s. Se intersectaron medidas de aire repetidamente con muestras desconocidas para corregir la discriminación por masas y la contaminación de Ar atmosférico, usando un ratio de 40Ar/36Ar de 295.5. Cada paso y medida de aire fue precedido de una medida de blanco. Para la reducción de datos, se usó el software NGX-Red 1.0® y AgeCalc 1.9® desarrollado por el CICESE, Ensenada, México. Encima de la sustracción del blanco, los datos isotópicos de Ar fueron corregidos por discriminación por masas, y para calcio, potasio y cloro se realizaron reacciones de interferencia. Los parámetros usados para corregir las reacciones de neutrones inducidos fueron: (39Ar/37Ar)Ca = 6.50 x 10-4; (36Ar/37Ar)Ca = 2.55 x 10-4; (40Ar/39Ar)K = 0. La masa 36 también fue corregida para cloro derivado 36Ar [35Cl(n, γ) 36Cl → 36Ar + β– with t1/2 3.01 × 105 a]. Los isotopos de 37Ar y 39 Ar fueron corregidos por decaimiento radioactivo. Se uso el decaimiento constante recomendado por Steiger y Jäger, (1977) en todos los cálculos, adicional a cálculos de straight-line presentados por York et al., (2004). Todos los errores fueron reportados a 1 σ estándar, los errores, mesetas y edades de isócrona incluyendo la incertidumbre en el parámetro J fueron integrados. Las edades de integración fueron calculadas adicionando todas las fracciones del experimento en 20 calentamiento por pasos. Las edades de meseta (plateaus) fueron seleccionadas en la base de los valores MSWD con probabilidad de corte 0.005 (Wendt y Carl, 1991), las edades de meseta fueron calculadas como peso promedio entre mínimo de tres consecutivas fracciones que representaran el 50% o mas de perdida de 39Ar. Los errores del peso promedio y regresiones que tuvieran valores de MSWD mayores que 0.05 de probabilidad del valor de corte fueron multiplicados por 1 σ error por la raíz cuadrada de MSWD. Los errores también fueron reportados al 95% en intervalo de confianza, calculados y multiplicados por 1 σ error por su apropiado Valor de t Student. Petrografia de inclusiones fluidas y microtermometría Los estudios de inclusiones fluidas (IF) incluyeron petrografía detallada para la determinación de las diferentes fases, formas y tamaños de las inclusiones. Se uso el Microscopio óptico Olympus® BX-50 con Qimaging Micropublisher 5 Mp equipado con cámara digital a Peltier-cooled CCD para la caracterización de las inclusiones en láminas gruesas pulidas. Para la obtención de temperaturas mínimas de atrapamiento o temperaturas de homogenización y temperaturas de congelamiento se utilizó el microtermómetro platina Linkam THMSG-600 (2) acoplado a microscopio Olympus BX-51. Para la determinación de temperaturas máximas de atrapamiento por población de IF se utilizó el método de decrepitación térmica con el equipo Decriptrómetro BGS Modelo 04. Los estudios de inclusiones fluidas fueron llevados a cabo en el Laboratorio de Geofluidos del CGEO. Isotopía estable Diez muestras de cuarzo de ~15g cada una fueron seleccionadas para los estudios isotópicos de cada evento mineralizante. Las muestras fueron trituradas y cribadas en mallas 45 y 60. Las fracciones no magnéticas fueron separadas con el magneto isodinámico Frantz en el Laboratorio de separación de minerales del CGEO. Las muestras de cuarzo fueron separadas manualmente con microscopio binocular con el objeto de seleccionar los mejores cristales de cuarzo (sin impurezas). Finalmente, las muestras fueron lavadas con HCL, agua regia y acetona al 98% en 5 ciclos de 5 minutos cada una en una lavadora ultrasónica. 21 Las muestras fueron decrepitadas térmicamente desde 0 a 900ºC con una rampa de calentamiento de 5ºC por minuto en la línea de vacío “Ultra Vacuum Glass Line” (10-9 – 10- 10 mbar), se siguió el procedimiento de Montoya et al., (en preparación) en el Laboratorio de Isotopía Estable, Departamento de Geología, UNAM campus Ciudad de México. Se separaron 3 poblaciones de agua de inclusiones fluidas 100ºC - 300ºC, 300ºC – 545ºC, 545ºC – 900ºC por muestra. Cada muestra de agua fue guardada separadamente en tubos de cuarzo en ultra vacío. Los isótopos de δ2H y δ18O fueron medidos en el equipo Analizador Isotópico de Inyección Automatizada DLT -100 en el Laboratorio de Isotopía Estable, Departamento de Geología, UNAM campus Ciudad de México. Los resultados fueron reportados por mil relativo a los estándares V-SMOW y VSLAP (Coplen, 1988 y Coplen et al., 2006) con una precisión de ± 2 ‰ for δ2H and ± 0.2 ‰ for δ18O. Isotopía de gases nobles Se seleccionaron 10 muestras de cuarzo de vetas para analizar los volátiles contenidos en las inclusiones fluidas. La preparación de las muestras se realizó siguiendo el procedimiento de Di Piazza et al., (2015) y Rizzo et al., (2015) desarrollado por el Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo (INGV-Palermo). Los gases atrapados en las IF en cuarzo fueron obtenidos por medio de trituración a 200ºbar en un sistema de vacío siguiendo los procedimientos de Kurz, (1986) y Hilton et al., (1993, 2002). Los gases asociados a la fragmentación mecánica fueron removidos de la línea de vacío. Los isótopos de Helio (3He and 4He) y Neón (20Ne, 21Ne and 22Ne) fueron medidos separadamente en dos diferentes tubos en el espectrómetro de masas (Helix SFT-Thermo). Los valores de 3He/4He fueron reportados en R/Ra (donde Ra es el valor del radio del aire de 3He/4He, el cual es igual a 1.39•10–6). La incertidumbre analítica del radio isotópico de He es entre 1.5 y 15%. Los isotópos de Ar (36Ar, 38Ar and 40Ar) fueron analizados con un espectrometro de masas multicolector (GVI Argus) con una incertidumbre analítica menor 0.4%. La incertidumbre en la determinación del contenido elemental He, Ne y Ar fue menor al 5%. Lo valores de los blancos para He, Ne y Ar fueron <10–14, <10–15 y <10–13 mol, respectivamente. 22 Capítulo 2: New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico Paula Montoya-Loperaa, Luca Ferraria, ⁎ , Gilles Levressea, Fanis Abdullinb, Luis Matac a Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, Qro., Mexico b CONACyT–Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, Qro., Mexico c First Majestic Silver Corp., Tayoltita, Dgo., Mexico A R T I C L E I N F O Keywords: Sierra Madre Occidental San Dimas mining district Epithermal deposit Geochronology Volcanic stratigraphy A B S T R A C T The San Dimas district is a world-class silver-gold low-sulfidation epithermal deposit located in the central part of the Sierra Madre Occidental of Mexico, within the eastern part of the Gulf of California extensional province. Previous works assumed a single period of mineralization between∼38.5 and 31.9Ma, which is at odds with the existence of vein systems with two different orientations and Ag/Au ratios. We present a re-evaluation of this district based on new zircon U/Pb and apatite fission-track ages as well as petrographic and field observations of mineralization styles. Our study also includes two new prospective areas of Causita and Mala Noche located to the south of the main district. Within the Lower Volcanic Complex, we identify a Late Cretaceous volcanic succession (∼77 to 69Ma) correlative with the Tarahumara formation from southern Sonora and coeval with the San Ignacio batholith exposed to the west. This volcanic succession hosts Au-rich mineralized sub-volcanic felsic bodies yielding slightly younger ages and is covered by Paleocene intermediate lavas with hypabyssal intrusions with ages around 48Ma. A voluminous intrusive suite (Piaxtla batholith and associated dike swarms) was emplaced in the region between ∼49 and 44Ma. Early extensional basins were filled by a continental sedimentary sequence (Palmas formation), which yielded U/Pb age peaks at 66 and 56Ma from detrital zircons and a maximum depositional age of ∼43Ma. The last magmatic activity, as in the rest of the Sierra Madre Occidental, consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of ∼31.5–29Ma and 24–20Ma. NNW–SSE extensional fault systems expose the mineralization and tilted all the succession prior to the em- placement of a ∼24Ma ignimbrite package. This late Oligocene extension, associated to the early stage of the Gulf of California rift, is confirmed by apatite fission-track dating of samples from the Piaxtla batholith, which consistently indicate an episode of cooling at 25–23Ma followed by a second episode at ∼12.5Ma. Our absolute ages and geologic mapping allow to infer that an older, WSW–ENE trending normal fault system with up to 1 km of displacement must exist between the San Dimas district and the Causita area to the south. This fault system, currently buried beneath Oligocene–Miocene ignimbrites, may have controlled the intrusion of the Piaxtla batholith and played a crucial role in the preservation of large vein systems of San Dimas in a tectonic depression setting. The main epithermal mineralization is associated with two kinds of structures: Ag/Au veins with WSW–ENE to E–W orientation and Au/Ag veins with NNW–SSE to N–S orientation. The dominant Ag/Au veins slightly post- date the Piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization. The NNW–SSE to N–S Au/Ag veins are most probably associated with the Oligocene silicic volcanism. 1. Introduction A proper understanding of the formation and preservation of mi- neral deposits in continental settings implies assessing the dynamic interaction among magmatic, tectonic, erosional, and depositional processes. This approach is particularly important in the case of western Mexico, which has undergone a complex tectono-magmatic history since the Cretaceous (Ferrari et al., 2017). In this region, the Sierra Madre Occidental (SMO) geologic province hosts numerous important porphyry and epithermal deposits mostly occurring in its western side, https://doi.org/10.1016/j.oregeorev.2018.12.020 Received 13 August 2018; Received in revised form 4 December 2018; Accepted 28 December 2018 ⁎ Corresponding author. E-mail address: luca@unam.mx (L. Ferrari). Ore Geology Reviews 105 (2019) 273–294 Available online 02 January 20190169-1368/ © 2019 Elsevier B.V. All rights reserved. ELSEVIER OREGEOI..OCY REVIEWS J,.,r,,alb-Coonprd•:n,,..,Stooicsof °"'c:.ne... ... 1°"'fa~ a portion that represents the eastern rifted margin of the Gulf of Cali- fornia (Fig. 1). Several reviews and classifications of the SMO miner- alization have been proposed in past decades. The initial attempts re- lied on the concept of metallogenic provinces or belts, where mineralization is essentially tied to a geographic domain and produced by the interaction of arc magmatism with a certain type of basement terrane (e.g. Damon et al., 1981, 1983; Campa and Coney, 1983; Salas, 1994). More recent reviews (e.g. Staude and Barton, 2001; Camprubí, 2013; Camprubí and Albinson, 2007) placed each mineralization type into a specific magmatic-tectonic episode since Mesozoic, implicitly recognizing the dynamic nature of this plate margin and the possibility of superposition of mineralization events. Several studies have also been devoted to classifying the SMO epithermal deposits based on fluid geochemistry and physical parameters (e.g. Smith et al., 1982; Clarke and Titley, 1988; Albinson et al., 2001; Camprubí and Albinson, 2007). However, reviews and analytical studies are significantly limited by the fact that most parts of the SMO lack a good geochronologic control and a detailed geologic description of veins systems, which may jeopardize a correct interpretation of a mineral deposit development. The San Dimas mining district, centered at the town of Tayoltita, is a world-class silver-gold epithermal deposit and possibly the archetype of such deposits in the SMO (Locke, 1918; Davidson, 1932; Henshaw, 1953; Clarke, 1986). Mined since the 18th century, the San Dimas district contains more than 200 discovered Ag-Au veins, located in five blocks separated by major NNW-striking normal faults. Despite being a world reference for low-sulfidation Ag-Au epithermal veins, the mi- neralization model for San Dimas has not been improved significantly since the middle of the past century. The current stratigraphic overview is based on field and mine observations, supported by petrographic and geochronologic studies (Nemeth, 1976; Henry and Fredrikson, 1987; Enriquez and Rivera, 2001a,b; Henry et al., 2003). The rocks hosting the mineralization are not dated and the Ag-Au veins have been tra- ditionally considered to be developed in a single mineralization episode between ∼41 and ∼31Ma (Enríquez and Rivera, 2001b; Enriquez et al., 2018). However, most these ages, obtained by the K–Ar method, are not always reliable given the widespread hydrothermal alteration of the district. The difference in vein orientation with variable Ag/Au ratio through the district are also at odd with the idea of a single miner- alization episode. In this study, we present a revision of the geology and stratigraphy of the San Dimas district as well as of adjacent areas based on fieldwork at surface and in mine interior, supported by a detailed petrographic study and U/Pb and apatite fission-track ages of the whole geologic column. This provides a robust geologic and geochronologic context Fig. 1. Geodynamic context of western Mexico and main porphyry and epithermal deposits of the Sierra Madre Occidental volcanic province. The blue rectangle represents the study area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 274 ■ Unexended core of SMO ~ Active trench 4 Paleo Trench / Plate boundary * Porphyry deposits * Epithermal deposits towards a better understanding of the mineralization events that will be described in detail in a forthcoming paper (Montoya-Lopera et al., submitted for publication). 2. Previous studies 2.1. Regional geology and tectonic setting As a physiographic province, the SMO comprises a high plateau with an average elevation exceeding 2000m above sea level that cover an area ∼1200 km long and ∼200 to 400 km wide extending from the Mexico–US border approximately to Latitude 21° N, where it intersects the Trans Mexican Volcanic Belt (Fig. 1). The western part of the SMO high plateau is cut by normal faults systems that are part of the Gulf of California rift and where most ore deposits are exposed. As an igneous province, the SMO includes Late Cretaceous to early Miocene rocks formed during two main periods of continental magmatic activity (McDowell and Keizer, 1977; Ferrari et al., 2017). The first period, concurrent with the Laramide orogeny, produced a dominantly inter- mediate intrusive suite and its volcanic counterpart, associated with a normal supra-subduction magmatic arc active between ∼100 and 50Ma (Gastil, 1975; McDowell et al., 2001; Henry et al., 2003; Ortega- Gutiérrez et al., 2014). These rocks, traditionally named Lover Volcanic Complex (LVC) (McDowell and Keizer, 1977), formed the Sonora, Si- naloa, and the Jalisco batholiths, as well as the Late Cretaceous to Paleocene volcanic succession of the Tarahumara Formation in Sonora (Wilson and Rocha, 1949; McDowell et al., 2001), and equivalent rocks in the Jalisco Block (Ferrari and Rosas-Elguera, 2000; Valencia et al., 2013) (Fig. 1). Volcanic successions of this age have been inferred in Sinaloa and Durango, but no radiometric ages have been provided so far. After a transitional period that lasted until the late Eocene (Ferrari et al., 2017), volcanism became dominated by rhyolitic ignimbrites with less basaltic lavas, building one of the largest silicic volcanic provinces on Earth (Bryan and Ferrari, 2013). Defined as the Upper Volcanic Supergroup (UVS) (McDowell and Keizer, 1977), these rocks were emplaced mostly in two episodes of ignimbrite flare up at ∼35 to 29Ma along the entire province and at ∼24 to 20Ma in its southern part (Ferrari et al., 2002, 2007; McDowell and McIntosh, 2012). Mafic lavas with both asthenosphere and mantle lithosphere affinity are found interspersed within the ignimbrite successions, often associated with normal faulting (Ferrari et al., 2017). Extensional basins and associated continental sedimentary deposits formed between ∼27 and ∼15Ma in a NNW-trending belt along the western half of the SMO, spanning the western part of Sonora (McDowell et al., 1997; Nourse et al., 1994; Gans, 1997; González León et al., 2000; Wong et al., 2010; Murray et al., 2013) and most of Sinaloa and Nayarit (Ferrari et al., 2013) (Fig. 1). The temporal and spatial association of the silicic (or felsic) to bimodal magmatism of the UVS with crustal extension supports the idea that these processes represent the beginning of the rifting process that led to the formation of the Gulf of California (Bryan and Ferrari, 2013; Ferrari et al., 2013, 2017). The San Dimas mining district lies within the central part of the SMO, near the Sinaloa–Durango state border (Fig. 1). The geology of this part of the SMO is summarized in Fig. 2 and is briefly described in the following based on recent regional works presented in Ferrari et al. (2013, 2017). The basement predating the continental batholiths is exposed in the western part of the region in the state of Sinaloa and consists of strongly folded metasedimentary and meta-volcanic rocks, deformed granitoids, phyllite sandstones, quartzites, and quartz-biotite- muscovite schists with ages spanning from Jurassic to Early Cretaceous (Henry and Fredrikson, 1987; Henry et al., 2003). These rocks are lo- cally covered by Albian–Cenomanian limestones north of Mazatlán (Bonneau, 1970). The LVC consists of granite, granodiorite and diorite intrusive rocks exposed in the coastal areas and along the lower course of the main rivers, with ages progressively younger to the east. They form two main plutonic complexes: the San Ignacio batholith exposed towards the coast with mostly Late Cretaceous to early Paleocene ages; and, the Piaxtla batholith, cropping out towards the east along the Piaxtla and Presidio rivers with mostly Eocene ages (Fig. 2). The plu- tonic rocks were extensively studied by Henry (1975), Henry et al. (2003), who published many K–Ar and four U/Pb ages. Volcanic rocks consisting dominantly of ignimbrites with less lava flows are intruded by the Piaxtla batholith. Andesite lava flows cover these successions and the San Ignacio batholith and are sometimes intruded by the Eo- cene plutonic rocks. None of these volcanic successions have been dated so far. Continental conglomerates and sandstones fill intermontane basins and separate the LVC from the UVS. The latter consists of two succes- sions of silicic ignimbrites with minor basaltic lavas and some rhyolitic domes that covers the eastern part of the region. The first ignimbrite succession, mostly exposed towards the east in the Durango state, has been dated at ∼32 to 30Ma (McDowell and Keizer, 1977; McDowell and McIntosh, 2012; Ferrari et al., 2013). The second ignimbrite package, defined as the El Salto-Espinazo del Diablo succession by McDowell and Keizer (1977), yielded Ar–Ar ages of 24–23.5Ma (McDowell and McIntosh, 2012) and is only exposed in the western part of the region. A NNW-trending extensional fault system, named Pueblo Nuevo–Tayoltita (Ferrari et al., 2013), separates the undeformed pla- teau of the SMO to the east, mostly in the Durango state, from the faulted and highly incised terrain to the west, in the Sinaloa state (Fig. 2). In this 90-km-wide, coast-parallel, extensional belt the El Salto- Espinazo del Diablo ignimbrite succession filled pre-existing valleys and lies in angular unconformity (20–30°) over the ∼32 to 30Ma ignim- brite successions, which indicate that a first extensional phase of de- formation took place in the late Oligocene (Ferrari et al., 2013). Large volume rhyolitic domes with ages of ∼29 to 28Ma are aligned along the Pueblo Nuevo–Tayoltita fault system (Fig. 2). This coast-parallel extensional belt is characterized by several late Oligocene to middle Miocene graben, filled with conglomerates and some rhyolitic domes and basaltic lava flows (Ferrari et al., 2013). 2.2. Local geology Previous knowledge on the geology and geochronology of the San Dimas district is summarized in the following. The LVC has been tra- ditionally divided into informal geologic units primarily based on field observations. From base to top, these are the “Socavón rhyolite”, the “Buelna andesite”, and the “Portal rhyolite”, defined as a sequence of interlayered tuffs and lesser lava flows of felsic to intermediate com- position (Locke, 1918; Davidson, 1932; Henshaw, 1953). These rocks are overlain unconformably by a succession of andesitic lavas named “Productive andesite” which is intruded by intermediate rocks called the “Arana intrusive andesite” and the “Arana intrusive diorite” (Henshaw, 1953) as well as by a felsic suite consisting of the “Piaxtla Granite” (partly mapped as Candelero granodiorite in Henry et al., 2003) and “Santa Lucia”, “Bolaños”, and “Santa Rita” dikes. Enríquez and Rivera (2001b) reported K–Ar ages obtained in a commercial la- boratory for these intrusions. The intermediate intrusions yielded feldspar K–Ar ages ranging from 39.9 ± 1 to 36.6 ± 1Ma, which are likely the result of partial resetting given that these intrusions lie near the core of the mineralized area. Henry et al. (2003) obtained a U/Pb age of 47.8 ± 1.0Ma and hornblende and biotite K–Ar ages for six samples of the intrusive suite along the Piaxtla valley ranging from 51.2 ± 1.6 to 43.9 ± 0.3Ma. In the Tayoltita area, biotite con- centrates yielded K–Ar ages of 45.9 ± 1.2 and 45.1 ± 1.1Ma (Enríquez and Rivera, 2001b), and an Ar–Ar age of 46.3 ± 0.1Ma (Enriquez et al., 2018). The LVC is separated from the younger rocks by a major erosional unconformity, marked by the so called “Las Palmas” and “Camichin” units made of conglomerate and red beds (Davidson, 1932; Henshaw, 1953). The overlying UVS, consists of a voluminous package of P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 275 ignimbrites, breccias, and less lava flows (Henshaw, 1953). Enríquez and Rivera (2001b) obtained a K–Ar age of 24.5 ± 0.9Ma for the distinctive “Guarisamey andesite”, located at the base of the sequence, and an age of 20.3 ± 0.8Ma for the upper part of the ignimbrite succession. The structural context has been addressed by Ballard (1980), who investigated the structural control of mineralization in the Tayoltita mine, and by Horner and Enriquez (1999), who studied the structural geology and tectonic control for the whole district. The most prominent structures are major north–northwest–trending normal faults with op- posite vergence that divide the district into five blocks tilted to the ENE or WSW (Enriquez and Rivera, 2001a) (Fig. 3). All the major faults exhibit northeast–southwest extension, and dips that vary from nearly vertical to approximately 55° (Horner and Enriquez, 1999). E–W to WSW–ENE striking fractures, perpendicular to the major normal faults, are filled by quartz veins, dacite porphyry dikes, and pebble dikes, all cut by rhyolite porphyry dikes which occupy N–S to NNW–SSE trending fissures (Smith et al., 1982). Horner and Enriquez (1999) grouped the development of major faults, vein and dikes into three deformational events: 1) event D1, represented by tension gashes with E–W to EN- E–WSW orientation with a slight right-lateral offset, developed in the late Eocene. These structures host the first hydrothermal vein systems. 2) Event D2, produced N–S-trending right-lateral strike-slip to trans- tensional faults due to a rotation of the maximum horizontal principal stress to a ∼NE–SW position. In this stage, inferred to have occurred in the early Oligocene, a second set of hydrothermal veins developed. 3) Event D3 produced the major block faulting that affected the entire district along NNW–SSE-striking normal faults, which in some cases reactivated the former strike-slip faults during the late Oligocene–- Miocene period. These faults hosted bimodal dikes, which are part of the UVS. Fig. 2. Regional geologic map of central Sierra Madre Occidental showing the main extensional structures and published ages (modified from Henry and Fredrikson, 1987, and Ferrari et al., 2013). SIG—San Ignacio graben; CG—Conitaca graben; CHG – Concordia half-graben; VUHG – Villa Unión half-graben; MN—Mala Noche; Cs—Causita. The geologic section includes the ages of intrusive rocks in the Piaxtla valley (projected along the trace of the section) to show the sharp variation between the San Ignacio y Piaxtla batholiths. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 276 o Gulf of California 20 40km D Coastal plain D Late Pliocene basalts - 16-10 Ma basalts D Graben clastic filling Espinazo-El Salto ignimbrite secuence (24 Ma) Oligocene and early Miocene domes (a) and intrusive rocks ~ Eocene and Oligocena ~ ignimbrites Eocene volcanic (a) and intrusive (b) rocks ¡"""111 Laramide are (Late K-Pc) @.....1111 a) volcanic b) intrusive rocks D Pre-Laramide basement ? ---- WSW-ENE extension (29-24 Ma)--- ➔ ~ WSW-ENE extension (20-11 Ma) --------~ Mesa Cacaxtla shield va/cano Town Toll highway Federal Highway Secondary road Master fault Fault O/igocene ignimbrite ? sequence (33-29 Ma) • 40 Ma i--- - --;-- -~~-------+------~ - ------•• .------ . - - ,-----------, 50 Ma i-- -------------------.~~•~ •~ *~ •- *-~~ --------------1 San Ignacio batholith (94-61 Ma) Piaxtla batholith (49-43 Ma) SO Ma 1-- ----,---~,~----.-,-~-.-~-*--.--"o_=_=_=_=_=~-=-=_=_=_=a2-5:::::::::::5.o-km--l 70 Ma i-- -----------:c 1 :----•------ ---------- - ------l A. K-Ar(Hb) e K-Ar(Bio) * U-Pb(Zr) 3. Analytical techniques 3.1. Geologic mapping and sampling A revision of the geologic cartography was carried out by detailed mapping (at a scale 1:10,000) along key transects along roadcuts that crossed all the geologic units of the district. Geologic observations were also made along the Piaxtla river and adjacent creeks. We also made observations along a transect to the south of the district up to the Presidio river valley to compare the stratigraphy of San Dimas with that of the Causita prospective area and the inactive Mala Noche mining area (Fig. 3). Based on the result of fieldwork and the revised strati- graphy we selected twenty-nine representative samples for petrographic and geochronological analyses. Some of these samples come from exploratory drillings provided by Primero Mining Corp (recently acquired by First Majestic Silver Corporation) that were chosen to obtain information from the lowermost part of the succession. 3.2. Petrography Twenty-one thin-sections were analyzed petrographically. The pet- rographic study was made using an Olympus® BX-50 optical microscope with a Qimaging Micropublisher 5 Mp digital camera equipped with a Peltier-cooled CCD. Modal analyses were carried out using an average count of 600–800 points for igneous rocks. For rocks with porphyritic texture, phenocrysts modal proportions (crystal > 1mm) are based on Fig. 3. A) Geologic map of San Dimas mining district and Causita-Mala Noche areas, with location of published and new ages. B) Geologic section. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 277 ® es a an 1n rus1ve roe s igocene ignimbrites (a) and lavas dimentary deposits (Eocene): Palmas Fm., b) Camichin unit sive suite (49-38 Ma) Piaxtla granite; b) El Cristo granit diorite intrusion desitic group (66-47 Ma) NW 2000m San Dimas Río Piaxtla 1000m 30km 23°59.196 ' 105°48.258 " ■ O/igocene ignimbrites Tahuramara formation (77-69Ma) ■ O/igocene domes ■ Piaxtla suite (49-44 Ma) ■ ■ Andesitic group (66-47 Ma) ■ Portal member Bue/na member Socavan member SE a minimum of 800 points per thin-section. Samples were described according to the subcommission on the Systematic of Igneous Rocks standards (Le Maitre et al., 2002) and classified according to the Streckeisen QAP triangle for plutonic and volcanic rocks (Le Bas and Streckeisen, 1991) (Fig. 4). The composition of plagioclase was esti- mated by Michel-Levy’s method wherever possible. Sedimentary rocks were classified according to the conventional FQR triangle for detritus samples (Folk, 1974). 3.3. U/Pb geochronology and zircon geochemistry Twenty-nine rock samples with stratigraphic control were selected for U/Pb dating. The samples were crushed, powdered, and sieved (200–50 mesh). Heavy minerals were concentrated using conventional techniques. Zircon crystals were hand-picked under a binocular mi- croscope and mounted with EpoFix® in a 2.5-cm-diameter plastic ring, and then, were polished. Laser ablation target points were selected based on cathodoluminescence images to identify zircon cores and overgrowth zones. Single-spot analyses were performed with a Resonetics RESOlution® LPX Pro (193 nm, ArF excimer) laser ablation system, coupled to a Thermo® Scientific iCAP® Qc quadrupole ICP-MS at Laboratorio de Estudios Isotópicos (LEI), at Centro de Geociencias, Campus Juriquilla, UNAM. For every zircon, each spot analysis consists in the acquisition of 15 s of background signal (gas blank), 30 s of ab- lation, and 15 s to allow the signal to reach the base-line again (wash- out). The spot diameter was of 33 μm, using a fluency of 6 J/cm2 with a repetition rate of 5 Hz. Together with those isotopes required for the U/ Pb age calculation (206Pb, 207Pb, 208Pb, 232Th, and 238U), LA–ICP-MS permits to detect additional elements simultaneously such as major and trace elements and rare earth elements, etc. These compositional data may also be used to obtain information about the different magmatic pulses in term of mineral fertility. ICP-MS tuning follows the para- meters reported in Solari et al. (2010) and Ortega-Obregón et al. (2014). Corrected isotope ratios and ages with errors were calculated with Iolite (Paton et al., 2011) using the VizualAge data reduction (Petrus and Kamber, 2012). Chemical compositions in zircons were obtained based on NIST standard glasses. “91500” zircon (Wiedenbeck et al., 1995) was used as a main reference mineral for zircon U/Pb analyses. For igneous samples, ∼35 zircons were analyzed. The in- trusive and volcanic ages are reported based on the weighted mean crystallization age into two standard deviations (Ludwig, 2008). Ana- lyses outside of two standard deviation were discarded. Discordant percentages higher than twenty percent were not considered. For vol- canic rocks, the preferred age was considered that of the youngest zircon populations. For igneous samples, crystals with noticeably old ages were interpreted as zircons inherited from the basement. For se- dimentary samples, ∼100 zircons were analyzed. The maximum age of sedimentation was associated to the younger zircons analyzed. The different family ages were reported using histograms and probability density diagrams. 3.4. Apatite fission-track thermochronology Apatite fission-track (AFT) dating was performed at LEI Lab, using LA–ICP-MS-based technique (Hasebe et al., 2004; Donelick et al., 2005). Details of the methodology is described in Abdullin et al. (2018). In this experiment, Durango F-apatite, with an age of 31.4 ± 0.5Ma (Hurford, 2019), was used for ζ-equivalent calibration (Hasebe et al., 2004; Donelick et al., 2005, Vermeesch, 2017) as well as for Cl mea- surements in unknown apatite samples (taking Durango as a primary standard, with 0.43 ± 0.03wt% of Cl in Durango; Goldoff et al., 2012). Single-grain AFT ages with 1SE were calculated using IsoplotR (Vermeesch, 2017, 2018). The central (mean) AFT ages and different age peaks were obtained using RadialPlotter of Vermeesch (2009). 4. Results 4.1. Introduction In this section we present our revision of the stratigraphy of the San Dimas mining district that integrates the results of new mapping and the petrographic and geochronologic studies. Table 1 presents the re- sults obtained for twenty-nine representative samples of the strati- graphy of the San Dimas district and adjoining areas. Experimental results are presented in Figs. 5–8. For igneous rocks (Figs. 5, 6 and 8) we chose the mean age of the dominant population as the most reliable age, but Concordia diagrams are also presented to illustrate the quality of the data. Full analytical data are presented in Table 1a in the ap- pendix. For sedimentary rocks we present probability density diagrams of detrital zircons (Fig. 7). AFT results are shown on Fig. 9 and Table 2a in the appendix. The petrographic characteristics of each geologic unit are presented in Table 2. Macro- and micro-photographs of the samples are presented in the appendix (Supplementary material, Figs. 1A–10A). In our geological re-evaluation, the stratigraphic column is divided into five groups (Fig. 10): 1) the silicic to intermediate ignimbrites and lavas from the lower part of the LVC and the San Ignacio batholith; 2) the Andesitic Group, composed by intermediate lavas and hypabyssal intrusions; 3) the Piaxtla batholithic intrusion and associated dikes, 4) Fig. 4. Petrographic classification (Le Bas and Streckeisen, 1991) of studied samples. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 278 Syenite A QAP diagram plutonic rocks -· SD-018. - o CB-01. SD-080. es0-104 ~ esD-101 \ Granodlorite Quartzdiorite SD-044. \ Alkalit p A QAP dlagram volcanlc rocks o SD-106. CAU-16_001. SD-056. SD-065(1). SD-049. ,# ~~ Rhyolte Rhyodac:b • SD-04 • SD-<>35 SD-079 • MN-14_24b. Qtztrachyte .,.. ..... eao-102 - eso-023 es0-14(2J Latite basalt p Table 1 Summary of new U-Pb ages. Sample Geologic unit and location Long. W (datum WGS84) Lat. N (datum WGS84) Elevation (m asl) Rock type Age (Ma) Error (Ma) Age type Comments Sierra Madre Occidental silicic large igneous province – Upper Volcanic Supergroup CS-06 Second flare-up. First ash flow above a conglomerate covering tilted ignimbrites at Causita 105°46′45.08″ 24°0′47.72″ 2595 Ash flow tuff 26.2 0.29 Tuffzirc average MSWD=0.77. Mean of 14 grains of the dominant population (max age) SD-102 Silicic dike intruded by mafic dike, riverbed near La Puerta 106°00′23.57″ 24°04′4.45″ 415 Silicic pyroclastic dike 27.0 1.00 Tuffzirc average MSWD=0.16. Mean of 24 grains of the dominant population SD-106 Silicic lava near riverbed. Road Tayoltita to San Ignacio 106°08′14.40″ 23°59′31.23″ 290 Porphyritic rhyolitic lava 29.0 0.54 Tuffzirc average MSWD=0.57. Mean of 26 grains of the dominant population CB-01 Shallow intrusion just north of Cebollas, below ignimbrites 105°48́20.94″ 24°42́46.02″ 1200 Fine-grained, hb-rich syenite 31.3 0.48 Tuffzirc average MSWD=3.1. Mean of 14 grains of the dominant population 30.0 0.76 Lower intercept MSWD=1.4 SD-034 Guarisamey andesite member right above SD 035. Piaxtla river east of Guarisamey 105°53′10.53″ 24°08′21.87″ 600 Porphyritic latite basaltic lava 31.0 1.10 Tuffzirc average MSWD=0.59. Mean of the 4 younger grains. Large xenocrystic population at ∼43 to 45Ma. Three Late Cretaceous xenocrysts. SD-035 First volcanic unit above sedimentary unit. Piaxtla river east of Guarisamey 105°53′9.43″ 24°08′13.25″ 595 Reddish ignimbrites with plag, san, qz 31.5 0.80 Tuffzirc average MSWD=1.9. Mean of 6 the younger grains. Probable antecrystic population at ∼32Ma Volcano-sedimentary unit SD-029 Camichin unit, road Tayoltita to Guarisamey 105°54′20.58″ 24°07′19.07″ 975 Lithic tuff 43.2 2.30 Max deposition age, mean of the 2 younger zircons. One peak at 56Ma. 20 zircons in the range 53–68Ma, one at 140Ma and two > 500Ma SD-031 Palmas Formation, road Tayoltita to Guarisamey 105°54′16.28″ 24°07′26.96″ 965 Lithic arkose 52.1 1.40 Max deposition age, mean of the 5 younger zircons. Two peaks at 56 and 65Ma. 99 Zircons in the range 52–82Ma. One at 95Ma. Lower Volcanic Complex Piaxtla intrusive suite SD-023 “Santa Rita”-type dike cutting Piaxtla batholith, 3 km west of Tayoltita 105°56′50.26″ 24°05′08.30″ 543 Porphyritic andesite with plag, hb, bio and qz 45.0 0.51 Tuffzirc average MSWD=0.25. Mean of 24 grains of the dominant population SD-032 “Bolaños”-type dike cutting Piaxtla batholith, road to Guarisamey, 3 km east of Tayoltita 105°54′08.25″ 24°07′43.96″ 790 Porphyritic dacite with qz, plag, hb 45.4 0.66 Tuffzirc average MSWD=0.41. Mean of 21 grains of the dominant population SD-040 Piaxtla batholith, San Francisco tunnel 105°55′49.24″ 24°06′36.25″ 463 Fine grained granite with qz, plag, bio, hb, px 45.2 0.85 Tuffzirc average MSWD=1.5. Mean of 27 grains of the dominant population SD-101 Piaxtla batholith, riverbed at La Puerta 106°00′29.15″ 24°04′17.85″ 417 White, coarse grained granodiorite with qz, plag, feld, hb, px, bio 47.0 0.75 Tuffzirc average MSWD=1.4. Mean of 29 grains of the dominant population SD-079 Porphyritic andesite, “Santa Rita” dike, Santa Rita mine near entrance fo the main tunnel. 105°54′16.77″ 24°07′11.02″ 800 Porphyritic rhyodacite 47.5 0.93 Tuffzirc average MSWD=4.9. Mean of 27 grains of the dominant population SD-065(1) Porphyritic andesite, “Santa Lucia” dike, San Luis tunnel between Santa Lucia and San Salvador veins 105°58′21.58″ 24°08́11.27″ 799 Porphyritic rhyodacite 48.0 0.67 Tuffzirc average MSWD=1.4. Mean of 20 grains of the dominant population SD-080 Porphyritic andesite, “Santa Lucia” dike, San Luis tunnel between Santa Lucia and San Salvador veins 105°58′21.58″ 24°08́11.27″ 799 Porphyritic quartzomonzonite 48.0 0.46 Tuffzirc average MSWD=1.3. Mean of 24 grains of the dominant population SD-016 Piaxtla batholith, “El Cristo” facies, near entrance of the tunnel. Intrusive contact with Socavón member 105°56′15.30″ 24°05′34.96″ 573 Fine grained granite with qz, alk feld, plag, hb 49.0 0.43 Tuffzirc average MSWD=0.25. Mean of 35 grains of the dominant population. 9 xenocrysts in the range 74–65Ma (age of Tarahumara Fm) (continued on next page) P . M o n to y a -L o p era et a l. Ore Geology Reviews 105 (2019) 273–294 279 Table 1 (continued) Sample Geologic unit and location Long. W (datum WGS84) Lat. N (datum WGS84) Elevation (m asl) Rock type Age (Ma) Error (Ma) Age type Comments SD-104 Piaxtla batholith, riverbed 7 km west of La Puerta 106°03′44.40″ 24°02′20.50″ 350 Medium to fine grained granodiorite with plag, qz, bio, hb 49.1 0.94 Tuffzirc average MSWD=0.94. Mean of 21 grains of the dominant population San Ignacio batholith SD 107 San Ignacio batholith, 16 km east of San Ignacio. Locally completely altered to unconsolidated sand 106°16′9.50″ 23°57′17.83″ 253 Medium grained granodiorite 64.2 0.90 Tuffzirc average MSWD=3.2. Mean of 25 grains of the dominant population Tarahumara formation SD-049 Portal member. San Jose tunnel, recess 598 105°59′53.92″ 24°07′26.12″ 878 Lithic rhyodacite ignimbrite 69.0 1.70 Tuffzirc average MSWD=0.92. Mean of 4 grains of the dominant population SD-014(2) Buelna member, 4 km SSW of San Dimas 105°58′15.89″ 24°07′05.51″ 665 Porphyritic andesitic lava 69.0 0.85 Tuffzirc average MSWD=3.6. Mean of 16 grains of the dominant population SD-056 Porphyry intrusion within the Socavón member, stream at the Sinaloa-Durango border 105°59′50.07″ 24°06′51.79″ 620 Porphyry rhyodacite 73.0 1.50 Tuffzirc average MSWD=1.7. Mean of 14 grains of the dominant population SD-004 Socavón member, Tayoltita to San Dimas road 105°55′49.12″ 24°08′08.39″ 1176 Lithic porphyritic rhyolitic lava 75.0 0.77 Tuffzirc average MSWD=1.02. Mean of 19 grains of the dominant population SD-020 Socavón member, near Piaxtla river 2 km west of Tayoltita 105°56′49.18″ 24°05′23.49″ 570 Quartz latite lava 75.4 0.80 Tuffzirc average MSWD=0.57. Mean of 20 grains of the dominant population MN-14_24b Mala Noche drillhole. This rock is observed intruding the whole drilling 105°44′47.981″ 23°54′15.923″ 1038 Px-rich porphyritic quartz latite 75.5 1.00 Lower intercept MSWD=4.7 MN-14_24 Mala Noche drillhole. 105°44′47.95″ 23°54′15.906″ 950 Andesitic lava 76.1 0.63 Tuffzirc average MSWD=4.6. Mean of 28 grains of the dominant population CAU-16_001 Causitas drillhole. This rock includes clast of the whole sequence. Dated as detrital 105°46́56.21″ 24°0́58.24 2358 Lithic rhyodacite lava 76.5 Peak in PDD 91 zircones in the range 70–83Ma, one at 91Ma and one at 131Ma CS-01 Lava hosting the an epithermal vein at Causita. 105°46′58.50″ 24° 0′59.08″ 2518 Rhyodacite ignimbrite 77.5 Peak in probability density diagram 93 zircons in the range 71–84Ma, one at 90.5 Ma and four at 132–139Ma 77.7 0.88 Tuffzirc average MSWD=5.3. Mean of 31 grains of the dominant population Preferred ages in bold. P . M o n to y a -L o p era et a l. Ore Geology Reviews 105 (2019) 273–294 280 Fig. 5. Histograms and concordia diagrams of U-Pb ages for zircons of the Lower Volcanic Complex. Errors in calculated ages are 2σ. Black ellipses are data points not used in calculating the weighted mean. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 281 Socavón member at San Dimas 0.021 130 0.019 =i 0.017 :1l ~ 0.015 ~ 0.013 0.011 2a error ellipses o.oos,~~~--------~ 0.05 0.07 0.09 0.11 0.13 0.15 207Pb/235U 130 SD-04 Boxes 2a U/Pb age (Mean): 75.0± 0.77Ma MSWD= 1.02 120 80 1, , ,, , 11111111 1111 70 11 11 60L..---------- Porphyry intrusion in Socavón member 0.0145 =i 0.0135 :1l 'a Poo.0125 ~ 0.0115 0.0105 92 64 2crerrorellipses 0.0095~~-~--~--~-~ 0.02 0.06 0.10 0.14 0.18 0.22 0.26 0.30 207Pb/235U 92----------- 64L..---------~ Buelna member 0.022 0.018 110 0.014 70 0.010 2crerrorellipses 0.006 L..~-------~-~ 0.02 0.06 0.10 0.14 0.18 0.22 207Pb/235U SD--014 (2) Boxes 2cr 130 ~:t~~ei~ean): 69.0±0.85Ma J 1 m110 ~ g j 90 n, ¡ ¡ lrl 70h--H"1H-<+l4-1-1--'..1.J.-'-'- ----l ,o c_ _________ _, Portal member ::, :1l 0.034 0.030 0.026 i 0.022 ~ 0.018 0.014 O.Q10 0.0115 0.0105 0.0095 210~---------~ SD-049 Boxes 20' 190 ~ft~~e0~~;an): 69.0± 1 . 7 Ma 180 :'" 170 140 i150 '";130 g j 110 I I 00 2crerrorellipses 70 f-~,-JC-J-=------ 50 L..---------- 0.10 0.14 0.18 0.22 0.26 207Pb/235U Sinaloa balholilh 207Pb/235U 76 SD-107 Boxes 2cr 72 U/Pb age (Mean): 64.2± O. 9 O Ma MSWD= 3.2 55 c_ _________ _J Socavón member at Piaxtla river 0.017 115 SD-020 Boxes 2a U/Pb age (Mean): 75.4± 0.87Ma MSWD= 0.57 110 105 0,015 0.011 2aerrorellipses 60 0.009 L..~---------_j 55 c_ ________ __J O.OS 0.07 0.09 0.11 0.13 207Pb/235U Socavón equivalent at Causita 0.024 ~ 0.020 'a il; ~ 0.016 0.012 110 O.Q08L..-"'-~--~--~-..., 0.03 0.05 0.07 0.09 0.11 0.13 0.15 0.17 207Pb/235U 140 CS-01 Boxes 2cr ~;i~~e8~~ean): 76.3± 0.8Ma ¡ 11 120 100 i 80 . 1 1 ~ 60 1,1111r11111 ¡, ! 40 20 Socavón equivalent - Causita drilling 0.022 0.020 13_ □-- . 0.018 0.016 0.014 0.012 0.010 0.008CL..---------__J 0.05 0.07 0.09 0.11 0.13 0.15 0.17 207Pb/235U Socavón equivalent - Mala Noche drilling 0.0130 0.0126 0.0122 0.0118 0.0114 0.0110 0.0135 0.10 207Pb/235U 82•-------~-~ MN-14 24 Boxes 2a U/Pb a'ge (Mean): 75.94± 1O.79 Ma 80 MSWD= 7.5 tU 78 ~ . g> 76 111111 i 1111 74 72 70 0.14 Socavón equivalent - Mala Noche drilling "'---------~ 2a error ellipses MN-14_24b Boxes 2cr 85 U/Pb age (Mean): 76.53± 0.61 Ma 1 MSWD= 8.6 1 83 ~ 0.0125 • ~ ;: ' " 1, ,,11111 1111111111 11111111 11 'a . "- g ~ i 0.0115 lnterceptsat 75.5±1.0 & 3134±640 Ma 75 111 111111111111 111111' ,1111 ;: ¡11 MSWD=4.7 0.0105 59 c_ ________ _j o.os 0.07 0.09 0.11 0.13 207Pb/235U the Las Palmas and Camichín continental sedimentary deposits, and 5) the Oligocene to early Miocene silicic ignimbrites, rhyolite domes and mafic lavas belonging to the UVS. It is important to note that a large part of the district was affected by hydrothermal alteration, mainly in the form of propylitization (chlor- ite–epidote–calcite–pyrite). This alteration is pervasive at a local scale, increases in intensity close to the mineralized structures and it is overprinted by phyllic alteration close to vein structures. 4.2. Lower volcanic complex 4.2.1. Late Cretaceous volcanic sequence The oldest rocks belong to a thick volcanic succession exposed in the lower part of the valleys north of the Piaxtla River with a total thickness of approximately 2 km. The succession is composed by a series of an- desitic to rhyolitic lava flows, tuffs, and breccias defined as the Socavón, Buelna and Portal members. The Socavón member consists of an alternating suite of rhyolitic and andesite lavas locally intruded by mineralized felsic porphyritic bodies. It commonly presents a por- phyritic texture, reddish to gray in color, and consists of quartz > orthoclase > plagioclase > pyroxene > biotite and hornblende in a Fig. 6. Histograms and concordia diagrams of U-Pb ages for zircons of the Piaxtla intrusive suite. Errors in calculated ages are 2σ. Black ellipses are data points not used in calculating the weighted mean. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 282 20 Piaxtla Andesitic Laramide magmatism Pre-balholithic basement suite group 18 16 O SD 031 - Palmas Formation U/Pb max age: 52.1± 1 . 4 Ma 14 (mean of the 5 younger grains) 111 ii 12 ~- ,, a :. 10 cr ., .Q g E :::, ~ z 8 6 4 2 o 30 40 50 60 70 80 90 100 110 120 130 140 150 20 18 16 D SD 029 - Camichín member U/Pb max age: 43.2± 2. 3 Ma 14 (mean of the 2 younger grains) ;a CD ii 12 :.:· CD ,, .. a cr ., 10 ., .Q g; E :::, ~ z 8 6 4 2 o 30 40 50 60 70 80 90 100 11 O 120 130 140 150 Age (Ma) Ore Geology Reviews 105 (2019) 273-294 A Piaxtla batholith at La Puerta 0.010 82 0.0086 040 Boxes 20 a SD-101 Boxes 20 U/Pb age (Mean): 45,2+ 0.85Ma U/Pb age (Mean): 47.0+ 0.75Ma 58) MSWD= 1.5 | 0.0082 52 | MSWD=1.4 ' , 54 0.0078 50 TI! ¿ g | E 3 3” = 5% | e 0.0074 348 ell : ; MN: ETA Só au | 8 5 | | S 0.007 y Lt 10D ] g 0.0070 qe ! | | BI : , 4 | 0.0068 44 | 0.006 Á 36/- 38 | 0.0062 o 42 20 error ellipses o 20 error ellipses 0.005 4 0.0058 40 0.01 0.03 0.05 007 009 0411 0.00 0.02 0.04 0.06 0.08 010 0.12 0,14 207Pb/235U 207Pb/235U Bolaños dike at Guarisamey Santa Rita dike 53 SD-032 Boxes 20 55 SD-079 Boxes 20 0.0082 U/Pb age (Mean): 45.4 0.66Ma 0.022 140 U/Pb age (Mean): 47.61 1.0 Ma 51 | MSWD=0.41 EE. 53 | MSWD=6.0 0.0078 0.018 Y 7 5 | 49 > z 2 100 Ts 09 E 0.0074 Sy S 0014 e Z | I IM E : il E AMI per S lll | TT 0.0070 Ss ae | 0.010 a 5 43 0.0066 43 0.006 4 20 20 error ellipses 0.0062 d ' 41 0.002 39 0.03 004 005 008 007 008 0.00 0.04 0.08 0.12 0.16 207Pb/235U 207Pb/235U Santa Rita dike Santa Lucia porphyry intrusion 0.0082 51 0.0086 52/ $D-023 Boxes 20 SD-065(1) Boxes 20 P IP age (Mean): 45.0:0.51Ma 80 U/Pb age (Mean): 47.35+ 0.49 Ma 0.0078 5o/ 49 7 0.0082 MSWD= 0.88 / 56 > 0.0078 > _ E 3 0.0074 E" 53 E 8 2 3 0.0074 2 E E Cl | g z | UI 00070 3 lll 1 8 E aL | 3 A | 0.0070 | TEE 0.0066 : di 0.0066 4 / 20 error ellipses / 20 error ellipses 0.0062 L——40 4 0.0062 40 0.034 0.038 0.042 0.046 0.050 0.054 0.058 0.062 0.025 0.035 0.045 0.055 0.065 0.075 0.085 207Pb/235U 207Pb/235U Piaxtla batholith "El Cristo” facies Santa Lucia porphyry intrusion 0.0115 5 78) SD-016 Boxes 20 / SD-080 Boxes 20 U/Pb age (Mean): 49.01 0.43Ma 70 U/Pb age (Mean): 48.11 0.46Ma 0.012 74. MSWD= 0.25 pl 0.0105 Doo 51. MSWD= 1.19 10 | FÉ | / 2 E 66 l 3 80 g* L| | | | 8 0.010 Z 3 / Ez - N — / T 5 o 62 £ / o | [l | | l z 2 ¿2 0.0085 Í DS 47 8 = o [ - A ¿ 58 Ñ É 0.008 . _ 54 0.0075 45 . > qa ES | E a) —— dol > tana 0.0065 43 e 20 error ellipses / 20 error ellipses 0.006 ¿ 2 0.0055 / 41 0.00 0.02 004 006 008 0:10 012 0.02 0.04 0.06 0.08 0.10 207Pb/235U 207Pb/235U Piaxtla batholith westernmost part 68 | SD-104 Boxes 20 0.0086 U/Pb age (Mean): 49.11 0.04Ma 64. MSWD= 1.6 0.0082 $0 > mm 3 5 5 0.0078 5 5 S 5 ||| | [ NA 8 ] GIO ] ] ] | 0.0074 D 4 [PTETES al! 0.0070 ] 40 0.0066 a : : : % 0.03 0.04 0.05 0.06 0.07 008 0.09 207Pb/235U Fig. 7. Histograms and probability density diagrams for detrital zircons from the Las Palmas (upper panel) and Camichin (lower panel) continental sedimentary deposits, with indication of the main magmatic pulses in the San Dimas region. P. Montoya-Lopera et al. re eology evie s 5 19) –294 283 Piaxtla batholith • San Francisco tunnel 0.009 ::, ~ 0.008 ~ :g N 0,()()7 0. 06 . 3 . .07 .09 . b/ 35U 62~--------~ SD-040 oxes a / b e ( ean): . ± . a 8 SWD= 1.5 4 111 11 1 !so 1l, ~46 ~ 1111 2 8 ¡I 34 l os i e l uari ey 3~--------~ -032 oxes a 0. 082 / b e ean): . ± a SWD= 0.41 0. 078 . 74 0.0070 0. 0 6 b/ 35U nta it i e 0.0082 ~------5-2 _ ___ _ 0. 078 i . 74 <¡:: il; ~ 0.0070 0.0066 0.0062 ~~ ~ ~~~~~~~~~ . 4 . 8 . 2 . 6 . 0 . 4 . 8 . 2 b/ 35U 51 ~--------~ S -023 xes a U/Pb age (Mean): 45.0± 0.51 Ma MSWD= 0.25 49 {47 ID il' ! 45 t-+mt+++++H++++++t+H+++t+H,......, 43 41 ~--------~ i xtla atholil l ri t " i s 0.012 ¡¡: gJo,0 :a a. ~ 0.008 a rr rellips s . 2 .04 .06 .08 . .12 b/235U 8 -016 xes a / b e ean): . ± a I ' S D• . 5 I 111 70 1111 rn 6 ~ ID 2 il' ! 58 54 42~--------~ i xtla atholit est r ost art 0.0006 0. 082 ~ ~ 0. 078 a. ~ 0. 074 0. 070 -104 xes a / b e ean): . ± 94- a 4 D= . ., ! ffi ID il' 52 ¡¡ "' 40 " I 40 1 0.0066~~ ~ ~~~~~~~~ 36~--------~ . 3 . . 5 . . 7 .08 . 9 / ~ i xtla tholit l a uerta 0.0086 ~----------- 0. 082 0.0078 . 74 0.0070 0.0066 0.0062 2aerrorellipses 0.0058 ~~~ ~ ~~~~~~~~ O.DO 0.02 0.04 0.06 0.08 0.10 0.12 0.14 b/ 35U 54~--------~ 42 0~--------~ Santa ita dike 5 5 -079 oxes a . 2 140 / b age ( ean): . ± . O a 53 MSWD= 6.0 1111 I 0.018 51 0 ! 49 0.014 ID g1 47 ;; 0.010 ~ 45 . 6 1 a re lips 0. 02 9 0. 0 0.04 0.08 0.12 . 6 b/ 35U anta cia r hyry i .0086~----~ ------ -065(1) xes a 60 /Pb ge ean):47.35±0.49 a 0.0082 S D= 0.88 0. 078 ~ . 74 ~ 0.0070 0. 0 6 a re lips 0.0062 ~~ ~ ~~~~~~~~~ . 5 . 5 . 5 . 5 . 5 . 5 . 5 b/235U 0~--------~ anta cia r hyry i tr si 0.0105 i0.0095 ~0.0 ~ 0. 075 0.0065 60 ae renips 0.04 0.06 0.08 b/ 35U 3 -080 Boxes 2a / b e ( ean): . ± . ~a 1 D= r ]¡¡ j ¡ 1 ¡11 j j 43 1 groundmass of fine plagioclase and glass with lithic fragments at the base. This member crops out throughout the district with up to 800m of thickness. The alteration of mineralized porphyritic bodies is associated with pervasive secondary biotite overprinted by sodic–calcic (albi- te–actinolite–epidote) alteration around sinuous quartz, pyrite, and chalcopyrite veins. The mineralized bodies are exposed in the western Block in the Contraestaca area (Fig. 3). We obtained two ages of 75.4 ± 0.8 and 75.0 ± 0.7Ma for the Socavón member (SD 020 and 004) and an age of 73.0 ± 1.5Ma for a porphyry dacitic intrusion (SD 056) (Fig. 5). The latter sample displays a wide range of grain ages from 69 to 82Ma. The Buelna member is a sequence of intermediate lavas characterized by porphyritic-fine texture and bedding, which com- monly contain anhedral to subhedral crystals of plagioclase > quartz > hornblende > biotite in a groundmass of fine plagioclase. This member crops out throughout the district on top of the Socavón member with a thickness ranging from 20 to 100m. One sample from the type locality yielded an age of 69.8 ± 0.85Ma (SD 014-2) (Fig. 5). The Portal member is a sequence of rhyolite flows and tuffs with a lithic-rich layer at the base and a thickness of 50–250m. It contains anhedral crystals of quartz > plagioclase > biotite in a fine-grained sericitized groundmass. We obtained an age of 69.0 ± 1.7Ma (SD 049) (Fig. 5) for a sample of the Portal member from the interior of the mine. Volcanic rocks lithologically similar to the Socavón member were Fig. 8. Histograms and concordia diagrams of U-Pb ages for zircons of the Upper Volcanic Supergroup. Errors in calculated ages are 2σ. Black ellipses are data points not used in calculating the weighted mean. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 284 ~ '2i ~ lgnimbrite of the second flareup 0.010 => 0.008 ~ a. ~ 0.006 0.004 0.002 0.00 0.02 2o error enipses 0.04 207Pb/235U 0.06 0.08 65 CS-06 Boxes 2a U/Pb age (Mean): 26.4± 0.6Ma 55 MSWD= 5.2 15 Silicic dike at La Puerta o 20 25 30 35 40 P;¡e(Ma) ~ M ~ U W ~ M M ~ ~ ~ ~ ~ ~ Age(Ma) 0.0064 40 c---------,,--,,-, , ------------7 Boxes 2a 0.0060 0.0056 36 0.0052 0.0048 0.0044 0.0040 0.0036 2a error ellipses SD-102 38 U/Pb age {Mean): 27.0± 1 . O Ma MSWD= 0.16 36 1400 200 . ,: ::.: -·. ... 26 28 30 P;¡e(Ma) 28 0.0032 22~---------~'~-----------~ 0.00 0.04 0.08 207Pb/235U 0.12 0.16 Guarisamey andesite 0.014 0.012 al ~ O.D10 :a il; ~º-º-~- 0.006 0.004 2a error ellipses 0.002 ~~-~-~-~~-~- 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 207Pb/235U 85 SD-034 Boxes 2a U/Pb age (Mean): 31.0± 1 . 1 Ma 75 MSWD= 0.59 65 Í 55 w ~ 45 ] 35 25 15 Lowermost ignimbrite of the first fiare up 0.0058 0.0054 ~ / (") / ~ p.úoso ~( N\~_QQ46 0.0042 0.04 0.06 207Pb/235U 38----------- SD-035 Boxes 2cr U/Pb age (Mean}: 31.5± 0.80Ma 36 MSWD= 1.9 "34 ~ g> 32 ] ++<++++++++<++++++++<-+---< 30 28 26~---------~ Rhyolite lava • Piaxtla river 0.0054 0.0050 0.0046 0.0042 0.0038 ~-~~-------- O.ü18 0.022 0.026 0.030 0.034 0.038 0.042 0.046 207Pb/235U SD-106 Boxes 2o 33 U/Pbage(Mean):29.0±0.54Ma MSWD= 0.57 31 rn ~ g, 29 r-+tttttttttttt-1+'~~----, 25~---------~ Subvolcanic body at Cebollas 0.0055 0.0053 ~ 0.0051 '2i ~ 0.0049 0.0047 30 0.0045 /, ( 0.00430.02 0.03 0.04 207Pb/235U lntercepts al 29.98±0.76 Ma MSWD=1.4 0.05 0.06 34 CB-01 Boxes2a U/Pb age (Mean): 31.3±0.48Ma MSWD= 3.9 1 1 29 28~---------~ mapped south of the San Dimas district (Fig. 3). The main outcrops are in the creek west of Causita and in the Presidio River valley in the Mala Noche mining area. Two samples from the Causita area (CAU 16 001, CS 01) and two more from drillings in the Mala Noche area (MN 14-24, MN 14-24b) yielded tightly clustered ages comprised between 77.7 ± 0.88 and 75.5 ± 1.0Ma (Fig. 5, Table 1), which confirm the correlation with the Socavón member. 4.2.2. San Ignacio batholith Our new Late Cretaceous ages for the lower volcanic succession at San Dimas make them coeval with the intrusive assemblage forming the San Ignacio granodiorite as defined by Henry et al. (2003), which is exposed west of the district along the Piaxtla river in the homonymous area (Fig. 2). Along the riverbed the batholith is clearly distinguishable from the younger Piaxtla batholith for its pervasive alteration and a more mature erosional landscape. The dominant lithology is a medium- grained equigranular granodiorite which contains anhedral biotite, hornblende, and clinopyroxene. We have dated one sample from the eastern part of the batholith, 16 km east of San Ignacio obtaining an age of 64.2 ± 0.9Ma (SD 107) (Fig. 5), which is in line with previous K–Ar ages reported by Henry et al. (2003). 4.2.3. Andesite group Andesitic subvolcanic bodies and lava flows are found intruding and covering, respectively, the Late Cretaceous succession of the San Dimas district with a cumulative thickness comprised between 200 and 850m. Observations in mine interior indicate that the group display a range of texture without intrusive contacts between them. The sequence com- monly starts with an andesite lava agglomerate with a fine texture, rich in lithic fragments (“Productive andesite”), which passes transitionally to a porphyritic texture (“Intrusive andesite”) and then to a granular texture (“Arana diorite”). Regardless of the texture the andesite group is normally composed by plagioclase > hornblende > pyroxene > quartz ± biotite in an aphanitic groundmass (Fig. 11). Due to high hydrothermal alteration, these andesite bodies could not be dated by the 40Ar–39Ar method and because of their chemistry, zircon crystals are very rare. After several attempts we could separate a few zircons, which yielded a ∼63Ma age. Although insufficient to provide a sta- tistically robust age, this value is consistent with the stratigraphic po- sition of the Andesite group. Porphyritic andesite intrusions, composed by medium to fine crystal of plagioclase > quartz > biotite > hornblende > pyroxene and chlorite in a fine plagioclase groundmass cut the andesite group in the Santa Lucia mine. We have dated two of these bodies (SD 065-1, SD 080), which yielded indistinguishable ages of ∼48Ma. These crosscutting relationships and the few dated zircons indicate that the andesite group was emplaced in the Paleocene-early Eocene time span. 4.2.4. Piaxtla intrusive complex Intrusive bodies of dioritic, granodioritic and granitic composition were grouped into the Piaxtla intrusive complex. This intrusive complex is part of a batholith well exposed in the Piaxtla river that intrudes the lower volcanic succession and the Andesite Group. The Piaxtla intrusive rocks present a wide range of texture, from equigranular to porphyritic (in dikes). The dominant lithology consists of medium to coarse grained granite to granodiorite. The oldest body, called “El Cristo granite”, is exposed just south of the Piaxtla River near Tayoltita (Fig. 3). El Cristo is a fine-grained granite rich in K-feldspar that yielded an age of 49.0 ± 0.43Ma (SD 016) (Fig. 6). A similar age of 49.1 ± 0.94Ma was obtained in the westernmost part of the batholith along the Piaxtla river (SD 104) (Fig. 6). The main Piaxtla body consists of a medium to coarse grained granite with quartz > plagioclase > feldspar > biotite, but richer in pyroxene and hornblende. Two samples taken ∼8 km apart gave slightly different ages of 47.0 ± 0.75Ma and Fig. 9. Apatite fission-track results for samples SD-040 and SD-101. Radial plots were constructed using RadialPlotter of Vermeesch (2009). Ngr – number of apatite grains dated; P(χ2) – chi-squared probability test; Ns – number of spontaneous tracks counted to calculate the track densities in individual apatites. The track density (ρs) versus 238U concentration (analogue to isochron) and the single-grain age versus Cl content diagrams are also presents. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 285 SD-040 (Ngr = 47) 10 50 CentralAFTage= 18.1±1.1 (1cr)Ma R = 0.38 and R' = 0.14 Dispersion = 28% " 9 P(x') = O.DO 8 40 l . . 'ñi' .. ~ E 6 a, 30 u "' 1 ni 1:, 5 e: ~ "i~ ~ 4 o "' 20 ri f- ~ LL < peak-1: 24.7±1.9 Ma (ca. 54%) 2 i 10 o '.e peak-2: 12.6±1.2 Ma (ca. 46%) (fl t 38 30 5% o SD-040 (Ngr = 47) with fo error bars SD-040 (Ngr = 47) o o Vo o 10 20 30 40 50 60 70 80 90 100 0.5 1.5 2 7 Ns 16 238U [ppm] chlorine [wt.%] SD-101 (Ngr = 27) 10 50 Central AFT age = 17.4±1.3 (fo-) Ma R = 0.77 and R' = 0.59 Dispersion = 29% " 9 P(x') = 0.00 8 40 'ñi' 1 "" ~ E 6 o a, 30 "' u ni 1:, e: ~ o o o -~ ~ 4 o o "' 20 _, ri o f- 3 LL f c,00 o o < 2 fl'l'0oo o 10 peak-1: 23.3±1.9 Ma (ca. 54%) peak-2: 12.4±1.1 Ma (ca. 46%) ºcP °" 1 ," /' 1 118% SD-101 (Ngr = 27) o with fo error bars SD-101 (Ngr = 27) Vo o 20 30 40 50 60 70 80 90 100 0.5 1.5 2 9 Ns 39 2,su [ppm] chlorine [wt.%] Table 2 Petrography summary. Sample Geologic unit and location Long. W Lat. N Elevation (m asl) Rock type Age (Ma) Petrography description Sierra Madre Occidental silicic large igneous province – Upper Volcanic Supergroup CS-06 Second flare-up. First ash flow above a conglomerate covering tilted ignimbrites at Causita 105°46′45.08″ 24°0′47.72″ 2595 Ash flow tuff 26.2 ± 0.29 Ash flow tuff. Sequential porphyritic texture, hialocrystaline with subeuhedral crystals of quartz (84%) > plagioclase (An 9%) > feldspar (Alk+Alb 7%) > biotite in a vitreous matrix. Clasts of andesite ignimbrite with epidote alteration and andesite ignimbrites with sericite and pyrite alteration. Strong sericite alteration overpirnted by epidote and Fe oxides in clasts. Two types of pyrites are presented, fine and coarse grain less than 1%. SD-102 Silicic dike intruded by mafic dike, riverbed near La Puerta 106°00′23.57″ 24°04′4.45″ 415 Silicic pyroclastic dike 27.6 ± 1.00 Dacite pyroclastic dike. Sequential porphyritic texture, hialocrystaline and anahedral crystals of quartz (53%) > biotite > plagioclase (An 32%) > feldspar (Alk+Alb 15%). Strong pervasive sericite alteration. Two types of pyrites are presented: coarse and fine grain both less than 1%. SD-106 Silicic lava near riverbed. Road Tayoltita to San Ignacio 106°08′14.40″ 23°59′31.23″ 290 Porphyritic rhyolitic lava 29 ± 0.54 Rhyolitic lava with quartz “eyes”. Sequential porphyritic texture, holocrystaline and subeuhedral to anahedral crystals of quartz (70%) > feldspar (Alk+Alb 17%) > plagioclase (An 13%) > biotite. Strong diseminated sericite alteration overpirnted by pervasive moderated epidote. Two types of pyrite were found: 1) palid coarse grain (< 0.01%) and fine grain (< 0.01%). Overgrown quartz textures are present CB-01 Shallow intrusion just north of Cebollas, below ignimbrites 105°48́20.94″ 24°42́46.02″ 1200 Fine-grained, hb-rich syenite 31.2 ± 0.43 Syenite. Coarse to medium grain, equigranular texture, holocrystaline and anahedral crystals of feldspar (Alk+Alb 58%) > plagioclase (An 22%) > quartz (19%) > hornblende. Strong pervasive alteration of epidote, sericite and weak chlorite and calcite. Medium grain sphalerite is presented (< 3%) with fine pyrite inclutions (< 2%), Fe oxides were also found. 30 ± 0.76 SD-034 Guarisamey andesite member right above SD 035. Piaxtla river east of Guarisamey 105°53′10.53″ 24°08′21.87″ 600 Porphyritic latite basaltic lava 31 ± 1.1 Latite basaltic lava. Porphyritic texture, holocrystaline of subeuhedral plagioclases (An 74%) > feldspar (Alk+Alb 23%) > quartz (3%) in a microcrystaline matrix (30%) with Fe oxides. Moderated sericite and biotite alteration in crystals and weak calcite. Fine anahedral pyrite is presented less than 1%. Broken crystals of plagioclase and quartz are presented. SD-035 First volcanic unit above sedimentary unit. Piaxtla river east of Guarisamey 105°53′9.43″ 24°08′13.25″ 595 Reddish ignimbrites with plag, san, qz 31.5 ± 0.80 Reddish rhyodacite ignimbrite. Porphyritic texture, hialocrystaline with anahedral crystals of feldspar (Alk+Alb 40%) > plagioclase (An 31%) > quartz (29%). Strong pervasive sericite alteration overprinted by moderated chlorite and weak calcite. Two types of pyrite are presented: 1) fine grain in matrix (< 3%) and 2) coarse grain asociated to coarse quartz (< 1%). Coarse chalcopyrite is also presented less than 1% Volcano-sedimentary unit SD-029 Camichin unit, road Tayoltita to Guarisamey 105°54′20.58″ 24°07′19.07″ 975 Lithic tuff 43.2 ± 2.30 Lithic tuff. Lithic fragments of porphyritic andesites, traquite andesites, rhyolites and sedimentary rocks inmerse into a vitreous matrix. Strong sericite alteration in matrix. Coarse grain pyrite is presented less than 1% associated to the lithic fragments. SD-031 Palmas Formation, road Tayoltita to Guarisamey 105°54′16.28″ 24°07′26.96″ 965 Lithic arkose 52.1 ± 1.40 Lithic arkose. Detritic rock with clastic texture in a feldspar cement. Sandstone grain size of feldspar > lithic fragments > quartz. Sub-rounded to rounded clasts and inmadure. Lithic fragments of porphyritic andesites, felsic intrusions, volcanic and sedimentary rocks. Strong pervasive alteration of sericite, chlorite, epidote. Lower Volcanic Complex Piaxtla intrusive suite SD-023 “Santa Rita”-type dike cutting Piaxtla batholith, 3 km west of Tayoltita 105°56′50.26″ 24°05′08.30″ 543 Porphyritic andesite with plag, hb, bio and qz 45 ± 0.51 Porphyritic andesite. Porphyritic texture, holocrystaline and subeuhedral crystals of plagioclase (An 57%) > feldspar (Alk+Alb 29%) > quartz (14%) > hornblende > biotite in a microcrystaline matrix of plagioclase. Strong alteration in crystals of sericite overprinted by moderated biotite and weak calcite and chlorite. Broken coarse pyrites are less than 1%. Overgrown quartz and graphic textures are present SD-032 “Bolaños”-type dike cutting Piaxtla batholith, road to Guarisamey, 3 km east of Tayoltita 105°54′08.25″ 24°07′43.96″ 790 Porphyritic dacite with qz, plag, hb 45.4 ± 0.66 Porphyritic dacite. Sequential and porphyritic texture, holocrystaline and anahedral crystals of plagioclase (An 53%) > quartz (25%) > feldspar (Alk+Alb 22%) > hornblende in a microcrystaline matrix of silice. Strong pervasive sericite (continued on next page) P . M o n to y a -L o p era et a l. Ore Geology Reviews 105 (2019) 273–294 286 Table 2 (continued) Sample Geologic unit and location Long. W Lat. N Elevation (m asl) Rock type Age (Ma) Petrography description alteration overprinted by moderated chlorite in crystals. Coarse euhedral pyrite is less than 1%. SD-040 Piaxtla batholith, San Francisco tunnel 105°55′49.24″ 24°06′36.25″ 463 Coarse grained granite with qz, plag, bio, hb, px 45.2 ± 0.85 Coarse grain granite. Coarse equigranular texture, holocrystaline and subeuhedral crystals of quartz (46%) > feldspar (Alk+Alb 30%) > plagioclase (An 24%) > biotite > hornblende. Two events of plagioclase and biotite are presented: 1) with alteration, 2) without alteration. Strong alteration in crystals of sericite is overprinted by moderated calcite and chlorite. Accesory minerals, apatite, zircon and opaques. Coarse euhedral pyrite and chalcopyrite are presented. Overgrown quartz and graphic textures are present SD-101 Piaxtla batholith, riverbed at La Puerta 106°00′29.15″ 24°04′17.85″ 417 White, coarse grained granodiorite with qz, plag, feld, hb, px, bio 47 ± 0.75 Coarse grain granodiorite. Coarse equigranular texture, holocrystaline and subeuhedral crystals of plagioclase (An 44%) > quartz (42%) > feldspar (Alk+Alb 13%) > biotite > anfibol. Weak pervasive sericite and chlorite alteration. Coarse pyrite less than 1%. Overgrown quartz and graphic textures are presented. SD-079 Porphyritic andesite, “Santa Rita” dike, Santa Rita mine near entrance fo the main tunnel. 105°54′16.77″ 24°07′11.02″ 800 Porphyritic rhyodacite 47.5 ± 0.93 Porphyritic rhyodacite. Sequential porphyritic texture, holocrystaline with anahedral crystals of plagioclase (An 38%) > quartz (31%) > feldspar (Alk+Alb 31%) > piroxene > anfibole. Strong pervasive sericite and chlorite alteration overprinted by moderated epidote and weak calcite. Coarse euhedral pyrite is less than 0.1%. SD-065(1) Porphyritic dacite, “Santa Lucia” dike, San Luis tunnel between Santa Lucia and San Salvador veins 105°58′21.58″ 24°08́11.27″ 799 Porphyritic rhyodacite 48 ± 0.67 Porphyritic rhyodacite. Sequential porphyritic texture, holocrystaline with subeuhedral crystals, from coarse to Medium grain, of quartz (54%) > feldspar (Alk+Alb 30%) > plagioclase (An 15%) > piroxene > anfibol > biotite. Strong diseminated sericite alteration overprinted by moderated chlorite, epidote alteration and weak calcite. Coarse euhedral pyrite is presented less than 5%. SD-080 fPorphyritic andesite, “Santa Lucia” dike, San Luis tunnel between Santa Lucia and San Salvador veins 105°58′21.58″ 24°08́11.27″ 799 Porphyritic quartzomonzonite 48 ± 0.46 Porphyritic quartzomonzonite. Sequential porphyritic texture, holocrystaline with anahedral crystals of plagioclase (An 46%) > feldspar (Alk+Alb 35%) > quartz (18%) > biotite > piroxene > anfibole. Strong diseminated sericite and chlorite, moderated epidote and weak calcite alteration. Fine euhedral pyrite is less than 0.1%. SD-016 Piaxtla batholith, “El Cristo” facies, near entrance of the tunnel. Intrusive contact with Socavón member 105°56′15.30″ 24°05′34.96″ 573 Fine grained granite with qz, alk feld, plag, hb 49 ± 0.43 Fine grain granite. Fine equigranular texture, holocrystaline with crystals of quartz (40%) > feldspar (Alk+Alb 34%) > plagioclase (An 26%) > hornblende. Moderated pervasive sequndary biotite overprinted by weak sericite, chlorite and epidote. Fine and coarse pyrite are presented less than 1%. Overgrown quartz textures are presented. SD-104 Piaxtla batholith, riverbed 7 km west of La Puerta 106°03′44.40″ 24°02′20.50″ 350 Medium grained granodiorite with qz, plag, bio, hb 49.1 ± 0.94 Granodiorite. Medium equigranular texture, holocrystaline with crystals of quartz (49%) > plagioclase (An 36%) > feldspar (Alk+Alb 16%) > biotite > anfibole. Weak pervasive chlorite, sericite and calcite alteration. Coarse euhedral pyrite was found less than 0.3%. Sinaloa batholith SD-107 Sinaloa batholith, 16 km east of San Ignacio. Locally completely altered to unconsolidated sand 106°16′9.50″ 23°57′17.83″ 253 Medium grained granodiorite 64.16 ± 0.90 Tarahumara formation SD-049 Portal member. San Jose tunnel, recess 598 105°59′53.92″ 24°07′26.12″ 878 Lithic rhyodacite ignimbrite 69 ± 1.70 Lithic rhyodacite ignimbrite. Porphyritic texture, hialocrystaline with subeuhedral to anahedral crystals (20%) of quartz (47%) > feldspar (Alk+Alb 27%) > plagioclase (An 26%) > biotite. Angular clasts of coarse and tabular porphyritic andesites and agregates of quartz. Strong sericite in crystals, moderated calcite and weak chlorite in halos around crystals. Fine euhedral pyrite is less than 1%. SD-014(2) Buelna member, 4 km SSW of San Dimas 105°58′15.89″ 24°07′05.51″ 665 Porphyritic andesitic lava 69 ± 0.85 Porphyritic andesitic lava. Sequential porphyritic texture. Holocrystaline with anahedral to euhedral crystals (30%) of plagioclase (An 61%) > feldspar (Alk+Alb 29%) > quartz (10%) > hornblende > biotite in a afanitic matrix of tabular plagioclases (70%). Weak pervasive chlorite, sericite and epidote. Coarse Py less than 1%. SD-056 105°59′50.07″ 24°06′51.79″ 620 Porphyry rhyodacite 73 ± 1.5 (continued on next page) P . M o n to y a -L o p era et a l. Ore Geology Reviews 105 (2019) 273–294 287 Table 2 (continued) Sample Geologic unit and location Long. W Lat. N Elevation (m asl) Rock type Age (Ma) Petrography description Porphyry intrusion within the Socavón member, stream at the Sinaloa-Durango border Porphyry rhyodacite. Holocrystaline with anahedral crystals (30%) of quartz (58%) > feldspar (Alk+Alb 25%) > plagioclase (An 17%) > biotite in a afanitic matrix (70%) of quartz (60%) and feldspar (10%). Strong pervasive sericite alteration in matrix, moderate turmaline and chlorite alteration in crystals and weak calcite. Two events of mineralization are presented (less than7%): 1) coarse grain of Py, Cpy, Ga, Sph and 2) fine grain of Cpy, Ga, Sph. Overgrown quartz textures are presented. SD-004 Socavón member, Tayoltita to San Dimas road 105°55′49.12″ 24°08′08.39″ 1176 Lithic porphyritic rhyolitic lava 75 ± 0.77 Lithic porphyritic rhyolitic lava. Holocrystaline with anahedral to sub euhedral crystals (65%) of feldspar (Alk+Alb 57%) > quartz (31%) > plagioclase (An 12%) > hornblende > biotite in a microcrystaline matrix (35%). Rounded clast of andesites and rhyolites. Strong pervasive sericite alteration overprinted by moderate chlorite and weak calcite and turmaline. Three types of pyrite are presented: 1) broken coarse Py (less than1%), 2) fine elongate Py (less than1%) and 3) very fine anahedral Py (less than3%). Overgrown quartz textures are presented. SD-020 Socavón member, near Piaxtla river 2 km west of Tayoltita 105°56′49.18″ 24°05′23.49″ 570 Quartz latite lava 75.4 ± 0.8 Quartz latite lava. Porphyritic texture, holocrystaline with medium crystals (90%) of plagioclase (An 57%) > feldspar (Alk+Alb 37%) > piroxene > anfibole > biotite > quartz (7%) in a afanitic plagioclase matrix (10%). Moderate sericite and chlorite alteration in crystals. Two types of pyrites are presented: 1) fine anahedral grains (less than1%) and 2) very fine pyrite in inclutions in the former pyrite (less than1%), coarse anahedral chalcopyrite is also presented less than 1%. MN-14_24b Mala Noche drillhole. This rock is observed intruding the whole drilling 105°44′47.981″ 23°54′15.923″ 1038 Px-rich porphyritic quartz latite 75.5 ± 1.0 Porphyritic quartz latite. Holocrystaline, medium to fine anahedral crystals of feldspar (Alk+Alb 45%) > plagioclase (An 37%) > quartz (18%) > piroxene > biotite. Strong pervasive sequndary biotite alteration overprinted by moderate pervasive calcite and sericite. Coarse anahedral pyrite is presented less than 3% and Fe oxides in pots. Quartz rapakibi texture was found. MN-14_24 Mala Noche drillhole. 105°44′47.95″ 23°54′15.906″ 950 Andesitic lava 76.06 ± 0.63 Andesitic lava. Sequential porphyritic texture. Holocrystaline with sub euhedral to anahedral crystals of plagioclase (An 81%) > feldspar (Alk+Alb 13%) > quartz (7%) > hornblende in a matrix of fine anahedral plagioclase. Strong pervasive calcite alteration overprinted by moderated sericite and chlorite. Coarse anahedral pyrite is less than 4%, Fe oxides lower than 1%. Overgrown quartz textures are presented. CAU-16_001 Causitas drillhole. This rock includes clast of the whole sequence. Dated as detrital 105°46́56.21″ 24°0́58.24 2358 Lithic rhyodacite lava 76.5 Lithic rhyodacite lava. Sequential porphyritic texture. Holocrystaline with anahedral crystals of quartz (61%) > feldspar (Alk+Alb 23%) > plagioclase (An 16%) > biotite in a microcrystaline quartz matrix. Rounded lithic fragments of andesites and quartz agregates. Strong pervasive sericite alteration is overprinted by moderate chlorite and calcite alteration. Coarse euhedral pyrite and Fe oxides are less than 2% pervasive. Overgrown quartz textures are presented. CS-01 Lava hosting the an epithermal vein at Causita. 105°46′58.50″ 24° 0′59.08″ 2518 Lithic rhyodacite ignimbrite 77.73 ± 0.88 Lithic rhyodacite ignimbrite. Caotic porphyritic texture. Hialocrystaline with crystals of quartz (61%) > plagioclase (An 25%) > feldspar (Alk+Alb 15%) > biotite. Strong, pervasive sericite alteration. Rounded lithic clasts of quartz agregates. Coarse, euhedral pyrite is also present less than 1%. P . M o n to y a -L o p era et a l. Ore Geology Reviews 105 (2019) 273–294 288 45.2 ± 0.85Ma (SD 101, SD 040), with the older age belonging to the westernmost sample (Fig. 6). Several dike families associated with the Piaxtla batholith cut the Late Cretaceous volcanic succession, the Andesite Group and, some- times, the main Piaxtla body. Although they are known by different names they have similar crosscutting relationships, intermediate com- position, and only differ in texture and relative mineral abundances. The Santa Lucia dikes are exposed in the Santa Rita mine intruding E-W striking fractures that crosscut a massive porphyry andesite. The dike is a porphyritic andesite, composed of medium to fine phenocrystals of plagioclase > quartz > hornblende > pyroxene and biotite in a fine plagioclase groundmass. Santa Rita dikes are found in many areas of the district also intruding E-W striking fracture. They consist of porphyry andesite with medium to coarse phenocrystals of plagioclase > quartz, but richer in pyroxene and hornblende than the Santa Lucia. The Bolaños dikes are also intruded into E–W-striking fractures, have a dacitic composition and are also rich in pyroxene and hornblende, with the latter showing coarse phenocryst texture. The U-Pb age support the field observation that all these dikes belong to the same magmatic pulse as the Piaxtla batholith, with the Santa Lucia slightly older than Santa Rita and Bolaños dikes. As mentioned before two Santa Lucia dikes yielded ages indistinguishable within the error clustered at∼48Ma (SD 065-1, SD 080) (Fig. 6). Samples from the Santa Rita dikes and Bolaños dikes also yielded identical ages (within error) of ∼45Ma (SD 32, SD 23) (Fig. 6). 4.2.5. Sedimentary formation Two sedimentary formations have been recognized separating the LVC from the UVS. The Las Palmas formation is composed by con- glomerates, sandstones, red beds and mudstones. The Las Palmas for- mation lies unconformably on the Andesite Group in eastern part of the district. The Camichin formation crops out in the eastern part of the district (Santa Rita mine and Guarisamey area), ranging in thickness between 50 and 300m. It has been described as an alternating sequence of andesitic tuffs and volcano-sedimentary deposits, but in the field, we only observed the latter lithology. We have dated detrital zircons from both units (Fig. 7). For the Las Palmas formation (SD 031) we obtained a maximum age of deposition of 52.1 ± 1.4Ma with peaks at ∼56 and ∼64Ma. A sample from the Camichín formation previously mapped as a tuff turned out to be a sandstone (SD 029), which yielded a maximum Fig. 10. New stratigraphic columns for the San Dimas and Causita-Mala Noche areas based on fieldwork and U-Pb geochronology presented in this work. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 289 Tayoltita - San Dimas o o o . ·o o o Silicic ignimbrites (Second ignimbrite flareup) ~24Ma Polimictic conglomerate Mafic dikes UPPER VOLCANIC SUPERGROUP Silicic ignimbrites and lava domes (First ignimbrite flare-up) ~31.5 -29 Ma Sandstone and conglomerate Camichin unit Max age ~43 Ma Sandslone and conglomerate Palmas Fm. Max age ~52 Ma Andesitic group ("Productive andesites" and porphyry andesitic intrusions) ~63-48 Ma Porphyry inlrusions ~67 Ma Tarahumara Fm. Portal member ~69 Ma Buelna member ~69 Ma Socavón member ~75.5 - 73 Ma Andesitic and dacitic dikes -45 -44 Ma Piaxlla batholilh ~49 - 45 Ma Sinaloa balholilh ~80 - 62 Ma LOWER VOLCANIC COMPLEX Causita - Mala Noche Silicic ignimbrites (Second ignimbrile flareup) ~25-24 Ma Polimictic conglomerate Silicic ignimbrites and lava domes (First ignimbrile flare-up) -30-31 Ma Cebollas shallow intrusion ~30 Ma Tarahumara Fm. Porphyry intrusions ~75.5 Ma Socavón member ~77.7 - 76 Ma depositional age of 43.2 ± 2.3Ma with a major peak at∼56Ma. These peaks in detrital zircons age at the beginning and end of Paleocene point to important magmatic pulses that will be discussed later. 4.2.6. Upper volcanic supergroup Unconformably covering the continental sedimentary formations and the Andesite Group are two packages of silicic ignimbrites and domes, with intercalation of minor amount of mafic lavas, fed by dikes of bimodal composition. A volcanic conglomerate and an angular un- conformity separate the two packages. The base of the UVS is clearly exposed along the Piaxtla River near Guarisamey, where the homon- ymous member it is composed by a rhyolitic ignimbrite and a series of andesitic lavas. The ignimbrite contains feldspar > plagioclase > quartz as the main crystals in the glassy matrix. The andesite lavas have a holocrystalline porphyritic texture with subhedral plagioclase phe- nocrystals and rare quartz. We obtained an age of 31.5 ± 0.8Ma for the first ignimbrite capping the sedimentary formation (SD 035) and an age of 31.0 ± 1.0Ma for the andesite agglomerate that lies just above (SD 034). This latter unit displays a large population of zircons with ages of the Piaxtla intrusive suite, which were considered as inherited xenocrystals (Fig. 8). Despite being based on four crystals only, we consider our U/Pb age reliable since the andesite flow lies in direct contact with the ignimbrite dated 31.5Ma and no paleosoil was ob- served in between. The difference with the ∼24Ma K–Ar age reported by Enríquez and Rivera (2001b) for this unit is likely due to the fact that the andesite is highly altered, which may have resulted in Ar loss. A shallow granitic intrusion 10 km to the SSE of Guarisamey yielded an age of 31.3 ± 0.50Ma (CB 01), which suggests it represents an in- trusive equivalent of the first ignimbrite package. Henry et al. (2003) reported similar K–Ar dates on biotite for a granodiorite and a quartz diorite dike about 10 km to the north, in the Piaxtla river valley. Rhyolitic domes that cover the flat-lying ignimbrite succession NE of San Dimas are undated. However, we have dated a rhyolite flow that rest unconformably over the San Ignacio batholith west of the district at 29 ± 0.54Ma, an age very similar to that obtained by K-Ar by McDowell and Keizer (1977) and by U/Pb by Ferrari et al. (2013) for the massive rhyolite dome of Las Adjuntas, located 55 km southeast of San Dimas. NNW–SSE-striking felsic and mafic dikes cut most of the succession of the district. The felsic dikes are locally known as “Rebo” and consists of pyroclastic intrusions rich in quartz > plagioclase > biotite in a glassy matrix. These dikes are particularly abundant in the proximity of the fault system at the western limit of the district (Don Porfirio fault) at La Puerta (Fig. 11). A sample from one of these dikes at this location yielded an age of 27.0 ± 1.0Ma (SD 102) (Fig. 8). The mafic dikes are locally known as “San Luis” and consist of basaltic lavas with por- phyritic texture and plagioclase phenocrystals in a microlithic groundmass. The K-Ar age of 29.5 ± 0.7Ma reported by Henry and Fredrikson (1987) for an andesitic dike west of the district is probably representative for these intrusions. For the ignimbrite succession that cover the Oligocene ignimbrites in angular unconformity we obtained an age of 26.2 ± 0.29Ma. For this sample, we exclude a Pb loss as there is no correlation between higher U content and younger ages (Fig. 8). In addition, this sample belongs to an ignimbrite overlaying a conglomerate and lays in angular unconformity with the first ignim- brite package. The slightly older age obtained with respect to the cor- relative El Salto-Espinazo del Diablo succession (24–23.5Ma; McDowell and McIntosh, 2012) may be due to subtle zircon inheritance in the form of antecrysts, as it is quite common for this ignimbrite package in other part of the Sierra Madre Occidental (Bryan et al., 2008; Ferrari et al., 2013). Enríquez and Rivera (2001b) report a K–Ar age of 20.3 ± 0.8Ma for the uppermost ignimbrite of the capping sequence Fig. 11. Structural map of San Dimas mining district showing the mains fault systems, veins, and dikes, with stereograms showing their orientation (lower hemi- sphere). P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 290 Tayoltita block O ,.-., f N ~e::, ~ ¡g_v · • Eocene and Oligocene veins Eocene dikes Normal faults - j Km NW of Guarisamey (Fig. 3). This age, obtained on plagioclase, is un- common for the region, where the second ignimbrite flareup only consist of the 24–23.5Ma El Salto–Espinazo del Diablo succession. The younger age can be due to Ar loss, taking into account that the closure temperature of plagioclase can be as low as 200 °C (e.g. Cassata and Renne, 2013). 5. Tectonics As described in Section 2.2, the tectonic setting of the San Dimas district is dominated by major tilted blocks separated by NNW striking normal fault system that expose the mineralization (Fig. 10). The Li- moncito, Guamuchil, and the Arana faults, located in the central and eastern part of the district, dip to the WSW with inclination varying from 80° to 55°, the Don Porfirio fault dip toward the ENE with a high angle and the Peña fault is almost vertical. Rocks in the Tayoltita and Central blocks are typically tilted 30–35° to the ENE, whereas in the Western block they are tilted 10–15° to the WSW. Most mineralized veins (61 mapped structures) strikes E–W to ENE–WSW, with only a few in the easternmost part of the district (Tayoltita block) striking∼N–S (5 mapped structures) and in the western block striking NNE–SSW (5 mapped structures) (Fig. 10). The E–W to ENE–WSW striking veins formed before the Oligocene, as they do not cut the UVS and are exposed by the NNW striking normal fault systems. However, they almost did not change their inclination because are approximately orthogonal to the tilting. These veins cut the Late Cretaceous volcanic succession and the Andesite group, and some also cut the oldest part of the Piaxtla batholith (El Cristo granite) dated at ∼49Ma. Based on these crosscutting relations the veins can be limited to the late Eocene. Our ∼45Ma ages for the “Santa Rita” and “Bolaños” dikes (Table 1), whose strike is parallel to the veins, suggest that they could be part of the same episode. Recent Ar–Ar dating of adularia (Enriquez et al., 2018) points to a slightly younger age of ∼41Ma. The associated stress regime would have been characterized by NW–SE σHmin and NE-SW σHmax, with a possible transtensional deformation regime in some areas, as observed by Horner and Enriquez (1999) (Fig. 10). NNW striking normal faults and block tilting post-date the first ig- nimbrite package of the UVS. However, the angular unconformity and the conglomerate that separate it from the second ignimbrite package indicate that faulting occurred between ∼30 and ∼24Ma, as it has documented in several areas of Sinaloa (Ferrari et al., 2013). This is also confirmed by the 27.0 ± 1.0Ma age we obtained for a felsic dike in- truded parallel to and in proximity of the NNW striking Don Porfirio fault system (Fig. 10) (SD 102, Table 1). Taking into account this age, the crosscutting relations, and the fact that they are only moderately tilted, we consider NNW–SSE to N–S striking veins and dikes as em- placed during the initial stage of the Oligocene extension. Assuming that the veins and dikes were the result of a pure extensional opening, they would indicate an ∼E–W to WNW–ESE extension regime (Fig. 10) that have been also reported for the western SMO at the beginning of the Gulf of California rift (Ferrari et al., 2013, 2017; Duque-Trujillo et al., 2015). Further west, the ignimbrite package of the second ig- nimbrite flareup is also tilted (Henry, 1989; Henry and Fredrikson, 1987; Henry and Aranda-Gómez, 2000), implying that a second ex- tensional episode affected this area. The result of AFT dating on two samples at different altitudes of the Piaxtla batholith provide further support to this multiple-stage exten- sional history (Fig. 9). The dated samples show consistent cooling ages with a first group of ages at the end of Oligocene (24.7 ± 1.9 and 23.3 ± 1.9Ma) and a second one at the end of middle Miocene (12.6 ± 1.2 and 12.4 ± 1.1Ma). On a larger scale, our new mapping and absolute dating south of the district point to a major structure roughly parallel to the Piaxtla river. In fact, the Late Cretaceous volcanic succession lies at a maximum of ∼1500m of elevation in the San Dimas area whereas it crops out at ∼2600m in the Causita area to the SSE (Fig. 3). The easiest explanation for a difference of ∼1100m of elevation between the San Dimas and Causita blocks is the presence of a major ENE–WSW striking and NNW dipping normal fault system running somewhere in the Piaxtla river valley, here named Piaxtla fault zone. A structure with this orientation but opposite dip (i.e. down to the SSE) is drawn some km south- southeast of the river in the geologic map of Henry and Fredrikson (1987) but not described. In the field, no clear fault zone is observed in Piaxtla riverbed, which between Tayoltita and San Ignacio is always cut into the Piaxtla granite. We infer that this fault system is older than the ∼32 to 30Ma ignimbrites and thus buried beneath them. In Fig. 3 we draw this inferred normal fault system along the trace drawn by Henry and Fredrikson (1987) but we assume that it has an opposite dis- placement. In the Tayoltita area the intrusion contact of the Piaxtla batholith in the NNW side of the valley has a lower elevation with respect to the SSE side, which suggest that the batholith may have intruded into a pre- existing normal fault zone. If extended toward the west the Piaxtla fault zone would limit a WSW-ENE trending horst of the pre-Laramide basement that crop out along the coast between Mesa Cacaxtla and Mazatlán but not to the north and to the south (Fig. 2). On a regional scale is very possible that other fault systems orthogonal to the Gulf of California may exists and controlled the segmentation of the rift in the following extensional stage. This structural grain may be inherited from even older deformation episodes. Henry (1986) reported ENE trending folds and thrust faults, orthogonal to the paleo-subduction zone in Jurassic to Early Cretaceous rocks in Sinaloa, that would have formed by oblique convergence prior to 100Ma. 6. Discussion and conclusion 6.1. Revision of the San Dimas district stratigraphy Our new ages and geologic observations provide tighter constraints on the magmatic and mineralization pulses in the San Dimas district and allow to correlate its stratigraphy with other regions of the SMO. One of the main contributions is the recognition of the presence of a Late Cretaceous volcanic activity in the area. Previous studies were unable to date the LVC succession. Some workers suggested that it was older than the Piaxtla intrusive complex (Henshaw, 1953; Nemeth, 1976; Henry and Fredrikson, 1987; Enríquez and Rivera, 2001b) but mostly Eocene in age, by correlation with andesites dated at 51.6 Ma south of Durango by McDowell and Keizer (1977). The ages of the Socavón, Buelna, and Portal members obtained in this work clearly demonstrate that these rocks are part of the Late Cretaceous magmatic episode of the so-called Laramide arc. In particular, they share the same time span, lithology and composition of the Tarahumara formation of Sonora (Wilson and Rocha, 1949; McDowell et al., 2001). As in Sonora this Late Cretaceous volcanic succession is exposed to the east of the age-equivalent Laramide batholiths (Fig. 2), which suggest a wider volcanic arc that was eroded toward the coast. For the following Andesite group Enríquez and Rivera (2001b) re- ported ages between 38.8 and 36.6Ma using the K–Ar method. Based on these ages, the authors linked the andesitic magmatism with the epithermal mineralization events, which produced similar ages (38.5–31.9Ma). Given the widespread alteration of these rocks these ages are unreliable and contradicts our new U-Pb ages and crosscutting relations. In fact, we obtained a few zircons of early Paleocene age for the “Intrusive andesite” and, more importantly, two reliable U-Pb ages of ∼48Ma for intrusive bodies that cut the Andesite group. Therefore, we have relocated this group in the Paleocene to Early Eocene. In this context we consider that the K-Ar ages of Enríquez and Rivera (2001b) were the result of partial resetting of the dated minerals. We observed that the Piaxtla intrusive complex intrude the whole LVC from the base to the top and was formed by different magmatic pulses, separated based on texture, composition and age. Our U/Pb age P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 291 confirm earlier K–Ar and U/Pb ages (Enríquez and Rivera, 2001b; Henry et al., 2003) and show that the development of the batholith took place in several millions of years from ∼49 to ∼44Ma. A novel con- tribution is the dating of the “Santa Rita” and “Bolaños” intermediate dikes, that were previously thought to be somewhat younger than the Piaxtla suite but turned out to be concurrent with the last intrusion stage. These ages are relevant because document a period of ∼13Ma without any magmatic activity in the area, during which previous au- thors suggested the occurrence of a mineralization event. The occurrence of the continental sedimentary Las Palmas and Camichín formations points to an early stage of extension with the formation of intermontane basins in the Eocene. The rock fragments and different zircon populations indicate that these clastic units were mostly derived from volcanic edifices (domes and compound volca- noes) that form the Andesite Group and, to a lesser extent, the Piaxtla intrusive suite, most likely the Santa Rita and Bolaños dikes. A con- tinental sedimentary deposits lithologically equivalent to the Las Palmas formation and in the same stratigraphic position is reported south of the San Dimas district in the Mala Noche area (Servicio Geológico Mexicano, 2002), where it is called “Palmarito conglom- erate”. At a regional level it can be correlated with many discontinuous continental sedimentary deposits that mark a period of the erosion and low volcanic activity between the bulk of LVC and the UVS (McDowell and Keizer, 1977; McDowell and Clabaugh, 1979; Ferrari et al., 2007). The new U/Pb ages for the base of the ignimbrite succession at San Dimas are consistent with those of the Durango sequence, whose Ar–Ar ages range from 31.7 to 29.9Ma (McDowell and McIntosh, 2012). A slightly older age of 32.5 Ma was obtained by Ferrari et al. (2013) for a tilted ignimbrite sequence east of Mala Noche. This confirms that the eastward prolongation of the Durango sequence at least up to the Durango–Sinaloa state boundary. 6.2. Timing of igneous activity Henry et al. (2003) interpret that igneous activity was mostly con- tinuous between ∼100 and 45Ma in Sinaloa. Although at a regional level some ages of igneous bodies fall between ∼61 and ∼49Ma and our detrital zircons show a peak in this age interval, along the Piaxtla river valley a distinction can be clearly made between the San Ignacio and the Piaxtla batholiths. Along the river, we observe a clear change in morphology (much less relief) and increase in alteration (intrusive rocks often reduced to sand) and mineral composition (mafic minerals more abundant) between the two batholiths. Ages also change rather abruptly (Fig. 2). The Piaxtla batholith is exposed for ∼35 km to the northeast of the contact with ages restricted between 49.6 and 43.7Ma, whereas the San Ignacio batholith has ages between ∼67 and ∼61Ma to the southwest (Henry et al., 2003, this work). Supporting this in- ference, sample 199 of Henry et al. (2003), from what we mapped as the Piaxtla batholith, is a tonalite that yielded a biotite age of 49.3 ± 0.6Ma; however, just 2 km to the south sample 152 of Henry et al. (2003) is a granodiorite that yielded a hornblende age of 67.1 ± 1.5Ma. Interestingly, this same sample yielded a biotite age of 51.3 ± 0.6Ma, a large difference of 16Ma that cannot be due to normal cooling but rather to a thermal resetting of the biotite system due to the nearby Piaxtla intrusion. Our new ages also confirm a gap in igneous activity between ∼43Ma (youngest age of the Piaxtla intrusive suite) and ∼32Ma (first ignimbrite of the UVS), which is also marked by the deposition of the Las Palmas and Camichín sedimentary suc- cessions. 6.3. Implication for the mineralization events at San Dimas A detailed structural, geochemical and geochronologic study of the vein system is the focus of a forthcoming paper. However, the data presented in this work provide some constraint on the mineralization events of the San Dimas district. San Dimas has been traditionally classified as a classic Au–Ag epithermal low sulfidation vein system developed during a single mineralization episode during in a late stage of the LVC magmatism (Henshaw, 1953; Smith and Hall 1974; Smith et al., 1982; Clarke, 1986; Clarke and Titley, 1988). Enríquez and Rivera (2001b) obtained similar Late Eocene K–Ar ages for andesitic intrusions and some of the veins, which led them to associate the two events and conclude that the mineralization occurred in the ∼38 to 32Ma time span. However, the minerals dated by these authors were adularia and feldspar, whose closure temperature for the K– Ar system (∼150 to 200 °C; Love et al., 1998) is well below the temperature of the hydrothermal fluids circulating in the district (∼260 °C; Clarke and Titley, 1988). This means that the late Eocene ages of Enríquez and Rivera (2001b) should be considered minimum ages, likely resulting from partial resetting. In this work we have dated the main geologic units of San Dimas using zircon, whose closure temperature for the U/Pb system (∼900 °C) is much higher than the temperature of any hydrothermal fluid. Our data place the Andesite Group into the Paleocene–early Eocene time span, as it is also confirmed by the age peaks in the continental sedi- mentary deposits of the Las Palmas formation. Although we did not directly date the mineralization, the most important magmatic event postdating the host rock of the Andesite Group is the Piaxtla intrusive suite, emplaced between ∼49 and ∼44Ma, which we consider the most likely thermal source for the dominant ENE-WSW trending vein system. These veins occupied fractures and faults that developed before the first ignimbrite flare up of the UVS that may have also controlled the emplacement of the Piaxtla batholith. Recently, Enriquez et al. (2018) obtained Ar–Ar ages of 41 ± 0.2 and 37.8 ± 0.2Ma on adu- laria from two veins of the E–W system with high Ag/Au ratio. The first age confirms the K–Ar age of 40.9Ma on adularia from an undefined mine at Tayoltita obtained by Henry et al. (2003). The new Ar–Ar ages show a well-defined plateau, and indicate that at least part of the Ag/Au mineralization occurred ∼3 to 6Ma after the emplacement of the Piaxtla intrusive suite. On the other hand, NNW–SSE to N–S veins emplaced in the eastern part of the district must be associated with the similarly oriented normal faults responsible for the block tilting in the San Dimas area. As mentioned in the previous section, these extensional faults are partly coeval with the silicic volcanism of the flare up dated at ∼32 to 29Ma. If this is the case, the NNW–SSE to N–S veins should differ from the rest of the district in term of Ag/Au ratio and other structural and chemical features. In conclusion, the data presented in this work show that the San Dimas district was developed during multiple mineralization events tied to magmatic and tectonic pulses that affected the central part of the Sierra Madre Occidental. Acknowledgements This research is part of the PhD project of the first author at Universidad Nacional Autónoma de México (UNAM) Postgraduate Program and was funded by Consejo Nacional de Ciencias y Tecnología (CONACYT), Mexico, Grant CB 237745-T to L. Ferrari. We thank Primero Mining (presently First Majestic Silver Corp.) for sharing un- published information and for logistical support. We thank Chris Henry for his thorough review that contribute to clarify several aspects of the work and two anonymous reviewers that helped to improve its pre- sentation. A special thanks to Nicolas Landón for his strong support in the initial phase of the research and to Miguel Pérez for sharing his knowledge of the ore geology of the central SMO. We also thank Luigi Solari and Carlos Ortega for assistance with U-Pb datings and Juan Tomás Vazquez for the elaboration of thin sections. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2018.12.020. P. Montoya-Lopera et al. Ore Geology Reviews 105 (2019) 273–294 292 References Abdullin, F., Solari, L., Ortega-Obregón, C., Solé, J., 2018. New fission-track results from the northern Chiapas Massif area, SE Mexico: trying to reconstruct its complex thermo-tectonic history. Rev. Mex. Cienc. Geol. 35 (1), 79–92. Albinson, T., Norman, D.I., Cole, D., Chomiak, B., 2001. Controls on formation of low sulfidation epithermal deposits in Mexico: constraints from fluid inclusion and stable isotope data. In: New Mines and Discoveries in Mexico and Central America 8. Society of Economic Geology Special Publication, pp. 1–32. Ballard, S., 1980. 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Ore Geology Reviews 105 (2019) 273–294 294 44 Capítulo 3: New geological, geochronological and geochemical characterization of the San Dimas mineral system: evidence for a telescoped Eocene- Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev New geological, geochronological and geochemical characterization of the San Dimas mineral system: Evidence for a telescoped Eocene-Oligocene Ag/ Au deposit in the Sierra Madre Occidental, Mexico Paula Montoya-Loperaa, Gilles Levressea, ⁎ , Luca Ferraria, Teresa Orozco-Esquivela,b, Gabriela Hernández-Quevedoa,b, Fanis Abdullinc, Luis Matad a Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, 76230 Querétaro, Qro, Mexico b Laboratorio Interinstitucional de Geocronología de Argón (LIGAR), Campus UNAM Juriquilla, Querétaro, Mexico c CONACyT-Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, 76230 Querétaro, Qro, Mexico d First Majestic Silver Corp., San Dimas, Dgo., Mexico A R T I C L E I N F O Keywords: Telescoped deposits 40Ar-39Ar dating U-Pb dating Titanite fission track dating Zircon trace elements Sierra Madre Occidental A B S T R A C T The San Dimas district is a historical world class Ag/Au epithermal deposit located in the Sierra Madre Occidental (SMO) of western Mexico. San Dimas has been classified as a classic low to intermediate sulfidation Ag/Au epithermal deposit (quartz+ adularia+ sericite type) developed during a single hydrothermal pulse of ~10Ma, associated to the emplacement of intermediate intrusive bodies of Eocene K-Ar ages. However, this metallogenetic model includes several ambiguities, such as K-Ar cooling ages incompatible with the local and regional magmatic pulses, as well as wide differences in Ag-Au ratios between individual veins, which were also emplaced in two different structural systems. Based on a detailed study of mineralized veins, including new petrographic observations, new geochronological data (zircon U-Pb, adularia 40Ar/39Ar, and titanite fission track ages, FT), zircon trace-element composition, and geochemistry of gold and silver minerals, we demonstrate that San Dimas exhibits multiple mineralization events developed during different magmatic and tectonic episodes from Late Cretaceous to early Oligocene. The earliest episode is represented by Late Cretaceous copper – gold porphyry mineralization associated with fertile and oxidized magmas during the development of the Laramide arc. The second, most abundant ore mineralization is represented by a Ag-dominant vein system (adu- laria+ rhodochrosite type), which developed into east-west striking fractures. New 40Ar/39Ar ages on adularia as well as new titanite FT ages for this mineralization event, together with one published 40Ar/39Ar age indicate the formation of Ag rich veins at ~41–40Ma, shortly after the final emplacement of the Piaxtla batholith (~45Ma), which formed from multiple fertile and oxidized magma intrusions in an extensional environment. The third mineralization episode is represented by Au-dominant epithermal veins (quartz sericite type), which were emplaced at ~31Ma into north-south to NNE-SSE striking fractures. At regional scale these fractures also host rhyolitic domes associated with the first silicic ignimbrite flare up of the SMO in an extensional setting. The formation of the San Dimas district with its exceptionally rich Ag-Au mineralization is interpreted to be related to an extension-related late Eocene-early Oligocene regional uplift that allowed the overprinting of originally deeper Ag veins by a shallower Au mineralizing event. 1. Introduction Mexico has a special place in the precious metals mining industry as it is presently the largest silver producer in the world (Silver Institute, 2017). The size of the Mexican silver and gold anomaly is illustrated by the outstanding role of Mexico mining during XVI and XVII centuries and its position during last ten years as the largest silver (200 Moz/ year) and eighth largest gold (125 ton/year) producer worldwide. Most of this production comes from localities within the Sierra Madre Occi- dental province (SMO; Fig. 1). The timing of formation of epithermal deposits coincides with the distribution of the products of the three main Cenozoic volcanic pulses of the SMO, which record a broad mi- gration from the northwest to the southeast, where the last ignimbrite flare-up occurred (Ferrari et al., 2007; Ramos-Rosique et al., 2011; https://doi.org/10.1016/j.oregeorev.2019.103195 Received 29 March 2019; Received in revised form 18 October 2019; Accepted 22 October 2019 ⁎ Corresponding author. E-mail address: glevresse@geociencias.unam.mx (G. Levresse). Ore Geology Reviews 118 (2020) 103195 Available online 28 October 20190169-1368/ © 2019 Elsevier B.V. All rights reserved. ELSEVIER OREGEOI..OCY REVIEWS J,.,r,,alb-Coonprd•:n,,..,Stooicsof °"'c:.ne... ... 1°"'fa~ Fig. 1). The most fertile magmatic ore-forming events range in age from 36 to 28Ma and include all the giant Ag-Au-Sn and IOCG mining dis- tricts (Camprubí et al., 2003). The increasing comprehension of the genesis of historical Ag/Au districts in the SMO (e.g., San Dimas) is modifying several concepts central to the epithermal mineralization topic, such as the formation of Ag/Au deposits (Hayba et al., 1985; Simmons, 1991; Sillitoe, 1993), the classification into high, inter- mediate, and low sulfidation deposits (Sillitoe, 1993; Arribas et al., 1995; Camprubi and Albinson, 2007), and the existence of long-living hydrothermal events (Enriquez and Rivera, 2001; Camprubí et al., 2003; Velador et al., 2010; Enríquez et al., 2018), among others. The Mexican epithermal deposits are characterized by extremely variable Ag/Au ratios. These variations are observed from regional (i.e., among districts) to local scale, and might be even found within a single mi- neralized structure (Buchanan, 1980; Hayba et al., 1985; Sillitoe, 1993; Velador et al., 2010; Camprubí et al., 2003; Hall et al., 2014). At a regional scale, this variation has been interpreted to be related to crustal thickness (Camprubi and Albinson, 2007). At a local scale, au- thors agree that Ag/Au ratio variations in epithermal deposits are mainly controlled by boiling and water/rock interaction. The latter physicochemical processes lead to changes in temperature, pH, fS2 and/ or fO2 over time, which affect the transportation and deposition of gold and silver in hydrothermal fluids (Hynes, 1999; Enriquez and Rivera, 2001a; 2001b; Camprubi and Albinson, 2007; Velador et al., 2010; Moncada et al., 2012; Camprubí et al., 2003; Mango et al., 2014; Hall et al., 2014; Enríquez et al., 2018). However, Cole and Drummond (1986) estimated that the Ag/Au ratio variation related to boiling process ranges from 10 to 100, which, despite being conservative, cannot explain Ag/Au ratio variations within a single structure. A widely used classification divides epithermal deposits of Mexico into subtypes with different sulphidation state (Albinson and Camprubí, 2007). The geologic context of low sulfidation and high sulfidation Ag- Au epithermal deposits is well characterized and their relationship with rhyolitic domes or porphyries is also well established (Arribas et al., 1995; Sillitoe, 1999; Hedenquist et al., 1999; Sillitoe and Hedenquist, 2003; Einaudi et al., 2003; Simmons et al., 2005). However, a number of Mexican Ag/Au deposits are classified as of intermediate sulfidation. Their geological framework does not fit easily into the simple genetic/ chronological model proposed so far, and some have been already re- vised and reclassified in Levresse et al. (2017) and Zamora-Vega et al. (2018), who showed that their magmatic and mineralization history is more complex, and that some Ag-Au- districts, such as Ag-Zacatecas and Au-Plomosas, might be the result of telescoping of various mineraliza- tion events. The San Dimas district is a historical world class Ag/Au epithermal deposit located in the SMO, in the westernmost part of Durango State Fig. 1. Distribution of dated Mexican hydrothermal, skarn, porphyries and orogenic gold deposits from Eocene to recent (modified from Camprubi, 2013). Color shadows indicate the distribution of volcanic rocks of Paleocene-Eocene, Oligocene, early Miocene and recent ages (modified from Ferrari et al., 2012). P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 2 PALEOCENE EOCENE PALEOCENE EOCENE OL/GOCENE MIOCENE POST MIOCENE ~·w 102·w '.. ... ,..- .. , .. ◊ .. ◊ \ ◊ gg•w USA 27°N \ ...... ◊ MEXICO Legend Vein Skarn and Ag-Au Porphyries • ◊ .. ◊ • 1;J.. 24ºN Gulf of Mexico 21ºN Volcanic Rocks -.----1 1 ~--.J .----1 1 ~--.J 108ºW <> 105\W :o~~ Chihuahua ·.._ . ~/ • '. ... o "T' . • • •• •~ •• '.. USA : ~ ◊ \ 99ºW 96ºW 30ºN J> MEXI~ q, Durango e d 1 • 1" ...... ~ - e oro Zacatecas'IL.. San Luis ..... o Potosi 24ºN Gu/f of Guanajuat.;t. ... Real del 21ºN o. Monte GuadalaJara ... o ~ "t 0~ exico City Taxco • OLIGOCENE 108ºW 1()l;·w 102ºW 1 •. - .• • ·· ... .,1 ' ~ hihuah:a MIOCENE ToPRESENT \ MEx1cd ,, 99ºW 96ºW 30ºN \ 24ºN 21ºN near the boundary with Sinaloa State (Fig. 1). San Dimas is a good example for the geological complexity of the Mexican Ag/Au epi- thermal deposits. Currently, this deposit has been considered to re- present a long-lived intermediate to low sulfidation epithermal deposit (Enríquez et al., 2018). The metallogenic model is based on under- ground observations of a limited portion of the district (Tayoltita Block mainly; Fig. 2) and presents unresolved problems that need attention, such as unreliable K-Ar and 40Ar-39Ar cooling ages in an area affected by pervasive and widespread potassic alteration and, more importantly, differences in gangue minerals between individual veins (Henshaw, 1953), which were emplaced in two different structural systems (Horner and Enriquez, 1999), during an unusually long single epi- thermal event (~41 to 31Ma, Enríquez et al., 2018) that does not match with the local and regional magmatic pulses (Ferrari et al., 2018a; Montoya-Lopera et al., 2019). In this paper we present the results of a new detailed study of the Ag/Au mineralization of the San Dimas district. New petrographic and geochronological data (zircon U-Pb, adularia 40Ar/39Ar and titanite fission track ages), zircon trace-element composition, gold and silver geochemistry led to an improved model of the world-class San Dimas epithermal systems in relation to the geodynamic evolution of the SMO. 2. Regional geological setting The San Dimas mining district is located in the central part of the Sierra Madre Occidental (SMO), near the Sinaloa-Durango state border (Fig. 1). As a physiographic province, the SMO comprises a high plateau with an average elevation exceeding 2000m above sea level, extending from the Mexico-US border to the Trans-Mexican Volcanic Belt (Fig. 1). As an igneous province, the SMO includes Late Cretaceous to early Fig. 2. (A) Geologic map of San Dimas mining district and Causita-Mala Noche areas, with location of published ages (from Montoya-Lopera et al., 2019). (B) Simplified geological cross-section of the Ag/Au San Dimas district. P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 3 Oligocene and early Miocene domes (a) and intrusive rocks (b) Piaxtla intrusive suite (49-38 Ma) [!] 1000 a) Piaxtla granite: b) El Cristo granite e) diorite intrusion Andesitic group (66-47 Ma) -106º oo· -105° 56' Miocene rocks formed during two main periods of continental mag- matic activity (Ferrari et al., 2018a). The first period produced a dominantly intermediate intrusive suite and its volcanic counterpart, the so-called Laramide magmatic arc, which developed during east- verging subduction of the Farallon plate beneath the North America continent between~100 and 50Ma (Gastil, 1975; Henry et al., 2003; McDowell et al., 2001; Ortega-Gutiérrez et al., 2014; Valencia-Moreno et al., 2017). These rocks are traditionally grouped within the Lower Volcanic Complex (LVC; McDowell and Keitzer, 1977). After a transi- tional period that lasted until the late Eocene (Ferrari et al., 2018a), volcanism became markedly silicic and then bimodal, making the so- called Upper Volcanic Supergroup (UVS; McDowell and Keitzer, 1977). Silicic ignimbrites represent the overwhelming component of this vol- canism, which makes the SMO one of the largest silicic volcanic pro- vinces on Earth (Bryan and Ferrari, 2013). Most of these rocks were emplaced in two ignimbrite flare up episodes at ~35–29Ma along the entire province and at ~24–20Ma in the southern SMO (Ferrari et al., 2002, 2007; McDowell and McIntosh, 2012). Mafic lavas, often with an intraplate affinity, are found intercalated within the ignimbrite suc- cessions since 33Ma (Ferrari et al., 2018a; 2018b). 3. Local geological setting Since the eighteen century, the San Dimas district has been man- aged by different mining companies. The last one is First Majestic Silver Corp., which acquired the operation and exploration in 2017. Historical production of San Dimas has been estimated in 11 million ounces of gold and 745 million ounces of silver. The San Dimas stratigraphic column can be divided into two major igneous successions that correspond to the LVC and UVS, separated by erosional and depositional unconformities. A detailed description of the lithology, petrography and geochronology of the stratigraphic column is given in Montoya-Lopera et al. (2019) and is briefly summarized here. The Late Cretaceous to Eocene LVC, is composed of four members locally named “Socavón rhyolite”, “Buelna andesite”, and “Portal rhyo- lite” (Henshaw, 1953) and by the Andesitic Group as defined in Montoya-Lopera et al. (2019) (Fig. 2). The Socavón member consists of an alternating suite of rhyolitic and andesite lava flows dated at ~ 77–75Ma locally intruded by mineralized felsic porphyritic bodies (~75–73Ma) (Montoya-Lopera et al., 2019). The Buelna member is a sequence of andesitic lava flows and the Portal member is a sequence of rhyolite lava flows and tuffs, both dated at ~69Ma (Montoya-Lopera et al., 2019). Based on lithology and age, Montoya-Lopera et al. (2019) correlated these three members with the Tarahumara formation defined by Wilson and Rocha (1949) and McDowell et al. (2001), which is widely exposed in Sonora state and represent the volcanic counterpart of the Laramide arc. The Andesite Group has a thickness of over 800m and is formed by intermediate lava flows, tuffs, dikes and hypabyssal intrusions of Paleocene and early Eocene ages (Montoya-Lopera et al., 2019). The entire LVC volcanic column is crosscut and locally assimi- lated by the late Eocene Piaxtla batholith. The oldest intrusive bodies of the Piaxtla batholith (locally named El Cristo at Tayoltita) are fine- grained granites rich in K-feldspar that yielded ages of 49.1 and 49.0Ma (Henry et al., 2003; Montoya-Lopera et al., 2019). The main body consists of a coarse- to medium-grained granodiorite with U-Pb ages of 49.1 to 47.0Ma and a medium- to fine-grained granite dated at 45.2 Ma (Henry et al., 2003; Montoya-Lopera et al., 2019). The transition be- tween the LVC and the UVS is marked by the Las Palmas and Camichin continental conglomerates, sandstones, red beds and mudstones, with a maximum age of deposition of ~ 52 and~ 43Ma, respectively (Montoya-Lopera et al., 2019). The UVC unconformably covers the La Palmas and Camichin continental sedimentary formations, the Piaxtla batholith or/and the Andesite Group. It is composed by two successions of silicic ignimbrites with ages of ~31.5 to 29Ma and~ 24Ma re- spectively, with intercalations of minor amount of mafic lavas and continental conglomerate (Montoya-Lopera et al., 2019; Fig. 2). The lower ignimbrite succession is intruded by rhyolitic domes dated at ~29Ma (Ferrari et al., 2013) and crosscut by dykes of bimodal composition (Montoya-Lopera et al., 2019). 4. Previous model of the mineralization of San Dimas Ag-Au district The Ag/Au epithermal veins can be grouped into two sets striking E- W to WSW-ENE and NW-SE (Horner and Enriquez, 1999; Montoya- Lopera et al., 2019; Fig. 2). Both sets of veins pinch out, swell, bifurcate Fig. 3. (A) Au/Ag veins distribution and ages in the San Dimas mining district. Ages in black: K-Ar and 40Ar/39Ar in adularia and sericite from Enríquez et al., 2018; Ages in white: 40Ar/39Ar in adularia this study; Ages in green: U/Pb on zircon crystal LAICPMS, this study; (B) Discrimination diagram of Au/Ag in whole ore samples versus vein azimuth. Red: Au-dominant veins. Blue: Ag-dominant veins. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 4 2000 m [ID O.OS --M-a-la-N-oc_h_e_A-ra_n_a ___ 5_-L-u-is-, íl -~ Causita 40.0Ma 32.7Ma QJ o o eoo > .... e o ·.¡:¡ 0.04 ~ 0.03 Ol ~ :::, 1/25) associated to the N-S structures at the eastern and western sides of the district, with lower volume size (Fig. 3A and B). A few E-W striking large volume Ag-dominant struc- tures show significant Au enrichment toward deeper levels (Victoria, San Antonio, Roberta-Robertita veins; Fig. 3A and B) suggesting po- tential telescoping mineralization. In both sets, the Au/Ag ratios in- crease with depth. Finally, in the Sinaloa and Western blocks the Ag- dominant veins are characterized by higher Cu concentrations. Throughout the district a single economic horizon has been long re- cognized between~950 and~258m asl, roughly sub-parallel to the LVC stratigraphic upper contact and presenting the same general dip of ~35° to the east (Fig. 2). 5.2. Geological and petrographic characteristics of mineralization types 5.2.1. Cu-Au porphyry mineralization type In the Contraestaca area (Western Block; Figs. 2, 4 and 5A) we re- cognized for the first time subvolcanic porphyry Cu-Au mineralized bodies intruding the LVC Socavón member (Figs. 2, 3 and 5). Such porphyry mineralization is also observed as clasts in hydrothermal breccias of the Victoria vein roots, within the Andesite Group (Sinaloa Block). Samples of these hypabyssal intrusions are characterized by a Fig. 5. Mineral associations of the Cu-porphyry mi- neralization event. (A) Porphyry subvolcanic Cu-Au mineralized intrusion outcropping in the Contraestaca area (Western Block); (B) and (C) pet- rographic thin sections of andesite porphyry miner- alized intrusion (phenocrystals ≪ matrix) with dominant strong biotite (Bi) potassic alteration overprinted by subordinate patches of albite (Alb)+ sericite (Ser) and traces of epidote (Ep)+ chlorite (Chl) veinlets. (H) Andesite porphyry intrusion (matrix > phenocrystals) with dominant subordinate biotite potassic alteration overprinted by strong pervasive albite alteration and trace of chlor- ites and thin quartz veinlets. P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 6 porphyritic texture with discrete refractory quartz. They are dacite to andesite in composition and reddish to gray in color. Mineral para- genesis consists of quartz > orthoclase > plagioclase > pyroxene > biotite and hornblende phenocrysts in a groundmass of fine plagioclase and glass (Fig. 5 B and C). Three alteration processes were identified (1) a pervasive potassic alteration, mainly represented by secondary biotite replacing ferromagnesian phenocrysts and matrix minerals; (2) a sodic-calcic alteration with albite+ actinolite ± epidote nidus, patches or halos around pyrite, and chalcopyrite and thin veinlets stockwork (Fig. 5 B and C); and (3) a late propylitic al- teration with chlorite+ epidote+ calcite+ pyrite overprinting the previous events. Mineralization is dominated by pyrite and chalcopyrite in thin veinlets disseminated into the groundmass and magnetite veinlets from < 1mm to 10 cm in width. 5.2.2. Ag dominant hydrothermal mineralization type The most significant mineralization event in San Dimas district is the Ag-dominant hydrothermal vein mineralization (Fig. 3). These veins are developed in three different stages, called opening, filling and closing. The filling stage is the main mineralization event with the highest Ag values (Fig. 6A). Main hydrothermal textures observed, in all stages, are hydrothermal breccias and banding with gangue composed by mosaic quartz with undulatory extinction with less adularia-rhor- ochrosite carbonate. The alteration intensity varies from weak and pervasive. Alteration mineral assemblage is represented by coarse grains of chlorite and less sericite. Base metal sulphides are the earliest phases; they can be found disseminated in the breccia fragments and rimming and overgrown on fragments, in bands alternating with quartz and carbonate or in pots into the quartz matrix. Monosulphides are rare and they occur mostly as aggregates or as xenomorphic areas (Fig. 6B); they are, in decreasing abundance, pyrite–arsenopyrite, sphalerite, ga- lena and chalcopyrite. They are corroded by native silver, sulphides and sulphosalts. Silver sulphides (acantite, jalpeite and less polibasite) and electrum grains are also found free, filling fractures in quartz, adularia, and chlorite (Fig. 6C). Oxidation is scarce and locally limited to the very upper levels of the deposit. Locally in the Western and Sinaloa blocks, when the veins crosscut the Late Cretaceous porphyry intrusions, the mineralization is enriched in copper and bornite, with covelite and copper oxide completing the paragenesis. Fig. 6. Mineral associations of the Ag-dominant mineralization event. (A) and (B) Outcrops of vein textures of different stages at Victoria vein, Sinaloa Block. (A) Succession of crustiform and massive crowded quartz (stage i) in a crosscutting relationship with banded base metals (stage ii) and hydrothermal breccia (stage iii). (B) Three different hydrothermal breccias in a crosscutting relationship. (C) reflected light photomicrographs of replacement texture of sphalerite (Shp) in chal- copyrite (Ccp) and filling open space of acanthite (Ac) at stage i (Victoria vein, Sinaloa Block). (D) reflected light photomicrographs of anhedral electrum crystals filling open space fractures in mosaic quartz and coarse adularia at stage i (Victoria vein, Sinaloa Block). P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 7 5.2.3. Au-dominant hydrothermal mineralization type Six of the most representative Au-dominant vein structures were mapped in detail: San Luis and Arana veins in the Tayoltita Block, San Antonio and Guadalupe veins in the Western Block, and Mala Noche and Causitas veins in the prospecting area south of the district (Figs. 2 and 7A). These veins are typically symmetrically banded. Lat- tice texture comprises platy calcite and barite and its quartz pseudo- morphs filling fractures (Fig. 7A). These veins show typical interbanded and discontinuous layers of base metal mineralization ± native Cu, the latter increasing toward the Western Block (Fig. 7B). A weak to per- vasive sericite alteration is present throughout the veins. Gangue is represented by crustiform and mosaic euhedral quartz texture with undulatory extinction. The mineral abundance is pyrite > sphalerite > galena > chalcopyrite ± bornite ± native Cu in eu- hedral to anhedral coarse grains. Fig. 7B illustrate the copper reaction rims between chalcopyrite and galena. Overgrowths of pyrite on sphalerites represent the late mineral within the base metals sequence. Free electrum grains are found filling fractures as anhedral coarse crystals that show medium to fine grain sizes (Fig. 7C). 5.3. Trace element abundances and correlations in electrum, native silver, and silver sulphides from Au- and Ag-dominant veins Electron microprobe analyses (n=98) of electrum, native silver and silver sulphide minerals from mineralized veins were conducted to detect potential chemical variations. Samples were collected from structures that record Ag- and Au-mineralization events of different intensity: Victoria Ag-dominant vein, and San Luis, San Antonio and Mala Noche Au-dominant veins. Results for 12 major elements of the ore suite (Supplementary Table S2) are presented as Principal Compo- nent Analysis (PCA) in Fig. 8. Mineralogy and elemental relationships in the Ag- and Au-dominant mineralization events in all structures are broadly comparable. Silver is related to arsenic and base metals (Cu, Zn, Fe), and gold is pre- ferentially associated with bismuth and mercury (Fig. 8). However some chemical differences are noted. In Ag-dominant structures, silver mineralization is associated to tellurium, and gold mineralization positively correlates with antimony. In Au-dominant structures, silver mineralization is relatively depleted in tellurium and enriched in anti- mony (Supplementary Table S2), and silver positively correlates with both antimony and tellurium (Fig. 8). Besides, the Au/Ag ratio in electrum significantly increases in the Au-dominant structures (Supplementary Table S2). 5.4. Geochronology of the porphyry and epithermal mineralization events 5.4.1. Cu-Au porphyry event In an attempt to date the Cu porphyry mineralization found in the LVC, suitable exploration drill hole core samples were collected from the Victoria vein roots (VIC16-PC-1 and VIC13-PC-2; Figs. 2 and 3) in the Sinaloa block, and from two mineralized clasts (PM-1 and PM-2) from the first stage hydrothermal breccia of the Victoria vein (Fig. 6 A). Thirty-nine analyses performed on zircon crystals from VIC16-PC-1 mineralized porphyry sample yielded 206Pb/207Pb ages ranging from 64.1 ± 2.5Ma to 893.0 ± 17.0Ma. Five analyses were discarded due to their high sigma errors. Within the remaining thirty-four concordant ages, twenty-two analyses form a tight cluster with a distribution tail towards the younger ages, and a weighted mean crystallization age (Ludwig, 2008) of 67.4 ± 1.1Ma. (n= 22; MSWD=1.16; Fig. 9, Supplementary Table S3). Thirty-four analyses performed on zircon crystals from VIC13-PC-2 mineralized porphyry sample yielded 206Pb/207Pb ages ranging from 63.0 ± 1.9Ma to 456.0 ± 10.0Ma. Five analyses were discarded due to their high sigma errors. Within the remaining twenty-nine concordant ages, twenty-one analyses form a tight cluster with a weighted mean crystallization age (Ludwig, 2008) of 67.6 ± 1.2Ma. (n= 21; MSWD=0.36; Fig. 9 Supplementary Table S3). The two mineralized clasts (M−1 and PM-2) show a porphyry texture and are interpreted to correspond to the Andesite Group host rock. Eleven (PM-1) and five (PM-2) zircon crystals were analyzed in these barren samples, respectively. Three analyses in both samples were discarded due to their high sigma errors. The remaining eight analyses in the PM-1 sample form a tight cluster and yield 206Pb/207Pb ages ranging from 64.0 ± 2.3Ma to 68.5 ± 2.9Ma with a weighted mean Fig. 7. Mineral associations of the Au-dominant mi- neralization event. (A) Field view of the Causitas Au vein, with zoom to lattice calcite texture developed at a late stage of mineralization. (B) Reflected light photomicrograph showing the paragenetic associa- tion of base metals, replacement textures of spha- lerite in chalcopyrite and galena and reaction rims on chalcopyrites due to galena. These minerals fill open space fractures into mosaic quartz (San Luis Vein, Tayoltita Block). (C) Reflected light photomicrograph of a native gold crystal filling a fracture in mosaic quartz (San Luis vein, Tayoltita Block). P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 8 crystallization age of 66.1 ± 1.8Ma (n=8; MSWD=1.15; Fig. 9, Supplementary Table S3). The remaining two analyses of sample PM-2 yield 206Pb/207Pb ages of 65.7 ± 2.7Ma and 70.0 ± 4.0Ma (Fig. 9, Supplementary Table S3). As a whole, U-Pb dating of Cu-porphyry and related clasts in veins indicate a coherent period of magmatic intrusion ranging from 67 to 63Ma. The presence of porphyry clasts in Ag-dominant veins suggest a spatial overlap of the two mineralizing events and explains the in- creased Cu content in the silver mineralization in these specific areas of the district. Fig. 8. Principal component analysis for 12 major elements determined in electrum, silver and silver sulphide grains from the (A) Ag-dominant mineralization event, and (B) Au-dominant mineralization event. Fig. 9. U-Pb concordia diagrams and weighted mean U-Pb ages for zircons from two drill hole porphyries and two mineralized porphyry clasts (PM-1 and PM-2) from the Victoria vein. Errors in calculated ages are 2σ. Black ellipses and black bars are data not used in calculating the ages. P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 9 Ag - dominant veins (Ag/Au >25) Au - dominant veins (Ag/Au<25) y 0.8 0.6 0.4 0.2 O.O -0.2 -0.4 -0.6 Ag Ag-event Fe Victoria vein ...._ Au-event - Au Bi ... ... ... X - [fil Ag --.-----,.--..--~-~---r------.---T"""""' 1 1 -0.30 -0.1 O.ü30 0.026 ::::i 0.022 00 rr, N ~ ID fl 0.018 0.010 O 0.1 0.3 0.50 0.7 0.9 -0.30 -0.1 Drill hole porphyry samples VIC-16-PC-1 VIC-13-PC-2 180 O.ü30 180 0.026 0.022 140 100 0.018 0.014 0.010 60 0.014 ~ 0.006 0 _002 20 o error ellipses 0.006 0_002 20 2o error ellipses 0.00 0.04 0.08 0.12 0.16 0.20 80r------------, 80 VIC-16-PC-1 75 67.4 ± 1.1 Ma 75 ro MSWD of 1.16 ;:E 70 i::: ~ 70 t 65 11 60 60 65 55~-------~ 55 (error bars are 2a) 0.00 0.04 0.08 0.12 0.16 0.20 207pb/235u VIC-13-PC-2 1 l 1 67.6 ± 1.2 Ma MSWD of0.36 1 1 1 (error bars are 2a) • San Luis vein - San Antonio vein • Mala Noche vein Ag-event Au-event -- Au 1 1 1 1 1 0.1 0.3 0.50 0.7 0.9 Porphyry clast samples 0.030 PM-1-2 180 0.026 0.022 140 0.018 0.014 0.010 60 0.006 20 . 0 _002 2o error elhpses 80 75 70 65 60 55 0.00 0.04 0.08 0.12 0.16 0.20 207pb¡235u PM-1 PM-7 66.1 ± 1.8 Ma MSWDof 1.1' ,_ - ,_ 1 ,- ,- - 65.7 ± 2.7 Ma (error bars are 2a) 5.4.2. Ag- and Au-dominant mineralization events Minerals suitable for dating of both the Ag- and Au-dominant mi- neralization events were identified in the petrographic study. Adularia is characteristic of the silver mineralization event, and sericite content increases in the gold-enriched veins structures. However, the separation and 40Ar/39Ar dating of pure hydrothermal sericite was technically too complicate because of the fine size and the alteration of the crystals. For adequate dating of both events, we sampled and dated the best-pre- served adularia crystals most distant from the sericite aureole of the Ag- dominant structures, and the best-preserved adularia crystals in the more strongly altered sericite aureole in telescoped Ag-Au-mineralized structures. Adularia separated from Victoria (n=3), Robertita (n=1), Roberta (n=1), and Jessica (n=1) veins was dated using the 40Ar-39Ar step-heating technique. The paragenetic position and the purity of all dated adularia were initially evaluated by cath- odoluminescence and petrographic studies. The new 40Ar-39Ar ages are synthesized in Supplementary Table S4 and Fig. 10, and in Fig. 12 to- gether with the available literature data. All analyzed adularia samples display staircase-shaped age spectra from the lowest to the highest temperature step. In three experiments an apparent slight age decrease is observed at mid temperature steps (SD-41 stage 2, SD-42 and SD-43; Fig. 10 and Supplementary Table S4). Four experiments meet the criteria for a plateau crystallization age. In these cases, the inverse isochron ages are indistinguishable at the 95% confidence interval from the plateau ages (Fig. 10; Supplementary Table S4). Nevertheless, the steps in the inverse isochron diagrams concentrate at high 39Ar/40Ar values with little spread, which result in poorly defined 36Ar/40Ar intercepts and larger age errors. On these grounds, the plateau age is taken as the preferred age. For adularia related to the Ag-dominant event (sample SD-41; Robertita vein) a plateau age of 38.14 ± 0.19Ma (95% conf.) was obtained, whereas adularia from the Au-dominant event in the Roberta and Jessica veins, (samples SD42 and SD-43, respectively) yielded plateau ages of 32.05 ± 0.22Ma (95% conf.) and 32.14 ± 0.18Ma (95% conf.) (Fig. 10). In three experiments from Victoria vein (SD-48 stage 1–2-3), the staircase shape is too pronounced to determinate plateau ages. In these cases, isochron ages are not well defined neither and we interpret the higher temperature ages steps as minimum crys- tallization ages of 34.35 ± 0.10 to 38.91 ± 0.14Ma (2σ). The oldest age from sample SD-48 stage 1 is comparable to the plateau age de- termined for adularia from Robertita vein and with the oldest 40Ar/39Ar ages reported in literature (ca. 40Ma; Enríquez et al., 2018; Figs. 10 and 12). The younger plateau ages at ~ 32Ma obtained for adularia from the Roberta and Jessica veins are also comparable to the K-Ar and 40Ar/39Ar ages reported in the literature (~32Ma; Enríquez et al., 2018 and references therein; Figs. 10 and 12 and Supplementary Table S4). Finally, in all the seven experiments, the low temperature steps ages point to effects of argon loss at ages of 22 to 26Ma. These ages coincide with a regional extensional event, contemporary with a major ignim- brite flare up, which might have led to a crustal thermal anomaly (Ferrari et al., 2013, 2018a; Montoya-Lopera et al., 2019). 5.5. Titanite fission-track thermochronology To better constrain the Ag-dominant mineralization event, titanite fission-track dating was performed on a sample from the Eocene Piaxtla granite (SD-16), assuming that the hydrothermal event likely had an effect on the thermochronologic record of the batholith. Sample SD-040 was previously dated using U-Pb zircon geochronology revealing an age of ~ 49Ma (Montoya-Lopera et al., 2019). The results show a very a high P(χ2) value of 0.95, compatible with a single and short-lived thermal event. The model ages distribution ranges from~45 to~35Ma, with a very well-defined Gaussian maximum at 40.3 ± 1.1Ma (Fig. 11). Fission track length modification occurs in titanite below ~310 °C Fig. 10. Step heating 40Ar/39Ar spectra for adularia crystals separated from Victoria, Roberta, Robertita and Jessica veins. Spectra of all samples display variable degrees of perturbation with low temperature steps indicating argon loss. Plateau ages include the % of released 39Ar, other ages correspond to single steps. Age errors are 95% confidence intervals. tp: plateau age; tc: inverse isochron age; Wm: weighted mean. P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 10 42 Fumace 38.91 ± 0.14 Ma ro 38 step heating ~ 34 .E AJ 35.47 ± 0.08 Ma Q) 30 ci '---..Laser step heatlng C) <1'. 26 22 SD-48, sta e 1; Victoria vein o.o 0.2 0.4 0.6 0.8 1.0 42 ro 38 ~ 34 38.14 ± 0.19 Ma e 64% of 39Ar ·;¡; 30 ca.39 Ma Ji 26 22 SD-41, sta e 2; Robertita vein o.o 0.2 0.4 0.6 0.8 1.0 42 ro 38 36.30 ± 0.08 Ma ~ 34 .E 30 Q) C) <1'. 26 22 SD-48, sta e 2; Victoria vein 42°·0 0.2 0.4 0.6 0.8 1.0 ro 38 34.35 ± 0.10 Ma ~ 34 .E Q) 30 C) <1'. 26 22 SD-48, stage 3; Victoria vein o.o 0.2 0.4 0.6 0.8 Fraction of 39Ar released 1.0 42 ro 38 ~ 34 e Q) 30 32.05 ± 0.22 Ma C) 65% of 39Ar <1'. 26 22 SD-42; Roberta vein o.o 0.2 0.4 0.6 0.8 1.0 42 ro 38 ~ 34 e Q) 30 32.14 ± 0.18 Ma C) <1'. 26 80% of 39Ar 22 SD-43; Jessica vein o.o 0.2 0.4 0.6 0.8 Fraction of 39 Ar released 1.0 (Coyle and Wagner, 1998). The hydrothermal FT reset age on titanite from the Piaxtla granite is in good agreement with the 40Ar/39Ar ages on adularia for the Ag-dominant event. By contrast, the Au-dominant hydrothermal event is not recorded in the FT age of titanite, suggesting lower hydrothermal temperatures for this event. In conclusion, the in- tegration of our new 40Ar/39Ar ages on adularia and FT ages on titanite pin-point the occurrence of the Ag-mineralizing event at 40 ± 1.1Ma (Fig. 11), well separated from the Au-dominant event that occurred in the early Oligocene. 5.6. Zircon geochemical signatures In this section, the evolution through time of the chemistry of zircon crystals in the local stratigraphic column (Montoya-Lopera et al., 2019) is used to establish possible correlations between magma chemistry and the Eocene and Oligocene mineralizing events dated at ~40Ma and~32Ma (this study), respectively. Within the local stratigraphy, the hydrothermal events occur at the beginning and at the end of a ca. 10Ma magmatic lull. As proposed for other regions worldwide (Ballar et al., 2002; Dilles et al., 2015; Lu et al., 2016), the zircon trace element chemistry can be used to identify periods of fertile magmatism and to define potential time relationships between magmatic and hydro- thermal/mineralization events through the use of Eu/Eu* and (Ce/Nd)/ Y) versus U/Pb zircon ages plots (Ballar et al., 2002; Dilles et al., 2015; Lu et al., 2016). Zircon U/Pb ages (Montoya-Lopera et al., 2019) and trace element compositions (this study; Supplementary Table S5) from Late Cretaceous to late Oligocene volcanic and sedimentary units in the district are presented in Fig. 12. The evolution of the zircon Eu/Eu* and (Ce/Nd)/Y) ratios through time strongly point to the occurrence of three fertile magmatic events: (1) a hydrated and oxidized event related to Late Cretaceous arc mag- matism; (2) a hydrated and oxidized event associated to Eocene mag- matism (from 50 to 43Ma; Eu/Eu*> 0.3 and (Ce/Nd)/Y > 0.01), which is characterized by a gradual increase in the Eu/Eu* ratio with time and correspond to the construction of the Piaxla batholith; (3) a hydrated and reduced event (Eu/Eu*> 0.3 and (Ce/Nd)/Y < 0.01) corresponding to the emplacement of early Oligocene UVS rhyolitic ignimbrites and domes (Fig. 12). Zircon chemistry clearly illustrates the most likely magmatic sources for the hydrothermal pulses and thus brackets the age of the mineralization events. It also allows identifying the chemical differ- ences between the Eocene and Oligocene events that favored the Ag- or the Au-mineralization style. The Eocene fertile period with hydrated and oxidized magmatism favored the transport and concentration of silver metals. The continuous increase of Eu/Eu* and (Ce/Nd)/Y ratios during the formation of the Piaxla batholith suggests that the age of the silver mineralizing event may be bracketed between the last zircon age of ~43Ma and the first hydrothermal mineral age (adularia) at ~41Ma. On the other hand, the Oligocene chemical anomaly con- firms the field relationship between rhyolitic domes and gold veins. The zircon chemistry of early Oligocene volcanics suggests a hydrated and reduced magma, favoring the transport and concentration of gold me- tals. 6. Discussion and conclusions The first mineralization event recognized in San Dimas district is related to Cu-porphyries intrusions associated with the formation of the Laramide magmatic arc during Late Cretaceous to early Paleocene times. Two different pulses are recognized, the first at ~73–75Ma (Montoya-Lopera et al., 2019) and the second, more intense, at ~67 to 63Ma. These ages coincide with the ages of the Cu-porphyry deposits of northwest Mexico (Sonora state) and southwestern U.S.A (Arizona, New Mexico) (Valencia-Moreno et al., 2017 and references therein). The Cu- porphyry stocks are probably associated with subduction-related Lar- amide batholiths, well exposed to the west in Sinaloa. The porphyries recognized at San Dimas probably represent the southeastern most Cu- porphyries intrusions of the Cordilleran belt. In Sinaloa, Cu-porphyry intrusions are generally scarce, and geographically restricted to the coastal area (e.g. the Cosalá and La Azulita deposits Valencia-Moreno Fig. 11. Titanite fission track radial plot for sample SD-040 from the Piaxtla granite. The blue band represents the age of the Ag-dominant hydrothermal mineralizing event. The ρs versus 238U graph indicates the confidence of the single event age determination. The U-Pb zircon crystallization age of the Piaxtla batholith and the age of Au-dominant veins are showed for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 11 Piaxla batholith, SD-040 Ngr = 20; D = 0%; P(x2 ) = 0.95 Central TFT age = 40.3 ± 1. 1 Ma 2 -2 Ag-dominant veins (less perturbed ages ca. 41-39 Ma) a/t 15 12 10% Ua o 1 2 3 4 s 6 7 a g 10 45 41 40.3 39 5.0 4.5 p5 (X106 cm·2) 4.0 o 3.5 3.0 2.5 :b 2.0 1.5 ~ '1:18 «iP 1.0 0.5 U-238 (ppm) 100 200 300 et al., 2017). The discovery of Cu-porphyry intrusions in San Dimas extends the exploration targets to the east, below the Oligocene vol- canic cover. The following mineralization events, the Ag and Au hydrothermal events, were distinguished and characterized on the basis of a detailed petrographic, geochemical and geochronologic study of the vein sys- tems. Detailed geological field work indicates strong structural control on metal deposition, which exhibit a wide range of Au/Ag ratios, ran- ging from Ag-dominant to Au-dominant mineralized structures (Figs. 2, 3, 6 and 7). Ag- and Au-mineralization types were emplaced in two different structural systems (Horner and Enriquez, 1999). Ag-dominant mineralization structures are mainly hosted in E-W sigmoidal tension gashes and geographically distributed in proximity to the Eocene Piaxtla batholith. Au-dominant mineralization structures are mainly hosted in N-S faults systems that have undergone multiple reactivations and are geographically associated with Oligocene rhyolitic domes. The Ag- and Au-dominant events are characterized by different vein texture, mineralogy, and Au-Ag ratios. The Ag-dominant veins are composed by various quartz and breccia events and are characterized by adularia and rhodochrosite alteration. The Au-dominant veins are monogenetic and characterized by intense sericitization. In both Ag- and Au-dominant events, trace element chemistry show distinct metals affinities sug- gesting different metal sources and/or P-T-fO2 mineralizing fluids conditions. Ag- and Au-dominant veins present comparable metal as- sociations in two different patterns: a) silver sulphide minerals asso- ciated to base metals, and b) electrum gold grains with As, Sb, Hg signature. Strong negative correlations between silver and base metals illustrate the chemical evolution of fluids during the Ag hydrothermal event, which is in good agreement with the petrographic observations. The Au epithermal event is clearly disconnected from the Ag event regarding metal association. The presence of tellurium in the silver association points to a connection to subduction processes and mantle wedge enrichment (Sanders and Brueseke, 2012). The Au, Hg, Sb, and Bi association is commonly related to shallow epithermal systems (Gray et al., 1991; Bornhorst et al., 1995). The increase of Sb in the Ag- dominant veins suggests a more distal position from the thermal source. This last observation is corroborated by the geographic distribution of the veins (Figs. 2 and 3). With increasing distance of the mineralized structure from NNW structural trend that bounds to the east the San Dimas district, a more pronounced Sb enrichment is observed during the Oligocene Au-epithermal event. Although Ag and Au mineralization exhibit structural, petrographic and chemical differences, “pure” veins (i.e., belonging only to one of these events) are uncommon. Most of the mineralized veins present a “mix or sum” of the two events characteristics. Therefore, they could represent two end-members of a continuum process as claimed by several authors (Enríquez et al., 2018 and references therein) or they could represent a proportional sum of two different events. The development of a magmatic-hydrothermal system corresponds usually to a particular stage(s) of a more complex magmatic evolution (Simmons and Brown, 2006; Buret et al., 2016; Richards, 2018), in- cluding the evolution of magma in deep crustal intrusion zones, crustal melting processes, and emplacement of batholiths in higher crustal le- vels. These hydrothermal stage(s) could be unique, pulsatile, or Fig. 12. Synthesis diagram of the trace elements variation in zircon crystals through time. 238U-206Pb ages of single zircon crystals from Montoya-Lopera et al. (2019); K-Ar and 40Ar/39Ar ages from Enríquez et al. (2018) and this study; apatite and titanite fission tracks data from Montoya-Lopera et al. (2019) and this study. Local stratigraphic column from Montoya-Lopera et al. (2019). P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 12 Stratigraphy column Second fiare u -24 Silicic domes Andesitic group -63 to 50 Age Ma 25 80 85 o o o • Volcanic, Oligocene U/Pb zircon age • Plutonic, U/Pb zircon age i< Sedimentary, U/Pb zircon age • Volcanic, Cretaceous U/Pb zircon age Zircon crystal chemistry (Ce/Nd) /Y Eu / Eu* Epid,e,mal Au-doml~ í ml"erall¡ •Íoo ca. 30 Ma Mesothermal Ag-dominant mineralization ca. 40 Ma AgeMa Literatura 23.3±1.9 24.7±1.9 73.0±1.5 75.4±0.8 ~ io. :º : · ~ 32.1±0.2 34.3±0.1 36.3±0.1 38.1±0.3 38.9±0.1 40.3±1.1 65.7±2.7 Thermal events Miocene Extension Au-D, L.S. Ag-D, /.S. 66· 1 ±1-8 Cu-C-Porph 67.4±1.1 67.6±1.2 Age of mineralization events K-Ar, Sericite (Enriquez and Rivera, 2001) K-Ar, Adularia (Henry, 1975; Enriquez and Rivera, 2001) Ar-Ar, Adularia (Enriquez et al., 2018; this study) U/Pb zircon, plutonic (this study) FT-apatite and titanite (Montoya et al., 2019; this study) Au-D, L.S. Au-Dominant Low Sulfidation Ag-D, /.S. Ag-Dominant inter. Sulfidation Cu-C-Porph Cu-Cretaceous porphyry continuous over hundred thousand of years (Arribas et al., 1995; Garwin, 2002). They may occur in discrete periods of time, be over- printed by subsequent hydrothermal systems, or eroded by uplift and erosional processes (Garwin, 2002). Lovett (2003) and Sanchez-Alfaro et al. (2016) showed that in extreme cases gold veins may form over seconds by seismic depressurization. Metal precipitation may be so ef- ficient that a giant epithermal gold deposit such as Ladolam (>42 Moz) might have been formed in<55,000 years (Simmons and Brown, 2006). In general, the occurrence of long-living hydrothermal systems is not a precondition for the genesis of large hydrothermal deposits. The application of different geochronology methods to primary and secondary minerals represents a reliable tool to decipher the genesis of complex deposits. Such studies allow to understand and highlight the relationship between local/regional magmatic events and the hydro- thermal systems. Furthermore, reliable geochronological data is es- sential to understand the geodynamic context and evolution of a de- posit. Magmatic events are relatively easy to date by routine geochronological methods such as U/Pb zircon dating. However, dating of hydrothermal events is not straightforward and is often done by isotopic analysis of minerals with relatively low closure temperature (230 to 300 °C for K-Ar and 40Ar/39Ar in adularia and sericite) which are easily perturbed or may undergo recrystallization as in the case of hypogene alunite under supergene acidic conditions (Arribas et al., 1995; Lozano et al., 2018). This isotopic closure temperature range (lower than 300 °C) corresponds to most of the known epithermal sys- tems in Mexico and worldwide (Hayba et al., 1985; Simmons, 1991; Sillitoe, 1993; Arribas et al., 1995; Hedenquist and Arribas, 1999; Sillitoe and Hedenquist, 2003; Einaudi et al., 2003; Simmons et al., 2005; Camprubí and Albinson, 2007). Within a polyphase hydro- thermal system, long-lived or overprinted, older ages related to the initial mineralization event can be easily perturbed by subsequent hy- drothermal fluids, which may blur temporal distinctions between dis- crete and spatially related hydrothermal systems. One of the con- sequences could be an apparent long-lived hydrothermal system, rather than one characterized by superposed systems that may have acted episodically over a period of time. To be able to distinguish a long-lived, albeit pulsatile, hydrothermal system from time-discrete overlapping hydrothermal systems, the mineralizing hydrothermal system must be studied in terms of its source, the conducts transferring the metals and the trap, all occurring in a unique geological episode. Any mineralized system presenting significant internal differences in chemistry, struc- tures or ages would be the result of overlapping hydrothermal events. Based on our sampling methodology, the petrography of the San Dimas veins and the shape of the 40Ar/39Ar step heating experiments we infer that all studied samples present a variable grade of isotopic resetting. The oldest ages, being plateau ages or higher temperature steps, are interpreted as a minimum age of the Ag-mineralization event at ~ 39–40Ma. The younger plateau ages do not show a complete iso- topic resetting and are interpreted as a maximum age of the Au-mi- neralization event at ~ 32Ma. All the 40Ar/39Ar experiments yielding ages in the range 40 to 32Ma show the highest spectra perturbation, so we conclude that they have no geological significance. Magmatic zircon U/Pb ages and trace element ratios distribution allow linking the cor- responding magmatic and mineralizing hydrothermal events at ~41–40Ma and~30–29Ma. The Eocene-Oligocene magmatic evolution of the SMO and of most of central Mexico is comparable in age and chemistry and matches with the mineralization events. Eocene ages of mineralization, like those identified at San Dimas, are reported for other epithermal deposit in the SMO (~44Ma K-Ar age for the Ag-Pb-Zn Topia deposit, Loucks, 1991; Loucks et al., 1988; Loucks and Petersen, 1988), and epithermal and skarn deposits of central Mexico (~47Ma U-Pb age for the Zn-Ag Charcas deposit, Levresse et al., 2015; 42.36 ± 0.18Ma, 40Ar/39Ar age for the Ag-Pb-Zn Zacatecas district, Zamora-Vega et al., 2018; 46.2 ± 1Ma, K-Ar age for the San Martin Cu-deposit, Damon et al., 1981), which were developed along a crustal fault system that bounds the Mesa Central to the west-southwest (Nieto-Samaniego et al., 2005). With the exception of the Zacatecas district, all these deposits are ge- netically related to a geochemically anomalous plutonism with sig- nificant enrichment in large ion lithophile elements (LILE) and Ba (Chávez Cabello, 2005; Damon et al., 1981; González García, 2016; Mascuñano et al., 2013; Patterson, 2001; Poliquin, 2009; Rubin and Kyle, 1988; Velasco Tapia et al., 2011). The petrogenetic model pro- posed for these granitoids and related mineralization involves partial melting of a metasomatized mantle in a post-orogenic scenario (González García, 2016; Mascuñano et al., 2013; Velasco Tapia et al., 2011). Geodynamically, they illustrate the magmatic transition from the Cretaceous-Paleocene calc-alkaline magmatism related to normal supra-subduction arc dynamics to Oligocene-early Miocene anhydrous and bimodal magmatism related to lithospheric extension associated to a growing slab window (Ferrari et al., 2018a). In this framework, the role of the Eocene Piaxla batholith as a thermal and metal source for the Ag-dominant mineralization in the San Dimas district, strongly ques- tions the intermediate sulfidation epithermal model previously sug- gested for this event. In the San Dimas district as well as in the southern part of the SMO, the Au-dominant low sulfidation veins are clearly related to Oligocene rhyolitic domes, aligned within a long NNW-SSE extensional to trans- tensional corridor that marks the limit of the unextended core of the SMO (Tayoltita-Pueblo Nuevo fault system of Ferrari et al., 2013) (Fig. 2). This mineralizing event is comparable in age, structural and geological context to most of the historic Ag-mining districts in central Mexico (Ag-Au epithermal Zacatecas district: Tristán-González et al., 2012; Zamora-Vega et al., 2018; Ag-Au epithermal Fresnillo district: Velador et al., 2010; Ag-Au epithermal Guanajuato district: Moncada et al., 2012, 2017; Nieto-Samaniego et al., 2016; Angeles Moreno et al., 2017; Ag-Au epithermal Taxco district: Hernandez Vargas et al., 2017). A succession of magmatic and tectonic events must have been su- perposed to create a large Ag/Au deposit like the San Dimas district. The first mineralization event associated to Cu-porphyry intrusions developed as part of the Laramide magmatic arc in Late Cretaceous to early Paleocene times, similarly to other deposits in Sonora, Arizona and New Mexico. This first mineralization is uneconomic but was likely instrumental in the development of the second and most prolific Ag- dominated mineralization event, in which silver mineralization is ge- netically related to the end of the Piaxtla batholith emplacement at ~43Ma. The related hydrothermal system is structurally controlled, of relatively high temperature (up to 400 °C), neutral pH (adularia and calcite) and with intermediate sulfidation fluids. The building of the Piaxtla batholith is related to the initiation of lithospheric extension characterized by high rate of exhumation and erosion, which allowed the rapid uplift of the batholith and the related Ag mineralized veins to a shallower crustal level. The last mineralization event occurring at ~30–29Ma resulted in what is considered a classic Mexican low sulfidation epithermal Au vein deposit. Mineralized veins are geneti- cally related to rhyolitic domes emplaced along north-northwest transcurrent faults systems. The related hydrothermal system is struc- turally controlled, of relatively low temperature (up to 250 °C), low- acidic to neutral pH (alunite and sericite), with low sulfidation fluids and occurred at shallow depth. At a regional level, it coincides with the Oligocene-early Miocene synextensional voluminous bimodal vol- canism of the SMO with pulses at 31–29Ma, and 24–20Ma (Ferrari et al., 2018a). The detailed analysis of the classic San Dimas epithermal deposit presented in this contribution suggests that the traditional view for the formation of large polymetallic Mexican deposit need to be revised. As in San Dimas, it is very likely that the idea of a single, protracted hy- drothermal system cannot resist a detailed geological and geochrono- logical evaluation. The richness of the large silver and gold Mexican deposits could be explained by the superposition in space and time of specific geological events within a favorable geodynamic evolution. The formation of the San Dimas district with its exceptionally rich Ag-Au P. Montoya-Lopera, et al. Ore Geology Reviews 118 (2020) 103195 13 mineralization resides in the late Eocene to Early Oligocene uplift that allowed the overprinting of originally deeper Ag veins by a shallower Au mineralizing event and their final exposition favored by the Oligocene-early Miocene extension that produced the erosion of the local volcanic cover. More importantly, geology and fluid geochemistry of each mineralizing event are coherent within the geodynamic evo- lution of the area, showing a transition from a magmatic arc to a con- tinental extension regime. Acknowledgments This research is part of the PhD project of the first author at Universidad Nacional Autónoma de México (UNAM) Postgraduate Program. The research was funded by CONACYT grant CB 237745-T to L. Ferrari and DGAPA-PAPIIT grant IN106017 to G. Levresse. We thank Primero Mining (presently First Majestic Silver Corp.) for sharing un- published information and for logistical support. Special thanks to Nicolas Landón for his strong support in the initial phase of the research and to Miguel Pérez for sharing his knowledge on the ore geology of the central SMO. We also thank Carlos Ortega for assistance with U-Pb dating, Margarita Lopez (CICESE, Mexico) for assistance with 40Ar-39Ar dating, A. Susana Rosas Montoya y Miguel Angel García García for sample preparation for Ar-Ar dating, Carlos Linares for assistance with the Electron Probe X-Ray Microanalyzer at Laboratorio Universitario de Petrologia, Instituto de Geofisica, UNAM, Mexico, Marina Vega for as- sistance at Laboratorio de Fluidos Corticales, Centro de Geociencias, UNAM, and Juan Tomás Vazquez for the elaboration of thin sections. 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Ore Geology Reviews 118 (2020) 103195 15 60 Capítulo 4: Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes. . Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: Constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes Paula Montoya-Loperaa, Gilles Levresseb, ⁎ , Luca Ferrarib, Andrea Luca Rizzoc, Santiago Urquizaa, Luis Matad a Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, 76230, Queretaro, Qro., Mexico b Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, 76230, Queretaro, Qro., Mexico c Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, 90146 Palermo, Italy d First Majestic Silver Corp., Tayoltita, Dgo., Mexico A R T I C L E I N F O Keywords: San Dimas Ag–Au district Fluid inclusions D-O stable isotope Noble gases Telescoped ore deposits A B S T R A C T The San Dimas district is a world-class Ag/Au deposit, developed as a telescoped Eocene-Oligocene Ag/Au mineralization located in the Sierra Madre Occidental (SMO) of western Mexico. San Dimas exhibits multiple mineralization events during different magmatic and tectonic episodes from Late Cretaceous to early Oligocene. The well-preserved magmatic-hydrothermal system provides an excellent opportunity to determine the source of silver and gold, the evolution of the hydrothermal fluids, and the controls on the mineralization precipitation. Mineralogical, fluid inclusions (FI), stable and noble gases isotope analyses suggest that the San Dimas deposit consist of two different mineralization styles: 1) Ag-dominant epithermal Eocene veins that occurred at tem- peratures up to ~350 °C developed at ca. 2–3 km depth, associated to the final stages of intrusion of the Piaxtla batholith, with FI dominated by a crustal component, and 2) epithermal low sulfidation Au-dominant Oligocene veins which were developed at 250 °C, at shallower depths (< 1 km), associated to the feeding fractures of rhyolitic domes developed at the end of the main ignimbrite flare up of the SMO, with FI showing crustal fluids variably mixed with a magmatic component. Our results highlight the importance of a multidisciplinary ap- proach, such as field observations, geochronological and geochemical studies, to better understand the com- plexity of the hydrothermal magmatic processes involved in the formation of many Mexican ore deposits and their proper classification. 1. Introduction Since the early 20th century, economic geologists have recognized epithermal deposits as being important sources of silver and gold (Lindgren, 1922). More than 6000 tons of gold (Au) resources have been proven from worldwide epithermal systems and keep growing (Kerrich et al., 2000; Chen et al., 2003, 2012; Zhang et al., 2019). In the still valid original description of Lindgren (1922, 1933), an epithermal deposit is usually defined as the “sub-aerial volcanic-hosted types”. The typical characteristics of this kind of deposits are a structurally con- trolled extensional vein system, geographically and chronologically associated to a volcanic center (usually rhyolite domes) that formed in a shallow environment (< 1 km below the water table) and involving predominantly near neutral chloride waters (mostly of meteoric origin) at relative low temperatures (typically 150–300 °C) and low sulfidation (LS) mineralogy (Heald at al., 1987; Panteleyev, 1996; White, 2003; Pirajno, 2009). Later, Heald et al., (1987), Hedenquist (1987), and Simmons et al., (2005) among many others, described comparable mineralized structure related to deeper porphyries mineralization en- vironments. These are characterized by acidic magmatic fluids (high temperature and salinity), deeper metals precipitation, gangue and al- teration mineralogy (alunite, dickite, kaolinite) and related to high sulphidation (HS) state. These well-defined genetic models can be seen as two pure end-members of a common family, since many epithermal- like deposits do not fit in them. To solve this problem, John (1999), John et al., (1999) and Hedenquist and Arribas (2000) proposed a third epithermal class named intermediate-sulfidation (IS) type to explain and regroup the large variety of the structurally controlled deposits, https://doi.org/10.1016/j.oregeorev.2020.103427 Received 30 October 2019; Received in revised form 12 February 2020; Accepted 18 February 2020 ⁎ Corresponding author. E-mail address: glevresse@geociencias.unam.mx (G. Levresse). Ore Geology Reviews 120 (2020) 103427 Available online 21 February 20200169-1368/ © 2020 Elsevier B.V. All rights reserved. ELSEVIER OREGEOI..OCY REVIEWS J,.,r,,alb-Coonprd•:n,,..,Stooicsof °"'c:.ne... ... 1°"'fa~ from Cordilleran polymetallic to carbonate-base metal Au deposits (Einaudi, 1992; Leach and Corbett, 1994; Wang et al., 2019 and re- ferences therein). In Mexico, a review of epithermal deposit classification, including the classic San Dimas Au/Ag district, was proposed by Camprubi and Albinson (2007). The size of the Mexican silver and gold anomaly is illustrated by the outstanding play of Mexico mining during XVI and XVII centuries and its last ten years position as the largest silver (200 Moz/year) and eighth gold producer (125 t/year) worldwide. Most of this production comes from localities within the Sierra Madre Occi- dental province (SMO) and the magmatic events related to its forma- tion. The chronological distribution of epithermal deposits coincides with the three main Cenozoic volcanic pulses of the SMO, which record a broad migration from the northwest to the southeast, where the last ignimbrite flare-up occurred (Ferrari et al., 2007, 2018; Ramos-Rosique et al., 2010; Fig. 1). The age of the most fertile events ranges from 36 to 28 Ma and includes all the giant Ag–Au–Sn and IOCG mining districts (Camprubí, 2013). Few epithermal deposits in México are known to have formed under an acid chemical regime. In fact, HS deposits were only recognized in Sonora state, in close relationship to the Cu-por- phyry province (Camprubí, 2013). The recent re-evaluation of various historical Au/Ag districts (Guanajuato, Moncada et al., 2012, 2017; Fresnillo, Velador, 2010; Zacatecas, Zamora-Vega et al., 2018; San Dimas, Montoya et al., 2019b) is modifying several crucial concepts about timing, formation, and metals sources of the dominant inter- mediate and low sulfidation Mexican epithermal deposits. Some of them, traditionally classified as intermediate sulfidation, are now re- interpreted as telescoped deposits, formed during separate volcanic events sometimes under different geodynamic contexts (Zamora-Vega et al., 2019; Montoya-Lopera et al., 2019b). In this study, we report on the description and geochemistry of fluid inclusions (FI) hosted in the Eocene silver-dominant and Oligocene gold-dominant mineralizing deposits found in the San Dimas district (Fig. 1). Building on recent geological, geochronological and geo- chemical studies (Montoya et al., 2019a; Montoya-Lopera et al., 2019b), we describe the TPX evolution of the ore-forming fluids and the ore genesis, to finally propose a new metallogenic model for the San Dimas Ag/Au deposit. 2. San Dimas local geology and mineralization settings The San Dimas stratigraphic column can be divided into two major igneous successions that correspond to the Lower Volcanic Complex (LVC) and Upper Volcanic Supergroup (UVS) of the SMO, separated by Fig. 1. Regional geologic map of central Sierra Madre Occidental showing the main post-Eocene extensional structures and the principal mining districts (modified from Henry and Fredrikson, 1987, and Ferrari et al., 2013). Insert show the spatial distribution of the Sierra Madre Occidental (SMOc) volcanic province and the San dimas district (SDD) location. P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 2 D Coastal plain D Late Pliocene basalts 16-1 O Ma basalts D Graben clastic filling Espinazo-El Salto ignimbrite secuence (24 Ma) 177 Oligocene and early Miocene @_j_Q_J domes (a) and intrusive rocks ~ Eocene and Oligocene ~ ignimbrites í77 Eocene volcanic (a) and @_j!>_J intrusive (b) rocks r-........ Laramide are (Late K-Pc) @-..1111 a) volcanic b) intrusive rocks D Pre-Laramide basement 23º 00' Gu/f of California Town Toll highway Federal Highway Secondary road Master fault Fault erosional and depositional unconformities. A detailed description of the lithology, petrography and geochronology of the stratigraphic column is given in Montoya et al., (2019a) and is briefly summarized here. The Late Cretaceous to Eocene LVC, is composed of four volcanic members locally named “Socavón rhyolite”, “Buelna andesite”, “Portal rhyolite” and the Andesitic Group (Henshaw, 1953; Montoya et al., 2019a; Fig. 2). The LVC consists of an alternating suite of rhyolitic and andesite lavas flows locally intruded by felsic porphyritic bodies (Montoya et al., 2019a). The entire LVC volcanic column is crosscut and locally assimilated by the late Eocene Piaxtla batholith and its por- phyritic dikes swarm (49.1–45.2 Ma; Henry et al., 2003; Montoya et al., 2019a). The transition between the LVC and the UVS is marked by the Las Palmas and Camichin continental sedimentary formations with a maximum age of deposition of ~52 Ma and ~43 Ma, respectively (Montoya et al., 2019a). The UVC unconformably covers the La Palmas and Camichin formations, the Piaxtla batholith and the Andesite Group. It is composed by two successions of silicic ignimbrites with ages of ~31.5 to 29 Ma and ~24 Ma respectively, with intercalations of minor amount of mafic lavas and continental conglomerate (Montoya et al., 2019a). The lower ignimbrite succession is intruded by rhyolitic domes dated at ~29 Ma and bimodal dikes swarm (Ferrari et al., 2013; Montoya et al., 2019a). The San Dimas district is affected by two main fault systems with E- W and NNW-SSE orientation. Both systems present various reactivation events and are mineralized (Horner and Enriquez, 1999; Enriquez et al., 2018; Montoya et al., 2019a). The E-W fault system affects mainly the LVC and is overprinted by the NNW-SSE one. The main NNW-SSE striking normal faults divide the district in tilted fault blocks, dipping Fig. 2. (A) Geologic map of San Dimas mining district and Causita and Mala Noche southern extension, with location of published ages (from Montoya et al., 2019a) and sampled veins. (B) Simplified geological cross-section of the Ag/Au San Dimas district. P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 3 ~ 3000 2000 1000 o Oligocene and early Miocene domes (a) and intrusive rocks (b) dimentary deposits (Eocene): Palmas Fm.: b) Camichin unit sive suite (49-38 Ma) Piaxtla gran ite; b) El Cristo granite diorite intrusion Andesitic group (66-47 Ma) -106º oo· Central Block Roberta Rob~rtita vem -105° 56 ' Arana San Luis vein A' up to 30° to the east or to the west. Based on veins paragenesis sequence and 40Ar/39Ar ages, Montoya et al. (2019b) propose that mineralization occurs in two discrete hydrothermal events, a first Eocene Ag-dominant vein system (adularia + rhodochrosite type; ca. 41–40 Ma), mainly developed into E-W striking faults, followed by a Au-dominant epi- thermal vein mineralization (sericite type; ca. 30 Ma), which was em- placed into NNW-SSE striking faults concurrent with Oligocene rhyo- litic domes related to the final stage of the first SMO ignimbrite flare up. 3. Previous fluid inclusions studies of the San Dimas Ag–Au district Previous hydrothermal fluid studies at San Dimas mainly focused on the “bonanza” level Au-dominant structures of the Tayoltita Block (Fig. 2). Homogenization temperatures for quartz Au/Ag-mineralizing stage range from 250 °C to 310 °C, averaging 260 °C in all studies (Smith et al., 1982; Clarke and Title, 1988; Conrad et al., 1992; Enriquez and Rivera, 2001; Albinson et al., 2001; Churchill, 1980). Reported freezing point show a tide variation range from −0.11 °C to −1.5 °C. (Smith et al., 1982; Clarke and Title, 1988; Conrad et al., 1992; Enriquez and Rivera, 2001). Smith et al., (1982) report positive last solid fusion temperatures ranging from 0.3 °C to 2.9 °C, suggesting clathrate fusion and occurrence of CO2. They also describe hetero- geneous trapping and vapor and liquid phase homogenization pro- cesses, suggesting occurrence of boiling. Clark and Titley (1988), pre- sent a reverse correlation between Ag/Au ratio and FI salinity. FI data and field relationships indicate an approximate 400 to 1000 m depth range below the surface for the bonanza level at the time of vein for- mation (Smith et al., 1982; Clark and Titley, 1988). The δ18Oqtz values of Au/Ag mineralizing quartz event range from 3.9 to 9.5‰. Re- calculated δ18OH2O range from −2.9 to 3.7‰, indicating that meteoric water dominated the hydrothermal system (Smith et al., 1982; Conrad and Chamberlain, 1992). Gas spectrometry indicates that water con- stituted over 99.5 mol percent of the liquid and gas phases, with CO2 comprising most of the remaining gases, minor CO and traces of H2, CH4, N2, C2H6, H2S, C3H8, SO2 and NO (Smith et al., 1982). 4. Results Analytical techniques and detailed data are presented in supple- mentary files. 4.1. Decriptometry Fig. 3 presents the results of the decrepitation experiment of twenty quartz populations from nine Au- and Ag-dominant veins representative of the San Dimas district as well as its southern extension. All samples display a more or less defined bimodal distribution pattern. The two maximum peaks range from 200 to 220 °C and 400 to 440 °C (Fig. 3A). In a few samples a third high temperature peak roughly developed above 550 °C (Fig. 3A). Burlinson et al. (1983, Burlinson et al., 1988) suggest that peaks like these are not related to FI decrepitation but to quartz crystallization phase transition, therefore should not be con- sidered. Quartz cements from the “Bonanza” level are characterized by higher FI decrepitation count per 10 s than samples collected at the top or at the root of the veins. Plotting the 400 °C/200 °C peak intensity ratios versus Au/Ag ratio allows distinguishing both Ag- and Au-dominant hydrothermal events (Fig. 3B). The plot also shows a positive relationship between the 200 °C peak intensity with Ag concentration (Fig. 3B). 4.2. Vein and fluid inclusions petrography and microthermometry The Ag- and Au-dominant veins paragenetic sequence is detailed in Montoya et al. (2019b) and presented in Fig. 4. Ag-dominant veins are characterized by three stages of formation named open, filling and close stage. The opening stage shows open space filling quartz texture. The filling stage corresponds to the mineralization and is characterized by mosaic quartz texture. The closing stage present crack-seal and open space filling quartz texture (Fig. 4). Based on the petrographic and compositional features of the FI at room temperature, two types were identified: type I and type II. Type I is the dominant type recognized in both Ag- and Au-dominant events and corresponds to two phases liquid–vapor (LVaq), liquid dominant inclusions, with estimated vapor volume of 5 to 10%. Type I primary FI (LVaq) are either found as isolated inclusions, in small clusters, or in quartz growing plans. This population of FI yields very homogeneous volume ratios (Fig. 4 A,B, C). Typically, the FI are negative crystals to ovoid in shape and are less than 20 μm in length. Type II is recognized principally in the filling stage of the Ag-dominant event and corre- sponds to two-phases vapor dominant (VLaq) inclusions, with estimated vapor volume up to 90% (Fig. 4 D, E, E). Type II FI (VLaq) are crystal negative to ovoid in shape and are less than 30 μm in length. The visual estimate of the liquid-to-vapor ratio is highly variable. These FI are only found in small clusters. The opening stage presents the highest homogenization tempera- ture. The type I fluid inclusion homogenize to liquid between 128 °C and 320 °C, with a poorly defined modal distribution (median at ca. 240 °C; Fig. 5 A, B; table 1). Final ice melting temperatures occurs between −0.9 °C and 0.3 °C, corresponding to 0 and 1.57 wt% NaCl eq (Fig. 5 C). The filling stage presents two biphasic fluid inclusions types, liquid dominant (LVaq) and vapor dominant (VLaq). Final homogenization temperatures range from 122 °C to 304 °C with a well-defined unimodal distribution and a median value at ca. 140 °C (Fig. 5A, B). Final solid melting temperatures present a wider distribution, including ice and clathrate melting at temperatures varying from −0.5 °C to 5.6 °C, corresponding to 0 and 0.88 wt% NaCl eq (Fig. 5C). In the closing stage, type II (LVaq) fluid inclusions homogenize to liquid from 120 °C to 318 °C with a bimodal statistical distribution, with maximum values at 140 °C and 230 °C. Final ice melting temperatures are almost constant, varying between −1.9 °C and 0.0 °C corresponding to 0 and 3.22 wt% NaCl eq (Fig. 5C). In the Au-dominant veins, the biphasic LVaq FI in the quartz stage yield final ice melting temperatures ranging from −1.4 °C to −0.0 °C, corresponding 0 and 2.4 wt% NaCl eq. Total homogenization occurs in the liquid phases at temperatures ranging from 121 °C to 316 °C in a bimodal distribution (Fig. 5B, D and Table 1). On a diagram of last melting temperature (Tm) versus homo- genization temperature (Th) (Fig. 5C, D), the Ag- and Au-dominant FI from San Dimas district veins swarm overlap historic data trend com- pilations (Smith, 1982; Conrad et al., 1992; Enriquez and Rivera, 2001). Both Ag- and Au-dominant events record the same evolution from high to low homogenization temperatures with low salinity values, sug- gesting an adiabatic cooling process in a dominant meteoric environ- ment (Fig. 5C, D). The Ag-dominant filling stage presents low homo- genization and positive fusion temperatures indicating clathrate fusion and the presence of dissolved CO2. Micro-infrared spectrometry analysis was performed to show the CO2 concentration in the different events/stages of Au/Ag mineraliza- tion. In all Ag- and Au-dominant events analyzed samples, CO2 con- centration in FI vapor phase remain within the analytical noise, in- cluding in LVaq vapor dominant inclusions. However, CO2 concentration was detected in water liquid phase in analyzed fluid in- clusions from both mineralization events. CO2 concentration increases notably in the Ag-dominant filling stage event. 4.3. Fluid inclusions δ18O and δD isotope analysis δ18O and δD isotopic data are presented in Table 2 and Fig. 6 in ‰ deviation relative to the SMOW standard. Data are grouped according to mineralized stages from the older to the more recent. From Victoria P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 4 and Robertas Eocene Ag-dominant veins, we analyze six FI populations from opening (n = 1), filling (n = 4) and closing (n = 1) quartz cement stages. The analyzed δD values display a narrow range from −87.98‰ to −63.02‰ whereas the measured δ18O values display a relatively large range, from −8.79‰ to 10.34‰ (Table 2). δ18O-δD fluid inclu- sions isotopic signature of Robertas quartz filling stage was detailed following its thermal decrepitation pattern (Fig. 6). Fluid inclusions population in the range 110 °C − 310 °C shows δ18O-δD isotopic sig- nature of 10.34‰ and −87.98‰, whereas that in the 310 °C to 550 °C range has δ18O-δD signature of −0.54‰ and −88‰. Once plotted in the δ18O-δD diagram the results are distributed along a mixing line from the global meteoric water line (GMWL) to the primary magmatic water field. The opening and closing stages results plot closer to the GMWL than the filling (mineralizing) stage, which plot toward or within the Mexican active geothermal fields and epithermal deposits fields. Four samples from four different Au-dominant veins were analyzed (Arana vein, Causita vein, San Luis vein and San Antonio vein; Fig. 6; table 2). The δ18O-δD values range from −5.13‰ to 1.61‰ and from −72.20‰ to −45.81‰. In the δ18O-δD diagram the results obtained Fig. 3. (A) FI decrepitation diagram of Ag-dominant and Au-dominant quartz veins from Ag/Au San Dimas district. (B) 400/200 °C peak intensity ratio versus Au/Ag metals ratio sample diagram of Ag-dominant and Au-dominant quartz veins from the Ag/Au San Dimas district. P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 5 4000 ~ .---------, LVaq LVaq u .e c.. 3500 o ~ 3000 ..... e ....... ..... § 2500 o u e o -~ 2000 ..... 'i5.. ·;:: u ~ 1500 e o 'vi :::::, ~ 1000 ~ :::::, LL 500 o 100 o.os [ID o 0.04 ·,¡::; ro ,_ e 0.03 ·a:; > O) 0.02 ~ :::::, 0.01 <( 0.00 o + C02 ~ O) o ro e t;; o ~:-E U VI N C ,._. ro Ot; 200 300 400 500 600 700 Temperature (°C) • • • ••••••••• • Au-Dominant vein ~ . ~ -:::;~J .. •·· oº'\ , ..... • oA 1'."! • . :.•·· ... , ~ o,~ ..... 1-···· . . ..... . •····· • •• • ~ Ag-Dominant vein • • 1 2 3 4 5 6 7 8 9 400/200°( peak intensity ratio from Arana, Causita, San Luis and San Antonio Au-dominant veins overlap the Ag-dominant mineralized filling stage isotopic trend data and plot between the LS and IS Mexican epithermal deposit and active geothermal fields. 4.4. Chemistry and noble gas isotope composition of fluid inclusions 4.4.1. Chemistry For the Eocene Ag-dominant veins we analyzed quartz samples from Fig. 4. Photomicrographs image of fluid inclusions paragenesis. (A) Filling quartz stage (mineralization stage) from Roberta vein with primary FI trails along growing quartz plans; (B) Type 1 and type 2 asso- ciation; Type 2-Primary biphasic Liquid- dominant FI; Type 2-Primary biphasic gas- dominant FI; (C) Type 1-Primary biphasic Liquid-dominant FI associate with silver droplets; (D); Sulphide mineralizing quartz from San Antonio vein with primary bi- phasic Liquid-dominant FI clouds; (E) Type 1-Primary biphasic Liquid-dominant FI; (F) Type 1-Primary biphasic Liquid-dominant FI. Fig. 5. (A) Homogenization temperature histograms for FI from Ag-dominant opening, filling and close stages. (B) Homogenization temperature histograms for FI from Ag-dominant and Au-dominant quartz events. (C) Homogenization temperature and ice melting temperature plot for Ag-dominant quartz gangue. (D) Homogenization temperature and ice melting temperature plot for Au-dominant quartz gangue. Dark grey area: FI from Smith et al., (1982); grey area: FI from Clark and Titley (1988); light grey area: FI from Enriquez and Rivera (2001a). P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 6 Eocene Ag - dominant vein Oligocene Au - dominant vein 40 Ag-dominant (low [Ag]) ◊ Ag-dominant (high [Ag]) ~ Opening stage 10~~ ~~~~~ -1_ . , 1. ilillil 1. ., Au-dominant ■ 20 Filling and mineralizing stage o ....__L...L_.._,._..__.__._._L...L~~ ........ _._..__~--- 35-1 1 Close stage o . J1111 11 0+-,,....,o.--""1 5 150 200 250 300 350 100 150 200 250 300 350 Homogenization Temperature (ºC) 6 350 Homogenization Temperature (ºC) 6 G 5 o '-" ~ 4 ~ 3 180sMOW %o 0ligocene Au-dominant vein e Arana vein, Fillingstage e Causita vein, Fillingstage e San Antonio vein, Fillingstage e San Luis vein, Fillingstage SM0-0ligocene Au-dominant vein □ BACIS ( Albinson, 2001) • Palmajero (Albinson, 2001) ■ Panuco (Albinson, 2001) Eocene Ag-dominantvein Victoria vein(SinaloaBlock) T Open space stage e Filling stage • Clase stage Robertas(CenterBlock) * Fillingstage general overlap between the two groups of samples. 4.4.2. Noble gas isotope composition The Eocene and Oligocene quartz 4He/20Ne ratios vary from 1.2 to 20.7 and from 0.4 to 11.7, respectively, indicating a strong contribution from an atmospheric-derived component (Ozima and Podosek, 2002). 40Ar/36Ar ratios vary from 295.13 to 304.04 (average 298.99) and from 294.04 to 319.71 (average 300.66), respectively, supporting the in- dications from 4He/20Ne (Ozima and Podosek, 2002). 20Ne/22Ne and 21Ne/22Ne ratios from Eocene and Oligocene quartz give similar in- dications: 9.79 to 9.81 and 0.0285 to 0.0291, respectively. Fig. 8A is a plot of 21Ne/22Ne versus 20Ne/22Ne showing trajectories for radiogenic and nucleogenic Ne production extending outward from the air value. Within the error margin all samples plot near the air value. Some Oli- gocene Au-dominant results plot slightly below it, suggesting a small influence of degassing processes consistent with a hydrothermal com- ponent in the gas (Ballentine, 1997, and references therein). Fig. 8B plots Argon (mol/g) versus neon (mol/g) showing air and air saturated water line distribution (Ozima and Podosek, 2002). Within the error margin all samples plot near the air-line. The 3He/4He ratios not corrected for atmospheric contamination (R/Ra) from Eocene Ag-dominant (0.08–0.24 Ra, mean 0.19 Ra) are on average lower than for the Oligocene Au-dominant (0.07–1.19 Ra, mean 0.57 Ra), with a partial overlapping of values from both deposits (Fig. 8C). He corrected for air contamination (4Hecorr) versus Rc/Ra plot (Fig. 8C) shows that overall the two events are in partial overlapping. Au-dominant Oligocene samples are well distributed along a crust- magmatic mixing line, pointing out the predominance of crustal fluids and variable contributions of a “hypothetical local magmatic” reservoir at about 3 Ra. Ag-dominant Eocene samples present a lateral shift, suggesting a dominant presence of crustal fluids. In the 4He/40Ar versus R/Ra plot (Fig. 8D) we get similar information, confirming our previous inference of air-crust mixing for most of the samples, except Causita, San Antonio, and Arana samples from Oligocene Au-dominant veins suggesting Hemantle contribution in a 3-component mixing. Considering 3He/4He ratio values of 0.01–0.05 Ra for the crust and 8 Ra for a MORB-like mantle (R/Ra = 8 ± 1; Graham, 2002), the Hemantle contribution is estimated to be up to 1.7% and 24% in the Ag- and Au- dominant veins samples, respectively. Accordingly, the 3He/4He ratios corrected for atmospheric con- tamination (Rc/Ra) from Eocene Ag-dominant samples (0.07–0.13 Ra, mean 0.10 Ra) is lower than for the Oligocene Au-dominant samples (0.05–1.91 Ra, mean 0.81 Ra), with a partial overlapping of values from both deposits. 5. Discussion 5.1. Sources and evolution of Eocene Ag- and Oligocene Au-dominant ore- forming fluids The origin of the ore-forming fluids cannot effectively constrain the source of the ore-forming metals. However, they illustrate well the dynamic of the hydrothermal system, metals transport and precipita- tion conditions. Petrographic examination of FI assemblages provides evidence concerning the chemical and physical conditions of formation of hydrothermal deposits (Bodnar et al., 1985). Two FI assemblages and compositional features were identified in both the Eocene and Oligo- cene hydrothermal structures: 1) Type I, two phase liquid–vapor (LVaq), liquid dominant, with estimated vapor volume percent of 5 to 10, is the main dominant type recognized in both events; 2) Type II, two-phase vapor dominant (VLaq), with estimated vapor volume per- cent up to 90, is recognized principally in the filling stage of the Ag- dominant event in the highest Ag grade mineralized area. The presence of vapor-dominated inclusions, the heterogeneous liquid/vapor ratios and the hockey stick shape distributions in the Th vs Tm diagram suggest that boiling was an important process during silver precipita- tion. The chemistry of FI confirms that H2O is the major component. In addition, the lack of clear differences in salinity between the Eocene and Oligocene hydrothermal events confirms that Type I FI dominates within the two events and that Type II FI poorly or not contributed to the extracted gas mixture (Fig. 7). Fluid inclusions microthermometry and FTIR spectroscopy indicate that ore-forming fluid belongs to the H2O-NaCl-(CO2) system and has high to low temperature (ca. 320–120 °C) and low salinity. Eocene and Oligocene hydrothermal events homogenization temperature and sali- nity data are comparable to those described in the literature (Smith, 1982; Conrad et al., 1992; Enriquez and Rivera, 2001). The overlapping of their homogenization temperature ranges prevents using the micro- thermometry technique to discriminate both events. However, the Eo- cene Ag-dominant hydrothermal event is characterized by positive ice melting temperature. The low salinity of the solutions, the small amounts of dissolved CO2 content, and the absence of trapped minerals Fig. 7. Major and noble gases Eocene and Oligocene fluid inclusions composition. P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 8 Ag-Dominant Au-Dominant 100 % H - - -- ~ C02 99 • - 98 H2ü '-..! 97 96 95 % SD48(2) SD42 mean SD73 SD72 MN 14 mean SD48(1) SD48(3) SD67 CS2 (halite, sylvite) suggest that the main component is meteoric water. These evidences are corroborated by the chemistry of FI, which clearly indicates H2O as the major component and significantly low to absent CO2 content (Fig. 7). In the Eocene and Oligocene hydrothermal events, the FI microthermometric results reveal a continuous adiabatic cooling of the fluid and an increase of CO2 content during the Ag-dominant filling stage. δ18O-δD are considered to provide precise information about the sources and evolution of ore-forming fluids (Taylor, 1974). Plotted in the δ18O-δD diagram, both Eocene and Oligocene mineralizing FI are distributed along a mixing line from the global meteoric water line (GMWL) to the primary magmatic water field (Fig. 6). During the Eo- cene Ag-dominant hydrothermal system, the opening and closing stages data are close to the GMWL, showing the dominance of meteoric fluids, whereas the filling (mineralizing) stage results plot in or near the Mexican active geothermal fields. The δ18O shift to less negative values of these samples suggests a magmatic input in the mixing fluids isotopic equation. The δ18O-δD data of the Au-dominant hydrothermal events are similar to those of numerous SMO and Mexican LS and IS epi- thermal deposits (Fig. 6). The δ18O-δD data of the Au-dominant hy- drothermal event show a range of compositions indicating that ore fluids are partially magmatic-derived. The obtained δ18O values suggest mixing and the involvement of meteoric water into the ore fluid, which could lead to the lower δ18O values. This is also consistent with the measured low homogenization temperatures and salinities of fluids in the Au-dominant event. The He-Ar-Ne abundance and isotope ratios are a powerful tool in discriminating meteoric, magmatic and crustal reservoirs of the Fig. 8. Noble gases and helium isotope geochemistry diagrams. (A) 20Ne/22Ne vs. 21Ne/22Ne plot of the Eocene and Oligocene fluid inclusions. The MORB line represents mixing between atmospheric neon and that measured in the upper mantle (Sarda et al., 1988). The crust line represents the evolution of neon isotopic composition by nucleogenic production of 20Ne, 21Ne, 22Ne* in a crustal source having constant O/F ratio of 113 (Kennedy et al., 1990). MFL is the mass fractionation line depicting the isotopic variations of Ne produced by a diffusion-controlled degassing process. (B) Plot of Argon (mol/g) versus Neon (mol/g) including the air- and ASW-line distribution. (C) Plot of R/Ra versus 4He/20Ne, with mixing trend fitting the data. (D) Plot of R/Ra versus 4He/40Ar, with air and crustal fluid mixing trend. The assumed mantle and crustal production ratios are taken from Marty (2012) and Ballentine et al. (1994, 2002), respectively. Black stars are compiled data from worldwide mesothermal granite related deposits (references in table). P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 9 11.0 [Al 10.5 10. (l.) ~ ~ 9.5 ~ N 9.0 8.5 8.0 0.025 10 ~ ~ Epithermal deposits 9.9 9.8 •• • 9.7 1 1 0.027 0.028 0.045 0.055 21Ne¡22Ne MORB-like u er mantle Hypothetic Oligocene Magmatic Therme . . ~ V1ctona .. n Luis 1 1.0-12 1.0-13 1.0-14 0.03 1. 0-1 s ,.__, .................. ..__ ............... ..__ .................. ..._ ................. ....__ ...................... 0.085 1.0-13 1.0-12 1.0-ll 1.0-l0 1.0-09 1.0-08 Argon (moVg) .. , .. .. , Causitas San Antonio •• Arana • Mantle production ratio Mala Noche ~ CRUST - Mixing AIR - CRUSTAL fluids CRUST O .o 1 .____.___._.......,_......_.....__...__.__._ ......... u..u. _ _._--'-.................. 1.10-14 1.10-13 1.10-12 1.10-11 0.0001 0.001 0.01 0.1 He corr moVg 4He / 40 Ar Oligocene Au-dominant vein • Filling stage * Select epithermal deposit Eocene Ag-dominant vein Open stage Filling stage Close stage different fluids (Ballentine, 1997; Ozima and Podosek, 2002). Ar and Ne isotopic data as well as 4He/20Ne and 4He/40Ar ratios indicate that Ar and Ne in both Eocene and Oligocene FI samples are mainly atmo- spheric (table 3; Fig. 8C and 8D), with very minor nucleogenic pro- duction from halogens (F) and crustal contamination (K). The con- servative behavior of helium and the difference in isotopic composition between the crust (3He/4He = 0.01–0.05 Ra; Ballentine et al., 2002), MORB-like mantle (3He/ 4He = 8 ± 1 Ra; Sarda et al., 1988; Ballentine et al., 2002) and atmosphere (3He/4He = 1 Ra; Ballentine et al., 2002) make helium the most sensitive proxy for the determina- tion of the sources of the fluids trapped during mineralization and allow to illustrate possible interactions between these fluids and the country rocks (e.g., Mamyrin and Tolstikyn, 2013). Since Simmons et al. (1986), it is accepted that hydrothermal minerals can preserve mantle-derived 3He. However, there are evidences that in FI-free quartz crystals He may diffuse from the crystal lattice, especially at progressively higher temperatures (e.g., next to or above 400 °C; Shuster and Farley, 2005). On the other hand, the diffusivity through quartz lattice of 4He seems indistinguishable from that of 3He, providing no graphical evidence for the commonly expected inverse square root of the mass diffusion re- lationship between isotopes. This implies that even in case of a diffusive loss of helium, the 3He/4He should maintain the original signature. Another indication in support of the conservative behavior of He within FI of quartz crystals of hydrothermal origin comes from the evidence that FI may enhance noble gas retentivity in quartz crystals (Shuster and Farley, 2005). In Fig. 8, our data do not show any graphical evi- dence of He loss as well as 3He/4He fractionation (even considering 3He vs R/Ra; Table 3), suggesting that they can be used to constrain the origin of fluids trapped in Ag- and Au-dominant deposits. We notice that the lowest 3He/4He are recorded in the most He-rich samples, al- though there is not a unique trend of decrease of 3He/4He at increasing 4Hecorr concentration. This behavior strongly indicates mixing between crustal and magmatic fluids, rather than ingrowths of 4He from the radiogenic decay of U and Th. The 3He/4He not corrected for air contamination from San Dimas Eocene Ag-dominant and Oligocene Au-dominant FI present values spanning a wide range (0.07–1.19 Ra), which is in partial overlapping among a few samples from the two distinct deposits. When 3He/4He is plotted versus 4He/20Ne and 4He/40Ar (Fig. 8D, respectively), we de- duce that air-saturated groundwater is the main isotopic reservoir. However, there are clear evidences that fluids from both hydrothermal events mix with fluids from other sources (i.e., crustal and magmatic fluids). In particular, FI trapped in the Eocene Ag-dominant hydro- thermal deposits originate from a mixing mostly involving atmospheric and crustal fluids, with a slight contribution of mantle or magmatic 3He (Fig. 8C and 8D). This could imply that during the Eocene Ag-dominant hydrothermal event faults pathways only marginally reached the basement or magmatic reservoir. Instead, FI trapped in part of the Oligocene Au-dominant hydrothermal deposits track a 3-components mixing that involve atmospheric, crustal, and mantle fluids (Fig. 8C and 8D). In fact, during the Oligocene Au-dominant hydrothermal event continental extension had already produced a thinner crust and in- volved a deeper faults system (Ferrari et al., 2017; Montoya et al., 2019a, 2019b), creating effective pathways for the mantle derived gases (up to 24%, Table 3) to escape and be subsequently trapped in the mineralized veins. This evidence is also supported by the δ18O-δD va- lues of the Au-dominant hydrothermal event, which indicate that ore fluids are partially magmatic-derived. In order to figure out what could be the source of 3He excess in Causita, San Antonio, and Arana samples from Oligocene Au-dominant veins, we plot 4Hecorr concentration versus Rc/Ra (Fig. 8C), excluding in this way any effect due to atmospheric contamination. If we consider a possible crustal term having 3He/4He = 0.05 Ra and 4Hecorr ~2·10−12 mol g−1, we could fit Causita, San Antonio, and Arana samples assuming a binary mixing with a possible local magmatic term having 3He/4He ~ 3 Ra and 4Hecorr ~ 2·10−14 mol g−1. This term could correspond to a differentiated magmatic source that resided and de- gassed within the crust at the time of formation of quartz and that under cooling and ageing conditions lowered its original MORB-like 3He/4He values (Ballentine et al., 2002; Batista Cruz et al., 2019). This hypothesis has not strong constraints, so we cannot exclude that a magmatic body with higher 3He/4He and lower 4He concentration could have supplied fluids trapped in quartz samples. We are only proposing an alternative explanation to the direct rise of 3He-rich fluids from the local mantle. Compared to other studies, the range of 3He/4He values measured in San Dimas Eocene and Oligocene hydrothermal events is significantly lower than those of epithermal deposits and magmatic arc environ- ments (3He/4He : 0.1–10; Manning and Hosftra, 2017). In the specific case of the Oligocene Au dominant LS-epithermal event, the most comparable with similar deposits, the low 3He/4He ratios values could be explained by several reasons: (1) analyzed samples do not represent the highest Au grade areas, (2) quartz could present post-entrapment He ingrowth or exchange with external fluids rich in radiogenic 4He (Kendrick and Burnard, 2011), and (3) despite our attempt to remove secondary inclusions a variable proportion of secondary inclusions probably contributed to enrich FI in the crustal component, saturating the isotopic ratio end member. In hydrothermal systems, gold and silver transport and deposition depend on fluid composition, temperature, pressure, pH, oxygen and sulphur fugacity, and type and amount of dissolved sulphur and other species (e.g. Mikucki, 1998). The low salinity, low to intermediate sulphidation and near-neutral pH environments and occurrences of several sulphides and sulphosalts, gold–silver from both hydrothermal mineralizing fluids (Tmi = −1.9 to 5.6 °C) pleads in favor of silver and gold transport as AgHS- and Au(HS)2 complex (Zotov et al.,1995; Barnes, 1997; Cooke and Simmons, 2000; Stefansson and Seward, 2003, 2004; Pokrovski et al., 2014). The stability of Au(HS)2 complexes is greatly increased by the presence of low concentrations of chlorine (Zajacz et al., 2010). Near neutral to slightly alkaline solutions origi- nating in the field of maximum gold solubility as a bisulphide complex produced sericite as the most common potassium aluminosilicate in the alteration assemblages (Romberger, 1986). In the San Dimas district, the Au-dominant vein and telescoped structures are characterized by a well-defined sericite alteration halo, while Eocene Ag-dominant hy- drothermal structures are characterized by quartz-adularia-rhodochro- site gangue and chlorite alteration halo (Montoya et al., 2019b). So- lubility of AgHS- and Au(HS)2 is extremely sensitive to changes in temperature and pressure. Depressurization (which occurs during hy- draulic fracturing, boiling or as hot fluids ascend up faults) and/or rapid cooling (50 °C or more) decrease Au-Ag solubility by 90% (Zhu et al., 2011). The adiabatic decrease in homogenization temperature recorded in both events and the boiling process observed in Eocene and Oligocene events provided favorable conditions for the deposition of precious metals deposition. In absence of external geothermometer, pressure and temperature (P-T) trapping conditions could not be estimated for both mineralizing hydrothermal events. However, the chronology of the magmatism and hydrothermal events give us some key information to estimate miner- alization depth formation within a reliable geological evolution sce- nario. The space–time association of Oligocene rhyolitic magmatism with the Au mineralized veins, as well as the chemical and isotopic evidences for a meteoric fluids source (Montoya et al., 2019b), clearly indicate a connection to an air saturated groundwater under hydrostatic P-T conditions. The sericite alteration halos of Au-dominant veins are observed in the stratigraphic column up to the Las Palmas sedimentary formation and are sealed by the Oligocene ignimbrite flows, suggesting that the mineralization structure almost reach the paleosurface, and probably developed within 1 km depth. The synchronism of Eocene porphyric dikes and Ag mineralized veins, and the chemical evidences for a meteoric fluids source indicate a connection to an air saturated groundwater and degassing process P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 10 under hydrostatic P-T conditions, apparently in a shallow position. However, several authors report that fluids exist at deeper depth in magmatic arcs context (down to 10 km, Menzies et al., 2014 among other). Reliable mineral (chlorite) or isotopic equilibrium geotherm- ometers are needed to correct the minimum fluid inclusion homo- genization determination (up to 20 bars). In our case, the alteration induced by the later Oligocene hydrothermal event does not allow to obtain reliable data, but the local geological evolution is well con- strained and give us some reliable information. The Piaxla batholith mineral texture and its stratigraphic position, locally overlain by the Camichin sedimentary formation or by the Oligocene ignimbrite flows (~30 Ma, Montoya et al., 2019a), highlight an important extensional tectonic context since the Eocene. Continental conglomerates of Eocene to Oligocene age are widespread in all Central Mexico (Montoya et al., 2019a; Nieto-Samaniego, et al., 2019). At regional scale, the exten- sional tectonic and exhumation-erosion rate has been estimated at ca. ~0.3 mm/yr (Nieto-Samaniego, et al., 2019), in well agreement with the value for magmatic arc environments (Burkan et al., 2002). Taking into account these observations the difference in the maximum depth formation needed to overlap the Eocene and the Oligocene “bonanza” levels could be estimated at 2–3 km. Such conditions are in agreement to those reported in literature for comparable plutonic related vein deposits (Prokofiev and Pek, 2015; Nieva et al., 2019). At this depth, the silver mineralization trapping temperature, corrected for the hy- drostatic pressure, could be estimated at ~350 °C (Steele-MacInnis et al., 2012). 5.2. A new genetic model for the San Dimas district The San Dimas Ag–Au district is an example of a Eocene-Oligocene telescoped deposit that comprises two mineralizing events: (a) an Eocene Ag-dominant epithermal event (40–41 Ma; Montoya et al., 2019b) associated with the late stage intrusion of a batholith dated at 45–43 Ma (Henry et al., 2003; Montoya et al., 2019a); (b) an Oligocene Au-dominant low sulfidation epithermal event (ca. 30 Ma), associated with rhyolitic volcanism dated at 31–29 Ma (Montoya et al., 2019b). Zircon chemistry also illustrates the most likely magmatic sources for the hydrothermal pulses and thus brackets the age of the mineralization events (Montoya et al., 2019b). The integration of microthermometry and noble gases, oxygen and deuterium isotope data, in conjunction with a detailed geological re- evaluation and tectonic and petrographic studies, permit a coherent genetic model to be drafted. This model is summarized in Fig. 9. Eocene silver bearing veins are hosted in the Cretaceous to Paleocene Laramide arc volcanic sequence (Lower Volcanic Complex). Ag-dominant mineralization structures are mainly hosted in E-W sig- moidal gashes and geographically distributed in proximity with the Eocene Piaxtla batholith (Montoya et al., 2019a). The Ag-dominant veins are composed of different quartz and breccia events and are characterized by adularia and rhodochrosite gangue. The proximity of the epithermal alteration 40Ar/39Ar cooling ages (41–40 Ma) and the U/ Pb zircon ages of the Piaxla batholith last magmatic pulses (45–43 Ma) support a genetic relationship (Montoya et al., 2019a, b). The con- tinuous enrichment of the Eu/Eu* and (Ce/Nd)/Y) ratios through time, from 50 to 43 Ma in zircon crystals, strongly suggest that the most enriched magmatism occurred during the last and shallowest stage of the batholith formation (Montoya et al., 2019b). It is reasonable to suppose that the batholith building process not only provided the heat, part of the fluids and the metals, but also generated shallow extension and the formation of mineralized tension gashes during its ascent. The Piaxla batholith shallow position in the stratigraphic column suggest the enrichment of the magma in precious metals and metalloids by assimilation of the Cretaceous porphyry bodies (Montoya et al., 2019a), as recognized in other granite-related deposits in Northwest Europe (Nieva et al., 2019; Vallance et al., 2003). Furthermore, during the batholith emplacement the thermal anomaly remained elevated for a long period of time. In such geological environment, the mineralized fluids could come from three reservoirs: magmatic, metamorphic and/ or meteoric. Microthermometry results, O-D, H2O and CO2 concentra- tion, as well as noble gases trapped in FI, all point to the dominance of meteoric fluid, or air saturated groundwater in the hydrothermal system. He isotopes indicate that magmatic/mantle fluids are very low to negligible (< 2%), with a dominant contribution by a crustal re- servoir, in good agreement with the geological context. The low salinity of the fluids (Tmi = −1.9 to 5.6 °C; wt% NaCl eq = 0–3.22 wt%) pleads in favor of silver transport as a bisulphur complex (AgHS-; Zotov et al., 1995; Barnes, 1997). Hydrosulphide complex is dominant in neutral to alkaline pH and moderate to low salinity fluid (Akinfiev and Zotov, 2001; Gammons and Barnes, 1989; Seward and Barnes, 1997). In this situation, degassing may be more effective for silver deposition rather than conductive cooling or mixing. According to Stefansson and Seward, (2003), the hydrosulphide complex of silver can be stable in a wide range of temperature so that mixing or cooling may not drama- tically reduce its solubility. However, the gas separation during de- gassing leads to the loss of volatile H2S, which can effectively break the hydrosulphide complex and cause the deposition of silver (Seward, 1989; Stefansson and Seward, 2003). During the filling stage, hydraulic breccia and the evidence of heterogeneous trapping of FI illustrate the boiling/degassing process. Silver precipitation was probably linked to cooling temperature and pressure drop/boiling during the batholith ascent (Vallance et al., 2003). The Eocene Ag-dominant mineralization event does not fit into a classic metallogenetic model. Based on the mineral paragenesis and following the literature of Mexican deposits it would be classified as intermediate sulphidation silver epithermal deposit (Camprubí and Albinson, 1996). In fact, these deposits are commonly hosted in calc- alkaline andesitic-dacitic arcs and found at the margins of high-sul- phidation and/or porphyry deposits (John, 1999; 2001). The historical classification of San Dimas district as an IS epithermal deposit was based on mineralogical paragenesis and an incomplete knowledge of the chronological and geological context (Enriquez et al., 2018; Montoya et al., 2019b). If we accept the intermediate sulphidation character of the San Dimas Ag mineralization, the epithermal concept does not seem the better representation for such a deep (2–3 km), high temperature (up to ~350 °C) and plutonic-related deposit. However, the Eocene Ag-dominant mineralization event has also several char- acteristics that do not fit into the traditional “intrusion-related” type (IRGS; Lang and Baker, 2001; Hart, 2005), such as the location above a large and elongated batholith, the absence of regional mineralogical zonation with As–Au at the center of the batholith and Ag–Pb–Zn as- sociations at the periphery, the very low gold content, a low sulphur fugacity and the dominant meteoric fluids. The Oligocene Au-dominant mineralization structures are mainly hosted in N-S faults systems that have undergone multiple reactivations and are geographically and chronologically associated with Oligocene rhyolitic domes (at ca. 30 Ma, Montoya et al., 2019b). The Au-dominant veins are characterized by a single hydrothermal pulse, an intense sericitization and low-temperature formation (FI homogenization tem- peratures of 121 °C to 316 °C). The Au, Hg, Sb, and Bi metals association is commonly related to shallow epithermal systems (Gray et al.,1991; Bornhorst et al., 1995). The source of the fluids in this model, like classic LS epithermal models (Henley, 1986; Sillitoe, 1993; Hedenquist et al., 2000), is meteoric. The low salinity of the fluids (Tmi = −1.4 to 0.0 °C; wt% NaCl eq = 0–2.40 w%) argues in favor of gold transport as a bisulfide complex (AuHS-2; Zotov et al., 1995; Barnes, 1997). The paragenetic mineralogy, the low salinity of the fluids, as well as their gas content (H2O > 99.5 mol percent of the liquid and gas phases, with CO2 comprising most of the remaining gases, minor CO and traces of H2, CH4, N2, C2H6, H2S, C3H8, SO2 and NO; Smith et al., 1982), is characteristic of the low sulphidation epithermal type (Henley, 1986). Concerning the sources of the FI trapped in quartz, H2O reveals the dominant species as in Ag-dominant deposits. Ar-Ne-He isotopic P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 11 systematics illustrates a hydrothermal system dominated by air satu- rated groundwater. Helium isotopes indicate that a not negligible contribution to FI has a magmatic/mantle origin (up to 24%), although some samples show a classical crustal signature. The Oligocene Au- dominant mineralization event matches all the geological and geo- chemical characteristics of the gold low sulphidation epithermal de- posit model (Henley, 1986; Campbell and Larson, 1998). As a consequence, it seems likely that the development of the San Dimas Oligocene Au-dominant mineralization event and probably many other epithermal precious metal deposits are related to the activation of extensional or transtensional fault systems within a major crustal dis- continuity rooted in the lower crust or even in the mantle. The ascent of bimodal magma batches is accompanied by fluids rich in gas and sulphur, promoting the gold transport. The reactivation of the NNW- SSE structural corridor at ca. 30 Ma was an important event in the evolution of the SMO during a major pulse of crustal thinning asso- ciated to the ascent of asthenospheric melts (Ferrari et al., 2017). This process was accompanied by the melting of the mantle lithosphere and the lower crust and could explain the temporal and spatial association between the ascent of deep mineralizing fluids and silicic magmatism, and upper crustal extensional structures. Finally, our study highlights the necessity to integrate geological observations and independent chronological and geochemical techni- ques to better understand the complexity of the hydrothermal mag- matic processes involved in the formation of many Mexican ore de- posits. Fig. 9. Genetic model for the evolution for the Eocene-Oligocene telescoped San Dimas Ag–Au deposit. Boxes summarize the isotopic characteristics of the different reservoirs. Helium isotopic results are given in 3He/4He (Rac) ratios, and oxygen and deuterium in ‰ deviation relative to the SMOW standard. The synthesized geological cross-section is drawn from the East-West transect (modified from Montoya et al., 2019b). For legend, see Fig. 2. P. Montoya-Lopera, et al. Ore Geology Reviews 120 (2020) 103427 12 L-) ____ 50_M_a ______ 4_0_M_a _______ 3_0_M_a __ ~> 0km 2km 4km Laramide Magmatic Are E-W Fault and Fluid inclusions Agveins 122ºC to 320ºC -0.5°C to 5.6ºC -0.24%0 to 1 0.34%0 -87.98%0 to -63,02%0 295.13 to 304.04 0.08 to 0.24Ra 50Ma Paleo Water Table • • • • • • • • 43 Ma f 41 Ma íl Ag to 40Ma r> 69Ma 75 Ma 1 Continental extension NWFaultand Auveins 120.9ºC to 316.s°C -1 .4ºC to -0.0ºC -5.13%0 to 1.61 %o -72.20%0 to -45.81 %o. 294.04 to 319.71 0.07 to 1.19Ra 30Ma L---:;;o~,::,.--,= ::-"::,~~=-==-=,---t water Table 30Ma 41 Ma to 40Ma Continental Extension Laramide Are Magmatism c:::::J Oligocene ignimbrites, domes and dikes c:::::J Piaxtla granite and dikes c:::::J Eocene sedimentary deposits - Si na loa batholith Las Palmas and Camichin fms. Laramide Are volcanism Sedimentary Basement ~ Andesitic group (tuff, flow, porphyry) - Cretaceous rhyolite (flow and intrusion) and ignimbrites c:::::J Jurassic limestone - Triassic sandstone Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influ- ence the work reported in this paper. Acknowledgments This research is part of the PhD project of the first author at Universidad Nacional Autónoma de Mexico (UNAM) Postgraduate Program. The research was funded by CONACYT grant CB-237745-T and PAPIIT IV100117 to L. Ferrari. We thank Primero Mining (pre- sently First Majestic Silver Corp.) for sharing unpublished information and for logistical support. Special thanks to Nicolas Landón for his strong support in the initial phase of the research and to Miguel Pérez for sharing his knowledge or the ore geology on the central SMO. We also thank Marina Vega for assistance at Laboratorio de Fluidos Corticales, personnel at Stable Isotope Laboratory at Laboratorio Nacional de Geoquímica y Mineralogía, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad de México, and Juan Tomás Vazquez for the elaboration of thin sections. We thank Maria Grazia Misseri and Mariano Tantillo from INGV-Palermo for helping in samples preparation, H2O-CO2 concentration, and noble gases isotope analysis of fluid inclusions, as well as Fausto Grassa for useful discussions on data elaboration. Authors are special grateful to Seequent for providing an academic license of Leapfrog Mining. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2020.103427. References Akinfiev, N., Zotov, A., 2001. 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Ore Geology Reviews 120 (2020) 103427 14 75 Capítulo 5: Conclusiones En la presente tesis contribuimos a mejorar el conocimiento sobre la relación tectono- magmática y el desarrollo de multiples mineralizaciones en el distrito minero de San Dimas en tres aspectos principales: 1) Re-evaluación estratigráfica: se demostró que las unidades geológicas mas antiguas del distrito son pertenecientes al CVI con una edad Cretácico tardío y por lo tanto representan el equivalente volcánico del batolito de Sinaloa y pueden correlacionarse con la Formación Tahuramara de Sonora. Las secuencias lavícas del CVI son cubiertas por lavas intrusionadas por cuerpos hipoabisales de afinidad intermedia (Grupo Andesítico, GA) de edades Paleocénicas. GA es de particular importancia en el distrito debido a que eran asociados geneticamente a la mineralización por presentar ambos edades similares, adicional por ser las hospedantes de la mayor parte de la mineralización (Enríquez y Rivera, 2001b). Basados en las nuevas edades U/Pb en circón reportadas en este estudio, mas las relaciones de corte entre Piaxtla y GA, demostramos que estos cuerpos intermedios hacen parte de pulsos magmáticos desarrollados entre el Paleoceno al Eoceno temprano, y que las edades K-Ar, Ar-Ar reportadas por Enríquez y Rivera (2001b) eran el resultado del reseteo parcial de los minerales debido a la alta alteración hidrotermal en el área. Observamos también que el Batolito Piaxtla intruye todo el CVI desde la base hasta el techo, y que fue formado en diferentes pulsos magmáticos separados textural, composicional y geocronologicamente. Nuestras nuevas edades de U/Pb delimitan el Piaxtla entre ~49 a 44 Ma confirmando así las edades reportadas por Enríquez y Rivera (2001b) y Henry et al., (2003). Adicional a lo anterior, y como una contribucción importante del estudio, son las nuevas edades reportadas para los diques intermedios “San Rita” y “Bolaños”, los cuales hacen parte de los últimos pulsos magmáticos del Piaxtla. Estas edades son relevantes para el estudio, debido a que estan asociadas geneticamente con la mineralización. Basados en la ocurrencia de la formación sedimentaria de afinidad continental Las Palmas, la Fm Camichin, y a las nuevas edades U/Pb de las mismas, se demuestra el inicio de una extensión en el Eoceno. Regionalmente, correlacionamos estas formaciones sedimentarias con los depositos de capas rojas continentales que marcan un periodo de erosión y baja actividad volcánica entre el CVI y SVS reportado por 76 otros autores (McDowell y Keizer, 1977; McDowell y Clabaugh, 1979; Ferrari et al., 2007). Las nuevas edades U/Pb de la base de las sucesiones ignimbríticas del SVS en el distrito, son consistentes con las edades de las secuencias volcánicas de Durango reportadas previamente por otros autores en un rango de ~31 a 29 Ma ( McDowell y McIntosh, 2012), confirmando así la prolongación al Oeste de esta secuencia en la SMO. En conjunto la nuevas observaciones geológicas y edades de U-Pb demuestran que existen tres pulsos magmáticos mayores: 1) el magmatismo asociado al arco Laramide, 2) el magmatismo asociado al batolito Piaxtla y 3) el volcanismo sílicico y bimodal de la gran provincia silícica de la SMO. 2) Re-evaluación de la mineralización del distrito de San Dimas: la integración de una detallada reevaluación geológica, geocronológica, tectónica y petrográfica versus análisis multivariados, tales como geoquímica, microtermometría, gases nobles e isotopía estable, nos permitío reevaluar el origen del clásico epitermal de baja sulfuración de San Dimas y a su vez desarollar un nuevo modelo genético para el distrito (Montoya-Lopera et al., 2020). La mineralización del distrito de Ag-Au San Dimas es un ejemplo de un yacimiento telescopeado desarrollado en dos eventos mineralizantes separados en el tiempo: i) evento de vetas eocénicas (~40-41 Ma; Montoya et al., 2019b) dominantes en Ag, hospedadas en las secuencias lavícas del CVI y desarrolladas principalmente en sistemas extensionales incipientes de orientación E-W. Estos sistemas son tipo cuarzo-adularia-rodocrosita y se desarrollan en tres eventos de apertura, relleno y cierre. Basados en los resultados de REE en circones de la roca caja (relaciones Eu/Eu* y (Ce/Nd)/Y) Montoya et al., 2019b) y geocronología se asocian genéticamente las vetas de Ag dominante a los últimos pulsos hidratados y oxidados del batolito Piaxtla (~44 Ma Montoya et al., 2019a). Los resultados de microtermometría, O-D, H2O y concentración de CO2 así como tambien los resultados de gases nobles atrapados en IF, indican dominancia de fluidos meteóricos o aire saturado en el sistema hidrotermal (Montoya et al., 2020). Los isotópos de He indican bajo aporte de fluidos magmáticos o de manto (menos al 2%), con una contribucción importante cortical, lo cual estaría de acuerdo con el contexto geológico (Montoya et al., 2020). La baja salinidad de los fluidos (Tmi =- 1.9 to 5.6ºC; wt% NaCl eq = 0 to 3.22 wt%) favorece el transporte de Ag como complejo bisulfurado (AgHS-; Zotov et al., 1995; Barnes, 1997). En este contexto, la 77 degasificación sería mas efectiva para la depositación de Ag antes que el enfriamiento o mezcla (Montoya et al., 2020). Debido a la profunidad de emplazamiento de las vetas (2-3km), la alta temperatura de formación (mayor a ~350ºC) y fisicoquímica de los fluidos hidrotermales, el sistema eocénico dominante en Ag no podría catalogarse como clásico epitermal de baja sulfuración. ii) evento de vetas Oligocénicas dominantes de Au (~30Ma Montoya et al., 2019b), emplazados principalmente en sistemas estructurales de orientación NNW-SSE, se desarrollan como un solo pulso hidrotermal, tipo sericita-clorita (Montoya et al., 2019b). Estos sistemas de Au dominante se asocian geneticamente a domos riolíticos de afinidad reducida (~31-29 Ma Montoya et al., 2019b) desarrollados durante el inicio de la extensión que llevó a la apertura del Golfo de California. Las vetas de Au dominante se caracterizan por tener bajas temperaturas de homogenización (121ºC to 316ºC) (Montoya et al., 2020). La asociación mineral del Au con Hg, Sb y Bi sugiere un emplazamiento superficial (Gray et al., 1991; Bornhorst et al., 1995). La fuente de estos fluidos sigue los clásicos modelos epitermales de baja sulfuración (Henley y Hedenquist, 1986; Sillitoe, 1993; Hedenquist y Arribas, 2000). La baja salinidad de los fluidos (Tmi = -1.4 to 0.0ºC; wt% NaCl eq = 0 to 2.40 w%) favorece el transporte de Au como un complejo bisulfatado (AuHS-2; Zotov et al., 1995; Barnes, 1997). Por lo tanto, basados en las anteriores caracteristicas, seguido por el bajo contenido de gases en IF (IF bifasicas H2O > 99.5 mole percent, Montoya et al., 2020) indicarían que estas vetas se catalogarían como vetas epitermales de baja sulfuración (Henley y Hedenquist,1986). Con respecto a los sistemas isotópicos, los resultados de Ar-Ne-He indican un sistema hidrotermal dominado por aire saturado. Isótopos de He en gases de IF mostraron alta contribución de origen magmático/mantélico (encima de 24%), lo cual es coherente con el ambiente geológico (Montoya et al., 2020). Como consecuencia, las vetas de Au dominante se asocian a la activación de sistemas de fallas extensionales o transtensionales de alto ángulo las cuales permiten el ascenso de magma bimodal acompañado de fluidos ricos en sulfuros promoviendo el transporte de Au (Montoya et al., 2020). La reactivación de esos sistemas (NNW- SSE) comenzo para los 30 Ma durante la evolución de la SMO (Ferrari et al., 2017) 3) Re-evaluación geocronológica al sur de San Dimas 78 Observamos la presencia de miembro basal del CVI ~77-75 Ma (equivalente al Miembro Socavón en SD, ~75-73 Ma) en contacto discordante con la formación Palmarito (~52 Ma, correlacionada con la Fm Las Palmas), indicando una erosión casi completa del CVI o una no deposición del GA por ser ya un alto del basamento al sur de San Dimas (Montoya et al., 2019 a). Lo anterior trae consigo implicaciones importantes desde el punto de vista del yacimiento y la tectónica regional del área. Implicaciones desde el punto de vista tectónico: 1) el bloque al sur del Piaxtla, Mala Noche – Causitas, estaría por lo menos 1000 metros mas elevado; 2) a escala regional, el desarrollo de estructuras extensionales regionales EW en la SMO durante el Cretácico tardío al Oligoceno temprano debe ser estudiado con mayor detalle. Implicaciones desde el punto de vista del yacimiento: 1) las secuencias volcánicas Buelna, Portal y GA se habría exhumado en el bloque Causitas – Mala Noche para los ~40-41 Ma, edad de formación de las vetas de Ag dominante ; 2) San Dimas representa un bloque hundido donde se preserva la sobreimposición de los diferentes eventos mineralizantes desde la raíz hasta el techo, hecho que permitió la formación de un distrito excepcional de clase mundial. 79 Capítulo 6: Referencias Abdullin, F., Solari, L., Ortega-Obregon, C., & Sole, J. (2018). 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New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico. Ore Geology Reviews. V. 105, p. 273-294 - Resultados completos geocronología U-Pb en circón - Resultados completos geocronología Trazas de Fisión en apatito - Imágenes petrográficas completas macro y micro de toda la columna estratigráfica Descargar el material suplementario en la siguiente link https://www.sciencedirect.com/science/article/pii/S0169136818306930?via%3Dihub Anexo 2: Material suplementario del artículo: Montoya-Lopera, P., Levresse, G., Ferrari, L., Orozco-Esquivel, T., Hernán-Quevedo, G., Abdullin, F., Mata, L. 2019. New geological, geochronological and geochemical characterization of the San Dimas mineral system: Evidence for a telescoped Eocene-Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico. Ore Geology Reviews. V. 118, p. 1-15 - Procedimientos analíticos - Resultados completos de geoquímica en cristales de Au y Ag - Resultados completos de geocronología U-Pb en circón - Resultados completos de geocronología 40Ar-39Ar - Resultados completos geoquímica de elementos traza en circón (U-Pb) Descargar el material suplementario en la siguiente link https://www.sciencedirect.com/science/article/pii/S0169136819302975?via%3Dihub 87 Anexo 3: Material suplementario del artículo: Montoya-Lopera, P., Levresse, G., Ferrari, L., Rizzo, A.L., Urquiza, S., Mata, L. 2020. Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes. Ore Geology Reviews. V. 120, p. 1-14 - Procedimientos analíticos - Resultados completos de estudios de inclusiones fluidas - Resultados completos de geoquímica isotópica (isótopos estables) - Resultados completos de geoquímica isotópica (gases nobles) Descargar el material suplementario en la siguiente link https://www.sciencedirect.com/science/article/pii/S016913681930928X?via%3Dihub