Origin and evolution of ore-forming fluids of San Dimas deposits

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, stable and noble gases isotope analyses suggest that the San Dimas deposit consist of two different mineralization styles: 1) Ag-dominant mesothermal Eocene veins that occurred at temperatures > 300ºC 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 which were developed at 250ºC, at shallower depths (<1km), associated to the feeding fractures of rhyolitic domes developed at the end of the main ignimbrite flare up of the SMO. 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.圣迪马斯（San Dimas）矿床为世界级银金（Ag/Au）矿床，属于发育于墨西哥西部西马德雷山脉（Sierra Madre Occidental, SMO）的叠置始新世-渐新世银金矿化体。该矿床记录了白垩纪晚期至渐新世早期不同岩浆与构造事件期次内的多期矿化作用，其保存完好的岩浆热液系统为解析银金物质来源、热液流体演化过程以及矿化沉淀的控制机制提供了绝佳研究契机。矿物学、流体包裹体、稳定同位素与稀有气体同位素分析结果表明，圣迪马斯矿床存在两类截然不同的矿化类型：其一为以银为主的中温热液始新世脉体，形成于温度高于300℃、约3公里深度的环境，与皮亚克斯塔岩基（Piaxtla batholith）侵入活动的晚期阶段相关；其二为浅成低温低硫化型以金为主的渐新世脉体，形成于250℃、更浅的深度（<1公里），与西马德雷山脉主火成碎屑岩喷发鼎盛期结束阶段形成的流纹岩穹隆的导水断裂相关。本研究结果凸显了多学科研究方法（包括野外地质观测、年代学与地球化学研究）的重要性，有助于更深入理解墨西哥众多矿床形成过程中涉及的岩浆热液作用复杂性，并为矿床的合理分类提供支撑。