Geology, geochemistry, and 3D geological modelling of the Independencia - Los Bancos Ag-Au epithermal vein systems in the Palmarejo District, Chihuahua, Mexico

The Sierra Madre Occidental (SMO) is a Cretaceous-Tertiary silicic ignimbrite province in northwestern Mexico. It is the result of tectonic and magmatic events produced by the subduction of the Farallon plate under North America (80-40 Ma) and the opening of the Gulf of California (14-12 Ma), thus forming the SMO metallogenic provinces. The Late Eocene to Oligocene was a major period of metallic mineralization hosting various deposit types, principally Ag-Au (±Pb-Zn-Cu) epithermal veins, which have been economically significant. The study area lies in the Palmarejo district, operated by Coeur Mining Inc., which was the world’s fifth largest silver producer in 2013. Coeur Mining Inc. increased its resources with the acquisition of Paramount Gold and Silver Corp. as they shared property boundaries where the Independencia deposit is contiguous to the Don Ese deposit. Currently, mining activities are increasing in the Independencia deposit. It is important to understand the geochemical characteristics, sources, transport mechanisms, and depositional controls of mineralizing fluids for the application of mineral exploration. This study aims to provide new insights about the metallogenesis in this region by analyzing the: 1) geochemistry, 2) timing and origin of the ignimbrites (UVS) by providing zircon U-Pb ages and ϵHf (t) compositions by LA-ICP-MS, 3) statistical results of multi-element geochemical drill core data from the Independencia epithermal system, and 4) construct a 3D geological model of the Independencia – Los Bancos Ag-Au epithermal vein systems to evaluate the deposit and metal distributions. Recent studies in the northwest SMO have shown evidence which indicate that the Late Oligocene and Early Miocene volcanic activity, which produced the ignimbrite flare-up (UVS) is largely syn-genetic with the post-Laramidic Tertiary extension. The U-Pb geochronological data in this study record ages at ~ 28 Ma in the San Francisco area and at ~ 23 Ma in the Guerra Al Tirano and Guadalupe Norte areas. The ϵHf (t) compositions varying from -0.6 to +7.8 are indicative that the origin of magmas which generated large volumes of silicic volcanic rocks may have been evolved from mantle derived sources and mixing between sub-continental lithospheric mantle (SCLM) and evolved non-radiogenic crustal fluids, possibly stemming from the widespread hydrothermal activity associated to epithermal mineralization. The extensional periods in northwest Mexico provided the framework for the shallow emplacement of Ag-Au ± (Pb, Zn, Cu) epithermal vein deposits along NW-SE structural trends. The extensional fractures served as the migration paths for the ore fluids and eventual deposition. The mineralization age at the Independencia – Los Bancos deposit is determined to be ~ 23 Ma on the basis of the cross-cutting relationships between the mineralized veins and the district rhyolitic porphyry bodies, domes and dykes of similar origin. The statistical treatments conducted in this study show comparable results which revealed ore fluid characteristics of an intermediate-sulfidation epithermal deposit. The mineral assemblages of silver sulfosalts, electrum, sphalerite, are distinguished by the high proportional relationships between Ag-Au and Pb-Zn. The ore mineralogy of an intermediate-sulfidation epithermal deposit is partly controlled by the ore fluid composition and salinities ranging from 3.5 to 7.5 wt. % NaCl equivalent. Salinities of ore fluids observed in the Palmarejo district range from 1.3 to 7.8 wt. % NaCl equivalent. The geologic model aids to visualize the geometry of the deposit and observe the variability and distribution of precious metals which has many implications for exploration and mining