Geology and Mineralization of the San Sebastian Vein Systems, Saladillo Mining District, Durango Mexico: Examples of Concurrent Intermediate-Sulfidation and Low-Sulfidation Epithermal Environments

摘要

The Saladillo mining district in central Mexico is located at the intersection of three regional structural zones related to major basement structures; WNW-oriented structures possibly related to the Mojave-Sonora Megashear, NE- to ENE-oriented structures originating as a steep transfer fault zone during the fold-thrust deformation, and the NNW Parral-Sierra Madre terrane boundary. Clastic Proano-type Middle to Late Cretaceous sedimentary rocks belonging to the Caracol Formation host the Saladillo epithermal vein systems, which are overlain in portions of the district by Tertiary volcanics. This district lies within the heart of the Mexican Silver Belt which includes several plus billion ounce silver producing districts such as Fresnillo and Guanajuato. The district contains a series of high-grade epithermal veins that span a vertical extent of over 600 meters. As the district shows post-mineral tilting, the paleo-vertical extent of mineralization is estimated to be over 1 km. The district is an example of concurrent shallow, gold-silver dominated, low-sulfidation epithermal mineralization deposited within the upper 350 meters of the crust and deeper more base metal-rich intermediate-sulfidation mineralization formed at depths between 600 and>1,000 meters below the paleo-water table. The large vertical extent of mineralization observed in the district may reflect a significant lowering of the paleo-water table during deposition of mineralization. Production between 2001 and 2005 from the oxide portions of the intermediate-sulfidation Francine vein and low-sulfidation Don Sergio-Andrea vein totaled approximately 472,340 tonnes grading 12.9 g/t Au and 836.3 g/t Ag containing 196,456 oz Au and 12.7M oz Ag. Successful exploration since Hecla acquired the property in 1999 has resulted in the discovery of additional high-grade gold, silver, and base metal resources on the Francine, East Francine, Middle, North, and Andrea veins in the district. As of December 31, 2014 indicated resources total 2,192,700 tonnes containing 37,744,400 silver equivalent ounces and inferred resources total 3,375,900 tonnes containing 34,894,300 silver equivalent ounces. Including historic production, the Saladillo district initially contained a minimum of 98M silver equivalent ounces.