CONTROLS OF CARBONATE-REPLACEMENT AG-PB-ZN MINERALIZATION, SHAFTER DISTRICT, PRESIDIO COUNTY, TEXAS

The Presidio mine in the Shafter district produced 35 million ounces of Ag from 1882 to 1942, along with Pb and minor Au; a resource of 47 million ounces of Ag is presently being developed for production starting in mid-2012. The Shafter deposits occur in the Permian Mina Grande carbonates on the eastern end of a mineralized district centered on the Laramide Red Hills intrusive complex that contains porphyry Cu-Mo mineralization. The Shafter ore zone is elongated N60E and lies along the projected western extension of the long-lived regional Chalk Draw fault. The north-trending Mina Grande fault zone and associated en echelon faults appear to control the distribution of the mineralized zones within the Presidio Mine.

The Mina Grande carbonates have a complex diagenetic history that prepared the strata for hydrothermal mineralization. Post-Permian uplift resulted in karstification of the Mina Grande with associated dolomite dissolution and dedolomitization. Subsequent burial during the early Cretaceous filled the karst cavities with laminated clay. Laramide- and Chinati Mountains caldera-related tectonic activities in late Cretaceous to late Paleogene deformed the Permian through Cretaceous strata. Laramide fractures and diagenetic porosity zones provided conduits for hydrothermal solutions that deposited Ag-Pb-Zn sulfides.

A fluid inclusion study of calcite and quartz samples collected in the Presidio mine and of calcite, quartz, and sphalerite collected from deep cores between Shafter and Red Hills. Homogenization temperatures for primary fluid inclusions in sphalerite range from 190 to 300°C, with an average of 220°C. Likewise, sulfur isotope fractionation between sulfide pairs suggests a mineralization temperature range of 170° to 290°C. District metal zonation, fluid inclusion, and stable isotope data suggest a magmatic source, probably from a largely buried Laramide magmatic system that produced a hydrothermal system that cooled laterally (Red Hills to Shafter), vertically, and over time, perhaps in response to fluid mixing. Late-stage meteoric water infiltration resulted in oxidation of the sulfide ore to produce the silver deposit in its present form.