Summary:
The majority of mineralization corresponds to ore skarns: silicoaluminate of calcium, iron and manganese with variable quantities of lead, zinc, copper and iron sulfides. Economic ore include sphalerite (ZnS), galena (PbS) and small quantities of pyrargyrite (Ag3SbS3 ).
Mineralization is associated in time and space with the felsite intrusions in two zones known as the East and West camps, which lie on opposite flanks of a broad, doubly plunging anticline. The felsites are texturally and compositionally indistinguishable and have identical REE patterns. West camp mineralization consists of massive sulfide mantos and chimneys with lesser mineralized breccias and skarn bodies. East camp mineralization consists of dike contact skarns with subordinate massive sulfide bodies. Structural controls on mineralization in both camps include reactivated fold-related fracturing and faulting as well as intrusive contacts. Host-rock lithology (specially secondarily enhanced permeability) controls ore distribution in the upper and peripheral parts of the district.
Ten different ore types have been recognized in the district, but only the normal sulfides, silicates (calcic-iron skarns), and calc-silicate skarn ores were examined in this study. The ore mineralogy throughout the district is relatively simple, consisting of pyrrhotite, pyrite, sphalerite, and galena with minor amounts of calcite, quartz, fluorite, and in places calc-silicate gangue. Despite stark differences in trace metal contents and the amount and composition of the skarns in the two camps, the overall geologic character, fluid inclusion compositions, sulfur, oxygen and carbon isotope characteristics and alteration assemblages are similar enough to indicate that the ore fluids for both camps had a common origin.
West camp mineralization
Manto and chimney deposits are the most common types of mineralization in the West camp. The ore mineralogy consists of pyrrhotite, galena, sphalerite, and pyrite, replacing limestone with little alteration of the host rock. Skarn mineralization is more limited, present as either silicate or calc-silicate orebodies. Silicate orebodies consist of Ca-Fe silicates with Mn-rich compositions. The silicate orebodies are reported to have higher silver values than the manto ores. Ca-silicate orebodies consist of calcic skarn silicate minerals and an ore assemblage of galena, sphalerite, arsenopyrite, pyrite, and pyrrhotite. Trace amounts of chalcopyrite and pyrrhotite, argentopyrite, polybasite, and stephanite are also reported as discrete minerals in the calc-silicate ores.
East camp mineralization
Mineralization in the East camp is controlled by structures related to the San Antonio graben and the contacts of a series of felsite dikes that follow and cut across the graben faults. Skarn mineralization dominates and calc-silicate mineralogy is bilaterally and symmetrically zoned outward from the dikes. The dikes are widely converted to calc- silicates. Most of the sulfides in the East camp occur within the skarn, but podiform sphalerite + pyrite ± pyrrhotite bodies (± galena) commonly occur between the skarns and the enclosing limestone. Massive sulfide mantos composed of galena, sphalerite, and pyrrhotite, are texturally similar to the West camp ores and occur in contact with, and peripheral to, the skarns.