The geologic setting of the Santa Eulalia district is similar to other high-temperature, carbonate-hosted Ag-Pb-Zn deposits of northern Mexico. The host rocks are Cretaceous limestones intruded by felsite and diabase dikes and sills.

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 com ........

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