Geology and geochemistry of the Santa Eulalia mining district, Chihuahua, Mexico

Abstract

Santa Eulalia contains two separate, contrasting Pb-Zn-Ag deposits. The East Camp consists of a symmetrically zoned calc-silicate skarn with distal sulfide and tin-bearing orebodies; whereas the west Camp is composed of massive sulfide orebodies with minor proximal calc-silicate skarn and isolated intermediate calcic-iron skarns. Mineralization and alteration are zoned within each camp but do not overlap. Sulfide mineralization in both camps consists of pyrrhotite, sphalerite, galena, and pyrite with lesser amounts of arsenopyrite and chalcopyrite. The East Camp is apparently richer in zinc and copper than the West camp. Mineralization is temporally and spatially related to geochemically identical felsite intrusions which apparently have a common source. Mineralizing fluids followed these felsites through a thick section of evaporites and organic-rich shaly limestones into clean, homogeneous, relatively undeformed, limestone hosts. West Camp mineralization occurs along an interconnected network of vertically discontinuous tight fissures and sill contacts, whereas East Camp mineralization is located along vertically throughgoing faults and dike contacts. strata-bound, but not stratiform, mantos extend off discordant chimneys in both camps. Ore textures reveal that mineralization occurred primarily by limestone replacement. 21 Pressure-corrected primary fluid inclusion homogenization temperatures in fluorite range from 220 to 490 deg. C. Salinities are bimodal with high-salinity (>26.3 equivalent wt% NaCl) and low-salinity (1-12 equivalent wt% NaCl) populations. Mineralogical constraints indicate that the hydrothermal fluids were acid and reduced. Sulfur isotope analyses indicate that the ore fluids varied from -17 to +4 permil without correlation to iron-sulfide species, temperature, or salinity. Co-existing sulfides are commonly in isotopic disequilibrium. Sulfur isotopes from the West Camp are crudely zoned, but no consistent patterns exist in the East Camp. Oxygen and carbon isotope analyses of limestone wallrocks reveal a distinct isotopic alteration halo. A single analysis of gangue calcite from each camp indicates that the ore fluids contained non-carbonate-derived carbon and oxygen, possibly of magmatic origin. Metals were apparently transported as chloride complexes and deposited through coupled dissolution-precipitation replacement reactions. Most ore sulfur apparently came from diagenetic pyrite and sedimentary anhydrite, but some of the sulfur may have had a magmatic source. The metals probably came from the felsite parent magma and this magma may have also contributed fluids. Close similarities between Santa Eulalia and numerous other intrusion-related carbonate-hosted deposits in northern Mexico reinforce these interpretations.