Eco.omic Geology

The Fresnillo mine ore deposits consist of replacement chimney and manto bodies, dissem-inated sulfide bodies, and vein deposits hosted mainly in Cretaceous marine sedimentary and volcanic rocks. The base metal-silver deposits are zoned to the southeast away from a small quartz monzonite stock, with increasing silver content and decreasing base metal values with distance from the stock. Wall rocks adjacent o the stock exhibit silicification and calc-silicate alteration that has been partially replaced by the sulfide minerals. Vein wall rocks exhibit potassic, phyllic, argillic, and propylitic alteration with increasing distance from the veins. Chimney and manto bodies contain pyrite, pyrrhotite, sphalerite, chalcopyrite, galena, tetrahedrite, marcasite, arsenopyrite, and silver minerals. Chimney and manto bodies predate or are contemporaneous with base metal veins containing asimilar mineralogy. Younger silver-rich veins consist primarily ofpyrargyrite with lesser acanthite, proustitc, miargyrite, polybasite, tetrahedrite, galena, sphalerite, and chalcopyrite. Individual veins exhibit increasing silver and decreasing base metal concentrations with increasing elevation. Fluid inclusions in quartz, calcite, and sphalerite contain two phases, water and wate