Ore deposits at the Fresnillo Mine, Zacatecas, Mexico

Abstract

The Fresnillo mine ore deposits consist of replacement chimney and manto bodies, disseminated 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 to 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 a similar mineralogy. Younger silver-rich veins consist primarily of pyrargyrite with lesser acanthite, proustite, 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 water vapor, and are dilute (0-12 equiv wt % NaCl) solutions. Filling temperatures and salinities vary sharply between growth zones in ore and gangue minerals with distinct salinity increases in sphalerite. Filling temperatures in chimney, manto, and deep vein quartz and calcite range between 240 degrees and 350 degrees C while shallow and distal quartz, calcite, and sphalerite from veins yield temperatures between 140 degrees and 275 degrees C. Sphalerite geobarometry of samples from the Cueva Santa manto yielded a total pressure of 730 bars. Pressure correction for the fluid inclusion filling temperatures from samples in chimney and manto deposits is 70 degrees C. Silverrich veins formed from fluids that were boiling, indicating a depth of formation of 350 m below the paleosurface.Sulfur isotope analyses of galena and sphalerite from chimney, manto, and vein deposits exhibit overlapping ranges of values, with galena depleted in 34 S with respect to sphalerite. Galena delta 34 S values from all deposit types at Fresnillo vary between -4.1 and -8.0 per mil while sphalerite ranges between -2.0 and -5.0 per mil. The calculated delta 34 S (sub Sigma S) utilizing fluid inclusion filling temperatures is -3.0 + or - 0.4 per mil. Such a composition may have been derived from a slight shift in ore fluid pH (as indicated by the wall-rock alteration assemblages) or by increased log f (sub O 2 ) (as indicated by minor hematite and amethystinc quartz) from an original delta 34 S (sub Sigma S) of 0 per mil. Galena lead isotope data indicate that mantos and veins formed from the same deep crustal source.The fluctuating temperature and salinity values from growth zones in ore and gangue minerals, deep crustal source of the lead, and magmatic origin of the sulfur argue for a mineralizing system that derived its fluids from magmatic and meteoric sources. The magmatic fluids associated with the quartz monzonite stock spread upward and outward along major regional faults and bedding planes of the sedimentary rocks, mixing and cooling with meteoric fluids. The ore and gangue minerals may have been precipitated due to mixing of these two fluids. Boiling of ore fluids may have caused ore mineral precipitation in the silver-rich veins as suggested by sharp vertical increase in silver concentrations at the 350 m (below the present surface) level.