GREENHOOT, C.A., FRIEHAUF, K.C., and BARTON, M.D., Dept. of
Geosciences, University of Arizona, Tucson, AZ 85721
The Metates gold deposit in northwest Durango, Mexico, contains over 800 million metric tons of mineralized rock (0.62 ppm Au, 15 ppm Ag, 0.2% Zn) hosted by a quartz latite body and enclosing marine clastic sedimentary package.
Mineralization is spatially related to an Early- Cretaceous concordant body of porphyritic quartz latite (107 Ma, U/Pb). The enclosing 500 m sequence of dark shales (argillites), greywackes and conglomerates may represent marine turbidites deposited in a back-arc basin. Mesozoic rocks are deformed and unconformably overlain by unmineralized Tertiary volcanics. The latite body strikes northwest for more than a kilometer, and dips 30 degrees northeast; maximum thickness is about 300 meters. Massive latite is overlain by a heterolithic breccia, characterized by igneous and sedimentary matrix alternating at meter scale. Bedding within 20 meters of the upper and lower contacts is sheared and contorted. Contact metamorphism is absent. The latite may be a shallow syn-sedimentary sill, with peperite at the upper contact. Alternatively, the breccia may represent a debris flow overlying a submarine latite flow or dome.
Syngenetic lamellae of framboidal pyrite are present within the sedimentary rocks. Epigenetic veins include: (1) pyrite +/- chalcopyrite; (2) pyrite- arsenopyrite- +/- quartz +/- chalcopyrite; (3) sphalerite- pyrite- galena- siderite- quartz; and (4) quartz- pyrite +/- sphalerite. Sulfur isotope data suggest a magmatic sulfur source for vein sulfides. A magmatic fluid component is compatible with moderate salinity fluid inclusions (>5 wt % NaCl eq), and pervasive alteration of the quartz latite to sericite- quartz- pyrite. Pervasive silicification is common, but veins are nearly free of silicate minerals. Alteration in the sheet-silicate-rich sedimentary units presents as replacement of clays and feldspars by sericite.
Gold is present as micron sized inclusions within pyrite. SIMS microanalysis indicates gold in the pyrite crystal structure in very low concentrations, typical of background levels in many hydrothermal pyrites. Two conical zones of Au/As/Cu mineralization (with peripheral Zn) correlate with two zones of high vein abundance in (1) the latite body and (2) overlying coarse sediments. Ag mineralization (as pyrargyrite, tetrahedrite, and argentiferous galena) is irregularly distributed and inversely correlated with Au.
Abstract number 52376