GEOLOGY, GEOCHEMISTRY AND GENESIS OF SEDEX AU ORES ON THE CARLIN TREND, NEVADA

Sedimentary exhalative (sedex) Au mineralization on the Carlin trend is hosted in the Middle Devonian Upper Mud Member (UM) of the Popovich Fm. The UM is a carbonaceous limy mudstone with pyrite laminations that contain sphalerite, barite, tetrahedrite, chalcopyrite, and Au and isolated lenses of bedded barite and massive sulfide. Grains of native Au (<1 mm) are disseminated in mudstone and occur as inclusions in barite and base metal sulfides. Base metal sulfides also contain chemical enrichments of Au. Zinc, Cd, Cu, V, Se, Ni, Hg, and Mo correlate positively with Au. The footwall locally exhibits discordant sparry "zebra" dolomite and stratabound silicification with barite, base metal sulfides, and Au. The Upper Zone of the Rodeo deposit is hosted in the UM and contains sedex ore with up to 68 g/t Au. This ore type is interpreted to be sedex because; 1) it predates compaction and lithification of its host rocks; 2) the ore horizon is stratiform and highly anomalous in ore metals over 20 sq km; 3) barite has d34S and d18O values identical to other sedex barite deposits in NV; 4) ore sulfides have biogenic d34S (-20 to –10‰); 5) synsedimentary debris flows terminate mineralized beds; 6) local abrupt facies changes, large debris flows and slumps indicate synsedimentary faulting during mineralization, as is characteristic of sedex basins; and 7) the absence of alteration and d13C and d18O isotopic shift of primary carbonate minerals and Au inclusions in base metal sulfides in ore-grade rocks is strong evidence that Au was not introduced by younger Carlin-type mineralization. Petrologic, geochemical, isotopic, and fluid inclusion studies and chemical modeling indicate sedex ore fluids were reduced H2S-rich basin brines. In sedimentary basins, H2S is produced by thermochemical reduction of sulfate and removed by sulfidation of reactive Fe. Thus, carbonate and shale sequences with high organic C and sulfate and low reactive Fe concentrations are most likely to produce H2S- and Au-rich brines. Sedex Au is a previously unrecognized end member in the spectrum of sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, there is potential for large sedex Au deposits in sedimentary basins that are under explored for Au. In the Carlin trend, sedex Au may have been an important source for Eocene Carlin-type Au.