Geochemistry of Lower Paleozoic host rocks for sediment-hosted gold deposits, western United States.

摘要

Lower Paleozoic, passive margin strata host more gold deposits than other Paleozoic strata. Geochemical analysis of unaltered, unmineralized lower Paleozoic sedimentary rocks from Nevada, Arizona, and Australia suggests that the abundance of gold mineralization in lower Paleozoic strata is related to geochemical conditions during sedimentation. Siliciclastic strata show primary gold enrichments of 21 parts per billion (ppb) and 7 ppb (mean values) in Ordovician Valmy and Vinini strata, respectively. Gold contents of lower Paleozoic carbonate platform, shelf, and transitional facies are comparable to the average crustal abundance of ∼2 ppb. Gold concentrations of the siliciclastic rocks correlate stratigraphically with high total organic carbon, total metal contents, vanadium ratio [V/(V+Ni)], and authigenic uranium, and with low delta13C org and cerium anomaly. These geochemical parameters are indicative of a low-O2 paleo-environment, interpreted to result from high organic productivity in upwelling zones.; The origin of the gold enrichments in lower Paleozoic, siliciclastic strata is related to global conditions in the marine environment during the early Paleozoic. Hydrothermal and detrital inputs of gold to the oceans were higher than at other times. Organic activity in upwelling zones contributed to the accumulation and preservation of gold in marginal marine sediments. Lower Paleozoic strata are favorable hosts for gold mineralization because they are associated with gold-enriched, carbonaceous siliciclastic strata that may act as a source and/or trap for gold-bearing hydrothermal fluids.