Origin of Gold in Placer Deposits of the Sierra Nevada Foothills, California

摘要

The Sierra Nevada Foothills (SNF) region of northern California is one of the world's premier gold provinces. Gold is present both in bedrock lode deposits and in major Tertiary, primarily Eocene, and Quaternary alluvial placer accumulations. Gold nuggets in the auriferous gravels of northern California traditionally have been interpreted to be masses of native gold released by physical weathering from underlying lode deposits and incorporated in the stream bed load. This interpretation has numerous problems. (1) Large gold nuggets are extremely uncommon in primary hydrothermal gold deposits, even within the orogenic gold-quartz veins of the SNF. (2) Historic lode gold production from the SNF has been about 35 million ounces, while Tertiary placer production exceeded 65 million ounces with yet another 50 million ounces of placer resource stranded by extraction restrictions. (3) A large part of the placer gold accumulation is situated upstream and uphill of the lode deposits. (4) Tertiary channel gold averages 920 fineness, whereas gold from primary veins is 600-900 fineness. We suggest an alternative hypothesis that much of the placer gold in the SNF Tertiary gravels was sourced from the east, either from the great gold districts of Nevada or from districts long ago removed by erosion. Gold in most primary hydrothermal deposits is widely disseminated in host rock at ppm-level concentrations within pyrite or other sulfide minerals. As demonstrated in modern tropical environments, the transformation of gold from trace concentrations in sulfide to grains or masses of native metal occurs primarily by supergene processes during intense chemical weathering in the soil profile. Gold is readily dissolved, mobilized and reprecipitated by chloride, bisulfate or organic-acid solutions, often with microbial mediation. These supergene processes significantly increase the fineness of the supergene gold grains relative to that of gold in the primary source. The Paleogene climate of western North America was considerably warmer and wetter than present, much like modern tropical environments. Tertiary river channels were eroded into a surface of intense weathering, as evidenced by remnant laterite soil profiles throughout northern California. The lower lone Formation, the distal marine equivalent of the proximal alluvial auriferous gravels, is comprised largely of kaolinite and quartz: the products of intense chemical weathering. Plant and animal fossils in Tertiary sedimentary rocks record a distinctly more tropical climate. Multiple lines of evidence indicate that the Cretaceous to middle Cenozoic Sierra Nevada formed the steeper western flank of a gradually sloping high-elevation plateau-the Nevadaplano-that extended to central Nevada through the Oligocene time, and that the gold-bearing river channels extended from the Great Valley shoreline to head in what is now eastern Nevada. We propose that gold nuggets, formed from by pedogenic processes during intense weathering of disseminated Eocene and older gold deposits in Nevada during a warmer and more humid Tertiary climate, were transported westward in great river systems draining the Nevadaplano, and were deposited in the Tertiary channels as stream gradients decreased at the foot of the range. The SNF auriferous gravels combine these distal nuggets with gold nuggets derived locally from intense weathering of the bedrock veins.