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6-Three pipe retort at Dawson mine, location of sample 21DC1 7-Silica-carbonate (SC) type mercury deposits associated with serpentinite have high nickel and cobalt concentration, field I, as compared to hot spring (HS) type mercury deposits, field II. Other trace elements enriched in SC deposits define Factor 2 (Co, Ni, Mg, Fe, Cr, Mn) and discriminate these deposits from HS deposits which are enriched in Factor 1 elements (Al, K, Na, V, Ti). Samples from the Dawson and King and Avenal Creek areas plot in the field of silica carbonate type deposits but some samples from the King mine plot in the hot spring type field.

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The regional distribution of arsenic and 20 other elements in stream-sediment samples in northern Nevada and southeastern Oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area. Data were used from 10,261 samples that were originally collected during the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) program in the 1970s. The data are available as U.S. Geological Survey Open-File Report 02-0227.The data were analyzed using traditional dot maps and interpolation between data points to construct highresolution raster images, which were correlated with geographic and geologic information using a geographic information system (GIS). Wavelength filters were also used to deconvolute the geochemical images into various textural components, in order to study features with dimensions of a few kilometers to dimensions of hundreds of kilometers.The distribution of arsenic, antimony, gold, and silver is different from distributions of the other elements in that they show a distinctive high background in the southeast part of the area, generally in areas underlain by the pre-Mesozoic craton. Arsenic is an extremely mobile element and can be used to delineate structures that served as conduits for the circulation of metal-bearing fluids. It was used to delineate large crustal structures and is particularly good for delineation of the Battle Mountain-Eureka mineral trend and the Steens lineament, which corresponds to a post-Miocene fault zone. Arsenic distribution patterns also delineated the Black Rock structural boundary, northwest of which the basement apparently consists entirely of Miocene and younger crust.Arsenic is also useful to locate district-sized hydrothermal systems and clusters of systems. Most important types of hydrothermal mineral deposit in the northern Great Basin appear to be strongly associated with arsenic; this is less so for low-sulfidation epithermal deposits.In addition to individual elements, the distribution of factor scores that resulted from principal component studies of the data was used. The strongest factor is characterized by Fe, Ti, V, Cu, Ni, and Zn and is used to map the distribution of distinctive basalts that are high in Cu, Ni, and Zn and that appear to be related to the Steens Basalt. The other important factor is related to hydrothermal precious metal mineralization and is characterized by Sb, Ag, As, Pb, Au, and Zn. The map of the distribution of this factor is similar in appearance to the one for arsenic, and we used wavelength filters to remove regional variations in the background for this factor score. The resulting residual map shows a very strong association with the most significant precious metal deposits and districts in the region. This residual map also shows a number of areas that are not associated with known mineral deposits, illustrating the utility of the method as a regional exploration tool. A number of these prospective are...

Clear Creek, one of the major tributaries of the upper Sacramento River, drains the eastern Trinity Mountains. Alluvial plain and terrace gravels of lower Clear Creek, at the northwest edge of the Sacramento Valley, contain placer gold that has been mined since the Gold Rush by various methods including dredging. In addition, from the 1950s to the 1980s aggregate-mining operations removed gravel from the lower Clear Creek flood plain. Since Clear Creek is an important stream for salmon production, a habitat restoration program is underway to repair damage from mining and improve conditions for spawning. This program includes using dredge tailings to fill in gravel pits in the flood plain, raising the concern that mercury lost to these tailings in the gold recovery process may be released and become available to biota. The purposes of our study are to determine concentrations and speciation of mercury in sediments, tailings, and water in the lower Clear Creek area, and to determine its mobility. Mercury concentrations in bedrock and unmined gravels both within and above the mined area are low, and are taken to represent background concentrations. Bulk mercury values in flood-plain sediments and dry tailings are elevated to several times these background concentrations. Mercury in sediments and tailings is associated with fine size fractions. Although methylmercury levels are generally low in sediments, shallow ponds in the flood plain may have above-normal methylation potential. Stream waters in the area show low mercury and methylmercury levels. Ponds with elevated methylmercury in sediments have more methylmercury in their waters as well. One seep in the area is highly saline, and enriched in mercury, lithium, and boron, similar to connate waters that are expelled along thrust faults to the south on the west side of the Sacramento Valley. This occurrence suggests that mercury in waters may at least in part be from sources other than placer mining.