Geochemistry of Surface-Waters in Mineralized and Non-Mineralized areas of the Yukon-Tanana Uplands

Abstract

The U.S. Geological Survey (USGS) and Alaska Department of Natural Resources (ADNR) are continuing investigations on element mobility in mineralized and non-mineralized areas of the YukonTanana Upland in east-central Alaska. The chemistry of stream water is evaluated in the context of regional bedrock geology and geologic structure. Sampling sites were located in the Big Delta B2 quadrangle, which includes the mineralized areas of the Pogo claim block. The area is typified by steep, subarctic-alpine, boreal forest catchment basins. Samples were collected from catchments that either cross structural features and lithologic contacts, or are underlain by a single lithology. Waters are generally dilute (< 213 mg/L TDS), and are classified as Ca and Mg HCO3 to Ca and Mg SO4 2waters. Gneissic lithologies are more SO4 2dominated than the intrusive units. The major-ion chemistry of the waters reflects a rock-dominated aqueous system. Trace-element concentrations in water are generally low; however, As and Sb are detected near mineralized areas but in most cases rapidly attenuated downstream and processes other than simple dilution are controlling the concentrations of these trace elements. There is a tendency toward increasing SO4 2concentrations downstream in waters both proximal and distal to mineralized areas. More work is necessary to determine what proportion of the increase in SO4 2could be derived from the oxidation of sulfide minerals as opposed to water influenced by the underlying gneissic units.