GEOCHEMISTRY OF SURFACE-WATERS IN MINERALIZED AND NON-MINERALIZED AREAS OF THE YUKON-TANANA UPLANDS, ALASKA

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 Yukon-Tanana 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. Bedrock is composed of metamorphic rocks and younger plutons. 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 Ca²⁺ and Mg²⁺ - HCO₃^- to Ca²⁺ and Mg²⁺ - SO₄^2- waters. The metamorphic lithologies are more SO₄^2- dominated 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 detectable near mineralized areas but in most cases rapidly attenuate downstream and processes other than simple dilution control the concentrations of these trace elements. The SO₄^2- concentrations increase downstream near the mineralized areas but the concentration may not be directly related to mineralization. More work is necessary to determine what proportion of the increase in SO₄^2- concentration is attributable to the sulfide mineral oxidation, as opposed to water influenced by the underlying metamorphic units.