MULTI-MEDIA EXPLORATION GEOCHEMISTRY STUDY TARGETING CU-AU AND MO RESOURCES IN THE TYONEK QUADRANGLE, ALASKA RANGE, ALASKA

Stream and pond sediment, panned concentrate, and water were collected at and around select mineral occurrences in the Tyonek quadrangle, Alaska Range, Alaska. The study was designed to establish the geochemical signatures of mineral occurrences, including porphyry Au-Cu, breccia-hosted Au, and Mo systems, in different material, as well as to distinguish whether or not areas are prospective based on similar geology and geochemical signatures. Two different size fractions (-80 mesh and -230 mesh) were analyzed, and yielded generally similar results. Elevated concentrations of elements such as As, Au, Cd, Cu, Mo, Pb, ± Zn were detected in both size fractions in streams draining known occurrences as well as from several other locations. The 90^th percentile concentrations of these elements were ~ 2 times median values. Ore minerals in panned concentrates (including gold, molybdenite, arsenopyrite, and/or copper minerals) account for some high metal concentrations in sediments. Water samples were collected from most stream, pond and seep sediment sample sites and were analyzed by high-resolution ICP-MS methodology. Relative high concentrations of constituents (including Mo, Re, As, Tl, ± Cu, SO₄) at sites where elevated pathfinder element concentrations were also present in sediment and/or heavy mineral concentrates. Higher contrast between upper quartile and particularly 90^th percentile groups relative to median values were recognized as compared to sediments (e.g., 4 times median values for As, Mo, Re, SO₄). High As, Mo, and Re concentrations probably relate both to deposit mineralogy and the higher solubility of these metals (compared to Cu, Pb, Zn) under the predominantly oxidized and near-neutral pH conditions of the stream waters even though snow melt is a major source of water. Our results suggest that water chemistry, despite comprising very low absolute concentrations (micrograms per liter), is a viable exploration methodology for identifying drainages with mineral potential even in wet, mountainous terrain like that of southwestern Alaska.