GEOPHYSICAL AND HYDROLOGICAL SURVEY TO ASSESS THE ECONOMIC POTENTIAL OF A GOSSAN IN THE EASTERN UINTA MOUNTAINS, UTAH

When sulfide-bearing rocks are exposed to oxidizing conditions, they become destabilized, leaving behind a leached, altered, and replaced outcrop called a gossan. Many of these gossans form by the oxidation of ore minerals and have been known since antiquity to be associated with ore deposits. The objective of this study was to use geophysical and hydrological methods to assess the economic potential of a gossan on the southeastern margin of the Uinta Mountains in northeastern Utah. The South Flank Fault forms the boundary between the Uinta Mountain Group (Neoproterozoic) and a heavily brecciated wedge of the Madison Limestone (Mississippian), which hosts the gossan. Two vertical electrical soundings (employing the DC resistivity, induced polarization (IP), and spontaneous potential (SP) methods) were consistent with resistivity lows in the depth range 23-48 m, while one sounding was consistent with a chargeability high in the same depth range. Two total magnetic field profiles could not be modeled without assuming both normally and reversely magnetized subsurface bodies with tops ~20 m below the surface and magnetic susceptibilities about three orders of magnitude greater than those measured on the surface. A survey of the fluvial chemistry of Black Canyon Creek, which flows across the gossan and which receives groundwater input either from the porous gossan or the South Flank Fault, showed statistically significant increases in Cu and Ni and enrichment in ¹⁸O and ²H on the downstream side of the gossan. Collectively, the results from this study are consistent with sulfide ore bodies ~20 m below the surface.