David Sutterfield, Utah Valley University
When sulfide-bearing rocks are exposed to oxidizing conditions, they become destabilized, leaving behind a framework of leached, altered, and replaced host rock 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. However, the extent and quality of ore mineralization beneath a given gossan cannot readily be determined through surface sampling of minerals. Work conducted by mineral exploration professionals (in Africa, Australia, India, and the Middle East) has indicated that geomagnetic and geoelectric surveys of a gossan can be useful for constraining the shape, size, and economic potential of an associated ore deposit. Although gossans are found in Utah, there have been no published studies of these rock units either in terms of their economic potential or geophysical signature. The objective of this study was to carry out geomagnetic and geoelectric surveys to determine the geophysical signature of gossans exposed about 10 miles northwest of Vernal, Utah, on the southeastern margin of the Uinta Mountains, for the purpose of estimating the grade and depth of possible sulfide mineralization. The gossans overlie a heavily brecciated wedge of Mississippian Madison Limestone and are structurally bounded by the South Flank Fault, which forms the boundary between the Permian Weber Sandstone to the south and the Neoproterozoic Uinta Mountain Group metaquartzites to the north. Geoelectric measurements with an Iris Instruments Syscal Junior Resistivity System and inversion of a portion of the resistivity and chargeability data with the Interpex IX1D Sounding Inversion software shows a resistivity low (~200 Ω·m) and a chargeability high (~7 ms) below 23 m depth. Measurements of total magnetic field using a Geometrics G-856 Proton Precession Magnetometer were modeled with Interpex IX2D-GM Magnetic Interpretation Software and constrained with magnetic susceptibilities of exposed rocks measured in the field using a SM-20 Magnetic Susceptibility Meter. Models based upon a portion of the data show anomalies of amplitude about 100 nT and wavelength about 50 m, suggesting isolated bodies of elevated magnetic susceptibility (~0.08 SI units) with upper surfaces 20-30 m below the surface. Since, based upon the topography, the depth to the water table is also 20-30 m, the geophysical measurements are consistent with the presence of reduced sulfide bodies below this depth. Further work will include interpretation of remaining data and possible drilling for improved calibration of geophysical models.