PARAGENETIC AND GEOCHEMICAL STUDIES OF WELL P1-10, HUMBOLDT HOUSE GEOTHERMAL AREA, PERSHING COUNTY, NEVADA

Five exploratory geothermal wells have been drilled in the Humboldt House geothermal area (HHGA), located 200 km northeast of Reno NV, to assess geothermal potential. Nearly 1000 ft of core were recovered from well P1-10, which had a total depth of about 1500 ft. In the P1-10 core, lithologies are primarily conglomerate and silicified conglomerate. Paragenesis in the conglomerate is represented by authigenic kaolinite and amorphous aluminum-iron clays. Paragenesis in the silicified conglomerate is represented by a major silicification event that largely obscures the original conglomeratic texture. Pervasive veining, vuggy texture and major pyrite along fractures in the silicified conglomerate are observed. Some veins exhibit concentric layers of chalcedony and internal voids partially filled with zoned euhedral quartz crystals. Smectite, illite and corrensite clays also increase with depth. Abundant chloritized biotite occurs as micaceous flakes, in addition to minor amounts of chlorite rosettes. Clay minerals are generally observed to take place as a late-stage pore-filling material and generally coat detrital grains, thereby reducing secondary porosity. Petrographic evidence suggests that several episodic events of reducing, silica-rich fluids may have been active in the HHGA over time. Comparisons of major, minor, and trace element concentrations between the unsilicified and silicified zones are being used to characterize the geochemistry of fluid events identified by petrographic analyses. ICP-MS analyses show that Al generally increases in the unsilicified conglomerate where kaolinite is observed as the dominant secondary mineral. In the silicified conglomerate, Al, Fe, K and Mg generally increase with depth and correlate with increasing chloritized biotite content. In the silicified zone, trace elements Rb and Ba show similar trends and occur in levels as high as 373 ppm and 6854 ppm respectively. Concentrations of As are as high as 4086 ppm, and correlate strongly with total S and Fe concentrations. Cs, Sr, and W levels decrease in the silicified zone relative to the unsilicified zone. Continuing work will utilize stable isotope and trace element analyses of the vein filling cements to further characterize the hydrothermal fluids at HHGA.