ELEMENT MOBILITY IN THE CHEMICAL EVOLUTION OF A GEOTHERMAL SYSTEM, HUMBOLDT HOUSE GEOTHERMAL AREA, PERSHING COUNTY, NEVADA

Humboldt House geothermal area (HHGA), located in Pershing County Nevada, is characterized by upwelling of hot geothermal fluids along high-angle, range bounding faults and lateral dispersal into adjacent lacustrine basin sediments. Core was recovered from three geothermal wells during a 2003 drilling program to evaluate the chemical evolution of geothermal fluids. Approximately 1800 feet of core was recovered from wells P32-2, P10-1 and P3-1. The P32-2 core rocks are slightly altered pluvial/fluvial sediments. Both P10-1 and P3-1 are composed primarily of highly altered silicified conglomerates.

Chemical evidence from major and trace element distributions suggests several episodes of silica-rich fluids that leached major and trace elements from the host rock during alteration and deposited secondary mineral phases rich in base and precious metals. The general geochemical trend in both the major and trace element distribution in the cores shows a decrease in the concentration of the major and trace element as the silica concentration increases. These changes are especially evident in the trace element distribution between unaltered conglomerates and the highly altered silicified conglomerates in the P10-1 core. For example, concentrations of rare earth elements (REE) in silicified zones decrease significantly in comparison to the REE concentration in the unsilicified zone. Specifically, the concentration of Ce decreases from an average of 75 ppm in the unsilicified zone to an average of 21 ppm in the silicified zone and Nd decreases from an average of 33 ppm to 10 ppm. The concentrations of Fe, Al, Na and K also decrease considerably in the silicified zone. Concentrations of Au, Ag, Mo and As increase notably along with Si concentrations in highly-altered zones. Disseminated pyrite and a decrease in the positive Eu anomaly indicate that the silica-rich fluids were likely reducing.