USE OF NOBLE GAS GEOCHEMISTRY AND TRACE ELEMENT ANALYSIS TO DETERMINE IF MODERN GEOTHERMAL WATERS ARE RESPONSIBLE FOR ASSOCIATED EPITHERMAL AU DEPOSITS

Modern geothermal systems in the Great Basin of California and Nevada typically are characterized by a focused and identifiable magmatic heat source or a regionally dispersed amagmatic heat flux. Some young (< 7Ma), economically viable epithermal Au deposits are closely associated with modern, geothermal systems. The spatial association of the Au deposits to modern geothermal systems has led to speculation of an amagmatic origin for the deposits and that the modern geothermal systems could now precipitate Au in the subsurface. To evaluate this hypothesis, noble gas and fluid chemistry was determined for active, amagmatic geothermal systems in NW Nevada (Wabuska, San Emidio, Brady Hot Springs, Humboldt House, and Soda Lake). For comparison, modern magmatic geothermal systems in California (Clear Lake, Wilbur Springs, Lassen Peak, and Mammoth Mountain) were also sampled. Noble gases in both amagmatic and magmatic systems are dominated by atmospherically derived components that have mixed with deeper crustal and magmatic sources respectively. Helium isotopic data, after correction for atmospheric components (excess He), show a dominance of crustal helium (87 to 97 percent of the excess He component), supporting an amagmatic heat source for a majority of the Nevada geothermal systems, except for Soda Lake, where almost 54 to 59 percent of the excess He is mantle-derived. Compositions of noble gases trapped in quartz veins at Humboldt House suggest a possible link to a magmatic heat source in the metal-rich samples, with background crustal values present in the barren samples. Geochemical modeling of the water compositions predicts the precipitation of pyrite and silica in all systems. Gold and silver phases are close to saturation despite very low concentrations. Ore phases recognized within quartz veins from Humboldt House are consistent with the measured water composition. Nonetheless, the low dissolved concentrations would require very large fluid volumes to form a deposit of economic interest. Preliminary interpretation of the results indicates that existing gold deposits may have had a magmatic component at the time of mineralization that was distinct from the origin of the heat contribution to modern fluids .