Identifying magmatic versus amagmatic sources for modern geothermal systems associated with epithermal mineralization using noble gas geochemistry

Modern geothermal systems in the Great Basin of California and Nevada typically
are characterized as the result of a focused and identifiable magmatic heat source
or of a regionally dispersed amagmatic heat flux. Young (< 7Ma), economically viable epithermal Au deposits are closely associated with some modern, presumably amagmatic geothermal systems. This close spatial association of theAu deposits to the modern geothermal systems has led to speculation of an amagmatic origin for the deposits and that the modern geothermal systems could be actively precipitating Au in the subsurface. In an effort to clarify the possible role of amagmatic geothermal systems as the source of young to modern epithermal Au mineralization, noble gas and fluid chemistry samples were collected from active, amagmatic geothermal systems in Nevada (Wabuska, San Emidio, Brady Hot Springs, Humboldt House [Florida Canyon], and Soda Lake) as well as modern magmatic geothermal systems in California (Clear Lake, Wilber Springs, Lassen Volcanic National Park and Mammoth Mountain). Calculated silica geothermometery on the fluids from the Nevada geothermal systems agrees well with the measured down-hole temperatures from the reservoirs. The calculated reservoir temperatures of these systems span the range of 150 to 210°C, exceptWabuska which yields a temperature of 135°C. Noble gas compositions from both magmatic or amagmatic systems are characterized by atmospherically derived components that have mixed with deeper non-atmospheric sources. Helium isotopic data, after correction for atmospheric components (excess He), shows a dominance of crustal helium (87 to 97 percent of the excess He component) supporting an amagmatic origin, except in the case of the Soda Lake, where almost 54 to 59 percent of the excess He is mantle derived. Comparison of the He data with Au concentrations in fluid samples shows a positive correlation between gold and the mantle helium content in both the magmatic and amagmatic systems. This relationship is at odds with current theories that the modern, amagmatic geothermal systems are responsible for substantialAu transport that resulted in the spatially associated young epithermal deposits. This postulate is further supported from noble and bulk gas compositions from fluid inclusions in silica precipitates from Florida Canyon and Dixie Valley.