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The Geysers “felsite” (a designation of common usage in the geothermal community and thus retained for this article) is a young, wholly concealed, hypabyssal intrusive complex of batholithic dimensions within and beneath The Geysers steam field in the Mayacmas Mountains of northwest-central California (Fig. 1). The felsite was first penetrated by geothermal wells in the 1970’s, but because these air-drilled boreholes yielded extremely minute (typically <0.1 mm in diameter) cuttings, the igneous nature of the body long eluded recognition. Bailey (1946) speculated that fumaroles at The Geysers signaled such a hidden intrusive, but the first detailed descriptions and chemical analyses of the felsite were published by Schriener and Suemnicht of Unocal Corporation (1981).

The Geysers felsite shows a clear correlation with the extent and configuration of the currently exploited vapor-dominated geothermal system (Figs. 1 and 2). A portion of this system is actually hosted by the felsite (e.g. Beall and Box, 1992), and above the pluton the remainder of the steam reservoir occurs in rocks which are hydrothermally altered and mineralized in concentric zones centered on the deep intrusive (Hulen and Nielson, 1993; Hulen and others, 1992; Moore, 1992; Walters and others, 1992; Hebein, 1986; McLaughlin and others, 1983). This alteration records the prior presence of a high-temperature, liquid-dominated hydrothermal system which appears to have “dried out” to yield the modern vapor-dominant regime (e.g. Moore, 1992; McLaughlin and others, 1983). The surface projection of the steam field includes and is encircled by numerous but scattered, commonly hydrocarbon-rich mercury deposits (e.g.

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