Glass Mountain Federal 31-17, with a total depth (TD) of 2678 m, is one of three deep geothermal wells capable of commercial production at the sum-mit of the large, Pleistocene-Holocene, Medicine Lake shield volcano, on the eastern flank of the Cascade Range in northeastern California. Well 31-17 encountered strong flows of high-temperature (up to 260oC) geothermal fluids in intensely propylitized Quaternary volcanics. The volcanic sequence is dominated by basalt and basaltic andesite, but in-cludes some zones of rhyolite and dacite in the upper 900 m of the well. From 2460 m to TD, the mafic volcanics are contact metamorphosed and intruded by hornblende quartz diorite. The quartz diorite is probably part of a larger pluton that floors the active geothermal system. Hydrothermal alteration in well 31-17 shows a distinct vertical zonation, with an up-permost zeolite-smectite zone above a thick and in-tense argillic zone, which in turn overlies the pro-pylitic production zone. Modern temperature logs for the well show a cool isothermal interval through the zeolite-smectite zone, a steep conductive interval (up to 500oC/km) through the argillic zone (the cap on the geothermal system), and a near-isothermal (at about 250oC) zone to total depth in the propylitic interval. Fluid-inclusion temperature data (from low-salinity inclusions) show two distinct trends; one resembling the modern temperature profile; another reaching maxima of 373oC and also exceeding, by up to 20oC, temperatures appropriate for a boiling point curve emanating from the modern water table. These fluid-inclusion thermal maxima, the curve they roughly define, and the presence of deep, high-temperature alteration phases like actinolite, clinopy-roxene, talc, and biotite virtually mandate that the corresponding temperature regime was supported by a large, cooling pluton. If this igneous body is suffi-ciently young and large, it is also the likely still-cooling heat source for the modern geothermal system.

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