SAMPLE DESIGNATION: Well FF-52-32, ST-2, measured depth interval 9040-9060 ft, or 2755.3-2761.4 m. True vertical depth interval 8687-8707 ft, or 2647.7-2653.8 m.

SAMPLE LOCATION: South-central Geysers, about 3.5 km south-southwest of the crest of Cobb Mountain.

SUMMARY AND INTERPRETATION: Very weakly altered, probably porphyritic, biotite orthopyroxene granite. The same rock type that dominates the previously described samples.

SAMPLE TYPE: Mud-drilled cuttings

BACKGROUND INFORMATION: GDC-5 and Angeli-1, this well penetrated essentially impermeable rock without commercial steam entries. A hidden benefit of this situation, however, was that a substantial thickness of felsite was drilled with mud rather than air, so that cuttings were produced sufficiently large for meaningful, single-mineral age dating. Moreover, much of the texture of the rock is preserved by these (relatively) large cuttings, permitting inferences about their magmatic and hydrothermal histories which are painfully obscure from examination of air-drilled cuttings alone (typical chip diameter 40 microns). Like many Geysers cuttings samples, this one from FF-52-32 is contaminated by caved rock chips, cement, rust, and other foreign debris (8% in aggregate wt. %). The rock and mineral percentages given below have been recalculated to eliminate this contamination.

ROCK TYPE: Fine-grained, probably porphyritic, leucocratic, biotite-orthopyroxene granite, very similar texturally and mineralogically to examined for this study from wells Angeli-1, LF-23-ST-1, and GDC-5. This rock is much fresher, that is, less affected by hydrothermal alteration, than the otherwise similar one from GDC-5. It is hypidiomorphic-granular in texture, with local patches of micrographic quartz-feldspar intergrowth. It consists of the following primary minerals: quartz – 39%; plagioclase – 29%; potassium feldspar – 25%; orthopyroxene - 1%; biotite – 1%; ilmenite -- 0.5-1%; apatite and zircon – traces. All these minerals are, essentially, texturally identical to their occurrence in the GDC-5 sample (see companion write-up for the sample from this well), even to the turbid appearance of potassium feldspar; one or two zircons in the present sample, however, are up to twice the apparent volume of those from GDC-5, and the mafics are obviously less abundant than in that sample.

ALTERATION AND MINERALIZATION: This rock is comparatively unaltered and unmineralized. Biotite is locally, sparsely, and variously altered to chlorite and sphene; orthopyroxene to (deuteric) biotite and hydrothermal actinolite or chlorite. Some plagioclase crystals are dusted with microscopic flakes of illite; others contain irregular patches and stringers of apparently secondary potassium feldspar, with or without minor hydrothermal epidote.

FLUID INCLUSIONS: Same as in the granite cuttings from well GDC-5.