SAMPLE DESIGNATION: Well LF-23, ST-1, measured depth interval 9540-9560 ft, or 2907.7-2913.7 m. True vertical depth interval is 9388-9408 ft (2861.3-2867.4 m).

SAMPLE LOCATION: Northern Geysers, about 2 km west of the felsite-penetrating portion of well Angeli-1. Just west of the 1.2-1.1 Ma Cobb Mountain felsic volcanic center.

SUMMARY AND INTERPRETATION: A highly contaminated cuttings sample of biotite hornblende orthopyroxene granite, identical in many respects to the one penetrated in the lowermost portion of well Angeli-1 (see companion description). The hornblende may be of deuteric origin. This granite is quite fresh, almost unaffected by hydrothermal alteration. It is also monolithologic, lacking the granodiorite dike (?) drill chips found in the Angeli-1 sample. The sample was retrieved from a position about 100 m below the top of the felsite in this part of The Geysers.

SAMPLE TYPE: Drill cuttings, the relatively large size (up to 0.6 mm) suggests that the well may have been mud-drilled rather than air-drilled.

BACKGROUND INFORMATION: This sample was retrieved from a little over 100 m below the contact of The Geysers felsite with overlying metagraywacke-hornfels. It is highly contaminated, containing nearly 40% introduced materials, including cement, steel, rust, geothermal scale, and Franciscan-assemblage rock fragments probably from more than a km higher in the well. Rock and mineral percentages reported below are recalculated to eliminate the contaminants.

ROCK TYPE: Leucocratic biotite hornblende orthopyroxene granite, very similar to the granite sampled in the drilled interval 11,400-11,420, but without associated granodiorite chips. Grain size of the rock is larger than the drill-chip size, but textures of composite-mineral chips suggest that the granite is a fine- to medium-grained rock. It is apparently hypidiomorphic granular, and consists of the following phases: quartz – 40%; plagioclase – 30%; potassium feldspar – 25%; orthopyroxene – 2%; hornblende and biotite – 1% each; actinolite, epidote, zircon, apatite, chevkinite or allanite – traces. There are also traces of a delicately acicular translucent reddish-brown phase which may be an iron-titanium oxide.

Plagioclase is commonly twinned but rarely zoned. Potassium feldspar includes orthoclase and microperthite. Quartz is clear and shows undistorted extinction. Orthopyroxene is pale green in transmitted light, commonly shows well-developed elongate cleavage, and is commonly altered to hornblende, biotite, actinolite, and chlorite in various combinations (the first two minerals are of deuteric origin; the last two are hydrothermal phases). Hornblende is grass-green to brownish-green, strongly pleochroic in this value, and in part appears to be a primary mafic mineral. Biotite is strongly pleochroic, greenish- to reddish-brown, and is commonly altered to chlorite with minor sphene. Zircon is euhedral, doubly terminated, and forms stubby laths up to 30 X 20 microns. Apatite is also euhedral, forming slender prisms up to 50 X 5 microns. Chevkinite or allanite is pleochroic from light brownish-yellow to nearly opaque reddish brown; it forms highly irregular crystals, often with embayments and crenulations, up to 50 X 30 mm.

HYDROTHERMAL ALTERATION AND MINERALIZATION: The rock is sparingly altered and mineralized. Potassium feldspar and quartz are largely unaffected. Plagioclase is locally and very weakly altered to illite, epidote, and chlorite. All the primary and deuteric mafic mienrals are locally altered to chlorite, with or without traces of epidote. Chloritized biotite inevitably contains a trace of sphene.

Vein fragments occur in this sample, but they are very rare. They comprise quartz, epidote, actinolite, and a trace of ferroaxinite in various combinations.

FLUID INCLUSIONS: Moderately abundant in primary quartz and feldspar, and of certain secondary origin. Most are either single phase vapor or two-phase, liquid plus vapor with vapor dominant. By contrast with those in the Angeli-1 felsite, few of the inclusions in this sample contain daughter crystals. All are equant and isotropic, probably halite and/or sylvite. The birefringent daughters of the Angeli-1 sample are absent.