SAMPLE DESIGNATION: Well Angeli-1, measured depth interval 11,400-11,420 ft, or 3474.6-3480.6 m. True vertical depth interval is 11,343-11363 ft, or 3457.2-3463.3 m.

SAMPLE LOCATION: Northeast-central Geysers, on the western flank of the 1.2-1.1 Ma Cobb Mountain volcanic center.

SUMMARY AND INTERPRETATION: This sample is from one of the deepest (if not the deepest) felsite penetrations in The Geysers. It is quite representative of granite throughout the felsite, but also contains a few chips of the felsite’s other main rock type, granodiorite. Based on relationships between these two phases in other deep wells in this part of the steam field, it is believed likely that the main mass of the granodiorite is less than 100 m below the sample depth. The granodiorite chips in this mostly granite sample probably represent dikes.

SAMPLE TYPE: Drill cuttings, up to 0.6 mm in diameter. The relatively large size of many of the chips suggests that air-drilling was not applied in this well, at least in this depth realm.

BACKGROUND INFORMATION: The sample was retrieved from approximately 100 ft (30 m) below the contact between felsic plutonic rock and overlying hornfels (metagraywacke precursor). The hole, a non-producer of steam, was terminated at 11,460 ft (3492.8 m), equivalent to a TVD of 11,403 ft (3552.3 m). The sample contains abundant rust, lost-circulation material, cement, and other contaminants. Rock and mineral percentages have been recalculated to eliminate these introduced materials.

ROCK TYPES: There are two igneous rock types in this sample. The dominant (90%) one is biotite-orthopyroxene + hornblende granite; the other is orthopyroxene-biotite-hornblende granodiorite (8%). Hornfels chips in the sample (2%) are probably caved debris. The granite consists of approximately 38% quartz; 30% plagioclase; 20% K-feldspar; 2% each biotite, orthopyroxene, and tourmaline; 1% each hornblende, actinolite, epidote, chlorite, and illite; and traces of apatite, zircon, sphene, and an unknown, acicular, possible iron-titanium phase. Its average grain size exceeds that of the cuttings diameters, and is therefore >0.6 mm; based on the geometries of grain boundaries in composite-grain drill chips, it is estimated that the rock is fine-grained, hypidiomorphic-granular, and perhaps 1-1.5 mm in average grain diameter. It may or may not be porphyritic. The grains are interpreted to be subequant, with smooth or straight grain boundaries. Potassium feldspar in the granite includes both orthoclase and "string" microperthite. Plagioclase is commonly twinned but uncommonly and simply zoned; it is also relatively free of fluid or solid inclusions; some is partially altered to tourmaline, chlorite, and illite. Orthopyroxene is pale green in transmitted light; it is commonly partially altered (deuterically) to pleochroic pale to medium grass-green hornblende with or without greenish- to reddish-brown biotite and gray-green chlorite. There are a few grass-green hornblende grains which are monomineralic, but these may well be wholly replaced orthopyroxene. Biotite grains are pleochroic from pale, slightly greenish-brown to nearly opaque, slightly greenish- to reddish brown; some is partially altered to chlorite and microcrystalline sphene.

The granodiorite is texturally distinct from the granite in larger chips. The distinction is impossible to make for chips smaller than 50 microns in diameter. The granodiorite is fine-grained, subhedral-granular, and has ragged-edged plagioclase laths arrayed in a decussate, or pseudo-diabasic texture. The plagioclase, which is the main mineral in this rock, is also very turbid-appearing due to an abundance of fluid and solid inclusions. These solid inclusions comprise hornblende, biotite, and chlorite after both of these primary mafics. Hornblende, orthopyroxene, and biotite are the same color and texture as in the granite, but are finer-grained and tend to be subhedral to euhedral. These three mafics also account for up to 12% of this more mafic rock type, as opposed to about 5-7% in the granite. The granodiorite also contains traces of very acicular apatite, finer-grained than its counterpart in the granite.

HYDROTHERMAL ALTERATION AND MINERALIZATION: Both the granite and granodiorite chips are only weakly altered, with the granodiorite the more altered of the two rock types. Alteration consists of partial chloritization of mafics (with neoformation of sphene after biotite); and weak tourmalinization, illitization, and chloritization of plagioclase, particularly in granodiorite. There are numerous chips of hydrothermal veins, and some of the igneous rock chips are similarly veined. Vein minerals include quartz, tourmaline, epidote, ferroaxinite, chlorite, prehnite, and actinolite.

FLUID INCLUSIONS: Primary quartz and plagioclase in both igneous rock types contain abundant, large (up to 15 microns) fluid inclusions, some of which are apparently pure vapor, while others contain a large vapor bubble and numerous daughter crystals. Two separate isotropic cubic daughters are present in some inclusions. Others contain one such phase and one or two birefringent daughters which remain to be identified. Quartz in the vein chips contains single-phase vapor inclusions and two-phase, liquid plus vapor inclusions without daughter products. Large vapor bubbles in the latter suggest high entrapment temperatures.