SAMPLE DESIGNATION: Well GDC-5, measured depth interval 7800-7820 ft, or 2377.3-2383.4 m. True vertical depth interval is 7681-7701 ft, or 2341.1-2347.2 m.

SAMPLE LOCATION: Southwest-central Geysers, near the southwestern margin of the productive steam reservoir.

SUMMARY AND INTERPRETATION: Fine-grained, possibly porphyritic, biotite-orthopyroxene granite, which has undergone moderate potassic and other hydrothermal alteration. The chief alteration mineral is secondary biotite, replacing principally the primary variety but also orthopyroxene. Primary biotite which has been so altered was also locally and subsequently altered to a mix of hydrothermal albite (?) and ilmenite.

SAMPLE TYPE: Cuttings, but by contrast with most of those from The Geysers, these are relatively large, up to a mm in diameter (avg. abt. 0.6 mm).

BACKGROUND INFORMATION: This Unocal well did not yield commercial quantities of steam. The cuttings sample described here was retrieved from nearly 300 m beneath the top of the felsite at this locale. The sample is contaminated with rust, steel, cement, and caved rock fragments from higher portions of the well. The rock and mineral percentages set forth below have been recalculated to eliminate the contamination.

ROCK TYPE: Potassically altered, biotite orthopyroxene granite. In general, the GDC-5 cuttings are smaller than the mean grain size of the granite, but textures and boundary relationships in multi-grain cuttings suggest that the rock may have an average grain size of about 0.7 mm. It may well be a porphyry, as some larger chips (>1 mm) consist entirely of quartz or feldspar. Primary mafic minerals make up 7% of this granite, which is therefore considerably less leucocratic than those sampled by well LF-23, ST-1 (see companion descriptions). This granite is hypidiomorphic-granular for the most part, with local near-graphic intergrowths of quartz and orthoclase. It consists of the following primary minerals: quartz – 35%; plagioclase – 30%; potassium feldspar – 27%; biotite – 4%; orthopyroxene – 3%; ilmenite, apatite, zircon, and sphene – trace. There are also traces of "sea-urchin"-like aggregates of an unknown acicular mineral which may be an iron-titanium oxide. This mineral also occurs in all the other samples examined for this study; its dramatic texture makes it much more likely that the age-dated granites are approximately coeval. Quartz in this rock is clear, with only a few fluid inclusions. Slightly more translucent is plagioclase, which is commonly twinned and locally shows a crude concentric zoning. Potassium feldspar – orthoclase and microperthite – is very turbid-appearing due to the presence of myriad, sub-micron-sized fluid (and solid?) inclusions. Orthopyroxene is pale pink to (most commonly) pale green in transmitted light, and many crystals have a well-developed prismatic cleavage. Most grains of this mineral are partially altered to both deuteric and secondary biotite, the distinction between which is explained below. Primary biotite grains are very pleochroic from pale orange to deep, nearly opaque orange- to reddish-brown. This biotite is widely replaced by the secondary variety, along with ilmenite, albite, and chlorite (see below). Zircon and apatite are texturally similar to their occurrence in the granite of LF-23, ST-1, but are slightly more common in this rock.

HYDROTHERMAL ALTERATION AND MINERALIZATION: Feldspars are only weakly altered; some mafic minerals extensively so. Brownish-green to grass-green, microcrystalline aggregates of secondary biotite replace principally the primary variety, but also rim and internally replace some orthopyroxene crystals. Replacing both primary and secondary biotite, but only where the two occur together, are irregular aggregates of hydrothermal albite (? – need microprobe analysis) and ilmenite, the latter as disseminated, highly irregular grains averaging about 0.15 mm in maximum dimension. Bright green, fibrous actinolite totally replaces pyroxene (? -- total replacement, but shape is suggestive). Green chlorite, with anomalous red/blue birefringence, partially to totall replaces primary and secondary biotite. Some plagioclase crystals are very sparsely replaced with microcrystalline illite flakes; others enclose alteration patches of secondary potassium feldspar; still others are marred by patches of epidote and chlorite. Primary ilmenite is altered partially to sphene. A few chips consist partially or exclusively of hydrothermal veinlet-fillings, comprising, in various combinations, adularia, quartz, epidote, actinolite, and ferroaxinite.

FLUID INCLUSIONS: Up to 20 microns in diameter, most are exclusively or dominantly vapor, and many of these form "veils" along healed fractures, preserving evidence of past boiling. A few inclusions are three-phase, with a large vapor bubble, liquid, and a cubic isotropic daughter mineral, probably halite.