SAMPLE DESIGNATION: Well LF-48, depth 8089 ft (2465.4 m)

SAMPLE LOCATION: Central Geysers steam field, about 4 km southwest of Cobb Mtn., and about 2.5 km southwest of the corresponding felsite sample from well Angeli-1.

SUMMARY AND INTERPRETATION: Biotite-orthopyroxene granite porphyry, very fine-grained, with phenocrysts of quartz, potassium feldspar, oligoclase, orthopyroxene, and biotite scattered throughout a generally equigranular, blocky-textured granitic matrix. Many of the phenocrysts are highly corroded-appearing, and some even appear fragmentary, like xenocrysts. Suspect they were engulfed in a younger magma with which they were out of equilibrium. Microcrystalline aplitic stringers in the matrix, some of which wrap around these phenocrysts, could alternatively be interpreted as quench textures, which would be in line with the alleged disequilibrium textures of the phenocrysts discussed above.

This particular core presents a quandary, because drill chips from several hundred m above the coring site are principally granodiorite of an entirely different texture, composition, and color index. If we assume that the cuttings were properly collected and truly representative of their drilled interval, this relationship would suggest either that (1) the core represents a granite xenolith in the granodiorite; or (2) the well passed through a large mass of granodiorite, and was just getting into the granite again at the depth the core was collected (the bottom of the hole). Intrusive relationships in other felsite-penetrating wells in the area suggest that alternative [2] is the more likely. In either case, it would seem likely that the younger granodiorite would have great potential to "reset" the age of the cored granite.

SAMPLE TYPE: Diamond drill core (conventional rotary, not wireline).

BACKGROUND INFORMATION: One of three felsite cores retrieved by Unocal Geothermal during the early 1990's. The others are from wells GDC-12 and DV-2.

ROCK TYPE: Leucocratic, biotite-orthopyroxene granite porphyry, consisting of 8% conspicuously larger phenocrysts (described below) embedded in a fine-grained granitic matrix. Phenocrysts comprise quartz, orthoclase and microperthite, oligoclase, biotite, and orthopyroxene. Quartz phenocrysts range from 2 to 6 mm (avg. 3 mm), are irregular to rounded and strongly embayed, commonly with very ragged borders invaded by the matrix material. Orthoclase and microperthite phenocrysts are anhedral to subhedral, 2-5 mm in diameter, and like quartz also have commonly corroded, ragged-appearing margins; some are irregular and could be fragmentary xenocrysts. Many are characterized by simple (Carlsbad?) twinning. These potassium feldspar phenocrysts commonly form glomerocrysts with others of their kind or with oligoclase, biotite, and orthopyroxene in various combinations. Oligoclase phenocrysts are similar in shape and size to those of potassium feldspar, but many show ultrafine albite twinning and simple concentric zoning (the former extending through the latter in many cases). Orthopyroxene phenocrysts (up to 2 mm in maximum dimensions) are lath-like to highly irregular, the former tending to be elongate, with a typical aspect ratio of 4:1. The pyroxene is nearly colorless in transmitted light, but shows a subtle pleochroism from peach to pale gray green. All examples of this mineral in this rock are highly corroded, and many are intergrown with biotite and, less commonly, other phenocryst phases. Biotite phenocrysts are smaller than the others, averaging 0.8-1.2 mm in diameter. They are pleochroic, medium orange-brown to deep, nearly opaque orange brown. All are ragged-appearing.

The groundmass of this rock has an average grain size of 0.2-0.3 mm. It consists mainly of quartz, orthoclase, and oligoclase with minor biotite and orthopyroxene as well as traces of ilmenite, zircon, and apatite. The groundmass has an equigranular, anhedral to subhedral texture, locally appearing "blocky". Quartz is apparently the latest phase to have crystallized, and is invariably anhedral’ the feldspars are subhedral; biotite and orthopyroxene are also anhedral for the most part. Apatite forms euhedral prisms ranging from 0.01 to 0.15 mm in length, with aspect ratios of 4:1 to 10:1. Locally throughout the groundmass, there are irregular patches which are much more finely crystalline, averaging as little as 0.03 mm. These appear to be aplitic segregations, but where they clearly surround phenocrysts could also be quench textures.

The estimated primary mineral composition of this granite porphyry is as follows: quartz – 38% oligoclase 27%; potassium feldspar 30 %; orthopyroxene – 2%; biotite – 2%; ilmenite – 1%; apatite and zircon – trace.

HYDROTHERMAL ALTERATION AND MINERALIZATION: Weak in this sample, although moderate in other sections of the core. The most obvious is microcrystalline, green to brown, secondary biotite and bright green acicular actinolite (along with a trace of epidote) partially replacing some orthopyroxene, and secondary biotite and chlorite, with minor ilmenite, partially to totally replacing primary biotite. Some of the oligoclase phenocrysts and presumably a portion of that mineral in the matrix have been replaced partially by secondary potassium feldspar. Potassium feldspar whether in phenocrysts or groundmass has a "dusted" appearance, probably due to micron-scale alteration and/or myriad sub-micron-sized fluid inclusions. Oligoclase is locally very sparsely altered to flakes and fibers of illite or microcrystalline muscovite.

The rock is sparsely cut by microveinlets consisting principally of the same actinolite described above, with minor chlorite, epidote, and ferroaxinite. The chlorite appears to replace rather than be intergrown with the actinolite.

FLUID INCLUSIONS: Abundant in quartz, all of secondary origin, up to 20 microns in diameter but typicall <5 microns in diameter. Three principal varieties: (1) Exclusively or dominantly vapor, the latter with a trace of liquid: (2) Two-phase, liquid plus vapor, with a 2 or 3/1 L/V ratio; (3) Three- and four-phase inclusions, with liquid, a large vapor bubble, and up to three daughter crystals – an isotropic salt, a pale-brown birefringent phase, and an apparently fibrous phase (biotite?).