Abstract The Geysers-Clear Lake area, located about 150 km north of San Francisco, is mainly underlain by Jurassic and Cretaceous rocks of the Franciscan assemblage (composed dominantly of a melange of gray wacke and argillite with lesser amounts of altered mafic igneous rocks, radiolarian chert, serpentine, limestone, and very minor blocks of blueschist, eclogite, and amphibolite), the Great Valley sequence (mainly siltstone and argillite in the Clear Lake region), and associated ophiolitic rocks that accumulated in marine settings, and that later were deformed at an obliquely convergent subduction margin (McLaughlin, 1977,1981; McLaughlin and Ohlin, 1984; Thompson, 1989). Later strike-slip movement on northward-propagating faults of the San Andreas transform system, some of which pass through The Geysers-Clear Lake area, cut and offset the thrust sheets, and shut down subduction at the latitude of Clear Lake about 3 Ma (Atwater, 1970; Blake and others, 1978; Dickinson and Snyder, 1979). Simultaneously, behind a northward-migrating triple junction, there also has been a northward migration of volcanic centers in the Coast Ranges of California above a window of anomalously shallow asthenosphere, with the most recent volcanic activity focused in the Clear Lake region (Dickinson and Snyder, 1979; Hearn et al., 1981; McLaughlin, 1981; Fox et al., 1985).

Abstract Quartz and chalcedony are the silica minerals commonly found in hydrothermal ore deposits. However, in many places there is textural evidence that chalcedony formed after amorphous silica, probably with poorly crystalline cristobalite or opal- CT as an intermediate phase. Fournier (1973) and apparently White (1965) used the term β -cristobalite both for poorly crystalline cristobalite and for opal- CT. Poorly crystalline cristobalite shows broad X-ray diffraction peaks centered at about 4.1 and 2.5 A. Opal-CT also shows these same X-ray peaks plus an additional low-tridymite peak at about 4.3 A (Jones and Segnit, 1971). Quartz is the stable form of silica at pressure- temperature conditions found in convecting hydrothermal systems. Faceted quartz crystals generally grow in solutions that are not greatly supersaturated with silica, indicating relatively slowly changing conditions. In contrast, the deposition of amorphous silica requires high degrees of silica supersaturation with respect to quartz, and generally indicates large and rapid changes in the physical or chemical nature of the solution. These large and rapid changes may also affect the capacity of a solution to transport and deposit ore. There are various ways to bring about this supersaturation such as rapid cooling (generally with decompressional boiling), mixing of different waters, pH changes, and reaction of the solution with volcanic glass. Each of these processes will be discussed in the subsequent sections. Much of what follows is taken from Fournier (1985).

Abstract The factors affecting the transport and deposition of carbonate in hydrothermal systems have been discussed in detail by Holland and Malinin (1979). Solubilities of carbonates are strongly influenced by pH, P CO2 temperature, and the presence of other dissolved salts. The alkali carbonates, Na, K, and Li, are relatively soluble at all temperatures and generally precipitate only where there is extreme evaporation. In contrast, the alkaline earth carbonates, Ca, Mg, Sr, and Ba, are moderately to sparingly soluble and commonly precipitate in hydrothermal systems. Calcite is by far the most abundant and important carbonate found in the epithermal environment, and more solubility data at hydrothermal conditions are available for it than for any of the other carbonates. Therefore, after briefly reviewing the system CO 2 -water, the discussion will focus on the transport and deposition of calcite in hydrothermal solutions. The behaviors of other moderately to sparingly soluble carbonates in hydrothermal solutions are similar to that of calcite.