Little information has been published on the hydrothermal alteration minerals occurring at depth in the Geysers steam field , California. Steiner (1958) reported the occurrence of wairakite from a well; McNitt (1964) identified pyrite , sericite , calcite , quartz, siderite , apatite and chlorite in cores of Franciscan graywacke and greenstone. Recently, Union Oil Geothermal Division furnished a set of well cores from the cap rock overlying the steam reservoir for geophysical studies (Lockner -- e t a l . , 1980); sample localities are shown in Figure 1. Cores of metagraywacke and greenstone from 4 wells---Thermal 10 (TH10) , Lakoma Fame 6 (LF6), D & V 1 (DV1) and Horner State (HS1)---were compared to unaltered Franciscan metagraywacke (GP5) from surface exposures (Table 1). Several previously unreported alteration minerals were found in the cored rocks, including epidote, tremolite - actinolite , prehnite and tourmaline. This note describes the observed alteration minerals and some of the factors that controlled their growth.

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