Vein and alteration assemblages from eight Coso wells have been collected and their fluid-inclusion gases analyzed by quadrupole mass spectrometry. Four major types of alteration were sampled: 1)
young calcite-hematite-pyrite veins; 2) wairakite or epidote veins and alteration that are spatially associat-ed with deep reservoirs in the main field and eastern wells; 3) older sericite and pyrite wallrock alteration;
and 4) stilbite-calcite veins that are common in cool-er or marginal portions of the geothermal area.

The gas compositions of the fluid inclusions display systematic differences among the secondary assem-blages. The highest N 2 /Ar ratios are found in some calcite vein samples (N 2 /Ar up to 5173) and in epidote assemblages (N 2 /Ar ratios of 1000 to 1427). These high N 2 /Ar ratios suggest the presence of a magmatic component in the fluids that produced these assem-blages. Sericite-pyrite altered samples, and stilbite and calcite-hematite veins have fluid-inclusion gas compositions with N 2 /Ar ratios less than 100, indicat-ing that these assemblages formed from meteoric flu-ids. High CO 2 and low H 2 O contents (less than 98 mol %) of inclusions in wairakite and sericite-bearing assemblages suggest deposition from boiling fluids. Low CO 2 /CH 4 ratios of inclusions in stilbite veins indicate formation from crustal fluids.

The gas chemistry reflects mixing between two end-member fluids; one enriched in magmatic volatiles, and a second represented by meteoric fluids. With the exception of the epidote assemblage, the fluid-inclu-sion gases plot in the basaltic field on a He-N 2 -Ar diagram. This signature is similar to the present-day gas analyses from steam samples taken from both the Devilís Kitchen fumarole area and from Coso produc-tion wells.

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