| Title | TRACING FLUID SOURCES IN THE COSO GEOTHERMAL SYSTEM USING FLUID-INCLUSION GAS CHEMISTRY |
|---|---|
| Authors | Susan Juch Lutz , Joseph N. Moore, Michael C. Adams, David I. Norman |
| Year | 1999 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Coso |
| Abstract | 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. |