TRACING FLUID SOURCES IN THE COSO GEOTHERMAL SYSTEMUSING FLUID-INCLUSION GAS CHEMISTRY

摘要

Vein and alteration assemblages from eight Cosowells have been collected and their fluid-inclusiongases analyzed by quadrupole mass spectrometry.Four major types of alteration were sampled: 1)young calcite-hematite-pyrite veins; 2) wairakite orepidote veins and alteration that are spatially associat-edwith deep reservoirs in the main field and easternwells; 3) older sericite and pyrite wallrock alteration;and 4) stilbite-calcite veins that are common in cool-eror marginal portions of the geothermal area.The gas compositions of the fluid inclusions displaysystematic differences among the secondary assem-blages.The highest N 2 /Ar ratios are found in somecalcite vein samples (N 2 /Ar up to 5173) and in epidoteassemblages (N 2 /Ar ratios of 1000 to 1427). Thesehigh N 2 /Ar ratios suggest the presence of a magmaticcomponent in the fluids that produced these assem-blages.Sericite-pyrite altered samples, and stilbiteand calcite-hematite veins have fluid-inclusion gascompositions with N 2 /Ar ratios less than 100, indicat-ingthat these assemblages formed from meteoric flu-ids.High CO 2 and low H 2 O contents (less than 98mol %) of inclusions in wairakite and sericite-bearingassemblages suggest deposition from boiling fluids.Low CO 2 /CH 4 ratios of inclusions in stilbite veinsindicate formation from crustal fluids.The gas chemistry reflects mixing between two end-memberfluids; one enriched in magmatic volatiles,and a second represented by meteoric fluids. With theexception of the epidote assemblage, the fluid-inclu-siongases plot in the basaltic field on a He-N 2 -Ardiagram. This signature is similar to the present-daygas analyses from steam samples taken from both theDevil’s Kitchen fumarole area and from Coso produc-tionwells.