Fluid inclusion gas composition of some mineral deposits and a geothermal area

Abstract

Gas compositions of fluid inclusions in minerals from hypothermal tungsten and gold quartz veins and from a geothermal area were analyzed by the combination of a vacuum extraction system and gas chromatography. Major gas species are CO 2, CH 4 and N 2, and gas concentration varies from 1 to 10, but mostly 1 to 3, in molality. Fluid inclusions of tungsten deposits are rich in CO 2, as is expected from microscopic observations. They have a tendency to be enriched in CO 2 with a lower CH 4/CO 2 ratio in the mineralized zone, but less enriched in CO 2 with a higher CH 4/CO 2 ratio in the peripheral zone. Hypothermal gold deposits, which have a close genetic relation with granitoids, are also rich in CO 2. A deposit that occurs mainly in sedimentary rocks has a high CH 4/CO 2 ratio, presumably because of reduction by carbonaceous matter in the sedimentary rock. The same technique was applied to the study of anhydrite from a geothermal area and results were compared with the gas composition of the present geothermal fluid. H 2 concentration of the trapped fluid was estimated to be 1 × 10 −5 (mole fraction) in the center of activity and 1 × 10 −7 on the periphery. The present results indicate that the study of fluid inclusion gases can supplement temperature and salinity data of fluid inclusions, the acquisition of which is occasionally laborious.