Fluid Inclusion Analysis At Valles Caldera - Sulphur Springs Geothermal Area (Sasada & Goff, 1995)

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Exploration Activity: Fluid Inclusion Analysis At Valles Caldera - Sulphur Springs Geothermal Area (Sasada & Goff, 1995)

Exploration Activity Details
Location Valles Caldera - Sulphur Springs Geothermal Area
Exploration Technique Fluid Inclusion Analysis
Activity Date - 1995

Usefulness useful
DOE-funding Unknown

Exploration Basis
A fluid inclusion study of hydrothermal minerals and quartz phenocrysts from core hole VC-2A was undertaken to examine evolutionary processes of the hydrothermal system involved in the formation of the vapor zone that exists below Sulphur Springs.
Microthermometric data were collected from 618 fluid inclusions in hydrothermal quartz, fluorite, and calcite and magmatic quartz phenocrysts in intracaldera tuffs. Homogenization temperatures (Th) and final melting point of ice (Tm, 183 inclusions) were measured using a USGS heating and freezing stage accurate to +/- 2°C on heating runs and +/- 0.1°C on freezing runs. Bubble behavior of select inclusions (35 inclusions) was observed under the microscope during crushing to semiquantitatively estimate their CO2 content. Primary and secondary aqueous inclusions identified in hydrothermal minerals are generally low salinity, two phase (vapor and liquid), liquid-rich and/or vapor-rich inclusions when observed at surface conditions. Cogenetic liquid-rich and vapor-rich inclusions encountered in samples collected from twelve depth intervals indicate boiling of the geothermal fluids (except in samples from 522 m depth). Homogenization temperatures of these inclusions are several tens of degrees higher than the present thermal profile, and indicate that the paleo-water table dropped approximately 320 m to its present depth of 120 m sometime after hydrothermal molybdenite was precipitated from liquid water between 25-125 m depth at \leq 0.66 Ma. Secondary halite-saturated, three phase (vapor, liquid, and halite), liquid-rich inclusions were only observed along healed fracture planes in quartz phenocrysts from 363 m depth. The variability in the salinities of inclusions from the hydrothermal minerals and the high salinities of the inclusions from the quartz phenocrysts are inferred to have resulted from intensive boiling of the hydrothermal fluid caused by rapid decompression of the reservoir, likely associated with the sudden drop in the water table that occurred as the intracaldera paleolake drained away when the southwestern wall of the caldera was breached at about 0.5 Ma. This event initiated erosion of at least 200 m of the overlying caldera-fill strata, and resulted in a shift to vapor-dominated conditions in the upper levels of the Sulphur Springs reservoir as liquid-stable conditions retreated towards greater depths.

Additional References