FLUID INCLUSION EVIDENCE FOR RAPID FORMATION OF THE VAPOR-DOMINATED ZONE AT SULFUR-SPRINGS, VALLES CALDERA, NEW-MEXICO, USA

Microthermometric measurements were obtained for 618 fluid inclusions in hydrothermal quartz, fluorite and calcite and magmatic quartz pheno crysts in intracaldera tuffs from the VC-2A core hole in order to stud y evolutionary processes of the Sulphur Springs hydrothermal system in the Valles caldera. Relatively high T-h values in samples from shallo w depths indicate erosion of about 200 m of caldera fill since deposit ion of hydrothermal minerals at shallow depths in the Sulphur Springs hydrothermal system, accompanied by a descent in the water table of th e liquid-dominated reservoir. For samples collected below the current water level of the well, the minimum values of homogenization temperat ure (T-h) fit the present thermal profile, whereas minimum T-h values of samples from above the water level are several tens of degrees high er than the present thermal profile and fit a paleo-thermal profile fo llowing the boiling point curve for pure water, as adjusted to 92 degr ees C at 20 m below the present land surface. This is attributed to de velopment of an evolving vapor zone that formed subsequent to a sudden drop in the water table of the liquid-dominated reservoir. We suggest that these events were caused by the drainage of an intracaldera lake when the southwestern wall of the caldera was breached about 0.5 Ma. This model indicates that vapor zones above major liquid-dominated geo thermal reservoirs can be formed due to dramatic changes in geohydrolo gy and not just from simple boiling.