AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geochemical Modeling of the Near-Surface Hydrothermal System Beneath the Southern Moat of Long Valley Caldera, California
(1) University of Nevada, Reno, Reno, NV.
Geochemical reaction path and mass balance modeling techniques were used to test the hypothesis that an eastwardly flowing plume of thermal water in the southern moat of the Long Valley caldera system reacts with hydrothermally-altered intra-caldera tuffs and mixes with non-thermal groundwater. Our conceptual model was based on hypotheses in the literature and utilized published geochemical and petrologic data. Mixing of thermal and non-thermal waters and reaction with wall rock were simulated using the reaction path code EQ3/6. Mass balance calculations were conducted to estimate the extent of water-rock interaction between the intra-caldera tuffs and fluids. A mixing ratio of 82% thermal and 18% non-thermal water closely matches fluid compositions in Casa Diablo fluids as well as minerals observed in petrographic studies. Results of this study show that the mineralogy and fluid chemistry observed in the shallow reservoir at Long Valley caldera is formed by an open system of fluid-rock interaction. Further, calcite precipitated in the system serves as a sink for high levels of CO2 generated by the deeper magmatic system. Our study shows that processes acting in a geothermal system can be effectively quantified using geochemical modeling and mass balance calculations.