Collaborative Research: Time-Dependent Hydrothermal Convection within the Great Basin, Nevada
Is groundwater flow restricted to high permeability fault planes driven by free-convection? Or, is flow driven by water-table topographic gradients with some combination of matrix and fault controlled fluid circulation? Are these systems fundamentally transient and self-sealing due to gaunge mineralization, or , is permeability enhanced by carbonate dissolution? To evaluate these problems we will develop a suite of 3D, single-phase, hydrothermal models using a new parallel finite element code (PGEOFE) for two field sites within the Great Basin. We will utilize a new three-dimensional (3-D) geologic model of the modern Beowawe geothermal system and, with USGS colleagues, develop another 3-D geologic model of the Eocene Carlin system; two sites with rich , isotopic, geochemical and geothermal data sets. Using LaGrit mesh generation software, these hydrogeologic models will honor known fault geometries, widths, and stratigraphy. This mesh generation software will allow us to honor low grid aspect ratios (required by the AZTEC parallel solver package) as the grid transitions from strata to faults while allowing sufficient refinement to minimize numerical grid dispersion when solving the heat and mass transport equations. We will use multiple constraints including temperature profiles, shallow heat flow maps, fluid/rock ä 18O composition, and the age of hot springs deposits to constrain our models. At Beowawe, by dating pollen in the sinter terrace and adualria in exposed veins nearby we can get a sense of the timing of episodic discharges of hot fluid up the faults and on the duration of such discharges recorded by the sinter deposits. More sophisticated reactive-transport geochemical models will be developed using PFLOTRAN to evaluate porosity-enhancing carbonate dissolution reactions and ?sealing gangue precipitation reactions in each system. Our models will be validated by comparing modern and fossil geochemical and thermal proxies . The project will support two graduate students at New Mexico Tech and University of Missouri at Columbia. Initial results will be presented to industry representatives from Beowawe power plant, Barrick and Newmont gold mines for their input and feedback to improve the final result. Results will be published at the Geological Society of Nevada 2010 symposium and scientific journals to reach local and international audiences.
