Collaborative Research: Time-Dependent Hydrothermal Convection within the Great Basin Nevada

New Mexico Institute Of Mining And Technology, Socorro NM

Investigators

Abstract

Groundwater flow systems within the Great Basin, Nevada are remarkable in many respects. Despite the arid conditions and relatively low permeability volcanic rocks and basin fill, this region hosts active geothermal systems and world class Eocene age gold deposits. Temperature profiles, fluid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems share many common features including the absence of a clear magmatic fluid source, discharge areas restricted to fault zones, and remarkably high temperatures ( >200 °C) at shallow depths (200-1500 m). Many of the Great Basin geothermal systems exhibit some of the highest shallow crustal heat flow levels ever recorded ( > 2000 mW/m2 at Beowawe) within the continental crust. Geochemical and isotopic data collected at the Beowawe geothermal system suggests that fluid circulation is deep (> 5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small ä18O (< 2.5 ?) shifts from the meteoric water line (MWL). Fossil ore-forming hydrothermal systems associated with Carlin-type gold mineralization have similar temperature patterns but exhibit fluid-rock interactions with larger ä18O shifts of 5 to 20 ? from the MWL. The goal of this proposal is to understand the three-dimensional plumbing, fluid flow impelling mechanisms, and temporal evolution of modern and fossil hydrothermal flow systems within the Great Basin. We wish to evaluate two end member questions regarding the nature of hydrothermal circulation within the Great Basin: A) Is flow restricted to high permeability fault planes driven by free-convection? or B) is flow driven by water-table topographic gradients with some combination of matrix and fault controlled fluid circulation? Because of the lack of broad low heat flow anomalies adjacent to Great Basin geothermal fields, we suspect that these flow systems must be episodic in nature due to permeability reduction associated with silica mineralization. By including the systematics of silica precipitation (and associated porosity/permeability reduction), we hope to constrain the duration of these geothermal systems. 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 develop geologically/geophysically constrained, three-dimensional hydrothermal models of the modern Beowawe and fossil Carlin geothermal systems; 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. A unique feature of the proposed work is that 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 test our models. We will also develop more sophisticated reactive-transport geochemical models using PFLOTRAN incorporating porosity-enhancing carbonate dissolution reactions to constrain how long the Carlin flow systems remained active before becoming clogged by gangue mineralization. By dating organic matter (pollen) within the hot springs deposits at Beowawe using 14C dating methods, we hope to determine whether or not these hot springs deposits formed during a single event or in several episodes. Our study may help document the existence of time-dependent natural convection systems within the Great Basin. Understanding the mechanisms and patterns of fluid circulation within this region is of great societal relevance because this region may soon host our nation?s high level nuclear wastes. The project will support two graduate students at New Mexico Tech and University of Missouri at Columbia.