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1 Geothermal systems in the Great Basin, western United States: Modern analogues to the roles of magmatism, structure, and regional tectonics in the formation of gold deposits Mark F. Coolbaugh* Great Basin Center for Geothermal Energy, MS 178, University of Nevada, Reno, NV Greg B. Arehart Department of Geosciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, NV James E. Faulds and Larry J. Garside Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV ABSTRACT Western North America produces over one-third of the world s geothermal power, and significant increases in power production are expected as additional plants come on line. Many geothermal systems in western North America derive their heat from magmas or cooling intrusions that occur in variety of tectonic settings, including a triple junction, volcanic arc, hot spot, and pull-apart zones in strike-slip systems. The interior of the Great Basin however, is characterized by widespread amagmatic geothermal activity that owes its existence to high crustal heat flow and active extensional tectonics. Even though magma-heated geothermal fluids have higher concentrations of some trace metals, including As, Li, B, and Cs, than extensional (amagmatic) fluids, both fluid types in the Great Basin have recently, or are currently, depositing gold. Quaternary to Pliocene-aged gold deposits with adjacent high-temperature ( 150 C) active geothermal systems occur at Long Valley, California, and Florida Canyon, Wind Mountain, Dixie Valley, and other locations in Nevada. Prolonged uplift of mineralized zones along range-front faults suggests that extensional systems, although possibly episodic, have lifetimes measured in millions of years. The total known gold inventory in deposits younger than 7 Ma in the Great Basin exceeds 12 million ounces. Many Great Basin geothermal systems are aligned along northeast-trending belts hundreds of kilometers long that are likely related to ongoing northwest-directed crustal extension. However, the highest-temperature extensional systems and the most productive young gold deposits are aligned along northwest trends sub-parallel to the dextral Walker Lane shear zone. A transitional transtensional setting in which rightlateral fault motion along the Walker Lane splays into extensional northeast-striking normal fault systems may promote deep fracturing and the circulation and heating of meteoric fluids to form hydrothermal systems. Key Words: Great Basin, geothermal, gold, Quaternary, mineral deposits * , mfc@unr.nevada.edu Coolbaugh, Mark F., Arehart, Greg B., Faulds, James E., and Garside, Larry J., 2005, Geothermal systems in the Great Basin, western United States: Modern analogues to the roles of magmatism, structure, and regional tectonics in the formation of gold deposits, in Rhoden, H.N., Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, p

2 1064 Mark F. Coolbaugh, Greg B. Arehart, James E. Faulds, and Larry J. Garside INTRODUCTION Western North America is richly endowed with Cenozoic metallic ore deposits, including world-class porphyry copper/ molybdenum districts and Carlin-type sedimentary rock-hosted, disseminated gold deposits. Of major importance, western North America is also richly endowed in active geothermal systems. Approximately 20 geothermal systems supply over a third of the world s geothermal electrical energy, even though several major systems, including those at Yellowstone and Lassen Volcanic National Parks, are withdrawn from development. Western North American geothermal systems occupy diverse tectonic settings, including a slab gap induced by a migrating triple junction (The Geysers), an active volcanic arc (Mt. Lassen and Mt. Meager in the Cascades), a possible hot spot (Yellowstone), major pull-aparts in a strike-slip fault system (Cerro Prieto and the Salton Sea along the San Andreas fault system), and a broad zone of enhanced heat flow and crustal extension (Great Basin). Included are some of the largest known continental geothermal systems in the world. The Geysers and Cerro Prieto are ranked first and second in world geothermal power producing capacity, but only because the geothermal potential of Yellowstone is untapped. The Salton Sea system may shift from fourth to third, if a new power plant comes on line as planned in In the Great Basin of the western United States, 15 geothermal systems have a combined power producing capacity of roughly 600 MWe, and 10 additional geothermal systems have demonstrated economic potential and/or are under active exploration and development. It appears that many active geothermal systems in the Great Basin are either forming gold deposits now or have done so in the recent past (i.e., Quaternary). By studying these young gold-producing geothermal systems, we can gain insights into the roles that magmas or deeply circulating meteoric fluids play in providing heat and metals to hydrothermal systems. The tectonic settings and structural controls of modern geothermal systems offer examples of how mineral belts form and the structural conditions necessary for mineral deposition. The Great Basin has a long history of paleogeothermal activity and associated gold mineralization. Similar environments that fostered deep circulation and deposition of metals in the past may have generated epithermal ore deposits. GREAT BASIN GEOTHERMAL SYSTEMS Two types of geothermal systems, magma-heated and extensional, in the Great Basin have temperatures and fluid flows sufficient to support power plants. As described by Koenig and McNitt (1983) and Wisian et al. (1999), magmaheated systems are those geothermal systems closely associated with young ( 1.5 Ma) silicic volcanic rocks along the margins of the Great Basin, whereas extensional-type systems occur throughout the Great Basin (Fig. 1) and are not associated with volumetrically significant young volcanic rocks that could have provided a source of heat. In most places of the world, convective geothermal systems do not attain temperatures of 200 C or higher without an upper crustal magmatic heat source (Arehart et al., 2003). The Great Basin appears to be an exception; 6 known extensionaltype geothermal systems with no known magmatic affinity have measured or estimated temperatures exceeding 200 C and 17 known systems have measured or geochemical temperatures exceeding 180 C. Active extensional tectonics and high crustal heat flow (Koenig and McNitt, 1983; Wisian et al., 1999) may allow meteoric fluids to penetrate along permeable fractures to greater-than-normal depths into hotter-than-normal crust to reach these anomalous high temperatures. The ultimate cause of high heat flow in the Great Basin is debatable, and previous authors have attributed it to highly attenuated crust associated with extension (Lachenbruch and Sass, 1977, 1978), passing of the Yellowstone hot spot (Suppe et al., 1975), and mantle upwelling (Lachenbruch and Sass, 1977). Recent evidence for lower crustal magma injection beneath Lake Tahoe (Smith et al., 2004) and along the Rio Grande rift in New Mexico (Cordell and Kottlowski, 1975; Fialko and Simons, 2001) lend credence to suggestions by Blackwell (1983) and Lachenbruch and Sass (1978) of lower crustal basaltic sill emplacement as a means of at least locally supplying the high heat flow. In any case, high heat flow alone does not appear sufficient to explain the unusual concentration of relatively high-temperature geothermal systems in the Great Basin because similarly high heat flow is found inboard of the western continental margin throughout western North and Central America, from Alaska to Costa Rica (Blackwell and Richards, 2004). Instead, it is believed that active extensional tectonics play a key supporting role in providing the fracturing and permeability necessary for fluid circulation to form economic high-temperature (defined here as temperatures 150 C) extensional geothermal systems (Koenig and McNitt, 1983; Wisian et al., 1999). Many extensional geothermal systems in the Great Basin, including those at Desert Peak, Blue Mountain, Soda Lake, Stillwater, and the Fish Lake Valley, do not exhibit surface manifestations of geothermal activity such as hot springs or fumaroles that would indicate the presence of a subsurface geothermal reservoir. These geothermal systems were originally discovered through drilling of water, oil, temperature gradient, or mineral exploration wells. Factors conspiring to conceal geothermal activity include deep water tables, near-surface impermeable cap rocks, and laterally flowing groundwater in aquifers that can capture, dilute, and/or entrain rising geothermal fluids (Sass et al., 1971). Because of these factors, Coolbaugh and Shevenell (2004) estimated that potentially economic, but undiscovered, geothermal resources in Nevada were several times those currently known. Extensional geothermal systems occur over a large portion of the Great Basin (Fig. 1), and it can be challenging to ascertain the location of the undiscovered blind systems. However, many features, including the presence

3 Great Basin geothermal activity and gold deposits 1065 Figure 1. Active geothermal systems of the Great Basin. Magma-heated geothermal systems are those occurring adjacent to young silicic volcanic rocks < 1.5 Ma. Extensional geothermal systems occur elsewhere. B-A = Black Rock-Alvord Desert trend, N-R = Newcastle-Roosevelt trend. HSZ = Humboldt structural zone. of active faults, recent volcanic activity, earthquakes, high gravity gradients, and high temperature gradients, are useful for predicting geothermal activity and thus the presence of blind geothermal systems, but none of these features are perfect in terms of their uniqueness or, in the case of drilling, their cost effectiveness. More geothermal systems are waiting to be discovered, and some of them are likely be found while drilling for precious metals, because, as described below, young gold/silver mineralization and geothermal systems sometimes occur together and evidence suggests that the former is being produced by the latter. STRUCTURAL ENVIRONMENT OF GEOTHERMAL SYSTEMS IN THE GREAT BASIN Regional structure From a plate tectonics perspective, crustal strain is currently focused on the margins of the Great Basin, as evidenced by global positioning system (GPS)-based geodetic velocity measurements (Kreemer et al., 2004) and earthquake activity (Fig. 2). Quaternary silicic volcanic activity and magma-heated geothermal systems are restricted to these same margins (Fig. 1). A broader and more diffuse zone of extension characterizes the interior of the Great Basin, as evidenced by basin and range-style deformation. A clockwise rotation of the direction of extension is indicated by geodetic velocity studies (Bennett et al., 2003; Hammond and Thatcher, 2004, 2005) and by the fact that the long axes of horsts and grabens gradually shifts from north-northwest in the northeastern Great Basin, to northsouth in the central Great Basin, to north-northeast in the northwestern Great Basin (Fig. 1). The origin of extension in the Great Basin remains somewhat controversial; possible causative mechanisms include back-arc spreading behind Cenozoic volcanic arcs (e.g. Karig, 1971), gravitational collapse of thickened crust into regions of thinner crust (e.g. Coney and Harms, 1984), mantle upwelling beneath the Great Basin (Gans et al., 1989), and tensional shadowing by thick crust in Colorado and Wyoming of east-west-directed compression on the North American plate (Humphreys and Dueker, 2004). Crustal deformation in the Great Basin acquires a transtensional character near its western and northwestern margins, due

4 1066 Mark F. Coolbaugh, Greg B. Arehart, James E. Faulds, and Larry J. Garside Figure 2. Crustal extension and earthquakes are focused along the margins of the Great Basin. All earthquakes of magnitude 4.0 and higher are shown as small black dots. Warmer colors indicate relatively greater amounts of crustal dilation. The color scale was derived by combining (adding) crustal dilation estimated from Quaternary faults (Kreemer et al., 2004; Machette et al., 2003) to crustal dilation estimated from geodetic global positioning system (GPS) measurements (Kreemer et al., 2004; Blewitt et al., 2003). Adding fault rates to GPS-derived rates provides a means of obtaining a geographically more representative estimate of crustal extension. Higher rates of dilation occur along the western and eastern margins of the Great Basin. Note the broad area of greater extension predicted for the northwestern Great Basin, and its general correlation with region of transfer between strike-slip faulting and normal faults (Fig. 3), higher temperature geothermal systems (Fig. 4) and young gold deposits (Fig. 5). to the influence of dextral strike-slip faulting along the Walker Lane (Fig. 3). Recent findings (Faulds et al., 2004) suggest that the amount of strike-slip motion along the Walker Lane decreases northward, as strike-slip motion is transferred into a broad zone of north-northeast-trending normal faults in the Humboldt structural zone, the central Nevada seismic belt, and other structures in northwestern Nevada, northeastern California, and southeastern Oregon (Faulds et al., 2004; Fig. 3). Geothermal patterns The greatest concentration of high-temperature geothermal systems occurs in the northwestern quarter of the Great Basin (Fig. 4), broadly coincident with the transition from Walker Lane-style transtension to the more regional west-northwestdirected extension (Figs. 2, 3). Some Great Basin geothermal systems are aligned along northeast-trending belts hundreds

5 Great Basin geothermal activity and gold deposits 1067 Figure 3: Dextral strike-slip motion from the Walker Lane is transferred into a series of northeast-striking normal faults in the northern and northwestern Great Basin. Those normal faults are concentrated in the Humboldt structural zone (HSZ), the central Nevada seismic belt (CNSB), and the Black Rock Desert (BRD) and Surprise Valley (SV) belts. Geothermal fields cluster in the northwestern Great Basin, directly northeast of the northwest terminus of the Walker Lane. White circles are geothermal systems with maximum temperatures of C; grey circles have maximum temperatures > 160 C. ECSZ = eastern California shear zone. Figure taken from Faulds et al. (2004).