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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 89557-0088
Greg B. Arehart
Department of Geosciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, NV 89557-0088
James E. Faulds and Larry J. Garside
Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557-0088
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
*E-mail, [email protected]
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. 1063–1081.
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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 2007. 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
Great Basin geothermal activity and gold deposits
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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
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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
Great Basin geothermal activity and gold deposits
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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 100–160°C; grey circles
have maximum temperatures > 160°C. ECSZ = eastern California shear zone. Figure taken from Faulds et al. (2004).