Inventory of structural settings for active geothermal systems and late Miocene (~8 Ma) to Quaternary epithermal mineral deposits in the Basin and Range province of Nevada

Abstract

We have completed a comprehensive inventory of the structural settings of known geothermal systems (>200 total; ≥37°C) and late Miocene (~8 Ma) to Quaternary epithermal mineral deposits in the extensional to transtensional region of Nevada. The structural settings are important to characterize, because many geothermal systems and epithermal mineral deposits are hidden beneath the surface, and thus these settings can be used as an exploration tool to find new systems or deposits. Of the known geothermal systems in Nevada, for example, ~37% are blind or hidden (no surface hot springs or fumaroles). We catalogued geothermal systems into eight major groups, based on the dominant pattern of faulting. Of the ~214 known systems in Nevada, we found that step-overs or relay ramps in normal fault zones are the most common setting, hosting ~39% of the systems. Step-overs are characterized by multiple, commonly overlapping fault strands, increased fracture density, and thus enhanced permeability. Other common settings include a) normal fault terminations (~23%), where horsetailing generates a myriad of closely spaced faults and thus increased fracture permeability; and b) fault intersections between normal faults or between normal faults and transverse oblique-slip faults (~21%), where multiple minor faults typically connect major structures, and fluids can flow readily through highly fractured, dilational quadrants. Less common settings include: a) accommodation zones (~5%); b) displacement transfer zones (~4%) at the ends of strike-slip faults; c) pull-aparts in strike-slip faults (~4%); d) bends in normal faults (~1%); and e) major range-front normal faults (~1%). Pull-aparts and displacement transfer zones are more abundant in the transtensional western part of the region within and near the dextral shear zone of the Walker Lane. Quaternary faults typically lie within or near most of the geothermal systems. Controlling faults in the systems most commonly strike north-northeast (~54% of systems) approximately orthogonal to the regional extension direction. Northerly striking faults (~N10°W to N10°E) are also relatively common and serve as the primary controlling structure in ~32% of the systems. Most late Miocene to recent epithermal mineral deposits in Nevada occupy similar structural settings. Step-overs are the most common type of setting, followed by fault terminations, accommodation zones, and displacement transfer zones. Similar to active geothermal systems, the formation of epithermal deposits is favored by complex structural settings. These data can be used to guide exploration strategies, because economic concentrations of precious metals typically occupy only a small percentage of their respective districts. The favorable structural settings for geothermal systems and epithermal mineral deposits (e.g., step-overs, fault terminations, and accommodation zone) correspond to long-term, critically stressed areas, where fluid pathways more likely remain open in networks of closely spaced, breccia-dominated fractures. Hydrothermal systems are rare along the main segments of normal faults due to reduced permeability in zones of clay gouge and periodic release of stress in major earthquakes. Accommodation zones, displacement transfer zones, and pull-aparts are disproportionately associated with higher temperature and/or power-producing geothermal systems. These settings appear to be especially favorable for enabling fluids to circulate to and from relatively great depths (up to 5+ km). Notably, many higher temperature systems are hybrids containing more than one type of favorable setting.