A Dual Hypothesis for Thermal-Fluid Advection in the Northern Steamboat Geothermal Field, Nevada--Upflow in Ancient Breccia Pipes: Distributed Outflow in a Low-Angle Extensional Fault Zone

Abstract

Results from drilling and geologic logging of six recently completed, shallow- to intermediate-depth wells in the northern Steamboat geothermal field strongly support the premise (Hulen and Johnson, 2004; Johnson and Hulen, 2005) that commercial thermal-fluid entries in this traditionally shallow (<350 m) part of the resource are controlled by an areally extensive, low-angle extensional fault zone. The new geologic data—when integrated with corresponding static temperature/ pressure logs and other borehole information—additionally suggest that this gently-inclined, fault-controlled thermal aquifer could be fed from greater depth by an upflow zone exploiting relict permeability in a Cu-Mo-mineralized, magmatichydrothermal breccia pipe of probable late Cretaceous age. Key findings underpinning our new dual hypothesis for thermal-fluid advection at Steamboat can be summarized as follows: (1) As confirmed by each well, the reservoir rock in this part of the field is late Cretaceous Sierran granodiorite; (2) A moderate-temperature (that is, within the 150-180oC range) commercial thermal-fluid entry zone was encountered in all six wells (separated by up to 1.6 km) at ~1200 m elevation and within ~350 m of the modern ground surface. This entry zone (one of lost-circulation when drilled) is typically but not invariably signaled at shallower depths by silicified cataclasite along with euhedral quartz crystals in epithermal veins and breccia cements; (3) Above the fluid-entry zone (but not systematically linked to it), the granodiorite in at least three of the wells hosts highly anomalous combined concentrations (0.1-1.4% over 3-m depth intervals) of relatively coarse-crystalline (up to 3 mm) molybdenite and chalcopyrite; (4) The one well drilled through the entry zone and deeper into the granodiorite encountered— as anticipated and below 540 m depth—the sole, spent-fluid injection zone for the entire developed southern sector of the Steamboat field. The injection zone is identified here, for the first time, as a highly porous, chalcopyrite- and molybdenite-bearing granodiorite breccia overprinted by drusy epithermal quartz. The breccia is texturally reminiscent of those found in Mesozoic to early Cenozoic, Cu-Mo-mineralized breccia pipes at numerous locations along the North and South American cordillera. Many of these ancient pipes have surprisingly sizable relict porosities, and if placed conceptually in a modern geothermal system would certainly channel (or store) correspondingly high thermal-fluid volumes: This may be the actual scenario at Steamboat.