STABLE AND CLUMPED ISOTOPE RECORD OF FAULT-RELATED FLUID FLOW AND MINERALIZATION IN THE BLUE MOUNTAIN GEOTHERMAL FIELD, BASIN AND RANGE, NEVADA USA

We investigate the geochemical record of fault-related fluid flow and mineralization preserved by calcite cements, scale and waters in the active Blue Mountain Geothermal field, Nevada USA. The study site is located at a Basin and Range fault intersection that has experienced extension since ~12-8 Ma episodically to the present. Geothermal fluids in this blind hydrothermal system are pumped from five wells to produce ~40 MW of electricity. Research during exploration, development and operation of the field provides outstanding context, core and well data, and the opportunity to sample 1) modern geothermal waters and calcite precipitates (scale), and 2) fault-related cements at depth. δ¹⁸O, δD and pH are consistent across modern well and power production system waters (mean δ¹⁸O = -14.7 ± 0.5 ‰; mean δD = -129 ± 3 ‰, VSMOW), except for one well that reflects infiltration of evaporative surface waters. Modern calcite scale samples filtered from the production system likely formed during boiling events; calcite δ¹⁸O values of -8.7 to -8.2 ‰ VPDB combined with elevated apparent clumped isotope temperatures of 184-200°C predict parent fluid δ¹⁸O compositions that are higher than the observed production water values, suggesting boiling-related kinetic isotope effects on both oxygen and clumped isotopes. For the sub-surface fossil cements, cathodoluminescence of some samples shows banding parallel to the fracture walls as well as brecciation, indicating that the cements record variations in the composition and source of fluids that moved through the fractures as they opened episodically. Most cements indicate paleofluid temperatures of around 150°C, with several wells peaking at above 200°C, and parent water δ¹⁸O values range from -4 to +8 ‰ suggesting significant water-rock interaction. The consistency of these temperatures is likely related to upwelling of fluids in the convective hydrothermal system, and the similarity of the clumped isotope temperatures to modern geothermal fluid temperatures of ~160-180°C suggests average reservoir temperatures have changed little since precipitation of the calcite cements. In contrast, cement associated with fault gauge records a cooler temperature of 11°C and anomalous δ¹³C and δ¹⁸O_water values, which may suggest a fault-controlled pathway for downwelling meteoric fluid.