SILICIFICATION DUE TO DISCHARGE OF METEORIC-HYDROTHERMAL FLUIDS INTO A SHALLOW DETACHMENT FAULT AT THE SOUTHERN WHITE PINE RANGE, EAST-CENTRAL NEVADA

Previously done stable isotope studies (Toneva et al., 2007) of carbonates from the Currant Gap (CGD) Detachment Fault (Francis et al., 2005) at the Southern White Pine Range documented a complex history of hydrothermal activity at 200-400°C during Oligocene-Miocene brittle deformation and the intrusion of magma into the lower plate. Our recent isotopic studies of >50 samples from veined and brecciated silica replacement zones characterizing the CGD indicate deposition of hydrothermal quartz from meteoric-hydrothermal fluids (δD ~ –100‰ ; δ¹⁸O ~ –11‰) that migrated upward (e.g., down-T). Silica replacement zones include, from lowest structural levels to highest: (S1) a zone of thin ‘spider’ veins (<5 mm) within the top of the Cambrian Upper Pole Canyon Limestone, (S2) a fault-bounded ‘spine’ rock, a brecciated amalgam of dolomite, limestone, and quartzite in a Si matrix, (S3) a zone of complete Si-replacement with clasts of Ordovician Eureka Quartzite and earlier phases of Si matrix formed during previous episodes of silicification, and (S4) jasperoid breccia. Quartz δ¹⁸O values (+10.7 to +14.4‰) and fluid inclusion δD values (~ –100‰) from Eureka Quartzite reflect their depositional provenance. Consistently low quartz δ¹⁸O values from S1 (–1.9 to +2.6‰) and S3 (–4.4 to +2.0‰) suggest high water/rock ratios. Intermediate water/rock ratios are indicated by a wider range of quartz δ¹⁸O values (+2.0 to +10.8‰) for the S2 ‘spine’ rock. Veins hosted by the upper plate Ordovician Lincoln Peak Formation record the lowest quartz δ¹⁸O values (–4.4 to –0.8‰). Very low quartz fluid inclusion δD values (–170 to ­–140‰) from all Si-replacement units attest to the pervasive nature of meteoric-hydrothermal fluid circulation through the CGD. Hydrothermally altered lower plate felsic sills and shales contain clay and zeolite minerals, indicating low temperature (200-300°C) alteration. Low δD values of sericite and biotite (~ –150) and non-equilibrium quartz (+11‰) and feldspar (+4 to +10‰) δ¹⁸O values from the altered intrusive rocks indicate the infiltration of moderate amounts of meteoric water into the lower plate. A compilation of stable isotope data from East-Central Nevada suggests that meteoric-hydrothermal fluids may have infiltrated much of the northern Basin and Range upper and middle crust during Tertiary extension.