STABLE ISOTOPE EVIDENCE FOR MIXING BETWEEN METEORIC AND MAGMATIC WATERS ADJACENT TO A SHALLOW DETACHMENT FAULT IN THE SOUTHERN WHITE PINE RANGE, NEVADA

Recent mapping of the southern White Pine Range by Francis et al. (2005) indicate the presence of a major detachment fault (the Currant Gap fault) in an area that earlier workers (e.g. Moores et al., 1968) interpreted to be dominated by the Currant Summit left-lateral strike-slip fault. The PIMA infrared spectrometer has identified alteration patterns associated with hydrothermal events related to faulting. Dolomite is restricted to the upper plate and the detachment fault whereas calcite is the dominant carbonate in the lower plate, but this distribution may be stratigraphically controlled. Low-temperature alteration (200-300°C) of the lower plate is indicated by the presence of illite, montmorillonite and zeolites in felsic sills and shale from the Pole Canyon and Pioche Formations. Values of calcite d¹⁸O (-3.5‰ to +0.1‰) from upper-plate veins indicate large-scale infiltration of fresh meteoric water through this zone. The migration of mixed meteoric and magmatic water through the lower plate is indicated by calcite d¹⁸O values between –4.4‰ and +19.0‰. A 17‰ range of d¹⁸O values (+0.5‰ to +17.5‰) from multiple generations of carbonate-hosted calcite veins 0-10 meters above lower-plate intrusive felsic sills indicates a multi-stage fluid history involving variable mixtures of meteoric and magmatic waters near the upper contacts of these intrusions. No veins were observed to have d¹⁸O values above +20.0‰; this argues against the infiltration of formation waters through the system. These data agree with recent mapping that suggests the Currant Summit Fault may not exist. The tight range of observed d¹³C values (-2.7‰ to +1.2‰) rules out the incorporation of carbon from low-¹³C organic sources into these fluids. Our stable isotope results are similar to those obtained from nearby detachment faults in the Ruby Mountains (Fricke et al, 1992) and the Snake Range (Losh, 1997), and from the Carlin trend of gold mineralization (Hofstra and Cline, 2000).  All of these areas were affected by large amounts of meteoric-hydrothermal fluids. Thus, the integrated stable isotope dataset related to Tertiary hydrothermal systems from the northern Basin and Range indicates that most of the upper and middle crust in this region was infiltrated by meteoric-hydrothermal waters during extension.