CONTRASTING MIDDLE AND UPPER CRUSTAL DETACHMENT FAULT HYDROTHERMAL SYSTEMS: VALHALLA COMPLEX, BRITISH COLUMBIA AND THE SOUTHERN WHITE PINE RANGE, NEVADA

Field stable isotope studies of two detachment faults, Slocan Lake Fault (SLF) in B.C. and Currant Gap Fault (CGF) in Nevada, reveal major differences in hydrothermal processes. Hydrothermal systems were driven by the introduction of heat into the mid-to-shallow crust by lower plate anatexis and/or magmatic intrusions that released volatiles which interacted with large amounts of meteoric-hydrothermal fluid in the detachment fault and within the upper plate. The CGF is characterized by brecciation, stratigraphic attenuation and omission, and extensive Si replacement of carbonate that intensifies toward the detachment surface. Lower plate isotopic heterogeneity, isotopically variable high-δ¹⁸O host-rock and low-δ¹⁸O vein carbonates at the CGF and the dominance of clay and zeolite alteration minerals indicate channelized circulation of meteoric-hydrothermal fluids at 200-300°C. A 17‰ range of δ¹⁸O values (+0.5 to +17.5‰) from multiple generations of calcite veins within 1-2 m of altered felsic sills (δD = –120‰ to –150‰) indicate a complex fluid history involving variable mixtures of meteoric-hydrothermal and magmatic waters in the CGF lower plate. The higher-temperature (350-450°C) SLF system, characterized by mylonitization overprinted by brecciation with mica and chlorite alteration, has a 3-stage hydrothermal evolution: Isotopically homogeneous aqueous fluids (δ¹⁸O = +10) were expelled at lithostatic pressure from the lower plate into the upper plate through oblique-sinistral faults. As brittle deformation along SLF became dominant, metamorphic fluids mixed with even larger quantities of downward circulating, hydrostatically pressured meteoric-hydrothermal fluids (δ¹⁸O = –15‰). Hydrothermal activity at stranded ductile portions of SLF was associated with post-detachment plutons. A high W/R system that advected heat from the lower plate over a period of 1-3 m.y. is suggested by quantitative modeling of SLF ¹⁸O/¹⁶O data (Holk and Taylor, 2007), consistent with that of thermochronological studies (Gordon et al., 2009). Thermal input by magmas, along with the evolution in association with systematically changing permeabilities along detachment faults appear to be the primary controls on hydrothermal activity in highly extended crustal regimes.