INFILTRATION OF METEORIC-HYDROTHERMAL FLUIDS INTO THE LOWER PLATE OF A SHALLOW DETACHMENT FAULT AT THE SOUTHERN WHITE PINE RANGE, EAST-CENTRAL NEVADA

Stable isotope studies of the recently defined Currant Gap detachment fault (Francis et al., 2005) at the southern White Pine Range indicate a complex history of hydrothermal activity during brittle deformation and the intrusion of magma into the lower plate. Hydrothermally altered lower plate felsic sills and shales contain illite, montmorillonite, and zeolites, indicating low temperature (200-300°C) alteration. Low hydrogen isotopic ratios of muscovite (dD = -120) and biotite (dD = -150) in the altered intrusive rocks indicate the infiltration of meteoric-hydrothermal fluids (dD ~ -100; d¹⁸O ~ -11) into the lower plate, but some high-²H biotites from these intrusions (dD = -75) indicate the preservation of magmatic dD values. Calcite and dolomite d¹³C values (-2.7 to +1.2) are independent of d¹⁸O; indicating minimal decarbonation of limestones and/or dolostones during hydrothermal activity and the incorporation of low-¹³C organic carbon is ruled out.  Values of calcite d¹⁸O (-3.5ä to +0.1ä) from upper-plate veins indicate large-scale infiltration of fresh meteoric water through this zone. Upper plate host limestone and dolostone experienced variable degrees of ¹⁸O/¹⁶O exchange (d¹⁸O = +6.4 to +26.1) with the lowest values nearest to the detachment zone. The migration of mixed meteoric and magmatic water through the lower plate is indicated by vein calcite d¹⁸O (-4.4 and +19.0) and this is matched by a slightly smaller range for host-rock carbonate d¹⁸O (+0.2 to +17.1) with the highest values associated with the intrusive sills and the lowest values found near brittle faults. This is consistent with the 17ä range of d¹⁸O values (+0.5 to +17.5) observed for multiple generations of carbonate-hosted calcite veins 0-10 meters above lower-plate intrusive felsic sills; thus indicating a complex fluid history involving variable mixtures of meteoric and magmatic waters for the lower plate of this complex. The occurrence of vein calcite d¹⁸O values (-4.4 to -2.7) formed in equilibrium with modern Nevada meteoric water (d¹⁸O ~ -11) suggests that these detachment faults served as recharge zones for hydrothermal fluids. 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.