18O/16O Evidence for Contrasting Hydrothermal Regimes Involving Magmatic and Meteoric-Hydrothermal Waters at the Valhalla Metamorphic Core Complex, British Columbia

Abstract

Modeling of published oxygen isotope analyses of 115 whole-rock, quartz, and feldspar samples from the Valhalla metamorphic core complex provides evidence for three distinct stages of hydrothermal activity affecting the detachment faults and upper plate. (1) Lithostatically pressured aqueous fluids having δ18O ~10 per mil were expelled from the lower plate and moved upward into the upper plate through an oblique-sinistral transfer zone between the southern terminus of the Columbia River detachment fault and the northern terminus of the Slocan Lake detachment fault. These fluids ascended through fractures and exchanged with Slocan Group shales and graywackes to produce the marked zonation of vein siderite δ18O (11.4–18.4‰) observed in previous studies of the Sandon group of Ag-Pb-Zn lode deposits. (2) Later, as brittle deformation along the Slocan Lake detachment fault became the dominant tectonic process, these magmatic and/or metamorphic fluids intermittently mixed with even larger quantities of downward circulating, hydrostatically pressured meteoric-hydrothermal fluids. In the upper plate Nelson granodiorite adjacent to the Slocan Lake detachment fault, δ18O values of quartz (8.9–11.9‰) and feldspar (−5.0 to +9.6‰) define a steep array in δ-δ space (slope = 25), suggesting that synextensional meteoric-hydrothermal activity along the Slocan Lake detachment fault (initial δ18Owater ~−15‰ and T ~300°C) was relatively short-lived (~106 yr), but very intense, with W/Ropen-system = ≤1.05 and fluid flux = 0.1 to 3 m/yr. The δ18O values of quartz (10.8–12.4‰) and feldspar (6.4–10.1‰) from lower plate greenschist mylonites indicate that small quantities of the low 18O meteoric-hydrothermal fluids penetrated into the lower plate. The systematic decrease in feldspar δ18O toward the Slocan Lake detachment fault in both the upper and lower plates shows that these low 18O waters were channeled along the detachment fault. (3) Meteoric-hydrothermal activity along the Valkyr shear zone postdates ductile deformation and was associated with the emplacement of the Coryell plutons in the upper plate. The δ18O values of quartz (7.6–11.0‰) and feldspar (0.0–8.6‰) from the Valkyr shear zone define a much shallower array in δ-δ space (slope = 2.3) than those at the Slocan Lake detachment fault, indicating a higher temperature and/or longer lived episode of hydrothermal activity. The geographic distribution of pronounced disequilibrium δ18O effects in Coryell quartz (5.8–8.2‰) and feldspar (−4.4 to +7.4‰) indicates that meteoric-hydrothermal activity was most intense along inferred coeval extensional fracture zones that lie beneath Arrow Lake. This modeling suggests that transfer zones that accommodate deformation between detachment faults may be a very attractive exploration setting for lode-type mineral deposits.