Sodium metasomatism and magnesio-riebeckite mineralization in the Wilson Ridge pluton
The Wilson Ridge pluton contains a rare occurrence of hydrothermal sodic amphibole. hydrothermal amphibole occurs as fracture coatings, veinlets, dense stockwork-type networks, and diffusive alteration fronts. Electron microprobe analyses of the hydrothermal amphibole reveal a range of Na-rich compositions, most notably magnesio-riebeckite. Magnesio-riebeckite in the Wilson Ridge pluton formed by Na-metasomatism related to subsolidus hydrothermal alteration involving Na-rich fluids of meteoric origin. Whole-rock geochemistry of samples from zones of hydrothermal amphibole show enrichment of Na2O (>9.6 wt. %) and depletion of K2O (0.15 wt. %) and CaO (2.4 wt. %) relative to unaltered host rock. Secondary fluid inclusions associated with zones of hydrothermal amphibole contain daughter crystals of halite and up to three other unknown crystals. Oxygen isotope studies suggest that granitoid rocks containing hydrothermal amphiboles have lower delta18O values (+3.68Ã¢â‚¬Â° to +6.97Ã¢â‚¬Â°) than unaltered granitoid rocks (delta 18O values +6.04Ã¢â‚¬Â° to +9.23Ã¢â‚¬Â°).
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