Serpentinite Dehydration as a Source of Fluids and Metals In Hydrothermal Gold-Bearing Veins In the Bullrun Creek District, Northeastern Oregon

Hydrothermal gold-bearing veins occur at the contact between equigranular tonalite of the 36 Ma Bullrun Creek stock and serpentinized peridotite. They consist of Fe-actinolite + apatite + magnetite ± Ca-pyroxene. The veins are thickest at the contact, become thinner and eventually pinch out with distance into the tonalite. Gold concentrations are greatest at the contact, and gold is typically associated with late magnetite and Fe-actinolite. These observations suggest that the adjacent serpentinite may have provided at least some of the fluid and metals now observed in the ore veins.

This study focused on a ~16 m wide outcrop of serpentinite, into which a ~2 m thick dike of porphyritic tonalite intruded. In comparison to unaltered serpentinite at either end of the outcrop, serpentinite adjacent to the dike has low total Fe, MgO, and H2O, and higher SiO2, CaO, and CO2. Loss of H2O in these rocks is suggestive of thermal dehydration during dike emplacement. Results of thermal analyses indicate that serpentine (as opposed to anthophyllite or talc) was the principal dehydrating phase adjacent to the dike, and that dehydration occurred at temperatures below the tonalite solidus. It is suggested that breakdown of serpentine and an Fe-bearing phase (e.g., orthopyroxene, magnetite) supplied Mg and Fe, respectively, that was removed by the dehydration fluid. These results suggest that dehydration of serpentinite adjacent to the Bullrun Creek stock may have provided the fluid and metals (Mg and Fe) that formed the hydrothermal veins within the pluton. The presence of numerous gold mines within the serpentinized peridotite suggests that serpentinite dehydration might have supplied the Au observed in the hydrothermal veins as well. Similarities between these veins and Fe-oxide CuAu deposits worldwide suggest that these results may have implications for the origin of Feoxide Cu-Au mineralization elsewhere.