Uralitization (Ca-Pyroxene to Actinolite) and Gold Deposition in the Bullrun Creek District, Northeastern Oregon

At Mine Ridge in northeastern Oregon, hydrothermal gold-bearing veins within the 36 Ma Bullrun Creek stock occur normal to the contact between tonalite and serpentinite and exhibit a wide range of mineralogies, from early Ca-pyroxene (hedenbergite) + apatite (barren) to late Fe-actinolite + apatite + magnetite (gold-bearing). Native gold is intergrown with Fe-actinolite and magnetite. Ca-pyroxene (hedenbergite/diopside) displays variable degrees of uralitization to Fe-actinolite. Vein thickness and gold values are greatest adjacent to the serpentinite, and decrease with distance into the pluton.

Based on these observations, we suggest that gold deposition was intimately related to uralitization, and may have resulted from a redox reaction of the type:

4Fe2+ + 2Au+ + 4Heden + 4H2O → Act + Mt + 2Au° + 6H+ + 2Ca2+

This reaction is consistent with the observed paragenesis, and suggests that uralitization at Mine Ridge: (1) consumed Fe2+ and Mg2+ and liberated Ca2+, (2) resulted in deposition of magnetite and gold, and (3) lowered the pH of the resultant solution. The system was sulfur-poor, suggesting Au, Fe, and Ca were transported as chloride complexes (e.g., FeCl+, AuCl2-, CaCl2).

The source of the fluids and metals (Fe and Mg) is uncertain, but preliminary results suggest that dehydration of adjacent serpentinite may have provided the necessary components. The mineralogy of these veins is similar to that observed in Fe-oxide Cu-Au deposits worldwide; these results therefore suggest that metamorphic dehydration fluids may play a vital role in Fe-oxide Cu-Au mineralization.