GOLD-BEARING AMPHIBOLE-APATITE-MAGNETITE VEINS IN THE BULLRUN DISTRICT, NORTHEASTERN OREGON: A POSSIBLE EOCENE FE-OXIDE CU-AU DEPOSIT

Gold mineralization in hydrothermal veins within the 38 Ma Bullrun Creek stock (BRC) was associated with a Ca+Fe+Mg-rich gangue assemblage.  The veins are generally <10 cm wide, and all share a common orientation normal to the contact between tonalite and serpentinized peridotite.  Early hydrothermal activity was characterized by Ca-rich pyroxene (diopside + hedenbergite; Mg/(Mg+Fe) from 0.37 to 0.55), and fluorapatite.  A second hydrothermal stage resulted in partial to complete uralitization of pyroxene, resulting in the formation of actinolite/ferroactinolite and magnetite.  Gold mineralization was associated with these sulfur-poor stages.  Late sulfide mineralization resulted in molybdenite + pyrite + chalcopyrite + minor cobaltite.

Two fluid inclusion populations are observed in apatite.  Primary inclusions (L+V±S; S=halite) are elongate parallel to the c-axis and exhibit negative crystal habits.    They have T_mi between -40 and -65 °C, with most grouped near the eutectic temperatures of CaCl₂±NaCl-bearing fluids (-50 to -52 °C).  T_fm ranged from ~-10 to -42 °C, indicating wide arrays in salinity and/or Ca/(Ca+Na).  Several inclusions turned orange-red upon decrepitating, suggesting the fluids were also Fe-bearing.  Primary inclusions homogenized in the liquid phase at temperatures between ~250 and 340 °C.  Secondary inclusions (L+V±S; S=halite) are irregular in shape and follow annealed cracks into adjacent minerals.  They are predominantly NaCl±KCl-bearing, with salinities between 11 and 32 wt.% NaCl, and homogenize in the liquid phase between 305 and 330 °C.  Primary fluid compositions are consistent with the Ca+Fe+Mg-rich gangue assemblage; post-emplacement leaching of these ions from late-stage dikes within the BRC is suggested by isocon diagrams.  Primary inclusion fluids are unlike magmatic fluids responsible for coeval porphyry Cu mineralization in a small stock adjacent to the BRC.

Hydrothermal veins in the Bullrun district possess most of the hallmarks common to Fe-oxide Cu-Au deposits worldwide (e.g., low S, Ca+Na+Fe-rich fluids).  Unlike Fe-oxide Cu-Au deposits elsewhere, however, BRC fluids lack CO₂, and did not result in extensive sodic alteration.  If classified as a member of the Fe-oxide Cu-Au class, then it represents one of the youngest examples yet described.