PARAGENESIS AND FLUID INCLUSIONS IN THE MINE RIDGE FE-OXIDE CU-AU DEPOSIT, NE OREGON

Gold-bearing hydrothermal veins in the Eocene Bullrun Creek stock (BR) in the Blue Mountains province occur along the slopes of Mine Ridge, normal to the northeast contact between the host tonalite and serpentinized peridotite. The adjacent and coeval Grouse Creek stock (GC) is also tonalitic, but is host to porphyry Cu mineralization and surrounded by a zone of propylitic alteration. Dikes of porphyritic GC tonalite intruded the BR in several localities, including areas adjacent to its contact with ultramafic rocks. Early calcic alteration in the BR is manifest as a broad spectrum of vein mineralogy, from diopside + apatite + magnetite +/- actinolite to actinolite + apatite. Gold is observed in apatite. Early diopside was replaced by actinolite, and several apatite crystals were broken by thin actinolite veins. Magnetite and abundant nonmetamict allanite (100s-1000s of crystals per thin section) appear to be associated with this stage. A later stage of sulfide mineralization (pyrite, chalcopyrite, cobaltite, and molybdenite) followed.

Apatite in the hydrothermal veins contains at least two populations of fluid inclusions. Primary inclusions (L+V+/-S) are elongate parallel to the c-axis, commonly with aspect ratios >20:1, and exhibit initial T(ice melting)<-50 degC, which suggests a CaCl2 + NaCl-rich composition. Some inclusions contain small cubic daughter crystals that are probably halite. These fluids homogenize in the liquid phase at T(h) between 310 and 370 degC. Secondary inclusions (L+V+/-S) typically follow fractures, are NaCl-rich, and homogenize in the liquid phase at lower temperatures (160-300 degC). Rare fluid inclusions of indeterminate composition in diopside homogenized at T(h) between 406 and 512 degC.

Characteristics of the inclusion fluids observed in the Mine Ridge deposit are distinct from those associated with porphyry Cu mineralization in the GC. Isocon diagrams indicate that contact metamorphosed serpentinite experienced overall mass loss (water?) but a gain in Ca and Na. In contrast, altered GC dikes from Mine Ridge lost Ca, Na, and Fe. The available data suggest connections between contact metamorphism of serpentinite, hydrothermal leaching from adjacent tonalitic rocks, and precipitation of Ca- and Fe-rich mineral assemblages at Mine Ridge.