The Union Bay Platinum Prospect, SE Alaska, a Hydrothermal PGE Deposit Christopher J. Van Treeck and Rainer J. Newberry
Department of Geology and Geophysics, The University of Alaska Fairbanks, ftcjv@uaf.edu PGMs in Alaskan-type Complexes (ACs) are supposed to occur within chromiterich dunites of magmatic origin. At the Union Bay AC, PGEs occur with magnetite (not chromite) in pyroxene-rich rocks, deposited at temperatures of ~700 to <400oC. PGEbearing magnetite occurs in veins and irregular pods that cut across ultramafic rock layering and commonly parallel through-going ferric diopside veins. The latter are compositionally distinct from magmatic clinopyroxene, contain primary fluid inclusions, and apparently represent early stage hydrothermal alteration.
Earliest hydrothermal magnetite occurs as direct replacements of clinopyroxene and yields ilmenite-magnetite temperatures of ~575-700oC. Magnetite with lower ilmenite-magnetite temperatures (~575-475oC) replaces clinopyroxene with a hornblende alteration rim. PGMs identified in this magnetite include Pt3Fe, OsS2, RhAsS, Os-Ir, Ir-
Pt, Rh-Fe, PtSb, and PtIrS. Latest PGE-bearing magnetite yields magnetite-ilmenite temperatures of < 475o, and is rimmed by interlayered chlorite and serpentine. Elemental correlations with Pt at Union Bay are erratic, in keeping with the variable forms of Pt-Pd
deposited at different temperatures and in different host rocks. Pt and Pd themselves are variably correlated; high and low Pt/Pd zones are present.
Highest-temperature magnetite at Union Bay displays extensive ilmenite and spinel exsolution; that deposited at lower temperatures is nearly pure Fe3O4. Hydrothermal phologopite and hornblende contain significant Cl-; the latter is richer in Na and Fe3+, and poorer in Fe2+ than magmatic hornblende. A likely scenario for ore deposition at Union Bay involves a quartzundersaturated fluid with moderately high Cl-, moderate pH, and fO2 slightly below magnetite-hematite. Fluid reaction with olivine and clinopyroxene rich rocks produces a reaction envelope of amphibole and (or) chlorite, depending on temperature. Reaction with silicates causes an increase in solution pH, which causes both magnetite and PGE deposition from chloride complexes. The fluid origins are unknown, but absence of
quartz suggests derivation from a mafic-ultramafic source.
