AGE,CHARACTER AND ORIGINS OF PGE AND RELATED MAFIC-ULTRAMAFIC ROCKS OF THE E-CENTRAL ALASKA RANGE

Mafic-ultramafic rocks of the Mount Hayes Quad are of three major types; with contrasting ages, mineralogies, PGE's, and likely origins. All contain variable phlogopite and hornblende and are superficially identical. These similar-looking rocks have very different potential for PGE deposits. A Triassic suite (~200-240 Ma, 40Ar/39Ar on 5 rocks) predominates W of ~ 145.5W. These rocks vary from dunite through harzburgite to troctolite and olivine-bearing gabbro-norite; orthopyroxene and olivine are notable constituents. Whole rock and mineral compositional data indicates the suite is calc-alkalic and reduced, with olivine as iron rich as Fo63. Magmatic sulfide with Ni > Cu, characteristically associated with troctolite and troctolitic gabbro-norite, is the predominant PGE host. Limited replacement of pyrrhotite and pentlandite by secondary pyrite, marcasite, and niccolite indicates little hydrothermal reactivation of the ores. The PGEs typically occur as arsenides or sulfo-arsides, usually with Pd>Pt. Chondrite-normalized PGE plots broadly resemble those of typical gabbroic-associated magmatic ores. Essentially no PGM placers are present. Two Cretaceous mafic-ultramafic suites occur E of ~ 145.5W. The older (~123-129 Ma, 40Ar/39Ar on 5 rocks) suite varies from wherlite to gabbro; orthopyroxene is notably absent. The rocks display a calcic, oxidized trend and contain Fe3+-rich pyroxene, hornblende, and biotite and abundant primary magnetite. The suite resembles zoned ultramafic complexes; the 10 km dia. “circular anomaly” E. of Summit Lake may be one. Magmatic sulfide is absent: lode PGM anomalies are very rare. However, placer PGM as Pt-Fe >> Os-Ir alloys are common associates. Chondrite-normalized PGE plots from rare lodes show extreme Os-Ir-Ru depletions. The third and least common suite, also E of ~ 145.5W, displays slightly younger (121-119 Ma, 40Ar/39Ar on 4 rocks) ages and displays an alkalic, reduced character. Rocks of this suite contain both ortho- and clino-pyroxene and little olivine; pyroxenite and K-spar- gabbro-norite are most typical. Associations with PGE are uncertain; no lodes or placers are clearly associated.