Zoned Cr, Fe-spinel from the La Perouse layered gabbro, Fairweather Range, Alaska

Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr₂O₃ to rims with less than 11 wt.% Cr₂O₃. Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al₂O₃ and TiO₂. Also notable are the concentrations of MnO and V₂O₃, which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900°C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma.