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Van Treeck, C.J., 2009

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Van Treeck, C.J., 2009, Platinum group element enriched hydrothermal magnetite of the Union Bay Alaska-type ultramafic intrusion, southeast Alaska: University of Alaska Fairbanks, M.S. thesis, xvi, 188 p., illust., maps.


The Union Bay Alaskan-type Complex represents a completely different type of platinum-group-element (PGE) concentration from the classic Alaskan-type complex PGE mineralization model, with PGEs localized in vein/veinlet magnetite, commonly within the olivine + pyroxene portions of the body. PGE concentrations of 18 g/t occur in magnetite veins and pods that cross magmatic layering and have an irregular morphology in both outcrop and thin section. Fe-Ti oxide geothermometry indicates that this mineralization formed at temperatures between 675 and 475 degrees C. A variation in the amount of ilmenite and Mg-Al spinel exsolution in magnetite correlates with temperature estimates; lower temperature magnetite has less exsolution. Magnetite veins with a formation temperature of less than 600 degrees C are surrounded by an alteration envelope of hydrous silicates that vary with temperature and contain appreciable amounts of Cl. Interaction of a PGE-Fe-Cl-rich fluid with clinopyroxene and olivine increased the fluid's pH and decreased PGE and Fe solubility, depositing a PGE-enriched magnetite in the wehrlite and clinopyroxenite of the Union Bay Alaskan-type complex. Potentially this fluid remobilized the PGE from disseminated euhedral magnetite of the peripheral units to form PGE-enriched magnetite veins in the near-central portion of the complex.

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