Geology and Geochemistry of Volcanogenie Massive Sulfide Deposits and Related Igneous Rocks, Prince William Sound, South-Central Alaska

The Prince William Sound region is located approximately 100 km east-southeast ofAn-chorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Histori-cally, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologi-cally and are characteristically stratiform to strata bound. The deposits vary in size from less than i million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type nvironments. The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite