Magmatic Cu-Ni-PGE mineralization at a convergent plate boundary: Preliminary mineralogic and isotopic studies of the Duke Island Complex, Alaska

Abstract

The Duke Island complex is one of more than thirty ultramafic bodies of Cretaceous age located in a belt of_∼40 km width that stretches along the 560 km length of the Alaskan panhandle. Recent drilling operations have detected the presence of large volumes of low-grade massive and disseminated sulfide mineralization hosted within an olivine clinopyroxenite body that appears to cross-cut other ultramafic rock types. Assays show a maximum of 2.08% Cu, 0.25% Ni, and 1 gram/ton combined Pt + Pd, Prior to the recent discoveries Alaskan-type ultramafic complexes were thought to be poor prospects for world-class magmatic Cu-Ni-platinum group element (PGE) sulfide deposition. One reason for this assessment relates to the fact that although PGE placers are often found in association with the ultramafic complexes, base metal sulfides have only rarely been found in the host rocks. PGE mineralization appears to be primarily associated with chromite-rich occurrences in dunitic portions of the complexes, and most of the PGEs are housed in alloys, tellurides, or antimonides. For these reasons the complexes are thought to have formed in very low fS₂ environments. A second reason for not suspecting the presence of magmatic sulfides is that the linearly arranged Alaskan-type complexes are thought to have formed in compressive regimes related to subduction processes. Very few large magmatic Cu-Ni-(PGE) sulfide deposits are associated with subduction zones, although some researchers believe that komatiites (host to many Ni-sulfide deposits) may have formed in Archean to Proterozoic subduction zones. Two pre-requisites for the formation of large Cu-Ni-(PGE) deposits are: 1) a supply of readily available S from country rocks and 2) localization of immiscible sulfide liquids in magma conduit systems where they have the opportunity to exchange metals with large volumes of metal-bearing magma. An analysis of the tectonic and petrologic environments of the Duke Island complex suggests that immediate country rocks are indeed sulfide-bearing (some may include Triassic volcanogenic massive sulfide deposits), and that the complexes probably served as feeder systems for arc-related volcanic activity. Sulfur isotopic values of the mineralization at Duke Island range from-15 to 4.6‰, and are strongly suggestive of the involvement of country rock sulfur. Nickel contents of olivine (Fo_77–85) are variable; grains associated with a high proportion of sulfide are Ni-depleted (to values_<100 ppm), whereas grains in sulfide-deficient zones contain up to 1000 ppm Ni. Our preliminary data suggests that the complex may represent an excellent target for the localization of Cu-Ni-(PGE) mineralization in a tectonic environment that in the past may have been overlooked in terms of sulfide potential.