Mineralogic and Isotopic Studies of Cu-Ni Sulfide Mineralization Associated with the Duke Island Ultramafic Complex, Southeastern Alaska

Indiana University, Bloomington IN

Investigators

Abstract

The Duke Island complex is one of more than thirty ultramafic bodies of Cretaceous age located in the Alaskan panhandle. Recent drilling operations detected large volumes of low-grade massive and disseminated sulfide mineralization hosted within an olivine-clinopyroxenite body. Assays show a maximum of 1.95% Cu, 0.25% Ni, and 1 gram/ton combined Pt + Pd. Prior to this discovery, Alaskan-type ultramafic complexes were considered poor prospects for Cu-Ni-platinum group element (PGE) sulfide deposition. 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. 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, and an analysis of the tectonic and petrologic environments of the Duke Island complex suggests that immediate country rocks are sulfide-bearing and probably served as feeder systems for arc-related volcanic activity. Thus 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. Detailed petrographic, electron-microprobe, PGE, stable isotopic, and Re/Os isotopic studies of the sulfide mineralization will be undertaken to evaluate its petrogenetic evolution. Drill cores collected in 2001 and 2003 will be available for our project. Geochemical studies will assess the nature and causes of sulfide liquation and separation from a silicate magma (and if this process occurred at all), the source of sulfur involved in the mineralization, the overall nature of country rock contamination in the ore-forming processes, the possible importance of hydrothermal processes in sulfide genesis, and the source and evolution of Os (and potentially related metals) in the sulfide system. Preliminary analyses indicate that interaction with country rocks has been important in sulfide formation, and that hydrothermal processes may have played a significant role as well. Results of this study will be applicable not only to other Alaskan-type complexes, but will offer an interesting comparison in terms of tectonic environments relative to other world-class Cu-Ni-(PGE) deposits. Broader implications of the proposed research include the educational opportunities presented to both graduate and undergraduate students who will be involved in the field and analytical portions of the research. Collaboration with industry partners will be imperative for the success of the research. Quaterra Resources will provide access to drill cores and chemical analyses, and will provide field transportation in Alaska. Results from this study will have an impact on further exploration and potential economic development in southeastern Alaska, and will be important for the assessment of limited Ni and PGE resources in the United States.