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Thakurta, Joyashish, 2008

Isotopic and mineralogical studies of sulfide mineralization and hydrothermal alteration in the Duke Island ultramafic complex, southeastern Alaska

Bibliographic Reference

Thakurta, Joyashish, 2008, Isotopic and mineralogical studies of sulfide mineralization and hydrothermal alteration in the Duke Island ultramafic complex, southeastern Alaska: Indiana University, Bloomington, Ph.D. dissertation, 237 p.


The Duke Island Complex of southeastern Alaska is an ultramafic complex where recent exploration has revealed significant zones of sulfide mineralizations. The high MgO content of the olivine and clinopyroxene in the host ultramafic rocks can be modeled by fractional crystallization of a picritic or ankaramitic parental magma generated in the mantle wedge under high degrees of partial melting in a convergent tectonic setting. The composition of the sulfides, recalculated to 100% sulfide, averages 0.48% Ni, 1.4% Cu, and ~1 ppm combined Pt and Pd. The presence of sulfides in the complex indicates that f O2 conditions were more reduced than those commonly ascribed to arc-related magmas. The mass of sulfide mineralization, together with the sulfur isotopic values indicate that external sulfur was added to the parent magmas at staging chambers. Os isotopic data from sulfides and C isotopic compositions of graphite in the olivine clinopyroxenites also indicate that the parental magmas were contaminated by crustal material. However, oxygen isotopic values of clinopyroxene together with the REE data suggest that bulk assimilation of country rocks has not occurred and the contamination was selective, involving C- and S-bearing fluids, or graphitic sulfides. Since the Duke Island Complex and other similar 'Alaska-type' complexes are parts of conduit systems through which large volumes of metal-bearing mafic magmas have passed, they have the potential to host large concentrations of sulfide-rich Cu-Ni-PGE mineralization. Assimilation of organic-carbon-bearing sedimentary country rocks acts to reduce the f O2 of the magma and promote sulfide stability. Incorporation of sulfur from country rocks is also essential to produce large masses of metal-rich sulfides. The zones of hydrothermal alteration in the ultramafic rocks of Duke Island are characterized by the presence of serpentine, along with minor magnetite, chlorite, and talc. Hydrogen and oxygen isotope data from serpentine indicate that the hydrothermal fluids were dominantly meteoric. The observed range and heterogeneity in the oxygen isotopic signatures in different textural domains prove the involvement of multiple fluids and only partial attainment of isotopic equilibrium. The hydrogen isotope data are relatively less variable and suggest homogenization by the last phase of hydrothermal activity.

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