ORIGIN OF PLATINUM GROUP ELEMENT MINERALIZATIONS IN SULFIDE-ABSENT AND SULFIDE-PRESENT ALASKAN-TYPE COMPLEXES

Alaskan-type complexes are ultramafic igneous intrusive bodies formed in convergent tectonic settings and are thought to have been staging magma chambers for subduction zone volcanoes. These complexes are characterized by lithological zoning and are often associated with placer deposits enriched in platinum group elements (PGE). PGE mineralization has been found in Alaskan-type complexes either in sulfide-absent, chromite-rich dunite cores (e.g. Nizhni Tagil and Union Bay) or as sulfide segregations in peridotite and olivine clinopyroxenite units (e.g. Duke Island and Salt Chuck).

PGE concentrations found in sulfide associations sampled from the Duke Island Complex in southeastern Alaska differ significantly from those reported from sulfide-absent associations in other Alaskan-type complexes. The sulfide zones of the Duke Island Complex are on average at least 10 times more enriched in PGE than sulfide-absent zones in other Alaskan-type complexes. In the Duke Island sulfides, Pt and Pd together account for 98% of the PGEs and average about 1 ppm. Sulfide-absent Alaskan-type complexes are characterized by extreme depletion of Ru and consequently Ru/Ir ratios <1. However Ru/Ir ratios in the Duke Island sulfides vary in a wide range from 0.1 to 5.

In sulfide-absent Alaskan-type complexes PGEs most commonly occur as Pt-Fe alloys strongly enriched in Ir and with variable amounts of Os in solid solution. The PGE concentrations are directly associated with the occurrence of chromites. High fugacity of oxygen (>QFM+2) and high activity of Fe in the parental magma strongly favor the stability of PGE-alloys under conditions of low sulfur-fugacity. In contrast, the parental magma of the Duke Island Complex underwent high degrees of assimilation of sulfur and carbon rich country rocks that lowered the oxygen fugacity, caused sulfide-saturation and consequently led to the formation of an immiscible sulfide liquid that selectively concentrated the PGEs from the silicate magma. Thus, the relative concentrations of PGEs in the sulfide zones of the Duke Island Complex are functions of their initial concentrations in the parental magma and the strong distribution into the sulfide liquid.