GEOCHEMICAL CONSTRAINTS ON THE CONCENTRATIONS OF PLATINUM GROUP ELEMENTS IN URAL-ALASKAN TYPE COMPLEXES

Several Ural-Alaskan type (also called Alaskan-type) complexes around the world, particularly in Russia, are associated with platinum group element (PGE) enriched placer deposits. Although many of these deposits have been mined periodically, the mechanism of concentration of PGEs has not been satisfactorily explained. Most of these complexes occur in subduction zone environments and are characterized by concentric lithological zones with dunite in the center and successive zones of wehrlite, olivine clinopyroxene and clinopyroxenite towards the rim. In the absence of sulfide minerals in most instances, it is popularly believed that the PGEs are derived from the chromite-rich dunite cores of these complexes.

The Duke Island Complex, in southeastern Alaska, is a Ural-Alaskan type complex, but it is characterized by the presence of sulfide deposits, mostly in the olivine clinopyroxenite unit. Mantle-normalized PGE plots of the Duke Island sulfide horizons show a high degree of similarity with the Uralian examples. The sulfide zones in olivine clinopyroxenite show an average Pt + Pd content of 1 ppm (recalculated to 100% sulfide), but the sulfide-absent zones, including the dunite core, are remarkably poor in PGE. Numerical calculations indicate that the Pt and Pd contents of the parental magma were about 1 ppb and 2 ppb respectively. These are much lower than the PGE contents of most of the known mantle-derived magma types, but are comparable to arc-picrite magmas reported from subduction zone settings.

The depletion of PGE in the parental magma could either be explained as a primary characteristic that was inherited from refractory lherzolitic source rocks in the mantle wedge or as a product of PGE removal by fractional crystallization of olivine and spinel in addition to separation of sulfide liquid. We propose a model of magmatic evolution by fractional crystallization and sulfide liquid segregation in a shallow magma chamber environment to explain the PGE characteristics observed at Duke Island Complex. According to this model, the PGE contents of the sulfide horizons are consistent with a second phase of sulfide liquid separation from the parental magma which was caused by crustal contamination at shallow levels.