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