An Isotopic Study of the Ni-Cu-Pge-Rich Wellgreen Intrusion of the Wrangellia Terrane - Evidence for Hydrothermal Mobilization of Rhenium and Osmium

Re-Os, Sm-Nd, Rb-Sr, and oxygen isotope systematics of the Wellgreen intrusion in the Wrangellia terrane were investigated in an effort to deduce the origin of this mafic-ultramafic sill and its attendant Ni-Cu-PGE deposit. Radiogenic initial Os ratios (1.06-1.82) and Sr ratios (0.7044-0.7062), and heavy delta(18)O (7.3-7.9 parts per thousand) suggest alteration of the intrusion by hydrothermal fluids that carried radiogenic Os-187 and Sr-87 from the surrounding country rocks. The great majority (>99%) of the Os, however, and by inference the other PGEs, derived from a mantle-derived magma that suffered little or no interaction with the crust prior to crystallization. Initial Nd isotope ratios are not as variable (epsilon(Nd(t)) ranges from 2.02-4.49) and suggest that the rocks were derived from a light-rare-earth-element depleted mantle source. The Nd results, together with Os data from relatively undisturbed wehrlites, are compatible with either a plume (RICHARDS et al., 1991) or island are (SAMSON et al., 1990) model for Wrangellia.This study contributes to a growing body of evidence that documents the ready mobilization of Os in hydrothermal fluids. The potential effects of this mobility must be carefully evaluated prior to invoking crustal assimilation to explain variable and radiogenic Os initial ratios in layered intrusions.