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 i ntrusion in the Wrangellia terrane were investigated in an effort to d educe 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) sugg est alteration of the intrusion by hydrothermal fluids that carried ra diogenic Os-187 and Sr-87 from the surrounding country rocks. The grea t majority (>99%) of the Os, however, and by inference the other PGEs, derived from a mantle-derived magma that suffered little or no intera ction with the crust prior to crystallization. Initial Nd isotope rati os are not as variable (epsilon(Nd(t)) ranges from 2.02-4.49) and sugg est that the rocks were derived from a light-rare-earth-element deplet ed mantle source. The Nd results, together with Os data from relativel y 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 eff ects of this mobility must be carefully evaluated prior to invoking cr ustal assimilation to explain variable and radiogenic Os initial ratio s in layered intrusions.