Larry J. Hulbert

Rhenium and osmium isotopes in black shales and Ni-Mo-PGE-rich sulfide layers, Yukon Territory, Canada, and Hunan and Guizhou provinces, China

Rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of Devonian black shale and an associated Ni-enriched sulfide layer from the Yukon Territory, Canada. The same composition information was also obtained for samples of early Cambrian Ni-Mo-rich sulfide layers hosted in black shale in Guizhou and Hunan provinces, China. This study was undertaken to constrain the origin of the PGE enrichment in the sulfide layers. Samples of the Ni sulfide layer from the Yukon Territory are highly enriched in Re, Os, and other PGE, with distinctly higher Re/192Os but similar Pt/Re, compared to the black shale host. Re-Os isotopic data of the black shale and the sulfide layer are approximately isochronous, and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales. Samples of the Chinese sulfide layers are also highly enriched in Re, Os, and the other PGE. Re/192Os are lower than in the Yukon sulfide layer. Re-Os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 Ma, similar to the depositional age of the black shale host. The osmium isotopic data suggest that Re and PGE enrichment of the brecciated sulfide layers in both the Yukon Territory and in southern China may have occurred near the time of sediment deposition or during early diagenesis, during the middle to late Devonian and early Cambrian, respectively.

An isotopic study of the Ni-Cu-PGE-rich Wellgreen intrusion of the Wrangellia Terrane: Evidence for hydrothermal mobilization of rhenium and osmium

Abstract 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 δ 18 O (7.3–7.9%.) suggest alteration of the intrusion by hydrothermal fluids that carried radiogenic 187 Os and 87 Sr 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 ( ϵ 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 arc ( 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.