Platinum-group elements geochemistry of felsic rocks

Abstract

The mechanisms by which the platinum group elements (PGE) concentrate and fractionate during magmatic processes, especially in felsic magmatic systems, is poorly understood. The felsic igneous rocks are often linked to hydrothermal Cu-Au mineralization and the presence or absence of sulfide saturation during fractionation is critical to the ore forming processes. PGE can be a sensitive indicator for sulfide saturation due to their high compatibility with sulfide melts. This research investigated the geochemistry of the PGE and Re in two volcanic suites (Ambae and Pual Ridge lavas) and one intrusive suite (Boggy Plain zoned pluton) that show large compositional variations from mafic to felsic in order to improve our understanding of the behavior of PGE and Re during crystal fractionation, and to apply the lesson learnt for distinguishing between productive and barren felsic systems.

Analyses of PGE and Re in felsic rocks were carried out using a modified precise NiS fire assay - isotope dilution method. The Ambae and Pual Ridge studies showed that Cr spinel is an important host for Rh, Ir, Os and Ru in oxidized ultramafic-mafic arc magmas and that a Pt-alloy can crystallize from an arc-type basaltic-andesitic magma, and fractionate Pt from Pd. The timing of sulfide saturation is identified by the systematic depletion of all PGE in evolving magmas and a marked decrease in Pd/Cu. Sulfide saturation followed after the onset of magnetite crystallization may be a common feature of tholeiitic (or transitional tholeiitic) suites that follow an iron-enrichment trend. In addition, loess samples from China were analyzed for PGE and Re to constrain the PGE and Re abundances in the upper continental crust. The results lead to a revised PGE composition of the upper continental crust, especially for Ru and Rh. These results are of importance for understanding PGE abundances in felsic igneous rocks that form by direct melting or assimilation of continental crust. -- provided by Candidate. Show more