Abundances of platinum group elements in native sulfur condensates from the Niuatahi-Motutahi submarine volcano, Tonga rear arc: Implications for PGE mineralization in porphyry deposits

Abstract

Some porphyry Cu–Au deposits, which are enriched in Pd, are potentially an economic source of Pd. Magmatic volatile phases are thought to transport the platinum group elements (PGEs) from the porphyry source magma to the point of deposition. However, the compatibilities of the PGEs in magmatic volatile phases are poorly constrained. We report PGE and Re contents in native sulfur condensates and associated altered dacites from the Niuatahi-Motutahi submarine volcano, Tonga rear arc, in order to determine the compatibility of PGEs and Re in magmatic volatile phases, and their mobility during secondary hydrothermal alteration. The native sulfur we analyzed is the condensate of a magmatic volatile phase exsolved from the Niuatahi-Motutahi magma. The PGEs are moderately enriched in the sulfur condensates in comparison to the associated fresh dacite, with enrichment factors of 11–285, whereas Au, Cu and Re are strongly enriched with enrichment factors of ∼20,000, ∼5000 and ∼800 respectively. Although the PGEs are moderately compatible into magmatic volatile phases, their compatibility is significantly lower than that of Au, Cu and Re. Furthermore, the compatibility of PGEs decrease in the order: Ru>Pt>Ir>Pd. This trend is also observed in condensates and sublimates from other localities.PGE mineralization in porphyry Cu–Au deposits is characterized by substantially higher Pd/Pt (∼7–60) and Pd/Ir (∼100–10,500) than typical orthomagmatic sulfide deposits (e.g. Pd/Pt ∼0.6 and Pd/Ir ∼20 for the Bushveld). It has previously been suggested that the high mobility of Pd, relative to the other PGEs, may account for the preferential enrichment of Pd in porphyry Cu–Au deposits. However, the low compatibility of Pd in the volatile phase relative to the other PGEs, shown in this study, invalidates this explanation. We suggest that the PGE geochemistry of Pd-rich Cu–Au deposits is principally derived from the PGE characteristics of the magma from which the ore-forming fluid exsolved. Pd-rich porphyry Cu–Au deposits are associated with highly oxidized magmas. Prior to sulfide saturation Pd, Au and Cu behave as incompatible elements and concentrate in the melt with fractional crystallization, whereas Pt is depleted by early crystallization of a Pt-rich alloy and the other PGEs by the co-crystallization of the Pt-rich alloy and Cr spinel. As a consequence the Pd/Pt and Pd/Ir in the evolving melt and the magmatic volatile phases that exsolve from that melt, increase with increased fractionation. The high Pd content and high Pd/Pt (∼7–60) of Cu–Au porphyry ores therefore require the parent magma to have undergone extensive sulfide-undersaturated fractional crystallization prior to volatile exsolution.Our study also showed that the altered dacites contain PGE abundances that are similar to those of fresh dacites although Pt and Rh are slightly enriched in the altered dacites, which indicates low mobility of PGEs during secondary hydrothermal alteration.