On the diversity of the PGE content in chromitites hosted in ophiolites and in porphyry-Cu systems: Controlling factors

Abstract

The present study on the diversity of the platinum-group elements (PGE) content in chromitites and porphyry-Cu deposits from Greece and everywhere (literature data) aims to define factors controlling precious metal enrichment in ophiolite complexes and the unknown Pd and Pt potential in porphyry Cu systems, through an integrated set of mineralogical and geochemical approaches. The PGE content in chromitites associated with ophiolite complexes is commonly low (a few 100s of ppb). However, a significant PGE-enrichment (up to 10s of ppm) is a feature of relatively small chromite occurrences, regardless their major element composition. Plots of the Pd/Ir versus normalized Pt/Pt∗ ratios defined separate fields corresponding to various chromitite types. Large chromite deposits exhibit low Pd/Ir values, reflecting low degree of fractionation (primitive magmas), negative (Pt/Pt∗<1) Pt/Pt∗ (partial melting of already depleted mantle) and increasing partial melting trend from high-Al towards to high-Cr deposits. The association with hydrous silicates (Cr-bearing hydrogrossular, chlorite and serpentine) and texture features of Os, Ir, Ru-rich (compatible) or IPGE-minerals, coupled with very low Pd/Ir and negative Pt/Pt∗ values were attributed to an origin by partial melting of already depleted mantle and increased partial melting degree, during a subsequent crustal/mantle recycling stage, via fluid-driven processes. Assuming that the increasing Pd/Ir reflects a fractionation trend of parent magma, a significant enrichment in Pt and Pd (incompatible) or PPGE, occurring in small occurrences (either high-Cr or high-Al type) may be a sensitive fingerprint for the orientation in the mantle sequence of the ophiolites and exploration for chromite.Commonly the PGE content in porphyry Cu-Mo-Au deposits is less than 10ppb. However, elevated (Pd+Pt) contents (over 6ppm in sulfide-rich samples) are a characteristic feature of certain porphyry Cu-Au alkaline intrusions. Evidence of a favorable tectonic setting (post-collisional) for the origin of porphyry Cu-Au-Pd-Pt-elevated porphyry deposits include elevated (up to hundreds of ppm) contents in Cr, Co, Ni and Re, and elevated 187Os/188Os ratios, reflecting mixing and contribution to the magma composition. The oxidized nature of parent magmas (abundant magnetite) and the evolved mineralized fluid system in porphyry deposits are considered to be critical requirements for the transportation and deposition of sufficient Au and PGE. Elevated Pd, Pt and Au contents in porphyry deposits combined with the association of the major Pd-mineral (merenskyite) with bornite and chalcopyrite (major Cu-minerals) are encouraging economic factors for the contribution to the global (Pd+Pt) production.