Enrichment of Platinum-group Elements (PGE) and Re-Os Isotopic Tracing for Porphyry Copper (Gold) Deposits

摘要

Platinum-group elements(PGE) in PGE-rich porphyry copper(gold) deposits are mainly Pt and Pd,whereas the concentrations of other PGE(Ru,Rh,Os,Ir) are significantly low.Moreover,Pt and Pd mainly exist in sulfides in the forms of crystal lattice or tiny platinum-group mineral(PGM) inclusions.The present data show that there is a positive relationship between Pt and Pd concentrations and Cu(Au) in porphyry copper(gold) deposits.The comparison of chondritenormalized PGE distribution patterns between the ore-bearing porphyry intrusions and ore-barren porphyry intrusions in arc setting,187Os/188 Os,87Sr/86 Sr and S isotopes for porphyry copper(gold) deposits shows that PGEs were mainly derived from the mantle,and fluids from subduction zones devoted trivial PGE to the magma.The porphyry copper(gold) deposits associated with subducted events are most probably enriched in PGE,whereas those related to crustal thickening,lithospheric delamination or underplating rarely concentrate PGE.The osmium isotopic compositions in porphyry copper(gold) deposits reveal that(187Os/188Os)i values are highly variable and not lower than those of primitive upper mantle(PUM) and mantle peridotite,however,osmium concentrations are commonly lower than mantle peridotite,suggesting that parental magmas of some porphyry intrusions had experienced crustal contamination during magma evolution.Experimental investigations have proved that PGE exist in the forms of Cl- and HS- complexes during transportation and migration of the oreforming fluids.This paper summarizes previous studies including crucial controlling factors and mechanisms for PGE enrichment,and points out that the mantle-derived magmas parental to porphyry intrusions are the prerequisite for PGE enrichment in porphyry copper(gold) deposits.Favorable physical and chemical conditions(including salinity,temperature,pressure,pH,and oxygen fugacity) in hydrothermal fluids crucially control the PGE enrichment,and sulfur concentrations of melts play important roles in this process as well.