Platinum Group Elements Deposits: Types, Bushveld Complex and Origin
Platinum Group Elements Deposits
Platinum group elements (PGE) deposits: these elements comprise a group of siderophile and chalcophile geochemically coherent elements that includes Os, Ir, Ru, Rh, Pt, and Pd. Au ... in terrestrial environments, their concentrations vary from the sub-ppb level in felsic to intermediate rocks, up to 1 to 100 ppb in mafic and ultramafic rocks. Economic deposits typically contain between 5 and 10 ppm of PGE, with enrichment factors of the order of 1000, similar to those of gold deposits.
Types of Deposits
Almost all identified reserves correspond to two types of deposits intimately associated with Fe-Ni-Cu sulfides:
- Stratiform deposits in large layered complexes (Bushveld).
- Deposits exploited as a by-product of Fe-Ni-Cu sulfide deposits (Sudbury).
Deposits in Stratiform Igneous Complexes
Their characteristics are the presence of sulfide disseminations in silicate rocks at specific horizons, and their association with chromite, either in massive chromitite layers or in strata with small amounts.
Bushveld Complex
PGE mineralization occurs in horizons (Merensky Reef, UG2 chromitite layer, and Platreef) and in platiniferous ultramafic pipes.
Merensky Reef
It occurs near the roof of the critical zone and forms the base of the Merensky cyclic unit. It is a feldspathic pyroxenite layer of large thickness, coarse-grained to pegmatoidal. The highest concentrations of platinoids are found in proximity to the two associated chromitite layers. The EGP grade is closely related to its highest concentration in sulfides.
Origin of the Merensky Deposits
Two models have been proposed:
- Orthomagmatic: EGP-sulfide mineralization as an integral part of magmatic crystallization, with some modifications by postcumulus processes. Separation of an immiscible sulfide melt enriched in EGP and its accumulation on or near the floor of the magma chamber. The mixing of magmas gives the best explanation.
- Hydrothermal: late-magmatic fluids rich in volatiles exsolved from intercumulus melt were implicated in the localization of EGP.
UG2 Chromitite
It is the most continuous and richest layer. It occurs as an accumulation of bronzitite at the base of the UG2 cyclic unit. Most EGP minerals are associated with base metal sulfides, although some also occur within the opx gangue.
Ultramafic Pipes
Masses of transgressive post-cumulus ultramafic rocks. EGP mineralization is essentially located in the core zone. Origin through postcumulus metasomatic infiltration due to magmatic-hydrothermal fluids rich in volatiles.
Potholes
Transgressive depressions, circular to elliptical in shape, with dipping walls that penetrate the stratigraphy of the footwall.
Composition of the Ores
Most EGP values in mineralizations occur as discrete platinum group minerals.