Platinum Group Element Segregation and Mineralization in a Noritic Ring Complex Formed from Proterozoic Siliceous High Magnesium Basalt Magmas in the Vestfold Hills, Antarctica

Abstract

Siliceous High Magnesium Basalt (SHMB) magmas have been implied to play critical roles in the formation of platiniferous horizons in layered intrusions such as the Bushveld Complex, South Africa. However, their role in the formation of such complexes and particularly in the generation of the Cu–Ni–Platinum Group Element (PGE) sulphide mineralization has been deduced rather than proven. In this paper, we present an example of Cu–Ni–PGE sulphide mineralization which was unequivocally formed from SHMB magmas. This example is provided by the Noritic Ring Complex of the northern Vestfold Hills of Antarctica. The Noritic Ring Complex is co-magmatic with the early Proterozoic High Magnesium Tholeiite (HMT) dykes which were previously demonstrated to have formed from SHMB magmas. Chilled margins from the HMT have very similar PGE (average values: 16.0 p.p.b. Pd, 15.2 p.p.b. Pt, 0.98 p.p.b. Ru, 0.34 p.p.b. Ir) and other chemical features common to SHMB volcanics and intrusives from Western Australia, as well as to sills from the Bushveld Complex that are believed to have formed from the magmas that formed the Critical Zone of the Complex. The high Pd and Pt contents combined with the low S contents (291 p.p.m.) of the HMT dykes indicate that they were formed from S-undersaturated magmas, a fundamental requirement for the formation of major Cu–Ni–PGE sulphide mineralization. Sub-economic Cu–Ni–PGE sulphide mineralization occurs as disseminated sulphides and sulphide lenses within the last intrusive phase of the Noritic Ring Complex, the Rubbly Norite, which is characterized by the presence of up to 40 modal % orthopyroxene phenocrysts as well as xenoliths of country rock. It is suggested that the SHMB magma which formed the Rubbly Norite lagged behind the rest of the SHMB magma and spent some time in a residence chamber below the Noritic Ring Complex in which it underwent cooling, assimilation of country rock fragments and extensive crystallization of orthopyroxene which accumulated along the floor of the temporary magma chamber. These factors caused S saturation of the SHMB magma and the generation of the Cu–Ni–PGE sulphides which accumulated along with the orthopyroxene phenocrysts; the orthopyroxene- and sulphide-bearing magma was subsequently emplaced into the Noritic Ring Complex magma chamber. Economic concentrations of Cu–Ni–PGE sulphides did not form in the Noritic Ring Complex because the SHMB magmas did not interact with an abundant source of S.