A Record Of Assimilation Preserved By Exotic Minerals In The Lowermost Platinum-Group Element Deposit Of The Bushveld Complex: The Volspruit Sulphide Zone

Abstract

Low-grade platinum-group element mineralisation in the Volspruit Sulphide Zone is sulphide-poor (<5 vol. %), distributed over a ~60 m-thick horizon in the lowermost cumulates of the northern limb of the Bushveld Complex. Unlike any other platinum-group element (PGE) deposit of the Bushveld Complex, the Volspruit Sulphide Zone is hosted exclusively within harzburgitic and dunitic cumulates in the Lower Zone of the Rustenburg Layered Suite.

Here, we present a petrological investigation on the distribution of PGEs and chalcophile metals in mineralised pyroxenite cumulates from the Volspruit Sulphide Zone, to determine the origin of the PGE mineralisation in ultramafic cumulates and evaluate whether Volspruit-style mineralisation could occur in the stratigraphically lowest, ultramafic portions of other layered intrusions.

Electron microscopy of pyroxenite cumulates revealed (1) chromite inclusions containing dolomite, albite, monazite, Pb-chlorides, base metal sulphides and Pt-As minerals, (2) the presence of exotic microxenocrysts (<300 μm diameter) in the pyroxenite matrix such as grains of CaCO3, U-Th-oxide and Mn-ilmenite, and (3) base metal sulphide assemblages enclosing grains of primary galena, sphalerite and Pb-chlorides.

Systematic mapping of high-density mineral assemblages in pyroxenite cumulates across the Volspruit Sulphide Zone identified 196 precious metal mineral grains (Pt-, Pd-, Rh-, Au- or Ag-minerals), 98 Pb-sulphide grains (± Se, Cl), 27 Pb-chloride grains (± K, Se, Te, S), as well as 1 grain of Pb-telluride, 1 monazite grain and 1 grain of U–Pb-Th oxide. Trace element analyses of base metal sulphides reveal the highest S/Se values in pyrrhotite and chalcopyrite yet recorded in the Bushveld Complex. While some base metal sulphides are enriched in PGEs, the overall low-grade of the deposit and inferred fertile ultramafic magma(s) require relatively low R-factors (mass of silicate to sulphide melt) compared to other sulphide-poor PGE deposits, with a calculated R factor of ~500–3000.

We consider that the presence of exotic inclusions in chromite, exotic microxenocrysts, and Pb/Zn/Cl grains enclosed within primary base metal sulphide assemblages provide strong evidence for crustal contamination in the Volspruit Sulphide Zone. The Malmani dolomite and the Black Reef quartzite within the lower Chuniespoort Group (2.2–2.4 Ga) are the most likely source of xenocrysts, assimilated in a staging chamber beneath the main Grasvally chamber, in which the Volspruit Sulphide Zone developed. It is possible that the Malmani dolomite contained an enrichment of Pb, Zn, Cl, and S minerals prior to assimilation. The assimilation of dolomite and limestone would locally increase the fO2 of the magma, triggering chromite crystallisation. The sudden removal of Fe from the melt, coupled with the addition of external sulfur triggered saturation of an immiscible sulphide melt in the ultramafic Volspruit magma. Chromite and base metal sulphides were subsequently emplaced into the main Grasvally magma chamber as a crystal-bearing slurry. Therefore, we consider it is possible for PGE mineralisation to occur in the ultramafic portion of any layered intrusion intruding in the vicinity of carbonate units. Even if this style of mineralisation in the lowermost portions of layered intrusions is sub-economic, it may reduce the grade or opportunity for PGE mineralisation higher up in the local magmatic stratigraphy, or in later magma emplacement events sourced from the same reservoir.

The technique of specifically searching for microxenocrysts could be applied beyond layered intrusion research, to identify the range of crustal contaminants in other magmatic systems where macro-scale xenoliths are neither sampled nor preserved.