Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

Abstract

The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma-mass spectrometry. These data, coupled with wholerock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt-As and Pd-Bi-Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400-800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE teno...

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

“…The Te contents of magmatic sulfides are generally in the range of a few ppm (e.g., Dare et al 2010;Djon and Barnes 2012;Piña et al 2013, Prichard et al 2013Smith et al 2014). Our data show that around the Au zone, Te concentrations are in the tens to a few hundred ppm (Table 2; Fig.…”

“…Queffurus and Barnes 2015). For comparison, Se is generally present in sulfide minerals in the range of tens of ppm, up to around 250 ppm in some sulfide minerals (e.g., Dare et al 2010;Piña et al 2013, Prichard et al 2013Smith et al 2014) and up to 455 ppm in sulfides of the Lac des Isles Complex (Djon and Barnes 2012). The S/Se ratios of the Platinova Reef (mostly 190-800) are lower than any other magmatic sulfide deposit currently recorded (see review by Queffurus and Barnes 2015; Fig.…”

“…Laser ablation-ICP-MS is one of the most effective and advanced tools in determining the fine scale mineralogy and composition of minerals in general, and the application to determining the base, precious and semimetal contents of sulfide minerals has become a standard tool in characterizing magmatic sulfide ores (e.g., Ballhaus and Sylvester 2000;Holwell and McDonald 2007;Godel et al 2007;Godel and Barnes 2008;Hutchinson and McDonald 2008;Dare et al 2010;Holwell et al 2011;Piña et al 2013, Prichard et al 2013Smith et al 2014). In addition to determining the bulk compositions of sulfides, the technique also allows for the identification of whether elements are present in solid solution, or as discrete minerals or microinclusions.…”

Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton

“…The compositions and details of analytical methods for these standards are presented in Prichard et al (2013) and further procedural details are available in Smith et al (2014 Table B). The small size of many sulphides prevented repeat analyses and therefore 1σ precision based on TRA counting statistics are typically 2-16% (concentrations 10-100 ppm), 10-27% (1-10 ppm) and 23-80% (< 1 ppm).…”

Evaluation of Platinum Group Elements (PGE) Potentials of Ultramafic Rocks of the Paleoproterozoic Nyong Series, Southeast Cameroon

“…When the melts are S-saturated, the immiscible sulfide liquids will separate from the primary melt as the temperature and pressure decrease, and be retained in the residual mantle, resulting in a PGE-poor melt because of the siderophile nature of PGE. When S-undersaturated magma is formed by high degree of partial melting, the PGE remain in the primary melt, leading to PGE, especially PPGE enrichment (Smith et al, 2014). S-undersaturation would require at least 25 % partial melting of the mantle source (Keays, 1995).…”