Where are all the PGEs in the Platreef? The use of Laser Ablation ICP-MS in revealing trace element mineralogy David Holwell 1,2 and Iain McDonald 1 1 School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK. 2 SRK Exploration Services Ltd, 16 Park Grove, Cardiff, CF10 3BN, UK.

The Platreef

Sulphides

PGM Over 2200 PGM located and analysed on SEM ~99% Pt/Pd phases, eg moncheite (PtTe 2 ), kotulskite (PdTe), sperrylite (PtAs 2 ) No Os bearing PGM at all Where is all the Os, Ir, Ru, Rh? Samples with Pt/Pd ratios ~0.3, but no Pd phases

LA-ICP-MS system Best technique for detecting trace amounts of PGE to a few tens of ppb. Laser beam is 40µm Can ablate lines, spots, shapes, even words

Ablation pits pyrrhotite pentlandite pyrite chalcopyrite

LA-ICP-MS analysis We analysed the sulphides: pyrrhotite FeS pentlandite (FeNi) 9 S 8 chalcopyrite CuFeS 2 For the following elements: S, Cu, Ni, Co, Zn (monitors sulphide composition/phase) the PGE and Au Semi-metals As, Bi, Te, Sb, Se (to identify any PGM present) Using a New Wave Research UP213 UV laser coupled to a Thermo X Series ICP-MS. Analyses performed using a 40μm laser spot at a frequency of 10Hz.

TRA spectra Laser on

Os, Ir, Ru in pyrrhotite

Pentlandite exsolution flames

Polyphase analysis

PGE in Platreef BMS Pyrrhotite (ppm) OsIrRuRhPdPtAu bdl Pentlandite (ppm) OsIrRuRhPdPtAu bdl Chalcopyrite (ppm) OsIrRuRhPdPtAu bdlbdl0.1bdl0.30.1bdl Limits of detection for all PGE and Au <0.1ppm

Mass balance PGE in whole-rock re-calculated to the equivalent in 100% sulfide Plotted together with sulfide concentrations If an element in sulfide plots the same as the bulk rock (100% sulfide), it is held wholly in the sulfide If an element plots below that of the bulk rock, it must be present as discreet phases Confirmed with PGM observations (Holwell and McDonald, 2007, CMP)

PGM microinclusions

Polyphase analysis

Consistent with a magmatic origin The association of IPGE- bearing BMS with Pt/Pd PGM around the margings/ at grain boundaries: Most ‘primary’ style of Platreef mineralization Fractionating sulphide droplets

PGE in BMS This study has revealed that in the typical magmatic sulphide assemblage of pyrrhotite, pentlandite and chalcopyrite in the Platreef: Pyrrhotite hosts Os, Ir and Ru in solid solution Pendlandite hosts Rh and Pd, with some Os, Ir, Ru in ss Chalcopyrite hosts no appreciable PGE No phases contain Pt in ss, but all contain PGM microinclusions, commonly Pt-Bi phases Or: Pt occurs as PGM microinclusions and as PGM Pd occurs mainly as PGM, but also within pentlandite Rh occurs mainly in ss in pentlandite, occasionally as PGM Os, Ir and Ru very rarely occur as PGM and are concentrated in ss in pyrrhotite and pentlandite

Chalcopyrite: no PGE in ss Pentlandite: Pd and Rh in ss, some Os, Ir, Ru Pyrrhotite: Os, Ir and Ru in ss Pt only present as discreet PGM Pt and some Pd PGM present around BMS margins

‘Primary’ mineralization Fractionated polyphase blebs of sulfide Formed directly from the in situ fractional crystallization of a PGE-rich sulfide liquid

Conclusions Pentlandite and pyrrhotite hold virtually all the bulk Os, Ir, Ru and Rh in the Platreef sulfides and formed from the cooling and crystallization of a PGE rich sulfide liquid. Some Pd is also locked up in solid solution in pentlandite Chalcopyrite contains no PGE Pt and Au do not occur in solid solution within sulfides and form discreet PGM (or electrum). ‘Primary’ assemblages are present throughout the Platreef pyroxenites, however, they may be altered by hydrothermal activity…

Experimental studies sulphide liquid e.g. Fleet et al. (1993), Barnes et al. (1997), Peregoedova (1998), Mungall et al. (2005) Pt, Pd, Au- rich melt iss crystallises from the Cu-rich liquid, however Pt, Pd and Au are incompatible in iss and concentrate with other trace elements 900°C Ni-mss iss Fe-mss Os, Ir, Ru, Rh compatible in mss, Pt, Pd, Au remain in Cu-rich residual liquid 1000°C liquid Cu-rich mss Pt, Pd, Au Os, Ir, Ru, Rh PGM Os, Ir, Ru, Rh are likely to be present in solid solution in cooling products of mss. Pt, Pd, Au form discreet PGM and electrum. <650°C pn po cpy

Argide interferance Co, Ni, Cu and Zn produce polyatomic argide complexes (e.g. 99 CoAr, 101 NiAr, 103 CuAr, 105 CuAr, 106 ZnAr) These interfere with the isotopes of Ru, Rh and Pd Corrections are applied