Where are all the PGEs in the Where are all the PGEs in the Platreef?Platreef?

The use of Laser Ablation ICP-MS in The use of Laser Ablation ICP-MS in revealing trace element revealing trace element

mineralogymineralogy

David HolwellDavid Holwell1,21,2 and Iain McDonald and Iain McDonald11

11School of Earth, Ocean and Planetary Sciences, Cardiff University,School of Earth, Ocean and Planetary Sciences, Cardiff University,

Main Building, Park Place, Cardiff, CF10 3YE, UK.Main Building, Park Place, Cardiff, CF10 3YE, UK.22SRK Exploration Services Ltd, 16 Park Grove, Cardiff, CF10 3BN, UK.SRK Exploration Services Ltd, 16 Park Grove, Cardiff, CF10 3BN, UK.

d_holwell@hotmail.comd_holwell@hotmail.com

The PlatreefThe Platreef

SulphidesSulphides

PGMPGM

• Over 2200 PGM located and analysed on SEM

• ~99% Pt/Pd phases, eg moncheite (PtTe2), kotulskite (PdTe), sperrylite (PtAs2)

• 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 systemLA-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 pitsAblation pits

pyrrhotite

pyrrhotite

pentlandite

pyrite

chalcopyrite

LA-ICP-MS analysisLA-ICP-MS analysis

• We analysed the sulphides:

• pyrrhotite FeS

• pentlandite (FeNi)9S8

• chalcopyrite CuFeS2

• 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 spectraTRA spectraLaser on

Os, Ir, Ru in pyrrhotiteOs, Ir, Ru in pyrrhotite

Pentlandite exsolution Pentlandite exsolution flamesflames

Polyphase analysisPolyphase analysis

PGE in Platreef BMSPGE in Platreef BMSPyrrhotite (ppm)

Os Ir Ru Rh Pd Pt Au

1.0 1.8 7.8 0.5 0.1 0.6 bdl

Pentlandite (ppm)

Os Ir Ru Rh Pd Pt Au

0.6 1.2 5.2 15.0 119.2 0.5 bdl

Chalcopyrite (ppm)

Os Ir Ru Rh Pd Pt Au

bdl bdl 0.1 bdl 0.3 0.1 bdl

Limits of detection for all PGE and Au <0.1ppm

Mass balanceMass 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 microinclusionsPGM microinclusions

PGM microinclusionsPGM microinclusions

Polyphase analysisPolyphase 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 Fractionating sulphide dropletsdroplets

PGE in BMSPGE 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’ mineralizationPrimary’ mineralization

•Fractionated polyphase blebs of sulfide

•Formed directly from the in situ fractional crystallization of a PGE-rich sulfide liquid

ConclusionsConclusions

• 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 studiesExperimental studiessulphideliquid

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 elements900°CNi-mss

iss

Fe-mss

• Os, Ir, Ru, Rh compatible in mss,

• Pt, Pd, Au remain in Cu-rich residual liquid

1000°CliquidCu-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°Cpn

po

cpy

Argide interferanceArgide interferance

• Co, Ni, Cu and Zn produce polyatomic argide complexes (e.g. 99CoAr, 101NiAr, 103CuAr, 105CuAr, 106ZnAr)

• These interfere with the isotopes of Ru, Rh and Pd

• Corrections are applied