232 by linfengfengfz


									Where are all the PGEs in the Platreef?
 The use of Laser Ablation ICP-MS in
  revealing trace element mineralogy

         David Holwell1,2 and Iain McDonald1

  1Schoolof Earth, Ocean and Planetary Sciences, Cardiff University,
          Main Building, Park Place, Cardiff, CF10 3YE, UK.
 2SRK Exploration Services Ltd, 16 Park Grove, Cardiff, CF10 3BN, UK.

The Platreef

• 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 system

•Best technique for detecting trace
amounts of PGE to a few tens of
•Laser beam is 40µm
•Can ablate lines, spots, shapes,
even words
                      Ablation pits




                LA-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 spectra
 Laser on
Os, Ir, Ru in pyrrhotite
Pentlandite exsolution flames
Polyphase analysis
              PGE in Platreef BMS
Pyrrhotite (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 balance
•PGE in whole-rock re-calculated
to the equivalent in 100% sulfide
•Plotted together with sulfide

•If an element in sulfide plots the
same as the bulk rock (100%
sulfide), it is held wholly in the
•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
PGM microinclusions
Polyphase analysis
Fractionating sulphide droplets

                 •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
                               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
Pd and Rh in ss, some Os, Ir, Ru

                                                         Pt only
                                                         present as

                                                         Pt and some
                                                         Pd PGM
                                                         around BMS
Chalcopyrite:                      Pyrrhotite:
 no PGE in ss                      Os, Ir and Ru in ss
‘Primary’ mineralization

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

• 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
                        Experimental studies
              sulphide               e.g. Fleet et al. (1993), Barnes et al. (1997),
               liquid                Peregoedova (1998), Mungall et al. (2005)

   Os,                    Pt,
   Ir,                    Pd,
   Ru,                    Au               • Os, Ir, Ru, Rh compatible in mss,
   Rh         Cu-rich
               liquid           1000°C     • Pt, Pd, Au remain in Cu-rich residual liquid


                                            • iss crystallises from the Cu-rich liquid,
                                            however Pt, Pd and Au are incompatible in
Pt, Pd, Au-                                 iss and concentrate with other trace
rich melt      Ni-mss            900°C      elements

                                           • Os, Ir, Ru, Rh are likely to be present in solid
                                           solution in cooling products of mss.
      PGM       cpy
                                           • Pt, Pd, Au form discreet PGM and
                pn              <650°C     electrum.
                 Argide 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

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