Download Issue PDF - Platinum Metals Review

doi:10.1595/147106709X480913
•Platinum Metals Rev., 2010, 54, (1)•
appears to be almost universal in the magmatic sulfide
deposits reviewed here. The iss recrystallises to
chalcopyrite with no PGEs in solid solution, which is
also a universal feature of the natural sulfides.
Crystallisation of the semimetal rich liquid on
cooling forms discrete PGMs around the margins of
the sulfide blebs and in small veinlets injected into
the surrounding silicates,which explains the tendency
for PGMs to be present around the margins of sulfide
blebs (Figure 5(d)).Later alteration of the sulfide
blebs, with replacement by secondary amphiboles
around the margins of base metal sulfide blebs,
appears not to affect the early formed PGMs, which
are then isolated as satellite grains within secondary
silicates around the base metal sulfide blebs,and are
therefore no longer spatially associated with sulfides.
This relationship of Pt- and Pd-bearing PGMs
occurring as satellite grains around the margins of
sulfide blebs has long been recognised in natural
ores (34, 35).
The presence of trapped microinclusions of Pt-Bi-
Te minerals in all sulfide phases implies that Bi and
Te were present within the initial sulfide liquid at
high temperature. In this sense, the differences
between the Platreef studies by Holwell and
McDonald (25) and Hutchinson and McDonald (26)
show a fundamental difference in the sources of certain
semimetals (primary magma versus external
country rocks) and the profound influence that has
had on the resultant mineralogy. The key factor in the
mineralogical differences in Platreef ores from
Turfspruit and Overysel in the northern Bushveld
Complex is the proportion of semimetals within the
mineralising sulfide liquid.Where there was a limited
amount, Pt partitioned into the semimetal-rich melt
with a little Pd,and most Pd was accommodated with-
Glossary
Term
Definition
amphibole
A group of hydrated double silicate minerals containing calcium,
iron, magnesium, sodium and aluminium
bleb
A irregular bubble-like mass of one or more minerals
chalcophile An element that has a high affinity for sulfur and thus readily
forms sulfide minerals
chalcopyrite CuFeS 2 , a sulfide ore of copper
chromitite A rock with high concentrations of chromite ((Fe, Mg)Cr 2 O 4 )
feldspathic
Containing feldspar (aluminium silicates with potassium, sodium,
calcium or barium)
felsic
Rock rich in silicate minerals such as feldspar and quartz
footwall
The layer of rock beneath a vein or expanse of ore
fugacity
Tendency of a compound in a mixture to vaporise or escape from
the liquid phase
komatiites
Ultramafic rocks low in silica, potassium oxide and alumina and
high in magnesium oxide
mafic/ultramafic Rock rich in silicate minerals containing iron and magnesium,such
as olivine and pyroxene
ophiolite
Oceanic crust rocks that have been moved onto a continental crust
orthomagmatic Referring to the stage where silicates crystallise from magma
pentlandite
(Fe, Ni) 9 S 8 , a sulfide mineral of nickel
pyroxenite
Rock with high concentrations of pyroxenes (silicates of iron,
magnesium and calcium)
pyrrhotite
Fe (1–x) S, a sulfide mineral of iron (0≤x≤0.2)
34 © 2010 Johnson Matthey