Platinum-group elements form a great variety of platinum-group minerals (PGM). Different PGM are often produced by different processes. Therefore the type of PGM found may indicate its origin as, for example, formation during early or late magmatic crystallisation and often the primary PGM have been replaced during subsequent low temperature alteration and/or surface weathering.
Despite its great age the Bushveld Complex is relatively unaltered and undeformed because of its size. Even here the PGE that were magmatically concentrated in Ni-Cu-Fe-sulphide in the Merensky reef are now contained
Fig. 3 Photographs taken using a scanning electron microscope of (A) a homogeneous Pt-sulphide with a smooth out line exsolved from a Cu-Fe-sulphide (Cpy) surrounded by chromite (Ch) and plagioclase (Pl) and (B) a mottled inhomogeneous altered Pt-Pd-oxide with a ragged outline enclosed in the low temperature alteration mineral serpentine (Serp) adjacent to chromite (Ch).
in secondary minerals. These include discrete PGM sulphides enclosed by PGE-poor Ni-Cu-Fe-sulphides that are themselves associated with chromite layers. These PGM were produced by re-equilibration during cooling with PGE being expelled (or exsolved) from the crystal structure of their Ni-Cu-Fe-sulphide hosts during cooling after all the magma has crystallised (Fig. 3A) [Prichard et al. (2004b)]. Bi-, Te-, Sb- and As-bearing PGM may have formed either directly by crystallisation from a fractionated sulphide liquid or are the result of later exsolution. In the Shetland ophiolite it is clear that Bi, Te, Sb and As were introduced during low temperature regional metamorphism. The PGE came originally from a magmatic source but have been mobilised to form Pt-, Pd- and Rh-bearing bismuthides, tellurides, antimonides and arsenides and they are surrounded now by low temperature alteration minerals such as serpentine and chlorite [Prichard et al. (1994)].
The association of PGM with low temperature carbonate has been described from a number of localities including the Raglan PGE-bearing massive sulphide deposit in ultramafic lavas in Cape Smith northern Canada. Here Pt and Pd tellurides, antimonides and arsenides occur in carbonates [Seabrook et al. (2004)]. In the Shetland ophiolite, at the very Pt- and Pd-rich locality at Cliff, PGM occur surrounded by Ni carbonate associated with chromite (Fig. 1C). In Jinchuan, a major nickel and PGE deposit in China, Se has been introduced during low temperature alteration to replace palladium-bismuthides by palladium selenides [Prichard et al. (2004c)].
The alteration of PGM sulphides, bismuthides, tellurides, arsenides and antimonides to PGE-alloys appears to be the next stage in the alteration sequence. This can be observed where PGM are in contact with secondary minerals such as amphibole and chlorite. Other alloys may also form at this stage including combinations of Au, Ni, Cu, Fe with or without PGE. For example, in the Freetown complex Pt-sulphides alter directly to Pt-Fe alloys [Bowles et al. (2002)].
A final stage of alteration, probably caused by surface weathering and oxidation, is the production of PGE oxides (Fig. 3B). Often these are located around other PGM. Now PGE oxides are being described much more commonly [Auge and Legendre (1994); Moreno et al. (1999); Ortega et al. (2004)] but the processes of their formation are not understood. PGE oxides that have been described so far tend to be poorly crystalline amorphous minerals that may replace earlier minerals.
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