AUTHORS: James Mungall
The world’s principal sources of platinum group elements (PGE) are stratiform deposits that are generally conformable with the layering in large layered igneous intrusions. The origins of stratiform PGE deposits remain controversial after several decades of intensive petrological study. Two apparently diametrically opposed hypotheses are popular; both depend on the extreme compatibility of PGE with sulfide liquid to provide a proximal physical mechanism for concentrating them in a confined layer. One viewpoint holds that the sulfide melts formed in the magma and collected downwards on top of the crystal pile. Other researchers maintain that the PGE are collected from within the cumulate pile and advance upwards in a chromatographic front, perhaps until they reach the top of the cumulate sequence. The controversy presents an interesting example of a stalemate between viewpoints that are both well supported by observation and theory, but seem to be mutually incompatible. A resolution will likely draw on ideas from both camps.
KEYWORDS: PGE, layered intrusions, hydrothermal, magmatic sulfide
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