A Model for PGE Enrichment Due to the Splitting of Freezing Magma Chambers by Suspended Crystal Loads

Exploration & Mining Geology, Vol. 6, No. 2, 1997




The mechanism by which stratiform ore deposits of PGE (platinum-group
elements) are laid down in ultramafic igneous complexes is not completely clear.
Early studies by the author and coworkers on numerical analyses of the
convective cooling of low-Rayleigh-number magma chambers examined the effect of
suspended crystal load on fluid density, and raised the possibility that the
growth of the suspended crystal load during cooling could split a magma chamber
into several convecting layers. The lower layers would have a higher content of
refractory phases than the upper layers, which would consist of more evolved
melt. This paper considers numerical modelling, as well as theoretical and
experimental work in other fields dealing with suspended loads in fluids. The
calculations, which are based on the physics of convecting magmas, yield
quantitative relationships consistent with the observation that economically
interesting stratiform PGE deposits are restricted to large magma bodies, and
also yield estimation of the depth in a magma chamber at which certain grades of
mineralization may occur.
Keywords: PGE, Ultramafic igneous complexes, Magma chambers, Suspended
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