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Genetic aspects of the PGE-bearing MG Chromitite layers of
the Bushveld complex - are those the world-class PGE ore
bodies of the future?
Gauert, C.D.K
Dept. of Geology, University of the Free State, Bloemfontein, R.S.A., gauertcdk@ufs.ac.za
ABSTRACT
Traditionally UG2 chromitite layer and Merensky Reef (MR) have been mined for platinum, whereas the MG1 as
well as the LG6 layers for chrome. This is due to the higher PGE and base metal sulphide content of the former and
the large chrome concentrations of the latter. Lately mining companies took interest in the so far neglected PGEs in
the LG6 and MG1, mostly as by-products to chrome.
A closer look at whole rock and PGE geochemistry, cumulate textures, and cumulus mineral chemistry at selected
intersections both in the eastern as well as the western Bushveld Complex indicates a lateral homogeneity of the
observed features.
Highest total PGE concentrations of more than 5000 ppb are measured in the MG1, whereas PGE enrichment
above 3000 ppb was also achieved in the MG2, MG3 and MG4. This applies both to the Tweefontein as well as to
the Kroondal sections indicating a relative homogeneous effectivity of enrichment processes in laterally over wide
distances separated layers both of the western and eastern Bushveld Complex.
Variable incompatible trace element ratios indicate mixing of liquids of distinct magma lineage below and above
the MG2, a phenomenon which is observed both at Kroondal and Tweefontein.
Although the MG chromitite layers due to their stratigraphic and absolute vicinity to UG2 and MR and their
regular intersection in exploration programmes, would be an easy target to mine, extraction seems to be not yet
feasible due to the still existing more feasible upper layers, i.e. UG2 and MR. Extraction as by-product to chromite
is aggravated by the abundance of chromite causing difficult flotation, and the complex liberation textures in
interstices, requiring a very fine grain size to grind down to.
Scavenging of PGEs by exsolved sulphide globules from the magma at the elevation of the MG layers and the
LG7/6 was deteriorated by hampered magma mixing due to missing density contrasts between resident and
subsequent magma pulses. Thus PGE concentrations in the above layers are still regarded as sub-economic,
although this status might change with time.
KEYWORDS: Bushveld Complex, MG chromitite layers, magma mixing, cumulus minerals, PGE-bearing
potential.
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