Pressure fluctuations and the formation of the PGE-rich Merensky and chromitite reefs, Bushveld Complex |
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| Authors: | R G Cawthorn |
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| Institution: | (1) School of Geosciences, University of the Witwatersrand, PO Wits, , 2050, South Africa |
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| Abstract: | The Merensky Reef and the underlying Upper Group 2 chromitite layer, in the Critical Zone of the Bushveld Complex, host much
of the world’s platinum-group element (PGE) mineralization. The genesis is still debated. A number of features of the Merensky
Reef are not consistent with the hypotheses involving mixing of magmas. Uniform mixing between two magmas over an area of
150 by 300 km and a thickness of 3–30 km seems implausible. The Merensky Reef occurs at the interval where Main Zone magma
is added, but the relative proportions of the PGE in the Merensky Reef are comparable to those of the Critical Zone magma.
Mineral and isotopic evidence in certain profiles through the Merensky Unit suggest either mixing of minerals, not magmas,
and in one case, the lack of any chemical evidence for the presence of the second magma. The absence of cumulus sulphides
immediately above the Merensky Reef is not predicted by this model. An alternative model is proposed here that depends upon
pressure changes, not chemical processes, to produce the mineralization in chromite-rich and sulphide-rich reefs. Magma was
added at these levels, but did not mix. This addition caused a temporary increase in the pressure in the extant Critical Zone
magma. Immiscible sulphide liquid and/or chromite formed. Sinking sulphide liquid and/or chromite scavenged PGE (as clusters,
nanoparticles or platinum-group minerals) from the magma and accumulated at the floor. Rupturing of the roof resulted in a
pressure decrease and a return to sulphur-undersaturation of the magma. |
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| Keywords: | Bushveld Complex Platinum mineralization Pressure fluctuations Sulphide immiscibility Chromite |
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