Genesis of the PGE-Enriched Merensky Reef and Chromitite Seams of the Bushveld Complex
Published:January 01, 2011
Anthony Naldrett, Judith Kinnaird, Allan Wilson, Marina Yudovskaya, Gordon Chunnett, 2011. "Genesis of the PGE-Enriched Merensky Reef and Chromitite Seams of the Bushveld Complex", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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The total grams of platinum group elements (PGE) in the Critical zone of the Bushveld when projected to the original horizontal and expressed as g/m2 is relatively uniform for the Critical zone of the Bushveld Complex as a whole, varying from 157 to 171 g/m2 in the western Bushveld and from 105 to 116 in the eastern Bushveld. However, in the interval from the top of the UG-2 to the top of the Merensky Reef, the northwestern Bushveld is twice as rich as the southwestern Bushveld, which is again significantly richer than the eastern Bushveld. This latter...
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Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis
Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the