Thomas Oberthür, 2011. "Platinum-Group Element Mineralization of the Main Sulfide Zone, Great Dyke, Zimbabwe", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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The Great Dyke of Zimbabwe hosts the world's second largest reserve of platinum-group elements (PGE). Economic PGE mineralization is restricted to sulfide disseminations mainly in pyroxenites of the P1 layer, the Main sulfide zone, which is currently under extensive exploration. The producing Ngezi and Mimosa mines are the lowest cash-cost platinum mines in the world.
The Main sulfide zone has a fine structure made up by a number of successive, geochemically distinct layers and typical vertical element distribution patterns characterized by a general upward zoning sequence in the order → Pd Pt → base metal sulfides. With some overlap, a number of sublayers can be distinguished in the PGE subzone and in the base metal sulfides subzone of the Main sulfide zone. These layers and the element decoupling patterns are regarded to represent first-order, primary magmatic features of sulfide accumulation and concomitant scavenging of PGE in relationship to their different (and probably variable) sulfide/silicate partition coefficients.
The PGE are bimodally distributed in the Main sulfide zone: Large proportions of Pd and Rh are hosted in pentlandite, whereas Pt is dominantly present in the form of discrete platinum-group minerals (PGM). The distribution patterns of the various PGM within the Main sulfide zone suggest that a large fraction of the PGE, primarily concentrated in sulfide at magmatic conditions, was redistributed following the crystallization of sulfides in the subsolidus stage. PGE expelled from the annealing sulfides formed PGM with reactant partners like As, Te, and Bi, whose proportions and availabilities differ regionally. It is assumed that these reactions took place under largely isochemical conditions; however, chemical gradients within the Main sulfide zone and magmatic-hydrothermal fluids may have supported the small-scale redistribution of the PGE and the reactions that formed the PGM.
The current work on the Great Dyke emphasizes the role of sulfide generation and accumulation on PGE concentration which take place in the course of the magmatic evolution of layered intrusions. Sulfur saturation leading to sulfide segregation appears to be the most important factor in the primary magmatic concentration of the PGE. The enrichment of the economically most important Pd group PGE (PPGE) i.e., Pt, Pd, and Rh, is sulfide controlled. The geochemical offset patterns are regarded to reflect a first-order, dominantely magmatic control of the mineralization as these patterns are observed persistently over wide areas of the Great Dyke. In contrast, the different sulfide and PGM assemblages are viewed to represent second-order reaction products that came into existence downtemperature during annealing of the mineral assemblages. The variability of PGM assemblages appears to be controlled mostly by the presence of semimetals.
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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