Geology of the Northern Bushveld Complex and the Setting and Genesis of the Platreef Ni-Cu-PGE Deposit
Iain McDonald, David A. Holwell, 2011. "Geology of the Northern Bushveld Complex and the Setting and Genesis of the Platreef Ni-Cu-PGE Deposit", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
Download citation file:
The Platreef is one of the largest and most valuable Ni-Cu-PGE orebodies on Earth. It is located at the base of the northern limb of the 2.06 Ga Bushveld Complex and stratigraphic relationships with other limbs of the complex and stratiform orebodies such as the Merensky Reef and UG2 chromitite are not clear. The Bushveld Complex intruded along the axis of the >2.9 Ga Thabazimbi-Murchison lineament and this may have acted as a barrier between the northern limb and the rest of the complex for some or all of the intrusion history. Research since the turn of the millenium has demonstrated that the Platreef represents a sill or complex of sills intruded into basement granite-gneiss and sediments of the Transvaal Supergroup. Different sills display variable lithologic units, thicknesses, bulk chemical signatures, and mineralization arising from different inputs of magma and the effects of local wall-rock contamination. Chilling and injection of Main zone gabbronorites took place into already solidified and deformed Platreef, indicating a major break in time between these events. Aspects of mineral chemistry and bulk geochemistry and Nd and Os isotopes in the Platreef overlap completely with the Merensky Reef but not the Upper Critical zone. Conventional and mass independent S isotopes suggest a mantle source of S that was overprinted by addition of local crustal S where the Platreef intruded pyrite-rich shales. Assimilation and introduction of external S is viewed as an ore-modifying process, not as the primary trigger for mineralization. The genesis of the Platreef is more likely to have involved introduction of PGE-rich sulfide droplets with the intruding Platreef magma. These sulfides may have been derived from the same magma(s) that formed the Merensky Reef and which injected up and out along the intrusion walls as the chamber expanded. Alternatively, the sulfides may have formed in pre-Platreef staging chambers where they were upgraded by repeated interactions with batches of Lower zone magma before being expelled as a crystal-liquid-sulfide mush by an early injection of Main zone magma, prior to the formation of the bulk of the Main zone which crystallized above (and partially eroded) the solidified Platreef.
Figures & Tables
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