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
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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.