World-class Ni-Cu-PGE deposits: key factors in their genesis

Abstract

Magmatic Ni-Cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma, and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce an ore deposit. The appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals, and then is concentrated in a restricted locality so that the resulting concentration is of ore grade. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 × 2 × 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, trans-crustal fault and immediately below the centre of a 3.5 km-thick volcanic basin. Studies of the overlying basalts have shown that lavas forming a 500 m-thick sequence within these have lost 75% of their Cu and Ni and more than 90% of their PGE. Overlying basalts show a gradual recovery in their chalcophile element concentrations to reach “normal” values 500 m above the top of the highly depleted zone. The ore-bearing Noril'sk-type intrusions correlate with those basalts above the depleted zone that contain “normal” levels of chalcophile elements. The high proportion of sulfide (2–10 wt.%) associated with the Noril'sk-type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+8–+12 ∂34S) are explicable if the ore-bearing bodies are exit conduits from high level intrusions, along which magma has flowed en route to extrude at surface. The first magma to enter these intrusions reacted with much evaporitic sulfur, at a low “R” value and thus gave rise to sulfides with low metal tenors. Successive flow of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows. The Voisey's Bay deposit lies partly within a 30–100 m-thick sheet of troctolite, interpreted as a feeder for the 1.334 Ga Voisey's Bay intrusion, and partly at the base of this intrusion, where the feeder adjoins it. Studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was Ni depleted but that subsequent pulses were much less depleted. Trace element, Re-Os and S and O isotope data, and mineralogical studies indicate that the magma pulses interacted with country gneiss, probably principally in a deeper level intrusion, extracting SiO2, Na2O, K2O and possibly sulfur form the gneiss, which accounts for the magma becoming sulfide saturated. The Jinchuan deposit of north central China occurs within a 6 km-long dyke-like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma. Flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion. The West Australian komatiite-related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma. The sediments are sulfide-rich, and may have contributed substantially to the sulfide of the ores. The mineralisation in the Duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite- and sulfide-bearing sediments. No magma flow channels have been identified so far, and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade. When most major Ni-Cu sulfide deposits are compared, they prove to have a number of features in common; olivine-rich magma, proximity to a major crustal fault, sulfide-bearing country rocks, chalcophile element depletion in related intrusive or extrusive rocks, field and/or geochemical evidence of interaction between the magma and the country rocks, and the presence of or proximity to a magma conduit. The features are thought to explain the three key requirements (sulfide immiscibilty, adequate mixing between sulfides and magma, and localisation of the sulfides) discussed and have important implications with respect to exploration.