Key factors in the genesis of Noril'sk, Sudbury, Jinchuan, Voisey's Bay and other world‐class Ni‐Cu‐PGE deposits: Implications for exploration

Abstract

Magmatic Ni‐Cu sulfide deposits form as the result of the 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 a massive concentration of magmatic sulfide. The appropriate physical environment is required to concentrate sulfides from a large mass of magma into one place. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 X 2 X 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, transcrustal fault and immediately below the centre of a 3.5 km‐thick volcanic basin. An anticlinal axis that transects the axis of the basin at a high angle has brought these intrusions to surface to give rise to the two major ore junctions, Noril'sk and Talnakh. Studies of the overlying basalts have shown that basalts forming a 500 m‐thick sequence have lost 75% of their Cu and Ni and more than 90% of their PGE. Basalts above this show a gradual recovery in their chalcophile element concentrations to reach ‘normal’ values 500 m above the highly depleted zone. Two groups of mineralised bodies have been identified as correlative with these basalts: the poorly mineralised Lower Talnakh‐type bodies,which resemble the highly depleted basalts; and the ore‐bearing Noril'sk‐type intrusions which correlate with the overlying, essentially undepleted basalts. 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 to +12 834S) are explicable if the ore‐bearing bodies are exit conduits from high‐level intrusions, along which magma has flowed en route to surface. The Lower Talnakh bodies are interpreted as intrusions along which magma flow stopped earlier than along those of the Noril'sk type. The first magma to enter the high‐level intrusion reacted with much evaporitic sulfur, at a low ‘R’ value and thus gave rise to sulfides with low metal tenors. Successive pulses 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. The flow direction along the conduits is shown by the direction in which the tenor of disseminated sulfides decreases. Sulfides have settled from the moving magma to form separate injections of liquid sulfide, up to 3.5 X 1.5 X 0.05 km in size. Recent reflection seismic studies at Sudbury have shown the Sudbury Igneous complex to have been much more extensive than originally supposed. Nd and Sr isotopic studies on rocks of the Sudbury Igneous Complex and Re‐Os studies on the ores have indicated the incorporation of much country rock gneiss in the complex. Debate has centred around whether the Sudbury Igneous Complex is entirely an impact melt or the consequence of the mixing of primary magma with 50 or more wt% impact melt; the most recent evidence favours the latter hypothesis. 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. Flow of magma into the intrusion has resulted in olivine and sulfide being retained within this keel. The Voisey's Bay deposit lies partly within a 30–100 m‐thick sheet of troctolite, which is interpreted as a flat‐lying part of a feeder for an adjacent intrusion, and partly at the base of the intrusion, where the feeder adjoins it. Ore types range from disseminated sulfides in troctolite, which increase downward, grading into massive ore. The latter is underlain by a breccia composed of fragments of gneiss, unmineralised troctolite and peridotite in a troctolitic matrix that, in places, contains appreciable sulfide. When most major Ni‐Cu sulfide deposits, including those at Kambalda, Western Australia, are viewed in the light of studies at Noril'sk, Sudbury, Jinchuan and Voisey's Bay, three factors become apparent: (i) the concentration of sulfides in channels or conduits through which much magma has flowed (feeder conduits for intrusions are much more prospective targets for exploration than the base of the intrusions themselves); (ii) the interaction of the source magma with country rocks, either leading to the incorporation of sulfur, or the felsification of the magma in question; and (iii) fractional crystallisation of sulfide liquid giving rise to Cu‐rich ores which may be far removed from the ‘source’ ore.