Formation of magmatic nickel-sulfide ore deposits and processses affecting their copper and platinum-group element contents

Nickel-copper sulfide ore deposits are found at the base of mafic and ultramafic bodies. All their host rocks, except the Sudbury Igneous Complex, are thought to be mantle derived melts. The Sudbury Igneous Complex is thought to be the product of complete melting of continental crust. In the case of mantle-derived magmas, a high degree of partial melting of the mantle serves to enrich the silicate magma in Ni and platinum group elements (PGE). This magma must then be transported to the crust by an efficient process in order to reduce the possibility that Ni is removed from the magma by crystallization of olivine. Once the magma is emplaced into the crust, S from some source must be added to bring about saturation of the base metal sulfide liquid. An ideal site for all of these processes is where a mantle plume intersects a continental rift. The plume provides a large volume of magma, produced by a high degree of partial melting. The normal faults of the rift provide easy access to the crust so that the magma is transported efficiently. In many cases rifts contain sedimentary rocks rich in S, thus providing an ideal source of S for sulfide saturation. The heat from the plume can lead to melting of a large volume of rift sediments and release of S from the sediments to the Ni-PGE–rich primary magma. In the case of the Sudbury Igneous Complex, a very large volume of superheated magma formed by flash melting of the crust. This melting event was the result of the impact of shock waves from the explosion of a large meteor in the atmosphere. In both the case of mantle-derived magma and the case of the Sudbury Igneous Complex, once sulfide liquid formed as suspended droplets in the silicate magma it must have interacted with a large volume of mafic magma in order to become enriched in Ni, Cu, and PGE in the sulfide. This enrichment occurred when the droplets were transported or when they were suspended in eddies. The magma from which the Sudbury Igneous Complex formed was superheated and base metal sulfide liquid formed at approximately 200°C above the magma liquidus. Thus the Sudbury sulfide liquid had more time to equilibrate with the silicate magma than a sulfide droplet in a mantle-derived magma. This extra time and the huge volume of silicate magma in the melt sheet allowed the sulfide liquid to maximize the concentration of Ni, Cu, and PGE. The efficiency of the metal collection step in the case of the Sudbury Igneous Complex counterbalanced the fact that this crustalderived magma had lower Ni and PGE contents than most mantle-derived magmas. The sulfide droplets collected at the base of intrusions and lava flows because they are denser than the silicate magma. The largest concentrations are typically found in locations where there are changes in the geometry of the contacts between intrusions or flows and the country rock. In some cases the accumulated sulfide liquid fractionated to form an Fe-rich monosulfide solid-solution (mss) cumulate and a Cu-rich sulfide liquid which later crystallized as an intermediate solid solution (iss). As a result of crystal fractionation of mss many Ni sulfide orebodies show a strong zonation with respect to Cu and PGE. During mss fractionation Os, Ir, Ru, and Rh concentrated in the mss cumulate and Cu, Pt, Pd, and Au concentrated in the Cu-rich sulfide liquid. The partition coefficient for Ni into mss is close to 1; thus, mss fractionation would not have caused large variations in Ni concentrations. The silicate magma solidified at or above 1,000°C whereas the Cu-rich sulfide liquid solidified at ~900°C. Thus, at many localities the Cu-rich sulfide liquid appears to have migrated into dilatent spaces in the footwall or the hanging wall to form veins that extend into the country rock for up to 2 km. At subsolidus temperatures a number of processes modify the orebodies. Both the mss and iss are not stable below 600°C. As the sulfides cooled mss exsolved to form pyrrhotite and pentlandite (±pyrite), and iss exsolved to form chalcopyrite and pyrrhotite (±cubanite, ±pyrite). Most of the PGE and chalcophile elements that originally partitioned into mss or iss are not readily accommodated in the structure of pyrrhotite, pentlandite, and chalcopyrite; therefore, they exsolve from the mss and iss at low temperature and form a wide variety of platinum group minerals (PGM). During deformation stress may focus in the structurally incompetent massive sulfide units, which are generally located at the lower contact of the mafic or ultramafic host rock. In this situation the massive sulfides may then be displaced relative to the host rocks. Finally, during greenschist to amphibolite metamorphism, olivine is unstable and Ni released from the olivine will partition into disseminated sulfides, thereby upgrading the sulfides

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