Magmatic Sulfide Deposits

Magmatic Nickel-Copper-Platinum-group element 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 magma.

Magmatic sulfide ores form as the result of droplets of an immiscible sulfide-oxide liquid developing within silicate magma and then becoming concentrated in a particular location. Certain elements, notably the Group VIII transition metals Fe, Co, Ni, Pd, Pt, Rh, Ru, Ir and Os together with Cu and Au, partition strongly into the sulfide-oxide liquid, and thus become concentrated with it. An understanding of deposits of this type requires an understanding of the chemical and physical processes that are involved. To help in this, we first discuss (section 2.1) factors governing the solubility of sulfide in mafic and ultramafic melts. This is followed by a discussion of the partitioning of elements between sulfide-oxide liquid and silicate magma or olivine, and the influence of mass ratios between sulfide liquid and silicate melts (“R” and “N” factors) on the composition of sulfides. Some relevant phase equilibria are discussed in section 2.3. Section 2.4 is devoted to fractional crystallization of sulfide melts which is particularly relevant to the formation of valuable PGE-rich zones within deposits such as are known at Noril’sk and Sudbury and elsewhere. In a final section, 2.5, the poorly understood problem of how sulfur enters a magma from an external source is briefly discussed.

Komatiites are rocks that have been the subject of intensive investigation since they were first recognized as a separate class during the 1960’s (Viljoen and Viljoen 1969; Naldrett and Mason 1968; Nesbitt 1971). Initially they were only recognized in sequences of Archean rocks, where they constitute as much as 10% of the total volcanic succession. However, soon they were identified as important components of Proterozoic and Phanerozoic successions (Arndt and Nisbett 1982). In general the younger the komatiites, the lower the MgO content of the magma responsible for them, although the Paleocene komatiites on Gorgona Island (off the coast of Ecuador) developed from magma containing up to 24 wt% MgO (Ganser et al. 1979). It is also clear that komattiites constitute a progressively lower proportion of the volcanic succession in progressively younger successions. This has been attributed to the cooling of the earth’s mantle with time.

Flood basalt volcanism is a feature of the later stages of the Earth’s evolution, beginning in the mid to late Proterozoic. Specific episodes are characterized by the immense areas that they cover (commonly hundreds of thousands km²) and the short duration of the magmatic activity (up to 15 My in the Proterozoic and, typically, 1–2 My in the Phanerozoic). Lava facies dominate over explosive and intrusive facies and uniform tholeiitic basalts predominate among the rock types. The flood basalt provinces are typically, but not universally, equant in shape. These eruptions represent the most catastrophic igneous events in the Phanerozoic history of the earth. The magmatism occurred both in oceanic and continental environments. Oceanic flood-basalt provinces can be much more extensive than continental provinces. The largest oceanic province (the Outang-Java plateau) comprises over 60 millions km³ (Mahoney and Coffin, eds 1997) while the largest continental province (the Siberian trap) comprises about 4 millions km³ (Masaitis 1983) (see Fig. 4.1). However continental flood-basalt events had the greater effect on geological history because they resulted in ecological catastrophes that brought with them major mass extinctions. The Siberian trap define the biostratigraphic boundary between the Paleozoic and Mesozoic (Czamanske et al. 1998 and their references), the Karoo-Ferrar between the Lower and Middle Jurassic (Duncan et al. 1997), and the Deccan — between the Mesozoic and Cenozoic (e.g. Courtillot et al., 1986, 1988).

The deposits of the Pechenga area are associated with a major Early Proterozoic rift system that is referred to in Russia as the Pechenga-Varzuga sedimentary-volcanic belt (e.g. Smolkin et al. 1995a), and referenced as the Polmak — Opukasjarvi — Pasvik — Pechenga — Imandra/Varzuga — Ust’Ponoy greenstone belt in some of the Western literature (e.g. Melezhik et al. 1995). This belt traverses the northeastern part of the Fennoscandian shield for a distance of about 700 km (Fig. 5.1). It includes a series of depressions filled by Early Proterozoic sedimentary and volcanic rocks (the Polmak, Pasvik, Pechenga, Imandra-Varzuga and Ust’Ponoy Structures) that occur within a reactivated Archean basement, that is cut by dikes and granitoid intrusions in intervening areas. The belt developed at about 2.3 Ga, after the emplacement and partial erosion of a series of peridotitegabbronoritic layered intrusions, (Mt. Generalskaya, Monchegorsk, Pana and Fedorova Tundra which are the equivalents of the 2.5–2.4Ga intrusions of Northern Finland — see Chapter 9). Rifting started in the eastern and central parts of the Imandra-Varzuga structure and propagated to the west. The greatest amount of spreading along the rift occurred in the Pechenga area (Smolkin 1992, 1993, 1997). Northwest-oriented compression took place from 1.75–1.70 Ga and the belt, especially the southern part, suffered intense deformation and greenschist to amphibolite metamorphism (Smolkin at al. 1995a).

In the summer of 1993, two prospectors, Albert Chislett and Chris Verbiski, who had been sent to Labrador by Diamond Fields Resources Ltd. to conduct stream sampling for diamond indicator minerals, discovered a gossan with the blue staining characteristic of copper on a hill-top, 45 km southwest of the village of Nain. An initial 5-hole drill programme was conducted to investigate the occurrence in October 1994 and one of the holes intersected a 41.2 m zone of semi-massive to massive sulfide grading 2.96% Ni, 1.89% Cu and 0.16% Co within a wider 71.0 m zone which graded 2.23% Ni, 1.47% Cu and 0.12% Co. A major drilling program was started by Diamond Fields Resources Ltd in January 1995 and by July 1995, 31.7 million tonnes of ore grading 2.83% Ni, 1.68% Cu and 0.12% Co had been outlined at surface in the area known as the “Ovoid” and “Mini-Ovoid”. In October 1995 a second major discovery, known as the “Eastern Deeps”, was made during a stratigraphic drilling program. The deposit was acquired by INCO Ltd in August 1996. Current reserves plus inferred and indicated resources stand at 136.7×10⁶ tonnes grading 1.59 wt% Ni, 0.85 wt% Cu, 0.06 wt% Co. The success at Voisey’s Bay led to a huge surge in exploration in this part of Labrador in 1995, 1996 and to a lesser extent in 1997. Many Ni-bearing occurrences were located and explored, and some of these are discussed in the closing sections of this chapter.

The Jinchuan Ni-Cu deposit in Northwest China, with ore reserves of 500 million tons at Ni and Cu grades of 1.2 wt% and 0.7 wt%, respectively, is ranked as the third largest economic Ni-Cu deposit in the world after Noril’sk and Sudbury (see Table 1.1, Chap. 1). Ores of the deposit are relatively rich in Cu with an average ratio Ni/Cu = 1.76 (see Table 1.1) and have moderately high PGE concentrations with a total PGE and Au content of around 1 ppm in the sulfide ores (Sun 1986; Yang 1989) and (Pt+Pd)/(Ni+Cu) ratios (PGE in g/t, Ni and Cu in wt%) in the three different ore bodies of 0.04 to 0.45. However, the intrusive rocks that host the sulfide ores are all ultramafic, ranging from dunite to olivine pyroxenite, and, previously, had been thought to have formed from an ultramafic magma with an MgO content of >30 wt% (S.G.U. 1984); this is quite different to the general features of other Cu and PGE rich Ni sulfide deposits.

The first recorded mention of mineralization in the vicinity of what subsequently became the town of Sudbury occurs in a report by Alexander Murray of the Geological Survey of Canada. As Dr Murray wrote:

PGE accessible to mankind are derived from the earth’s mantle, with possibly a component coming from the core. This chapter is concerned with (a) the distribution and behavior of PGE in the mantle and in magmas derived from the mantle, (b) the characteristics of deposits derived directly or indirectly from these magmas, and (c) the methods by which, and environments in which the PGE have been concentrated.

The magmatic model for the formation of magmatic sulfide deposits carries with it concepts that have many implications for exploration — some simple, qualitative, and in common use and others not so simple, more quantitative, and not yet in common use. Examples of the application of these concepts form the subject of much of this chapter. PGE deposits have different characteristics, and bring with them different criteria for recognizing likely ore-bearing environments. In order to avoid confusion, Ni-Cu deposits are discussed first, followed by a discussion of PGE deposits.