Concentrations of platinum group elements (PGE) are found in a variety of geologic settings, ranging from high-tem- perature magmatic through hydrothermal (McCallum et al., 1976) to late diagenetic or sedimentary (Kucha, 1982). The only deposits from which substantial quantities of PGE are produced are those associated with mafic or ultramafic rocks. Identified resources within these are summarized in Table 8.1.
The deposits are divisible into those in which the PGE are obtained as a byproduct, as is the case with Noril'sk-Talnakh discussed above, and those in which PGE are the principal product. At the time of writing the Merensky Reef of the Bushveld Complex is the most important producer amongst the second class of deposits. Production is also obtained from the somewhat similar J-M reef of the Stillwater Complex. Both of these are stratiform horizons containing from 0.5% to 3% sulfide. Significant production is now also obtained from the UG-2, which is the most important (from the point of view of grade and total contained PGE) of a number of PGE-enriched chromitites in the layered complexes. The Pla- treef is a sulfide enriched zone at the base of the Bushveld Complex where it transgresses down through quartzites and carbonate-rich country rocks of the Pretoria Series to come to rest on Archean granitic gneisses. It is not currently in production.
Possible other deposits with a similar setting include the mmerahzed layers of the Penikat layered intrusion of northern Finland (Alapieti and Lahtinen, 1986). A stratabound, PGE-enriched zone also occurs in
Figures & Tables
Ore Deposition Associated with Magmas
Magmatic sulfide ores are thought to form as the result of droplets of an immiscible sulfide-oxide liquid forming 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. A number of factors may influence the concentration of this liquid, but the dominant one is gravitational settling, since the liquid has a density of >4 in comparison with a value of <3 for its host silicate magma.
To help in the understanding of deposits of this type, in this book we first discuss the phase relations of simple sul- fide-oxide liquids and activity-composition relations within them. We then discuss the solubility of sulfide in mafic and ultramafic melts, followed by the partitioning of elements between silicate magma and sulfide-oxide liquid. The oxidation state and volatile content of a silicate magma can have a major influence on the segregation of a sulfide-oxide liquid and the distribution of metals so that this forms the focus of a second chapter.
Magmatic sulfide deposits can be viewed in terms of their associated mafic or ultramafic bodies and the tectonic settings into which these were emplaced. The scheme shown as Table 1. 1 is adapted from that of Naldrett, (1989). In it, bodies are divided into whether they were emplaced in a rifted continental environment (category II), a cratonic environment (category III) or an active orogenic belt (category IV) . Archean greenstone belts still represent an enigma in terms of present-day tectonics. For example, were komatiites erupted through continental crust (Arndt, 1986a; Compston et al., 1986) or do they represent the floor of a primitive ocean (de Witt et al., 1987)? Thus a separate category (category I) has been created for the syn-volcanic activity in this environment.
Experience in Archean greenstone belts has shown that mafic and ultramafic bodies fall into two main classes, komatiites and tholeiites, and that the tholeiites constitute two distinct sub-classes, one with picritic average compositions and chilled margins and the other very rich in anorthositic gabbro. The komatiites are host to Ni sulfide ores in Western Australia, Zimbabwe and Canada; these ores and their origin are discussed by C.M. Lesher in this volume. Examples of mineralization associated with the picritic sub-class of tholeiites include