The Geochemistry of the Platinum-group Elements in Mafic and Ultramafic Rocks
Published:January 01, 1989
E. A. Mathez, C. L. Peach, 1989. "The Geochemistry of the Platinum-group Elements in Mafic and Ultramafic Rocks", Ore Deposition Associated with Magmas, James A. Whitney, Anthony J. Naldrett, James M. Robertson
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The platinum-group elements (PGE) ruthenium, osmium, rhodium, iridium, palladium and platinum together with iron, cobalt and nickel form Group VIII of Mendeleev's periodic table. All are strongly siderophile and chalcophile, and their distribution in the crust and upper mantle is largely determined by the distribution of sulfides. In the case of PGE, there are few specific data on how they distribute themselves among sulfides, silicates and other phases or on their solubilities in high-temperature, volatile-rich fluids. In fact, our understanding of how these elements behave in response to igneous differentiation or metamorphism is based on empirical, primarily geochemical, observations. For these reasons, models devised to explain the development of PGE- rich horizons in the layered intrusions are not well constrained.
Their generally siderophile and chalcophile nature should not obscure the fact that in some environments the PGE are volatile. This is clearly demonstrated by the enormous enrichments in iridium (along with gold, silver and certain other trace metals) in Kilauea volcanic gases relative to lavas (Olmez et al., 1986). In the Bushveld Complex, the presence of hydrothermal platiniferous pegmatite pipes (e.g., the Driekop) and the complexities of PGE distribution within the UG-2 chromitite and Merensky Reef imply some control by fluids. Again, lack of data reduce us to little more than speculation on the specific roles of fluids in the genesis of ores in the layered intrusions.
The purpose of this chapter is to assemble all the PGE data relevant to processes occurring in layered complexes. The available data
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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