E. A. Mathez, 1989. "Vapor Associated with Mafic Magma and Controls on Its Composition", Ore Deposition Associated with Magmas, James A. Whitney, Anthony J. Naldrett, James M. Robertson
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It seems a matter of faith that fluids are responsible for any number of nature's deeds that otherwise have no obvious explanation, a case in point being the petrogenesis of platinum group element (PGE) deposits in layered intrusions. One confusion that persists is that fluids associated with all magmas are similar. They are not, of course. Those associated with acid systems are aqueous, contain high concentrations of dissolved silica and salts and had in many instances a profound influence on metallogenesis. In contrast, the fluids associated with mafic magmas are initially C02-rich and evolve to more water-rich compositions, and their role in redistribution of metals in mafic layered intrusions is uncertain.
In and around many granitoid bodies, such as those that yielded porphyry coppers, both the geology and hydrothermal mineral assemblages lend themselves to theoretical deduction of the nature and petrogenetic role of fluids. However, in complexes such as the Stillwater and Bushveld, key relationships that would allow us to make similar deductions are lacking, and inferences must be based in part on knowledge gained by experiment and analogy to natural systems. Too, the high-temperature, fluid-rock interactions in mafic systems have not been so thoroughly scrutinized.
Sulfur must be included in a discussion of volatiles since it is a component of volcanic gas and sulfides are the major ore minerals. This is particularly so because the stability of sulfide-oxide liquids in equilibrium with silicate melts is sensitive to oxidation state of the system, and oxidation state also influences the
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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