E. A. Mathez, 1989. "Interactions Involving Fluids in the Stillwater and Bushveld Complexes: Observations from the Rocks", Ore Deposition Associated with Magmas, James A. Whitney, Anthony J. Naldrett, James M. Robertson
Download citation file:
Nearly all mafic igneous bodies exhibit evidence for the presence of volatile-rich melts and fluids. This is true for thin sills, in which localized pods of pegmatitic and gran- ophyric rocks are common, as well as for the layered intrusions. In the Bushveld and Stillwater Complexes, volatile elements, either as a separate vapor or simply dissolved in the melt, were clearly involved in the petrogenesis of the PGE-rich horizons because the constituent rocks are pegmatitic and contain magmatic, volatile-bearing minerals. There is also circumstantial evidence that volatiles were involved in processes of PGE enrichment or redistribution. The specific roles of fluid in these processes are enigmatic because there is a lack of knowledge about the composition of fluid under near-solidus conditions, the ability of fluid to transport PGE and the physical and chemical interactions of fluid with partially molten to completely crystalline cumulates.
A conception of how fluids and volatile-rich melts behave in the environment of a crystallizing layered intrusion must originate from observations of the rocks themselves. The point of this chapter is to bring together such observations. To make this task managable, the description is restricted to the Stillwater and Bushveld Complexes. A reference stratigraphic section of the former is presented in Fig. 9.1, and for the Bushveld reference may be made to von Gruenewaldt et al. (1985). Implicit here is the conception, first expressed by Howland et al. (1936) in the above quotation, that the processes which guided the evolution of the two complexes were essentially
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