Many hypotheses for the formation of porphyry and skarn deposits and other deposits which are spatially and temporally associated with felsic igneous rocks suggest that the magmatic “volatile constituents” exert strong controls on the quantity, chemistry and style of the mineralization found in these systems. Many workers have summarized the data on the relationships between metallization and parameters such as rock type, ore metal content, and halogen content of the associated pluton(s). More recently, thermodynamic parameters such as fo2 and the fugacities of water, HF, and HCl have been related to the nature of the associated deposits.
In our laboratory, my coworkers and I have examined the partitioning of molybdenum and tungsten between water- saturated, high silica melts and minerals such as magnetite, ilmenite and rutile as a function of fo2 at 800°C and 1 kb (Tacker and Candela, 1987; Bouton et al., 1987; Bouton and Candela, in prep.). In this paper, our experimental results will be combined with previous work on felsic igneous rocks and associated ores in an effort to shed light on the role of fo2 in magmatic-hydrothermal ore genesis. Further, some of the models discussed in Candela (this volume) will be used to suggest links between the volatile constituents of magmas, their source regions, and the ore systems they spawn.
This chapter is not meant to be a review of the work done to date on the igneous petrology of the volatile constituents or their relationship to ore genesis. Rather, I will explore the implications
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