Years ago it was assumed that nearly all metallic ore deposits were associated in some way with magmatic systems. Even deposits such as the Mississippi Valley lead/zinc deposits were hypothesized to be related to some granitic source at depth. Then, in the early 1970's, magmatic models began to fall out of favor as many economic geologists recognized that igneous processes were not required for the generation of many ore deposits. Today, the pendulum of opinion has returned to a middle position. We recognize that there are a number of metallic ore deposits whose existence does not necessarily require a direct magmatic input. There are, however, a number of classes of deposits whose close temporal and spatial relationships suggest a genetic connection with silicic plutonic or volcanic events.
Porphyry systems are disseminated deposits of copper, molybdenum, lead, and zinc with relatively low gold and silver. The mineral assemblages are mainly oxidized sulfide assemblages. In many cases there are several periods of mineralization involved in the formation of an economic deposits. In most cases, a protore stage in which the ore minerals are deposited in an intricate series of healed veins and veinlets is first developed. Where this stage is preserved, the fluids appear to have been hot and saline. Isotopic studies suggest that these early volatile phases were partly magmatic in origin. Thus, the protore stage of porphyry systems may be thought of as a deuteric magmatic process. Many porphyry systems are thought to be the roots of old volcanic
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