Granitoid Textures, Compositions, And Volatile Fugacities Associated With The Formation Of Tungsten-Dominated Skarn Deposits
Published:January 01, 1989
J. D. Keith, Middelaar W. van, A. H. Clark, C. J. Hodgson, 1989. "Granitoid Textures, Compositions, And Volatile Fugacities Associated With The Formation Of Tungsten-Dominated Skarn Deposits", Ore Deposition Associated with Magmas, James A. Whitney, Anthony J. Naldrett, James M. Robertson
Download citation file:
Perhaps a dominant theme that has been expressed in the last few years concerning the petrogenesis and metallogeny of granitoids in western North America concerns the relative proportions and composition of various types of crustal material that have been incorporated in batches of magma (Keith et al., 1985a; Barton, 1987; Farmer and DePaolo, 1983; Ague and Brimhall, 1985, 1986, 1987; Christiansen et al., 1986; Stein and Hannah, 1985; Newberry and Swanson, 1986). Many workers suggest that these processes of crustal melting or contamination by crustal components exert one of the strongest controls on the character or grade of mineralization that ultimately may be produced by that magma. However, broad distinctions between classes of mineralization (i.e. porphyry Cu versus W skarn) may largely be a function of depth of crystallization and correlative water content of the magma (Einaudi et al., 1981; Burt et al., 1982; Barton et al., 1988; Newberry and Swanson, 1986). The controversy over how metal ratios in a deposit are related to granitoid composition is particularly applicable to W, Mo, and Sn deposits because of their close spatial association to granitoids and strong evidence that the dominant portion of the metals and the hydrothermal fluid are derived from the magma. This being the case, the question that must be asked is which magmatic characteristics are set (or modified) during assimilation of crustal material, that have some control on the metal ratios or grade of mineralization, versus those which are "inert" in terms of effecting mineralization. Perhaps the
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