Re-Os Isotope Geochemistry of Magmatic Sulfide Ore Systems
D. D. Lambert, J. G. Foster, L. R. Frick, E. M. Ripley, 1999. "Re-Os Isotope Geochemistry of Magmatic Sulfide Ore Systems", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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The formation of magmatic Cu-Ni-Co-platinum-group element (PGE) sulfide deposits is dependent on mantlederived silicate magmas (komatiites and basalts) attaining sulfide saturation. Because the sulfur content of the upper mantle is considered to be low (250 ppm; McDonough and Sun, 1995), it is likely that, for moderate to high degrees of partial melting (≥25%), much of earth’s mantle-derived magmatism is sulfide undersaturated at the time of separation from the mantle residue (Morgan and Baedecker, 1983; Keays, 1995; Lesher and Stone, 1996). Thus, many models for the petrogenesis of giant magmatic sulfide deposits associated with mafic-ultramafic rocks propose that sulfide saturation and immiscible sulfide ore formation were a consequence of assimilation of crust or crustally derived sulfur into sulfide-undersaturated mafic-ultramafic magmas which transported base and precious metals from the mantle to the upper crust (see Naldrett, 1989, and Barnes et al., 1997a, for summaries).
Constraining the sources of sulfur and metals in magmatic ore deposits is important for understanding the dynamic and potentially open-system behavior of their parental magmatic systems. This information can lead to improved or enhanced exploration strategies in prospective new terranes based on the recognition of key geodynamic processes of ore formation (Duke, 1990; Barnes et al., 1997a; Lambert et al., 1998a). Important tools that can be used in this regard are the stable and radiogenic isotope systems. The study of the sulfur isotope composition of magmatic sulfides is the best method of directly tracing the source(s) of sulfur in ore deposits. In many cases, S isotope data demonstrate that assimilation of crustally derived sulfur into sulfide-undersaturated mantle-derived magmas is an important step in the genesis of magmatic sulfide deposits
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Lead (Pb) isotope compositions of sulfide minerals coupled with rocks associated with an ore deposit provide critical constraints on the source of metals and fluid pathways in a fossil hydrothermal system (Heyl et al., 1966; Stacey et al., 1968; Gulson, 1986; Sanford, 1992). Lead isotope compositions of sulfide minerals also provide chronologic information, either absolute or relative, for ore deposition (for example, Carr et al., 1995) and can also be used as an exploration tool during prospect evaluation (Gulson, 1986; Young, 1995). These varied applications of Pb isotopes to achieve an understanding of the ore genesis process are too diverse to be adequately discussed in a single overview chapter. Instead, this chapter focuses attention on what Pb isotopes tell us about (1) the sources of Pb nd other metals in ore deposits, (2) the interaction between hydrothermal fluids and wall rocks, (3) the influence of basement rocks and tectonic setting on Pb sources in ore deposits in magmatic arcs, and (4) the application f crustal-scale Pb isotope variations to an understanding of regional controls on ore deposition Before Pb isotopes pertinent to understanding ore genesis can be examined, we must review some basic principles of Pb isotope geochemistry (Fig. 1). Elegant discussions of U-Th-Pb geochemistry are presented by Doe (1970), Faure (1977), Zartman and Haines (1988), Garipy and Dupr (1991), and Dickin (1995). The following discussion is simplified from these sources. Three isotopes, 208Pb, 207Pb, and 206Pb, are partly the radiogenic daughter products from the radioactive decay of one isotope of thorium (232Th 208Pb*) and two isotopes of uranium (238U 206Pb* and 235U 207Pb*). (Note that an asterisk (*) after an isotope denotes that it is the product of radioactive decay of a parent isotope over time and is not the total abundance of the isotope in a sample.) The abundance of radiogenic isotopes has grown since the earth formed some 4.56 billion years ago (Fig. 1), building upon an initial concentration. The fourth isotope of Pb, 204Pb, is stable and has no long-lived parent isotope nordoes it decay to another isotope. Time-integrated growth of radiogenic Pb isotopes from an arbitrary starting time, t1, to an ending time, t1, in an environment where there has been no migration of U, Th, and their daughter products, is described by standard decay equations: These equations simply show that the measured present-day Pb isotope composition is equal to the sum of the initial