Metallogenesis in Continental Margins: Re-Os Evidence from Porphyry Copper Deposits in Chile
Joaquin Ruiz, Ryan Mathur, 1999. "Metallogenesis in Continental Margins: Re-Os Evidence from Porphyry Copper Deposits in Chile", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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Re-Os isotope studies of molybdenite and other more common sulfides have yielded new geochronological and tracer information on the origin of the ore-forming metals for base metal porphyry mineralization. Because the new geochronological data are on the sulfides and not the associated alteration minerals or magmatic rocks, it is possible to constrain the mineralizing events within the context of the magmatic history of the mineralized district or region. Re-Os studies of base metal porphyry mineralization in Chile and Arizona, the two premier base metal porphyry regions in the world, indicate that the ore formation is episodic within the overall life of the magmatic arc. In Arizona, mineralization occurs in two distinct periods: ca. 75 Ma in the northwest part of the state, and ca. 55 Ma in the southeast. In Chile, the new Re-Os data on sulfides support previous K-Ar work that date mineralization at ca. 35 to 40 Ma in the northern part of the country and ca. 3 to 5 Ma in the south. The age data on sulfides typically place mineralization in the later stages of the magmatic history of the studied districts.
Pyrite, chalcopyrite, and bornite from the base metal porphyry deposits have Re concentrations between 0.01 and 4,500 ppt and Os concentrations from 0.01 to 200 ppt. There is no obvious correlation between geographical setting or age and the concentration of Re and Os. There is also no obvious correlation between the Re content of the ore and its Pb isotope signature.
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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