Integrative Geochronology of Ore Deposits: New Insights into the Duration and Timing of Hydrothermal Circulation
John T. Chesley, 1999. "Integrative Geochronology of Ore Deposits: New Insights into the Duration and Timing of Hydrothermal Circulation", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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Our understanding of the formation of mineral deposits and fluid circulation has increased greatly over the last few decades, through the use of increasingly sophisticated hydrogeochemical and hydrothermal fluid circulation models. However, for these models to be valid, accurate knowledge of the age of a deposit, the lifetime of the hydrothermal system(s) and an understanding of the tectonic, emplacement and/or cooling history of possible sources of heat, metals, and fluids is necessary. Some of these most basic questions concerning ore deposits remain poorly constrained despite long and intense scrutiny. The inability to tie down the timing of ore deposition and multiple episodes of hydrothermal circulation has led to a number of conflicting theories for fluid movement, fluid origin, and metal sources (e.g., Slack, 1976; Skinner, 1979; Sangster, 1986). In part, some of the conflicting theories are due to the lack of a regional framework that establishes the timing and duration of the circulation of mineralizing fluids, and to the lack of dating techniques to apply to well-documented paragenesis within an ore deposit or district. Recent advances in analytical methods and new geochronometers now allow, not only accurate dating of a mineral deposit, but in some cases potential resolution in time between different paragenetic sequences of mineralization within a single deposit (Snee et al., 1988; Chesley et al., 1993; Brannon et al., 1996b).
This chapter is an attempt to demonstrate that through the application of multiple geochronologic techniques, the temporal relationship between igneous intrusions and associated hydrothermal mineralization or timing of largescale crustal fluid flow can be refined. This chapter is divided into two parts. The first part is a discussion of different geochronologic methods.
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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