40Ar/39Ar Geochronology of Supergene Processes in Ore Deposits
From the viewpoint of economic geology, weathering promotes the supergene enrichment of ore deposits and the dispersion of ore and tracer elements in the regolith surrounding an orebody. The reactive nature of the mineralogy associated with ore deposits and the structural complexity of mineralized areas promote the development of extremely deep (in excess of 400 m) and complex weathering profiles. To understand the history and to evaluate possible mechanisms and pathways for the migration of ore or pathfinder elements in supergene enrichment zones, it is often useful to unravel the complex superposition of processes that can occur during weathering. Dating authigenic minerals present in the weathering profile contributes to this goal. In this review, I will illustrate the application of 40Ar/39Ar geochronology of supergene minerals in dating weathering processes.
The supergene minerals most commonly dated by the 40Ar/39Ar method are alunite-group sulfates (alunite and jarosite) and hollandite-group Mn oxides (cryptomelane and hollandite) (Table 1). In addition to their abundance in weathering profiles and their relatively high K contents, alunite-group sulfates and hollandite-group Mn oxides often host ore elements. In some manganese deposits, hollandite-group oxides are the most abundant ore minerals (e.g., in lateritic Mn deposits). In weathering profiles overlying precious and base metal deposits, alunite-group sulfates and hollandite-group Mn oxides often contain lead, silver, copper, and zinc whereas cobalt and nickel are concentrated in supergene Mn oxides associated with lateritic Ni-Co deposits. Dating these supergene minerals provides direct information on the age and duration of the geochemical conditions conducive to dissolution, transport, and redeposition of ore elements in the weathering environment.
Figures & Tables
Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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