THE ROLE OF EXHUMATION IN THE TEMPORAL DISTRIBUTION OF ORE DEPOSITS--A DISCUSSION

Sir: Kesler and Wilkinson (2006) have taken an interesting approach to understanding the temporal distribution of ore deposits—in this case epithermal, porphyry copper, and orogenic gold deposits—by examining the role of exhumation, the conjunction of burial, uplift, and erosion that exposes ore deposits and their host rocks at the surface. Calculation of exhumation rates of Phanerozoic deposits, which developed in crust over negatively buoyant subcontinental lithospheric mantle (SCLM; Poudjom Djomani et al., 2001), shows an excellent match with the known distribution of epithermal and porphyry copper deposits, which are generally accepted to have formed at less than 1 km and 5 km depths, respectively (Seedorff et al., 2005; Simmons et al., 2005). However, there appear to be some problems with the methodology as applied to orogenic gold deposits. First, Kesler and Wilkinson (2006) use a modal age of 160 ± 15 Ma for exposed orogenic gold deposits, which is a period in Earth history when there are few, if any, absolute dates for large orogenic gold deposits (appendix in Goldfarb et al., 2005). In fact, without a description of how the best-fit curves were applied to the age-frequency histograms (fig. 2, Kesler and Wilkinson, 2006), it is unclear how the stated theoretical modal age for orogenic gold deposits, as well as for porphyry copper ores, was calculated. Second, a “review of the literature” is stated to lead to a modal formation depth for orogenic gold deposits of 10 km and this is shown to fit with the Goldfarb et al. (2005) estimate of 5 to 20 km, which is thus nicely separated from the 0.5- to 5-km range for porphyry deposits. Unfortunately, the referenced paper clearly gives a range of 2 to 20 km for orogenic gold deposits and we would argue that Phanerozoic orogenic gold deposits are as …