Discussion of “Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands” by D.C. Bradley and D.L. Leach

Abstract

The interesting and useful study by Bradley and Leach (2003) of tectonic controls on Mississippi Valley-type (MVT) mineralization in orogenic forelands is marred by a surprising disregard for published evidence for the age of these deposits. This problem is of major importance, because the ages of the deposits control their relation to tectonic features. Here are three examples of the problem, showing how they relate to the suggested tectonic models. 1. Bradley and Leach (2003, p. 657 and p. 663) cite the Canadian Rockies as an example of MVT deposits formed in anAndean-type orogen based on Laramideage paleomagnetic poles for Pine Point, KickingHorse and Robb Lake. They ignore Devonian Rb/Sr ages for sphalerite from Pine Point because they ‘‘may date late Devonian clays entrapped in the much younger sphalerite.’’ This statement is referenced to Symons et al. (1998b), which contains nothing about Pine Point. Perhaps they meant to refer to Symons et al. (1998a), which states (p. 79) that ‘‘Garven and Sverjensky (1994) noted that the (Rb-Sr)method is prone to contamination by colloidal clay particles entrapped in sphalerite....’’ However, Garven and Sverjensky (1994, p. 1,150) say only that ‘‘Other workers dated earlystage sphalerite at Pine Point as Devonian using a RbSr method, but this type of analysis may be prone to contamination by clay particles incorporated during rapid precipitation of colloform aggregates of metal sulfides.’’ None of these studies contains any information on clay minerals at Pine Point. Nakai et al. (1990, 1993), which were omitted by Bradley and Leach (2003), deal with this issue specifically and show that inclusions of clay minerals are unlikely to account for Rb-Sr compositions of MVT sphalerite. If Cordilleran MVT deposits are Devonian in age, they could have formed when the western margin of North America was ‘‘...dominated by long-lived regional extension’’ (Nelson et al. 2002). 2. Bradley and Leach (2003, p. 657 and Fig. 5) cite the Cevennes and Maestrat areas as examples of MVT deposits formed in an inversion-type orogen of Santonian to Miocene-age in the Pyrenees. In support of this interpretation, they refer to U-Pb dating by Grandia et al. (2000), isotopic ages on fluorite in Leach et al. (2001), and paleomagnetic measurements by Lewchuk et al. (1998). Sample and analytical data on the fluorite dated by Leach et al. (2001) are, to our knowledge, not published or available for evaluation. However, the U-Pb isochron age of about 62.6 Ma Editorial handling: L. Meinert