A new indicator of glacial dispersal: Lead isotopes

Abstract

Lead isotope ratios from glacigenic sediments have been successfully used as glacial indicators to define dispersal patterns in mineralized areas of the Canadian Shield and the Appalachian Orogen. Sediments adjacent to and down ice from volcanogenic massive sulphide (VMS) deposits have Pb isotope ratios that reflect an input of Pb from the deposits. These ratios are distinctly different from those of the surrounding country rocks. Vertical sections overlying one of the deposits show that the Pb isotope signature is preserved in much of the succession, although incursions of Pb from other sources are also identified by Pb isotope ratios that are much higher than those of the deposit. Sections away from known deposits have isotopic ratios characteristic of the country rocks. Pb from some VMS deposits can form an ideal glacial indicator. The reasons for this are: (i) the deposits can be considered point sources; (ii) the Pb in old VMS deposits is isotopically different from the country rocks; (iii) the Pb abundance in the deposits is significantly enriched relative to average crustal abundances; (iv) the Pb isotope ratios are independent of the Pb concentration in samples containing low to moderate amounts of Pb; (v) isotope ratio measurements can be made on small quantities of material; and (vi) the isotope ratios of till enriched in VMS-derived Pb are not measurably affected by weathering.