A NEW INDICATOR OF GLACIAL DISPERSAL - LEAD ISOTOPES

Lead isotope ratios from glacigenic sediments have been successfully u sed as glacial indicators to define dispersal patterns in mineralized areas of the Canadian Shield and the Appalachian Orogen. Sediments adj acent to and down ice from volcanogenic massive sulphide (VMS) deposit s 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 th at the Pb isotope signature is preserved in much of the succession, al though incursions of Pb from other sources are also identified by Pb i sotope ratios that are much higher than those of the deposit. Sections away from known deposits have isotopic ratios characteristic of the c ountry rocks. Pb from some VMS deposits can form an ideal glacial indi cator. The reasons for this are: (i) the deposits can be considered po int sources; (ii) the Pb in old VMS deposits is isotopically different from the country rocks; (iii) the Pb abundance in the deposits is sig nificantly enriched relative to average crustal abundances; (iv) the P h isotope ratios are independent of the Ph concentration in samples co ntaining low to moderate amounts of Ph; (v) isotope ratio measurements can be made on small quantities of material; and (vi) the isotope rat ios of till enriched in VMS-derived Pb are not measurably affected by weathering.