Pb isotopic ratios and elemental abundances for selective leachates from near-surface till: implications for mineral exploration

Abstract

Pb isotope ratios obtained from fine-grained fractions ( < 63 and < 2 gmm from near-surface ( < 1 m depth) till surrounding ore deposits show isotopic overprinting from the underlying sulfide mineralization, and provide a new approach to mineral exploration for massive sulfide deposits (VMS) in glaciated terrains. In this study, Pb isotopic measurements, and selective leaching of 6 near-surface till samples down-ice from the Chisel Lake (Manitoba) and Manitouwadge (Ontario) VMS deposits were carried out in order to determine the location and nature of the Pb within till. Elemental abundances from selective leachates for all 6 samples display similar patterns and show that chalcophile elements (Cu, Ni, Pb and Zn), derived predominantly from the underlying VMS deposits, occur as (i) adsorbed/exchangeable metals; (ii) associated with oxyhydrous Fe and Mn; (iii) crystalline Fe oxides, and/or (iv) silicate. Despite the relative proximity of some of the till samples to the VMS deposits, only a very small component of the chalcophile elements is present as sulfide. This result is consistent with those from studies of weathered (oxidized) tills, which show that labile minerals such as sulfides have been completely destroyed and their chemical constituents reprecipitated or scavenged locally by clay-sized phyllosilicates and secondary oxides/hydroxides. Pb isotopic ratios for selective leachates from till samples with VMS-like (anomalous) signatures are similar to those from ore (galena) within the proximal VMS deposits. This indicates that the Pb is of a secondary nature and was probably scavenged and deposited after destruction of original sulfide minerals during till formation. The lack of a predominant sulfide-held Pb component within the selective leachates supports this interpretation. In contrast, Pb isotopic ratios for the same selective leachates from “background” samples are significantly higher and show that the Pb is not derived from proximal VMS deposits but from a more radiogenic source. Till samples were also leached using 2.5 M HCl (ldconventional” leaching). The Pb isotope ratios from the conventional leachates are similar to those obtained from the selective leachates, and show a large difference in Pb isotopic ratios between anomalous and background samples. We propose, therefore, that the conventional leaching rather than selective leaching or complete dissolution of a particular grain-size fraction be adopted for mineral exploration purposes using glacial sediments. The results from this study support the effective use of Pb isotope ratios from near-surface till as an exploration tool despite the weathered nature of the latter. We feel that this represents a more cost-effective technique over traditional geochemical prospecting methods, if used in conjunction with Pb abundance data.