PB ISOTOPIC-RATIOS AND ELEMENTAL ABUNDANCES FOR SELECTIVE LEACHATES FROM NEAR-SURFACE TILL - IMPLICATIONS FOR MINERAL EXPLORATION

Pb isotope ratios obtained from fine-grained fractions (< 63 and < 2 m u m) from near-surface (< 1 m depth) till surrounding ore deposits sho w isotopic overprinting from the underlying sulfide mineralization, an d provide a new approach to mineral exploration for massive sulfide de posits (VMS) in glaciated terrains. In this study, Pb isotopic measure ments, and selective leaching of 6 near-surface till samples down-ice from the Chisel Lake (Manitoba) and Manitouwadge (Ontario) VMS deposit s were carried out in order to determine the location and nature of th e Pb within till. Elemental abundances from selective leachates for al l 6 samples display similar patterns and show that chalcophile element s (Cu, Ni, Pb and Zn), derived predominantly from the underlying VMS d eposits, occur as (i) adsorbed/exchangeable metals; (ii) associated wi th oxyhydrous Fe and Mn; (iii) crystalline Fe oxides, and/or (iv) sili cate. Despite the relative proximity of some of the till samples to th e VMS deposits, only a very small component of the chalcophile element s is present as sulfide. This result is consistent with those from stu dies of weathered (oxidized) tills, which show that labile minerals su ch as sulfides have been completely destroyed and their chemical const ituents reprecipitated or scavenged locally by clay-sized phyllosilica tes and secondary oxides/hydroxides. Pb isotopic ratios for selective leachates from till samples with VMS-like (anomalous) signatures are s imilar to those from ore (galena) within the proximal VMS deposits. Th is indicates that the Pb is of a secondary nature and was probably sca venged and deposited after destruction of original sulfide minerals du ring till formation. The lack of a predominant sulfide-held Pb compone nt within the selective leachates supports this interpretation. In con trast, Pb isotopic ratios for the same selective leachates from ''back ground'' 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 (''conventional'' leach ing). The Pb isotope ratios from the conventional leachates are simila r to those obtained from the selective leachates, and show a large dif ference in Pb isotopic ratios between anomalous and background samples . We propose, therefore, that the conventional leaching rather than se lective leaching or complete dissolution of a particular grain-size fr action be adopted for mineral exploration purposes using glacial sedim ents. The results from this study support the effective use of Pb isot ope ratios from near-surface till as an exploration tool despite the w eathered nature of the latter. We feel that this represents a more cos t-effective technique over traditional geochemical prospecting methods , if used in conjunction with Pb abundance data. Copyright (C) 1996 El sevier Science Ltd