Application of lead isotopes in till for mineral exploration: A simplified method using ICP-MS
Elemental abundances and Pb isotopic ratios for 2.5N HCl leachates from Chehalis valley basal till samples (<0.063 mm fraction) highlight down-ice glacial dispersion of volcanogenic massive sulphide (VMS) occurrences. Despite the relatively young age of the Seneca VMS deposit and surrounding volcanic rocks (Middle Jurassic), the contrast in Pb isotopic ratios between tills derived from country rocks and tills containing ore material is 3-7%. This contrast is 2-3 orders of magnitude above the analytical uncertainties of state-of-the-art multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Our simplified method, in which Pb isotopic ratios are measured directly on bulk 2.5N HCl leachate solution by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), is consistent with the MC-ICP-MS results. It also has low analytical uncertainties (0.4-0.6%) capable of distinguishing between tills derived mainly from country rocks and those containing ore materials. Direct-leachate measurements using a quadrupole ICP-MS lack the resolution for tracing <10% isotopic contrast. Although elevated Pb, Zn, Cu, and Ba abundances in till overlying the Seneca deposit identify the latter, they show different down-ice dispersion patterns. In contrast, Pb isotopic ratios for leachates from the <0.063 mm till fraction appear to be robust indicators of down-ice glacial dispersion for the VMS mineralization. The Pb abundances and isotopic compositions are consistent with derivation of the Chehalis valley tills from isotopically heterogeneous local bedrock sources mixed with variable proportions of lead from VMS mineralization. The relatively inexpensive method of determining Pb isotopic ratios in tills by measuring 2.5N HCl leachates using HR-ICP-MS constitutes a robust exploration tool for a broad range of concealed Pb-rich deposits including relatively young deposits.
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