Lead Isotope Geochemistry of Shales from the Wolverine Volcanogenic Massive Sulfide Deposit, Yukon: Implications for Pb Isotope Vectoring in Exhalative Ore Systems

Abstract

Combined whole-rock lead isotope and high-precision trace element data are reported for shales from the Wolverine deposit and surrounding prospects (e.g., Puck, Fisher, Sable) to elucidate the source of Pb in the shales and their potential utility as an exploration vector. Shales exhibit distinct variations in time-integrated U/Th/Pb evolution with proximity to mineralization. Distal samples resemble the local crustal Pb isotope composition of the northern Cordilleran continental crust (i.e., shale curve). More proximal samples exhibit evidence for hydrothermal activity, but were not significantly mineralized. They have very high U/Th and U/Pb and anomalous present-day 206 Pb/204Pb (>20) coincident with elevated Y/Ho, Zr/Hf, and carbonate alteration. These signatures were derived from oxygenated seawater and imprinted on the shales by oxygenated, CO2-rich hydrothermal fluids that contained uranyl carbonate ions (e.g., UO2(CO3) 3 4– ). It is envisioned that this was due to shallow-level (within 50 m of the seafloor), near-vent ingress of oxygenated seawater associated with hydrothermal venting. A third population of samples is most proximal to mineralization. These, too, have trace element signatures (i.e., high Y/Ho, Zr/Hf, CO2-U enrichment) inherited from seawater, but they do not exhibit anomalous present-day 206Pb/ 204 Pb, being more juvenile in 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208Pb/204Pb. This isotope composition coincides with enrichments in Pb, Zn, Cu, Tl, Sb, W, and Sn (±Cu, Mo). The Pb enrichment associated with mineralization effectively buffered these samples from appreciable Pb isotope evolution, despite overprinting by uranyl carbonate. The juvenile Pb associated with the shales proximal to mineralization argues for deep-penetrative circulation of hydrothermal fluids and leaching of Pb from mafic igneous rocks at depth and/or influence from juvenile magmas at depth (i.e., magmatic fluids). Both of these processes were likely critical for the genesis of mineralization at Wolverine. This study illustrates the utility of quadrupole inductively coupled plasma-mass spectrometry (ICP-MS) for providing sufficiently precise and accurate Pb isotope determinations for district-scale metallogenic evaluations. Furthermore, the coupling of solution ICP-MS Pb isotopes with high-precision trace element geochemistry from the same digest provides a powerful tool for regional exploration studies for hydrothermal mineralization in shale basins.