Reconnaissance thallium isotope study of zinc-lead SEDEX mineralization and host rocks in the Howard's Pass district, Selwyn Basin, Yukon: potential application to paleoredox determinations and fingerprinting of mineralization

Abstract

The commonly accepted genetic model for sedimentary exhalative (SEDEX) lead-zinc deposits requires a marine basin with anoxic (no oxygen) and euxinic (free H2S) conditions, in order to provide sulphur to bond with the metals. Recent work on the largest SEDEX district in Canada, the Howard's Pass district, Selwyn Basin, Yukon, has cast doubt on the universality of this model, as the water column may have been suboxic during mineralization. Paleoredox indicators based on bulk geochemical compositions can be contradictory or equivocal. Developments in non-traditional metal stable isotope analysis have shown the potential of certain isotope systems (e.g. molybdenum, uranium, thallium) as paleoredox indicators. We have conducted a bulk geochemical and thallium isotopic traverse through a mineralized intersection (and immediate stratigraphic footwall and hanging wall) from the Anniv East vent-distal SEDEX deposit, Howard's Pass district. The epsilon-205Tl values range from -7.5 to -4.0 for unmineralized samples and -3.6 to -2.6 for mineralized ones. There is good general agreement between certain redox sensitive elements, venerable redox indicators and epsilon-205Tl for the unmineralized host rocks (reflecting suboxic or oxic conditions for the most negative values). Thus, the application of thallium isotopes as a redox indicator in seafloor hydrothermal deposits in sedimentary (or volcano-sedimentary) settings shows great promise. The heaviest values are for mineralized samples, likely reflecting contributions of isotopically heavy thallium from the mineralizing fluids. Preliminary data indicate that thallium isotopes as a redox indicator in mineralized samples is untenable, but it has the potential to fingerprint sulphides precipitated by various processes.