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

Abstract

Recent work on the largest sedimentary exhalative (SEDEX) district in Canada, the Howard's Pass district (HPD) in Selwyn Basin, Yukon, has shown that a persistently euxinic water column was not the source of sulphur in the mineralization, and that the redox conditions of the water column (and shallow subsurface) varied from anoxic to suboxic to euxinic. We have conducted a bulk geochemical and molybdenum isotopic traverse through a mineralized intersection in a single drill hole from the Anniv East vent-distal lead-zinc SEDEX deposit in the HPD. Bulk geochemical redox proxies generally show that redox conditions varied between (intermittently to permanently) suboxic, dysoxic, anoxic and euxinic. Herein we present new molybdenum isotope data for these same samples, and evaluate the efficacy of the application of this isotopic system to determining redox conditions of the water column and shallow subsurface, and examine the utility of molybdenum isotopes in providing genetic information for SEDEX deposits. Molybdenum isotope values range from delta-98Mo -0.29 to 2.27 permille. There is good general agreement between the molybdenum isotope values and certain redox sensitive elements and established redox indicators for the unmineralized and mineralized host rocks; these relationships reflect suboxic or oxic conditions for the most negative values, and anoxic and euxinic conditions for the most positive values. This indicates that the application of molybdenum isotopes as a redox indicator in seafloor hydrothermal deposits in sedimentary (or volcano-sedimentary) settings shows great promise. Unlike thallium isotopes, molybdenum isotopes do not appear to be of use in fingerprinting SEDEX mineralization.