Interpreting geochemical paleoredox proxies in mudstones associated with Sediment-Hosted Massive Sulphide (SHMS) deposits

In this study, a suite of geochemical redox proxies have been used to evaluate ambient basinal conditions during a period of Late Devonian hydrothermal activity in the Selwyn Basin (Canada). Organic-rich mudstones were collected from two correlated drill-holes; one intersects a 35 m thick sequence of SHMS mineralization, while the other intersects un-mineralized strata. All samples contain a high proportion of reactive Fe (Fe-HR/Fe-T > 0.38), indicating water column anoxia. Greater pyritisation in the mineralised succession (median Fe-Py/Fe-HR = 0.86), relative to un-mineralised samples (median Fe-Py/Fe-HR = 0.72), could represent more locally stagnant, euxinic conditions. However, delta Mo-98(NIST) values (0.33 to 0.69 parts per thousand) are uniform between both drill-holes and substantially lower than coeval Late Devonian seawater approximate to 1.5 parts per thousand). The low abundance (<< 5 ppm) of Mo in mineralised samples indicates hydrothermal Mo input was negligible; in the un-mineralised mudstones fractionation of authigenic Mo from seawater is consistent with deposition under noneuxinic conditions. In the absence of water column euxinia, high primary productivity and enhanced organic carbon burial were the key variables for promoting sulfate reduction in the sub-surface and trapping metals; therefore, when, how and where these conditions develop in sedimentary basins are the most important consideration when targeting prospective host rocks for SHMS systems in the geologic record.