Redox-controlled selenide mineralization in the Upper Old Red Sandstone

Selenium concentrations occur in organic-rich lithologies such as coal, and as secondary accumulations from the oxidative mobilization, migration and subsequent concentration upon encountering reductants such as organic matter or H₂S. Here we assess the redox mineralization of copper and lead selenide phases in the cores of reduction spheroids in Devonian sedimentary red beds. We propose that the selenide mineralization occurred as descending meteoric fluids, supplied with selenium from Carboniferous coal seams, migrated through a major fault plane into underlying sandstones where localized, microbially-induced reducing conditions prevailed. These findings and other occurrences of selenide mineralization between Devonian and Carboniferous strata on the British Isles suggest a widespread selenide mineralization system between strata of that age.