Molybdenum-isotope signals and cerium anomalies in Palaeoproterozoic manganese ore survive high-grade metamorphism

Molybdenum (Mo) and its isotopes have been used to retrieve palaeoenvironmental information on the ocean–atmosphere system through geological time. Their application has so far been restricted to rocks least affected by severe metamorphism and deformation, which may erase or alter palaeoenvironmental signals. Environmental Mo-isotope signatures can be retrieved if the more manganese (Mn)-enriched rocks are isotopically depleted and the maximum range of δ(98)Mo values is close to the ~2.7‰ Mo-isotope fractionation known from Mo sorption onto Mn oxides at low temperature. Here, we show that the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil, contains Mn-silicate–carbonate ore and associated graphitic schist that likely preserve δ(98)Mo of Palaeoproterozoic seawater, despite a metamorphic overprint of at least 600 °C. The extent of Mo-isotope fractionation between the Mn-silicate–carbonate ore and the graphitic schist is similar to modern Mn-oxide precipitates and seawater. Differences in δ(98)Mo signals are broadly reflected in cerium (Ce) anomalies, which suggest an oxic–anoxic-stratified Palaeoproterozoic ocean.