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 delta Mo-98 values is close to the similar to 2.7 parts per thousand 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 delta Mo-98 of Palaeoproterozoic seawater, despite a metamorphic overprint of at least 600 degrees 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 delta Mo-98 signals are broadly reflected in cerium (Ce) anomalies, which suggest an oxic-anoxic-stratified Palaeoproterozoic ocean.