Chemical environments of deposition of ancient iron- and manganese-rich sediments and cherts

Abstract

Despite detailed classification, criteria to distinguish between marine and freshwater environments of deposition in fine-grained, non-clastic sedimentary rocks are scanty. Data on recent lacustrine and marine chemical precipitates reveal that these two environments can be distinguished in terms of their composition in the CaO–MgO–Na2O–K2O system provided carbonates and detrital components associated with them are trivial. Lacustrine ferromanganese precipitates display low Na2O and K2O but a high CaO/(CaO + MgO) ratio (>0.70) similar to the world average river water. In contrast, diagenetically mature marine ferromanganese precipitates and nodules show a lower (<0.60) CaO/(CaO + MgO) ratio, in general, and variable Na2O and K2O. Recent pelagic cherts reveal an average CaO/(CaO + MgO) ratio comparable to that of ocean water, whereas those affiliated to ocean floor hydrothermal activity span a wide range of composition in CaO–MgO–Na2O–K2O space and mostly avoid the lacustrine field. Extension of the observed relations to the ancient Archean BIFs and cherts from (1) Isua supracrustals, Greenland, (2) Yilgarn Block, Australia, (3) Montana, USA, and Proterozoic BIFs from (4) Hamersley Basin, Australia, (5) Labrador Trough, Canada, (6) Biwabik iron formations, USA and the Late Proterozoic (7) Upper Tindhir Group iron formation, Alaska, and many others reveal a marine signature corroborating the inferences based on REE and isotope studies. However, chemical data on a number of deposits, including the late Proterozoic BIFs, Mn-deposits and cherts from the Damara Orogen, Africa, BIFs from Rapitan Formation, Northwest Territories, Canada, and several Proterozoic Mn-deposits and cherts from India, suggest a freshwater chemical environment of deposition. The concurrence in the results obtained from studies on iron and/or manganese deposits and coexisting cherts from several Archean, Proterozoic and Phanerozoic basins suggests that the chemistry of the more abundant siliceous sedimentary rocks can provide an additional tool for palaeogeographic reconstruction.