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

Despite detailed classification, criteria to distinguish between marine and freshwater environments of deposition in fine-grained, non-elastic sedimen tary rocks are scanty. Data on recent lacustrine and marine chemical precip itates reveal that these two environments can be distinguished in terms of their composition in the CaO-MgO-Na2O-K2O system provided carbonates and de trital components associated with them are trivial. Lacustrine ferromangane se precipitates display low Na2O and K2O but a high CaO/(CaO + MgO) ratio ( >0.70) similar to the world average river water. In contrast, diageneticall y mature marine ferromanganese precipitates and nodules show a lower (<0.60 ) CaO/(CaO + MgO) ratio, in general, and variable Na2O and K2O. Recent pela gic cherts reveal an average CaO/(CaO + MgO) ratio comparable to thar of oc ean water. whereas those affiliated to ocean floor hydrothermal activity sp an a wide range of composition in CaO-MgO-Na2O-K2O space and mostly avoid t he lacustrine field. Extension of the observed relations to the ancient Arc hean BIFs and cherts from (1) Isua supracrustals, Greenland, (2) Yilgarn Bl ock, Australia, (3) Montana, USA, and Proterozoic BIFs from (4) Hamersley B asin, 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 ba sed on REE and isotope studies. However, chemical data on a number of depos its, including the late Proterozoic BIFs, Mn-deposits and cherts from the D amara Orogen, Africa, BIFs from Rapitan Formation, Northwest Territories, C anada, and several Proterozoic Mn-deposits and cherts from India, suggest a freshwater chemical environment of deposition. The concurrence in the resu lts obtained from studies on iron and/or manganese deposits and coexisting cherts from several Archean, Proterozoic and Phanerozoic basins suggests th at the chemistry of the more abundant siliceous sedimentary rocks can provi de an additional tool for palaeogeographic reconstruction. (C) 1999 Elsevie r Science B.V. All rights reserved.