Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1- to 15-m-thick beds within the approximate to 2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shiel d, Russia. Drillcore material together with natural exposures reveal that t he 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' s equence formed in a complex combination of shallow-marine and non-marine, e vaporitic environments. Dolomite-collapse breccia, stromatolitic and micrit ic dolostones and sparry allochemical dolostones are the principal rocks ho sting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) an d delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousan d (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coars ely crystalline magnesite. All varieties exhibit anomalously high delta C-1 3 values ranging from +9.0 parts per thousand to +11.6 parts per thousand a nd delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structu reless micritic magnesite forms as a secondary phase replacing dolomite dur ing early diagenesis, and replaced dolomite before the major phase of buria l. Crystalline and coarsely crystalline magnesite replacing micritic magnes ite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Mag nesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup c oastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-1 3 values reflect a combined contribution from both global and local carbon reservoirs. A C-13-rich global carbon reservoir (delta C-13 at around +5 pa rts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) i s associated with evaporative and restricted environments with high bioprod uctivity.