ABSTRACT
Finely-laminated micritic, stromatolitic magnesite, and structureless micritic, crystalline and coarsely crystalline magnesite forms a series of 1 to 15 m thick beds within the ca. 2.0 Ga Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. The 680 m thick formation is composed of a stromatolite-dolostone-‘red bed’ sequence formed in a combination of shallow marine and non-marine, evaporitic environments. Dolomite collapse breccia, stromatolitic and micritic dolostones, and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. The magnesite and host dolostones are marked by δ13C values from +7.1‰ to +11.6‰ (V-PDB), and δ18O ranging from 17.4 to 26.3‰ (V-SMOW). Laminated and structureless micritic magnesite replaces dolomite during early diagenesis prior to the major phase of burial Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from seawater-derived brine, diluted by meteoric fluids in sabkha to playa lake environments. Extremely high δ13C values of dolomite and magnesite reflect a combined contribution from both global and local carbon reservoirs. A 13C-rich global carbon reservoir (δ13C at around +5‰) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in 13C (up to +12‰) is associated with evaporative and restricted environments. The Palaeoproterozoic sabkha/playa magnesite studied is unlikely to be of economic significance.
