Isotopic Composition (δ13C, δ18O) and Genesis of Carbonates from Phosphorite Deposits in the Lesser Karatau (Kazakhstan)

The study of structural features along with carbon and oxygen isotope compositions made it possible to discriminate lithological rocks in deposits of the Karatau phosphorite basin (Kazakhstan) into the following types: high- and low-grade phosphorites; unaltered and altered dolomites (with and without phosphate) hosting phosphorites of the Chulaktau Formation (lower dolomite and Fe–Mn horizons); dolomites of the underlying and overlapping rocks of the Malokaroi and Dzhanat groups and the Shabakta Formation; calcareous schists, sandstones, as well as secondary calcite. The carbonate material in rocks of the studied deposits is heterogeneous and characterized by wide variations of the δ13C (–7.6 to 1.6‰ PDB) and δ18O (15.4 to 27.0‰ SMOW) values. The results obtained suggest that phosphorites and host rocks underwent intense postsedimentary transformations. Isotopic (δ13C, δ18O) composition of the carbonate material in rocks of the siliceous and productive (phosphorite) horizons, as well as, to a large extent, the Fe–Mn horizon of the Chulaktau Formation is nonprimary and related to the presence of carbonates of superimposed (secondary) generations during the late diagenesis or catagenesis. In this regard, data on the carbon isotope composition obtained for rocks of the studied Karatau deposits cannot be used for the chemostratigraphic reconstructions.