Organa-Geochemical Studies of Proterozoic Stromatolitic Phosphorite and the Intercolumnar Dolomite from the Aravalli Group, India

Stromatolitic phosphorites and carbonates from the Proterozoic Aravalli sediments of Western India, have been subjected to organo-geochemical studies, involving a modified gc-ms technique. The stromatolitic phosphorites have an average value of 0.22% TOC which is significantly different from the average TOC value (0.64%) for all types of phosphorite from the Indian subcontinent. With only a few exceptions, the isoprenoids-pristane and phytane and the n-alkanes, account for about 30% of the total amount of volatiles (1-20 ppm). Normal alkanes occur in the range of n-c₁₂ to n-c₃₁ with maxima between n-c₁₇ and n-c₂₁ . No odd/even predominance has been observed. Besides pristane and phytane, other isoprenoids identified in these rocks are iSO·C₁₆ and iSO-C₁₈. The low pristane to phytane ratio of 1.14 suggests a reducing micro-environment within the laminae of shallow-water cyanobacterial/algal stromatolites. The phosphate is assumed to be precipitated within the pore-waters under anoxic conditions through the activities of biological agents, which is supported by geological and geochemical data. The overall basinal environment, however, remained oxidizing, as is evident from the associated sedimentary structures.