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Precambrian Deposits |
National Geophysical Research Institute, Hyderabad — 500 007, India
Late Archaean greenstone belts of the Dharwar supergroup are uniquely characterized by a marker horizon that consists of Fe + Mn formations, carbonates (± stromatolites), and phyllites (± organic matter). Microfossils are found in cherts associated with stromatolites and banded Fe-Mn carbonate. Manganese-bearing arenites-argillites-carbonates have a varied mineralogy that is reflected by variations in chemical composition. Trace elements characteristic of felsic and volcanic rocks such as Zr, Hf, Rb, Sr, U, Th, Ni, Cr, Co, V, and Sc are low or high depending on the amount and type of clastic debris deposited with the chemical precipitates. REE also are influenced by the clastic fluvial and hydrothermal fluxes. Banded Fe + Mn carbonate-chert rocks (BMF) contain lower amounts of REE compared to samples containing ripidolite, which have higher Al2O3,
REE, Zr and Hf contents. Both positive and negative Eu, La and Ce anomalies are observed which reflect variable mixing of hydrothermal solutions with ambient ocean water and divergent clastic material. The data indicate that FeO, MnO and SiO2 were added to the Archaean proto-ocean by hydrothermal solutes produced along a spreading ridge axis. The redox potential of these proto-oceans was very low near the ridge axis, but increased towards the shelf, due to higher organic productivity and photosynthesis reflected by the presence of stromatolites, organic matter, and microbiota. Precipitation of stable Mn oxides at the margin of the greenstone belts is probably the consequence of the high redox potential.
