Goswami and 4 co-authors, reported in 2017 that dolostone of Vempalle Formation near Tummalapalle hosts large uranium deposit (>100,000 tonnes with an average grade of 0.045%U3O8). It is a unique type of uranium deposit because carbonate formations have been considered to be among the least uraniferous of all the rocks of the Earth’s crust due to mobility of uranium in aqueous fluid in the presence of carbonate and bicarbonate ions. Vempalle dolostone hosts syn-sedimentary uranium mineralization in the form of discrete uranium phases (pitchblende and coffinite) associated with collophane, and adsorbed uranium in organic matter. The organic matter has played dual role of concentrating uranium from solution and also chemically reducing it to pitchhblende and coffinite.
Pyrite, chalcopyrite and molybdenite are also found in mineralised dolostone. These appear to have been concentrated in dolostone under oxygen deficient environment. The two mineralised bands correspond to two stages of terrestrial supply from uplifted basement provenance. Enrichment of phosphate can lead to eutrophication of fresh and inshore marine waters, leading to algae bloom because of excess nutrients.
Over time, geological processes can supply phosphor in to ocean sediments. The algae during that time absorbed phosphates binding the phosphate into organic compounds. After death, those algae decayed and in that way phosphates were returned to the sediments after forming compound with calcium in the form of collophane. Nash et al., 1981 suggested that concentration of uranium by adsorption onto organic matter or a mineral surface preceded reduction and accelerated the process of reduction of uranium. In addition it has also provided the reducing environment necessary for uranium precipitation.
Siliciclastic impurities in dolostone have also reduced the Eh-pH conditions favorable for uranium precipitation. Undoubtedly the organic matter has been oxidized, but under natural groundwater chemistry and flow conditions the time for oxidation is long enough so that the deposits of paleo-Proterozoic age have been preserved.
Based on the present studies, it can be concluded that:
1. Very low TOC, H.I. values and higher O.I. values represent overmature kerogen of type IV category.
2. Organic matter plays a dual role of concentrating uranium from solution and also of chemically reducing it to insoluble pitchblende and coffinite.
3. FTIR spectra do not show bonding between uranium and organic component, therefore uranium is present as adsorbed phase with organic matter.
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