Rare earth element signatures of Paleoproterozoic Sallopat phosphorites of Aravalli Basin, India: implications for diagenetic effects and depositional environment.

As a repository of REE, the Paleoproterozoic phosphorite of the Aravalli Basin in India is explored time to time. Paleoproterozoic phosphorite deposits of the Sallopat sub-basin Banswara district of the Aravalli Supergroup show moderate REE concentrations much lower than that of average marine phosphorites. Different ratios and PAAS-normalized REE distribution patterns reflect MREE enrichment over LREE and HREE in these phosphorites. MREE enrichment confirmed by (Sm/Yb)N versus (Sm/Pr)N relationship and attributed to the selective uptake by cyanobacteria and mixing of riverine inputs. Y/Ho, La/Ce and Er/Nd ratios confirm that the REE in Sallopat phosphorites might have been affected by the argillaceous terrigenous and plagioclase-rich sediments probably derived from Banded Gneissic Complex. Diagenetic modeling indicates that REE were concentrated during early diagenetic processes with limited weathering influence. Moderate negative to low Ce anomaly depicts oxic to sub-oxic conditions of seawater due to oxidation of Ce3+ to Ce4+ whereas Eu3+ reduced to Eu2+ indicating positive Eu-anomaly and reducing conditions as well during the deposition. These anomalies may be due to the stratified restricted marine environment of upwelling and mingling of organic-rich anoxic deep water with oxic upper seawater prior to the formation of phosphorite deposits in the embayment. Enrichment of REE in Sallopat phosphorite relies on various physico-chemical conditions. These phosphorites may have been formed in restricted marine water which was influenced by extra clastic sediment input carried out by fluvial systems. [ABSTRACT FROM AUTHOR]