Major, trace and rare earth element geochemistry of Archaean carbonate sediments of Tanwan group rocks of the Bhilwara supergroup, India: implications for seawater geochemistry and depositional environment

Geochemistry of the sediments deposited during the Archaean Era is potentially useful to decipher the Earth’s pristine seawater chemistry and the depositional environment. This study highlights the geochemical evolution, redox conditions, and the sedimentary environment based on carbonate sediments of the Tanwan group rocks, Bhilwara supergroup, Rajasthan, India, deposited during 2.9–2.5 Ga. The petrographic features and variation in major elements of these carbonate sediments indicate extensive dolomitization during this time. The elemental ratios such as Mg/Ca, Fe/Al, Fe/Sr, and Mn/Sr ratios and their mutual relationships suggest anoxic conditions during precipitation of the carbonates. The anoxia is well supported by the redox-sensitive element ratios such as Ni/Co, V/Cr, V/V + Ni, V/V + Cr, and Th/U, which show reducing/anoxic sedimentary environments during 2.9–2.5 Ga. The post-Archaean Australian Shale (PAAS) normalized anomalies, such as Ce anomaly, Gd anomaly, and Eu anomaly, showing average 1–1.48 values, indicate minor positive anomalies in the Archaean seawater. The rare earth elements and yttrium (REY) content in these sediments show no significant contamination, mixing, and alteration because Dissolution II (carbonate leachate) is characterized by Zr from 0.015 ppm to below detection limit (BDL), Th < 1 ppm, Sc < 1.5 ppm, and Al₂O₃ < 1%. The Y/Ho ratio > 26 represents insignificant detrital input during the precipitation of the carbonates. The Y/Ho ratio versus Nd also supports this contention. The positive anomalies for Y (i.e. Y/Ho > 26), Gd anomaly (Gd/Gd*), and La anomaly (La/La*) confirmed the preservation of anoxic pristine marine seawater signatures in these carbonates of the Bhilwara supergroup that were mainly deposited during the Neoarchaean Era.