Phosphorite Ore Enrichment Due to Secondary Alteration in the Jhamarkotra Stromatolitic Phosphorites, Aravalli Supergroup, Northwestern India

The Paleoproterozoic Aravalli Supergroup in northwest India hosts one of the oldest phosphorite deposits on Earth, located in the Jhamarkotra Formation, which was deposited after ca. 1762 Ma. Secondary enrichment is identified in the eastern region, resulting in upgradation of phosphate content, while primary stromatolitic columns are well-preserved in the western area of the Jhamarkotra mines. In this study, drill-core samples were collected from the unaltered western Block B and the upgraded eastern Block E to understand the alteration process. Petrographic studies reveal evidence of structural deformation and alteration. Elemental mapping of petrographic thin sections, employing SEM-EDS, indicates that dolomite has been leached out, resulting in phosphorite upgrading in the E-block. The major element oxide data support the leaching of dolomite. In the upgraded E-block, the weighted average P₂O₅ content nearly doubled (from 21% to 38%), while the MgO content decreased from 21% to 4% compared to the B-block. REE+Y contents in Block E are increased with minor Ce and Eu anomalies developed compared to the B Block. The U and Sr concentrations are also increased in Block E phosphorites. The petrographic and geochemical studies indicate that phosphorite enrichment was driven by structurally controlled, low-temperature hydrothermal alteration in the Jhamarkotra mines.