Reconstruction of hydrothermal fluid composition of the Kamthai carbonatite complex, India using lattice strain model: Implications for LREE/HREE fractionation

Abstract

The composition of the fluid phase is often a poorly constrained variable in many ore deposits. In this study, we used calcite-fluid partition coefficients extrapolated to the P-T conditions of mineralization using the lattice strain model to reconstruct the composition of the hydrothermal fluids associated with different stages of the hydrothermal alteration and secondary rare earth element (REE) mineralization in the Kamthai carbonatite complex of western India. We identified three generations of calcite including magmatic, two generations of secondary/hydrothermal, as well as partially reequilibrated magmatic grains. The magmatic and partially reequilibrated calcite are unusually enriched in the REE, particularly the LREE, and Sr, which were incorporated via coupled cationic substitutions from highly differentiated brine- and incompatible element-enriched carbonatitic melts. Calcite and carbocernaite were the important primary phases that sequestered the REE from the carbonatite magma. Textural and geochemical evidence suggest that magmatic calcite and carbocernaite underwent extensive partial to complete reequilibration through dissolution and reprecipitation during hydrothermal alteration. The prominent chemical changes during the alteration of calcite involved loss of substantial REE, Sr, Fe, and Mn, and transition from LREE-dominated to LREE-poor compositions. The REE released during hydrothermal alteration were incorporated in secondary REE minerals such as REE-(fluor)carbonates, REE-(hydroxyl)carbonates, ancylite, and cerianite. The reconstructed chemistry of the carbonatite melt and the hydrothermal fluid, together with the trace element chemistry of secondary calcite, point towards progressive decrease in the REE contents. However, the HREE were conserved and ƒO2 increased from the most primitive to the most evolved fluid. The REE were redistributed as alkali‑carbonate-hydroxyl complexes, which can explain the LREE/HREE fractionation observed between the early and the late-stage hydrothermal fluid. Precipitation of the LREE-bearing secondary minerals is attributed to a decrease in temperature of the hydrothermal fluid as a consequence of mixing of the carbonatite-derived fluid with a relatively cooler and oxidized fluid. Such mixing explains the observed negative Ce-anomaly in hydrothermal calcite.