Rare earth element precipitation induced by non-redox transformation of magnetite to hematite: Microtextural and geochemical evidence from the Kamthai carbonatite complex, western India

Abstract

Hydrothermal rare earth element (REE) precipitation in carbonatite is generally attributed to increase in pH or decrease in temperature when REE-bearing acidic fluids interact with carbonates. Here, we document microtextures comprising intergrowth of bastnäsite, hydroxyl-parisite, röntgenite, synchysite, and pyrochlore, with calcite that pseudomorphically replaces patches of hematite with cellular boxwork-type structure, in calciocarbonatites from the Kamthai alkaline complex in western India. The nature of the REE mineralization grades from proximal bastnäsite-dominated to distal hydroxyl-parisite dominated in the boxworks. The microtextural relations and trace element chemistry of hematite, magnetite and calcite, and C-O isotope composition of carbonate are suggestive of extensive low-temperature hydrothermal alteration of the carbonatites. The hematite boxwork structure and the REE mineral-calcite intergrowths often have squarish outlines and are interpreted to have pseudomorphed primary magnetite during fluid-rock interaction. We propose a new mechanism of REE precipitation in magnetite-rich carbonatites involving influx of acidic hydrothermal fluids, which scavenged the REE and other trace elements from magmatic carbonates and apatite. These acidic fluids were responsible for the protonation of magnetite and leaching out of Fe2+, converting them to hematite through a non-redox transformation. The reaction results in 32% volume reduction for every mole of magnetite consumed, generating significant rock porosity, which further aided and abetted fluid-rock reaction and hydrothermal alteration. More importantly, it consumed proton, which increased the fluid pH triggering precipitation of bastnӓsite-group minerals (including bastnӓsite, parasite, röntgenite, synchysite). Close to the magnetite-hematite reaction front, fluor-dominated bastnӓsite-group minerals (mainly bastnӓsite) appear, while away from such fronts, mineralization was enriched in hydroxyl-bastnӓsite group minerals (mainly parisite) as a consequence of decreasing activity of F‐ and/or increasing activity of OH‐ with the progress of magnetite-to-hematite transformation.