High MREE-HREE solubility in a carbonatite-derived hydrothermal fluid: Evidence from fluorite-hosted fluid inclusions in the Amba Dongar carbonatite complex, India

Abstract

Recent experimental works suggest that complexation with alkalis, carbonate, and hydroxyl groups enhances rare earth element (REE) transport and HREE/LREE fractionation, seen in carbonatite-hosted REE deposits. The Amba Dongar carbonatite complex in western India hosts fluorite and REE-fluorcarbonate mineralization occurring as vug/vein fillings or as disseminated ores associated with calcite, dolomite, apatite, baryte, and quartz. In this study, we analyzed the major and trace element composition of fluorite-hosted fluid inclusions, and the trace element chemistry of calcite and fluorite, using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) to characterize the nature of the hydrothermal fluid and the processes of mineral-fluid interaction. Fluorite contains aqueous bi-phase (liquid + vapor) fluid inclusions of variable size that furnish low salinities (0.4–2.2 wt% NaCl equivalent) and liquid-vapor homogenization temperatures (130 °C–155 °C). The fluid inclusions have high concentrations of alkalis (Na, K), Si, Al, Mg, and Mn, and their chemistry resembles orthomagmatic fluid derived from evolved carbonatite melts. They also have elevated MREE and HREE concentrations relative to the LREE, that is significantly higher than in many other hydrothermal settings. Such fluids with fractionated LREE/HREE were likely derived from residual brines post the LREE-mineralization stage, when the HREE still remained in solution. This report of hydrothermal fluids with elevated MREE and HREE solubility from natural carbonatites supports experimental findings that alkali‑carbonate-hydroxyl complexation helps to mobilize REE and fractionate the LREE from MREE and HREE. Magmatic calcite has LREE-enriched REE patterns while those of hydrothermally-altered calcite is nearly flat. The secondary calcite also has elevated concentrations of Na and K, which suggests that the alkali-rich fluids scavenged the REE from magmatic calcite and redistributed them into more insoluble REE-fluorcarbonates. The REE patterns of fluorite mimic those of its fluid inclusions, with the inclusions containing higher and more variable REE concentrations, suggesting that the REE signals of fluorite are coming from the ablation of fluid inclusions, and not from fluorite themselves.