The Composition of the REE mineralizing Fluid in the Amba Dongar Carbonatite Complex: Constraints from Chemistry of Fluorite and its Fluid Inclusions

Abstract:

The Amba Dongar carbonatite complex in western India hosts fluorite and rare earth element (REE) mineralization. The mineralization mostly comprises light REE-bearing minerals such as synchysite, parisite, florencite, and bastnäsite, which occur as vug/vein fillings or in disseminated form associated with hydrothermal fluorite, quartz, calcite, apatite, and barite. We use the trace element chemistry of fluorite and its fluid inclusions to characterize the nature of the mineralizing fluid. Both primary and pseudo-secondary bi-phase (L+V) fluid inclusions (<10 to 70 μm in size) are identified. Microthermometric measurements furnish salinities of 0.4–2.0 wt.% NaCl equivalent and homogenization temperatures of 130–155°C. Two types of REE patterns are identified in fluorite. In one, depletion of the light-REE and enrichment of the middle and heavy-REEs are noticed with convex upward patterns at the middle-REE. The second type is characterized by nearly flat to slightly light-REE enriched patterns. The REE patterns of fluorite mimic those of the fluid inclusions with the inclusions having significantly higher and more variable REE concentrations. This suggests that the REEs may not be hosted in the fluorite structure and their REE signature may be the result of ablation of fluid inclusions with the lower concentrations being due to dilution of the fluid signature during the ablation process. This implies that REE patterns (but not concentrations) of fluorite may be used as a proxy for the REE pattern of the fluid. The two distinct REE patterns retrieved from the fluid inclusions reflect the fluid composition during two stages of the hydrothermal alteration. The inclusions having flat/slightly light-REE enriched patterns correspond to pre- or syn-mineralization stage while those with light-REE depleted patterns represent late-stage fluids post light REE mineralization.