Nd, Pb, and Sr isotope systematics of fluorite at the Amba Dongar carbonatite complex, India; evidence for hydrothermal and crustal fluid mixing

Carleton University, Department of Earth Sciences, Ottawa, ON, Canada

The Cretaceous Amba Dongar Complex contains one of the largest fluorite deposits (11.6 million tons at 30 wt % CaF ₂ ) associated with any carbonatite. Fluorite mineralization postdates carbonatite emplacement and takes the form of replacement hydrothermal veins concentrated at the carbonatite-country rock contact. Sr isotope ratios from the Amba Dongar fluorites reveal large isotopic variations that can only be attributed to reaction between a carbonatite-derived, F-rich fluid and continental crust (or crustal-derived fluid). The extremely variable initial ⁸⁷ Sr/ ⁸⁶ Sr ratios (0.70910-0.71729) are intermediate between those of the carbonatites (0.70549-0.70628) and the host Bagh sandstones (0.75359-0.78274); they correlate negatively with Sr abundances- dances. Sr isotope ratios from intrasample fluorites show large isotopic variations (0.70910-0.71425) that cover almost the entire range of values found for all of the fluorites measured from Amba Dongar. The Pb isotope ratios from the fluorites are also variable and differ from those of the country rocks. Compared to the carbonatites, most contain similar ²⁰⁷ Pb/ ²⁰⁴ Pb and ²⁰⁸ Pb/ ²⁰⁴ Pb ratios but more radiogenic ²⁰⁶ Pb/ ²⁰⁴ Pb values. Galena and pyrite coeval with fluorite mineralization have Pb isotope ratios which overlap those from most of the fluorites, but which are isotopically distinct from one another. In contrast, initial ¹⁴³ Nd/ ¹⁴⁴ Nd isotope ratios from most of the fluorites are relatively uniform (0.51240-051247), quite different to those from the surrounding Bagh sandstones (0.51122-0.51149), but similar to those from the carbonatites (0.51248-0.51253). The new isotopic results are partly consistent with a model for fluorite deposition involving interaction between ground waters within the surrounding country rocks and an F-rich, carbonatite-derived fluid (Deans and Powell, 1968). The variation in the Sr isotope composition for the fluorites suggests that certain physio-chemical parameters of the mineralizing fluids were changing with time of deposition.

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