Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, (87)Sr/(86)Sr) data from Kangankunde, Malawi

Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and (87)Sr/(86)Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8–15 wt% REE(2)O(3), comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The (87)Sr/(86)Sr values (0.70302–0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (δ(18)O + 3 to + 5‰(VSMOW) and δ(13)C − 3.5 to − 3.2‰(VPDB)). In contrast, dolomite in the same samples has similar δ(13)C values but much higher δ(18)O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar δ(18)O and (87)Sr/(86)Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: δ(18)O + 7.7 to + 10.3‰ and δ(13)C −5.2 to −6.0‰; (87)Sr/(86)Sr 0.70296–0.70298) is not directly linked with the REE mineralisation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00410-017-1412-7) contains supplementary material, which is available to authorized users.