Abstract
The Tundulu carbonatite complex in southeastern Malawi was intruded during the late Jurassic to early Cretaceous over three episodes. During the first and second episodes, the major rock types were calcite carbonatites, ankerite carbonatites, and apatite carbonatites. These rocks experienced hydrothermal alteration at the close of the second episode, during which quartz-baryte veins containing a significant level of rare earth fluorocarbonates were deposited. Veins in calcite carbonatites contain abundant synchysite [(La, Ce, Nd)Ca(CO 3) 2)F] with subordinate amounts of parisite [(La, Ce, Nd) 2Ca(CO 3) 3F 2] and bastnäesite [(La, Ce, Nd)CO 3F], crystallising in the order synchysite parisite → bastnäesite. Some of the parisite is retrograde in origin, having formed as an alteration product from synchysite. Only synchysite has been identified in veins hosted by apatite carbonatites. However, bastnäesite predominates the hydrothermal assemblage in ankerite carbonatites. Parisite and synchysite have been found only as a fibrous fringe between wallrock ankerite and bastnäesite. The crystallisation sequence seen in calcite carbonatites represents a progressive depletion of both Ca 2+ and CO 2− 3 in the fluids from which this mineral suite precipitated, with Ca 2+- and CO 2− 3-poor phases precipitating last. Clearly, the predominance of bastnäesite in veins hosted by ankerite carbonatites suggests insufficient Ca 2+ and CO 2− 3 in the fluids for the ubiquitous precipitation of synchysite and parisite. These observations are consistent with a model in which hydrothermal fluids reacted with the various wallrocks, which then released different amounts of Ca 2+ and CO 2− 3, that subsequently reacted with REEs in the fluid to form the various fluorocarbonates. As such, the Tundulu carbonatites provide a natural laboratory in which compositional phase relationships of rare earth fluorocarbonates can be related to variations in the activities of Ca 2+ and CO 2− 3 within the Ln(CO 3)F-CaCO 3-F 2(CO 3) −1 system.
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