A comparison of the fenites at the Chilwa Island and Kangankunde carbonatite complexes, Malawi

Abstract

Abstract Carbonatites are igneous carbonate rocks. They are the main source of the rare earth elements (REE) that are essential in low carbon and high technology applications. Exploration targeting and mine planning would both benefit from a better understanding of the processes that create the almost ubiquitous alkaline and REE-bearing metasomatic aureoles in the surrounding country rocks. Using scanning electron microscopy and whole-rock geochemistry, we investigated the composition and mineralogy of the fenite aureoles developed around the REE-poor Chilwa Island carbonatite and the REE-rich Kangankunde carbonatite, which intrude similar country rocks in the Chilwa Alkaline Province of Southern Malawi. Although common characteristics and trends in their mineralogy and composition may be typical of fenites in general, there are significant differences in their petrography and petrogenesis. For example, the mineralogically diverse breccia at Kangankunde contrasts with the intensely altered potassic breccia of Chilwa Island. This might be caused by differing sequences of fluids expelled from the carbonatites into the aureoles. The main REE-bearing mineral in fenite is different at each complex, and reflects the characteristic REE-bearing mineral of the main carbonatite: fluorapatite at Chilwa Island; and monazite at Kangankunde. Each fenite has distinctive mineral assemblages, in which the relative abundance of the REE-bearing minerals appears to be determined by the mineralogy of their respective host carbonatites. At both localities, the REE minerals in fenite are less enriched in lanthanum and cerium than their equivalents in carbonatite, a characteristic that we attribute to REE fractionation within fluids in the aureole. Identifying the mineral assemblages present in fenite and understanding the sequence of alkaline and mineralising fluid events could therefore be useful in predicting whether a fenite is associated with a REE-rich carbonatite. Detailed studies of other aureoles would be required to assess the reliability of these characteristics.