Fluid-rock interaction in the Kangankunde Carbonatite Complex, Malawi: SEM based evidence for late stage pervasive hydrothermal mineralisation

The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO2-rich and alkali chloride-carbonate-bearing fluid at ∼250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis.