Geology of the Ngualla carbonatite complex, Tanzania, and origin of the Weathered Bastnaesite Zone REE ore

Abstract

The late Mesoproterozoic Ngualla carbonatite complex in southwest Tanzania comprises a central magnesiocarbonatite plug surrounded sequentially by an annular calcite carbonatite intrusion and fenitised felsic igneous country rocks. The calcite carbonatite contains phlogopite-rich (glimmerite) enclaves interpreted as fenitised wallrock xenoliths that have contributed silicate minerals, apatite and magnetite through dispersal and interaction, mainly within the calcite carbonatite magma. Ultramafic magmas were emplaced into the magnesiocarbonatite magma chamber before complete solidification of the magnesiocarbonatite. Contemporaneity allowed the two magmas to mingle. Rounded enclaves of hematite-barite in the magnesiocarbonatite are tentatively attributed to magma immiscibility. Following complete solidification of the calcite carbonatite, and overlapping late crystallization of the magnesiocarbonatite plug, late magnesiocarbonatite dikes and ultramafic dikes were emplaced, some of the latter as diatremes.Crystallization of ferroan dolomite in the magnesiocarbonatite plug resulted in residual magmatic concentration of Si, Ba, F and rare earth elements (REE), and crystallization of barite, quartz, calcite, fluorite and REE fluorocarbonates in miarolitic cavities. Concentrations of (total) rare earth oxides (TREO) in the unweathered magnesiocarbonatite are 1 to 2%. REE ore with 3 to 6% TREO resulted from weathering, during which CaCO3 and MgCO3 were leached from ferroan dolomite leaving a porous goethite-rich residue containing barite and bastnaesite, the latter having replaced primary synchesite. Other commodities with potential economic significance include phosphate and niobium, both of which were enriched by residual accumulation over the calcite carbonatite as a result of karstic weathering.Although weathering was a critical factor in the formation of REE ore at Ngualla, the primary proto-ore resulted mainly from in situ igneous processes. This genetic model is different from that used to account for many carbonatite-hosted ore bodies, which result from late-stage hydrothermal processes. Examples of hydrothermal rare earth deposits include those of the late Jurassic to early Cretaceous Chilwa Province, located 800 km south of Ngualla. The differences in ore-forming processes may reflect the relative ages of the carbonatites and a deeper level of erosion at Ngualla.