The cancrinite-syenite/carbonatite complex of Lueshe, Kivu/NE-Zaire: petrographic and geochemical studies and its economic significance

The Luesche cancrinite-syenite/carbonatite complex is located in NE-Zaire about 50 km SSW of Lake Edward. The alkaline rocks have intruded metamorphic rocks of Burundian age (1.100 m.y.). The country rocks are quartzites and mica schists bearing chlorite, garnet, Kyanite and staurolite, which indicates P-T conditions of some 4 kbar and 430-500°C, maximum 6 kbar and 620°C. The K/Ar age of the biotites from the carbonatite is found to be 516 m.y. The core of the alkaline complex is composed of cancrinite-syenites which vary from place to place with respect to the contents of garnet, amphibole, aegirine, sodalite and cancrinite. Significant variations of the pyrochlore content are also observed. The core is surrounded by fine grained calcitic carbonatite (alvikite) which often shows a banded texture and grades into coarse grained calcitic carbonatite (søvite). In the SE of the søvite ring there is a separate dolomitic carbonatite body (beforsite) while at the contacts between the alkaline rocks and the country rocks a wide range of low to high grade fenites enriched in Na or K occurs. Within the alkaline complex the highest niobium values are found associated with metasomatic rocks of the contact zones such as carbosyenites, pyroxentic rocks and fenites. The main niobium mineral is a Ca, Na-pyrochlore enriched in Ti and/or Sr. The geochemical distribution patterns of the major elements as well as of the trace elements, especially the REE, suggests that the carbonatites are of primary magmatic origin derived from mantle material which was affected by later remobilizations, most likely as a result of postmagmatic hydrothermal solutions. Regarding the cancrinite-syenites, there is no evidence for a comagmatic formation with the carbonatites. They are rather of subcrustal origin; formed in the course of various metasomatic processes where the alkalic carbonate melts or volatile carbonate phases interacted with the metamorphic country rocks to form rheomorphic syenites which subsequently intruded. From an economic point of view only the lateritic zone derived from weathering of the carbo-syenites, pyroxenitic rocks and fenites is considered as ore due to the concentration of pyrochlore in the course of weathering. Ore grades vary from 1 % to 7 % Nb₂O₅. The ore is composed of clay minerals, iron oxides and phosphates. The weathering profile may be up to 150 m deep, whereby a gradual change with depth from crandallitic ore to crandallitic-apatitic ore and to apatitic ore in the bottom section occurs. Ore reserves amount to some 8 million tons with an average of 2.5 % Nb₂O₅. By these figures Lueshe is one of the few high grade niobium deposits in the world.