Petrology And Geochemistry Of Rocks Hosting Niobium -Tantalum,Tin And Tungsten Bearing Mineral In Runyankezi Area Northern Burundi

Abstract

Runyankezi area occur in the northern part of Burundi in Kirundo and Ngozi Provinces. It occurs in the Meso- to- Neoproterozoic Karagwe-Ankole belt that has a huge mineral potential. Runyankezi is historically known to have mineralization of cassiterite, columbo-tantalite, and more rarely Tungsten. This belt is composed of metasedimentary rocks subjected to different episodes of deformations and metamorphism. The lack of knowledge regarding characteristics of tantalum, tin, and tungsten (3Ts) bearing rocks in Runyankezi and the genesis of their associated mineralizations was the base of the choice of this research. The main objective of this research was to investigate the geological context of the 3Ts-bearing host lithologies and their related mineralization and the petrogenesis of the 3Ts‟ minerals in Runyankezi area in northern part of Burundi. The tools used in this research included geological field observations for stratigraphy and micro-structure, rock sampling, and geochemical analysis. Geological field observations revealed that all mineralization of 3Ts in the study area is from pegmatites. Petrography showed that principal minerals in rocks hosting 3Ts in the Runyankezi are Li-rich tourmaline (Elbaite), muscovite, biotite with zircon inclusion, hornblende, feldspars (orthoclase), sericite, and quartz. It is noted that muscovite, biotite, sericite, and tourmaline occur as a result of hydrothermal alteration. Geochemistry showed that 3Ts bearing rocks are Subalkaline and peraluminous. Trace elements (High Field Strength Elements and Large Ion Lithophile Elements) and rare-earth elements (REE) analyses revealed that pegmatites with high mineralization are of Lithium-Caesium-Tantalum type (LCT-Type) and are far from the granitic intrusion. They are associated with S-type granites. Their formation took place at low temperature and are evolved with Ta>Nb. These pegmatites occur in Buvyukana. In contrast, pegmatites with low concentration are of Niobium-Yettrium-Fluorine type (NYF-Type) and are close to granitic intrusion. They are associated with I-type granites. Those granites are not mantellic but crustal with high Rb concentration, the formation of derived pegmatites took place at high temperature, they are not more evolved, with Nb>Ta and occur in Nyanza-Burenge site;  Those pegmatites have undergone post-magmatic alteration evidenced by Rb/Sr and Ba/Rb ratios;  The genetic setting of Tin, Tungsten, and Columbo-Tantalite is associated with hydrothermal alterations. These alterations are sericitization, albitization, tourmalinization, muscovitization, kaolinization, and greissenization