Thesis defense of Seconde NTIHARIRIZWA

The mineralization in Rare Earth Elements of the Gakara region (Burundi): structural control, petrological and geochemical characterization, model metallogenic

Abstract

The Gakara deposit in Burundi is one of the richest deposits of Rare Earth Elements (REE) in the world. The East African rift zone is the well-known host of many carbonatites and associated alkaline complexes, which have been in place for many hundred million years. The main deposits of REE mined today in the world are associated with carbonatite complexes. In these complexes, mineralization is rarely part of the primary mineral assemblage; they precipitate from fluids evolved from magmatic intrusions. To better understand the mechanisms of emplacement of REE mineralization in the Gakara area, we used a multidisciplinary approach:
i) A geological study based on field observations and a study of the mineralogical composition and textures of different facies of the ore, using conventional microscopic observation tools.
ii) In-situ U-Th-Pb dating by La-ICP-MS to determine the ages of primary mineralization (veins of bastnaesite, REE-fluorocarbonate) and hydrothermal alteration (monazitic alteration, REE-phosphate).
iii) A detailed study of the fluid inclusions trapped in bastnaesite and quartz crystals associated with the monazitic alteration to characterize the mineralizing fluids and thus specify their nature and physicochemical conditions of emplacement.
iv) A study of stable isotopes of carbon and oxygen on bastnaesite and quartz to give a complementary indication of the origin of the mineralizing fluids and the mechanisms involved.
v) The integration of geophysical and geochemical data acquired by Rainbow Company during its prospecting campaigns to compare them with field geology data in order to better understand the formation mechanisms of the Gakara deposit as well as to discuss the identification of new mineralized zones.

All the data acquired in these different studies and the main observations allow to establish a global metallogenic model of Gakara area as resulting from the deposition of REE in hydrothermal conditions in the upper crust, from exsolved fluids of a probably carbonatitic magma, fluids having subsequently undergone CO2 vapour – aqueous brine separation. This phase separation was probably the main precipitation mechanism of REE minerals. The implication of surrounding fluids, with very low salinity, possibly coming from the surface, is suggested. Mineralization took place during the Pan-African tectonic event, at about 600 Ma, and was affected by recent deformations, possibly associated with the development of the East African Rift.