Fault rocks and differential reactivity of minerals in the Kanawa Violaine uraniferous vein, NE Nigeria

The Kanawa Violaine uraniferous vein occurs in a narrow granitic shear zone. Mylonitic fault rocks at the centre of the vein grade outwards into cataclasites and fault breccias. The mineralization is associated with pervasive silicification and phyllosilicate alteration of the feldspar phases. The uranium mineralization occurs as uraninite-rich veinlets within brittle structures. ars were the phenocryst phases most affected by alteration during the brittle-ductile deformation of the host rock. Plagioclase was extensively altered to micas, chlorite ± epidote, ±albite. Alkali-feldspar deformed mainly by transgranular fracturing as a result of shearing to yield clasts with lensoid shape. Quartz shows little evidence of brittle deformation but extensive in situ recrystallization. A mylonitic foliation is defined by monomineralic lenses. Microcracks in the feldspar are oblique to this foliation. eenschist-facies secondary mineral assemblage, pervasive silicification, deformation mode of alkali-feldspar and the presence of quartz subgrains point to deformation of granites in the epizone by simple shearing within a hydrothermal-fluid-infiltrated medium. Alteration temperatures did not exceed 250 °C. The hydrothermal fluid remobilized and subsequently concentrated the uranium. This fluid was enriched in Si4+, Na+ and K+, possibly derived from plagioclase alteration. This lead to association of the ore with phyllosilicate and silicification alterations.