Mesostructural and microstructural evidences for a two stage tectono-metallogenetic model for the uranium deposit at Mika, Northeastern Nigeria: A research note

The Mika uranium deposit is of the Central Massif vein-type. The deposit consists of a series of subparallel veins, similar to Lilljuthatten deposit (Sweden), with variable ore grade. Two main structural trends occur within the vicinity of Mika namely: (1) vertical N160E-N180E or N-S normal fault trend in which the brecciated granite has been permeated and cemented by silica ribbons; (2) N130E trend associated with reddish brown hematitized siliceous breccias filling open fractures. The N130E trend is younger, and it is a local, passive shear trend developed at the bends of the N-S faults. On a microscopic scale, samples from the N-S trend are rich in crude cubic-tetragonal prismatic crystals of uraninite and/or pitchblende ores occurring in association with a low-temperature greenschist mineral assemblage of chlorite, epidote, and mica. Conversely, samples from the N130E trend show widespread kaolinitisation, goethite formation and layering reminiscent of groundwater activity at shallow depths near the earth’s surface. The main uranium mineral in this zone is yellowish autunite. The model proposed here envisages that the mineralization was related to two separate processes. A first mineralization episode developed simultaneous with the greenschist mineral assemblage, and a second episode was related to remobilization and subsequent mineral concentration in the passive N130E open fractures by groundwater to form autunite. Such a model has far reaching implications for exploration. Although the secondary minerals along the N130E fractures are more obvious in the field, the potentially economic primary mineral concentrations may be concealed at greater depth in the N-S structures.