Structural controls and geochemistry of Ghumchi uranium mineralization, northeastern Nigeria

摘要

The Ghumchi uranium mineralization is ahydrothermal veintype deposit hosted by a small brittle-ductilegranitic shear zone. The mineralization is concordant to theMylonitic foliation and confined to north-south and N140 0E-N155 0E fault trends. Mylonites at zones of intersection of thesetwo trends as well as where these trends are truncated bylamprophyric dykes show elevated concentrations of ore. Thebrittle structures in the breccias and cataclasites zones, however,are barren. Under the microscope the mineralized mylonites aretypified, bookshelf-sliding microstructures in feldspars.Dissolution textures, veined structures and pervasive plagioclasealteration are restricted to the zones of fracture convergence.Comparative geochemical analysis of the wall-rock and ore-hostmylonites indicates that solution transfer processes wereoperational in the ore concentration stage. The mineralizedsamples, for example, are depleted in the alkalis Na2O and K2O3 ,but enhanced in TiO2, Fe2O3, Pb, V, Mo and As asopposed to freshgranite and altered but barren samples.