Element distribution patterns in the uranium occurrence at Mika, northeastern Nigeria

The Mika uranium occurrence is located in one of a series of NW-SE trending shear zones which hosts swarms of uraniferous Jurassic rhyolitic dikes, situated in Pan African brecciated granites within the peraluminous Mika granite complex of N. E. Nigeria. The bodies of mineralisation are about 100 metres long and up to four metres thick and are emplaced in parallel and steeply dipping faults which transverse granites and N-S trending rhyolite dykes within a narrow, intensely deformed and tectonised trending NW-SE. The U mineralisation associated with the rhyolite dykes contains predominantly meta-autunite and apatite, while those at the brecciated granites display variable mineralogy with meta-autunite, one or two generations of coffinite and colloformic pitchblende in open veins. This grades into a peripheral zone consisting of a network of haematized pitchblende, veinlets associated with siderite, chalcopyrite and pyrite. Lithochemical data for 28 elements/oxides in 296 host rock samples from surface outcrops and drill cores have been collected. Statistical evaluation shows a coherent association of: (i) U, Pb, Zn, Cu, P2O5, Fe2O3, Mo, (ii) CaO, Zr, Sr +/- Y, Mo, V, As and (iii) MgO, K2O +/- TiO2, Rb in the mineralised rocks thus reflecting the presence of haematised phosphate in association with sulphides and apatite in the granites and rhyolites. A strong relationship between the mineralising fluids and the emplacement of the rhyolites is implied from the striking resemblance between the above element association in mineralised rocks and those of the altered/unmineralised rhyolites. The source for the mineralising fluids is inferred as of U and SiO2 oversaturated fluids emanating from crystallising rhyolitic melts expelled into faults and/or shear zones in the sorrounding country rock.