Natural losses of uranium and other constituents from partly altered uraninite, uranothorite, pyro-chlore and allanite in granitic rocks of Precambrian age (100–1100 Ma) were documented using a scanning electron microscope coupled with an energy dispersive spectrometer and an electron microprobe. Migration and precipitation of the liberated uranium along fractures and reactions between uranium and other ions to form secondary uranium-bearing minerals were studied in polished thin sections using microbeam techniques. Mode of formation of the most common secondary uranium ore minerals: autunite, torbernite, phosphuranylite, thorogummite and uranophane is discussed in relation to supergene enrichment and formation of secondary uranium mineralizations in weathered granites. The purpose of this paper is to describe natural decomposition of primary radioactive minerals in granitic rocks, as well as migration and redeposition of the liberated uranium and other ions on the basis of field and laboratory studies.