[en] Field and laboratory studies of specimens collected in 1978 and 1979 in the Baie-Johan-Beetz area, Quebec are summarized. Radioactive syenitic and granite rocks are characterized by their heterogeneous character, namely, variable grain size, petrochemistry, mineralogical composition, size and nature of xenoliths, and by abundance and distribution of radioactive ore and rare-earth elements (REE) minerals. There are apparent associations between early plagioclase- and biotite-bearing syenitic phase and uraninite and phosphate minerals, and between later microcline-quartz phases and REE-bearing minerals. Two types of favourable host rocks for uranium mineralization are white syenitic pegmatites, characterized by relatively low silica content, high concentrations of alumina and the ratio Fesup(II)/Fesup(III)>1; and pink to red granite pegmatites, characterized by variable proportions of plagioclase, microcline, albite and quartz, the ratio Fesup(II)/FEsup(III)<1, and by alteration of primary iron- and uranium-bearing minerals. The uranium mineralization is a factor of geochemical evolution of host rocks and of the nature of paleosome remnants. Most of the uraninite grains alter during late stages of pegmatitic activity. The U/Th ratio in the Baie-Johan-Beetz area is relatively high, the average being 4, but varies from 0.3 to 10 depending on the abundance of secondary uranyl-bearing minerals in the rock, and on redistribution and losses of uranium during deuteric alteration and weathering. (auth)