Genetic significance of inclusions and fracture fillings in magmatic and metamorphic rocks from selected Canadian uranium occurrences

Radioactive mineral inclusions constitute deposits with disseminated uranium mineralization, whereas radioactive crusts and fracture filings form autochthonous or allochthonous vein type deposits. In selected radioactive occurrences, notably at the contact between metamorphic and pegmatitic rocks, disseminated and vein type mineralizations may occur together. Under conditions of high grade metamorphism thorium rich uraninite crystallizes at early stages in mafic minerals, such as pyroxene and amphibole. Uraninite and uranothorite grains crystallize during the main pegmatitic phase in xenotime, cyrtolite and other accessory minerals, and during the late pegmatitic stage in interstitial feldspar, tourmaline and REE mineral aggregates. Thorium poor, Y bearing uraninite, associated with pyrochlore and partly replaced by uranothorite, enclosed in rims of calcite, fluorite, REE mineral aggregates and/or diverse layer silicates and sulphides, crystallizes in allanite, in titanite and in interstices, notably between biotite and muscovite during the late pegmatitic hydrothermal and deuteric phases. Alteration of early crystallized radioactive minerals, associated with mobilization of U and other ions, takes place during the deuteric stage and continues after fracturing and subsequent hydration and weathering of host rocks. During advanced alteration, elements liberated from altered minerals migrate along fractures and precipitate as secondary U minerals and as chemically complex U-Pb-Th-REE-Zr-Ti-Fe bearing crusts. During the ultimate stages of alteration and replacement by phyllosilicates and oxides, radioactive elements removed from their host can become a potential source for allochthonous vein type deposits. Isotopic ages of minerals range from 230 to 1090 Ma. 27 refs, 5 figs