Geological setting and genetic aspects of uranium occurrences in the Kaipokok Bay-Big River area, Labrador

Geol. Surv. Can., Ottawa, Ont., Canada

The metasedimentary and metavolcanic rocks of the Aphebian Aillik Group host a number of uranium occurrences which are lenticular concordant zones of finely disseminated pitchblende. The occurrences fall into two groups represented by the Kitts and Michelin deposits which are being considered for development. The Kitts deposit and several other occurrences are in an argillaceous-tuffaceous unit, up to 100 m thick and traceable for 20 km. The Michelin deposit and a number of other occurrences are in rhyolitic volcanic rocks which underlie an area 120 km long and 25 km wide.The uranium mineralization predates at least the final phase of the Hudsonian orogeny that affected the Aillik Group approximately 1,600 m.y. ago and is probably older. This is indicated by the geological settings, structures and textures of the occurrences, and radiometric age determinations. It is postulated that the group represented by the Kitts deposit is syngenetic sedimentary in origin and was formed by precipitaton of uranium in a reducing environment as indicated by the presence of pyrrhotite and graphite in the host rocks. The mineralization in the rhyolitic rocks is characterized by a pronounced enrichment of soda and a depletion of potash in the mineralized rocks in comparison with the unmineralized rocks. It is postulated that the uranium mineralization was contemporaneous with the synvolcanic alkali metasomatism of the host rocks. Uranium was precipitated by a titanium-bearing mineral, probably sphene, and occurs as pitchblende intergrown with sphene. Later Hudsonian deformation, metamorphism, and intrusions induced local redistribution of uranium, seen as pitchblende in veins, fractures, shears, and high-grade aggregates in the occurrences of both the groups. The ultimate source of uranium is considered to be the acidic magma that gave rise to the abundant volcanics of the Aillik Group.

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