Abstract

Five miles east of Hunter bay, Lac La Ronge, is an irregular pegmatite sill, about 40 feet thick, introuding metamorphosed sediments. It is composed of three facies: an upper margin two inches thick of fine-grained (1/4 to 1/2 inch) oligoclase and quartz; next is ten feet or more of coarse (4 to 6 inches) crystals, chiefly salmon-pink microcline with some peach colored oligoclase, both graphically intergrown with quartz; and, a central band four feet, or more, thick of salmon-pink microcline-perthite crystals up to two feet long with interstial glassy quartz. The same two outer zones are repeated below the central zone.The central band is not radioactive, but monazite is found in one area in the upper band of the intermediate facies, and uraninite is found in the upper marginal facies and in the immediately adjacent coarse mica schist. At a gable-like part of the upper contact of the sill, over a length of eight feet, and across a width of one to four inches, numerous crystals of uraninite up to one inch in diameter have been obtained. Along the same contact is a similar, but less rich occurrence, and a scintillometer survey obtained a number of above-background readings elsewhere near the inferred upper contact of the sill.The concentration of the uraninite on the hanging wall of the sill may have occurred partly at the time of consolidation of the marginal facies and partly during the final solidification of the sill.

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