Evidence of the origin of the Blind River uranium deposits

Abstract

The 10 U mines now in production in the Blind River area of Ontario have provided considerable new evidence on the origin of the deposits and have been the subject of a number of studies by government, company, and research geologists. An attempt is here made to coordinate and assess the evidence and conclusions presented by the various workers. Commercial ore is almost exclusively restricted to quartz pebble conglomerate beds near the base of the lower Huronian quartzites, and a direct relationship exists between maximum conglomerate development and grade of U. The detrital material was derived from Archean basement rocks to the NW. and was probably deposited under deltaic conditions near a northward-migrating shore line. The ore minerals are uraninite, brannerite, and minerals of the gummite group in variable proportions. Brannerite is the most generally accepted as detrital mineral but even here the evidence is not altogether convincing. Age determinations show that definite detrital minerals, such as zircon and monazite, have an age corresponding to that of the Archean basement while the U minerals show an age closer to that of the deposition of the lower Huronian sediments. Pending more convincing evidence that the brannerite is detrital, it is concluded that: 1) The U minerals were deposited contemporaneously, or shortly after, deposition of the conglomerate beds, but as precipitates from supergene solution rather than as detrital minerals, 2) The present U minerals resulted from recrystallization or reaction with other minerals after burial. 3) Hydrothermal action does not appear to have played a more significant role than aiding such recrystallization and the redistribution of U within the conglomerate beds.