Geology of the pitchblende deposits of Port Radium, Great Bear Lake, N.W.T.

Abstract

The geology of the McTavish Arm area of Great Bear Lake, Northwest Territories, Canada, is reviewed and a geologic map is presented. The geology of the Labine Point region is described in detail. Intrusive feldspar porphyry bodies are haloed by contact metasomatic zones in which the lower Echo Bay cherty sediments have been enriched in Ca, Al, Mg and Fe by the development of ferromagnesian minerals. The sediments have been locally severely deformed by intrusion of the porphyry bodies. A body of massive crystalline tuff in the mine area is believed to be the remnant of a volcanic vent. The ore deposits occur in a lenticular network of northeast trending shear zones. Differences in wall rock competency have resulted in the development of relatively wide tensional fracture zones at various places along the shear systems. The pitchblende ore bodies occur in these zones of greater than usual dilation. The mineralization of the Port Radium deposits occurred in five stages: I, hematite-quartz; II, pitchblende-quartz; III, quartz-cobalt nickel arsenides; IV, copper sulphides-chlorite; V, carbonate-silver. Deposition of pitchblende at Port Radium took place in dilated portions of the vein zones in the following manner: release of pressure in dilated zones caused loss of CO2; loss of CO2 resulted in drop of bicarbonate ion content and consequent decrease in the solubility of the uranium as a complex with carbonate; the uranium flocculated as the uranium colloid precipitate, pitchblende. Brecciation and some redistribution of the pitchblende by later solutions is illustrated. Comparison is made between the mineralogy of Port Radium and other pitchblende deposits. Possible sources of error in the lead isotope age determination of Port Radium pitchblende are discussed. Ages determined are believed to be too great by a factor of about three.