Isotopic studies of lead-depleted pitchblende, secondary radioactive minerals, and sulfphides from the Rabbit Lake uranium deposit, Saskatchewan

Uranium-lead and lead isotopic studies have been made on different types of pitchblende, on secondary Pb-rich and Pb-poor uranium-bearing minerals, and on sulphides from the Rabbit Lake uranium deposit in northern Saskatchewan. Most specimens have been selected on the basis of their mineralogy and Pb/U ratios, which varied between 1/2 and 1/665. All concentrates yielded discordantages. Six samples contained an excess of radiogenic lead and yielded Pb/U ratios above the concordia curve. P-1 (primary) pitchblende samples were found to be depleted of radiogenic lead and the data on even the best material can be interpreted in terms of a discordia line which yields intersections at about 1281 and 440 Ma. A second discordia interects the concordia curve at 1085 Ma and was obtained on concentrates containing several types of pitchblende and secondary Pb-rich and Pb-poor uranyl-bearing aggregates that fell above, and at the lower end of, the concordia curve. This discorida intersection agrees well with previously published ages. The important event related to the replacement of pitchblende by sulphides, selenides, and arsenides took place longer than 800-900 Ma ago. Reactions between remobilized uranium and altered silicates, and between the uranium and silica to form uraniferous phyllosilicates and coffinite occurred 440 and ca 200 Ma ago. Crystallization of hydrous uranyl-bearing aggegates continues at the present time. The Rabbit Lake deposit has been affected by superimposed alterations and recurring fracturing leaving fragments of partly resorbed and Pb-depleted pitchblende as the only remnants of the original intensive and widespread uranium mineralization