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

Abstract

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 (radiogenic galena and pyrite) from the Rabbit Lake uranium deposit in northern Saskatchewan. Most specimens have been selected on the basis of their mineralogy and Pb/U ratios as determined by electron microprobe analyses. The Pb/U ratio varied between 1/2 and 1/665 as a result of diverse episodes of crystallization, differential losses of uranium and radiogenic lead, recrystallization of remobilized uranium and lead in different proportions in secondary radioactive minerals, and possible loss or enrichment of radon gas.All concentrates yielded discordant ages. 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, thus grading into the Pb-depleted pitchblende of type P-2, and the data on even the "best" material can, thus, only be interpreted in terms of a discordia line which yields intersections at about 1281 and 440 Ma.A second discordia intersects the concordia curve at 1085 Ma and was obtained on concentrates containing several types of pitchblende including some samples difficult to distinguish in polished section from those of the 1281–440 Ma line, and secondary Pb-rich and Pb-poor uranyl-bearing aggregates that fell above, and at the lower end of, the concordia curve. This discordia intersection agrees well with previously published ages.The important event related to the replacement of pitchblende by sulphides, selenides, and arsenides, accompanied by marked losses of radiogenic lead and mobilization of uranium from the partly-resorbed "primary" pitchblende of type P-1, took place at a time no greater 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 aggregates, including precipitation of amorphous crusts in fractures of argillized rocks, 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.