Paragenesis and Isotopic Composition of some Gangue Minerals from the Uranium Deposits of Eldorado, Saskatchewan

Abstract

A paragenetic sequence of the common mineral constituents of the uraniferous vein deposits of the Fay Mine and Bolger open pit of Eldorado Nuclear Ltd., in northwestern Saskatchewan, is presented.Pitchblende associated with hematite, pyrite, chalcopyrite, bornite, galena, and clausthalite in quartz ± calcite ± dolomite gangues is found in veins cutting folded Tazin metamorphics, adjacent to the St. Louis Fault. Fluid inclusion studies reveal a cooling sequence from the initial phase of mineralization at 440 ± 30 °C down to the final stages at around 80 ± 10 °C. Stable isotope studies, augmented by petrographic data, serve to distinguish at least five generations of carbonate in the ore veins. Early generations of dolomite have apparently not undergone any significant recrystallization or isotopic readjustment and exhibit original restricted δ 18 O (SMOW) values of 9.5 to 11.0‰ and δ 13 C (PDB) values of −0.60 to 0.55‰. Isotopic evolution via equilibration with wall rocks has resulted in the deposition of late-stage calcites characterized by δ 18 O (SMOW) values of + 13.2 to + 14.5% and δ 13 C (PDB) values of −15.9 to −16.8‰. The strong decrease in δ 13 C may possibly be explained by a decrease in the amount of CH 4 present in the system. It is felt that the deposits were generated by metamorphic hydrothermal fluids with initial δ 18 O (SMOW) values of + 6 to + 8‰, which during cooling underwent isotopic exchange with the host metamorphic rocks and became depleted in 18 O. The final stages of mineralization were possibly effected by some influx of isotopically lighter surface waters into an otherwise essentially 'closed' system. Present day waters from the Fay Mine have δ 18 O (SMOW) values of −16.0 to −16.2‰ and could represent products of a continuing evolution of the hydrothermal fluids.