MINERAL CHEMISTRY AND OXYGEN ISOTOPIC ANALYSES OF URANINITE, PITCHBLENDE AND URANIUM ALTERATION MINERALS FROM THE CIGAR LAKE DEPOSIT, SASKATCHEWAN, CANADA

The Cigar Lake unconformity-type U deposit is one of the largest and h ighest grade U deposits in the Proterozoic Athabasca Basin, northern S askatchewan, Canada. Cigar Lake has recently been the focus of an inte rnational, 3-a, collaborative program in which this U deposit was stud ied as a natural analogue for a spent nuclear fuel repository. The dep osit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults an d the unconformity between Athabasca group sandstones and Aphebian met asediments. Three stages of U mineralization have been identified base d on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably alter ed to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite. U-Pb chemical ages of the 3 stages of U mineralization fro m Cigar Lake coincide with the 3 major fluid events that precipitated day and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout t he entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende hav e the lowest delta(18)O values that range from -30.1 parts per thousan d to -15.2 parts per thousand; whereas, stage 3 uraninite has delta(18 )O values ranging from -10.0 parts per thousand to -3.4 parts per thou sand. Uranyl oxide hydrate minerals have delta(18)O values that range from -11.3 parts per thousand to -8.2 parts per thousand; whereas, ura nyl minerals have much higher delta(18)O values. Based on U-Pb chemica l ages, delta(18)O values, and petrographic relationships of U alterat ion minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in t he Athabasca Basin. Therefore, the Cigar Lake ore deposit, although su rrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin. The 3 stages of ore formation and associated alte ration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary con text for the evaluation of the Cigar Lake deposit as a ''natural analo gue'' for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield. (C) 1997 Elsevier Science Ltd.