U AND Pb ISOTOPE ANALYSIS OF URANIUM MINERALS BY ION MICROPROBE AND THE GEOCHRONOLOGY OF THE MCARTHUR RIVER AND SUE ZONE URANIUM DEPOSITS, SASKATCHEWAN, CANADA

A method for in situ U–Pb isotopic analyses by secondary ion mass spectrometry (SIMS) has been developed for uranium minerals with a range of chemical compositions. This method combines the advantages of conventional U–Pb dating (i.e., use of concordia) and in situ analysis, and therefore is ideally suited for the study of chemically complex and fine-grained uranium oxides associated with uranium deposits. An ion-yield normalizing coefficient ( SIMS) that accounts for variation in relative ion-yields with chemical composition of the mineral of interest was calculated using uraninite standards that cover a range of U and Pb compositions, and an appropriate empirical mass-bias model was developed. The coefficient SIMS varies as a function of wt% PbO, requiring two working curves to define the relationship between the 206Pb+/238U+ and 207Pb+/235U+ values measured by SIMS versus the “true” 206Pb/238U and 207Pb/235U values: 207Pb+/235U = 0.762 ± 0.015 (207Pb+/235U+)0.69±0.02 206Pb/238U = 0.333 ± 0.007 (206Pb+/238U+)0.64±0.02 The application of this technique to unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, demonstrates that at the microscale, these deposits preserve the initial age of mineralization (1486 to 1519 Ma) and a temporal record of accretion and breakup of supercontinents. Prior to in situ analyses, this detailed chronological record was obscured by the wide variability in U–Pb and Pb–Pb data obtained by micro-drilling and conventional isotopic analyses due to mixing of different generations of minerals.