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 (alphaSIMS) 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 Ph compositions, and an appropriate empirical mass-bias model was developed. The coefficient aSIMS varies as a function of wt% PbO, requiring two working curves to define the relationship between the Pb-206(+)/U-218(+), and Pb-207(+)/U-231(+), values measured by SIMS versus the true (206)pb/U-238 and Pb-207/U-235 values: Pb-207(+)/U-235 = 0.762 +/- 0.015 (Pb-207(+)/U-235(+))(0.69 +/- 0.02) Pb-206/U-238 = 0.333 +/- 0.007 (Pb-206(+)/U-238(+))(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.