|Title:||A rapid in situ method for determining the ages of uranium oxide minerals: evolution of the Cigar Lake deposit, Athabasca basin|
|Authors:||M. Fayek, T.M. Harrison, M. Grove, and C. Coath|
|Publication:||Int. Geol. Rev., v. 42, p. 163‐171.|
|Publish Date:||Feb 2000|
The authors present a rapid and accurate technique for making in situ U‐Pb isotopic measurements of uranium oxide minerals that utilizes both electron and ion microprobes. U and Pb concentrations are determined using an electron microprobe, whereas the isotopic composition of Pb for the same area is measured using a high‐resolution ion microprobe. The advantages of this approach are: mineral separation and chemical digestion are unnecessary; homogeneous uranium oxide standards, which are difficult to obtain, are not required; and precise and accurate U‐Pb ages on ⁓10µm spots can be obtained in a matter of hours. The authors have applied their method to study the distribution of U‐Pb ages in complexly intergrown uranium oxides from the unconformity‐type Cigar Lake uranium deposit, Saskatchewan, Canada. In situ U‐Pb results from early formed uraninite define a well‐correlated array on concordia with upper and lower intercepts of 1,467±63 Ma and 443±96 Ma (±1σ), respectively. The 1,467 Ma age is interpreted as the minimum age of mineralization and is consistent with the age of clay−mineral alteration (⁓1477 Ma) and magnetization of diagenetic hematite (1,650 to 1,450 Ma) that is associated with these unconformity‐type uranium deposits and early diagenesis of the Athabasca Basin sediments. In situ U‐Pb isotopic analysis of uraninite and coffinite can document the Pb*/U heterogeneities that can occur on a scale of 15 to 30µm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits.