Composition and Chemical Microprobe Dating of U—Th-Bearing Minerals. Part 2. Uraninite, Thorite, Thorianite, Coffinite, and Monazite from the Urals and Siberia

摘要

To develop further chemical microprobe timing of U—Th-bearing minerals on the basis of upgraded measurement techniques and special age calculation, uraninite, thorite, thorianite, coffinite, monazite from several localities in the Urals and Siberia have been dated. The samples were taken from granitic rocks of the Pervomaisky pluton in the Central Urals; the pre-Jurassic basement of western Siberia and Yamal Peninsula; carbonatite-like dolomite from the Karabash ultramafic massif in the Southern Urals; granitic pegmatites of the Lipovsky vein field; and quartz—sulfide veins of the Pyshma—Klyuchevsky Cu—Co—Au deposit in the Central Urals. Scrutiny of the composition and chemical heterogeneity of mineral grains is a necessary stage of chemical dating aimed at the estimation of the degree of closeness of the U—Th—Pb system and unbiased screening of analytical data. The condition (Si + Ca)/(U + Th + Pb + S) ～ 1 was used as evidence for significant secondary alteration of monazite; the negative correlation between Pb and Th or U +Th in uranitite was used for the same purpose. The positive correlation between Pb and U, along with low concentrations of Ca, Si, and Fe admixtures, implies that the stoichiometric composition of thorite is close to 100%. The reliability and accuracy of the chemical dating of minerals with high contents of radioactive elements can be enhanced by using bimineralic or multimineralic isochrons, e.g., monazite—uraninite, uraninite—coffinite, etc. The results obtained have been compared with the available isotopic ages of the studied minerals; the compared data are satisfactorily consistent.