CHEMICAL COMPOSITION AND AGE OF THE ACCESSORY URANINITE FROM GRANITOIDS OF THE CRYSTALLINE BASEMENT OF THE SOUTHERN YAMAL

Abstract

The relevance of the work is due to the need to improve the method of chemical dating in application to high-uranium accessory minerals that cannot be date by isotopic methods of research. The purpose of the work: the study of the accessory uraninite chemical composition from granitoids of the crystalline basement of Southern Yamal and the determination of its age. Methodology of the research. Quantitative analysis of the chemical composition of zircon is performed with the X-ray spectral electron probe micro-analyzer CAMECA SX 100 (electron beam diameter is from 1 μm, BSE, SE, Cat modes, determination of elements from beryllium to uranium). Spectra were obtained using inclined wave spectrometers; intensity measurement was carried out according to analytical lines: Th Ma , U Mb , Pb Ma , Y La , Si Ka , Ce La , La La , Nd La . Age calculation was carried out according to the known methods of foreign authors in addition to authors own developments. Results. The chemical composition of uraninite is quite stable and quite contaminated, the main impurities are thorium (ThO2 up to 5 wt.%), lead (PbO up to 3 wt.%) and yttrium (Y2O3 up to 3.7 wt.%). Based on the impurity content in Verkhnerechensky uraninite, its unit cell parameters were calculated as 5.475 ±5.476 Å. A substantial lead content and high crystallinity of the substance allows the use of this mineral as a mineral-geochronometer. The calculated weighted average age of uraninite is 258.7 ± 3.4 Ma (MSWD = 0.19); and isochron age is 259 ± 8 Ma (MSWD = 0.34). Conclusions. Microprobe compositions of uraninite were obtained, and the Late Permian age of granitoids was determined by chemical dating. The given chemical composition data show that Verkhnerechensky uraninite differs sharply from most of its analogues from other rocks, but it correlates quite well with accessory uraninites from S-type granites. The mineral is very close to synthetic uraninite and is not metamict; it also does not contain significant amounts of more oxidized uranium (UO3 и U3O8).