Uranium even isotopes in kimberlites, enclosing and overburden rocks of the Zolotitskoye ore field (Arkhangelsk diamondiferous province)

Abstract

To confirm the presence of geophysical anomaly the isotopic composition of uranium has been studied from the samples extracted from the exploration wells drilled in different years in the territory of the Zolotitskoye ore field of the Arkhangelsk diamondiferous province (ADP). The study was based on the heterochronous deposits of the geologic profile and managed to obtain data on isotopic composition of uranium. For the quaternary deposits and coal formations, which are overburden ones in the Zolotitskoye ore field kimberlites, and in the Vendian rocks, which enclose kimberlites, the average values of isotopic ratio $^{234}$U/$^{238}$U did not exceed~1. Most samples from the near-contact zone, which are represented by the upper Vendian deposits and by the peripheral part of crater facies of pipes, contained excessive amounts of the daughter isotope $^{234}$U. The generation of high $\gamma = ^{234}$U/$^{238}$U values is explained as being related to the geologic structure of the near-contact zone. The particular features of the $^{234}$U isotope distribution are consistent with the tectonic and substantial changes in the rocks within the near-contact zone. This phenomenon has also been evidenced by several other investigators. The generation mode of non-equlibrium uranium with the excessive $^{234}$U content is explained by the dynamics and circulation of underground waters along the tectonic fractures bounding the pipes. The empirical data obtained in the course of the laboratory tests allow to conclude with a certain degree of scepticism that non-equilibrium uranium in enclosing rocks relates to kimberlite magmatism in the ADP territory. In turn, non-equilibrium uranium with the excessive content of $^{234}$U, which was discovered in the geologic profile of enclosing Vendian deposits, may be considered as the local indirect evidence of ADP kimberlite bodies. However, when estimating the prospecting areas for availability of the kimberlite magmatism in the geologic profiles of exploration wells, the non-equilibrium state of uranium should be considered only in conjunction with the kimberlite signs.