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 = 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.

Radon over Kimberlite Pipes: Estimation of the Emanation Properties of Rocks (Lomonosov Diamond Deposit, NW Russia)

“…It is known that among the magmatic formations, kimberlites are characterized minimal content of radioactive elements [35]. In addition, the kimberlites of the pipes of the Zolotitsky field have been characterized by an extremely low average concentration of uranium, 0.67 mg•kg −1 [19], which, in terms of the activity of 226 Ra under conditions of radioactive equilibrium in the uranium-238 chain, does not exceed 10 Bq•kg −1 .…”

“…In our experiment, the regression model has the form y = 19.138x − 32.696 and is characterized by a positive slope equal to 19.138 and a coefficient of determination R 2 = 0.8786. The positive slope indicates that with an increase in the level of radon production by 1 unit, the volumetric activity increases by 19.138 Bq•m −3 . This value is theoretical and can vary depending on a number of parameters.…”

“…In this regard, emanation methods began to be widely used in geochemical, geophysical and geodynamic studies to track geological processes [4]. Emanation methods are also used in the search for mineral deposits, including kimberlites [16][17][18][19]. Emanation studies carried out in the territory of the kimberlite fields of the Arkhangelsk diamondiferous province (Russian Federation) showed that, in the soil horizons above the kimberlite bodies, there are increased values of the volumetric activity of radon that are several times higher than the background values [20].…”

“…Emanation studies carried out in the territory of the kimberlite fields of the Arkhangelsk diamondiferous province (Russian Federation) showed that, in the soil horizons above the kimberlite bodies, there are increased values of the volumetric activity of radon that are several times higher than the background values [20]. It was shown that the nature of the distribution of radon over kimberlite pipes has a subcircular structure, which is associated with the development of faults and fracturing in the enclosing rocks of the near-pipe space [19,21]. Previously obtained data have shown high prospects for the application of emanation methods in the search for kimberlite pipes in the territory of the Arkhangelsk diamondiferous province [22].…”