Uranium-thorium mineralization process of the Ukrainian shield suture zones and evolution of the composition of depth fluids in Precambrian

Abstract

Тhe geochemical properties of uranium and thorium and the geological structure of the ore oc currences of the Golovanevskaya and Ingulo-Inguletskaya suture zones are correlated with changes in the composition of mantle melts and fluids in the Archean-Paleoproterozoic. It is shown that the formation of uranium and thorium deposits on the Ukrainian shield became possible in the Proterozoic, when oxidized and acidic melts prevailing in the Archean mantle were replaced by neutral and alkaline water-chloride-potassium. This led to the accumulation of tho rium (to a greater extent) and uranium (to a less extent) in granite magmas of the layer of partial melting of the crust, where they entered as chloride complexes. The ore occurrences of monazite in microcline granites and pegmatites of the Berdichev and Kirovograd complexes formed in the period 2.1-1.9 billion years ago are associated with these processes. The enrichment of crustal centers of melting with uranium occurred most intensively after 2.0 billion years ago (a maximum of 1.8-1.65 billion years ago), when uranyl-carbonate ions entered the crust as a part of mantle carbonate-fluoride-sodium fluids, they were mixed with melts of existing melting centers. A change in the pH of the medium led to the decomposition of complex compounds and the release of the uranyl group, reduction of uranium to U4+ in the presence of graphite and magnetite, and at geochemical barriers with an acid reaction, and the activity of hydroxyl ions ensured the creation of a brannerite structure.