Fluid inclusions in late minerals from the paleovalley-type uranium deposits of the West Siberian ore region: Thermochemical characteristics and genetic applications

Abstract

Comprehensive microthermometric investigations revealed similar temperature ranges (280–120°C) for the formation of late carbonates in the Khokhlovskoe, Semizbai, and Malinovskoe deposits of the West Siberian uranium ore region. A close chemical similarity was definitely established between the solutions of fluid inclusions and thermal nitrogen-methane waters with elevated CO2 concentrations typical of this region in general. It was noted that such CO2-rich mineral waters (Yessentuki no. 4 type) are common in the Mesozoic sequences of the Shadrinsk region, where Transuralian uranium deposits occur, and are similar in composition and temperature to the modern CO2-rich formation waters of the host sequences of the Khokhlovskoe deposit. The mineralogical and geochemical features of newly formed late minerals and uranium ores were considered as the most probable reflection of the exfiltration of such thermal solutions into the host levels. Two late mineral assemblages were distinguished: (1) hematite-calcite and (2) goethite-berthierine and goethite-smectite-chlorite with siderite or goethite-kaolinite-illite with siderite; they occur both in the host sequences and in the underlying basement rocks. The development of the latter assemblage causes a significant change in rock color (bleaching); it is widespread and was observed in all the deposits. It was shown that these altered rocks and uranium ores (especially high-grade) are very similar in mineral and chemical composition to the products of acid leaching and accompanying mineralization, which could be related to low-temperature argillization. It was suggested that exogenic epigenetic processes of ancient soil-bedrock oxidation contributed certainly to the development of uranium mineralization, and the modern character of the uranium ores and their host rocks is related to a large extent to the influence of hydrothermal CO2-rich solutions related to the neotectonic activation of the region. This resulted in the development of their specific mineral and chemical compositions and corresponding technological characteristics. It seems expedient to estimate the possible contributions of exogenic and endogenic factors to the formation of the uranium mineralization rather than oppose the roles of these processes of different stages.