Mineral System of the Streltsovka Caldera Uranium Deposits (East Transbaikalia)

Abstract—Various hypotheses, including alternative ones, have been put forward in the literature about the sources of ore substance and ore-forming fluids in the deposits of the Streltsovka ore field, which hosts the world’s largest uranium reserves, in excess of 250 000 t U: (1) uranium transport by ascending flow of ore-forming fluids that have separated from a deep subcrustal or intracrustal uranium-bearing felsic magma chamber and (2) uranium mobilization from uranium-bearing rocks of the volcano-tectonic structure of the Streltsovka caldera via postvolcanic thermoconvective circulation of the ore-forming fluids. For both hypotheses, the authors previously developed computer models of the paleohydronamic formation conditions of the largest in the ore field Antei–Streltsovka deposit, which presumed different uranium transport mechanisms with forced and free thermal convection of fluids, respectively. Both models yielded calculation results consistent with data on the reserves and formation temperatures of the Antei–Streltsovka ores. However, since the computer models are a simplified image of ore-forming systems, such agreement between the modeling results and a natural prototype only indicate the possibility of the proposed hypotheses on the formation conditions of the deposits. To solve the problem of their reality, additional information is necessary. Therefore, the authors of this paper have attempted a comparative analysis of the proposed alternative interpretations of the formation of the Antei–Streltsovka deposit using a methodology elaborated within the mineral systems concept. The results of our analysis substantiate the idea of successive forms of uranium transport by magmatic melt and ore-forming fluid to the mineral system of the Streltsovka ore field. The deep magmatic source was a feeder chamber for uranium transport by magmatic melts to the upper horizons of the crust with the formation of uranium-bearing rocks of the subvolcanic chamber and volcanic eruptions of the Streltsovka caldera. After uranium transport via magmatic melts, its subsequent redistribution occurred in the paleohydrothermal system with free thermal convection of fluids in the residual temperature field of the Streltsovka subvolcanic chamber. In this case, in the thermoconvective fluid circulation loop, conjugate processes of uranium mobilization could have taken place: (1) from the consolidated subvolcanic chamber, (2) from granitoid rocks of the caldera’s basement, and (3) from igneous felsic rocks in the caldera’s volcano-sedimentary cover. The coparticipation of these three potentially highly productive uranium sources in the ore mineralization process explains the origin of the unique uranium ore reserves of the Streltsovka ore field deposits.