Uranium Sources and Fluid Transport in Volcanic Mineralized Systems: an Example of Streltsovka Caldera, Russia with Reflection on Dornot, Mongolia

According to the IAEA classification (Geological Classification …, 2018), uranium deposits of the Streltsovka caldera in the South-Eastern Transbaikalia belong to the volcanogenic (volcanic-related) type based on the characteristics of host rock and/or ore-localizing structure. The implementation of such a descriptive taxonomy in classification of uranium deposits has a long history, but its limitations and the need for transition to models of uranium mineral systems (Descriptive Uranium …, 2020) are becoming increasingly evident. Such a transition for the deposits of the Streltsovka caldera is relevant and logical, bearing in mind a representative example of the space-time relationship of uranium ores in the basement and in the cover of the caldera, which is manifested in the Antei-Streltsovka mineral system (Pek et al., 2020). One of the key points of the mineral-system approach is the reconstruction of the environment of origin and dynamics of system-forming elements interaction in the “source → transport → deposition” sequence. We consider in the article two models of this interaction: (i) the model of forced convection of fluids with a deep-seated magmatic source of uranium and (ii) the model of free thermal convection of fluids with crustal sources of uranium. The hypothesis of thermoconvective circulation of fluids in the residual thermal field of the caldera magma chamber, which is verified by numerical models, substantiates the supply of uranium from three sources of its nearby mobilization: 1) subcaldera chamber, 2) basement rocks, and 3) volcanic rocks filling the caldera. Participation in the ore genesis of the three highly productive uranium sources sheds light upon the origin of the Streltsovka caldera unique ore reserves (~300 000 tU), as compared to the similar in age uranium mineralization of the Dornot ore field (33 000 tU) located about 500 km to the southwest in the territory of Eastern Mongolia (Geological Classification …, 2018).