On the Formation of Uniquely High-Grade Ores of Unconformity-Related Uranium Deposits in the Athabasca Basin (Canada): The Multistage Telescoped Ore Deposition Hypothesis

• Published in: Geology of ore deposits Vol. 63; no. Suppl 1; pp. S62 - S79
• Main Authors: Pek, A. A., Malkovsky, V. I., Petrov, V. A.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.12.2021; Springer Nature B.V
• Subjects: Basement rock; Basements; Cellular convection; Computer simulation; Convection; Deposition; Deposits; Earth and Environmental Science; Earth Sciences; Egress; Fault zones; Faults; Fluids; Free convection; Geochronology; Geological history; Geothermal gradient; Hydrothermal activity; Hydrothermal fields; Hypotheses; Mineral deposits; Mineral Resources; Mineralization; Ores; Oxidation; Precambrian; Renewal; Rocks; Sandstone; Sedimentary rocks; Unconformity; Uranium; Canada; interfault geothermal fluid convection; uranium deposits; Athabasca Basin
• ISSN: 1075-7015; 1555-6476
• DOI: 10.1134/S1075701522010044

— Unconformity-type deposits in the Canadian Proterozoic sedimentary Athabasca Basin, with total resources of >1.0 mln t of uranium and an average uranium content of 3.58%, are the most outstanding representatives of large-scale deposits with uniquely high-grade ores. Geochronological data record a long process of the deposits formation in the Athabasca Basin, the successive stages of which were separated by time periods of tens and hundreds of millions of years. It was assumed that the main ore mineralization was formed at the earliest stage 1590 Ma ago, while younger geochronological dates of ~1400, 1270, 1150, and 1000–850 Ma recorded discrete stages of remobilization of primary uranium. In this paper, we propose a hypothesis on the origin of the uniquely high-grade ores of the Athabasca Basin as a result of multistage telescoped deposition of uranium in structural traps created at the initial stage of the formation of deposits and subsequently renewed by seismotectonic impacts. The multistage interpretation was substantiated using computer simulation of fluid-dynamic conditions for the formation of the main structural types of egress- and ingress-style unconformity-related deposits, localized in sandstone and basement rocks, respectively; analysis of tectonodynamic environments of periodic renewal of hydrothermal activity at successive stages of the formation of deposits; consideration of fluid-dynamic processes of uranium mobilization from uranium-bearing basement rocks of the basin. Deposits of the egress and ingress types were formed due to oppositely directed upward and downward migration of mineral-forming fluids along fault zones in interfault thermoconvective cells. Since interfault thermal convection occurred in a regional geothermal field with a background geothermal gradient, the driving forces for thermoconvective fluid circulation persisted throughout the entire long-term multistage geological history of the formation of ore deposits in the Athabasca Basin. The mobilization of uranium from uranium-bearing basement rocks could have occurred during the migration of fluids along the path of their interfault flow with oxidizing conditions in association with egress and ingress ore mineralization in areas of the contour of thermoconvective fluid circulation with reducing conditions. The process of multistage deposition of uranium mobilized from the basement of the basin both at the first and subsequent stages of the formation of the deposits explained the positive correlation between the resources and uranium content, which provides a likely interpretation of the origin of the uniquely high-grade ores of the Athabasca Basin deposits.