Abstract

The unconformity-related U deposits associated with the Proterozoic Athabasca Basin are among the largest and richest U deposits in the world. The conventional genetic model suggests that mineralization occurred under deep-burial (>5 km), diagenetic-hydrothermal conditions at normal geothermal gradients (~35°C/km). Based on regional geochronostratigraphic and ore geochronological data, it is inferred that, at the time of primary U mineralization (≥ ca. 1540 Ma), the burial depths of the unconformity surface were likely <~3 km. The elevated fluid pressures (up to 1,500 bars) used to support the deep-burial model were probably overestimated due to misinterpretation of accidentally entrapped halite crystals as daughter minerals in fluid inclusions. The elevated fluid temperatures (180°–250°C) estimated from fluid inclusion and clay mineral geothermometry from both mineralized and barren areas, which were interpreted to have resulted from deep burial at normal geothermal gradients at the time of mineralization, may be alternatively explained by local or basin-scale elevation of geothermal gradients at the time of mineralization, followed by continued burial and/or temporarily increased thermal gradients after mineralization. The shallow-burial mineralization model can better explain the geologic characteristics of the unconformity-related U deposits, including development of pervasive clay alteration halos, breccia zones, and dissolution vugs locally filled with drusy quartz, as well as evidence of fluid boiling recorded by fluid inclusions. The modified model emphasizes the importance of combined basinal (development of brines) and deep-seated geodynamic factors for large-scale U mineralization. Recognition of these factors is important for U exploration in the Athabasca Basin and similar basins elsewhere.

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