Thermal profiles inferred from fluid inclusion and illite geothermometry from sandstones of the Athabasca basin: Implications for fluid flow and unconformity-related uranium mineralization

Abstract

The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (Th) range from 50° to 235°C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298°C, which are systematically higher than (partly overlapping) the Th values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion Th values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35°C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing.