Paper No. 7
Presentation Time: 10:10 AM
AN UPDATED GENETIC MODEL FOR THE KIANNA UNCONFORMITY-RELATED URANIUM DEPOSIT, SHEA CREEK, ATHABASCA BASIN, CANADA
The Athabasca Basin, Saskatchewan is host to the world’s highest-grade unconformity-related U deposits. Known deposits are primarily located along the eastern part of the basin. The Kianna deposit, Shea Creek Project, is an unconformity-related deposit in the western Athabasca Basin. It consists of disseminated to massive uraninite localized in three distinct zones: 1) perched, above the unconformity, entirely in sandstone; 2) at the unconformity, hosted by sandstone and basement rocks; and 3) below the unconformity, hosted by basement paragneiss. SIMS was used to obtain preliminary ages for uraninite and determine the source of sulfur. Basement-hosted uraninite is associated with hematite and illite and has a U-Pb age of 1448 ± 23 Ma. Uraninite intermixed with hematite at the unconformity precipitated at 390 ± 13 Ma, whereas perched uraninite, inter-disseminated with alumino-phosphate sulfate (APS) minerals, sulfides, and hematite, formed at 177 ± 20 Ma. Pyrite from the basement has δ34
S values from 2.1 to 4.7‰, suggesting that sulfur was derived from a metamorphic or igneous source. Sulfides associated with perched and unconformity-related mineralization have two populations of δ34
S values, from 1.5 to 7‰ and 17.1 to 21.8‰, suggesting that there were two sources of sulfur, metamorphic basement and the Athabasca basin (e.g. APS minerals).
We have developed an updated genetic model for all three styles of uraninite. This model builds on previous studies for the formation of unconformity-type deposits, which incorporate the fault-valve, and suction pump models for the movement of large quantities of fluid along faults due to cyclic variations in stress. There were three stages:
1) An oxidizing, basinal brine moved downward along a fault and precipitated uraninite in basement rocks (ingress). Interaction with the basement rocks caused physiochemical changes to the fluid (e.g. lower fO2). Mineral precipitation and increased frictional strength caused the fault to self-seal.
2) Increased fluid pressure and a change in stress caused the fault to re-open; the reducing fluid moved upward along the same structure and uraninite precipitated at the unconformity due to fluid mixing (egress).
3) Fluids continued upward along the fault, interacted with APS minerals and precipitated perched uraninite and sulfide minerals.