Fluorapatite cements have been documented at more than 18 localities in the Thelon Basin, Canada, occupying stratigraphic positions from the paleosol below the unconformity to well within the upper parts of the fluvial siliciclastic Thelon Formation. Ion microprobe analyses targeted using cathodo luminescence images to areas of higher uranium content on petrographically characterized cements provide the most precise pin on the timing of meso diagenesis in the northeastern part of the Thelon Basin at 1667 ± 5 Ma. Expulsion of phosphate-rich basinal brines was asso ciated with development of faults and local breccias in the lower strata where extensive quartz cementation took place before the apatite event. The new data do not support previous interpretations for multiple episodes of fluorapatite cementation within the basin despite the wide range of stratigraphic positions and variations in style of occurrence. The tectonic cause remains speculative, as does the relationship of the fluorapatite cements to the unconformity-style uranium mineralization. Thelon fluorapatite cements are older than those reported in the Athabasca basin, suggesting distinct fluid-flow histories occurred within the two basins. The difference in timing indicates that the basinwide events that generated the fluids with the chemistry capa ble of producing fluor apatite cement took place independently but under similar conditions within the Thelon and Athabasca basins.

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