Derome, and 5 co-authors, reported in 2005 that the richest U deposit in Saskatchewan, Canada, occurs in the McArthur River area, in the vicinity of the unconformity between the Athabasaca sandstones and an Archean to lower Proterozoic basement. Paleofluids related to the silicification of the sandstones and the formation of pre- and post-ore cements in breccias were studied using microthermometry, Raman microspectroscopy, and laser induced breakdown spectroscopy (LIBS) on individual fluid inclusions. A detailed reconstruction of the fluid composition in the system Na-Ca-Mg-Cl shows that two types of brines are responsible for the main quartz cements: an NaCl-rich brine (25 wt % NaCl, up to 14 wt % CaCl2, and up to 1 wt % MgCl2), which is interpreted as a primary formation water that was expelled from bedded evaporites; and a CaCl2-rich brine (5–8 wt % NaCl, 20 wt % CaCl2, and up to 11 wt % MgCl2), which is considered to have formed during the interaction between the NaCl-rich brine and Ca-rich minerals in the basement and was introduced into the fault system and mixed with the NaCl-rich brine during the critical stage of U deposition.
The present study shows that the nature and timing of fluid percolation within faults and at the unconformity controlled the genesis of the P2 uranium orebody. Two main types of brines are distinguished for the first time in the Athabasca sandstones and the underlying basement in the vicinity of uranium mineralization: a hot (min 190°C) NaCl-rich brine (about 25 wt % NaCl + up to 14 wt % CaCl2 + 1 wt % MgCl2) and a cooler (approx 140°C) CaCl2-rich brine (5–8 wt % NaCl + up to 20 wt % CaCl2 + up to 11 wt % MgCl2). Previous workers considered only one type of brine circulating at a temperature of about 200°C.
Quartz cementation at the base of the Athabasca basin occurred in three stages with an early diagenetic cementation associated with the NaCl-rich basinal brine; a later cementation (Qz2-Qz3), corresponding to a first brecciation event, was synchronous with the input of the CaCl2-rich brine. This calcic-brine episode may have been related to reactivation of basement faults, such as the P2 structure. The third step of cementation followed a second brecciation event and is represented by the post-ore Qz4 quartz crystals in which the NaCl-rich brine is again dominant. The two brines show evidence of mixing, especially in quartz Qz3. This quartz commonly hosts dravite (Drv1) and predates the chlorite-dravite (Drv2) brecciation event related to the main uranium mineralization event, as shown by the petrographic study and in accordance with previous results. According to most of the petrographic and paragenetic studies that have been carried out in the McArthur River deposit, euhedral quartz (Q2 of Kotzer and Kyser, 1995, Fayek and Kyser, 1997, and Q1 of Alexandre et al., 2003) is considered as postore. However, the present petrographic and fluid inclusion study has shown the existence of two generations of euhedral quartz, Qz3 and Qz4. The main stage of uranium deposition postdated the Qz2 stage and was probably synchronous with or immediately postdated the Qz3 stage and predated the Qz4 quartz cementation stage. Therefore, the two brines (NaCl- and CaCl2- rich brines), which were the dominant fluids trapped in Qz3 and Qz4, can be considered as representative of the main fluid types that circulated during the formation of uranium ores.
The chloride content of the NaCl-rich brine reaches 6.5 mol Cl–/kg H2O, and its Cl/Br ratio suggests that it was expelled from evaporitic layers and can be considered as a primary evaporitic brine. The NaCl-rich brine dominates over a large range of depths and laterally over distances of several hundreds of kilometers, suggesting a huge saline fluid reservoir at the scale of the basal sandstone formation within the Athabasca basin. The Ca enrichment resulted from fluid-rock interactions during the infiltration of the NaCl-rich brine into the basement lithological units.
The minimum P-T conditions during the maximum burial of the sandstones were 1,200 to 1,400 bar (5 km) and 190° to 235°C. In response to tectonic movement and breccia formation, the pressure decreased and enabled the CaCl2-rich brine (approx 140°C) to enter the fluid system and to mix with the resident warmer basinal NaCl-rich brine (min 190°C) in the breccia bodies. This critical change in the fluid regime is probably linked to major changes in the tectonic regime during a world-wide geodynamic event. The fragmentation of the supercontinent Columbia, associated with the development of anorogenic magmatism and continental rifting, which began at 1.6 Ga and continued until its final breakup at about 1.2 Ga, represents a major geodynamic event that may have been responsible for a critical change in fluid regime in the Athabasca basin, linked to the formation of uranium mineralization in the McArthur River area.
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