0361-0128/05/3556/1529-17 $6.00 1529 Introduction UNCONFORMITY-TYPE uranium deposits are the most prof- itable source of uranium because of their exceptionally high grade and large tonnage. At least 10 unconformity-type ura- nium deposits have been discovered in the Athabasca basin, northern Saskatchewan, Canada (Fig. 1), since the Rabbit Lake deposit was discovered in 1968 (Heine, 1986). In un- conformity-type uranium deposits, the ore is located close to the intersection of the unconformable contact between Archean to lower Proterozoic metamorphic rocks and a mid- dle Proterozoic sandstone cover with reverse faults that are rooted in the basement in graphitic metasedimentary rocks. Several processes, including supergene (Knipping, 1974) and hydrothermal processes (Little, 1974), have been proposed to account for the formation of the high-grade unconformity- type deposits. However, two genetic models have prevailed. The first model suggests that uranium deposition resulted from mixing between oxidized basinal brines and basement- derived reduced fluids (Pagel, 1975a; Pagel and Jaffrezic, 1977; Hoeve and Sibbald, 1978; Pagel et al., 1980; Hoeve and Quirt, 1987; Wilson and Kyser, 1987; Kotzer and Kyser, 1995; Fayek and Kyser, 1997). The second model proposes that ura- nium deposition was caused by the interaction of basinal brines with the reduced basement lithologies (Hoeve and Quirt, 1984; Komninou and Sverjensky, 1996; Fayek and Kyser, 1997). Although brines have been documented in Mixing of Sodic and Calcic Brines and Uranium Deposition at McArthur River, Saskatchewan, Canada: A Raman and Laser-Induced Breakdown Spectroscopic Study of Fluid Inclusions DONATIENNE DEROME, MICHEL CATHELINEAU, † MICHEL CUNEY, CÉCILE FABRE, THÉRÈSE LHOMME, CREGU and UMR CNRS-7566 G2R, BP239-54506 Vandoeuvre les Nancy cedex, France AND DAVID A. BANKS School of Earth Science, University of Leeds, Leeds LS2 9JT, United Kingdom Abstract The richest U deposit in Saskatchewan, Canada, occurs in the McArthur River area, in the vicinity of the un- conformity between the Athabasaca sandstones and an Archean to lower Proterozoic basement. Paleofluids re- lated to the silicification of the sandstones and the formation of pre- and postore cements in breccias were stud- ied 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 ex- pelled 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 pressure-temperature conditions of formation of the quartz cements are estimated to be 1,200 to1,400 bars and 190° to 235°C for the silicification events during the preore stage, and 500 to 900 bars after a pressure decrease from lithostatic conditions and slightly lower temperatures due to the mixing of the NaCl-rich brine with the cooler (approx 140°C) CaCl2-rich brine during the main stage of brec- cia sealing. Temperature and pressure drops combined with the effects of brine mixing appear to be key fac- tors for the main stages of quartz cementation and U deposition at the McArthur deposit. † Corresponding author: e-mail: michel.cathelineau@g2r.uhp-nancy.fr ©2005 Society of Economic Geologists, Inc. Economic Geology, v. 100, pp. 1529–1545 FIG. 1. Location of the main unconformity-type uranium deposits in the Athabasca basin (modified from Ruzicka, 1986).