Geometallurgical and Geological Evaluation of the High-Grade Polymetallic Unconformity-Related Cigar Lake Uranium Deposit

Abstract

The high-grade, polymetallic, unconformity-related, Cigar Lake uranium deposit, located in northern Saskatchewan, Canada, is the focus of a detailed mineralogical, geochemical and geospatial study aimed at geometallurgical characterization and reconstruction of its underlying geological history. The main mineralization event occurred before 1468±93 Ma and was likely syngenetic with the basin wide (ca. 1590 Ma) U mineralization event. Uraninite co-precipitated with common Pb-bearing arsenides, sulpharsenides and sulphides. Molybdenum was mobile during primary mineralization, crystallizing within Stage 1 uraninite. Relatively high and consistent δ34S values up to 14.6‰ indicate a significant basinal and marine SO42- contribution and an open-system with a well-mixed source. Low 207Pb/206Pb values (0.56–0.86) is imprinted on co-precipitated chalcopyrite, spatially and texturally associated with Stage 1 uraninite, suggesting that the U and Cu were sourced from Archean minerals. Syn-ore calcite equilibrated with a fluid having high δ18O values between +1.8 to +7.2‰, typical of diagenetic basinal brine in the Athabasca Basin, and low δ13C values ranging from -22.4 to -21.8‰, consistent with a source from underlying graphite. Stage 2 fluid incursion, constrained by U/Pb dates at 1270-1163 Ma resulted in crystallization, recrystallization and Pb absorption forming radiogenic (206Pb/204Pb ~1000) and selenite-bearing sulphides and sulpharsenides concurrent with enhanced clay alteration. Stage 3 alteration, with U/Pb and Pb/Pb dates between 947-755 Ma was responsible for extensive hematization and crystallization of anomalously radiogenic (206Pb/204Pb ~3000–20,000) bornite, chalcocite and galena. The most recent deposit alteration event, stage 4 (242-0 Ma), manifests as extensive coffinitization and re-mobilization of U as perched mineralization. The paragenetic stages coincide with major, far-field tectonic and regional geological events: (1) Stage 2 with the Grenville Orogeny, the Mackenzie dyke swarm and the Moore Lake olivine diabase, (2) Stage 3 with the breakup of the supercontinent Rodinia, and (3) Stage 4 with recent meteoric and glacial meltwaters. The paragenesis, geochronology and stable isotopes reveal a protracted history, with ongoing episodic fluid incursion, driven by far-field tectonics that resulted in alteration and re-mobilization of selected elements. Using the geometallurgical paradigm presented here, the geology at Cigar Lake can be used to optimize and reduce risk during long-term mine and mill planning.