C-O-H-N fluids circulations and graphite precipitation in reactivated Hudsonian shear zones during basement uplift of the Wollaston-Mudjatik Transition Zone: Example of the Cigar Lake U deposit

• Published in: Lithos 2017-12, Vol.294-295, p.222-245
• Main Authors: Martz, Pierre, Cathelineau, Michel, Mercadier, Julien, Boiron, Marie-Christine, Jaguin, Justine, Tarantola, Alexandre, Demacon, Mickael, Gerbeaud, Olivier, Quirt, David, Doney, Amber, Ledru, Patrick
• Format: Article
• Language: English
• Subjects: C-O-H-N fluids; Carbon isotopes; Earth Sciences; Graphite; Paleoproterozoic basement; Petrography; Sciences of the Universe; Shear zones
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2017.10.001

Graphitic shear zones are spatially associated with unconformity-related uranium deposits that are located around the unconformity between the strata of the Paleo- to Mesoproterozoic Athabasca Basin (Saskatchewan, Canada) and its underlying Archean to Paleoproterozoic basement. The present study focuses on basement-hosted ductile-brittle graphitic shear zones near the Cigar Lake U deposit, one of the largest unconformity-related U deposits. The goal of the study is to decipher the pre-Athabasca Basin fluid migration history recorded within such structures and its potential role on the formation of such exceptional deposit. Dominantly C-O-H(-N) metamorphic fluids have been trapped in Fluid Inclusion Planes (FIPs) in magmatic quartz within ductile-brittle graphitic shear zones active during retrograde metamorphism associated with the formation of the Wollaston-Mudjatik Transition Zone (WMTZ) between ca. 1805 and 1720Ma. Such fluids show a compositional evolution along the retrograde path, from a dense and pure CO2 fluid during the earliest stages, through a lower density CO2±CH4-N2 (±H2O) fluid and, finally, to a very low density CH4-N2 fluid. Statistical study of the orientation, distribution, proportion, and chemical characterization of the FIPs shows that: i) CO2 (δ13CCO2 around −9‰ PDB) from decarbonation reactions and/or partial water-metamorphic graphite equilibrium initially migrated regionally and pervasively under lithostatic conditions at about 500 to 800°C and 150 to 300MPa. Such P-T conditions attest to a high geothermal gradient of around 60 to 90°C/km, probably related to rapid exhumation of the basement or a large-scale heat source. ii) Later brittle reactivation of the shear zone at around 450°C and 25–50MPa favored circulation of CO2-CH4-N2(±H2O) fluids in equilibrium with metamorphic graphite (δ13CCO2 around −14‰) under hydrostatic conditions and only within the shear zones. Cooling of these fluids and the water uptake linked to fluid-basement rock reactions led to the precipitation at around 450°C of poorly-crystallized hydrothermal graphite. This graphite presents isotopic (δ13C−30 to −26‰ PDB) and morphological differences from the high-T metamorphic graphite (>600°C, −29 to −20‰ δ13C) derived from metamorphism of C-rich sedimentary material. The brittle structural reactivation and the related fluid migration and graphite precipitation were specifically focused within the shear zones and related damage zones. The brittle reactivation pro