Article Dans Une Revue Geochimica et Cosmochimica Acta Année : 2013

From evaporated seawater to uranium-mineralizing brines: Isotopic and trace element study of quartz-dolomite veins in the Athabasca system

Résumé

Stable isotope (O, H, C), radiogenic isotope (Sr, Nd) and trace element analyses have been applied to quartz–dolomite veins and their uranium(U)-bearing fluid inclusions associated with Proterozoic unconformity-related UO2 (uraninite) ores in the Athabasca Basin (Canada) in order to trace the evolution of pristine evaporated seawater towards U-mineralizing brines during their migration through sediments and basement rocks. Fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry (excepted for relatively small amounts of CO2 found in dolomite-hosted fluid inclusions). However, δ18O values of quartz veins (δ18O = 11‰ to 18‰) and dolomite veins (δ18O = 13‰ to 24‰) clearly indicate isotopic disequilibrium between quartz and dolomite. Hence, it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage (∼180 °C) and the dolomite stage (∼120 °C). The δ13C values of CO2 dissolved in dolomite-hosted fluid inclusions (δ13C = −30‰ to −4‰) and the δ13C values of dolomite (δ13C = −23.5‰ to −3.5‰) indicate that the CO2 dissolved in the mineralizing brines originated from brine–graphite interactions in the basement. The resulting slight increase in the fluid partial pressure of CO2 (pCO2) may have triggered dolomite precipitation instead of quartz. δ18O values of quartz veins and previously published δ18O values of the main alteration minerals around the U-ores (illite, chlorite and tourmaline) show that quartz and alteration minerals were isotopically equilibrated with the same fluid at ∼180 °C. The REE concentrations in dolomite produce PAAS-normalized patterns that show some similarities with that of UO2 and are clearly distinct from that of the other main REE-bearing minerals in these environments (monazite, zircon and aluminum phosphate–sulfate (APS) minerals). The radiogenic isotope compositions of dolomite (87Sr/86Sri = 0.7053 to 0.7161 and εNd(t) = −8.8 to −20.3) differ from one deposit to another, reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines. The previously published 87Sr/86Sri and εNd(t) values of UO2 compare with the most evolved dolomites, i.e. dolomites precipitated from brines that exchanged the most with the basement. This reinforces a close genetic link between dolomites and UO2 deposition and implies that UO2 deposition occurred in a cooling system during the transition from quartz to dolomite formation. The δ18O and δD values of the mineralizing brines (δ18O = −1‰ to 8‰ and δD = −150‰ to −50‰) are considerably shifted from that of their theoretical original values acquired during evaporation of seawater (δ18O = ∼−3‰ and δD = ∼−40‰). The positive δ18O shift is explained by protracted fluid–rock interaction within the basin and basement rocks. The negative δD shift is attributed to incomplete mixing between the U-mineralizing brines and low δD water. This low δD water was likely produced during the abiogenic synthesis of bitumen by Fisher–Tropsch-like reactions involving CO2 derived from brine–graphite interaction in the basement, and radiolytic H2. The resulting low δD brines have been equilibrated with alteration minerals. This may explain why some alteration minerals yield anomalously low δD values whose significance has long been debated.

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insu-00835882 , version 1 (13-02-2025)

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Antonin Richard, Philippe Boulvais, Julien Mercadier, Marie-Christine Boiron, Michel Cathelineau, et al.. From evaporated seawater to uranium-mineralizing brines: Isotopic and trace element study of quartz-dolomite veins in the Athabasca system. Geochimica et Cosmochimica Acta, 2013, 113, pp.38-59. ⟨10.1016/j.gca.2013.03.009⟩. ⟨insu-00835882⟩
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