Reduction kinetics of aqueous U(VI) in acidic chloride brines to uraninite by methane, hydrogen or C-graphite under hydrothermal conditions: Implications for the genesis of unconformity-related uranium ore deposits

Tochilinite/cronstedtite intergrowths are commonly observed as alteration products in CM chondrite matrices, but the conditions under which they formed are still largely underconstrained due to their scarcity in terrestrial environments. Here, we report low temperature (80 °C) anoxic hydrothermal experiments using starting assemblages similar to the constituents of the matrices of the most pristine CM chondrite and S‐rich and S‐free fluids. Cronstedtite crystals formed only in S‐free experiments under circumneutral conditions with the highest Fe/Si ratios. Fe‐rich tochilinite with chemical and structural characteristics similar to chondritic tochilinite was observed in S‐bearing experiments. We observed a positive correlation between the Mg content in the hydroxide layer of synthetic tochilinite and temperature, suggesting that the composition of tochilinite is a proxy for the alteration temperature in CM chondrites. Using this relation, we estimate the mean precipitation temperatures of tochilinite to be 120–160 °C for CM chondrites. Given the different temperature ranges of tochilinite and cronstedtite in our experiments, we propose that Fe‐rich tochilinite crystals resulted from the alteration of metal beads under S‐bearing alkaline conditions at T = 120–160 °C followed by cronstedtite crystals formed by the reaction of matrix amorphous silicates, metal beads, and water at a low temperature (50–120 °C).

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“…Oxygen gas was removed from the vessels according to the reaction (Dargent et al. ):

{normalO}_{2} false(normalg false) + {normalN}_{2} {normalH}_{2} false(aq false) = 2 {normalH}_{2} O false(aq false) + {normalN}_{2} false(normalg false) .

…”

Section: Methodsmentioning

confidence: 99%

“…To remove oxygen trapped in the porosity of the Teflon Ò bowls, the bowls were pretreated with a hydrazine solution (20 ppm N 2 H 2 ) at 200°C for 1 day. Oxygen gas was removed from the vessels according to the reaction (Dargent et al 2015):…”

Section: Hydrothermal Experimentsmentioning

confidence: 99%

Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments

Vacher

Truche

Fauré

et al. 2019

Meteorit & Planetary Scien

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“…Presumably, the Mackenzie event provided both the material, in the form of Ni-As rich magmas, and potentially the energy needed for hydrothermal activity to transport the metals to the Athabasca Basin, where they are redeposited in the form of arsenides, sulfides and sulfarsenides in the uranium deposits. In this context Ni-Co-As mineral precipitation seems to be dependent on structural controls and reducing environments, likely resulting from hydrogen produced by the radiolysis of water, similar to those that drove earlier U reduction and deposition (Dargent et al, 2015;Truche et al, 2018) because there are no occurrences of Ni-Co-As independent from U deposits in the Athabasca Basin. The Mackenzie event thus deeply modified the initial characteristics of the unconformity-related U deposits, being likely responsible for remobilisation of the primary stage 1 uranium oxides in the deposit, with partial to total resetting of the U-Pb uraninite age and formation of stage 2 uranium oxides (as well as the U oxides found within the WC530 rammelsbergite vein), and for massive deposition of nickel, cobalt, sulfur and arsenic in the form of arsenides, sulphides and sulfarsenides.…”

Section: Contribution Of Rammelsbergitementioning

confidence: 99%

Evaluation of rammelsbergite (NiAs2) as a novel mineral for 187Re-187Os dating and implications for unconformity-related U deposits

Chernonozhkin

Reisberg

Luais

et al. 2020

Geochimica et Cosmochimica Acta

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Rammelsbergite (NiAs2) is often present in ore deposits worldwide in association with other arsenides, sulphides and sulfarsenides. This work demonstrates the value of the application of the Re-Os isotopic system to the dating of rammelsbergite. Using the example of the Cigar Lake uranium deposit (Athabasca Basin, Saskatchewan, Canada), our results show that rammelsbergite concentrates significant and variable amounts of Re when crystallizing, up to 1.6 µg g-1 , while incorporating minimal common Os. Such characteristics make this mineral an ideal target for Re-Os isotopic dating, by both isochron and model age methods. Remarkable agreement of the Re-Os age of the rammelsbergite with the U-Pb age of the uranium oxide grains found inside one of the massive rammelsbergite veins confirms the applicability of the method. The selected rammelsbergite samples from the Cigar Lake uranium deposit yield a 187 Re-187 Os isochron age of 1239 +33/-20 Ma. These results challenge a longstanding petrogenetic model which postulates contemporaneous U oxide and Ni-Co-arsenide deposition in unconformity-related uranium deposits. The new Re-Os age of rammelsbergite veins indicates that arsenide crystallization postdated the deposition of the primary uranium oxides at 1461-1341 Ma, implying that uranium oxides and nickel arsenides, as well as other sulphides and sulfarsenides, were not precipitated contemporaneously during a single hydrothermal event as previously proposed. The 187 Re-Os data suggest that crystallization of rammelsbergite and co-genetic sulphides, arsenides and sulfarsenides was linked to the ca. 1270 Ma Mackenzie magmatic event, marked by the emplacement of mafic dikes throughout the Canadian Shield, including in the area of the Athabasca Basin. This study therefore suggests a new hypothesis for the origin of nickel, cobalt, sulfur and arsenic in unconformity-related U deposits of the Athabasca Basin. More broadly, the successful application of the 187 Re-187 Os rammelsbergite chronometer in this geological context suggests that its application in other ore deposit environments worldwide is likely to provide similar insights. In this respect, the 187 Re-187 Os chronometer applied to rammelsbergite, and potentially to other arsenides and sulfarsenides, represents a valuable complement to molybdenite dating because it can be used in systems that lack molybdenite.

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“…Such a model could explain why uranium was precipitated within the sandstones of the basin sequences, which generally lack phases capable of buffering oxidation state. Dargent et al [140] proposed that the reductant could have been hydrogen gas rather than methane. Pascal et al [66] demonstrated that the destruction of graphite and sulphides could generate enough methane (and H 2 S) to cause significant uranium deposition.…”

Section: Metal and Ligand Transport And Depositionmentioning

confidence: 99%

Unconformity-Type Uranium Systems: A Comparative Review and Predictive Modelling of Critical Genetic Factors

Bruce¹,

Kreuzer

Wilde³

et al. 2020

Minerals

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A review of descriptive and genetic models is presented for unconformity-type uranium deposits with particular attention given to spatial representations of key process components of the mineralising system and their mappable expressions. This information formed the basis for the construction of mineral potential models for the world’s premier unconformity-style uranium provinces, the Athabasca Basin in Saskatchewan, Canada (>650,000 t U3O8), and the NW McArthur Basin in the Northern Territory, Australia (>450,000 t U3O8). A novel set of ‘edge’ detection routines was used to identify high-contrast zones in gridded geophysical data in support of the mineral potential modelling. This approach to geophysical data processing and interpretation offers a virtually unbiased means of detecting potential basement structures under cover and at a range of scales. Fuzzy logic mineral potential mapping was demonstrated to be a useful tool for delineating areas that have high potential for hosting economic uranium concentrations, utilising all knowledge and incorporating all relevant spatial data available for the project area. The resulting models not only effectively ‘rediscover’ the known uranium mineralisation but also highlight several other areas containing all of the mappable components deemed critical for the accumulation of economic uranium deposits. The intelligence amplification approach to mineral potential modelling presented herein is an example of augmenting expert-driven conceptual targeting with the powerful logic and rationality of modern computing. The result is a targeting tool that captures the current status quo of geospatial and exploration information and conceptual knowledge pertaining to unconformity-type uranium systems. Importantly, the tool can be readily updated once new information or knowledge comes to hand. As with every targeting tool, these models should not be utilised in isolation, but as one of several inputs informing exploration decision-making. Nor should they be regarded as ‘treasure maps’, but rather as pointers towards areas of high potential that are worthy of further investigation.

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Reduction kinetics of aqueous U(VI) in acidic chloride brines to uraninite by methane, hydrogen or C-graphite under hydrothermal conditions: Implications for the genesis of unconformity-related uranium ore deposits

Cited by 34 publications

References 70 publications

Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments

Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments

Evaluation of rammelsbergite (NiAs2) as a novel mineral for 187Re-187Os dating and implications for unconformity-related U deposits

Unconformity-Type Uranium Systems: A Comparative Review and Predictive Modelling of Critical Genetic Factors

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