# Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool

Valérie Migeon, Caroline Fitoussi, Eric Pili, Bernard Bourdon

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Valérie Migeon, Caroline Fitoussi, Eric Pili, Bernard Bourdon. Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool. Geochimica et Cosmochimica Acta, 2020, 279, pp.238-257. 10.1016/j.gca.2020.03.046. hal-03084670

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# 1 Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool

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4 Valérie Migeon¹,² Caroline Fitoussi¹, Eric Pili² and Bernard Bourdon¹

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6 ¹Laboratoire de Géologie de Lyon (LGL-TPE), ENS Lyon, CNRS and Université Claude Bernard de Lyon, 7 46 allée d'Italie, 69364 Lyon Cedex 7, France.

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9 ²CEA, DAM, DIF, F-91297 Arpajon, France.

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## 11 Abstract

The ability to identify the sources of nuclear materials using various analytical methods has become a major target of nuclear forensics science. However, despite the emergence of novel tools developed in recent years, it has become obvious that accurate identification of sources requires multiple tools. Here, we have developed a new isotope tool based on Mo isotopes that could be used in nuclear forensics. Molybdenum is a trace element that is found in significant levels in uranium ores (0.5 to 4800 ppm), uranium minerals (0.02 to 6000 ppm) and uranium ore concentrates (UOCs, from 0.7 to 1400 ppm). The Mo isotope composition, reported as δ⁹⁸Mo was analyzed in these materials and shown to have a significant variability from -2.62 to +0.30‰ in uranium ores and from -3.50 to +0.46‰ in UOCs. In uranium ores, the largest Mo isotope fractionation is found in deposits of sedimentary origin while the few ores with a magmatic origin show limited Mo isotope fractionation. Thus, the Mo isotope ratios may ultimately be used to distinguish high temperature from low temperature uranium ores. The δ⁹⁸Mo values in UOCs also show a significant range that not only originate from variations in ore compositions but can also be attributed to ore processing such as leaching, solvent extraction, resin extraction and UOC precipitation. Laboratory experiments based on existing protocols of uranium ore separation demonstrate the existence of sizeable Mo isotope fractionation associated with these processes. Experiments with solvent extraction and precipitation of ammonium diuranate and peruranate show that the refined uranium fraction is enriched in light Mo isotopes in both cases. In the case of a uranium ore (SOMAÏR, Niger) and associated UOC, the observed fractionation can possibly be assigned to the combined effect of solvent extraction and precipitation, although Mo isotope variability in ores and UOCs may make such a diagnostic difficult. Ultimately, these experiments can be used to reconstruct the Mo isotope
composition of the ore, if there is sufficient knowledge about the ore processing flowsheet
or to provide information about the type of uranium ore. Alternatively, it could be used to
identify the process used for manufacturing a given UOC, if one can make an estimate about
the composition of the ore, probably in conjunction with other tools. Our study
demonstrates the potential of Mo isotopes as a useful tool in the field of nuclear forensics.

# 1. Introduction

Nuclear forensics aims at determining the age, provenance as well as industrial or storage history of uranium ores, uranium ore concentrates (UOCs) as well as other downstream compounds that are part of the nuclear fuel cycle. Similarly, post-detonation nuclear forensics aims at providing information from matter sampled after a nuclear explosion. The main goal is thus to attribute the material to a specific source and to identify its history. Several potential tracers and methodologies have already been identified for this purpose: uranium isotope signatures (Brennecka et al., 2010; Keegan et al., 2008; Uvarova et al., 2014; Wallenius et al., 2006), radiometric dating methods (Eppich et al., 2013; Morgenstern et al., 2002; Pointurier et al., 2013; Varga et al., 2015), oxygen isotopes (Fayek et al., 2011a, b), lead and strontium isotopes (Keegan et al., 2008; Švedkauskaitė-LeGore et al., 2007), sulphur isotopes (Han et al., 2013), anions (Keegan et al., 2012), as well as rare earth elements (Fryer and Taylor, 1987; Mercadier et al., 2011; Varga et al., 2010). Yet, many caveats in nuclear forensics tools have been identified. For example, Pb isotopes may be contaminated with common Pb and it is often difficult to extract a radiogenic lead signature typical of U ores. Similarly, Sr isotopes may be different for some deposits but rather constant for others. An important conclusion of these studies (Moody et al., 2014, Wallenius et al., 2006; Kristo et al. 2017; Varga et al., 2017) is that no single isotope tracer can provide unambiguous provenance and/or processing history information. In summary, a combination of these tracers is often required, and the need for new isotope tracers has also been strongly suggested (Kristo et al. 2017).

Molybdenum and uranium share a number of common geochemical properties. Over the past few years, their variations have been extensively used in marine, estuarine or
continental margin sediments as paleo redox proxies (Algeo and Tribovillard, 2009; Asael et al., 2013; Audry et al., 2007; Azrieli-Tal et al., 2014; McManus et al., 2006; Rahaman et al., 2010; Zhou et al., 2012). Indeed, Mo and U are redox sensitive elements: under oxidizing conditions, both are present as soluble U(VI) and Mo(VI) species in aqueous solutions, while they form insoluble U(IV) or Mo(VI)S₄²⁻ that adsorbs to solids under reducing conditions before being reduced to Mo(IV) (Kendall et al. 2017). Uranium is generally reduced by agents such as organic matter, sulphides (e.g. pyrite), hydrocarbons, or Fe-Mg bearing-minerals, etc. Molybdenum can form a variety of minerals hosted in uranium ores: molybdenite (crystallized MoS₂) and jordisite (amorphous MoS₂) are often associated with uraninite or pitchblende. Mo is also present in small quantities in pitchblende itself and in various uranium minerals (Augustithis, 1995). In roll-front deposits, molybdenum was inferred to precipitate as MoS₂, or hosted in pyrite (Thompson et al., 1978; Bullock and Parnell 2017) and Mo is often enriched in uranium ores relative to typical crustal levels (e.g. Keegan et al. 2008).

In the nuclear fuel cycle, molybdenum is an impurity that is difficult to separate during uranium extraction and purification processes, while its concentration is required to be lower than some specification limits (IAEA, 1993) because (i) volatile Mo fluorides would form during conversion to UF₆ and hamper proper ²³⁵U enrichment (Stone 2006) and (ii) Mo would absorb neutrons and decrease the yield of nuclear fission.

In uranium ores, U concentrations vary from a few hundred µg/g to 20 wt. %, while Mo concentrations vary from a few ng/g to more than 0.5 wt. % (Ceyhan, 2009; Gupta, 1992). Very few Mo isotope compositions have been reported so far for uranium ores in the literature, except for one determination by Migeon et al. (2018) and one by Rolison et al. (2019). In uranium ore concentrates, Mo concentrations must be less than the specification limit of 0.15 wt% (IAEA, 1993), corresponding to a Mo/U ratio < 2×10⁻³ (assuming a minimum U content of 65% wt. A large Mo isotope data base was obtained for UOCs by Rolison et al. (2019) exhibiting a large range of δ⁹⁸Mo values, with significant fractionation relative to the bulk silicate Earth (mean value -0.16 ‰, Willbold and Elliott 2017). This range was interpreted as reflecting variations in U ores combined with the effect of ore processing and possibly by Mo contamination by extraneous sources. A clear conclusion of that study was that Mo data on uranium ores is necessary while constraining the effect of U refining on Mo isotopes would also be important to better interpret the data.
Production of U ore concentrates from ores requires successive processes of extraction, purification and precipitation (IAEA, 1993). Extraction is first done by leaching, most commonly with sulfuric acid or with alkaline sodium carbonate solutions. Depending on the nature of the rocks, oxidizing reagents may be added to the leaching solution in order to increase U yields (IAEA, 1993). In a previous study, Mo concentrations and isotope compositions were reported for various leaching conditions (Migeon et al., 2018). It was shown that, when the leaching step does not extract Mo quantitatively, the leaching solution is enriched in heavy isotopes compared to the residual (leached) U ore.

Further uranium purification is achieved by solvent extraction and/or ion-exchange chromatography, followed by uranate precipitation. Anionic resins with a quaternary or tertiary amine function (Czegledi 1986; IAEA 1993) are used to concentrate and purify the uranyl sulphate-bearing species such as UO₂(SO₄)₂²⁻ or UO₂(SO₄)₃⁴⁻. As Mo also forms anionic species in solution, it is also adsorbed on the resins but is only partially eluted with the uranium fraction. During solvent extraction, both uranium and molybdenum are extracted in the solvent phase. NaCl solutions are then used to selectively remove uranium from the solvent, and only a fraction of the total Mo is expected to be back-extracted at this stage. The last step of uranium purification is the precipitation of UOC in the form of NH₄-, Mg- or Na-diuranates (-U₂O₇) using ammonia, magnesia or soda respectively, followed or not by calcination in order to form U₃O₈. Oxygen peroxide may also be used to precipitate a peruranate salt also known as uranium- or uranyl- peroxide or studtite: UO₄·nH₂O.

This study focuses on establishing Mo isotopes as a new tracer of uranium ore provenance and of ore processing for its application in the fight against illicit trafficking of nuclear materials and in nuclear forensics. For this purpose, we have determined the Mo isotope compositions in uranium ores, uranium minerals and uranium ore concentrates. In addition, we also determined experimentally the Mo isotope fractionation associated with the key purification processes of uranium in the nuclear fuel cycle: solvent extraction and precipitation. The effect of ion exchange on Mo isotopes has been documented in several previous studies (Anbar et al., 2001; Malinovsky and Vanhaecke, 2014; Migeon et al., 2015; Siebert et al., 2001; Wen et al. 2010; Tachibana et al. 2017, see Electronic Appendix for details) and was therefore not a focus in our study that complements another study focusing on Mo isotope fractionation associated with U ore leaching (Migeon et al., 2018).
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## 2. Samples

Various uranium minerals (Table 1), uranium ores (Table 2) and uranium ore concentrates (Table 3) were analysed in this study. Uranium minerals and ores originate from different types of ore deposits from various provenances as indicated in Table 2. The studied uranium minerals and ores represent various types of uranium deposits hosted either in granitic rocks, calcrete, sandstone, or phosphorite. Uranium ore concentrates are ammonium-, sodium- and magnesium-diuranates, triuranium octoxides and peroxides (Table 3). Details about the samples are given in the Electronic Annex. The ore samples that feed the COMINAK mill, Niger were taken in situ in the galleries of underground mines while the ores that feed the SOMAÏR mill, Niger were sampled after blending of large batches extracted from open-pit mines. Note that there is little information about the U ore reference materials obtained from CETAMA (Table 2), particularly their geographical origin.
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<table><caption>Table 1: Characteristics, Mo concentrations and isotope ratios of uranium minerals</caption><thead><tr><th>Sample ID</th><th>Mineralogy</th><th>Formula*</th><th>Mining district and locality</th><th>Nature</th><th>Mo (µg/g)</th><th>δ<sup>98</sup>Mo ± 2SE (n) (%)</th><th>Mo<sup>147</sup>Mo<sup>148</sup> (%)</th></tr></thead><tbody><tr><td>JU-1</td><td>Autunite</td><td>Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2.10H<sub>2</sub>O</td><td>Le Brugeaud, Bessines, France</td><td>Magmatic + low temperature alteration</td><td>0.5**</td><td>-0.27 ± 0.04 (2)</td><td>1.0x10<sup>-6</sup></td></tr><tr><td>JU-10</td><td>Pitchblende</td><td>UO<sub>2</sub></td><td>Unknown</td><td>Unknown</td><td>12**</td><td>-0.58 ± 0.12 (2)</td><td>1.4x10<sup>-5</sup></td></tr><tr><td>JU-11</td><td>Pitchblende</td><td>UO<sub>2</sub></td><td>High Atlas of Marrakech, Morocco</td><td>Unknown</td><td>447**</td><td>+0.57 ± 0.02 (2)</td><td>5.1x10<sup>-5</sup></td></tr><tr><td>JU-11g</td><td>Gangue</td><td></td><td>High Atlas of Marrakech, Morocco</td><td>Unknown</td><td>236.3</td><td>-1.97 ± 0.02 (2)</td><td>153</td></tr><tr><td>JU 12</td><td>Autunite</td><td>Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2.10H<sub>2</sub>O</td><td>Les Oudots, Morvan, France</td><td>Magmatic + low temperature alteration</td><td>0.1</td><td>—</td><td>154</td></tr><tr><td>JU 13</td><td>Autunite</td><td>Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2.10H<sub>2</sub>O</td><td>Compreignac, Haute Vienne, France</td><td>Magmatic + low temperature alteration</td><td>0.02</td><td>—</td><td>156</td></tr><tr><td>JU 15</td><td>Uranotile</td><td>CaO.2UO<sub>3</sub>.2SiO<sub>2</sub>.7H<sub>2</sub>O</td><td>Margnac, Haute Vienne, France</td><td>Magmatic + low temperature alteration</td><td>553</td><td>—</td><td>158</td></tr><tr><td>JU 16</td><td>Lodevite</td><td>Zn(UO<sub>2</sub>)<sub>2</sub>(AsO<sub>4</sub>)<sub>2.10(H<sub>2</sub>O)</td><td>Rivièral, Lodève, France</td><td>Sedimentary</td><td>0.1</td><td>—</td><td>159</td></tr><tr><td>JU 17</td><td>Pitchblende</td><td>UO<sub>2</sub></td><td>Unknown</td><td>Unknown</td><td>6086**</td><td>-0.51 ± 0.02 (2)</td><td>6.9x10<sup>-6</sup></td></tr><tr><td>JU 18</td><td>Pitchblende-Uraninite</td><td>UO<sub>2</sub></td><td>Morogoro, Tanzania</td><td>Metamorphic</td><td>1.1</td><td>—</td><td>162</td></tr><tr><td>JU19</td><td>Schoepite</td><td>UO<sub>3</sub>.2H<sub>2</sub>O</td><td>Margnac, Haute Vienne, France</td><td>Magmatic</td><td>0.7</td><td>—</td><td>163</td></tr><tr><td>MP01</td><td>Pitchblende</td><td>UO<sub>2</sub></td><td>Shinkolobwe, Katanga, DRC</td><td>Metamorphic + brines</td><td>424**</td><td>-0.34 ± 0.02 (4)</td><td>4.8x10<sup>-4</sup></td></tr></tbody></table>

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*Simplified theoretical formulas. **Mo concentrations measured by isotope dilution on the Neptune Plus MC-ICP-MS. Others concentrations were measured on the Element II ICP-MS. Uncertainty is less than 5 %. aMo/U determined from U concentrations calculated based on stoichiometry (simplified theoretical formulas) of uranium minerals.
<table><caption>Table 2: Nature, origin, U and Mo contents as well as Mo isotope ratios for uranium ores</caption><thead><tr><th>Sample ID</th><th>Nature</th><th>U minerals</th><th>Mining district & locality or provider</th><th>Mo (µg/g)</th><th>U (%)<sup>+</sup></th><th>Mo/U mass ratio</th><th>δ<sup>98</sup>Mo (%)<sup>+</sup><br>± 2SE (n)<sup>+</sup></th><th>181<br>182<br>183</th></tr></thead><tbody><tr><td>MIN D</td><td>Granite-<br>granodiorite<br>mylonite</td><td>UO<sub>2</sub> (pitchblende)</td><td>CETAMA*</td><td>19.8</td><td>0.065</td><td>0.0303</td><td>+0.30 ± 0.02<sup>+</sup></td><td>184</td></tr><tr><td>Namibia_1</td><td>Calcrete</td><td>Carnotite</td><td>Trekkopje, Namibia</td><td>0.5</td><td>0.004</td><td>0.0136</td><td>-0.21 ± 0.02<sup>+</sup></td><td>185<br>186</td></tr><tr><td>Niger_COM_1</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides</td><td>COMINAK, Niger</td><td>1.1</td><td>0.066</td><td>0.0020</td><td>-0.97 ± 0.02<sup>+</sup></td><td>187</td></tr><tr><td>Niger_COM_TP013</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides, Fe<br>oxides</td><td>COMINAK, Ebba North, Niger</td><td>11.9</td><td>24.8</td><td>4.8×10<sup>-5</sup></td><td>-0.47 ± 0.03<sup>+</sup></td><td>188</td></tr><tr><td>Niger_COM_TP016</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides</td><td>COMINAK, Ebba North, Niger</td><td>223</td><td>6.7</td><td>0.0033</td><td>-2.62 ± 0.02<sup>+</sup></td><td>190<br>191</td></tr><tr><td>Niger_COM_TP020</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides</td><td>COMINAK, Ebba North, Niger</td><td>4744.</td><td>0.8</td><td>0.6233</td><td>-2.62 ± 0.02<sup>+</sup></td><td>192</td></tr><tr><td>Niger_SOM_1</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides</td><td>SOMAÏR, Niger</td><td>732.</td><td>0.3</td><td>0.2327</td><td>-2.07 ± 0.02<sup>+</sup></td><td>193</td></tr><tr><td>Niger_SOM_TP038</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite, U-Ti oxides</td><td>SOMAÏR, Tamgak, Niger</td><td>1128</td><td>2.8</td><td>0.0441</td><td>-1.39 ± 0.02<sup>+</sup></td><td>194</td></tr><tr><td>MIN E</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite</td><td>CETAMA*</td><td>746</td><td>0.427</td><td>0.1747</td><td>-2.18 ± 0.02<sup>+</sup></td><td>195</td></tr><tr><td>Gabon_1</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite</td><td>Gabon**</td><td>1.94</td><td>0.063</td><td>0.0031</td><td>-0.15 ± 0.02<sup>+</sup></td><td>196</td></tr><tr><td>Mongolia_1</td><td>Sandstone</td><td>UO<sub>2</sub>, coffinite</td><td>COGEGOBI Mongolia</td><td>8.49</td><td>0.237</td><td>0.0036</td><td>-2.18 ± 0.06<sup>+</sup></td><td>196</td></tr><tr><td>RCA_1</td><td>Phosphorite</td><td>Apatite</td><td>Bakouma, Central African Rep.</td><td>0.47</td><td>0.047</td><td>0.0010</td><td>-0.09 ± 0.02<sup>+</sup></td><td>197</td></tr><tr><td>Jordan_1</td><td>Phosphorite</td><td>Apatite</td><td>Jordan**</td><td>16.7</td><td>0.009</td><td>0.1777</td><td>-1.55 ± 0.02<sup>+</sup></td><td>198</td></tr><tr><td>MIN B</td><td>Unknown</td><td>UO<sub>2</sub> (pitchblende), coffinite</td><td>CETAMA*</td><td>1.76</td><td>0.107</td><td>0.0017</td><td>-0.46 ± 0.06<sup>+</sup></td><td>199<br>n</td></tr></tbody></table>

geographical origin. **Unknown mining district. +U concentrations measured with Element II ICP-MS. Uncertainty is less than 5 %. $\Pi$

Number of analyses in parentheses.
<table><caption>Table 3: U and Mo concentrations, Mo isotope ratios of uranium ore concentrates</caption><thead><tr><th>Sample ID</th><th>Nature</th><th>Origin</th><th>U ore nature</th><th>Type of U source</th><th>Mo (µg/g)</th><th>U<sup>+</sup> (%)</th><th>Mo/U Mass ratio</th><th>δ<sup>98</sup>Mo ± 2SE (n)<sup>Π</sup> (%)</th></tr></thead><tbody><tr><td>Agaric</td><td>U<sub>3</sub>O<sub>8</sub></td><td>unknown<sup>**</sup></td><td>NA</td><td>NA</td><td>0.1</td><td>84.7</td><td>9.5×10<sup>-8</sup></td><td>-0.81 ± 0.02 (2)</td></tr><tr><td>Bolet<sup>*a</sup></td><td>U<sub>3</sub>O<sub>8</sub></td><td>unknown<sup>**</sup></td><td>NA</td><td>NA</td><td>4.2</td><td>84.7</td><td>5.0×10<sup>-6</sup></td><td>-0.33 ± 0.02 (8)</td></tr><tr><td>Chanterelle<sup>*a</sup></td><td>U<sub>3</sub>O<sub>8</sub></td><td>unknown<sup>**</sup></td><td>NA</td><td>NA</td><td>44.8</td><td>84.7</td><td>5.3×10<sup>-5</sup></td><td>+0.05 ± 0.02 (9)</td></tr><tr><td>Morille<sup>*</sup></td><td>U<sub>3</sub>O<sub>8</sub></td><td>unknown<sup>**</sup></td><td>NA</td><td>NA</td><td>149.2</td><td>84.7</td><td>1.8×10<sup>-4</sup></td><td>-0.20 ± 0.02 (16)</td></tr><tr><td>Emeraude<sup>*</sup></td><td>MgU<sub>2</sub>O<sub>7</sub></td><td>Lodève<sup>**</sup></td><td></td><td>Sedimentary</td><td>324.7</td><td>67.5</td><td>4.8×10<sup>-4</sup></td><td>-1.95 ± 0.02 (5)</td></tr><tr><td>Calcedoine<sup>*a</sup></td><td>MgU<sub>2</sub>O<sub>7</sub></td><td>COMINAK, Niger<sup>**</sup></td><td>Sandstone</td><td>Sedimentary</td><td>1372.0</td><td>69.5</td><td>2.0×10<sup>-3</sup></td><td>-1.82 ± 0.03 (4)</td></tr><tr><td>AKO 13088</td><td>MgU<sub>2</sub>O<sub>7</sub></td><td>COMINAK, Akouta, Niger</td><td>Sandstone</td><td>Sedimentary</td><td>1016.8</td><td>—</td><td>—</td><td>-1.83 ± 0.02 (5)</td></tr><tr><td>AKO 14086</td><td>MgU<sub>2</sub>O<sub>7</sub></td><td>COMINAK, Akouta, Niger</td><td>Sandstone</td><td>Sedimentary</td><td>732.5</td><td>63.6</td><td>1.2×10<sup>-3</sup></td><td>-2.44 ± 0.03 (5)</td></tr><tr><td>Grenat<sup>*a</sup></td><td>Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub></td><td>SOMAÏR, Niger<sup>**</sup></td><td>Sandstone</td><td>Sedimentary</td><td>455.2</td><td>67.3</td><td>6.8×10<sup>-4</sup></td><td>-2.84 ± 0.03 (9)</td></tr><tr><td>SOM 3079</td><td>Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub></td><td>SOMAÏR, Arlit, Niger</td><td>Sandstone</td><td>Sedimentary</td><td>768.3</td><td>—</td><td>—</td><td>-3.16 ± 0.03 (5)</td></tr><tr><td>SOM 3218</td><td>Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub></td><td>SOMAÏR, Arlit, Niger</td><td>Sandstone</td><td>Sedimentary</td><td>523.6</td><td>61.6</td><td>8.0×10<sup>-4</sup></td><td>-3.50 ± 0.04 (5)</td></tr><tr><td>ROS 7315</td><td>U<sub>3</sub>O<sub>8</sub></td><td>Rössing, Namibia</td><td>Granite</td><td>Magmatic</td><td>177.4</td><td>—</td><td>—</td><td>+0.46 ± 0.02 (5)</td></tr><tr><td>ROS 7525</td><td>U<sub>3</sub>O<sub>8</sub></td><td>Rössing, Namibia</td><td>Granite</td><td>Magmatic</td><td>217.8</td><td>69.2</td><td>3.0×10<sup>-4</sup></td><td>+0.40 ± 0.02 (5)</td></tr><tr><td>LAR 0604</td><td>UO<sub>4.2</sub>H<sub>2</sub>O</td><td>Langer Heinrich, Namibia</td><td>Calcrete</td><td>Granite weathering</td><td>0.7</td><td>—</td><td>—</td><td>-0.37 ± 0.06 (4)</td></tr><tr><td>NUF 512</td><td>U<sub>3</sub>O<sub>8</sub></td><td>Nufcor, South Africa</td><td>Gold ore</td><td>Magmatic</td><td>55.6</td><td>—</td><td>—</td><td>+0.36 ± 0.02 (6)</td></tr><tr><td>NUF 580</td><td>U<sub>3</sub>O<sub>8</sub></td><td>Nufcor, South Africa</td><td>Gold ore</td><td>Magmatic</td><td>82.3</td><td>69.4</td><td>1.0×10<sup>-4</sup></td><td>+0.29 ± 0.04 (4)</td></tr><tr><td>FB01</td><td>(NH<sub>4</sub>)<sub>2</sub>U<sub>2</sub>O<sub>7</sub></td><td>SIMO, Bessines, France</td><td>Granite</td><td>Magmatic</td><td>29.6</td><td>—</td><td>—</td><td>+0.20 ± 0.02 (7)</td></tr></tbody></table>

<footnote>*These reference materials are doped with Mo. **Samples distributed by CETAMA, most of them have unknown geographical origin. + U concentrations measured with Element II ICP-MS, except for CETAMA samples for which U analyses are those given in the certificate. Uncertainty is less than 5 %. Π Number of analyses in parentheses. All Mo concentrations were determined by isotope dilution. aThese samples were analyzed in Migeon et al. (2015).</footnote>
# 3. Experimental and analytical methods

## 3.1. Sample digestion

Uranium ores and separated uranium host rocks (gangues) were digested using a concentrated 1:1 $HNO_3$-HF mixture at ~140 °C in Savillex® beakers. After evaporation, samples were dissolved twice with hot concentrated HCl to remove remaining fluoride traces, and finally diluted in 8 M $HNO_3$.

First, uranium minerals and gangues were manually separated and crushed with an agate mortar. This step was carried out in plastic glove bags in order to avoid contamination of the operator and between samples. The autunite sample was digested in hot concentrated $HNO_3$, evaporated to dryness and then diluted in 8 M $HNO_3$. Small inclusions such as silicates were probably not dissolved during this process. The digestion solution was then centrifuged to isolate the uranium-bearing liquid phase. Centrifugation was repeated twice with deionized water to ensure a complete recovery of the dissolved uranium. All fractions were combined, evaporated and redissolved in 8 M $HNO_3$. In contrast, complete dissolutions of the gangue and minerals for the other samples were done with a mixture of concentrated 5:1 $HNO_3$-HF at ~140 °C in Savillex® beakers. This procedure is similar to that used for uranium ores. Uranium ore concentrates were solely dissolved with 8 M $HNO_3$ and no residual solids were observed.

Elemental concentrations were measured with a Thermo Element II ICP-MS at ENS Lyon using calibration with standard solutions with an analytical uncertainty of less than 5 %. More accurate molybdenum concentrations were measured by isotope dilution with a Neptune Plus MC-ICP-MS as detailed below.

## 3.2 Molybdenum separation and isotope measurements
Molybdenum purification from leachate solutions and sample residues was performed using a three-stage ion-exchange chromatography as described in Migeon et al. (2015). Prior to the Mo purification, a ⁹⁷Mo-¹⁰⁰Mo double-spike solution (DS) was added to the sample, such that 58.2 % of total Mo was from the DS and 41.8 % from the sample to minimize analytical uncertainties (Rudge et al., 2009). Molybdenum was separated from uranium, matrix and interfering elements on an anion exchange resin (2 ml of AG 1-X8, 100-200 mesh). Mo separation was improved by using an additional TRU-Spec resin column (1 ml, 100-150 µm). Samples were then loaded on a third column filled with AG 1-X8 anion exchange resin to eliminate phosphates released from the TRU-Spec resin. The purified Mo fraction was evaporated and diluted in 0.05 M HNO₃ for isotope composition analysis. Measurements were performed on a Thermo Neptune Plus MC-ICP-MS at ENS Lyon, equipped with Jet cones and with a CETAC Aridus II desolvating unit for sample introduction (Migeon et al. 2015). The sensitivity achieved was between 1200 to 1600 V/ppm (with an aspiration rate of ~150 µl/min) with a sample concentration of 30 ng/ml. Results are expressed using the delta notation δ⁹⁸Mo, in per mil, relative to the ⁹⁸Mo/⁹⁵Mo ratio of the NIST-SRM-3134 reference standard solution:

$$ \delta^{98}Mo = \left( \left( \frac{^{98}Mo}{^{95}Mo} \right)_{sample} / \left( \frac{^{98}Mo}{^{95}Mo} \right)_{NIST3134} - 1 \right) \times 1000 $$

During the same measurement period, other SRMs (standard reference materials, BHVO-2, BE-N, BR, AGV-1 and BX-N) were also measured, as reported in Migeon et al. (2015). To document further our analytical uncertainty, we provide Table S1 (Electronic Annex), giving the individual analyses of BHVO-2 and BE-N obtained over the course of this study. The results are comparable with published values (e.g. Willbold et al. 2016). The analytical uncertainties reported in the data tables correspond to standard errors (2 SE) of multiple measurements and when these measurements yielded uncertainties below the external reproducibility obtained with multiple digestions for BHVO-2 the uncertainty obtained with the latter (2 SD= ±0.02) is reported. One should note however, that the UOCs and U ores have very difference matrices compared with commonly used rocks used as SRMs. Thus, our study also includes data obtained for U-rich reference materials that are more akin to the materials analyzed in this study. These materials were obtained from CETAMA and can be used for comparison in future studies.
### 3.3 Solvent extraction of uranium

The effect of uranium solvent extraction on molybdenum concentrations and isotope ratios was investigated in the laboratory using synthetic Mo-U aqueous solutions. The source of Mo was the NIST-SRM 3134 solution, while the uranium solution was obtained from the Alpha Aesar Company. Mo and U were added with a 1:4 ratio, with 1 µg of Mo for 4 µg of U in a first series of tests and with 5 µg of Mo for 20 µg of U in a second series of tests to ensure there would be enough Mo for analysis (Table 4). As U and Mo yields were similar in both cases, Mo isotope ratios were determined only for the second series of tests (Table 4). The solvent extraction was realized with a starting solution containing a Mo/U ratio of 0.25, similar to those in the COMINAK or SOMAIR mills, Niger. The mixed Mo-U solutions were acidified and three acidic media were tested: 0.18 M H₂SO₄, 0.18 M H₂SO₄ + 0.15 M HNO₃ and 0.18 M H₂SO₄ + 1.5 M HNO₃ (Table 4). These conditions were chosen to match the leaching procedures developed by Migeon et al. (2018), which mimic industrial conditions.

The solvent mixture was prepared following the description given in Croisy et al. (2013). The solvent used for the extraction was prepared with a mixture of 0.2 M Tri-N-Octylamine (Alamine 336) and 5 vol. % of 1-Decanol diluted in kerosene, following Ajuria (1990) and IAEA (1993). The solvent was first conditioned by equilibrating 10 ml of solvent with 20 ml of 0.1 M H₂SO₄ in a bulb decanter with manual stirring over ~ 1 minute. For the extraction, the synthetic Mo-U acidified aqueous solutions were equilibrated with the solvent using 10 ml of preconditioned solvent and 10 ml of solution in a bulb decanter, with manual stirring over ~ 1 minute. Extraction yields were determined by measuring the concentrations of Mo and U that remained in the aqueous solutions after equilibration with the solvent followed by decantation. Results are reported in Table 4.

Back-extraction (stripping) of uranium was simulated with two types of solutions: 1.5 M NaCl and 1.0 M NaCl + 0.05 M H₂SO₄, following descriptions in IAEA (1993). Ten ml of these solutions were equilibrated with 10 ml of the loaded solvent in a bulb decanter, with manual stirring over ~ 1 minute. After decantation, the U and Mo yields were determined by measuring Mo and U concentrations in the back-extraction solutions. Before analysis, the back-extraction solutions were purified from their high sodium content using columns filled
with anion exchange resin (AG 1-X8). The U and Mo yields after back-extraction are given in Table 5.

A first preliminary series of measurements was conducted to determine the Mo concentrations in the various solutions to calculate the optimal spike-sample ratios. These tests gave crude estimates of Mo and U concentrations (after correction for a 15% Mo loss during column separation). During a second series of measurements, the Mo isotope ratios were determined using the double-spike method (note that double-spike was added before column chemistry) and the Mo yields could be determined precisely by isotope dilution. Note that the $\delta^{98}\text{Mo}$ values reported in Table 4 and 5 were only measured by mass spectrometry in the aqueous solutions. The Mo isotope ratios of the solvent after extraction ($\delta^{98}\text{MoS}_1$) and after back-extraction ($\delta^{98}\text{MoS}_2$) were then calculated using a mass balance equation:

$$ \delta^{98}\text{MoS}_1 = \frac{\delta^{98}\text{Mo}_{\text{initial}} - (\delta^{98}\text{Mo}_{\text{A1}} \times f\text{Mo}_{\text{A1}})}{1 - f\text{Mo}_{\text{A1}}} \quad (1) $$

$$ \delta^{98}\text{MoS}_2 = \frac{\delta^{98}\text{MoS}_1 - (\delta^{98}\text{MoA2} \times f\text{MoA2})}{1 - f\text{MoA2}} \quad (2), $$

where $\delta^{98}\text{Mo}_{\text{initial}}$ is the Mo isotope ratio of the solvent solution before extraction, $\delta^{98}\text{Mo}_{\text{A1}}$ and $\delta^{98}\text{Mo}_{\text{A2}}$, the Mo isotope ratios of the aqueous solutions after solvent extraction and after back-extraction, respectively; $f\text{Mo}_{\text{A1}}$ and $f\text{Mo}_{\text{A2}}$ are the fractions of Mo in the aqueous solutions used for extraction and stripping. The solution S1 from experiment Ei (which may have a fractionated Mo isotope composition) is used as a feed solution for the back extraction experiment BEi (i=1, 5). Thus, each entry of Table 4 are paired with a back-extraction experiment in Table 5 (except for experiment E4).

## 3.4 Uranium precipitation

The effect of uranium salt precipitation on molybdenum concentrations and isotope ratios in the residual solution was simulated using uranium solutions prepared from the UOC sample SOM 3079 (sodium diuranate), whose $\delta^{98}\text{Mo}$ value is -3.16 ± 0.03 ‰ (Table 3).
Aliquots of ~ 50 mg were completely dissolved in 2.5 ml of 0.25 M H₂SO₄ for each test. Two classically used reagents were tested to precipitate uranium: hydrogen peroxide (H₂O₂) and ammonium hydroxide (NH₄OH).

Starting with 1:1 molar ratios between solutions of 30 % hydrogen peroxide and uranium, a series of experiments was conducted with 10 %, 50 % and 100 % excess of hydrogen peroxide (Table 6) in order to cover the range of 35-100 % excess used in the uranium fuel cycle (Gupta et al., 2004; IAEA, 1993; Ajuria, 1990). The mixtures were stirred over 4 hours and the measured pH values were around 1.5 (Table 6), whereas the required pH was reported to be 3.5. In addition, experiments were also conducted by adding ammonium hydroxide to the uranium solution in order to reach a pH of 8 for instantaneous and total uranium precipitation (Ajuria, 1990). Some experiments were also conducted to determine yields for U and Mo precipitation over a range of pH between 5-6 and 7-8, with stirring over 39 hours (Table 6).

After each experiment, solutions and precipitates were centrifuged and supernatants were collected. Precipitates were washed with deionized water or using the corresponding precipitation solution. All liquid fractions were collected for concentration measurements. Mo concentrations in the precipitates were deduced by mass balance, given the small amount of precipitated solids. The Mo isotope ratio of the precipitates, δ⁹⁸Moₚrecip, was calculated using the following mass balance equation:

$$ \delta^{98}\text{Mo}_{\text{precip}} = \frac{\delta^{98}\text{Mo}_{\text{initial}} - (\delta^{98}\text{Mo}_{\text{sol}} \times f\text{Mo}_{\text{sol}})}{1 - f\text{Mo}_{\text{sol}}} \quad (3) $$

where δ⁹⁸Moₚinitial is the Mo isotope ratio of the solution before precipitation, δ⁹⁸Moₚsol, the Mo isotope ratio of the solution after precipitation, and fMoₚsol the fraction of Mo in the solution.
<table><caption>Table 4: Mo yields and Mo isotope fractionation for the extraction process during solvent extraction of U-Mo mixtures in three media⁺</caption><thead><tr><th colspan="9">Extraction</th></tr><tr><th>Mo (µg)</th><th>U (µg)</th><th>Estimated U<sub>S1</sub> (%)*</th><th>Estimated Mo<sub>S1</sub> (%)*</th><th>Experiment #</th><th>Mo<sub>A1</sub> (%)**</th><th>Mo<sub>S1</sub> (%)**</th><th>δ<sup>98</sup>Mo<sub>A1</sub> ± 2SE (‰) (n=5)</th><th>δ<sup>98</sup>Mo<sub>S1</sub> ± 2SE (‰)</th><th>Δ<sup>98</sup>Mo<sub>S1-A1</sub> (‰)</th></tr></thead><tbody><tr><td colspan="10"><strong>0.18 M H<sub>2</sub>SO<sub>4</sub></strong></td></tr><tr><td>1</td><td>4</td><td>99.8</td><td>74.2</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5</td><td>20</td><td>99.4</td><td>77.3</td><td>E 1</td><td>24.7</td><td>75.3</td><td>+0.25 ± 0.02</td><td>-0.08 ± 0.07</td><td>-0.33 ± 0.08</td></tr><tr><td>5</td><td>20</td><td></td><td></td><td>E 3</td><td>24.9</td><td>75.1</td><td>+0.23 ± 0.02</td><td>-0.08 ± 0.07</td><td>-0.31 ± 0.07</td></tr><tr><td>5</td><td>20</td><td></td><td></td><td>E 4</td><td>23.2</td><td>76.8</td><td>+0.23 ± 0.02</td><td>-0.07 ± 0.06</td><td>-0.30 ± 0.07</td></tr><tr><td colspan="10"><strong>0.18 M H<sub>2</sub>SO<sub>4</sub> + 0.15 M HNO<sub>3</sub></strong></td></tr><tr><td>1</td><td>4</td><td>96.9</td><td>41.1</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5</td><td>20</td><td>89.7</td><td>39.8</td><td>E 2</td><td>66.8</td><td>33.2</td><td>+0.09 ± 0.02</td><td>-0.18 ± 0.14</td><td>-0.28 ± 0.14</td></tr><tr><td>5</td><td>20</td><td></td><td></td><td>E 5</td><td>66.5</td><td>33.5</td><td>+0.08 ± 0.02</td><td>-0.16 ± 0.12</td><td>-0.23 ± 0.12</td></tr></tbody></table>

<footnote>*Experiments carried out with the NIST-SRM-3134 solution, with δ<sup>98</sup>Mo = 0 ‰ by definition. Mo isotope data were calculated relative to this solution. A1 refers to the aqueous solution and S1 to the solvent, both after extraction. *Calculated from the initial and final concentrations in the aqueous solutions. **Mo concentrations measured by isotope dilution on the Neptune Plus MC-ICP-MS.</footnote>
<table><caption>Table 5: Mo yields and Mo isotope fractionation for the back-extraction process during solvent extraction of U-Mo mixtures in two media +</caption><thead><tr><th colspan="10">Back-extraction</th></tr><tr><th>Back-extraction solution</th><th>Mo (µg)</th><th>U (µg)</th><th>Estimated U<sub>S2</sub> (%)<sup>*</sup></th><th>Estimated Mo<sub>A2</sub> (%)<sup>*Π</sup></th><th>Experiment</th><th>Mo<sub>A2</sub> (%)<sup>**</sup></th><th>Mo<sub>S2</sub> (%)<sup>**</sup></th><th>δ<sup>98</sup>Mo<sub>A2</sub> ± 2SE (‰) (n=5)</th><th>δ<sup>98</sup>Mo<sub>S2</sub> ± 2SE (‰)</th><th>Δ<sup>98</sup>Mo<sub>S2-A2</sub> (‰)</th></tr></thead><tbody><tr><td colspan="11"><strong>Extraction solution: 0.18 M H<sub>2</sub>SO<sub>4</sub></strong></td></tr><tr><td>1.5 M NaCl</td><td>5</td><td>20</td><td>—</td><td>~ 94</td><td></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>1.0 M NaCl + 0.05 M H<sub>2</sub>SO<sub>4</sub></td><td>5</td><td>20</td><td>72.1</td><td>~ 85</td><td>BE 1</td><td>89.3</td><td>10.7</td><td>-0.05 ± 0.02</td><td>-0.31 ± 0.40</td><td>0.26 ± 0.41</td></tr><tr><td>1.0 M NaCl + 0.05 M H<sub>2</sub>SO<sub>4</sub></td><td>5</td><td>20</td><td></td><td></td><td>BE 3</td><td>86.7</td><td>13.3</td><td>-0.08 ± 0.02</td><td>-0.03 ± 0.04</td><td>0.04 ± 0.04</td></tr><tr><td colspan="11"><strong>Extraction solution: 0.18 M H<sub>2</sub>SO<sub>4</sub> 0.15 M HNO<sub>3</sub></strong></td></tr><tr><td>1.5 M NaCl</td><td>5</td><td>20</td><td>—</td><td>~ 96</td><td></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>1.0 M NaCl + 0.05 M H<sub>2</sub>SO<sub>4</sub></td><td>5</td><td>20</td><td>79.7</td><td>~ 92</td><td>BE 2</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>1.0 M NaCl + 0.05 M H<sub>2</sub>SO<sub>4</sub></td><td>5</td><td>20</td><td></td><td></td><td>BE 5</td><td>93.2</td><td>6.8</td><td>-0.16 ± 0.02</td><td>-0.06 ± 0.05</td><td>0.10 ± 0.05</td></tr></tbody></table>

*Experiments carried out with the NIST-SRM-3134 solution, with a δ<sup>98</sup>Mo of 0 ‰. Mo isotope data were calculated relative to this solution. A2 refers to the aqueous solution and S2 to the solvent, both after back-extraction. *Calculated from the initial and final concentrations in the aqueous solutions. **Mo concentrations measured by isotope dilution on the Neptune Plus MC-ICP-MS. Π Approximate values, considering the Mo yield calculated by elemental analysis, including 15 % Mo loss during the purification from Na and U (see text for details). When available, isotope dilution data are more reliable.
<table><caption>Table 6: Results of precipitation experiments with hydrogen peroxide and ammonium hydroxide*</caption><thead><tr><th>Precipitation reagent</th><th>Volume (µl)</th><th>pH</th><th>Duration (hours)</th><th>U<sub>sol</sub> (%)</th><th>Fe<sub>sol</sub> (%)</th><th>Mo<sub>sol</sub> (%)</th><th>δ<sup>98</sup>Mo<sub>sol</sub>, ± 2 SE (‰)</th><th>Mo<sub>precip</sub> (%)</th><th>δ<sup>98</sup>Mo<sub>precip</sub> ± 2 SE (‰)</th><th>Δ<sup>98</sup>Mo<sub>sol.-precip.</sub> (‰)</th></tr></thead><tbody><tr><td>H<sub>2</sub>O<sub>2</sub> (10 % excess)</td><td>18</td><td>1.5</td><td>15</td><td>51</td><td>92</td><td>90</td><td>-3.14 ± 0.02</td><td>10</td><td>-3.40 ± 0.03</td><td>0.26 ± 0.03</td></tr><tr><td>H<sub>2</sub>O<sub>2</sub> (30 % excess)</td><td>21</td><td>1.5</td><td>15</td><td>42</td><td>91</td><td>89</td><td>—</td><td>11</td><td>—</td><td>—</td></tr><tr><td>H<sub>2</sub>O<sub>2</sub> (50 % excess)</td><td>24</td><td>1.5</td><td>15</td><td>33</td><td>87</td><td>88</td><td>—</td><td>12</td><td>—</td><td>—</td></tr><tr><td>H<sub>2</sub>O<sub>2</sub> (100 % excess)</td><td>32</td><td>1.5</td><td>15</td><td>25</td><td>88</td><td>88</td><td>—</td><td>12</td><td>—</td><td>—</td></tr><tr><td>H<sub>2</sub>O<sub>2</sub> (100 % excess)<sup>**</sup></td><td>32</td><td>1.5</td><td>4</td><td>39</td><td>99</td><td>92</td><td>—</td><td>8</td><td>—</td><td>—</td></tr><tr><td>NH<sub>4</sub>OH<sup>**</sup></td><td>570</td><td>8.0</td><td>Instantaneous</td><td>0.3</td><td>4</td><td>2</td><td>-2.31 ± 0.01</td><td>98</td><td>-3.18 ± 0.02</td><td>0.86 ± 0.02</td></tr><tr><td>NH<sub>4</sub>OH</td><td>570</td><td>7.0-8.0</td><td>39</td><td>0.1</td><td>0.01</td><td>14</td><td>-2.37 ± 0.01</td><td>86</td><td>-3.29 ± 0.02</td><td>0.92 ± 0.02</td></tr><tr><td>NH<sub>4</sub>OH</td><td>530</td><td>5.0-6.0</td><td>39</td><td>0.4</td><td>4</td><td>0.5</td><td>-2.28 ± 0.01</td><td>99.5</td><td>-3.17 ± 0.02</td><td>0.89 ± 0.02</td></tr></tbody></table>

* U<sub>sol</sub>, Fe<sub>sol</sub> and Mo<sub>sol</sub> are the U, Fe and Mo yields in the solution after precipitation. Mo<sub>precip</sub> is the calculated yield of Mo in the precipitate.

Δ<sup>98</sup>Mo<sub>sol.-precip.</sub> is the difference in per mil between δ<sup>98</sup>Mo values of the solution and the precipitate.

** Precipitate washed with deionized water.
# 4. Results

## 4.1 Uranium ores and ore concentrates

Uranium and molybdenum concentrations in uranium ores and uranium minerals show a large range from 40 µg/g to 0.88 g/g and < 10 ng/g to 6.1 mg/g, respectively (Tables 1 and 2). Overall, the Mo isotope ratios show no clear trend with the enrichment in Mo relative to U but there is a weak trend with Mo concentrations normalized to the average crustal Mo concentration (Figure 1).

As shown in Table 1, the highest Mo concentrations were found in pitchblende, which is a collomorph or botryoidal assemblage of UO$_{2+}$x and may therefore accommodate Mo in its structure. Two of the samples with high Mo concentrations come from mines where a close association with molybdenite has been described: Azegour (High Atlas of Marrakech, Morocco, Permingeat, 1952) and Shinkolobwe (Katanga, DRC, Thoreau and du Trieu de Terdonck, 1933). The range in δ$^{98}$Mo observed in molybdenites is very large (e.g. compilation in Willbold and Elliott, 2017) and encompasses the δ$^{98}$Mo measured in Mo-bearing UO$_{2}$ for Azegour and Shinkolobwe (Table 1). There is also a high Mo concentration in uranotile (aka uranophan) that is known to include other elements beyond Ca, Si and U (e.g. Novacek 1935). Most of the secondary uranium minerals have low Mo concentrations suggesting that the remobilization of uranium did not take place at the same P, T and pH conditions mobilizing Mo. Most of the uranium ores analyzed in this study are enriched in Mo, with Mo/U ratios frequently much larger in U ores (Table 2) than in U-bearing minerals (Table 1). As uranium ores are overall enriched in Mo relative to the upper continental crust (1.1 ppm), this suggests altogether that there are other hosts than uranium minerals for Mo in uranium ores (see discussion below).

The pitchblende (JU 11) and its gangue have contrasted δ$^{98}$Mo values equal to 0.57 and -1.97 ‰ respectively. The origin of this variation is difficult to interpret and would require a detailed mineralogical study. However, this reflects a large local scale heterogeneity in δ$^{98}$Mo that is similar to that found in uranium ores from Niger (see Table 2).

The uranium ore samples from sandstone-related ores have δ$^{98}$Mo values between -0.15 and -2.62 ‰ (Figure 2). A negative trend is observed for δ$^{98}$Mo values plotted as a function
of Mo/Mo<sub>crust</sub> ratios (Figure 1), with the Niger sandstones having the largest Mo enrichments.

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As shown in Table 3, triuranium octoxides and ammonium-diuranates have Mo concentrations generally lower than magnesium- and sodium-diuranates: 0.1-217.8 µg/g against 324.7-1372 µg/g. Mg- and Na-diuranates have negative δ<sup>98</sup>Mo values less than -1.82 ‰. Ammonium-diuranate and U<sub>3</sub>O<sub>8</sub> samples show a more limited range in δ<sup>98</sup>Mo, with values between 0.20 and 0.46 ‰ (Figure 3). The U<sub>3</sub>O<sub>8</sub> samples from CETAMA (Table 3) cannot be taken as representative of UOCs produced in the world industry regarding Mo isotopes, as i) their origins are unknown; Agaric being referred to as "high purity U<sub>3</sub>O<sub>8</sub>" could come from a pilot experiment, ii) all of these certified materials but one (Agaric) are spiked with Mo in order to serve as calibration material for analysis of impurities in uranium-rich materials. The uranium ore concentrates from the mills of COMINAK (AKO samples), SOMAÏR (SOM samples), Rössing (ROS samples) and NUFCOR (NUF samples) show a significant variability that reflect a combination of Mo isotope fractionations that are linked both to ore genesis and/or ore processing.

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## 4.2 Solvent extraction and back-extraction

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The data for solvent extraction and back extraction are reported in Table 4 and 5, respectively. The Mo yields obtained by solvent extraction were similar for the test with 1 µg of Mo and the test with 5 µg of Mo (Table 4). With 1.5 M HNO<sub>3</sub>, only 17 % of U and 6 % of Mo were extracted by the solvent, while a nitric acid concentration lower by a factor of ten yielded U extraction greater than ~90 % and Mo extraction around 33 %. The overall yield of the solvent extraction process for U is around 72 (including both extraction and back-extraction). The overall yield of solvent extraction for Mo depends on the nature of the extraction solution: 65-67 % with 0.18 M H<sub>2</sub>SO<sub>4</sub> and 31 % with 0.18 M H<sub>2</sub>SO<sub>4</sub> + 0.15 N HNO<sub>3</sub>.

During the extraction stage, Mo is enriched in heavy isotopes in the aqueous phase, leading to low δ<sup>98</sup>Mo values for the molybdenum in the solvent. As shown in Table 4, starting with an initial pregnant uranium solution with a δ<sup>98</sup>Mo value=0.00 ‰, δ<sup>98</sup>Mo values in the solvent range from -0.07 to -0.08 ‰ for extraction without nitric acid. During the

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back-extraction stage, Mo isotopes are only slightly fractionated between the solvent and the aqueous phase (Table 5).

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## 4.3 Experimental precipitation of uranium ore concentrates

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The results of precipitation tests with ammonium hydroxide and hydrogen peroxide are reported in Table 6 and illustrated in Figure 5. Precipitation tests with NH₄OH at a pH close to 8 left less than 1 % U in the residual solution while those with hydrogen peroxide were far less efficient (maximum U yield in precipitate was 75%). In residual solutions, δ⁹⁸Mo values showed an enrichment in heavy isotopes relative to the precipitated solid (Figure 5).

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Precipitation tests with H₂O₂ at pH 1.5 left high uranium contents in residual solutions ranging from 25 to 51 %. Mo co-precipitation with uranate salt is radically different for NH₄OH or H₂O₂ medium. The first experiment with NH₄OH led to a high concentration of Mo in the diuranate precipitate and relatively large Mo isotope fractionation between the solution and the uranium peroxide precipitate. In contrast, the second test with H₂O₂ led to a low Mo concentration in the precipitate and a smaller Mo isotope fractionation between the solution and the precipitate.

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# 5. Discussion

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## 5.1. Mo isotope signatures in uranium ores

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As shown above, with the exception of some pitchblende and uranotile minerals, the Mo concentrations in uranium minerals is low, which indicates some decoupling between U and Mo during uranium ore genesis. The known host minerals of Mo in uranium ores are jordisite, ilsemannite, powellite, molybdenite, molybdites, uranium molybdates, wulfenites and U-Ti-oxides (Gupta 1993; Wanty et al. 1987; Pribytkov 1960; Agrinier et al. 1972; Walker et al. 1963). With a few exceptions (U-Ti-oxides), this indicates indeed that Mo in uranium ores is often hosted in sulphides or molybdates. The presence of Mo in sulphides rather than

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in uraninites could be linked to the precipitation of Mo and U under different redox conditions, where U oxides becomes insoluble at slightly less reduced conditions than those for MoS₂ precipitation (Harshman, 1974; Bullock and Parnell, 2017). It has also been argued that Mo concentrations in uraninites could be a direct function of the oxygen content of the atmosphere (Frimmel et al. 2014), which has been variable over geological times. Under more reducing conditions, less Mo would be mobilized from source rocks by mineralizing fluids as it would remain locked in sulphides. When the fluid ends up forming U minerals, the amount of available Mo in solution is smaller and yields lower Mo concentrations in U oxides. These factors may explain partly the large variations in Mo contents in uraninites and in uranium ores and the decoupling between Mo and U mineralogy in ores. This decoupling has been observed directly in roll-front deposits by Bullock and Parnell (2017) who reported Mo and U concentrations in such uranium ore deposits, with decreasing Mo/U ratios with increasing U concentrations. Therefore, the process leading to U enrichment is not directly associated with Mo enrichment, underlining a decoupling between Mo and U.

Magmatic-related and some sedimentary uranium ores show differences in Mo isotope signatures. Uranium ores and minerals related to a magmatic environment (MIN D and MIN B granites, calcrete uranium deposit from Namibia, autunite from Bessines, France and pitchblende from Katanga, DRC) have δ⁹⁸Mo values between -0.34 and +0.30 ‰ (Figure 2), in agreement with the range reported in the literature for igneous rocks (-0.57 to +0.35 ‰: Greber et al., 2014; Siebert et al., 2003). Similarly to U isotope signatures (Uvarova et al., 2014), the δ⁹⁸Mo values of magmatic-related uranium ores are similar to those of the magmatic rocks (Willbold and Elliott 2017; Liang et al. 2015) from which they were derived. This indicates that there has been limited Mo fractionation during ore formation from magmatic rocks, which could either be the result of high temperatures or of quantitative transfer of Mo derived from magmatic source rocks to the ore deposit. The latter mechanism is hypothesized for the calcrete uranium deposit (sample Namibia_1) whose δ⁹⁸Mo values close to magmatic signatures suggest a quantitative transfer of Mo derived from the adjacent igneous rocks with a temperature associated with the formation of this deposit <50 °C (Uvarova et al., 2014).

Sedimentary uranium ores show a large range of δ⁹⁸Mo values between -0.09 and -2.62 ‰, with a weak relationship with Mo concentration (Figure 1). Some of these δ⁹⁸Mo signatures are even more negative than those found for Fe-Mn nodules (-0.75 to -1.25 ‰),
and must be compared with the range of δ⁹⁸Mo signatures known for suboxic (-0.6 to 1.5 ‰) or anoxic sediments (0.86 to 2.3 ‰), as reported by Arnold et al., (2004), Goldberg et al. (2009a), Goldberg et al. (2012); Goldberg et al., (2009b), Poulson et al. (2006), Siebert et al. (2006), Siebert et al., (2003). The large range in δ⁹⁸Mo values in sedimentary deposits is consistent with their formation at low temperatures with significant Mo isotope fractionation during ore deposition (Table 2). Mo enrichment in light isotopes seems to be associated with formation of uranium ores at low temperature, while the opposite trend is observed for uranium isotopes (Brennecka et al. 2010; Hiess et al., 2012; Uvarova et al., 2014).

In principle, Mo enrichment in light isotopes in U ore deposits could be related to kinetic isotope fractionation during precipitation. However, this interpretation would not be consistent with the U isotope data in similar deposits that show enrichment in U heavy isotopes. An additional question that arises is whether the Mo isotope fractionation is associated with final deposition of the uranium ore or whether the source of Mo in the ore is already fractionated. This point can be further examined in the case of Niger deposits for which there are more samples analysed and relevant geological information. The Mo isotope data for U ore samples from SOMAÏR and COMINAK uranium mines in Niger show a large range of variations, indicating that within the same U deposit, Mo isotopes are largely fractionated upon deposition and that the source of Mo itself (volcanic and magmatic rocks, Croisy et al. 2013) does not explain the whole range of δ⁹⁸Mo values. In the Niger deposits, pitchblende zones are located at the interface between reduced zones rich in pyrite and organic matter or uranium is also located in oxidized zones rich in goethite and hematite (Pons, 2015). These contrasted environments could produce some variability in Mo isotopes. First, goethite and hematite are known to adsorb preferentially light molybdenum isotopes, with Δ⁹⁸Mo of 1.3-1.7 ‰ and 2.0-2.6 ‰, respectively, between dissolved and adsorbed Mo (Goldberg et al., 2009). Second, molybdenum can also adsorb on pyrite surface as sulphur-coordinated species (Wanatabe and Takahashi, 2015). In roll-front deposits, it has been shown that high molybdenum concentrations are found in the pyrite-bearing reduced zone, with minerals such as MoS₂ polymorphs (molybdenite and jordisite). According to Tossel et al. (2005), the formation of sulphide Mo species that would then get adsorbed are sufficient to produce up to -1.7 ‰ of Mo isotope fractionation relative to the molybdate species at 25°C, corresponding to the variation in Mo isotope composition observed in the sandstones
558 deposits. More specifically, the experiments of Poulson et al. (2012) show that the
559 precipitation of MoS₂ or the adsorption of Mo onto pyrite leads to large Mo isotope
560 fractionation between 1 and 2.9 ‰, i.e. the Mo-bearing sulphide are enriched in light Mo
561 isotopes relative to the ore-forming solution containing Mo and U. This is overall consistent
562 with the observations made in the Niger uranium deposit where Mo is thought to be
563 associated with sulphides: the weak trend relating δ⁹⁸Mo to normalized Mo concentrations
564 (Figure 1) could result from Mo enrichment linked to Mo-bearing sulphides that are
565 themselves enriched in light Mo isotopes. In this context, the Mo/U variations and δ⁹⁸Mo
566 values would be directly linked to the local redox state of ore precipitation. These tentative
567 explanations are difficult to extrapolate to other localities where only one sample per
568 deposit is available.

569 The two phosphorite samples represent two rather different types of uranium deposits
570 with Bakouma corresponding to continental phosphorites in a paleokarst with uranium
571 contained in carbonate fluoroapatite (50% of the ore), while the Jordan phosphorites are of
572 marine origin. In the Bakouma continental phosphorite, the concentration of Mo is low, 0.47
573 µg/g with a δ⁹⁸Mo of -0.09 ‰. Molybdenum could be adsorbed to iron oxides or clays rather
574 than contained in the apatite. The marine phosphorite of Jordan is characterized by a higher
575 Mo content (16.71 µg/g) and a negative fractionation of -1.55 ‰. This could reflect
576 adsorption processes, similar to the sandstone deposits but more information would be
577 needed to interpret these results for phosphorites.

578 In summary, a large range in δ⁹⁸Mo values in uranium ores is a prerequisite if Mo
579 isotopes are to be used as a signature, for comparing unknown uranium ores to each other
580 or to a database and there is an indication that sedimentary U deposits exhibit more
581 negative δ⁹⁸Mo values Thus, there seems to be difference among various types of deposits
582 reflecting variable redox conditions and transport, especially in sandstone deposits. For
583 example co-deposition of Mo in uranium deposits in sulphides may induce a large Mo
584 isotope variations. More data on a wider range of uranium ore deposits would be desirable
585 to refine these inferences.

586

## 5.2. Mo isotope fractionation during uranium purification processes

588
The production of uranium ore concentrates involves a sequence of separation and purification processes. The main processes are: (i) leaching of uranium ore, (ii) resin ion-exchange chromatography and/or solvent extraction, and (iii) precipitation of uranium, followed or not by calcination to form $U_3O_8$ compounds. These steps aim at purifying uranium thereby decreasing the Mo/U mass ratios, from values up to 0.62 down to $1 \times 10^{-4}$ (Table 2 and 3, the CETAMA samples being excluded). The effect of leaching on Mo isotopes has been studied in Migeon et al. (2018) who showed that for acidic conditions (pH<2) which are the most common, Mo isotope fractionation during leaching is limited. It was beyond the scope of this study to focus on Mo isotope fractionation during ion exchange as there are many different types of resins used and literature data shows that it would require a complete separate study in itself (Electronic Annex).

Extraction of Mo in the solvent leads to a moderate enrichment in light isotopes for the solvent relative to the aqueous solution, with a $\Delta^{98}Mo_{S1\_A1}$ of -0.31 ± 0.08 ‰ for the experiment with no nitric acid (Table 4). In contrast, the back-extraction solution (1.0 M NaCl + 0.05 M $H_2SO_4$) of the first test show a weak Mo isotope fractionation, with $\delta^{98}Mo$ value of -0.08 ‰, relative to the initial Mo solution (NIST-SRM-3134). The overall solvent extraction process for the first test (E1-BE1) leads to a Mo recovery of 67.2 % and a $\Delta^{98}Mo$ of -0.08 ‰ for the final aqueous solution relative to the initial solution (using tests E3 and BE3 from Table 4 and 5). During back extraction, high Mo recovery from the solvent (~95 %) is not expected with NaCl 1.5 M, while it is observed in our back-extraction experiments (Table 5). Indeed, in the nuclear industry, NaCl solution is expected to be very selective at removing uranium from the solvent, with a minor amount of back-extracted molybdenum (IAEA, 1993). The reason for this discrepancy between our observations and those reported in the nuclear industry may stem from incomplete information about this exact process. Regardless of this aspect, our experimental results show that using simple experiments, it is possible to predict the Mo isotope composition associated with the process of solvent extraction depending on the yield. As solvent extraction is a very common method for purifying uranium and the flowsheets of numerous uranium mills is available in the literature (e.g. IAEA 1980, 1993, 2005), it should be possible to predict the shift in Mo isotopes due to solvent extraction in a given UOC, provided the isotope fractionation factor has been determined for the type of solvent used. A worked out example is given below in the case of Niger UOC (see section 5.3.2).
621
622
623
624 Mo isotope fractionation during uranate precipitation depends on reagents and pH.
625 According to Betenekov et al. (2016), Mo in solution at pH > 5 should be in the form of
626 molybdate ($MoO_4^{2-}$). Our experiments show a large amount of molybdate coprecipitation
627 during ammonium diuranate formation. The precipitation of ammonium diuranate could
628 avoid co-precipitation of Mo if done at a pH greater than 9 and it is likely that those
629 conditions are chosen for the industrial process (Nalewajek et al. 1986) as Mo contents in
630 uranium precipitates have to be limited in order to meet the standard requirements. In this
631 case, the effect of precipitation would have a larger impact on the $\delta^{98}Mo$ values.

632 Precipitation with hydrogen peroxide shows a distinct behaviour. In these conditions,
633 the relative fraction of molybdenum in the precipitate is less than 12 % and the Mo isotope
634 fractionation observed in this case is lower than with the precipitation with ammonia. This
635 low yield of Mo in the precipitate is probably explained by the selective complexation of Mo
636 by $H_2O_2$ as peroxomolybdate (Taube et al., 2002). The isotope fractionation factor between
637 the solution and the precipitate $\Delta^{98}Mo_{sol-precip}$ is equal to 0.26 ± 0.03 ‰.

638 Our experiments thus demonstrate that it is possible to predict the magnitude of Mo
639 isotope fractionation associated with uranium precipitation. Provided more experiments are
640 performed for other types of precipitates such as Na or Mg diuranates or at different
641 temperatures, it should be possible to predict in most cases by how much the $\delta^{98}Mo$ of the
642 solid is lowered due to precipitation. In addition, by cross checking with other nuclear
643 forensic tracers one could examine whether a specific type of precipitation was used to
644 prepare a given $U_3O_8$ concentrate. As all the other nuclear forensic tools, Mo isotopes should
645 not be used in isolation, rather they provide additional information that is not available with
646 other tracers.

647
648
649 5.3 Mo isotope composition in uranium materials: towards a fingerprint
650 in the nuclear fuel cycle

651
652

### 5.3.1 Mo isotope signature of uranium ores as a tool for indicating provenance

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655

656

657

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659

660

661

Mo isotope compositions in uranium ore concentrates show two groups based on our limited data set (Table 3, Figure 3). UOCs produced from magmatic-related deposits have $\delta^{98}Mo$ between -0.37 and +0.46 ‰, which may reflect the magmatic origin of the corresponding uranium ores, due to the similarity with the $\delta^{98}Mo$ range for crustal igneous rocks. In contrast, UOCs produced from sandstone deposits are fractionated towards negative $\delta^{98}Mo$ values (-1.82 to -3.5 ‰), similarly to the sedimentary uranium ores (Table 2). In a given mill, replicate samples show moderate variations in $\delta^{98}Mo$ values, probably resulting from variable Mo isotope compositions of the uranium ore, although mixing of ores in the mill must have dampened initial variations of the ores.

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664

665

666

667

668

669

670

One would require to extend the data base for U ores but our results indicate that Mo isotopes may enable to discriminate between different types of ores. Although we do not have a good understanding of the origin of these signatures, the existence of a larger data base of Mo isotopes in uranium ores would provide a useful additional tool for attribution of provenance as a complement of Sr, Nd and Pb isotopes (e.g. Kristo et al. 2017) and REE fingerprints (e.g. Mercadier et al. 2011 Spano et al. 2017; Svedkauskaite-LeGore et al., 2008). One obstacle may be that there can be a significant heterogeneity in ores that may limit the resolution of this tool by introducing overlaps, although blending is common practice at the ore feed of uranium mills. However, this characteristics is not specific to Mo isotopes.

671

672

### 5.3.1 Mo isotope signature of uranium ore concentrates and the role of ore processing

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682

683

In the nuclear industry, the processes used to concentrate and purify uranium can all modify the resulting Mo isotope composition of the refined uranium material, compared to the value of the original uranium ore. This study together with the study of Migeon et al. (2018) have shown that acid leaching, solvent extraction and precipitation can modify the Mo isotope signature of uranium ores. Thus, the separation processes could produce either a positive or a negative shift of $\delta^{98}Mo$ values of different uranium oxides produced along a given flowsheet. The magnitude of the Mo isotope fractionation will be a function of the yield of Mo separation, which is controlled by the reagents and the processes. Since the flowsheet of many uranium mills are in fact available in the literature (e.g. IAEA 1980, 1993), one could in principle reconstruct tentatively the overall $\delta^{98}Mo$ shift associated with a given
uranium mill. More detailed studies including different types of solvents and different forms
684
of uranium precipitation will be necessary to obtain fully quantitative information. This is
685
because the Mo yields and the isotope fractionation factors may vary with the nature of
686
chemicals used in each plant as well as the values of pH and Eh of the solutions which
687
control Mo speciation and this information is not always fully available. Yet our study shows
688
how a general framework for using Mo isotopes could be used for fingerprinting specific
689
processes. A contrario, if UOC samples are examined, the original Mo isotope composition of
690
the ore could possibly be inferred if the ore processing steps are roughly known. It is still
691
unknown to what level of uncertainty Mo isotope tracing could be brought due to
692
uncertainties in industrial practices.
693

694

One step that has not been studied in the literature is the calcination step. This could
695
result from a kinetic isotope fractionation of Mo linked with the preferential evaporation of
696
light Mo isotopes at high temperature, as U₃O₈ are produced from uranates by calcination in
697
an O₂-rich atmosphere at temperature up to 900°C (Eloirdi et al. 2014). Under such
698
conditions, it is known that Mo forms a volatile oxide (MoO₃, Gulbransen 1963) that could be
699
lost to the vapor and be enriched in light isotopes relative to the residual solids, with the
700
degree of fractionation depending on the local pressure in volatile Mo species. One could
701
note for example that for the only paired samples from Rolison et al. (2019) do not show any
702
Mo isotope fractionation after calcining ADU to produce U₃O₈, possibly indicating a back
703
reaction that reduces any Mo isotope fractionation due to evaporative loss.
704

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712

5.3.2 Comparison of Mo signatures in UOC and ores from Niger

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715

Despite an obvious role for ore processing, the large range found in UOC (Rolison et al., 2019; this study) reflects the overall large range of δ⁹⁸Mo values found in uranium ores and this initial range is shifted to lower (e.g. precipitation) or higher (calcination) during ore processing. This large range in Mo isotope compositions makes Mo isotope a discriminatory tool for nuclear forensics investigations, especially since δ⁹⁸Mo values from the same mill show a relatively narrow range (e.g. within NUFCOR, Rössing, SOMAIR and COMINAK mills).

The case of the two Niger mills can be examined further as Mo isotopes for several
uranium ores and UOC samples were measured. The uranium ores that feed the two mills
have very different δ⁹⁸Mo values, as well as U and Mo concentrations (Table 2). Mo/U ratios
range from 0.044 to 0.62 in uranium ores. Sialino et al. (1977) reported Mo/U ratios ranging between 0.1 and 0.3 for uranium ore mined in the Akouta deposit, which feeds the COMINAK mill. The overall uranium purification process leads to Mo/U ratios from 1.2×10⁻³ to 2.0×10⁻³ in the diuranates (Table 3). In the industrial process, uranium-rich and uranium-poor ores are mixed under different proportions to reach similar U (and probably Mo) concentrations for feeding the hydrometallurgy plant with a blend of nearly constant concentrations of these critical elements. To estimate a mean δ⁹⁸Mo value for the ore, a first approach is to calculate a concentration weighted average of the Mo isotope composition for the COMINAK and SOMAÏR uranium ore samples: the result is -2.61 ‰ for the COMINAK uranium ore while it is -1.66 ‰ for the SOMAÏR uranium ore. Thus, an enrichment in light Mo isotopes is observed in the UOCs produced from the SOMAÏR mill (-1.4 ‰ relative to the average of the uranium feed). In contrast, the explanation for the difference between the COMINAK ores (-2.61‰) and UOCs (-1.96‰) is not obvious and this may be due to the large variability in the Mo isotope composition of the ores (Table 2), as pointed out above. For example, if the ore that fed the mill at the time of the UOC production was -0.97 ‰ (as in Niger_COM_1), instead of -2.61‰, then the observations would be consistent with those of the SOMAÏR.

The COMINAK and SOMAÏR mills used similar processes of uranium purification, except for the precipitation process. In the COMINAK mill, molybdenum was recovered as a by-product before 1987 (IAEA, 1993; Sole et al. 2011). The leaching of uranium ores was developed to efficiently dissolve uranium and molybdenum. Chagnes et al. (2012) reported a Mo/U of 0.24 in the typical sulphate leaching solutions from the COMINAK mill, similar with ratios in uranium ores (Sialino et al., 1977). The acid leaching step is actually done with a mixture of H₂SO₄, HNO₃ and NaNO₃. This mixture is oxidizing and probably leads to a large molybdenum yield in the leaching solution. As a consequence, the Mo isotope fractionation during this step is expected to be limited (< +0.1 ‰, Migeon et al., 2018).

The leaching step at COMINAK is followed by solvent extraction. Similarly to our experiments, the solvent is a mixture of 0.2 M Tri-N-Octylamine (Alamine 336) and 5 vol. % Tridecanol diluted in kerosene. The back-extraction solution uses a 1.5 M NaCl solution, which is efficient for recovering uranium rather than molybdenum from the solvent phase. In our experiments, we obtained a high extraction yield of Mo, which is distinct from what is
described for the COMINAK or SOMAÏR mill. Furthermore, in the industrial process used at the COMINAK mill, up to five stages of countercurrent solvent extraction are used for both extractions and back-extractions to obtain reasonable uranium purification from molybdenum (Croisy et al., 2013; IAEA, 1993, see Figure 6 for a schematic view of the counter current multiple stages) and this is similar for the SOMAÏR mill (Sole et al. 2011). On the basis of this information, it is possible to estimate the Mo isotope fractionation associated with solvent extraction to examine whether this could be an important process for explaining the Mo isotope difference between uranium ore and uranium ore concentrates.

In what follows, we aim at calculating the cumulative effect of multiple stages of solvent extraction and back extraction on Mo isotopes using the results of our batch experiments. The goal was to predict the net effect of solvent extraction during UOC production in a real case. Based on the flowsheet for the uranium mill in COMINAK, it is possible to calculate the effect of five stages of counter current solvent extraction by writing simple mass balance equations (Figure 6). In such a counter current scheme, the clean solvent is fed in the last stage of extraction while the U rich aqueous solution is in the first stage. First, the mass balance equations for Mo or U concentrations can be written for stage i (from i = 0 for the initial step to i = 4 for the final one):

$$C_A^i X_A + \frac{(1-X_A)C_A^i}{D} = C_A^{i-1} X_A + \frac{(1-X_A)C_A^{i+1}}{D} \quad (4)$$

where C^i stands for the concentration in stage i, X_A stands for the mass fraction of U or Mo in the aqueous phase and D the partition coefficient between the aqueous phase and the solvent (=C_A/C_S). Five such equations can be written for the five stages of solvent extraction shown in Figure 7. It is then possible to solve a system of five equations with five unknowns which are the concentrations of Mo or U in the aqueous phase. Similarly, one can write five equations for the Mo isotope compositions in the aqueous phase corresponding to the five stages of solvent extraction:

$$\delta^{98}Mo_A^i X_{Ai}^{Mo\_f} + \delta^{98}Mo_S^i (1 - X_{Ai}^{Mo\_f}) = \delta^{98}Mo_A^{i-1} X_{Ai}^{Mo\_i} + \delta^{98}Mo_S^{i+1} (1 - X_{Ai}^{Mo\_i}) \quad (5)$$

where the parameters X_{Ai}^{Mo\_f} and X_{Ai}^{Mo\_i} represent the mass fraction of Mo in the aqueous phase after solvent extraction and the mass fraction of Mo in the aqueous phase prior to
778 solvent extraction, respectively. The δ⁹⁸Mo value in the aqueous solution is related to the
779 δ⁹⁸Mo value of the solvent with the following equation:

$$780 \qquad \Delta^{98}Mo = \delta^{98}Mo_A - \delta^{98}Mo_S \qquad (6)$$

781 The unknown parameters can be calculated using the results from the five equations
782 similar to equation (4) using the equations:

$$783 \qquad X_{Ai}^{Mo_i} = \frac{C_A^{i-1}X_A}{C_A^{i-1}X_A+(1-X_A)C_S^{i+1}} \qquad (7)$$

$$784 \qquad X_{Ai}^{Mo_f} = \frac{C_A^i X_A}{C_A^i X_A+(1-X_A)C_S^i} \qquad (8)$$

785 Using these equations, it is again possible to determine the δ⁹⁸Mo of the solvent and the
786 aqueous phase in all the five stages of extraction by solving numerically the five equations
787 (5). According to Sole et al. (2011) and IAEA (1993), most of the Mo is extracted by the
788 solvent which is consistent with our experiments reported in Table 4. However, during the
789 back-extraction, the Mo should have remained in the solvent fraction (Sole et al. 2011),
790 which is not what we observed during our experiments. A value of 0.3 was used for the Mo
791 partition coefficient during solvent extraction. To be consistent with the observed practice in
792 the Niger mills, we accordingly adjusted the D value of the back-extraction to match the
793 minimum Mo/U in UOCs equal to 7x10⁻⁴ (Table 3If one assumes that the precipitation will
794 lower the Mo concentration by a factor of 4 as in Weir and Genik-Sas-Berezkowsky (1983),
795 then the value of the solvent/aqueous solution partition coefficient for Mo should be equal
796 to 90.
797

798 In the case of a counter current system where most of the Mo is extracted by the
799 solvent, the δ⁹⁸Mo in the solvent is equal to the δ⁹⁸Mo of the initial aqueous solution (Figure
800 7b). This stems from having a aqueous/solvent solution partition coefficient for the
801 extraction less than 1. All the Mo that is not extracted in the first stage is recovered in the
802 subsequent stages but in mass balance this contributes to a small part of the total Mo
803 budget and it is not fractionated enough to greatly affect the final mass balance. In addition,
804 the δ⁹⁸Mo of the solvent fed in stage 1 depends on all the subsequent stages of extraction.
805 Similarly, the back-extraction was also modelled using a set of equations similar to those
806 used for extraction. In this case, the starting solution is the solvent fraction S1 and the
807 aqueous solution used for U recovery comes from the second stage of back-extraction
808 (Figure 6).
The results of the model calculations are shown in Figure 7. During the extraction, the final solvent (S1 in the Figure 7a) containing the Mo has a $\delta^{98}\text{Mo}$ almost equal to 0 ‰ because most of the Mo is contained in the solvent and during the back-extraction, the aqueous solution (A1) is shifted by -0.05‰ relative to the solvent initially containing the U. Thus, the overall effect is to offset the $\delta^{98}\text{Mo}$ value of the initial solution by -0.05 ‰. Our model calculations show that the observed Mo isotope fractionation should depend mostly on the nature of the solvent rather than the number of stages used in the solvent extraction: this is an important result as the number of stages in these processes is not always known. The observed Mo isotope fractionation is in the same direction but its magnitude is far too small to explain the observed values. This suggests that additional processes need to be taken into account.

In the COMINAK and SOMAÏR mills, magnesia and sodium hydroxide are used as precipitating reagents, producing magnesium-diuranate and sodium-diuranate respectively (IAEA, 1993). Both reactions are expected to be similar to ammonium diuranate precipitation with the exception that in the industrial process, Mo remains in the solution rather than coprecipitating with uranium. A patent by Nalewajek et al. (1986) shows that a high pH (9) during ammonium uranate precipitate prevents the premature precipitation of ammonium molybdate (note that pH of our experiments was lower than 9). The resulting Mg and Na diuranate precipitate should show a negative Mo isotope variation but the magnitude of this fractionation has not been determined experimentally. However, the structure of Mo in Mg or Na diuranate is expected to be similar to that in ammonium diuranate as the bond length and vibration frequencies of these compounds are similar, suggesting similar isotope fractionation factors for the three types of diuranates (Hardcastle and Wachs 1990). Thus, the effect of uranate formation would be to shift the $\delta^{98}\text{Mo}$ in the UOC by about -1‰.

The negative Mo isotope fractionation between uranium ores and uranium ore concentrates observed in the SOMAÏR mill can be explained by the cumulative effect of these two processes. The Mo isotope fractionation is probably null during the acid leaching and leads to enrichment in light isotopes during both solvent extraction and sodium diuranate precipitation. The combination of the two processes could lead to a cumulative Mo isotope fractionation of -1.05 ‰ which is sufficient to explain the observed difference between uranium ore concentrates produced by the SOMAÏR mill and the processed uranium ores. Thus, owing to the processes used in different mills, UOCs may have Mo
isotope signatures different from their uranium ores and this may stem from variable efficiencies in removing molybdenum during some key processes of the nuclear fuel cycle. In the case of COMINAK, the lack of resolvable difference between ore and UOC in $\delta^{98}\text{Mo}$ can be explained by ore heterogeneities since the sampling was done at a local scale (see section 2.). However, this could also stem from incomplete knowledge of Mo yields in the various separation stages. For example, our experiments demonstrate that the amount of Mo co-precipitated depends strongly on pH. The Mo yield in the precipitation is not known precisely as one can only estimate an overall yield based on Mo/U ratios. This aspect could be a limiting factor in future studies, despite the existing documentation about U mills worldwide.

This example illustrates that despite relatively limited information about the SOMAÏR uranium mill, it is possible to infer the effect of the various separation steps on the final Mo isotope composition of UOC. Thus, this shows the validity of Mo isotopes in recording a signature that is specific to a type of ore processing. This places Mo isotopes in a rather different category compared with other isotope tracers used in nuclear forensics. Sr, Nd and Pb are specific tracers of origin of ores (e.g. Kristo et al. 2017) while oxygen isotopes are thought to reflect the location of ore processing (Pajo et al. 2001; Fayek et al. 2011) with some possible caveats. The sensitivity of Mo to ore processing may represent a unique niche that is akin to sulphur isotopes that are thought to trace the utilization of sulfuric acid during ore leaching (Han et al. 2013). More generally, this type of approach could also be applicable with other isotopes or used by manufacturers as a diagnostic tool for industrial processes.

## 6. Conclusions

The molybdenum isotope composition of uranium ores and uranium ore concentrates is a promising fingerprint for future usage in nuclear forensics, nuclear safeguards and more generally the fight against illegal trafficking of products from the nuclear fuel cycle. Mo isotope ratios for uranium minerals and uranium ores from various mines as well as uranium ore concentrates from the industry span a range that extends to even more negative $\delta^{98}\text{Mo}$ than what has been observed in natural systems.
Similarly to U isotopes, there is a contrast in Mo isotope signatures between uranium ores formed under low temperature conditions where the redox chemistry of U and Mo may play a role in ore formation. However, the range of Mo isotope fractionation is arguably larger. Thus, the $\delta^{98}\text{Mo}$ values of some sedimentary uranium ores is shifted to negative values relative to magmatic ores or ores where transfer to the deposit involved quantitative transfer processes. This may provide a means of distinguishing these types of uranium ores. Depending on the ore processing flowsheet, uranium ores and UOC samples from a given mill may have similar Mo isotope compositions or significant shifts between them can be observed, indicating that one of the processes involved in the flowsheet yielded a Mo isotope fractionation related with Mo removal.

Industrial processes fractionate the Mo isotope composition yielding both positive (leaching) and negative (solvent extraction, precipitation) variations but this fractionation depends directly on the Mo recovery and on the type of process involved during purification. Calcination to $U_3O_8$ may lead to an enrichment in heavy Mo isotopes by kinetic isotope fractionation during evaporation. The processes used for extraction, purification, precipitation and calcination of uranium lead to specific Mo isotope fractionation. The associated isotope fractionation factors can be determined empirically from sampling and analysis of existing flowsheets or experimentally in the laboratory for existing or hypothetical systems. Ab initio calculations could also be used to derive additional values of Mo isotope fractionation factors.

Mo isotope ratios for a given type of uranium compounds produced from a given mill are similar within a narrow range of $\delta^{98}\text{Mo}$ values. The comparison of UOCs with the ores from which they derived is still difficult due to the currently limited database used in this study. As a consequence, a larger database of known uranium ores and UOCs will be necessary to fully exploit the possibilities afforded by Mo isotopes. Further studies will be necessary to quantify all the uranium concentration processes, such as alkaline leaching, effect of Mo adsorption on charcoal or calcination for reaching the full capabilities of this new isotope tracer. Yet our study shows that one can distinguish between uranium ore types and that provided one has an idea about the type of processing, it should be possible to reconstruct the Mo isotope composition of the ore based on UOC measurements.
904 Acknowledgements

905 We thank ORANO, Jean-Claude Bouillard (Sorbonne University, Paris), Maurice Pagel
906 (University Paris-Sud, Orsay) and Florent Barbecot (UQAM, Canada) for providing samples.
907 Anne-Laure Fauré (CEA) is thanked for help with shipment. Philippe Telouk (ENS de Lyon)
908 and Amélie Hubert (CEA) are thanked for their help with the Neptune Plus. Constructive
909 comments by two anonymous reviewers greatly helped improve this manuscript. This
910 project was funded in the framework of a CEA-ENS Lyon-CNRS-Université Claude Bernard
911 Lyon I collaboration and by the LABEX LIO (10-LABX-0066).
912
913

912

913

914 Figure captions

915 **Figure 1:** δ<sup>98</sup>Mo values plotted as a function of Mo/Mo<sub>crust</sub> ratios in uranium minerals and
916 ores. The Bulk Silicate Earth (BSE) value is represented by the blue star (McDonough and
917 Sun, 1995 and Willbold and Elliott 2017; Gao et al., 1998; Rudnick and Gao, 2004). The Mo
918 concentration in the BSE is 50 ppb, while the mean upper continental crust has 1.1 ppm Mo.
919

919

920 **Figure 2:** Mo isotope compositions in uranium ores from various localities. Error bars (± 2 SE
921 <0.06 ‰) are within the symbol size. The range in δ<sup>98</sup>Mo values reported for igneous rocks is
922 shown by the grey band (Greber et al., 2014; Greber et al., 2015; Siebert et al., 2003;
923 Willbold and Elliott 2017). The δ<sup>98</sup>Mo value of Bulk Silicate Earth (BSE) value is represented
924 by the solid line (Willbold and Elliott, 2017).
925

925

926 **Figure 3:** Mo isotope variations for uranium ore concentrates. Error bars (± 2 SE) are within
927 the symbol size. Bulk Silicate Earth (BSE) values are represented by the grey band and
928 igneous rock values show a range delimited by the dashed lines (Greber et al., 2015).
929

929

930 **Figure 4:** Schematic diagrams illustrating the solvent extraction experiments. The diagram
931 shows the Mo yields and Mo isotope compositions associated with the solvent and aqueous
932 phases for Test 1 (Table 4). Errors are < 0.1 ‰ (2 SE). Similar Δ<sup>98</sup>Mo<sub>A-S</sub> values are obtained for
933 both extraction and back-extraction processes. The initial solution is the NIST-SRM-3134,
934 with a δ<sup>98</sup>Mo of 0 ‰. The back extracted solution has a δ<sup>98</sup>Mo of -0.05 ± 0.02 ‰.
935

935
Figure 5: Results of Mo isotope fractionation during NH₄OH and peroxide precipitation showing the Mo isotope composition of residual solutions (square symbols) as a function of the fraction of Mo remaining in solution obtained during batch experiments (closed system). The calculated δ⁹⁸Mo of precipitates are plotted as solid circles. Error bars (± 2 SE) are smaller than the symbol size. The initial δ⁹⁸Mo of the SOM 3979 UOC (starting material) is represented by the dashed line, with δ⁹⁸Mo of -3.16 ± 0.03 ‰ (2 SE).

Figure 6: Schematic diagram illustrating an industrial countercurrent solvent extraction process. The loaded solution (A0) is injected in tank 1 and the organic solvent (S2) coming from tank 2 is injected in the tank 1. After mixing and chemical equilibration, the two immiscible phases are separated, the aqueous solution A1 is injected in the tank 2, while solvent S1 containing most of the U is sent to back-extraction. At the final stage (tank 5), fresh solvent S0 containing no U is injected in tank 5, while aqueous solution A4 is injected from tank 4. This type of process leads to a more efficient uranium extraction.

Figure 7: a. Results of model calculation for the five-stage countercurrent solvent extraction. The δ⁹⁸Mo value of the solvent S1 is not fractionated relative to the feed solution because most of the Mo is in the solvent. The solvent is shown as red crosses while the aqueous solution is represented by blue stars. b. Results of model calculations for the five-stage countercurrent back-extraction. The aqueous solution is shown as blue stars while the solvent is represented by red crosses. The δ⁹⁸Mo value of the aqueous solution A1 is fractionated relative to the feed solution of solvent S1 by approximately -0.05 ‰. These calculations can be used to predict the net effect of solvent extraction on Mo isotopes in a real U mill based on simple batch experiments performed in the laboratory.

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1203
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● Magmatic related

× Unknown locality

△ Phosphorites

■ Sandstones

■ Minerals

□ Gangue

BSE

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MIN D

Namibia_1

MIN B

RCA_1

Jordan_1

Mongolia_1

Gabon_1

MIN E

Niger_COMINAK

Niger_SOMAÏR

Pitchblende

Autunite
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Extraction (E1)

U=20 µg

Mo=5 µg

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