How to Understand in-situ U-Pb Isotopic Data of Uraninite and Pitchblende?

Graphical abstract

0 INTRODUCTION

Uraninite and pitchblende are the most important ore minerals in nearly all types of uranium deposits. Therefore, the U-Pb isotopic system of uraninite and pitchblende are widely used in dating uranium deposits. The ID-TIMS U-Pb analysis were usually adopted in early days (eg., Xia, 2019; Li et al., 1980), and in-situ U-Pb analysis, including LA-ICP-MS and SIMS, are mostly adopted in recently published papers (e.g., Wang et al., 2024; Zhang and Wang, 2023; Zhang et al., 2022; Zhu et al., 2022; Zheng et al., 2021; Guo et al., 2020; Wu et al., 2020; Bonnetti et al., 2018; Luo et al., 2015), besides the application and improvements in electron probe micro-analysis (EPMA) U-Th-Pb chemical dating on uraninite and pitchblende (e.g., Song et al., 2025; Martz et al., 2019; Luo et al., 2015; Dipak and Dieter, 2013; Alexandre and Kyser, 2005) However, the understanding to these in-situ U-Pb isotopic data of uraninite and pitchblende are confusing due to the weak U-Pb blocking system of uraninite and pitchblende. Beside the regular Pb loss similar to other U-bearing minerals like zircon (Wetherill, 1963,1956), uraninite and pitchblende are easily to suffer chemical alteration in near surface environment which is seldom in zircon. The vulnerability of uraninite and pitchblende is associated with the oxidation of U⁴⁺ to U⁶⁺ in oxidizing fluid or by auto-oxidation and the accumulation of radiogenic Pb²⁺ in crystals (Finch and Murakami, 1999). Hence, the correct understanding to U-Pb isotopic data of uraninite and pitchblende are more complex than those of stable minerals like zircon and apatite. Here we present four theoretical alteration models of U-Pb isotopic data of uraninite and pitchblende. In consideration of these new models, some published in-situ U-Pb isotopic data of uraninite and/or pitchblende are re-evaluated to distinguish typical wrong understanding on these data, and to propose suggestions in understanding these data correctly.

1 NEW THEORETICAL MODELS OF U-Pb ISOTOPIC SYSTEM

Like the U-Pb isotopic systems of zircon(Wetherill, 1963,1956), if the U-Pb systems of pitchblende and uraninite are always closed after their crystallization, the ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U plots will evolve concordial in a definite line to form the “concordia line” (Figure 1a).

The common situation of U-Pb isotopic systems is disturbed and closed again to accumulate daughter isotopes systematically. In such situations, the data in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U system will plot as a line that cut the “concordia line” to form an upper and a lower intercept (Figure 1a). If these data underwent loss of radiometric Pb isotopes, they plot below the “concordia line”(Wetherill, 1963,1956). If these data underwent loss of uranium isotopes, they plot above the “concordia line” (Wetherill, 1963,1956). The line constraining these data distributions, whether they are below or above the “concordia line”, is termed as “discordia line”(Wetherill, 1963,1956) with an upper and a lower intercept with the “concordia line” (Figure 1a). The upper intercepted age is the original crystallization age and the lower intercepted age is the alteration and/or recrystallization age (Wetherill, 1956). In practice, the upper intercepted age is believed to be reliable, while the lower intercepted age is believed to be unreliable, duo to complicated modifications of the U-Pb system by episodic Pb-loss(Wetherill, 1963,1956), volume diffusion (Tilton, 1960), radioactive dilatancy (Goldich and Mudrey, 1972) and/or chemical weathering (Stern et al., 1966), unless there is an independent evidence to prove the event at this age (Faure, 1977).

The above two situations are common in U-Pb isotopic data systems, whether they are in zircons or in pitchblende and uraninite. However, the following situations are seldom in zircon but common in pitchblende and uraninite. Due to the weakness in blocking U and Pb isotopes, uraninite and pitchblende are easy to suffer multiple isotopic alterations after their crystallization, besides the common hydrothermal alterations, even in near surface groundwater environment. Thus, we need to consider at least twice alterations and their possible data distribution models to understand the practical U-Pb isotopic data of uraninite and pitchblende.

In single stage firstly U-Pb alteration situation of pitchblende and uraninite without secondly alteration, the data distribute along “discordia line” as illustrated in Figure 1a. When overprinted the firstly and secondly U-Pb isotopic alterations, both of which can be either Pb loss or U loss, the situations would be much more complex. Therefore, we have four situations when considering twice alterations of U-Pb systems of uraninite and pitchblende. They are as follows.

(1) Firstly U-loss overlapped by secondly U-loss (Figure 1b);

(2) Firstly U-loss overlapped by secondly Pb-loss (Figure 1c);

(3) Firstly Pb-loss overlapped by secondly U-loss (Figures 1d, 1e);

(4) Firstly Pb-loss overlapped by secondly Pb-loss (Figure 1f).

In the No. (1) situation, the U-Pb isotopic systems of uraninite and pitchblende underwent two alterations, firstly U-loss overlapped by secondly U-loss. The firstly U-loss makes the data (red squares in Figure 1b) in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U move along the first discordia line (red line in Figure 1b) to above the “concordia line”. The secondly U-loss makes the data (red squares in Figure 1b) move along the second discordia line (green line in Figure 1b) to above the first discordia line (red line in Figure 1b) to form the final data distribution (yellow squares in Figure 1b). These data (yellow squares in Figure 1b) may form a line that has neither upper nor lower intercepts with the “concordia line”. Therefore, there is no geological meaningful ages in this situation.

In the No. (2) situation, the U-Pb isotopic systems of uraninite and pitchblende underwent firstly U-loss and secondly Pb-loss. Similar to that in No. (1) situation, the firstly U-loss makes the data (red squares in Figure 1c) in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U move along the first discordia line (red line in Figure 1c) to above the “concordia line”. The secondly Pb-loss makes the data (red squares) move along the second discordia line (green line in Figure 1c) to below the first discordia line (red line in Figure 1c) to form the final data distribution (yellow squares in Figure 1c). These data (yellow squares in Figure 1b) may form lines at different locations that cut the “concordia line” with both upper and lower intercepts (the upper purple dashed line in Figure 1c), or only with lower intercepts and without upper intercept (the lower purple dashed line in Figure 1c). In these scenarios, neither the upper nor the lower intercepted ages have geological significance, unless there is independent evidence to prove the event at these given ages. It is worth to note that the upper intercepted age, if it exists, is abnormally older than the true age. In some instances, these data will also locate near or just on the “concordia line” (yellow squares Figure 1c). These data may be appeared to be concordial, but they are older than the true age of crystallization, and thus have no geological significance.

In the No. (3) situation, the U-Pb isotopic systems of uraninite and pitchblende underwent firstly Pb-loss and secondly U-loss. Similar in Figure 1a, the firstly Pb-loss makes the data (red squares in Figures 1d, 1e) in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U move along the first discordia line (red line in Figures 1d, 1e) to below the “concordia line”. These data (red squares in Figures 1d, 1e) would be dispersed along the first discordia line (red line in Figure 1d) if they suffer different extend of Pb-loss, while they would be clustered in one or several segments on or near the first discordia line (red line in Figure 1e) if they suffer similar extend of Pb-loss. The secondly U-loss makes the data (red squares) move along the second discordia line (green line in Figures 1d, 1e) to above the first discordia line (red line in Figures 1d, 1e) to form the final data distribution (green squares in Figures 1d, 1e). In some instances, these data will locate near or just on the “concordia line” to form dispersed data (green squares in Figure 1d) or concentrated data clusters (green squares in Figure 1e). These data may be appeared to be concordial, but they are younger than the true age of mineral crystallization, and thus have no geological significance.

In the No. (4) situation, the U-Pb isotopic systems of uraninite and pitchblende underwent firstly Pb-loss and secondly Pb-loss. Similar in Figures 1a, 1d and 1e, the firstly Pb-loss makes the data (red squares) in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U moving along the first discordia line (red line in Figure 1f) to below the “concordia line”. The secondly Pb-loss makes the data (red squares) moving along the second discordia line (green line in Figure 1f) to below the first discordia line (red line in Figure 1c) to form the final data distribution (yellow squares in Figure 1f). These data (yellow squares in Figure 1f) may form lines at different locations that cut the “concordia line” with both upper and lower intercepts (the upper purple dashed line in Figure 1f), or only with lower intercepts and without upper intercept (the lower purple dashed line in Figure 1f). In these scenarios, neither the upper nor the lower intercepted ages have geological significance, unless there is independent evidence to prove the event at these given ages. It is worth to note that the upper intercepted age, if it exists, is abnormally older than the true age.

There is an important point that we need to notice. When the minerals containing U-Pb isotopic systems crystallized at young ages, especially younger than 100 Ma, the “concordia line” in ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U system is nearly a straight line with a little curve. In such a condition, the “discordia line” is nearly parallel to the “concordia line”, and the distance between the two are difficult to distinguish. Therefore, data underwent single alteration with Pb-loss or U-loss along the “discordia line” will appear to be concordial in the analytical errors. These data could be dispersed or concentrated clusters similar to those in Figure 1d and Figure 1e. In these cases, the concentrated data clusters are often treated as concordial to calculate concordial ages or weighted mean ages. These cases are absolutely wrong interpretations. It is common in practical uraninite and pitchblende U-Pb isotopic dating, which we need to stay alert to any of such mis-interpretations. Xiao (2022) also noted this phenomenon, and point out that the in-situ analyzed U-Pb data on pitchblende and uraninite will locate on the concordia line although they lost some radiogenic Pb. Because the analytical errors of in-situ measurement are greater than the deviation of discordia line to the concordia line. The similar phenomenon in zircon was also illustrated by Spencer et al. (2016), in which they showed that when original zircon age is 300 Ma and suffer Pb-loss, 45 of 50 data points seems to be Concordia, and when the original zircon age is 100 and 50 Ma, all the 50 data points seems to be concordia in analytical errors.

2 APPLICATION AND COMMENTS ON PRACTICAL CASES

In recent years, there are many published in-situ U-Pb isotopic data of uraninite and/or pitchblende that were used to confine ages of uranium mineralization. However, it would be misleading and confuse the understanding to uranium mineralization events in regional geological evolution, without critical evaluation and reasonable understanding to these U-Pb data. In light of the theoretical models proposed above, some practical cases are re-evaluated in assisting to understand the U-Pb isotopic data of uraninite and/or pitchblende reasonable and correctly.

2.1 Comment on U-Pb Isotopic Data of the Mianhuakeng Uranium Deposit in Zhuguang Area, North Guangdong Province, China

The Mianhuakeng deposit is a large granite-hosted vein type uranium deposit in South China. The mineralization is bounded in faults and fractures trending NNW, and occurs as pitchblende and secondary uranium minerals both related to mainly micro-crystalline quartz veins and minor with fluorite and calcite veins. The regional and deposit geology was documented in Zhong et al. (2023, 2019, 2018) and Zhang and Wang (2023).

Recently, Zhang and Wang (2023) presented LA-ICP-MS U-Pb isotopic data of uraninite from the Mianhuakeng deposit. They got lower intercepted ages in ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb plots at 62.2 ± 2.9 and 1.8 ± 0.3 Ma, and interpreted them as the uranium mineralization ages. When their data are plotted in ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U space (Figures 2a, 2b), they form fake discordia lines that in accordance with the theoretical models of Situation (4) and Figure 1f, in which the data plots are resulted from firstly Pb-loss overlapped by secondly Pb-loss. The fake discordia lines in Figure 2a and Figure 2b have both lower intercept ages (59.3 ± 8.4 and 1.32 ± 0.65 Ma) nearly identical to the lower intercept ages in ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb plots at 62.2 ± 2.9 and 1.8 ± 0.3 Ma (Zhang and Wang, 2023). Meanwhile, The fake discordia lines also show obvious much older upper intercept ages (1 613 ± 580 and 2 501 ± 980 Ma). These data distributions are in good agreement with the theoretical models in Figure 1f, which suggested that these data are results suffered twice Pb-loss. Therefore, the ages have no geological significance, and cannot be treated as correct mineralization ages.

Zhong et al. (2018) also dated the Mianhuakeng deposit by LA-ICP-MS U-Pb isotopes of uraninite, and got two weighted mean ²⁰⁶Pb/²³⁸U apparent ages at 60.0 ± 0.5 Ma (MHK1519) and 59.5 ± 1.0 Ma (CJ16153). When these data are plotted in the ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U space (Figures 2c, 2d), they show no reasonable geological significance. In Figure 2c for sample MHK1519, these data also give a fake discordia line with only a lower intercept age at 60.0 ± 1.1 Ma, which is identical to the weighted mean ²⁰⁶Pb/²³⁸U apparent age of Zhong et al. (2018) at 60.0 ± 0.5 Ma, and show no upper intercept age. The data of MHK1519 show two clusters in Figure 2c, with one cluster near or just on the concordia line, and another cluster below the concordia line. The former data cluster can be explained by the theoretical model of Situation (3) and Figure 1d, which is resulted from firstly Pb-loss overlapped by secondly U-loss. The several data points in the former cluster and those in the later cluster are all below the concordia line, which can be explained by the theoretical model of Situation (4) and Figure 1f that are formed by twice Pb-loss. Similar explanation is also suitable to the sample CJ16153 in Figure 2d. Therefore, the weighted mean ²⁰⁶Pb/²³⁸U apparent ages are neither concordia nor upper intercepted ages. They are not suggested to represent reasonable uranium mineralization ages. Besides, the author’s unpublished zircon fission track dating results show no thermal events at ~60 Ma in the Mianhuakeng deposit. The reliable uranium mineralization ages of the Mianhuakeng deposit were ~70 Ma (Huang et al., 2010), which can be confirmed by the author’s unpublished zircon fission track dating results.

2.2 Comment on U-Pb Isotopic Data of the Xianshan Uranium Deposit in Xiazhuang Area, North Guangdong Province, China

The Xianshi uranium deposit is a widely dated uranium deposit in the Xiazhuang uranium camp in South China. Details of its reginal geology and deposit geology were documented in Zhang et al. (2022), Pei et al. (2021) and Luo et al. (2015).

Recently, Zheng (2019) and Zhang et al. (2022) dated the ages of Xianshi uranium deposit separately by fs-LA-ICP-MS, SIMS and LA-ICP-MS U-Pb system of uraninite. Both the fs-LA-ICP-MS data of Zheng (2019) and LA-ICP-MS data of Zhang et al. (2022) form two Fake discordia lines in ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U space (Figure 3a). Both the two Fake discordia lines have lower intercepted ages and abnormal old upper intercepted ages. These data distributions can be explained by the theoretical models of Situation (2) and Figure 1c, and/or Situation (4) and Figure 1f. They are resulted in either from firstly U-loss overlapped by secondly Pb-loss or from firstly Pb-loss overlapped by secondly Pb-loss. Besides, Zheng (2019) also got SIMS U-Pb data of the Xianshi deposit, and treated the two weighted mean ages at 73.1 ± 2.9 and 46.1 ± 1.9 Ma as mineralization ages. When these data are plotted in ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U space (Figure 3b), they show two clusters both near or just on the concordia line. These data distributions can be explained by the theoretical model Situation (3) and Figure 1e, which are resulted from firstly Pb-loss overlapped by secondly U-loss. Another scenario would be also responsible for some of the data adjacent to the concordia line. It is the single alteration Pb-loss model in young ages that form the discordia line that cannot be distinguished from the concordia line in analytical errors. Whatever, these data cannot be treated as meaningful geological ages unless supported by independent dating results.

2.3 Comment on U-Pb Isotopic Data of the Xiwang Uranium Deposit in Xiazhuang Area, North Guangdong Province, China

The Xiwang deposit is another extensively dated uranium deposit in South China. Details of its reginal geology and deposit geology were documented in Zhang et al. (2022). Recently, Zheng (2019) and Zhang et al. (2022) dated the ages of Xiwang uranium deposit separately by fs-LA-ICP-MS, SIMS and LA-ICP-MS U-Pb system of uraninite. The data of Xiwang deposit from Zhang et al. (2022) formed a line in ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb plots and gave a lower intercepted age at 50.5 ± 1.7 Ma which is identical to the lower intercepted age at 51.8 ± 2.1 Ma when they are plotted in ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U space (Figure 4a). However, these data in Figure 4a form a fake discordia line, with a lower intercepted age and an abnormal old upper intercepted age, which can be explained by the theoretical models of Situation (2) and Figure 1c, and/or Situation (4) and Figure 1f similar to the scenario of Xianshi deposit in Figure 3a. Zheng (2019) got different uraninite U-Pb isotopic data by SIMS and fs-LA-ICP-MS method. Her SIMS U-Pb data are all near the concordia line, and were grouped into two clusters to present two weighted mean ²⁰⁶Pb/²³⁸U apparent ages at 42.3 ± 2.0 and 31.2 ± 2.2 Ma. However, these data can be explained by the theoretical model Situation (3) and Figure 1e, and/or the single alteration Pb-loss model in young ages similar to the scenario in Xianshi deposit in Figure 3b. The fs-LA-ICP-MS U-Pb data of Zheng (2019) are all below the concordia line, and show similar distribution to those of the Mianhuakeng deposit in Figure 2a and Figure 2b, although they don’t form a line to present intercepted ages. These data can be explained by the theoretical models of Situation (4) and Figure 1f, in which the data are resulted from firstly Pb-loss overlapped by secondly Pb-loss. Therefore, neither the intercepted ages nor the weighted mean ²⁰⁶Pb/²³⁸U apparent ages have geological significance unless they are supported by independent dating results.

3 CONCLUDING REMARKS

The in-situ U-Pb isotopic data of uraninite and pitchblende are easily to suffer complex and multiple alterations duo to their weak blocking of U and Pb in uraninite and pitchblende crystal structures. The correct interpretation to these data is crucial in understanding their geological significance. Several theoretical models are proposed to better distinguish the situations of U-Pb isotopic modifications of uraninite and pitchblende. Based on these theoretical models, several published data on uranium deposits in South China are re-interpreted. The theoretical models and comments on these examples show that the in-situ U-Pb isotopic data and their lower intercepted ages, weighted mean apparent ages should be evaluated critically to avoid mis-understanding of their geological significance. Independent dating results of uranium deposit are powerful in supporting the validity of U-Pb isotopic ages when attributed them to be reasonable uranium mineralization ages.

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