Li–Nb–Ta mineralization in the Jurassic Yifeng granite-aplite intrusion within the Neoproterozoic Jiuling batholith, south China: A fluid-rich and quenching ore-forming process

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Highlights

  • Yifeng granites and aplite dikes are related to Li-Nb-Ta mineralization.

  • The Nb–Ta mineralization of the Yifeng granite took place at 144 ± 5 Ma.

  • (Cs-bearing)-lepidolite, Mn-rich CGMs and montebrasite are present in the aplite.

  • The aplite is chemical and mineralogical analogue of the typical P-rich rare-metal granite.

  • The Li-Nb-Ta metallogenic belt occurs possibly in the northwest Jiangxi Province.

Abstract

The Neoproterozoic Jiuling batholith in northwestern Jiangxi Province, south China, is one of the largest batholiths in the Nanling Range, and it is intruded by Mesozoic granitic intrusions associated with mineralization. The Jurassic Yifeng granitic pluton intrudes to the southern part of the Jiuling batholith and comprises Li-rich muscovite (Li-Mus) granite, trilithionite (Tri) granite, and aplite dikes. The granitic rocks are enriched in rare metals (Li–Rb–Cs–Nb–Ta). We conducted whole-rock geochemical analyses, undertook columbite U–Pb dating, and examined the textures and chemistry of feldspars, micas and the accessory minerals montebrasite, columbite-group minerals (CGMs), microlite, cassiterite, and zircon. All three granitic rock types have high Li contents (up to ~6200 ppm), and the δ7Li values of the Li-Mus granite and aplite are −3.55 ± 0.14‰ and −5.34 ± 0.04‰, respectively. The isolated grains of CGMs and cassiterite in the aplite are magmatic products, but intense fluid activity in the later stages and quenching resulted in unique mineral aggregates of cassiterite + CGMs + zircon. The Nb–Ta mineralization of the Li-Mus granite took place at 144 ± 5 Ma according to columbite U–Pb dating, and this age is similar to that of the neighbouring Yichun granitic pluton to the south. Moreover, both the Yifeng and Yichun plutons contain P-rich rare-metal-bearing Li–F granitic rocks that contain P-rich feldspars, montebrasites (amblygonites), Cs-rich lepidolites, CGMs close to the Mn endmember, and Hf-rich zircons. It is inferred that a metallogenic district characterized by Li–Nb–Ta mineralization exists in northwestern Jiangxi Province, thus providing a reliable guide for further exploration.

Introduction

Numerous Jurassic granitic plutons in the Nanling Range of south China are generally composed of highly evolved, peraluminous granites associated with extensive W–Sn–Nb–Ta mineralization (Chen et al., 2008, Hua et al., 2003, Mao et al., 2008). Examples are the Yichun, Songshugang, Dajishan, Jiepailing, and Laiziling plutons (Li et al., 2015, Wu et al., 2017, Xie et al., 2016, Xie et al., 2018, Zhu et al., 2015). Dikes that intrude into the granitic plutons may reveal important information on hidden granitic chambers at depth and on the unique magmatic–hydrothermal systems associated with these shallow dikes, and examples include the Qigulingling rhyolite dike (Xie et al., 2015) and the Xianghualing No. 431 ongonite and topazite dike (Huang et al., 2015). Previous studies on the petrogenesis and mineralization of the rare-metal granitic rocks indicates that the enrichment of some typical elements (Li–F and Rb–Cs) and the textures and chemical compositions of minerals such as zircon, micas, and the oxide minerals could be used to constrain the fractionation of the granitic systems and the differentiation of the rare metals (Badanina et al., 2015, Borodulin et al., 2009, Cuney et al., 1992, Linnen and Keppler, 1997, Van Lichtervelde et al., 2007). Lithium, as an essential rare metal, commonly appears in the Li–F-bearing micas of these granites, and not only does its high content in a granite reflect the highly evolved nature of that granite, but it also points to the presence of other important volatile components that assisted in the enrichment of other rare metals in the magma, including Nb and Ta (Van Lichtervelde et al., 2007). Cesium is the element with the largest ionic radius in the same group 1A of the periodic table as Li, and primary Cs-rich minerals such as pollucite, londonite, and Cs-rich lepidolite are found mainly in highly evolved granitic rocks (Rao et al., 2009, Wang et al., 2007).

The northwestern part of Jiangxi Province is an important Li–Nb–Ta–Be metallogenic region in China where the Yichun, Xiushui, and other rare-metal deposits are found (Wang et al., 2018, Yin et al., 1995). The Yifeng deposit is also located in this region, near Yifeng County, including Nanmukeng, Baishuidong and Tong’an from north to south,. It is about 100 km north of the renowned Yichun granite which is associated Ta–Nb–Li mineralization. Several aplite dikes in the area have been exploited for ceramic materials because of the extremely fine grain size and high degree of whiteness of the dikes. These dikes intrude into Neoproterozoic metasedimentary rocks (Shuangqiaoshan Group), Neoproterozoic biotite granodiorites (Jiuling batholith), and the Late Jurassic two-mica granites (Guyangzhai intrusion). The dikes share numerous similarities with the Yichun granite such as high contents of rare metals and the presence of Cs-rich micas. However, these features have not been studied thoroughly (Zhang et al., 2016). Furthermore, Li-rich granites have been found in drill holes through the dikes, and in mining galleries.

Therefore, in the present study we focus on the Yifeng granite and aplite dikes. We used U–Pb isotope dating of the columbite group minerals (CGMs) to determine the age of the Nb–Ta mineralization. Combining the chemical and textural data for the micas, zircons, and oxide minerals within the granite and the shallow aplite dikes, we have been able to reveal in detail the petrogenesis of the rocks and the nature of the rare-metal mineralization in terms of magmatic fractionation and hydrothermal alteration. Our work also indicates that the relationship between the Yifeng granitic rocks and the Yichun granite requires further research.

Section snippets

Geological setting

The Nanling Range in southern China is well known for its hosted metallogenic districts. Granite bodies of Yanshanian (Jurassic – Cretaceous) age make up ~30% of the exposed area (i.e., >64 × 103 km2; Zhou et al., 2006) and they are commonly associated with extensive Sn–W–Nb–Ta mineralization (Hua et al., 2003, Xu and Zhu, 1988). The Neoproterozoic Jiuling batholith (ca. 820 Ma) in northern Jiangxi Province is one of the largest granitic batholiths in the Nanling Range, and it crops out over an

Analytical methods

Whole-rock major element compositions of the samples were measured using wet chemical analytical methods at the Analysis Center of the No. 230 Research Institute of the China National Nuclear Corporation (CNNC), Changsha, China. By using the approaches of the Chinese National standard protocol GB/T 14506-2010DZG93-05 (for detailed information, see Xie et al., 2015), all the reported element concentrations have errors less than 5%. Trace-element and REE concentrations were measured using a

Petrography and geochemistry of the Yifeng granite

The hand specimens of both Li-Mus granite and Tri granite are gray–white in color. The two types of granite have similar textures with a grain size of 0.5–1.0 mm, and their main minerals are quartz (40–45 vol%), albite (25–30 vol%), K-feldspar (~15 vol%), and mica (Li-rich muscovite or trilithionite, 10–15 vol%) (Fig. 3a–d). The albite is commonly subhedral with obvious polysynthetic twinning.

The aplite is fine grained and white in color (Fig. 3e). The main phenocryst minerals are quartz,

Feldspars

The K-feldspars and albites in the Yifeng granites are close to pure end-member compositions (Table 2) with the K-feldspars containing 2.2 mol% albite on average, and the albites containing just 0.45 mol% anorthite. The aplites contain albites with <0.42 mol% anorthite, and K-feldspar is rarely found. It is noteworthy that all the albites in these rocks have a high P2O5 content of ~0.44 wt% (Table 2).

Topaz

The topaz is abundant (~8 vol%) in the aplite as subhedral crystals in the groundmass (Fig. 4

U–Pb age of columbite-group minerals

From the Li-Mus granites we selected large, inclusion-free columbite grains that were greater than 40 μm in size. Nineteen spot analyses yielded a lower intercept age of 144 ± 5 Ma (2σ, MSWD = 0.1; Fig. 10; Table 9). Unfortunately, the CGMs in the aplites were too small for analysis.

Highly evolved granitic system in the Yifeng pluton

The Yifeng granitic pluton includes three types of granitic rock: Li-Mus granite, Tri granite, and aplite. The whole-rock compositions demonstrate that the Yifeng samples have high Al2O3 contents and ACNK (1.3–2.4) values, but extremely low total REE concentrations (~1 ppm) and Zr/Hf and Nb/Ta ratios. The characteristic minerals of these rocks include Li-rich minerals (Li-Mus, Cs-bearing-lepidolite, and montebrasite), Nb–Ta-bearing oxide minerals, and Hf-rich zircons. Such minerals are

Conclusions

  • (1)

    The Yifeng pluton is composed of three rock types: Li-rich muscovite granite, trilithionite granite, and aplite. They are all highly evolved and rich in rare metals (Li–Rb–Cs–Nb–Ta–Sn), and they are characterized by minerals such as Cs-rich lepidolite, montebrasite, Mn-rich CGMs, microlite, and cassiterite. U–Pb dating of the CGMs indicates an age of 144 ± 5 Ma for the Nb–Ta mineralization.

  • (2)

    The whole-rock and mineral compositions demonstrate that the Yifeng aplites are more evolved than the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the National Key R & D Program of China (Grant No. 2016YFC0600203) and the National Natural Science Foundation of China (Grant Nos. 41572045, 41672065 and 41230315). We appreciate Dr. Fangrong Zhang for his assistance in the field, Liang Li with the assistance with the Li-isotope analyses, and Prof. Robert Linnen for his helpful discussion. Prof. Karel Breiter and two anonymous reviewers are thanked for the constructive reviews.

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