Geochemistry of two high-lithium content coal seams, Shanxi Province, China

https://doi.org/10.1016/j.coal.2022.104059Get rights and content

Highlights

  • High Li-coals of two sites in Shanxi Province in China were presented.

  • Differing degrees of maturation seem to have little affect on the association of trace elements with inorganics.

  • The relationship between Rb vs Y + Nb was employed to distinguish the provenance of the sediment.

Abstract

Prospecting for trace and rare earth elements will only increase in the coming decades as the world expands into sustainable energy and the use of digital technology. A key component of finding these resources is understanding how they are distributed, mobilized and emplaced. The coal beds in Shanxi Province in China have already been identified as a resource with elevated trace elements, especially lithium. This study has extended the understanding of trace elements in the region from examination of two sites, one in the Ningwu coalfield (the No. 11 coal seam) and the other in the Qinshui Basin (the No. 8 coal seam). Lithium, in particular, but most trace elements, in general, were found to be associated with the inorganic fraction of the coal seams. Specifically, trace elements are highest in the sediments overlying the coal and in the interbedded partings. The coal beds themselves were also enriched, especially in Li, but their concentrations vary in direct proportion with ash yield. Differing degrees of maturation (the Ningwu site ~0.70% Ro; the Qinshui site ~2.50% Ro) seem to have little effect on the association of trace elements with inorganics. Using the relationship between Rb vs Y + Nb, the ultimate provenance of the sediment (and thus the trace elements) may be from the weathering products of regionally proximal granites. However, two high ash coal outliers, both from the Qinshui Basin sampling site, appear to have a different trace element source, as yet unidentified, compared to all other samples. The REY (rare earth elements and Y) patterns indicate hydrothermal fluids are also responsible for REY enrichments in the coal seams.

Introduction

From the beginning of this decade, the world changed significantly, both from the effect of the worldwide pandemic but also in terms of a global commitment to renewable energy. A key factor in securing sustainable and environmentally kind energy, however, is critical elements. Some of these elements, like copper, can occur in large, concentrated abundance and thus be mined at significant scale as ore (Berger et al., 2008). However, elements to run our batteries, smart phones and other electronics in our computer-based societies only occur in trace amounts, often in parts per million or even billion within the Earth's crust (Kennedy, 1999). To supply these important and critical elements, the search has expanded from conventional into unconventional ore bodies. Coal and coal-bearing sediment are two of those unconventional ‘ores’ (Sun et al., 2013a; Qin et al., 2015; Dai and Finkelman, 2018; Hu et al., 2018; Finkelman et al., 2019; Dai et al., 2021).

Coal and the palaeoenvironments within which they form are natural attractors for trace metals and rare earth elements and yttrium (REY) (Seredin and Dai, 2012). Organics and fine-grained inorganic sediments are especially good at ‘soaking-up’ these elements, either at the time of peat accumulation and deposition or later from basin brines after burial (Zhao et al., 2019; Dai et al., 2020a; Li et al., 2020). Equally, however, elements can be transported into the mire system from sediment-source regions (Dai et al., 2006).

The coal basins in Shanxi Province in China are well known for their elevated levels of critical elements, especially, but not limited to, lithium (Zhang et al., 2004; Sun et al., 2013b; Wang et al., 2019; Zhao et al., 2019; Liu et al., 2020; Di et al., 2022). The coal beds are known to have high proportions of critical elements but less is known how these elements are distributed, their stratigraphic distribution within the coal, and how they are emplaced and their ultimate origin. Thus, the objective of this study is to document trace element distribution in two locations, the Xinjing coalmine (XJ-8) in the northern Qinshui Basin and the Dongloutian open-pit mine (DLT-11) in the Ningwu coalfield, both in the Shanxi Province (Fig. 1) where the coals are of significantly different rank due to the influence of nearby magmatic activity (Yang et al., 1988). Coals at XJ-8 exhibit relatively high rank (anthracite), while DLT-11 coals are lower (bituminous). Coals in these two locations were reported to have high Li and other critical metals contents according to preliminary studies (Liu et al., 2020; Di et al., 2022). The mechanisms of emplacement and their provenance will be investigated.

Section snippets

Geological background

Pennsylvanian coals are widely distributed in Shanxi Province, north China, where there are six main coalfields (Datong, Ningwu, Hedong, Xishan, Huoxi, and Qinshui) (Fig. 1). The coal-bearing strata throughout Shanxi Province include the Pennsylvanian Benxi and Taiyuan Formations and the Lower Permian Shanxi Formation (Fig. 2). The Taiyuan Formation is composed of interbedded coal, siltstone, mudstone and limestone and is thought to have formed adjacent to lagoonal and carbonate platform

Sampling and analytical methods

Two coal beds, the Nos. 8 and 11 in the Taiyuan Formation were sampled on a bench-by-bench basis, which were differentiated based on macroscopic appearance. The sampling mines included Dongloutian open-pit mine of Pingshuo mining district in Ningwu coalfield and the Xinjing coalmine of Yangquan mining district in northern Qinshui Basin (Fig. 1). The sampling intervals are shown in Fig. 2 and thickness in Table 1. In the Dongloutian open-pit mine, eleven samples were collected from the No.11

Coal quality

Proximate analyses, total sulfur and vitrinite reflectance are given in Table 1. Both locations have samples that range from high to low ash yield. High-ash yield coal at both locations is generally >30% (dry basis [db]) whereas lithotypes designated only as coal range in ash yield from 6 to 23% (db). The volatile matter content of samples with <23% ash yield (db) for XJ-8 and DLT-11 range from 9 to 12% (dry, ash free [daf]) and 32–43% (daf), respectively. Total sulfur in samples from XJ-8 and

Discussion

Several trace elements at both the XJ-8 and DLT-11 sampling sites are enriched relative to average world hard coal (Ketris and Yudovich, 2009). However, it is Li, which is the most enriched for both coal and non-coal samples. It is well established that Li is high in coal seams throughout the basins and coalfields in the Shanxi Province, with concentrations exceeding 900 ppm in some samples (Liu et al., 2020 and references therein). The source of the Li is not clear, but we infer it to be

Conclusions

Analysis of coal, high-ash coal, parting and roof samples from two locations in the Shanxi Province were conducted to determine trace element distribution and to make some initial inferences on provenance has yielded the following conclusions:

  • 1.

    Most trace elements are associated with the inorganic fraction associated with coal seams at the two locations XJ-8 and DLT-11. Stratigraphic variation in trace elements, particularly Li, is a reflection of variation in the proportion of inorganics.

  • 2.

    Li is

CRediT authorship contribution statement

Beilei Sun: Conceptualization, Methodology, Validation, Funding acquisition, Writing – review & editing, Writing – original draft. Fangui Zeng: Resources, Supervision. Tim A. Moore: Formal analysis, Writing – review & editing, Writing – original draft, Visualization. Sandra Rodrigues: Formal analysis, Data curation, Software. Chao Liu: Writing – review & editing. Guoquan Wang: Formal analysis, Data curation, Visualization, Investigation.

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

It is with sadness that we dedicate this paper to our co-author, colleague and mentor, Prof. Fangui Zeng, who passed away recently. He was a humble, kind and highly intelligent man and will be hugely missed. This research was supported by the National Key Research and Development Program of China (2021YFC2902002), Shanxi Province Science and Technology Major Project (20191102001), and the National Natural Science foundation of China (Nos. U1810202 and 41872177). The authors would like to thank

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