Geological controls on the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons in late Permian coals from the Qiandongbei Coalfield, Guizhou Province, SW China

Highlights

• The studied coals are significantly enriched in REY-Zr (Hf)-Nb (Ta) assemblage.

• Detrital materials are derived from a mixture of felsic volcanic ash and eroded mafic materials.

• The joint influence of highly acidic solutions and volcanism governs the REY-Zr (Hf)-Nb (Ta) enrichment horizons.

• The volcanic ash erupted into peat bogs is geochemically similar to the peralkaline rhyolite.

Abstract

The abnormal enrichment of the REY (rare earth elements and yttrium)-Zr (Hf)-Nb (Ta) assemblage in Late Permian coals occurring in some coalfields in SW China has been extensively investigated, but the geological controlling factors of this assemblage are still under debate. This study reports on the mineralogical and geochemical features of coals and noncoal rocks collected from the Yudai and Jinqi coal mines within the Qiandongbei Coalfield in Guizhou Province to elucidate the geological controls on the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The studied coals show rather consistent clay mineral assemblage of kaolinite, together with varying amounts of quartz, gypsum, pyrite, and calcite. Notably, the coals are anomalously enriched in REY, Nb, Ta, Zr, and Hf and enriched in other incompatible elements (e.g., Ga, Th, and U). Phosphates (e.g., florencite) and, to a lesser extent, zircon are the primary carriers of REY. Zirconium and Hf are mainly incorporated into zircon and then anatase, the latter of which is a predominant carrier of Nb and Ta. The detrital materials originate from a mixture of the erupted felsic volcanic ash resembling a peralkaline rhyolite composition and the eroded materials of Emeishan Large Igneous Province (ELIP) basalts. The studied coals owe their highly enhanced concentrations of the REY-Zr (Hf)-Nb (Ta) assemblage to the input of felsic volcanic ash, wherein the highly acidic aqueous solutions allowed these elements to become mobile and be redistributed. Both geological processes governed the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The highly acidic solutions were possibly derived from the oxidation of peat bogs and volcanism-related acidic rain. The concentrations of REY, Zr, and Nb in the coal ashes exceed their corresponding cut-off grades; thus, these coals can be considered highly promising raw material sources for the REY-Zr (Hf)-Nb (Ta) assemblage.

Introduction

Critical elements, such as rare earth elements (REEs) and yttrium (REY), along with Nb, Ta, Zr, Hf, and Ga, have attracted much interest given their extensive application in modern electronics and other applications (Dai and Finkelman, 2018; Hower et al., 2016a; Lin et al., 2018a; Seredin et al., 2013; Seredin and Dai, 2012). Due to the growth of REY demand and the supply shortage of conventional types of REY ore, some countries are exploring new rare-metal deposits (Dai et al., 2014a; Dai and Finkelman, 2018). Fortunately, prior studies have demonstrated that coals may be thought to be a promising alternative source for some of these elements because the contents of some critical elements, in some cases, exceed the corresponding cut-off grade of conventional rare-metal deposits (Dai and Finkelman, 2018; Dai et al., 2012a, Dai et al., 2014a, Dai et al., 2018a; Hower et al., 2016a, Hower et al., 2020; Lin et al., 2018a, Lin et al., 2018b; Seredin et al., 2013; Seredin and Dai, 2012; Seredin and Finkelman, 2008).

Abnormal concentrations of the REY-Zr (Hf)-Nb (Ta) assemblage have been previously reported in some coal deposits around the world, especially in China (e.g., Dai et al., 2007, Dai et al., 2012a, Dai et al., 2013, Dai et al., 2014a, Dai et al., 2014b, Dai et al., 2016a, Dai et al., 2017a, Dai et al., 2018a; Li et al., 2019a; Liu et al., 2019; Zhao et al., 2019; Zhuang et al., 2012), Russia (e.g., Arbuzov et al., 2019; Nechaev et al., 2018, Nechaev et al., 2020; Seredin, 1996; Seredin and Dai, 2012; Seredin and Finkelman, 2008), and America (e.g., Hower et al., 1999, Hower et al., 2016b, Hower et al., 2018, Hower et al., 2020; Mardon and Hower, 2004). In most cases, the abnormal enrichment of REY-Zr (Hf)-Nb (Ta) has been almost entirely attributed to the input of alkaline or felsic volcanic ash, hydrothermal fluid influx, and terrigenous detrital materials (Dai et al., 2016a; Hower et al., 2016a, Hower et al., 2018; Liu et al., 2019; Zhao et al., 2019).

The Late Permian coals in SW China have drawn increasing attention because a few coal deposits contain highly enhanced contents of critical elements, especially REY-Zr (Hf)-Nb (Ta) assemblage (e.g., Dai et al., 2007, Dai et al., 2012a, Dai et al., 2014b, Dai et al., 2016a, Dai et al., 2016b, Dai et al., 2017a; Li et al., 2016, Li et al., 2017; Liu et al., 2019; Wang et al., 2019; Zhuang et al., 2012). Dai et al. (2016a) attributed the REY-Zr (Hf)-Nb (Ta) enrichment in the Late Permian coals in SW China to the input of alkaline volcanic ash that is intimately linked to the waning stage of the Emeishan mantle plume activity. Moreover, an influx of hydrothermal fluid was also considered to play a vital role in enhancing the contents of the REY-Zr (Hf)-Nb (Ta) assemblage in the Late Permian coals in SW China (Dai et al., 2014b, Dai et al., 2016b, Dai et al., 2017a; Li et al., 2017). Furthermore, an additional terrigenous detrital supply was also noted as a major cause of the enrichment of the REY-Zr (Hf)-Nb (Ta) assemblage in the Late Permian coals in SW China (Liu et al., 2019; Zhuang et al., 2012). Accordingly, with these interesting geochemical anomalies, the mineralogy and geochemistry of the coals in western Guizhou Province have been extensively examined (e.g., Dai et al., 2005; Li et al., 2016, Li et al., 2017; Liu et al., 2019; Zhuang et al., 2000) but rarely in the Qiandongbei Coalfield, NE Guizhou Province, possibly due to the presence of relatively thin coal seams in this region. There is abnormal enrichment of the REY-Zr (Hf)-Nb (Ta) assemblage in the coals from the Moxinpo, Songzao, and Guxu Coalfields (Dai et al., 2007, Dai et al., 2016b, Dai et al., 2017a), which are adjacent to the Qiandongbei Coalfield, but whether or not the above critical elements are enriched in the coals in the Qiandongbei Coalfield is still unclear. This study reports on the mineralogy and geochemistry of the Late Permian coals and host rocks in the Qiandongbei Coalfield and provides new insight into the source of felsic detrital materials, with an emphasis on the mutual influence of felsic volcanic ash and highly acidic aqueous solutions on the contents and distribution of minerals and high-field strength elements. It also offers an opportunity to determine whether the studied coals in the Qiandongbei Coalfield can be regarded as a promising source for certain critical elements.