Geological Controls on Enrichment of Rare Earth Elements and Yttrium (REY) in Late Permian Coals and Non-Coal Rocks in the Xian’an Coalfield, Guangxi Province

Abstract

The highly elevated concentrations of the rare earth elements and yttrium (REY), U, Mo, Se, and Pb in late Permian coals in some coalfields in Guangxi Province, South China, have been widely investigated; however, the mode of occurrence and enrichment mechanism of these critical elements are still under debate. This study investigates the mineralogical and geochemical compositions of coals and non-coal rocks from the Xian’an Coalfield in Guangxi Province to discuss the geological factors influencing the distribution of critical elements. The mineral in the studied coals consists mainly of quartz, and to a lesser extent, muscovite and kaolinite, with a trace amount of anatase. The coals are significantly enriched in REY, Pb, Se, Mo, and U and display the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and U–Mo-enrichment horizon (Horizon II) adjacent to the host rocks or partings. The REY, U, Se, and Pb show organic association while Mo is primarily hosted by Fe-sulfides within Horizon I. The U and Mo have a phosphate affinity within Horizon II. Both the input of pyroclastic and epiclastic materials and the leaching of acidic solutions jointly govern the distribution of the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and the U–Mo-enrichment horizon (Horizon II). The concentrations of REY in Horizon I exceed the cutoff grade of REY, and, therefore, the coals in the Horizon I can be regarded as promising raw materials of REY.

Geological Controls on Geochemical Anomaly of the Carbonaceous Mudstones in Xian’an Coalfield, Guangxi Province, China

“…Some previous studies have indicated that the Yunkai Upland located to the east of the Heshan, Yishan, Fusui, and Xian'an Coalfields (Figure 1B) is the dominant sediment source region providing felsic detrital materials into the late Permian coal-bearing basins in Guangxi Province, China [5,6]. However, a few studies have demonstrated that some detrital materials within the late Permian coal-bearing strata in Guangxi Province are probably derived from the basaltic materials from the Kangdian Upland [25], the eroded materials from the felsic igneous rocks at the top of the Kangdian Upland [26,27].…”

“…Uranium, Mo, V, As, and Se Uranium, Mo, and V are highly enriched in layer A and layer B and their contents are comparable to the U-Mo-V-rich coals intercalated within carbonate successions [3,5,8,46]. The highly elevated U content is commonly accompanied by Mo and V in the Heshan coals [1][2][3], Fusui coals [5], Yishan coals [13] (anomaly), and Shanglin coals [6], which were deposited in isolated carbonate platform environments. In the previous study, the highly enriched concentrations of U, Mo, and V are primarily governed by the infiltration of syngenetic or early diagenetic low-temperature hydrothermal fluids rather than the detrital materials from the sediment-source region and marine influence as confirmed by the following evidence:…”

“…The generalized stratigraphic column of the late Permian coal-bearing strata withi the Xian'an Coalfield [6]; (B) sampling column of borehole SL.…”

“…Coals formed in marine carbonate platforms are primarily distributed in South China, including Heshan [1][2][3][4], Fuisu [5], Xian'an [6], Yishan [7], Guiding [8], Yanshan (Yunnan Province) [9], and Chenxi (Hunan Province) coalfields [10], of which the first four are located within Guangxi Province, South China. These coals mostly contain highly enhanced concentrations of sulfur (especially organic sulfur) and, thus are classified as super-highorganic-sulfur (SHOS) coals [11,12].…”

“…The Late Permian is an important coal-forming period in Guangxi Province, South China. In recent years, the late Permian coal-bearing strata intercalated in marine carbonate strata in Guangxi Province, South China, have attracted much attention mainly due to the elevated concentrations of some critical elements, such as rare earth element and yttrium (REY), U, Mo, Se, V, and so on [3,5,6]. For example, the coals from the Heshan and Yishan…”