New Data and Evidence on the Mineralogy and Geochemistry of Wulantuga High ‐ Ge Coal Deposit of Shengli Coalfield, Inner Mongolia, China

: The mode of occurrence and origin of highly ‐ enriched trace elements, especially Ge, in Wulantuga high ‐ Ge coal deposit have been widely reported. In this study, coal samples and several coalified trunks embedded within the roof strata are collected, which provides a good opportunity to further confirm if Ge is mainly associated with organic matter. Minerals in coal samples are mainly quartz, kaolinite, montmorillonite, pyrite, and gypsum, along with trace albite, barite, chlorite, and Fe ‐ oxide, while those in coalified trunk samples include melanterite, pyrite, and gypsum, with traces of chlorite and magnesiocopiapite. Germanium, As, W, Sb, Hg, Be, and Cs are enriched in coal samples, and these elements are also enriched in the coalified trunks and roof glutenite. The elevated contents of Ge, As, W, Sb, and Hg were almost exclusively derived from the influx of hydrothermal fluids as evidenced by the presence of pyrite veins and chamosite as well as enhanced elemental associations of Ge ‐ W and As ‐ Sb ‐ Hg in the studied lignite samples. The coalified trunks in the study area should be taken into consideration due to the high contents of hazardous elements that cause potential environmental impacts during mining waste disposal and land reclamation. Funding Investigation, X.Z., B.L., N.M. and F.Z.; C.Y.; Writing–original C.Y.; Writing– X.Z.


Introduction
Critical elements, including rare earth elements, in coal have attracted significant attention in recent years due to their increasing prevalence in industrial technologies and their contributions to the economy [1]. Previous studies have found a number of coals being considered as an economic source of scores of critical elements, including Ge, Ga, U, V, Se, rare earth elements and Y (REY or REE if Y is not included), Sc, Y, Nb, Au, Ag, platinum group elements (PGEs), and Re, as well as some base metals, such as Al and Mg [1][2][3][4][5][6]. Further, there are cases where the industrial utilization of critical elements from coal has been successful, such as for Ga, Ge, Se, U, and V. Other critical elements, including Nb, Au, Ag, Re, Al, Mg, and REY, are considered to have prospects for the recovery from coal deposits [1].
The above studies on the Cretaceous Wulantuga coal deposit reported very high concentrations of Ge, exceeding 1500 mg/kg in some of the samples [7,9,10,15,18,[31][32][33][34][35]. These studies also reported co-enrichments in a number of elements, such as W, As, Sb, Hg, F, Be, Cs, Tl, Au, Ag, and PGEs, among others. These enrichments were mostly attributed to hydrothermal solutions affecting coal deposits.
With the process of mineral exploitation, the mining area of the Wulantuga open-pit mine has been expanded northward. In previous studies about coals from Wulantuga, both high-Ge and low-Ge, were sampled in the original area of the Wulantuga coal deposit [7,9,10,15,18,[31][32][33][34][35]37]. Now that the mining area has been expanded, it is necessary to investigate the property of coals in the new mining area. During geological surveys of the Wulantuga high-Ge coal deposit, we discovered large amounts of coalified trunks embedded within the roof glutenite of a high-Ge coal seam, which provides a good opportunity to further confirm if Ge is mainly associated with organic matter. In this paper, we report the mineralogical and geochemical data of coal and coalified trunks in the new mining area and put emphasis on the mode of occurrence, lateral distribution, and enrichment mechanism of Ge. Alternatively, the potential for resource recovery and environmental impact of coalified trunks are also be discussed.

Geological Setting
The Shengli Coalfield was deposited in the fault-controlled Shengli sub-basin of the Erlian Basins in northeastern Inner Mongolia [38,39], which is 45 km long and 7.6 km wide, with a total area of 342 km 2 ( Figure 1). The Wulantuga Ge-coal deposit only covers an area of 2.  The sedimentary sequences in the Wulantuga high-Ge coal deposit include Silurian, Devonian, Permian, Late Jurassic, Early Cretaceous, Neogene, and Quaternary strata (Figure 2), and detailed information of these strata has been described by previous studies such as Dai et al. [9], Du et al. [10], Sha [40], and Li et al. [31].
Tectonically, the high-Ge coal deposit is located within a monocline structure with an NW dip and is limited by normal faults on the N and S boundaries [21]. The coal seams extend NW from the eroded SE outcrop of the deposit across an N-dipping structure. The structural conditions allow the Wulantuga deposit to be worked in open pits. The deposit is surrounded on the NW by Quaternary basalts, on the SW by Hercynian granodiorites, and on the SE by Hercynian diorites (Figure 1).

Sampling
A total of fourteen samples was collected from the high-Ge Wulantuga open-pit coal mine in the Shengli Coalfield (Figures 3 and 4), including nine coal bench samples from the section of #6 coal (WZ-01 to WZ-09 from top to bottom), two glutenite samples from the roof of #6 coal (WLTG-02 and WLTG-04), two coalified trunk samples from the roof of #6 coal (WLTG-01 and WLTG-03), and a sample from mineralized infilling of a fracture of #6 coal (WLTG-PY). Each coal bench sample (WZ-01 to WZ-09) cut was about 10-cm wide and 10-cm deep ( Figure 4A). WLTG-03 was collected from the central part of the coalified trunk while WLTG-01 from the outer part of it ( Figure 4B,D,F). WLTG-02 was collected from the roof glutenite and WLTG-04 from the area adjacent to the coalified trunk ( Figure 4F). WLTG-PY was a pyrite sample collected from a fracture in the middle part of the No. 6 coal seam ( Figure 4A,C,E). All collected samples were immediately stored in plastic bags to minimize contamination and oxidation.  Subsequently, the samples were first crushed and ground to pass 1 mm sieve and then homogenized for maceral analysis. Then, the samples were crushed to powder samples (passing a 200-mesh sieve) for proximate analysis, geochemical, and mineralogical analyses. The powder coal with size < 1 mm was prepared and polished for the SEM-EDX analysis.
Maceral analyses were carried out in accordance with the classification of the International Committee for Coal and Organic Petrology (ICCP System 1994) [44][45][46] using conventional reflected-light, oil-immersion microscopy.
Mineral compositions of all samples were determined by means of powder X-ray diffraction (XRD). The reference intensity method was used to obtain semi-quantitative XRD estimation of the mineral contents in the studied samples [47]. A field emission-scanning electron microscope (QUANTA 200), equipped with an energy-dispersive X-ray spectrometer (EDX), was used to identify the morphology and modes of occurrence of minerals.
For major and trace element concentrations, a two-step acid-digestion method (first with HNO3 and secondly with mixtures of HNO3, HF, and HClO4) was used prior to determination, in order to retain any volatile elements of the studied samples in solution [48]. The resulting solutions were analyzed by inductively coupled plasma atomic-emission spectrometry (ICP-AES) for major element concentrations, and by inductively coupled plasma mass spectrometry (ICP-MS) for trace element concentration.
It is worth noting that due to the formation of highly volatile SiF6 during the digestion process and the consequent Si loss, Si concentrations were calculated through a mass balance method, combining the mineralogical and geochemical data, using Al as a reference [49]. The calculated Si and Al concentrations were respectively estimated by counting up the Si and Al concentrations in all minerals detected by XRD in the coal samples. The accuracy of this method was evaluated by the correlation of calculated Al concentration with that determined by ICP-AES.
The digestion of international coal reference material (South African coal reference material, SARM-19) and blanks were also carried out following the same procedure to check the accuracy of the digestion and the analytical methods. Mercury concentration was determined directly on solid samples using a LECOAMA 254 gold amalgam atomic absorption spectrometer (GA-AAS, LECO Corporation, St. Joseph, CA, USA).

#6 Coal
The minerals in #6 coal consist mainly of quartz (4.3%), and to a lesser extent, kaolinite (1.6%), pyrite (1.4%), and montmorillonite (1.9%), along with a trace of gypsum (0.7%) ( Table 3) ( Figure 5). In addition, trace amounts of albite, barite, chlorite, and Fe-oxide were also identified by SEM-EDX analysis ( Figure 6). Quartz occurs as a dominant constituent of mineral matter in almost all the #6 coal benches (Table 3) and is more abundant in the lowermost and uppermost coal benches compared to the middle ones ( Figure 5), probably indicating an influence of detrital matter input, as reported by Ward et al. [54].
Kaolinite ( Figure 6) occurs in the top and middle parts of the coal seam, while montmorillonite only in the upper part ( Figure 5) (Table 3). In a few cases, chlorite was also identified in some samples ( Figure 6). It contains a high Fe content and low Mg abundance, and thus is identified as chamosite. Previous studies concluded that chamosite is commonly formed in association with the coalification process [55][56][57][58], especially high coal rank coals. However, the studied coals are lignite and thus the chamosite may have formed by the interaction of kaolinite and Fe-Mg rich hydrothermal solution [59,60] or smectite and K-Al rich fluid [55].
Pyrite distributes randomly in the coal seam and shows an inverse variation with kaolinite throughout the coal section, except for samples WZ-08 and WZ-09 ( Figure 5). Pyrite and As-bearing pyrite were both observed by SEM-EDX ( Figure 6). Pyrite in the coals occurs mainly as individual euhedral crystals ( Figure 6C) and secondarily as framboidal aggregates ( Figure 6E) and disseminated particles ( Figure 6F), indicating its formation during early diagenesis stage [61,62]. In a few cases, pyrite is commonly found as dissolved pyrite crystals and agglomerate partially or totally oxidized to Fe-oxides ( Figure 6), probably as a result of oxidation of samples near the surface.
Gypsum occurs in all the coal seam and is randomly distributed throughout the coal seam ( Figure 5).

Coalified Trunks
Minerals present in the coalified trunk samples are mainly melanterite (16.4%), and to a lesser extent, pyrite (5.9%), and gypsum (2.0%) ( Table 3). Quartz is only present in the sample WLTG-03. Clay minerals such as kaolinite and chlorite are observed by SEM-EDX analysis, although below the XRD detection limit level (Figure 7). The contents of these minerals in WLTG-01 from the outer part of the coalified trunk ( Figure 4) are higher than that from the central part of the coalified trunk. From a macro perspective, these coalized trunks are obviously mineralized.
Pyrite in coalified trunks occurs as disseminated euhedral crystals (Figure 7), which are indicative of an early-epigenetic origin [61,62]. SEM-EDX analysis also evidenced the very frequent occurrence of globular Fe sulfide particles and high As-pyrite, and there are some crystal aggregates of these particles totally or partially dissolved with the morphology of pyrite (Figure 7). There are also partially dissolved pyrite crystals detected in melanterite (Fe-S-O) or magnesiocopiapite (Fe-Mg-S-O) (Figure 8). The occurrences of Fe-bearing minerals mentioned above indicate that pyrite in the coalified trunk may have been subjected to oxidization [63,64]. Elemental sulfur was also detected by SEM-EDX (Figure 7), which is another product from the oxidation of pyrite [36,62].
Melanterite is present at relatively high concentrations in the coalified trunks ( Figure 8 and Table 5). Melanterite is a hydrated ferrous mineral and occurs commonly in weathered coals as an oxidation product of pyrite [62,64]. The varying contents of melanterite in WLTG-01 (20.2%) and WLTG-03 (12.6%) indicate different weathering degrees between them. Under SEM-EDX analysis, melanterite, and to a lesser extent, Fe-oxides contain relatively high contents of As and Sb ( Figure 8). Furthermore, Mg-Al-bearing Fe sulfates were also detected by SEM-EDX (Figure 8). These sulfates were also reported in the previously-published literature [64].
Gypsum occurs as lath-like crystals within the organic matrix in the coalified trunk (Figure 8), indicating an epigenetic origin [65,66]. Syngenetic gypsum can also be found in coals influenced by sea water [67]. In this study, gypsum content in WLTG-01 is the highest (3.4%) compared to the other samples, probably because WLTG-01 is in the outer part of the coalified trunk and is most likely to be oxidized.
Chlorite, although below the detection limit of XRD analysis, in a few cases, is detected by SEM-EDX in the coalified trunk (Figure 7).

Major Elements
The concentrations of major and trace elements of the samples in this study are summarized in Table 4. The abundances of major elements of the studied coal samples are close to those of Chinese coals reported by Dai et al. [3] and of other Wulantuga coals (MY and M2, 3, and 4 profiles reported by Zhuang et al. [18] and Du et al. [10]).
Aluminum has a significant positive correlation with ash yield (r = 0.94) ( Table 5), indicating an inorganic affinity. Iron and S have a significant positive correlation (r = 0.97), and Fe/S ratio (0.81) is similar to the theoretical value for pyrite crystal (0.87), indicating they were most likely present in pyrite.
The coalified trunk samples are highly enriched in Fe, S, and Mg, which is in accordance with the high amounts of pyrite and Mg-bearing melanterite ( Figure 8). Aluminum, K, Ca and Na contents of WLTG-01 are higher than that of WLTG-03, which may be related to relatively high aluminum-silicate phases and gypsum contents in WLTG-01. The roof samples have relatively high Al, K, Na content, and relatively low Ca content compared to the coal samples.

Trace Elements
Thew concentration coefficient (CC = ratio of element concentration in investigated coals vs. world coals [68], which was proposed by Dai et al. [69]) is used in the study to determine the enrichment or depletion of trace elements (Figure 9). Compared to the average values for world coals [68], W is unsually enriched, with a CC > 100, while Ge, As, and Be are significantly enriched (10 < CC < 100). Hg, Sb, and Cs are enriched (5 < CC < 10), while Li, B, P, Sr, Y, Zr, and Lu are normal (0.5 < CC < 2). The remaining elements are depleted (CC < 0.5) compared to the world coals [68].
The coalified trunk sample from the central part (WLTG-03) contains very high Ge (1381 mg/kg), As (2889 mg/kg), Sb (1231 mg/kg), W (188 mg/kg), and Hg (15 mg/kg) contents. The content of Ge and As in the outer part of the coalified trunk sample (WLTG-01) is significantly lower than that in the central part of the coalified trunk, while the contents of Sb, Hg, Sr, Rb, Ba, and Pb are significantly higher than those in the central of the coalified trunk. The contents of Ti, V, Cr, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Se, Sn, Sb, Th, and Hg in WLTG-01 and WLTG-03 are significantly higher than the average contents in the coal seam profile in this study. A similar enrichment of a number of trace elements was also found in coalified trunks by Du et al. [10].
The roof sample (WLTG-02) is significantly enriched in Li, Sc, Ti, V, Cr, Ni, Ga, Rb, Zr, Nb, Sb, Cs, Ba, Pb, Th, and Hg when compared to the average value of the coal samples, and depleted in Be, B, P, Ge, As and W. Furthermore, the roof sample WLTG-04 adjacent to the coalified trunk contains higher contents of Ge, As, and Sb than another roof sample, WLTG-02, distal to the coalified trunk.
The modes of occurrence of trace elements in this study are similar to those reported by previous studies [9,10,18]: Germanium and W have a positive correlation coefficient with each other (r = 0.85), and they all have high negative correlation coefficients with ash (r = −0.72 and −0.73, respectively), indicating an organic affinity. Arsenic, Sb, and Hg have relatively significant correlation with ash (r = −0.31, −0.51, and −0.46, respectively) and slightly correlation with Fe (r = 0.31, 0.06, and 0.09, respectively), probably indicating a multiple source such as organic matter and pyrite, which is also attested to by the pyrite veins sample from the coal fractures (WLTG-PY) having the highest As content (3097 mg/kg). The correlation coefficients of Be-ash (r = 0.16) and Be-Ca (r = 0.63) suggest an association between Be and carbonate minerals. Cesium has a high correlation coefficient with ash (r = 0.69), suggesting an inorganic affinity.

Elevated Ge Contents
According to the isoline of Ge concentration in the Wulantuga Ge ore deposit from Du et al. [10], which is also shown in Figure 3, the average Ge content in the WZ profile from this study should be around 200 mg/kg. The average Ge content from coal bench samples in this study is 197mg/kg, which is in accordance with the fan-shaped distribution of Ge proposed by Du et al. [10].
There is a large difference between the macerals in the central and outer parts of the coalified trunk due to the high inertinite content in the outer part of the coalified trunk. In contrast, the concentration of Ge is much higher in the central part, indicating that Ge is most likely associated with huminite, which is confirmed by the correlation of Ge-huminite (r = 0.43) and Ge-inertinite (r = −0.45). This is also reported in previous studies [33,70]. For example, Etschmann et al. pointed out that Ge in low-rank coals is bound to the organic matrix by means of O functional groups [70], and the carboxylic acid and catechol functional groups as already suggested by Pokrovski et al. [71]. Wei et al. found that inertinite in two selected coal samples from the Wulantuga high-Ge pit mine have lower O and higher C compared to huminite, and Ge in these samples show a preferential enrichment in huminite relative to inertinite [33].
The previous studies have shown that the enriched Ge content in the Wulantuga coals was derived from the high-Ge hydrothermal solution [8][9][10]15,18,[31][32][33][34][35]37]. In the present study, the unusually elevated concentrations of Ge are most likely derived from the influx of hydrothermal solutions as attested by (1) the presence of pyrite vein in the fracture of coal seam; (2) elemental associations of Ge-W and As-Sb-Hg in coal samples; (3) the appearance of chamosite in studied lignite samples.

Elevated B Contents
Compared to other high-Ge coals in Wulantuga [9,10,18], the high-Ge coal in this study has a higher B content (94.3 mg/kg). Boron in the Wulantuga Ge-rich coals is considered to have an organic-inorganic mixed affinity [7,9]. In this study, the slightly negative correlations of B-ash (r = −0.15) and B-Ge (r = −0.14) (Table 5) also indicates an organic-inorganic mixed affinity of B. Further, the relatively high correlation of B-Fe (r = 0.64) and the high content of B in the pyrite vein sample (116 mg/kg) suggest that a portion of B is possibly associated with pyrite.
According to the previous study, the enriched B in Wulantuga coals were derived from acid water giving rise to elevated concentrations of B in both the Ge-rich and low-Ge coals in Wulantuga ore deposit [7]. In this study, boron was most likely derived from the Fe-rich solution involved in the pyrite formation, as confirmed by the relatively significant correlation of B-Fe (r = 0.64) and higher B content in the pyrite veins.

Environmental Impacts of Coalified Trunk
The coalified trunk contains significantly high contents of potential environmental hazardous elements (As, Sb, Hg, and S) in both the inner and outer parts of it (Table 4). During the fieldwork, the coalified trunks have been wasted and covered by a thin layer of sandstone in unexploited areas (Figure 4), and the coalified trunks once covered the mined coal have been wasted in a gangue dump near the open-pit mine. Previous studies have shown that both weathered coal gangue and burnt or unburnt waste dump from coal mine have potential impacts on the local environment [72,73] and human health [74]. However, the potential environmental impacts of coalified trunks in the Wulantuga Ge-coal deposit have never attracted attention. Although a great portion of pyrite in coalified trunks have been oxidized into Fe sulfates, there is still a certain amount of pyrite presented in coalified trunks (Table 3, Figures 7 and 8). Further, the amount of coalified trunks wasted in the gangue dump is considerable. Thus, the coalified trunks with significantly high contents of potential environmentally hazardous elements in the studying area should be taken into consideration during mining waste disposal and land reclamation.
In addition, the coalified trunk samples are significantly enriched in Ge (988.7 mg/kg on average). Considering the large amount of them wasted in the gangue dump and their high concentration of Ge, the coalified trunks may be another source of Ge-extraction in the Wulantuga open-pit mine.

Conclusions
The high-Ge Wulantuga #6 coal analyzed in this study is low-ash lignite. The macerals mainly include inertinite (59% on average) and huminite (40%), the former of which is mainly composed of semifusinite, fusinite, and inertodetrinite, and the latter of which is mainly gelinite and textinite. Minerals in coals consist mainly of quartz, kaolinite, pyrite, and montmorillonite, along with a minor amount of gypsum and traces of barite, chlorite, and iron oxides. Minerals present in coalified trunk samples primarily include melanterite, pyrite, gypsum, magnesiocopiapite, Al-Fe-Mg sulfates, iron oxides, quartz, kaolinite, and chlorite.
Compared to the averages of world coals, element Ge (23-492 mg/kg, 197 mg/kg on average), As (118-476 mg/kg, average 263mg/kg), and W (77-526 mg/kg, average 305 mg/kg), Sb (0.1-24 mg/kg, average 6 mg/kg), and Hg (0-2.5 mg/kg, average 0.7 mg/kg) are enriched in the Wulantuga #6 coal and highly enriched in the coalified trunks and roof glutenite. The elevated content of Ge is most likely associated with huminite and are most likely derived from the influx of hydrothermal solutions as indicated by the presence of pyrite veins and chamosite, as well as enhanced elemental associations of Ge-W and As-Sb-Hg in the studied lignite samples. Boron has a relatively higher content than other Ge-rich coals in Wulantuga high-Ge coal deposit and was most likely derived from the Fe-rich solution involved in the formation of pyrite.
Our findings have relevant implications, not only for the genetic patterns of the high Ge-W-As-Sb-Hg geochemical anomaly of the Wulantuga Cretaceous coals, but also for the potential of environmental impacts and Ge-extraction. This should be taken into account during the disposal of mining waste and land reclamation.