Mineralogical and Geochemical Characteristics of Late Permian Coals from the Mahe Mine, Zhaotong Coalfield, Northeastern Yunnan, China

This paper reports the mineralogical and geochemical compositions of the Late Permian C2, C5a, C5b, C6a, and C6b semianthracite coals from the Mahe mine, northeastern Yunnan, China. Minerals in the coals are mainly made up of quartz, chamosite, kaolinite, mixed-layer illite/smectite (I/S), pyrite, and calcite; followed by anatase, dolomite, siderite, illite and marcasite. Similar to the Late Permian coals from eastern Yunnan, the authigenic quartz and chamosite were precipitated from the weathering solution of Emeishan basalt, while kaolinite and mixed-layer I/S occurring as lenses or thin beds were related to the weathering residual detrital of Emeishan basalt. However, the euhedral quartz and apatite particles in the Mahe coals were attributed to silicic-rock detrital input. It further indicates that there has been silicic igneous eruption in the northeastern Yunnan. Due to the silicic rock detrital input, the Eu/Eu* value of the Mahe coals is lower than that of the Late Permian coals from eastern Yunnan, where the detrital particles were mainly derived from the basalt. Sn in the Mahe coals were mainly derived from the Kangdian Upland.


Introduction
Late Permian coals from the eastern Yunnan Province have recently attracted much attention, because of both the high female lung cancer rate caused by the indoor coal burning and the geological implication of mineral matter in the coals to the origin and evolution of Emeishan mantle plume.For the first aspect, Tian [1] and Tian et al. [2] found that the lung cancer risk was associated with crystalline silica released from the indoor coal burning.Dai et al. [3,4], Large et al. [5], and Wang et al. [6] observed high concentration of authigenic quartz (from nanometer to less than 20 μm in size) in the Xuanwei coals.For the second aspect, Dai et al. [3,4,7], Zhou et al. [8], and Wang et al. [6] conducted a systematic mineralogical and geochemical study of the Late Permian coals and tonsteins from the southwest Chongqing and eastern Yunnan and suggested that, after massive flood basalt eruption, the magma evolved from mafic to silicic or alkalinity.The previous studies mostly focused on the coals in the eastern Yunnan, only Dai and Chou investigated mineral compositions in the coals from the northeastern Yunnan [9].In this paper, we report the new data on the mineralogy and elemental geochemistry of the 5 coals in the Mahe mine, Zhaotong coalfield, northeastern Yunnan, China.

Geological Setting
The geological setting of the Late Permian coal basin from eastern Yunnan Province has been described in detail by several authors [9,10].The Emeishan mantle plume uplift and extensive flood basalt eruption resulted in the formation of the Kangdian Upland [11,12].Due to the existence of the Qinling sea trench to the north and Songpan basin to the west, the Kangdian Upland is the only possible source for the Xuanwei and Longtan/Changxing Formations in eastern Yunnan [12].The Mahe mine from the Zhaotong coalfield is situated in the northeastern Yunnan Province (Figure 1).The coal-bearing strata are mainly the Changxing Formation and the Longtan Formation of Late Permian age.The Changxing Formation, with a thickness of 47 m, is mainly made up of clastic sediments, including sandstone, mudstone, coal or limestone.It was deposited in a continent-marine transitional environment.There are four coal seams in Changxing Formation, named C1, C2, C3, and C4 in order from up to bottom, and most of them are too thin to be mined.The Longtan Formation has a thickness of 197 m and overlies the Middle Permian Xuanwuyan Formation (Figure 2).The Longtan Formation is comprised of mudstone, sandstone, gravel, and coal seams including C5, C6, and C7 coals.The No.5 coal is the major minable seam.Due to the less continuity of C6 and C7 coals in thickness, they could only be locally mined.
Mineral phases were investigated in coal briquette by optical microscopy and by a scanning electron microscope equipped with an Oxford energy-dispersive X-ray spectrometer (SEM-EDX, Hitachi, Tokyo, Japan), confirmed by X-ray diffraction (XRD, D/max 2500, Rigaku, Tokyo, Japan) on low-temperature ashes of coal (LTA).Low-temperature ashes of coal was performed using an EMITECH K1050 Plasma Asher (Quorum, Lewes, UK).XRD analysis of low temperature ashes was performed on a powder diffractometer with a Ni-filtered Cu-Kα radiation with a scintillation detector.The XRD pattern was recorded over a 2θ interval of 2.6°-70°, with a step size of 0.02°, and a 0.3 mm receiving slit.The X-ray diffractograms of LTAs of five coal samples were subjected to quantitative mineralogical analysis using Siroquant™ (Sietronics, Mitchell, Australia).Further details demonstrating the use of this technique for coal-related materials are given by Ward et al. [17,18] and Ruan and Ward [19].
Samples were crushed and ground by a shatterbox to pass 200 mesh for geochemical analysis.X-ray fluorescence (XRF) spectrometry (ARL Advant'XP+, ThermoFisher, Waltham, MA, USA) was used to determine the major element oxides including Al2O3, SiO2, Fe2O3, CaO, Na2O, MgO, K2O, TiO2, P2O5, and MnO as outlined by Dai et al. [20].Inductively coupled plasma mass spectrometry (X series II ICP-MS, ThermoFisher) was used to determine elements in coal samples as outlined by Dai et al. [7], except for Hg and F. Mercury was determined by DMA-80 (Milestone, Sorisole, Italy) in which samples are heated and the evolved Hg is selectively captured as an amalgam and measured by atomic absorption spectrophotometry.The detection limit of Hg is 0.005 ng, the relative standard deviation (RSD) from eleven runs on Hg standard reference is 1.5%, and the linearity of the calibration is in the range 0-1000 ng [21].Fluorine was determined by the ion-selective electrode method (ISE) following the procedures described in ASTM method D5987-96 [22].For quality control in fluorine determination, the standard reference materials GBW 11121 and GBW 11123 were analyzed with each batch of samples.

Minerals in the Mahe Coals
Minerals in the Mahe coals were observed by optical microscopy and SEM-EDX, confirmed by LTA + XRD.The quantitative XRD results from siroquant TM show that minerals in the Mahe coals are mainly comprised of quartz, chamosite, kaolinite, mixed-layer I/S, pyrite, and calcite, and followed by anatase, dolomite, siderite, illite and marcasite (Table 2).Quartz is the most common mineral in coal.Its concentration in the LTA of the Mahe coals varies from 8.9% to 50.4%, with an average of 38.2% (Table 2).Quartz in the Mahe coals has two origins: (i) authigenic and (ii) terrigenous or silicic volcanic ashes.The authigenic quartz is mainly present as disseminated irregular particles distributed in collodetrinite (Figure 3A), and is usually less than 20 μm in size.The microscopy observation shows that the authigenic quartz accounts for more than 90% of the total quartz.However, quartz of terrigenous origin, preserving well edges and angles and a completely euhedral crystal form (Figure 3B), occurs mainly as assemblages or discrete particles.It is larger in size (100 to 500 μm) than the authigenic quartz (Figure 3A) and the quartz of volcanic origin (20 to 100 μm) (Figure 3C).Based on the modes of occurrences, it is suggested that the source area of the terrigenous quartz is not far from the peat mire, because the frequently collision and friction suffered from a long distance transport would have resulted in a more rounded shape.β-form quartz is common in the Late Permian coals from eastern Yunnan, indicating a silicic volcanic ashes input [3,4,6].This is also the case in the Mahe coal samples.The β-form quartz in the Mahe coals is similar in shape and size with that in the Xinde and Taoshuping coals [3,6].It has a typical hexagonal form and a small size (less than 100 μm), and some of them occur as a doubly terminated bipyramidal form with an additional prism face (Figure 3C).Compared with common silicic volcanic ashes input indicated by the β-form quartz in the eastern Yunnan, mafic volcanic ashes input was only identified in the k21b coal from the Taoshuping mine [6].
Although chlorite is not common in coals [27], it was also observed in some high rank coals and formed by epigenetic processes [28,29].Mixed-layer I/S in coal could be altered to chlorite [27,30,31].Chamosite [Fe3 2+ Mg1.5AlFe0.5 3+ Si3AlO12(OH)6], a type of chlorite, is common in the Late Permian coals of eastern Yunnan [3,4,6,9].Chamosite is also present in all the Mahe coal samples (Figure 4), with a concentration from 2.3% to 14.4% (average 10.4%) (Table 2).The chamosite occurs mainly as cell-infillings alone (Figure 3D), sometimes exists with quartz and/or calcite.The coexisting of authigenic quartz and chamosite in the coals were mainly precipitated from the Fe-Mg-rich siliceous solution derived from the weathering of basalt, similar to the mechanism reported by Dai et al. [3] and Wang et al. [6].Dai and Chou [9] has shown that some chamosite is closely related to quartz and kaolinite in the Zhaotong coals, and was suggested to be derived from the interaction of kaolinite with Fe-Mg rich fluids during early diagenesis.Kaolinite and mixed-layer I/S are common in coal [32,33].Kaolinite was identified in the LTA of the samples MhC5a, MhC5b, MhC6a, and MhC6b, with concentrations of 26.6%, 13.6%, 54%, and 36.6%,respectively (Table 2).Kaolinite in the sample MhC2 is below detection limit of XRD and Siroquant.Mixed-layer I/S is present in all the LTA of the coal samples, ranging from 2.1% to 7.5% (average 4.1%), and much lower than those of kaolinite and chamosite (Table 2).Illite is only detected in the MhC2 sample.Kaolinite and mixed-layer I/S in the Mahe coals occur mainly as recrystallization particles, lenses, and thin beds, indicating a terrigenous origin.The kaolinite and mixed-layer I/S were derived from the weathering products of basalt from the Kangdian Upland [6,9].In addition, kaolinite as infillings of cells or fractures (Figure 3E) is similar to that in the Xinde coals, suggesting a authigenic origin [4].Mixed-layer I/S in the Changxing coals occurs not only as lenses or thin beds but also as infillings of maceral fractures.The mixed-layer infillings of fractures is of epigenetic origin and precipitated from hydrothermal fluids of a igneous intrusion [6].
Calcite is common carbonate in the Late Permian coals from eastern Yunnan.It occurs mainly as fracture/pores infillings, indicating an epigenetic origin of hydrothermal fluids [3,4,9].Although detrital calcite is very rare in coal because calcite can be easily-decomposed under acid conditions in the peat bog; however, syngenetic deposition of calcite (aragonite) is possible if a sediment source region mainly made up of carbonate rocks is located close to the peat mire [34,35].Detrital calcite was identified in the Taoshuping coals [6].The detrital calcite was probably blown by winds to the peat mire from the limestone of Middle Permian Maokou Formation [6], which underlies below the Xuanwuyan Formation.The content of calcite in the LTA of the Mahe coals varies from 4.9% to 21% (average 10%) (Table 2).It is mainly present as veins in macerals and display twin-striation characteristics under crossed polarized light (Figure 3F).In some cases, calcite fills the cells with quartz and/or chamosite.
Pyrite in the Mahe coals is dominated by disseminated fine or framboidal particles in macerals, and followed by massive particles of several micrometers in size (Figure 3G).Their modes of occurrences suggest a syngenetic origin [36].
Due to its low concentration, apatite was below the detection limit of XRD technique.It was only observed in the sample MhC2 under SEM-EDX.The apatite preserved distinct edges and angles, and has a big size of 220 μm in length and 60 μm in width (Figure 3H).However, apatite of silicic volcanic ashes origin in the Taoshuping coals is less than 5 μm in size [6].
The ratio of SiO2/Al2O3 in the Mahe mine ranges from 1.57 to 4.49, with an average of 2.24.It is much higher than that of Chinese average coals 1.42 [37] and the theoretic value of kaolinite (1.18).The higher value of SiO2/Al2O3 is attributed to the presence of abundant authigenic quartz.The CaO has a content of 0.61% to 3.14%, and the MnO has a content of 0.01% to 0.07%.CaO in the coals is mainly related to veins of calcite (Figure 3F).The significant relation coefficient between CaO and MnO (0.88) supports that they have a similar mode of occurrence.The concentration of Fe2O3 varies from 2.3% to 5.37%, with an average of 3.4%.The low positive relation coefficient between Fe2O3 and Sp,d ( , = 0.61) suggest that Fe is not only associated with pyrite, but also associated with chamosite.The average contents of K2O and Na2O are 0.32% and 0.14%, respectively.K2O and Na2O are probably attributed to mixed-layer I/S, and illite.Anatase is primarily responsible for TiO2 in the Mahe coals.
Although the average contents of potentially toxic elements F and Hg are lower than that of Chinese average coals [37], they are enriched in some coal samples.The MhC2 has a 246 μg/g fluorine, which is probably related to the P-bearing mineral apatite (Figure 3H).Fluorine in the Taoshuping coals also shows a significant positive relationship with phosphorous [6].Mercury in the Mahe coals varies from 254 to 320 ng/g (average 277 ng/g), which is much higher than that of the Xinde coals (average 44 ng/g).Because mercury in coal is usually related to sulfur-bearing minerals [39][40][41], the high total sulfur of Mahe coals (St,d = 2.86%, Table 1) is probably responsible for the elevated Hg, while the total sulfur of Xinde coal is only 0.16% [4].[37]; c, CC, the ratio of average elements in the Mahe coals to Chinese average coals [37]; d, Hg in ng/g.
The REY contents of the Mahe coals range from 100 μg/g to 336 μg/g as whole coal basis, with an average of 233 μg/g (Table 4).The Eu/Eu* value is from 0.78 to 1.02, with an average of 0.91 (Table 4).Three distribution types of REY are also present in the Mahe coal samples: L-type of MhC6a, M-type of McC2 and Mc5b, and H-type of MhC5a, respectively (Figure 5).The MhC6a shows an M-H type REY distribution pattern.In coal, L-type REY pattern is of terrigenous origin; M-type REY pattern is probably attributed to REY supply by hydrothermal solutions; and H-type REY pattern was resulted from the circulation of HREE-bearing natural water and finally adsorbed by organic matter [42].The L-type REY pattern of the sample MhC6a is also attributed to the terrigenous detrital input.Among the LTAs of the five coal samples, the McC6a has the highest content of kaolinite and mixed-layer I/S (illite) (57.3%), following by Mh6b (39.3%),Mh5a (28.7%),Mh5b (21.1%), and MhC2 (11.8%) (Table 2).Kaolinite and mixed-layer I/S as lenses and thin beds were derived from the weathering products of the basalt from the Kangdian Upland [6].The higher content of kaolinite and mixed-layer I/S (or illite) means that more terrigenous detrital input were received by the MhC6 coal during the peat mire accumulation than other coal seams.However, the lowest content of the kaolinite and mixed-layer I/S (illite) in McC2 suggests that the terrigenous detrital input has a less contribution to the mineral matter than the hydrothermal solution precipitation indicated by authigenic quartz and chamosite.Thus, the M-type REY pattern of MhC2 is mainly associated with the precipitation of the weathering Fe-Mg-rich solution of basalt.This is the similar situation for the sample MhC5a.The H-type REY pattern of MhC5a is attributed to its high content of organic matter (indicated by lowest ash yield 25.75%, Table 1).Because organic matter in coal favors HREE brought by the circulation of natural water [44].
However, in the northeastern Yunnan (Figure 1), mineral sources of the Mahe coals are attributed to not only (i) Emeishan basalt; (ii) silicic volcanic ashes; and (iii) hydrothermal fluids, but also Emeishan silicic rock.The kaolinite and mixed-layer I/S occurring as lenses and thin beds were derived from the weathering detrital residues of the basalt.It is also supported by the elevated concentrations of Sc, V, Cr, Co, and Ni in the Mahe coals, because Emeishan basalt are high in these elements [38].The abundant authigenic quartz and chamosite were precipitated from the weathering Fe-Mg-rich silicon solution of Emeishan basalt [6,45].The presence of β-form quartz in the Mahe coals suggests an input of silicic volcanic ashes during the peat mire formation.However, calcite as veins in the Mahe coals may have been precipitated by circulation of Ca-bearing meteoric fluids [46].
The euhedral quartz (Figure 3B) and apatite (Figure 3H) in the Mahe coals could not be attributed to the three sources discussed above.Firstly, quartz and apatite could not be the terrigenous detrital input of the weathering basalt residues.Because no quartz and apatite exists in the Emeishan basalt of the Kangdian Upland, which is mainly comprised of clinopyroxene, olivine, plagioclase, basaltic glasses or microlite, and magnetite [47].Secondly, the euhedral quartz is not precipitated by the weathering Fe-Mg-rich silicon solution of the basalt.The authigenic quartz is irregular in shape rather than a euhedral form.In addition, the authigenic quartz is smaller in size than the euhedral quartz (Figure 3A,B).Thirdly, the euhedral quartz and apatite are not attributed to the silicic volcanic ashes input.β-form quartz and apatite induced by the silicic volcanic ashes in the Late Permian coal from eastern Yunnan are much smaller in size (about 20-100 μm and less than 5 μm, respectively) [3,4,6] than that of the euhedral quartz and apatite in the Mahe coals (100-500 μm and 220 um in size, respectively) (Figure 3C,H).
In the Binchuan from the western Yunnan, the silicic rock occurring at the uppermost part of the basalt sequence means there were also small scale silicic igneous eruption after massive flood basalt eruption [48].Since quartz and apatite are common in silicic rock rather than basalt, it is inferred that euhedral quartz and apatite in the Mahe coals are probably related to the Emeishan silicic rock.However, the silicic sources of the Mahe coals is close to the peat mire in the eastern Yunnan rather than the faraway Binchuan area (more than 400 km between Binchuan and Mahe mine) from western Yunnan.Because the frequent collision and friction suffered from the the long-distance transport from western Yunnan would have resulted in more rounded shape of quartz and apatite.Thus, the euhedral quartz and apatite indicate there was silicic rock input during the Mahe coal accumulation.This further implies that there has been silicic igneous eruption in eastern Yunnan Province.
Implication of Eu/Eu* value also supports a silicic rock detrital input in the Mahe coals.As shown in Table 5, the Mahe coals show a slight negative Eu/Eu* anomaly, whereas other coals from the Xinde, Xuanwei, and Taoshuping mines have no or slight positive Eu/Eu* anomaly.The Eu/Eu* anomaly in coal, which is usually inherited from the parent rocks, is frequently used as an indicator of the terrigenous detrital sources [44,49].The Emeishan silicic rock has a strong negative Eu/Eu* anomaly with the value of 0.56, whereas the basalt shows a positive Eu/Eu* anomaly with the value of 1.41 (Table 5).The detrital particles of silicic rock input in the Mahe coals resulted in a lower Eu/Eu* value (0.91) of the Mahe coals than that of other Late Permian coals (1.04 of the Xinde coals, 0.99 of the Xuanwei coals, and 1.06 of the Taoshuping coals), in which the terrigenous detrital input was mainly derived from the weathering residue of basalt [3,4,6].[4]; Xuanwei from Dai et al. [3]; Taoshuping from Wang et al. [6]; Silicic rock is rhyolite from Xu et al. [48]; basalt from Xiao et al. [38].Number, sample number.

Conclusions
The Late Permian C2, C5a, C5b, C6a, and C6b coals from the Mahe mine are semianthracite with medium to high-ash yield and medium-high-to high-sulfur content.Minerals in the coals are mainly made up of quartz, chamosite, kaolinite, mixed-layer I/S, pyrite, and calcite, followed by anatase, dolomite, siderite, illite and marcasite.Compared with the Chinese average coals, the Mahe coals are high in Sc (4.38 μg/g), V (105 μg/g), Cr (45.7 μg/g), Co (19.0 μg/g), Ni (29.8 μg/g), Cu (70.4 μg/g), Ga (14.9 μg/g), and Sn (4.75 μg/g).The mineralogical and geochemical characteristics of the Mahe coals are attributed to four factors including Emeishan basalt, Emeishan silicic rock, silicic volcanic ashes, and hydrothermal fluid.The euhedral quartz and apatite particles in the Mahe coals mean that they were derived from silicic rocks.In addition, this further implies that there has been silicic igneous eruption in the northeast of Yunnan.

Figure 2 .
Figure 2. Stratigraphic column of the drill core from the Mahe mine, Zhaotong coalfield, northeastern Yunnan Province, China.

Figure 3 .
Figure 3. Minerals in the Mahe coals.(A) Disseminated irregular quartz particles in collodetrinite in the sample MhC2 (reflected light); (B) euhedral quartz in the sample MhC2 (reflected light); (C) β-form quartz of silicic volcanic ashes origin in the sample MhC2 (SEM, secondary electron image); (D) chamosite as cell-infillings in the sample MhC2 (SEM, secondary electron image); (E) kaolinite as cell-infillings in the sample MhC5a (SEM, secondary electron image); (F) calcite veins in the fractures of collotelinite with the twin-striation characteristics under crossed polarized light in the sample MhC2 (reflected light, oil immersion); (G) pyrite as disseminated or framboidal particles in the sample MhC2 (SEM, secondary electron image); (H) euhedral apatite in the sample MhC2 (SEM, secondary electron image).

Figure 4 .
Figure 4. Identification of minerals in the XRD pattern of the low temperature ashes (LTA) of the MhC2.

Figure 5 .
Figure 5. Distribution patterns of REY in the Mahe coal samples.REY are normalized by Upper Continental Crust (UCC) [43].
R o,ran , random vitrinite reflectance; M, moisture; A, ash yield; V, volatile matter; S t , total sulfur; ad, air-dry basis; d, dry basis; daf, dry and ash-free basis

Table 2 .
Mineral compositions of low-temperature ashes (LTA) of the Mahe coals by XRD analysis and Siroquant (%; on organic matter-free basis).

Table 3 .
Contents of oxides of major elements and trace elements in the Mahe coals from northeastern Yunnan, China (in μg/g unless as indicated).