Source and Enrichment of Toxic Elements in Coal Seams around Mafic Intrusions : Constraints from Pyrites in the Yuandian Coal Mine in Anhui , Eastern China

Pyrite, a mineral that can cause potential environmental issues in coal mining, is commonly found in coal seams around intrusions. In this paper, pyrites from the Yuandian Coal Mine (Huaibei Coalfield, Anhui, Eastern China) were studied using SEM, Raman and LA-ICP-MS. The pyrite morphologic and geochemical data suggest that (1) four pyrite generations are present (framboidal sedimentary pyrites (Py I) in the original coal, coarse-grained magmatic pyrites (Py II) in the intruding diabase, fine-grained metamorphic pyrites (Py III) in the intrusive contact aureole, and spheroid/vein hydrothermal pyrites (Py IV) in the cokeite); and (2) concentrations of cobalt, nickel, arsenic, selenium, lead and copper in the metamorphic pyrites are much higher than the other pyrite generations. We propose that mafic magmatism is the main contributor of the toxic elements to the intrusion-related cokeite at Yuandian.


Geological Setting
The Yuandian Coal Mine is located in the Northern Anhui Province, Eastern China (Figure 1a).The coal-bearing formations are located in the intersection of the Subei fault and the Fengxian-Guoyang fault (Figure 1b), hosted in upper Palaeozoic clastic and carbonate sequences [14].Eleven layers of coal seams are distributed in the Carboniferous Taiyuan and Shanxi groups and the Permian Shihezi group [9].Many diabasic-gabbroic sills have been documented in these coal seams [20].For seams in the middle part of the stratigraphy, for example, coal seam VIII, intrusion-related cokeite is found interlayered with the diabasic-gabbroic sills.Within coal seam VIII, there are residual (unaffected) coal seams (2.97 m thick) in the lower part of the seam (Figure 1c).The maximum vitrinite reflectance is about 1.53-2.06%,the ash content is about 8.2-14.7%, the volatile matter is about 7.83-8.37%,the moisture content is about 0.59-0.72%,and the carbon content is about 77.7-83.1%.For the intrusion-related cokeite (>0.95 m thick) in the upper part of the seam, the ash content is about 19.5-24.9%, the volatile matter is about 6.58-7.39%, the moisture content is about 0.54-0.66%,and the carbon content is about 64.7-76.5%.

Occurrence of Pyrite
In the coal mine, intrusion-related layered cokeite from coal seam VIII is in contact with micro diabasic sills along a narrow metamorphic aureole (Figure 2a).The diabase comprises plagioclase (>85%) and pyroxene (<15%) (Figure 2b), with the grain sizes changing progressively from medium-fine to very fine near the chilled margin (Figure 2c).The cokeite is composed of porous coke, mosaic carbon and microspheric carbon with high reflectance and anisotropy (Figure 2d).Pyrites in the diabase, cokeite and metamorphic aureole appear very differently-pyrites in the diabase occur as large subhedral grains (>200 µm in size) or thin rims around plagioclase (Figure 2e); pyrites in the metamorphic aureole are mainly fine-grained (<30 µm) and scattered around plagioclase (Figure 2f).Pyrites in the cokeite occur either as bands (>500 µm long and several microns wide) or spherical aggregates (20-100 µm) (Figure 2g).

Analytical Procedures of Pyrite
Pyrite samples were collected from boreholes in the Yuandian Coal Mine, and thin sections were prepared for the Raman spectroscopic analysis at the Instrument Analysis and Research Center of Anhui University.The samples were also observed using SEM (scanning electron microscopy) and their chemistry was determined using LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) at the Laboratory of Sample Solution in Wuhan.A scanning electron microscope (JSM-IT100, JEOL Ltd., Tokyo, Japan), coupled with an energy-dispersive X-ray spectrometer (X-MAX020, Oxford Instruments, Oxford, UK), was used to study the mineral morphology and elemental distribution.The samples were carbon-coated using a Quorum Q150T ES sputtering coater.The working distance of the SEM-EDS was 10 mm, the beam voltage was 20.0 kV, the aperture was 6 nm and the spot size was 5 nm.The images were captured via a retractable solid state backscattered electron detector.Laser Confocal Raman Microspectrometry (LC-Raman, InVia-Reflex, Renishow Co., Wotton-under-Edge, UK) was used to determine the spectral characteristics of pyrites, excited by the 532 nm line of an argon ion laser with 25 mW power and using a 50 µm slit of the Renishow InVia Micro-Raman instrument.The intrinsic resolution of the spectrometer was 1.0 cm −1 and calibrations were accurate to 0.15 cm −1 , with the scanning range being 100-500 cm −1 .The LA-ICP-MS system comprised an Agilent 7700 ICP-MS (Agilent Technologies, Santa Clara, CA, USA) coupled with a Resonetic RESOLution M-50 ArF-Excimer laser source (λ = 193 nm) (Coherent Inc., Santa Clara, CA, USA).The laser energy was 80 mJ with a frequency of 10 Hz and an ablation spot size of 44 µm.Both a double-volume sampling cell and a Squid pulse smoothing device were used to improve the data quality.The ablated aerosol was carried to the ICP source with He gas.Trace elements contents were calculated using the software ICPMS DataCal (version 10.2) (developed by the China University of Geosciences).The detection limits of most trace elements were 2-8 ppb, except for some elements, including Cr, Ni, Cu and Zn (detection limit: 1-5 ppm).The accuracy of the analysis was mostly better than 5%.

SEM
SEM back scattered electron (BSE) imaging of the pyrites from the studied samples appeared very differently.Pyrite crystals and pyrite veinlets in the diabase can be divided the coarse-grained and veinlet types (Figure 3a).The former is euhedral-subhedral (>200 µm), whilst the latter occurs as fine veins (<20 µm wide) around plagioclase (Figure 3b).While the boundaries between the coarse-grained pyrites and plagioclase are straight, those between the vein pyrites and plagioclase are irregular (Figure 3c).Pyrite crystals and pyrite veinlets in the metamorphic aureole can be divided into the fine-grained and veinlet types (Figure 3d).Fine-grained pyrites were mainly distributed in the groundmass of the diabase chilled margin, but not in either cokeite or phenocrysts in the chilled margin (Figure 3e).Only a few vein-type pyrites were found in the rim of mesocarbon microbeads in cokeite.Pyrite crystals and pyrite veinlets in cokeite occurred mainly as veinlets or spherical aggregates (Figure 3f).Pyrite veins commonly converged into bands of 500 µm long and several µm wide (Figure 3g).Spherical pyrite aggregates mainly filled in the pores of cokeite and encapsulated a large number of mesocarbon microbeads (MCMB) (Figure 3h).Pyrites in unaffected coal distal from the diabasic intrusion occurred dominantly as framboidal aggregates with a diameter of about 10 µm (Figure 3i).There were no fissures, and pore filled pyrites were found in the unaffected coal.

Raman
The Raman spectra of pyrites from the diabasic intrusion, metamorphic aureole, cokeite and unaffected coal are shown in Figure 4.The figure shows that three distinct Raman active modes, matching the previously published activity of Fe-[S 2 ] 2− Liberational Motion (Eg), Fe-[S 2 ] 2− Stretching Motion (Ag) and S-S Stretching Motion (Tg), are observed in all diagrams.However, there were clear differences in terms of the spectral intensity, frequency and shape.In order to facilitate identification, all Eg modes and Ag modes were incorporated into a correlation diagram (Figure 4) and transformed into Raman shift (∆ν), mode intensity (I) and half width at half maximum (FWHM) data by Lorenz fitting (Table 1).
The features of the above results can be summarised as follows: (1) the intensity ratio of Eg to Ag of pyrites increased significantly from the intrusion, via metamorphic aureole and cokeite, into unaffected coal.In addition, the Tg modes showed a weak and broad shoulder in all of the diagrams.( 2) Almost all of the Raman frequencies of Eg (343.0-344.2) and Ag (377.4-380.6)shifted towards high frequencies relative to the shifts of Eg (343) and Ag (377) in the previously published Renishaw database.Additionally, there was an increasing trend of the FWHM values of Eg and Ag with increasing distance from the intrusion.

LA-ICP-MS
The analytical results are shown in Table 2.The elements analysed include the total contents of iron and sulfur (FeS 2 ) and siderophile (V, Cr, Co, Ni) and sulphophile elements (As, Se, Bi, Sb) and other metals (Pb, Zn, Ag, Cu, REE).Most of these elements (Co, Ni, As, Se, Pb, Cu, Ag) had higher enrichment in the pyrites from the diabasic intrusion and metamorphic aureole than in those from the cokeite and coal.However, exceptions were encountered in some siderophile elements, for instance, the V content was very low (<4.2 ppm) in all of the pyrite types, but the Cr content was much lower in the pyrites from cokeite (0.32-1.2 ppm, average 0.64 ppm) than in the pyrites from the other occurrences (0.47-60 ppm, average 7.7 ppm).Both Co and Ni were not only strongly enriched in the pyrites from metamorphic aureole (Co = 148-1498 ppm and Ni = 93-1093 ppm) but were also strongly enriched in the pyrites from diabasic intrusion (Co = 2.3-416 ppm and Ni = 7.3-1051 ppm).The Co/Ni ratio of the pyrites from metamorphic aureole (1.0-2.9) was significantly higher than those of the pyrites from other occurrences (0.01-0.83).In addition, the contents of As (1.2-43 ppm) and Se (14-698 ppm) in the pyrites from metamorphic aureole were many times higher than those in the pyrites from other occurrences.Similarly, the Bi (0.03-28 ppm) and Sb (0.73-22 ppm) contents were higher in the pyrites from metamorphic aureole than from other occurrences.Many metallic elements were more enriched in the pyrites from diabasic intrusion and metamorphic aureole (especially the latter), for example, the Pb and Cu contents were 72-989 ppm and 9.2-478 ppm, respectively.In addition, REEs were strongly enriched (LREE = 0-258 ppm, HREE = 0-484 ppm) in the pyrites from unaffected coal.

Relative Age of the Different Pyrite Generations
Pyrites in the diabase appeared as coarse-grain euhedral-subhedral crystals and fine veinlets, with the former commonly embedded in plagioclase and the latter extended around plagioclase.This indicates that the former is likely co-magmatic and the latter, late/post-magmatic [46][47][48][49][50][51].In the metamorphic aureole, fine-grained pyrites were distributed mainly in the groundmass of the diabasic chilled margin, and only a few pyrite veinlets were found in/around the rims of mesocarbon microbeads in the cokeite.This indicates that the fine-grained pyrites were formed coevally with the groundmass [48][49][50][51][52][53], whilst pyrite veinlets were formed after the mesocarbon microbeads, which were post-magmatic as they crosscut the chilled margin [47][48][49][50][51]. Pyrites in the cokeite consisted mainly of veinlets and spherical aggregates, with the latter filling in the pores of cokeite and encapsulating large amounts of MCMB.This suggests that the pyrites in the cokeite were also likely formed after the mesocarbon [48][49][50][51][52][53][54].Pyrites in the unaffected coal were dominated by framboidal aggregates, indicating that the pyrites were likely early diagenetic stages [51][52][53].In summary, the pyrites from the Yuandian Coal Mine can be divided into four generations (Table 3), namely, the framboidal pyrites (Py I) in unaffected coal, coarse-grained pyrites (Py II) in diabase, pyrite veinlets (Py III) and fine-grained pyrites (Py III) in the metamorphic aureole, together with the pyrite veinlets (Py IV) and spherical pyrite aggregates (Py IV) in the intrusion-related cokeite.
Our results show that the pyrite grains from the intrusions, aureole and cokeite differ in trace element contents (those of the pyrite from unaffected coal were mostly below the detection limits).It is suggested that high Co/Ni ratios (>1) and high Se contents (>50 ppm) may reflect the hydrothermal genesis of pyrite [60][61][62][63].In our work, the Co/Ni ratio in Py III was >1 (Figure 5a) and the Se contents were 72.37-989.27ppm (Figure 5b), indicating that Py III may have been crystallized from magmatic-hydrothermal fluids [60][61][62][63].All evidence from the Raman spectra and trace elements above indicates that the pyrites in the diabase were likely of magmatic origin, whilst those in the metamorphic aureole may have been magmatic-hydrothermal fluid-derived, whilst the pyrites in the cokeite were likely of metasomatic origin.In brief, the genesis of pyrites at Yuandian included syngenetic-type (some metamorphosed along with the coalification) in the unaffected coal, magmatic-type (high temperature and pressure) in the diabasic intrusion, hydrothermal-type in metamorphic aureole and metasomatic-type with low temperature and low pressure in cokeite.

Geochemical Evolution of Pyrite
Pyrite compositions reflect the physicochemical conditions of the magmatic/hydrothermal/ sedimentary system in which the pyrites are formed [63-65], thus the morphological and chemical changes of the four Yuandian pyrite generations can shed light on the migration and enrichment of different elements across the diabasic intrusions and the coal seams.Our results show that most siderophile and sulphophile elements analyzed are markedly enriched in the pyrites from the intrusion and metamorphic aureole.Concentrations of Co, Ni, As, Se, Pb and Cu of the pyrite crystals and veinlets from the metamorphic aureole were much higher than those in the intrusion, but they were both higher than their counterparts in the cokeite and unaffected coal.This suggests that the pyrites in the metamorphic aureole were likely crystallized from a fluid rich in these elements [63][64][65].These elements (Co, Ni, As, Se, Pb and Cu) are likely magmatic-derived [64][65][66][67], as they were also enriched in the magmatic Py II (Figure 6).Meanwhile, these elements were relatively depleted in the pyrites from cokeite, suggesting that either these elements were scavenged by Py III before the fluids reached the cokeite to precipitate Py IV or that Py IV was precipitated from a fluid derived largely from the unaffected coal [63,64].The geochemical data points to the toxic elements in the coal seams around the intrusions at Yuandian, notably Co, Ni, As, Se, Pb and Cu, mainly originating from regional mafic magmatism [66][67][68][69].

Conclusions
In this study, four pyrite generations in the Yuandian Coal Mine were identified, namely the framboidal syngenetic pyrite crystals in the unaffected coal, coarse-grained magmatic pyrites in the diabasic intrusions, fine-grained pyrites in the metamorphic aureole, and spheroid void-filling pyrite aggregates in the cokeite.The geochemical analyses of pyrite crystals in the studied coals indicate that the concentrations of toxic elements (notably Co, Ni, As, Se, Pb and Cu) in the pyrites from the metamorphic aureole were much higher than those from other occurrences.This suggests that the toxic elements in the Yuandian coal seam are derived by hydrothermal fluids that originate from the intrusions within the basin.

Figure 5 .
Figure 5. Binary diagrams of Ni, Se versus Co contents for different pyrite from the Yuandian Coal Mine.(a) Ni versus Co contents for different pyrite; (b) Se versus Co contents for different pyrite.

Figure 6 .
Figure 6.Histograms of element abundance in the different pyrite generations from the Yuandian Coal Mine.(a) Siderophile elements; (b) sulphophile elements; (c) metallic elements.

Table 1 .
Raman spectral data of pyrites.

Table 2 .
LA-ICP-MS geochemical results (ppm) of the pyrites from the Yuandian Coal Mine.

Table 3 .
Occurrences of the different pyrite generations from the Yuandian coal mine.