Distribution Of Elements And Enrichment Mechanism Of Mineral Matter In The Wuhai C-P Coals

Mineral matter and elements in coal are the products of the processes associated with peat accumulation and various geological processes, thus they can provide valuable information on the coal genesis, rank advance, and geological evolution. The geochemical distribution characteristics of elements and enrichment mechanism of mineral matter associated with coal from the Wuhai coalfield, which is located on the northwestern margin of the Ordos Basin, were studied using the knowledge of coal geology, coal petrology, mineralogy, petrology, element geochemistry and organic geochemistry base on the data of XRF, LAT-XRD-Siroquant and SEM-EDX.Three factors, including sediment-source region, seawater influence and multi-stage hydrothermal fluid injections, were responsible for variations in the geochemical and mineralogical compositions of the Wuhai coals.The sediment-source region for the Wuhai Coalfield is the Alxa plate, situated to the northwest of the Wuhai Coalfield, rather than the Yinshan oldland that supplied terrigenous materials for most Permo-Carboniferous coal-bearing areas in North China Coal Basin. The differences in concentration of Th and Li and SiO2/Al2O3 ratio between the Wuhai coals and the coals from the eastern Ordos Basin may thus indicate different sediment source regions during peat accumulation. The Al2O3/TiO2 ratio is the most useful provenance indicator of sedimentary rocks. The Al2O3/TiO2 weight ratios of the host rock and partings from the Wuhai Coalfield range from 20.47 to 49 with an average of 28.63. We estimate that the SiO2 content of the gneisses in the sediment source region was between 56.8% and 71.12% with an average of 66.42%, indicating that the relevant parent rock(gneisses) is felsic. The most widely used chemical index to ascertain the degree of source rock weathering is the Chemical Index of Alteration(CIA). The calculated CIA values for the host rock and partings from the Wuhai Coalfield range between 85 and 99 with a mean of 92, indicating extreme weathering and hot and moist depositional environment.The coals from the Wuhai coalfield are much higher in Li(55.72μg/g), Be(2.42μg/g), Ga(10.84μg/g), Ge(3.56μg/g), Zr(104.61μg/g), Hg(0.224μg/g) and F(156.82μg/g). Compared to Chinese average coals, the concentration coefficients of trace elements is 1.75, 1.15, 1.66, 1.28, 1.17, 1.37 and 1.21. Compared to world hard coals, the concentration coefficients of trace elements is 4.64, 1.51, 1.87, 1.62, 2.91, 2.24 and 1.78.The Wuhai coals have a weak Eu negative anomaly. This may be closely associated with terrigenous control and seawater influence. Negative Eu anomalies may have been inherited from the source region, and the impact of seawater reduced the extent of negative anomalies. The LREY and HREY have been slightly fractionated in the Wuhai coal, which is associated with the homogenization of seawater to rare earth elements and yttrium. HREY have stronger ability to form soluble complexes and can be more easily to dissolve and migrate in seawater in comparison with LREY, due to shorter ionic radius of HREY. The REY distribution patterns for most of coal benches are of H-REY type. This may be attributed to the influence of seawater and hydrothermal fluid and stronger organic affinity of the HREY than that of the LREY. Alxa ancient land consisting of acidic magmatic rocks and precambrian metamorphic basement is enriched in LREY and these REY was transported into coal basin by colloidal solution and surface water at peat swamp accumulations. Accordingly, the supplies of terrigenous detrital materials are dominant contributor for L-type distribution patterns of the Wuhai coal benches.Some Wuhai coal and host rock and partings may have been subjected to leaching by groundwater. The roof sample and parting SHT15-2P in the No.15 coal of the Suhaitu Mine and the roof sample and partings HBC12-6P and HBC12-8P in the No.12 coal of the Huangbaici Mine have been subjected to leaching by groundwater. The content of REY of roof sample and partings is lower than that in the underlying coal bench. The fractionation of HREY and LREY in the partings and roof sample is much higher than that in their underlying coal benches. Cerium can be oxidized to Ce4+ and could be precipitated in-situ in groundwater leaching process. This may cause the partings to be higher in Ce content and Ce/Ce* value than those in the underlying coal benches. Clay minerals and REY-bearing organic compounds are the main modes of REY occurrence for the Wuhai coal.Multi-stage hydrothermal fluids may lead to some trace-element re-distribution between coal benches and partings. The higher element ratios(Yb/La, Nb/Ta, and Zr/Hf) in the benches of the Wuhai coals, compared to the overlying partings, is attributed to the re-deposition of the first element in each pair, because of the relatively more active leaching of that element from the partings, and then deposition in the underlying organic matter. Such re-distribution led to higher element ratios in the Wuhai coal benches than seen on average in world hard coals.The phases identified from the LTA of Wuhai coal samples are mainly represented by quartz, kaolinite, illite, pyrite, marcasite, apatite, calcite, dolomite, ankerite and anatase; with trace amounts of boehmite, jarosite, gibbsite, anhydrite and bassanite in some samples.The high proportion of fluorine and P2O5 in the Wuhai coals is mainly related to the syngenetic apatite derived from hydrothermal fluids. Apatite is rare in the coal, but it is quite common in Wuhai coals. Apatite is the most abundant in the mineral matter of the W9-12 bench(91.4%) compared to all the other coal benches in the seam sections studied. The bulk of the apatite in Wuhai coals occurs, sometimes along with anatase, as infillings in the pore spaces and cell cavities of macerals. It apparently formed by authigenesis at a relatively early stage of coalification, before compaction had significantly closed up the available pore space. Local Ph in Wuhai coal basin at the time of apatite precipitation may have played a part, since apatite is usually only precipitated in neutral to alkaline conditions. The phosphorus in Wuhai coals was probably derived mainly from phospho-proteins in the organic matter of the original peat deposits.The fact that the sulfur isotope values of organic and pyritic sulfur in the No. 9 coal seam are relatively low indicates that there was vigorous bacteria activity during peat accumulation. The weak reducing and alkaline environment in Wuhai peat swamp favored bacterial action and propagation. The rod-like pyritized bacteria identified here with SEM-EDX have three characteristics:(1) pyritized bacteria monomer with needle-like shape is about 10 um below in length.(2) pyritized bacteria aggregates with radial shape have a diameter of around 20 um.(3) some rod-like pyritized bacteria looking like wheat ears with the length between 5-15 um fills in the cell cavities, which growed close to each other from outside to inside making up a small pyritic geode in the coal.