| On the basis of comprehensive theories and methods of coal geology, coal petrology, petromineralogy, geochemistry and statistics, the distribution pattern and occurrence of minerals in coal/Tonstein from Upper Xuanwei Formation and tuff from Lower Xuanwei Formation were studied, the material composition characteristics of volcanic ashes with different sources were summarized, and the relationship between volcanic ash evolutionary sequence of Southwestern China and Emeishan mantle plume events were also discussed.Generally, the proportion of inertinite is lower than that of vitrinite in most coals, while their contents are close in coals from Laibin, Xuanwei, and the inertinite content even outweigh that of vitrinite in Bole coals. The inertinite of coals from these two sites are primarily fusinite, semifusinite and inertodetrinite, lepidophytofusinite can be observed, such a high content of inertinite implies that frequent conflagrations occurred in the peat swamps during Late Permian, it refers to volcanic activities of the period in southwestern area.The mineral and elemental composition characteristics of coal seams, host rocks and Tonsteins from Laibin and Bole are all different, which suggests their differences in origin. In general, the three factors that influence mineral enrichment are as follows:1) Hydrothermal fluid injection, including the syngenetic and the epigenetic,it can be reflected in the formation of coal seam. The dominant minerals in Laibin and Bole coals are authigenous, they exist as two types of occurrence: one is disseminated quartz in collodetrinite, these fine quartz particles are smaller than 10?m, some kept good quartz crystals, and there are framboidal pyrites and authigenic quartz particles distributed along the sedimentological layer; the other type of minerals was filled in the cells of fusinite and semifusinite, including quartz, berthierine/chamosite and calcite, besides, pyrite, florencite and kaolinite can be observed occasionally. Furthermore, a small amount of quartz exists in the fractures of organic matters, it suggests that these minerals were formed by hydrothermal fluid precipitation during the syngenetic/parasyngenetic stage. The cell-filling quartz and berthierine/chamosite precipitated from Fe- and Al-rich siliceous hydrothermal fluid, and extremely low kaolinite refers that the hydrothermal fluid were not from weathering solution of Kangdian Oldland. The calcites with twinning striation were filled in fractures, this implies its epigenetic hydrothermal origin. The significant enrichment of Si O2 and high value of Si O2/Al2O3 result from substantial authigenic quartz, the influence of hydrothermal fluid makes the REY pattern of most Laibin/Bole coals display a M-type/H-type enrichment with negative Eu anomaly.Tonsteins and host rocks suffered a mild influence of hydrothermal fluid, tonsteins were mainly influenced by epigenetic hydrothermal fluid, pyrite, chalcopyrite, Fe-oxysulfate and gypsum were filled in fractures, gersdoffite can be observed in kaolinite matrix. The kaolinite and berthierine/chamosite also exists as cell-fliing in host rock samples, and for B3-f sample, disseminated rhabdophane are distributed in kaolinite matrix or the surface of berthierine/chamosite, it suggests their authigenous origin.2) Volcanic ash input. It can be referred from the four layers of Tonsteins in Laibin/Bole coal seams and Bole floor C1-f. The Tonsteins in Laibin/Bole coal seams are primarily clay mineral-kaolinite, the content is 46-91.2%, only B1-3p sample contains 34.3% illite/smectite mixed layer, clay minerals exist as cryptocrystalline matrix, vermicular kaolinite and biotite pseudomorph also exist. The accessory minerals in Tonsteins include sharp angular quartz, anatase, zircon, monazite and apatite, the zircon crystal surface displays square or sharp angular, some contain bubble holes; for B1-3p sample, several apatite crystals can be seen, some are bamboo shaped, some cross sections are hexagonal, the mineral composition characteristics confirm the felsic volcanic ash origin, and the sediment environment is also acidic. The geochemical characteristics of Tonstein show an inadequate Ti O2, Sc and siderophile elements(V, Cr, Co and Ni) pattern, REY pattern shows significant negative Eu anomaly, these are typical features of acid rock. The XRD mineral composition, elemental composition and REY pattern characteristics of some Laibin/Bole roof samples(B2-00, C1-direct roof 1-2, C1-direct roof 1-1 and C1-direct roof 1) are similar to that of tonteins, it indicates their same source origin.The Bole floor sample(C1-f) is of high Ti O2, Sc and siderophile elements, and the content of Nb, Zr and some other elements are high than that of other roof and floor samples, it shows a similar feature with Emeishan high-Ti basalt-H-type enrichment with positive Eu anomaly, other roof and floor samples share a similar pattern. Kaolinite is the primary mineral according to XRD result of C1-f sample, which is as high as 84.4%, and anatase content is 9.9%; clay mineral kaolinite in C1-f sanmple exists as cryptocrystalline matrix through SEM observation, depositional layer can not be observed, large amount of anatase fragments are distributed in kaolinite matrix, high temperature fractures exist, the anatase is likely to be from volcanic shards, therefore C1-f sample can be regard as mafic tuffaceous argillite rock.3) Sediment-source region influence, normal sediment host rocks are mainly affected, coal comes second, and Tonsteins have little effect by source region. Emeishan basalts are the major terrigenous materials of the Laibin/Bole coals, geochemical evidence for this includes the enrichment of Sc and siderophile elements in coals and host rocks, and also high content of Ti O2 in the host rocks. The REY enrichment patterns in the host rocks are of the M-type and are characterized by positive Eu anomalies, is similar to that of Emeishan basalts; the other proof of influenced by sediment region is the REY distribution in part of coal samples, characterized by M-type enrichment and by an Eu-maximum or Gd-maximum. In addition, the terrigenous detrital quartz with certain roundness found in coals, and sedimentary layers of clay minerals and some plant fragments observed in B3-f from Laibin all due to normal deposition. The evidence shown that Tonstein was affected by the sediment region is the terrigenous quartz with certain roundness found in B3-5p.Based on mineralogical and geochemical characteristics the 56 samples of 701 drill hole from Xinde Mine can be divided into mineralized layer and non-mineralized layer, of which the mineralized layer was clay tuffaceous rock altered from alkali volcanic ash, and the non-mineralized layer was normal sedimentary clay rocks derived from the weathering products of Emeishan basalt.Whether belonged to mineralized layer or non- mineralized layer, clay minerals are the main mineral of all samples, which can be divided into two categories according to the main mineral is kaolinite or I/S mixed layer(illite). Berthierine is identified in most of the samples whereas berthierine/chamosite can be found in a small amount of samples, indicating the type of clay mineral is mainly influenced by the sedimentary environment regardless of the source of the sample, kaolinite was formed when the depositional environment is acidic, and illite or I/S mixed-layer formed in neutral or slightly alkaline environment. In addition, the variation trend of clay minerals appears to be kaolinite-illite-chlorite by the transformation of the late region diagenesis.The clay minerals in mineralized layers mainly exist as cryptocrystalline matrix, vermicular kaolinite can be seen, berthierine has two occurrence: one is distributed in kaolinite matrix as angular fragments, volcanic shards pseudomorph still remains; the other is flocculent or fibrous shapes, which results from authigenic origin. The berthierine/chamosite were mainly from Fe-rich hydrothermal fluid precipitation, EDX data illustrate that the Fe content in berthierine/chamosite is higher than that of berthierine. The quartz content in mineralized layers is quite low, they are mainly autogenetic and angular, and the latter indicates the volcanic origin. Titanium oxides commonly exist in mineralized layers, they can be categorized into three types: one is Nb-containing anatase, the second is Fe-containing anatase, and the last one is rutile, which is irregularly or network-shaped distributed in clay mineral matrix, and corrosion harbors can be seen, it further corroborates the volcanic origin of mineralized layers. The anatase is the only Ti-oxides exists in mineralized layers according to XRD results, which is 0.8-3.7%, its content in Group K3 and K4 is higher than that of Group K1 and K2, suggests that Nb and Fe were introduced into anatase through isomorphous substitution during the process of volcanic ash dissolution, and the crystal structures of anatase remained. In Group K3, intensive short prism shaped apatite is probably the product of P-rich parental magma crystallization differentiation. In addition, the mineralized layers were also affected by syngenetic and epigenetic hydrothermal fluid: Nb-minerals, zircon and florencite were formed through volcanic ash precipitation by hydrothermalism and sedimentation; while pyrite, gelenite and barite are epigenetic hydrothermal minerals. The most vital difference between non-mineralized and mineralized layers is that the former has higher anatase, which is 0.9-8.8%.The Ti O2/Al2O3 values of mineralized layer samples are between 0.02-0.08, whereas the samples of non-mineralized layer have a Ti O2/Al2O3 value of >0.08. The mineralized layer samples are significantly enriched in HFSEs(Nb, Zr, Th, U, etc.) and Ga, its content is 2-5 times the non-mineralized layer sampkes, which is the cause of positive anomalies of natural gamma curve for the mineralized layer. The Ti O2 and siderophile elements contents are lower in mineralized layer but to be higher in Group K3 and K4 than Group K1 and K2, and the non-mineralized layer are mainly enriched Ti O2 and siderophile elements. Thus, through the combination features of mineralogy and geochemistry suggests that the original material for the mineralized layer was derived from alkali volcanic ash, and the raw materials of different stages were also different, the mineralized layer from the research area performed an evolution trend of alkali-mafic to alkali-silicic, while the non-mineralized layer was derived from the weathering products of high-Ti basalts.Combined with previous studies we can find that volcanic ashes in the early Longtan/Xuanwei Formation were mainly of silicic, and the volcanic ashes in the late Longtan/Xuanwei Foemation were mainly of alkali, while the mafic volcanic ashes can be developed in the Late Permain of different layers in different regions, indicating the evolution difference of Emeishan basalts from different areas. Geochemical data shows that the alkaline ash are significantly enriched HFSEs and Ga, the mafic volcanic ash are rich in Ti O2 and siderophile elements, whereas these elements are all lower in the silicic volcanic ash. The origin of mafic ash maybe similar to the Emeishan high-Ti basalts and that the alkaline ash were similar to Emeishan silicic rocks. This study area of the dissertation is located in the eastern part of the Emeishan large igneous province, indicating this part were also evolved from high-Ti basalts to silicic rock. The deposition of Xuanwei Formation means the end of the main episode of flood basalts emplacement, therefore, the volcanic activity occurred after main phase of the Emeishan basalts, which is the product of the warning stages of Emeishan mantle plume or magma highly evolution. |
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