Based on analysis of the geological background and gas content data in Faer coalfield, we discuss the occurrence characteristics of coalbed methane (CBM) and its distribution along the stratigraphic sequence and in coal seams. We analyze the methane adsorption isotherm experiments, reservoir deformations and mercury intrusion porosimetry (MIP), and further discuss the reservoir physical properties. The results show that gas content and gas content gradient of CBM fluctuate with stratigraphic sequence that is mainly caused by the difference in absorption capacity of each coal seam and distributions of CBM reservoir pressure systems. The relatively low gas content gradients of those coal seams buried below 500 m depth result from the dissipation of CBM into the surrounding rocks of thin and unstable Nos. 28 to 34 coal seams and the lateral migration of CBM along the strata in high-elevation area. The Yangmeishu syncline and topography are the main controlling factors that affect the current distribution pattern of CBM content, which is higher in the north than in the south and has a northeastern trend. The theoretical gas saturation is generally lower than 60%, combined with well-developed brittle fractures and macropores, formed by structural deformation. The exploration prospects for CBM in Faer coalfield is good.
The nature of magma sources, time frame, and geodynamic settings of the late Early Paleozoic mafic magmatic activities in the South Qinling Orogen is not clearly understood. The Ziyang area with concentrated and representative outcrops of the late Early Paleozoic mafic dykes was selected as a study area, and detailed petrology, whole‐rock geochemistry, isotopic geochemistry, and baddeleyite U–Pb chronology were combined to reveal the origin of the mafic dykes. Baddeleyite U–Pb chronological study shows that the formation age of the mafic dykes is 425.4 ± 6.4 Ma, indicating they were formed in the Late Silurian. The mafic dykes in the Ziyang area are alkaline rock series, are generally characterized by enriched Rb, Ba, Sr, La, Ce, Nb, and Ta, and have similar geochemical characteristics to those of OIB‐type basalt. The spider diagram of trace elements shows that the mafic dykes are weakly depleted in K and strongly depleted in Zr, Hf, and Ti. The mafic dykes are composed of depleted Nd and Pb isotopes and moderately enriched Sr isotopes, with ε Nd ( t ) = +3.4 to +3.6, ( 87 Sr/ 86 Sr) i = 0.70382–0.70412, ( 206 Pb/ 204 Pb) i = 17.542–17.802, ( 207 Pb/ 204 Pb) i = 15.557–15.569, and ( 208 Pb/ 204 Pb) i = 37.531–37.880. The elemental and isotopic geochemistry shows that the mafic dykes in the Ziyang area did not undergo crustal contamination during formation and that their geochemical characteristics could reflect the characteristics of their source. Amphibole whose formation was related to metasomatism by carbonate fluids was likely present in the mantle source. Therefore, the mantle source might have underwent metasomatism by carbonate fluids. The mantle metasomatism occurred recently, so the metasomatism of mantle source was likely caused by a subducted ocean plate in the early Neoproterozoic. The mafic dykes in the Ziyang area were formed in an intracontinental rift environment, and mantle plume activities were the main cause of the intracontinental rifting.
The coastal area around Southeastern Fujian is located in the Changle-Nan'ao tectonic zone, where mafic dikes (swarms) were developed since late Mesozoic. To prove the popular view that the geometrical form and occurrence of mafic dikes should record the evolution of local stress field, a study was carried out on the mafic dikes from Chihu of Zhangpu and Haibian of Dongshan, and it was found that the Changle-Nan'ao tectonic belt might be a levorotatory ductile shear zone in the late Cretaceous, but was thereafter converted to the lefthanded fragile shear zone in the Paleocene; We believe that this was caused by the NW-trending subduction of the paleo-Pacific plate. Since late Cretaceous, the stress changes between shearing and tension in Southeastern Fujian, which indicates that the crustal extension is not continuous but proceeded by many stages; Through the low stress subduction of oceanic plate, crustal extension has dominated in Southeastern China.
The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W‐Cu‐As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12 Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn–Li–Be–Nb–Ta. The Cu–As are typical sulphophile elements and always coexisted with Au–Ag–Pb–Zn–Sb–Hg. It is rarely reported that a deposit preserves tungsten and copper–arsenic simultaneously in the southern Sanjiang region. In this study, the S–Pb isotopic ratios of sulphides, the H–O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He–Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the δ 34 S values of sulphides from ores are mainly between −6.17‰ and +0.02‰; the sulphur isotope compositions indicate that the ore‐forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The δ 18 O fluid values of ore‐forming fluids calculated from hydrothermal quartz range from −5.5‰ to +7.5‰, and the δD values of the fluid inclusions in quartz are from −81.7‰ to −50.2‰. The H–O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess 3 He/ 4 He ratios of 0.02–0.05 Ra, and their 40 Ar/ 36 Ar ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic 40 Ar. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore‐forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore‐forming fluids both promoted the formation of this rare DMJS deposit.
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