2020 Vol. 45, No. 1
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2020, 45(1): 1-16.
doi: 10.3799/dqkx.2019.224
Abstract:
The Neoarchean Qingyuan greenstone belt (QGB) is located in the northern margin of the North China Craton (NCC). It is characterized by the occurrence of the oldest Cu-Zn volcanogenic massive sulfide (VMS) deposits in China. Recent geochronological data indicates that the QGB also hosts a certain amount of Neoarchean BIF-type iron deposits. Hence,a detailed study on the enigmatic association of VMS and BIF deposits should be conducted urgently. In this study,the previous geological,U-Pb geochronological,element geochemical,as well as Nd-S-Fe isotopic data on the VMS,BIF,and associated lithologies of the QGB was reviewed in detail. Based on the review,we concluded the tectonic setting,source of ore-forming materials,and genesis of the QGB VMS-BIF paragenetic assemblage,and thus established a ore-forming model in a back-arc basin for the QGB VMS-BIF metallogenic system. Generally,this model is significant in assessing regionally metallogenic endowment,and also in better understanding Archean continental crust evolution and marine environment.
The Neoarchean Qingyuan greenstone belt (QGB) is located in the northern margin of the North China Craton (NCC). It is characterized by the occurrence of the oldest Cu-Zn volcanogenic massive sulfide (VMS) deposits in China. Recent geochronological data indicates that the QGB also hosts a certain amount of Neoarchean BIF-type iron deposits. Hence,a detailed study on the enigmatic association of VMS and BIF deposits should be conducted urgently. In this study,the previous geological,U-Pb geochronological,element geochemical,as well as Nd-S-Fe isotopic data on the VMS,BIF,and associated lithologies of the QGB was reviewed in detail. Based on the review,we concluded the tectonic setting,source of ore-forming materials,and genesis of the QGB VMS-BIF paragenetic assemblage,and thus established a ore-forming model in a back-arc basin for the QGB VMS-BIF metallogenic system. Generally,this model is significant in assessing regionally metallogenic endowment,and also in better understanding Archean continental crust evolution and marine environment.
2020, 45(1): 17-30.
doi: 10.3799/dqkx.2018.263
Abstract:
This paper proposes a new petrogenetic model for the MMEs (mafic microgranular enclaves) based on a study on the Tangjiangqiongguo pluton in the middle part of the North Lhasa Block,Tibet. The MMEs found in the Tangjiangqiongguo host granitic diorites are hornblende diorites usually presenting axiolite and non-compactly cement with the host rock,with an apparent clearance plane with host rocks. The MMEs have higher Na2O,CaO,MgO and Fe2O3T contents than the host rocks. The MMEs have distinct REE characteristics from host rocks,with higher REE contents (except La and Ce) and weaker fractionation between LREE and HREE. Both the MMEs and host rocks show characteristics of arc rocks with enrichment in LILEs (large ion lithophile elements) Rb,Cs,K,etc.,and Th and U elements,but depletion in HFSEs (high field strength elements) Nb,Ta and Ti elements. Zircon LA-ICP-MS U-Pb dating yields a 113.9±1.0 Ma age and a 110±1.1 Ma age for the MMEs and host rocks,respectively. Although the MMEs are 4 Ma earlier than the host rocks,they have consistent zircon Lu-Hf isotopes. Synthesizing above petrography,geochemistry and isotopic evidence,we suggest the MMEs are derived from the same source rocks with host rocks. The MMEs experienced a weak plagioclase fractional crystallization,but the host rocks experienced a relatively strong hornblende fractional crystallization in respective secondary magma chamber. The differential magma of the host rock wrapped and carried the earlier formed MMEs (in semiplastic) to subsurface. The most probable petrogenetic model for the Tangjiangqiongguo MMEs mingling between magmas of different periods sourced from the same rock.
This paper proposes a new petrogenetic model for the MMEs (mafic microgranular enclaves) based on a study on the Tangjiangqiongguo pluton in the middle part of the North Lhasa Block,Tibet. The MMEs found in the Tangjiangqiongguo host granitic diorites are hornblende diorites usually presenting axiolite and non-compactly cement with the host rock,with an apparent clearance plane with host rocks. The MMEs have higher Na2O,CaO,MgO and Fe2O3T contents than the host rocks. The MMEs have distinct REE characteristics from host rocks,with higher REE contents (except La and Ce) and weaker fractionation between LREE and HREE. Both the MMEs and host rocks show characteristics of arc rocks with enrichment in LILEs (large ion lithophile elements) Rb,Cs,K,etc.,and Th and U elements,but depletion in HFSEs (high field strength elements) Nb,Ta and Ti elements. Zircon LA-ICP-MS U-Pb dating yields a 113.9±1.0 Ma age and a 110±1.1 Ma age for the MMEs and host rocks,respectively. Although the MMEs are 4 Ma earlier than the host rocks,they have consistent zircon Lu-Hf isotopes. Synthesizing above petrography,geochemistry and isotopic evidence,we suggest the MMEs are derived from the same source rocks with host rocks. The MMEs experienced a weak plagioclase fractional crystallization,but the host rocks experienced a relatively strong hornblende fractional crystallization in respective secondary magma chamber. The differential magma of the host rock wrapped and carried the earlier formed MMEs (in semiplastic) to subsurface. The most probable petrogenetic model for the Tangjiangqiongguo MMEs mingling between magmas of different periods sourced from the same rock.
2020, 45(1): 31-42.
doi: 10.3799/dqkx.2019.191
Abstract:
The Nuri Polymetallic Deposit,Tibet,is the largest and first scheelite deposit in Gangdese,and there have been some controversies on the ore-forming source of this deposit. The Pb isotope analyses of molybdenite,chalcopyrite,and pyrites from the mine have been carried out to determine the source. The results show that the molybdenite and pyrites have the consistent Pb isotopes,with 206Pb/204Pb ratios ranging from 17.525-18.581,207Pb/204Pb ratios ranging from 15.621-15.661,208Pb/204Pb ratios ranging from 37.524-38.929,and the chalcopyrite with 206Pb/204Pb ratios ranging from 18.414-18.578,207Pb/204Pb ratios ranging from 15.619-15.642,and 208Pb/204Pb ratios ranging from 38.617-38.863,respectively,which have obvious grouping characteristics. The S and Pb isotope features indicate that the ore-forming sources are mainly derived from the mantle,and the Re-Os isotope characteristics of molybdenite indicate that the minerals are mainly derived from mantle. The melt from the lower crust of India continent partial melting migrated upward and mixed with the Fe3+ enriched fluid derived from the subducted oceanic crust,which metasomatized with Yarlung Zangbo MORB peridotite,and the Cu,Mo were released into the magma system after oxidezed by Fe3+,thus,Cu and other elements in biotite granite which partially melted in the thickened lower crust were extracted in the rising process,finally formed the deposit by contact metasomatism with the wall rock.
The Nuri Polymetallic Deposit,Tibet,is the largest and first scheelite deposit in Gangdese,and there have been some controversies on the ore-forming source of this deposit. The Pb isotope analyses of molybdenite,chalcopyrite,and pyrites from the mine have been carried out to determine the source. The results show that the molybdenite and pyrites have the consistent Pb isotopes,with 206Pb/204Pb ratios ranging from 17.525-18.581,207Pb/204Pb ratios ranging from 15.621-15.661,208Pb/204Pb ratios ranging from 37.524-38.929,and the chalcopyrite with 206Pb/204Pb ratios ranging from 18.414-18.578,207Pb/204Pb ratios ranging from 15.619-15.642,and 208Pb/204Pb ratios ranging from 38.617-38.863,respectively,which have obvious grouping characteristics. The S and Pb isotope features indicate that the ore-forming sources are mainly derived from the mantle,and the Re-Os isotope characteristics of molybdenite indicate that the minerals are mainly derived from mantle. The melt from the lower crust of India continent partial melting migrated upward and mixed with the Fe3+ enriched fluid derived from the subducted oceanic crust,which metasomatized with Yarlung Zangbo MORB peridotite,and the Cu,Mo were released into the magma system after oxidezed by Fe3+,thus,Cu and other elements in biotite granite which partially melted in the thickened lower crust were extracted in the rising process,finally formed the deposit by contact metasomatism with the wall rock.
2020, 45(1): 43-60.
doi: 10.3799/dqkx.2018.341
Abstract:
The Haisugou rock mass is located in the southern part of the Daxing'anling orogenic belt. The Haisugou Mo deposit and the Xiaojingzibei Cu and Mo deposit are found in the intrusive mass. At present,the understanding of the rock mass is insufficient,the genetic type is controversial and the difference between the two molybdenum deposits has not been discussed. In this study,zircon U-Pb geochronology and petro-geochemistry analyses were carried out on different rocky granites in Haisugou rock mass,and Re-Os isotope analyses on molybdenite in the mining area were conducted. The LA-ICP-MS zircon U-Pb dating results of the biotite granite in the Haisugou molybdenum mining area are 137.1±0.6 Ma and 143.6±0.8 Ma; the Re content of molybdenite in the Haisugou molybdenum deposit is 2.1×10-6,the molybdenite Re-Os model age is 143.9±2.9 Ma. The dating result of LA-ICP-MS zircon U-Pb of the monzonitic granite in the Xiaojingzibei copper-molybdenum deposit is 126.5±0.7 Ma.The granite in the Haishugou pluton belongs to high-K calc-alkaline series Ⅰ-type granite with metaluminous to weakly peraluminous characteristics. The main elements SiO2 content is 68.81%-77.18%,K2O+Na2O=6.80%-8.31%,CaO content ranges from 0.43% to 2.88%,MgO ranges from 0.15% to 1.32% and aluminum index (A/CNK) value ranges from 0.97 to 1.07. The rocks are relatively enriched in light rare-earth elements (La/Yb)N=6.19-10.74 and the rare earth distribution pattern is right-dipping with a moderate negative Eu anomalies with a δEu value from 0.37 to 0.81. By combining existing research results,It can be considered that the magma evolution experienced magma mixing and the fractional crystallization plays a dominant role. The time of diagenesis and molybdenum metallogenesis of the Haisugou intrusive mass coincides with the time of the third phase of the large-scale diagenesis and mineralization of the Silamulun metallogenic belt,indicating the corresponding tectonic background is the transition of the tectonic system (from the extrusion environment to the tensile environment) to the lithosphere thinning environment.
The Haisugou rock mass is located in the southern part of the Daxing'anling orogenic belt. The Haisugou Mo deposit and the Xiaojingzibei Cu and Mo deposit are found in the intrusive mass. At present,the understanding of the rock mass is insufficient,the genetic type is controversial and the difference between the two molybdenum deposits has not been discussed. In this study,zircon U-Pb geochronology and petro-geochemistry analyses were carried out on different rocky granites in Haisugou rock mass,and Re-Os isotope analyses on molybdenite in the mining area were conducted. The LA-ICP-MS zircon U-Pb dating results of the biotite granite in the Haisugou molybdenum mining area are 137.1±0.6 Ma and 143.6±0.8 Ma; the Re content of molybdenite in the Haisugou molybdenum deposit is 2.1×10-6,the molybdenite Re-Os model age is 143.9±2.9 Ma. The dating result of LA-ICP-MS zircon U-Pb of the monzonitic granite in the Xiaojingzibei copper-molybdenum deposit is 126.5±0.7 Ma.The granite in the Haishugou pluton belongs to high-K calc-alkaline series Ⅰ-type granite with metaluminous to weakly peraluminous characteristics. The main elements SiO2 content is 68.81%-77.18%,K2O+Na2O=6.80%-8.31%,CaO content ranges from 0.43% to 2.88%,MgO ranges from 0.15% to 1.32% and aluminum index (A/CNK) value ranges from 0.97 to 1.07. The rocks are relatively enriched in light rare-earth elements (La/Yb)N=6.19-10.74 and the rare earth distribution pattern is right-dipping with a moderate negative Eu anomalies with a δEu value from 0.37 to 0.81. By combining existing research results,It can be considered that the magma evolution experienced magma mixing and the fractional crystallization plays a dominant role. The time of diagenesis and molybdenum metallogenesis of the Haisugou intrusive mass coincides with the time of the third phase of the large-scale diagenesis and mineralization of the Silamulun metallogenic belt,indicating the corresponding tectonic background is the transition of the tectonic system (from the extrusion environment to the tensile environment) to the lithosphere thinning environment.
2020, 45(1): 61-71.
doi: 10.3799/dqkx.2018.302
Abstract:
Pegmatite-type uranium deposit is the most important uranium deposit in the eastern part of northern Qinling metallogenic belt. This paper presents a study of the metallogenic and geochronological characteristics of the ore body,ores,and accessory minerals. Uranium ore-body occurs in pegmatite dykes around the garnet-bearing leucogranite and uraninite is the major industry mineral. The accessory minerals of uranium bearing pegmatite are similar to those of garnet-bearing granitic rocks,but different from gneissic granitic rocks. The results of LA-ICP-MS zircon U-Pb dating of uranium bearing pegmatite is 404.3±1.4 Ma,Early-Devonian. Comprehensive data suggest that the Danfeng garnet-bearing granitic rocks were derived from partial melting of Qinling Group in the post-collision setting during Early-Devonian. The residue magmas were enriched in Th-U and other moderate-incompatible elements but low content of volatile component which have relative short migration. The pegmatite-type uranium was formed near the internal and external contact zone of the pluton. The pluton was emplaced as the stock in the surface which experienced the long time uplift and erosion after the Triassic. Then the uranium-bearing pegmatite dykes were distributed around the stock,which resulted in the Guangshigou uranium deposit. The intrusions at the top were emplaced as the batholiths in the surface after the anabatic uplift and erosion. Thus,the uranium-bearing pegmatites were developed in the internal contact zone,for example,Chenjiazhuang uranium deposit. Based on the metallogenic model,Damaogou and Zhifanggou areas are determined as two areas of metallogenic prospect of pegmatite-type uranium deposit.
Pegmatite-type uranium deposit is the most important uranium deposit in the eastern part of northern Qinling metallogenic belt. This paper presents a study of the metallogenic and geochronological characteristics of the ore body,ores,and accessory minerals. Uranium ore-body occurs in pegmatite dykes around the garnet-bearing leucogranite and uraninite is the major industry mineral. The accessory minerals of uranium bearing pegmatite are similar to those of garnet-bearing granitic rocks,but different from gneissic granitic rocks. The results of LA-ICP-MS zircon U-Pb dating of uranium bearing pegmatite is 404.3±1.4 Ma,Early-Devonian. Comprehensive data suggest that the Danfeng garnet-bearing granitic rocks were derived from partial melting of Qinling Group in the post-collision setting during Early-Devonian. The residue magmas were enriched in Th-U and other moderate-incompatible elements but low content of volatile component which have relative short migration. The pegmatite-type uranium was formed near the internal and external contact zone of the pluton. The pluton was emplaced as the stock in the surface which experienced the long time uplift and erosion after the Triassic. Then the uranium-bearing pegmatite dykes were distributed around the stock,which resulted in the Guangshigou uranium deposit. The intrusions at the top were emplaced as the batholiths in the surface after the anabatic uplift and erosion. Thus,the uranium-bearing pegmatites were developed in the internal contact zone,for example,Chenjiazhuang uranium deposit. Based on the metallogenic model,Damaogou and Zhifanggou areas are determined as two areas of metallogenic prospect of pegmatite-type uranium deposit.
2020, 45(1): 72-89.
doi: 10.3799/dqkx.2018.227
Abstract:
The Zhangjia uranium deposit is one of the representative uranium deposits in the northern Miaoershan uranium orefield. The detailed study of metallogenic ages of the Zhangjia uranium deposit is very important not only to understanding the regularity of uranium mineralization at regional scale of the Miaoershan uranium orefield but also to systematically investigating the tectonic setting and geodynamics of hydrothermal uranium deposits in granitoids in Southern China. Based on detailed microscopic observation,the vein pitchblende was studied by in-situ electron microprobe U-Th-Pb and LA-ICP-MS U-Pb isotopic and rare earth element analysis. Twenty spots electron probe micro-analysis (EPMA) show calculated U-Th-Pb chemical ages ranging from 55.3 Ma to 81.1 Ma. 19 of the 20 ages give a weighted average age at 71.4±1.9 Ma. Based on rare earth elements differences,34 spots LA-ICP-MS U-Pb isotopic ages can be classified into two groups. The 13 U-Pb isotopic data of the first group 15 data give a weighted 206Pb/238U average age at 69.4±4.9 Ma. The second group includes 19 data spots,16 of which give a weighted 206Pb/238U average age at 94.1±3.0 Ma. The weighted EPMA U-Th-Pb chemical age 71.4±1.9 Ma is identical to the weighted 206Pb/238U average age 69.4±4.9 Ma of first group 13 data spots calculated by LA-ICP-MS U-Pb isotopic method. Thus we believe that the Zhangjia uranium deposit was formed at some 70 Ma. The weighted 206Pb/238U average age 94.1±3.0 Ma of the second group of the LA-ICP-MS U-Pb isotopic data is believed to have no geological significance and was possibly affected by post-formation alteration and/or impurities minerals in the laser ablation process. In late Mesozoic era,the paleo-Pacific plate was subducted to the eastern Eurasia continent,which affected the tectonics,magmatism and metallogenisis of the South China block. The hydrothermal uranium deposits in grantoids of South China,including the Zhangjia uranium deposit,were mostly formed in 50 Ma to 80 Ma which corresponds to the oblique subduction of the paleo-Pacific Plate to the eastern Eurasia Continent.
The Zhangjia uranium deposit is one of the representative uranium deposits in the northern Miaoershan uranium orefield. The detailed study of metallogenic ages of the Zhangjia uranium deposit is very important not only to understanding the regularity of uranium mineralization at regional scale of the Miaoershan uranium orefield but also to systematically investigating the tectonic setting and geodynamics of hydrothermal uranium deposits in granitoids in Southern China. Based on detailed microscopic observation,the vein pitchblende was studied by in-situ electron microprobe U-Th-Pb and LA-ICP-MS U-Pb isotopic and rare earth element analysis. Twenty spots electron probe micro-analysis (EPMA) show calculated U-Th-Pb chemical ages ranging from 55.3 Ma to 81.1 Ma. 19 of the 20 ages give a weighted average age at 71.4±1.9 Ma. Based on rare earth elements differences,34 spots LA-ICP-MS U-Pb isotopic ages can be classified into two groups. The 13 U-Pb isotopic data of the first group 15 data give a weighted 206Pb/238U average age at 69.4±4.9 Ma. The second group includes 19 data spots,16 of which give a weighted 206Pb/238U average age at 94.1±3.0 Ma. The weighted EPMA U-Th-Pb chemical age 71.4±1.9 Ma is identical to the weighted 206Pb/238U average age 69.4±4.9 Ma of first group 13 data spots calculated by LA-ICP-MS U-Pb isotopic method. Thus we believe that the Zhangjia uranium deposit was formed at some 70 Ma. The weighted 206Pb/238U average age 94.1±3.0 Ma of the second group of the LA-ICP-MS U-Pb isotopic data is believed to have no geological significance and was possibly affected by post-formation alteration and/or impurities minerals in the laser ablation process. In late Mesozoic era,the paleo-Pacific plate was subducted to the eastern Eurasia continent,which affected the tectonics,magmatism and metallogenisis of the South China block. The hydrothermal uranium deposits in grantoids of South China,including the Zhangjia uranium deposit,were mostly formed in 50 Ma to 80 Ma which corresponds to the oblique subduction of the paleo-Pacific Plate to the eastern Eurasia Continent.
2020, 45(1): 90-107.
doi: 10.3799/dqkx.2018.287
Abstract:
The Jiling Na-metasomatic uranium deposit is one of the most important uranium deposits in NW China. Based on the deposit geological characteristics, we studied the inclusions in calcite, chlorite thermometer, isotope and geochemical characteristics in this paper. We considered that the Jiling uranium deposit was formed under post-collision background in the Early Paleozoic, which has experienced 5 stages, and includes 6 alteration zones from ore center to the edge of Na-metasomatic body. The ore bearing hydrothermal is rebalance water, which is generated from evolutive magma, reacted with wall rocks; its temperature is about 295℃, salinity is 2.99%-4.57% NaCleqv and density is 0.75-0.77 g/cm3.The hydrothermal fluid is rich in U6+, Na+, CO32-, and has strong dissolution ability to SiO2, Fe2+, Mn2+, K+, Rb, Sr, Mo, Ga, Zr and Ba. Fluid boiling is main mechanism of ore deposition, and the changing of pH and Eh has promoted material deposition at the same time.
The Jiling Na-metasomatic uranium deposit is one of the most important uranium deposits in NW China. Based on the deposit geological characteristics, we studied the inclusions in calcite, chlorite thermometer, isotope and geochemical characteristics in this paper. We considered that the Jiling uranium deposit was formed under post-collision background in the Early Paleozoic, which has experienced 5 stages, and includes 6 alteration zones from ore center to the edge of Na-metasomatic body. The ore bearing hydrothermal is rebalance water, which is generated from evolutive magma, reacted with wall rocks; its temperature is about 295℃, salinity is 2.99%-4.57% NaCleqv and density is 0.75-0.77 g/cm3.The hydrothermal fluid is rich in U6+, Na+, CO32-, and has strong dissolution ability to SiO2, Fe2+, Mn2+, K+, Rb, Sr, Mo, Ga, Zr and Ba. Fluid boiling is main mechanism of ore deposition, and the changing of pH and Eh has promoted material deposition at the same time.
2020, 45(1): 108-117.
doi: 10.3799/dqkx.2018.547
Abstract:
Yixingzhai gold deposit, located in Wutaishan-Hengshan gold district, is the most important and largest gold deposit in Shanxi Province. Spatial distribution of the four breccia pipes shows the well relation with gold mineralization in Yixingzhai deposit. However, robust constraints on the timing of the breccia pipes formation in this deposit are lacking. The relationship between the gold mineralization and breccia formation is also ambiguous. In this paper, systematic U-Pb dating of garnet crystals from skarn breccia and zircon grains from the quartz porphyry were carried out by using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). The garnet grains yielded the 207Pb-corrected weighted mean 206Pb/238U dates of 140±2 Ma, which is remarkably consistent with U-Pb age of zircon grains (141±1 Ma) from the quartz porphyry in this deposit. This demonstrates that the formation of breccia pipes was genetically related to the coeval magmatism. Therefore, we proposed that gas and hydrothermal fluid released from magma increase the fluid pressure, causing the formation of breccia pipes by cryptoexplosion of the wall rocks. The age of the breccia pipes is also consistent with the formation time of gold mineralization, suggesting that the hydrothermal gold mineralization may link with the fluids released from magma.
Yixingzhai gold deposit, located in Wutaishan-Hengshan gold district, is the most important and largest gold deposit in Shanxi Province. Spatial distribution of the four breccia pipes shows the well relation with gold mineralization in Yixingzhai deposit. However, robust constraints on the timing of the breccia pipes formation in this deposit are lacking. The relationship between the gold mineralization and breccia formation is also ambiguous. In this paper, systematic U-Pb dating of garnet crystals from skarn breccia and zircon grains from the quartz porphyry were carried out by using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). The garnet grains yielded the 207Pb-corrected weighted mean 206Pb/238U dates of 140±2 Ma, which is remarkably consistent with U-Pb age of zircon grains (141±1 Ma) from the quartz porphyry in this deposit. This demonstrates that the formation of breccia pipes was genetically related to the coeval magmatism. Therefore, we proposed that gas and hydrothermal fluid released from magma increase the fluid pressure, causing the formation of breccia pipes by cryptoexplosion of the wall rocks. The age of the breccia pipes is also consistent with the formation time of gold mineralization, suggesting that the hydrothermal gold mineralization may link with the fluids released from magma.
2020, 45(1): 118-130.
doi: 10.3799/dqkx.2018.583
Abstract:
The Tonglushan skarn Cu-Fe polymetallic deposit is located in the Edongnan metallogenic province along the Middle and Lower Yangtze River belt. Orebodies are hosted in the contact zone between the Tonglushan pluton and the Triassic carbonate rocks. Magnetite is widespread in the deposit and is one of the ore minerals. In this study, representative hydrothermal magnetite samples were collected from the exoskarn and endoskarn, as well as the igneous magnetite from the host intrusion, for detailed analyses of microtextures and elemental compositions. The hydrothermal magnetite grains commonly have Ti-spinel exsolutions textures and Si-rich bandings, and were weakly metasomatized by late stage hydrothermal fluids. The presence of Ti-spinel exsolutions textures indicates that the ore-forming fluids of the Tonglushan deposit may have high contents of Ti initially. The hydrothermal magnetite also has relatively high contents of Si, Al, Cr, V, Mn, Mg, Co and Ni. Si4+, Al3+, Mg2+, Mn2+ were incorporated into magnetite by a mechanism of isomorphism. However, the mechanism and intensity of isomorphism in different types of magnetite are various, indicating that element substitution was affected by fluids compositions, pressure and temperature physicochemical conditions. Al2O3/MgO ratios are less than 4 in magnetite from exoskarn, vary from 5-8 in magnetite from endoskarn, and are around 13 for igneous magnetite. Igneous magnetite within intrusion has highest V2O3(on average 0.31%), magnetite within endoskarn has intermediate values(on average 0.14%), and magnetite from exoskarn has lowest V2O3 values(on average 0.01%-0.03%). The Al2O3/MgO ratios and V2O3 contents clearly demonstrate that growth of magnetite was affected by composition of melts/fluids, composition of country rocks, and fluid-rock interactions. The trend of decreased temperature for magnetite from the hosting intrusion, endoskarn, and exoskarn, and the trend of elemental compositions in Tonglushan deposit show that magnetite is a good indicator mineral for skarn deposit.
The Tonglushan skarn Cu-Fe polymetallic deposit is located in the Edongnan metallogenic province along the Middle and Lower Yangtze River belt. Orebodies are hosted in the contact zone between the Tonglushan pluton and the Triassic carbonate rocks. Magnetite is widespread in the deposit and is one of the ore minerals. In this study, representative hydrothermal magnetite samples were collected from the exoskarn and endoskarn, as well as the igneous magnetite from the host intrusion, for detailed analyses of microtextures and elemental compositions. The hydrothermal magnetite grains commonly have Ti-spinel exsolutions textures and Si-rich bandings, and were weakly metasomatized by late stage hydrothermal fluids. The presence of Ti-spinel exsolutions textures indicates that the ore-forming fluids of the Tonglushan deposit may have high contents of Ti initially. The hydrothermal magnetite also has relatively high contents of Si, Al, Cr, V, Mn, Mg, Co and Ni. Si4+, Al3+, Mg2+, Mn2+ were incorporated into magnetite by a mechanism of isomorphism. However, the mechanism and intensity of isomorphism in different types of magnetite are various, indicating that element substitution was affected by fluids compositions, pressure and temperature physicochemical conditions. Al2O3/MgO ratios are less than 4 in magnetite from exoskarn, vary from 5-8 in magnetite from endoskarn, and are around 13 for igneous magnetite. Igneous magnetite within intrusion has highest V2O3(on average 0.31%), magnetite within endoskarn has intermediate values(on average 0.14%), and magnetite from exoskarn has lowest V2O3 values(on average 0.01%-0.03%). The Al2O3/MgO ratios and V2O3 contents clearly demonstrate that growth of magnetite was affected by composition of melts/fluids, composition of country rocks, and fluid-rock interactions. The trend of decreased temperature for magnetite from the hosting intrusion, endoskarn, and exoskarn, and the trend of elemental compositions in Tonglushan deposit show that magnetite is a good indicator mineral for skarn deposit.
2020, 45(1): 131-144.
doi: 10.3799/dqkx.2018.298
Abstract:
Biotite is not only a robust 40Ar-39Ar geochronometer, but also commonly used to constrain the physico-chemical conditions, magma source and evolution process, mineralization potential and tectonic settings of the parental magma. In this study, we have obtained the 40Ar-39Ar age and major element compositions of biotite from the most widely outcropped aegirine-nepheline syenite of the Saima alkaline complex. We further discuss the physicochemical property, the evolution process and tectonic settings of the rock by the combination of equilibrium calculation using pMELTS software on previously reported data. The biotite crystals from the aegirine-nepheline syenite have high Fe and Ti contents and belong to annite in both the APSE classification diagram and Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn) diagram for biotite. On the basis of the major element compositions of the biotite, the crystallization temperatures and oxygen fugacity (lgfO2) are estimated to range from 770 to 800℃ and -16 to -14, respectively, while the equilibrium calculation by pMELTS indicates that the magmatic system crystallized at >1 300℃ and the oxygen fugacity have decreased since the crystallization begun. The high temperature and decreasing oxygen fugacity prevent the early exsolution of a fluid phase, and the alkalis and volatiles are therefore retained in the melt and finally enriched in the hydrothermal fluids related to the late sodic lujavrite. Some of the biotite plates show oscillatory zoning, high TiO2, Na2O but low SiO2 contents and low Fe3+/(Fe3++Fe2+) ratio from the core to the rim, further indicating that the residual magmas after biotite crystallization would have lower temperature, lower oxygen fugacity but higher alkalis contents. The Saima alkaline complex is part of the E-W-trending alkaline rock belt at the northern margin of the NCC, and it is formed at~222 Ma as indicated by biotite 40Ar-39Ar age in post-collisional extension tectonic settings after the closure of the palaeo-Asian Ocean.
Biotite is not only a robust 40Ar-39Ar geochronometer, but also commonly used to constrain the physico-chemical conditions, magma source and evolution process, mineralization potential and tectonic settings of the parental magma. In this study, we have obtained the 40Ar-39Ar age and major element compositions of biotite from the most widely outcropped aegirine-nepheline syenite of the Saima alkaline complex. We further discuss the physicochemical property, the evolution process and tectonic settings of the rock by the combination of equilibrium calculation using pMELTS software on previously reported data. The biotite crystals from the aegirine-nepheline syenite have high Fe and Ti contents and belong to annite in both the APSE classification diagram and Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn) diagram for biotite. On the basis of the major element compositions of the biotite, the crystallization temperatures and oxygen fugacity (lgfO2) are estimated to range from 770 to 800℃ and -16 to -14, respectively, while the equilibrium calculation by pMELTS indicates that the magmatic system crystallized at >1 300℃ and the oxygen fugacity have decreased since the crystallization begun. The high temperature and decreasing oxygen fugacity prevent the early exsolution of a fluid phase, and the alkalis and volatiles are therefore retained in the melt and finally enriched in the hydrothermal fluids related to the late sodic lujavrite. Some of the biotite plates show oscillatory zoning, high TiO2, Na2O but low SiO2 contents and low Fe3+/(Fe3++Fe2+) ratio from the core to the rim, further indicating that the residual magmas after biotite crystallization would have lower temperature, lower oxygen fugacity but higher alkalis contents. The Saima alkaline complex is part of the E-W-trending alkaline rock belt at the northern margin of the NCC, and it is formed at~222 Ma as indicated by biotite 40Ar-39Ar age in post-collisional extension tectonic settings after the closure of the palaeo-Asian Ocean.
2020, 45(1): 145-155.
doi: 10.3799/dqkx.2018.278
Abstract:
The magmatic rocks, which is widely distributed in Fengcheng area, Liaodong peninsula, is always a hot spot for study of North China Craton (NCC) thinning in recent years. However, due to lack of precise age and geochemical constraints in previous research, its petrogenesis and tectonic setting still remain controversial. In this paper, three granite plutons were studied by the petrology, petrogechemistry and LA-ICP-MS zircon U-Pb ages. The Maokuishan monzogranite, Jiaoyangyu granodiorite and Qingliangshan monzogranite were emplaced with the ages of 127.2±0.6 Ma, 122.9±1.1 Ma and 129.0±0.8 Ma, respectively. These granites were all constrained to Early Cretaceous. The zircons of granites have the characteristics of oscillatory growth zonation and high ratios of Th and U (Th/U>0.4), indicating a magmatic origin for the zircons. Geochemical characteristics suggest that Maokuishan and Qingliangshan monzogranites are both metaluminum A-type and peraluminous high-K calc alkaline granites, and the major elements are characterized by high Si, alk and low Al, Fe, Ca and Mg. The chondrite-normalized REE patterns have a moderate negative Eu anomalies. Jiaoyangyu granodiorites are metaluminum-peraluminous granodiorites, and the trace elements are enriched in Nd, Tb, La, and depleted in Sr, P, Ti. Combined with geological background, the three granite plutons in Fengcheng area discussed above were all formed in anorogenic extensional tectonic setting during Early Cretaceous, and also is the epitome of the lithosphere thinning and cratonic destruction of the North China Craton.
The magmatic rocks, which is widely distributed in Fengcheng area, Liaodong peninsula, is always a hot spot for study of North China Craton (NCC) thinning in recent years. However, due to lack of precise age and geochemical constraints in previous research, its petrogenesis and tectonic setting still remain controversial. In this paper, three granite plutons were studied by the petrology, petrogechemistry and LA-ICP-MS zircon U-Pb ages. The Maokuishan monzogranite, Jiaoyangyu granodiorite and Qingliangshan monzogranite were emplaced with the ages of 127.2±0.6 Ma, 122.9±1.1 Ma and 129.0±0.8 Ma, respectively. These granites were all constrained to Early Cretaceous. The zircons of granites have the characteristics of oscillatory growth zonation and high ratios of Th and U (Th/U>0.4), indicating a magmatic origin for the zircons. Geochemical characteristics suggest that Maokuishan and Qingliangshan monzogranites are both metaluminum A-type and peraluminous high-K calc alkaline granites, and the major elements are characterized by high Si, alk and low Al, Fe, Ca and Mg. The chondrite-normalized REE patterns have a moderate negative Eu anomalies. Jiaoyangyu granodiorites are metaluminum-peraluminous granodiorites, and the trace elements are enriched in Nd, Tb, La, and depleted in Sr, P, Ti. Combined with geological background, the three granite plutons in Fengcheng area discussed above were all formed in anorogenic extensional tectonic setting during Early Cretaceous, and also is the epitome of the lithosphere thinning and cratonic destruction of the North China Craton.
2020, 45(1): 156-167.
doi: 10.3799/dqkx.2018.237
Abstract:
Koka granite, located in the west of the Nakfa region in Eritrea, is the main host rock of Koka gold deposit. The major elements of the granite are characteristiced by high SiO2 (67.94%-78.40%), Na2O+K2O (5.86%-8.76%), Al2O3 (11.05%-16.51%) and FeOT (2.46%-3.80%), weakly peraluminous to strongly peraluminous (A/CNK is 1.09-1.55), and low CaO (0.06%-1.85%), MgO (0.15%-0.39%). It is also enriched in LREEs and relatively depleted in HREEs, strongly depleted in Sr, P, Ti. The REE distribution curve shows characteristics such as swallow distribution and apparent negative europium anomalies. All of these indicate that the Koka granite has an affinity of A-type granite. Zircon LA-ICP-MS U-Pb dating shows the granite formed in early period of the Neoproterozoic with the diagenetic age of 851.2±1.9 Ma. This is different from the A-type granite which is widely distributed and related to the extension after collisional orogen (650-540 Ma). Combined with the regional geological research results, it is suggested that Koka granite was formed in a back-arc extensional environment which resulted from subduction. The zircon has a certain Ce positive anomaly, and variation range of the Ce4+/Ce3+ is 3.86-146.31, with an average of 32.4, indicating low degree of magmatic oxygen fugacity. Meanwhile the parental magma of Koka granite is "dry", suggesting that the metallogenic potentiality is low, and it is hard to form related large or super large deposits.
Koka granite, located in the west of the Nakfa region in Eritrea, is the main host rock of Koka gold deposit. The major elements of the granite are characteristiced by high SiO2 (67.94%-78.40%), Na2O+K2O (5.86%-8.76%), Al2O3 (11.05%-16.51%) and FeOT (2.46%-3.80%), weakly peraluminous to strongly peraluminous (A/CNK is 1.09-1.55), and low CaO (0.06%-1.85%), MgO (0.15%-0.39%). It is also enriched in LREEs and relatively depleted in HREEs, strongly depleted in Sr, P, Ti. The REE distribution curve shows characteristics such as swallow distribution and apparent negative europium anomalies. All of these indicate that the Koka granite has an affinity of A-type granite. Zircon LA-ICP-MS U-Pb dating shows the granite formed in early period of the Neoproterozoic with the diagenetic age of 851.2±1.9 Ma. This is different from the A-type granite which is widely distributed and related to the extension after collisional orogen (650-540 Ma). Combined with the regional geological research results, it is suggested that Koka granite was formed in a back-arc extensional environment which resulted from subduction. The zircon has a certain Ce positive anomaly, and variation range of the Ce4+/Ce3+ is 3.86-146.31, with an average of 32.4, indicating low degree of magmatic oxygen fugacity. Meanwhile the parental magma of Koka granite is "dry", suggesting that the metallogenic potentiality is low, and it is hard to form related large or super large deposits.
2020, 45(1): 168-179.
doi: 10.3799/dqkx.2018.181
Abstract:
In Fuping region of Shaanxi Province, a suite of radiolarian siliceous rocks is developed in Upper Ordovician Zhaolaoyu Formation, which is associated with deep water carbonate rocks. The geochemical characteristics of siliceous rocks are of great significance in revealing the source of silicon and tectonic setting. On basis of the field profile, the petrological and geochemical analyses of siliceous rocks in Upper Ordovician Zhaolaoyu Formation in Fuping region, the origin and tectonic setting are studied. It is found that Zhaolaoyu Formation is dominated by dark gray lamellose limestone, with lamellar radiolarian siliceous rocks, conglomerate limestone, calcarenite, patchy limestone and volcanic tuff. Siliceous rocks are cryptocrystalline-microcrystalline, containing radiolarian fossils. The geochemical test of the samples yields the value of Al/(Al+Fe+Mn) (average 0.68), MnO/TiO2 (average 0.18), ΣREE (average 46.41), δCe (average 0.95), δEu (average 1.13), LaN/YbN (average 1.15), LaN/CeN (average 1.08), suggesting that the tectonic environment is continental margin and the silicons are mainly of terrigenous origin. We believe that the siliceous rocks in Fuping region were formed on the northern margin of the Erlangping retroarc foreland basin, which was formed between the southern margin of North China Block and the North Qinling under the background of subduction of the Yangtze Plate to the North China Plate during the Ordovician.
In Fuping region of Shaanxi Province, a suite of radiolarian siliceous rocks is developed in Upper Ordovician Zhaolaoyu Formation, which is associated with deep water carbonate rocks. The geochemical characteristics of siliceous rocks are of great significance in revealing the source of silicon and tectonic setting. On basis of the field profile, the petrological and geochemical analyses of siliceous rocks in Upper Ordovician Zhaolaoyu Formation in Fuping region, the origin and tectonic setting are studied. It is found that Zhaolaoyu Formation is dominated by dark gray lamellose limestone, with lamellar radiolarian siliceous rocks, conglomerate limestone, calcarenite, patchy limestone and volcanic tuff. Siliceous rocks are cryptocrystalline-microcrystalline, containing radiolarian fossils. The geochemical test of the samples yields the value of Al/(Al+Fe+Mn) (average 0.68), MnO/TiO2 (average 0.18), ΣREE (average 46.41), δCe (average 0.95), δEu (average 1.13), LaN/YbN (average 1.15), LaN/CeN (average 1.08), suggesting that the tectonic environment is continental margin and the silicons are mainly of terrigenous origin. We believe that the siliceous rocks in Fuping region were formed on the northern margin of the Erlangping retroarc foreland basin, which was formed between the southern margin of North China Block and the North Qinling under the background of subduction of the Yangtze Plate to the North China Plate during the Ordovician.
2020, 45(1): 180-193.
doi: 10.3799/dqkx.2018.244
Abstract:
As a record of the process for ocean-continent transition related oceanic subduction, the magmatic rocks consisting of the basalts, andesites, dacites and rhyolites, were identified near the northwastern Xiazhuang-Shufan fault in Kaihua County, western Zhejiang, which is the eastern segment of Jiangnan orogeny. Geochemical analyses indicate that the basalts and andesites are mostly enriched in Ba, K, Rb, Th, U, Pb, but depleted in Sr, P, Nb, Ta, Ti. The basalts show high Nb contents of 11.8×10-6 to 15.2×10-6, Nb/Ta=15.36-18.10, and Nb/U=8.90-19.32. The andesites have higher Mg values with MgO contents ranging from 5.31% to 8.56%, Mg# ranging from 56.89 to 68.83, and FeOT/MgO=0.82-1.36. The dacites and rhyolites have higher Ga/Al ratios, FeOT/MgO ranging from 5.66 to 18.50 mostly, and high magma temperatures (837-920℃), reflecting the characteristics of A-type rhyolites. The U-Pb dating of zircon yields age of 800.5±9.2 Ma for the Nb-enriched basalt, 799.3±7.1 Ma for the high-Mg andesite, and 798.3±6.2 Ma for the A-type rhyolite, confirming that the magmatic activity of continental marginal arc occurred when the paleo-South-China Plate subducted northwestwardly in Neoproterozoic, and suggesting that the subducting movement continued to 800 Ma or later.
As a record of the process for ocean-continent transition related oceanic subduction, the magmatic rocks consisting of the basalts, andesites, dacites and rhyolites, were identified near the northwastern Xiazhuang-Shufan fault in Kaihua County, western Zhejiang, which is the eastern segment of Jiangnan orogeny. Geochemical analyses indicate that the basalts and andesites are mostly enriched in Ba, K, Rb, Th, U, Pb, but depleted in Sr, P, Nb, Ta, Ti. The basalts show high Nb contents of 11.8×10-6 to 15.2×10-6, Nb/Ta=15.36-18.10, and Nb/U=8.90-19.32. The andesites have higher Mg values with MgO contents ranging from 5.31% to 8.56%, Mg# ranging from 56.89 to 68.83, and FeOT/MgO=0.82-1.36. The dacites and rhyolites have higher Ga/Al ratios, FeOT/MgO ranging from 5.66 to 18.50 mostly, and high magma temperatures (837-920℃), reflecting the characteristics of A-type rhyolites. The U-Pb dating of zircon yields age of 800.5±9.2 Ma for the Nb-enriched basalt, 799.3±7.1 Ma for the high-Mg andesite, and 798.3±6.2 Ma for the A-type rhyolite, confirming that the magmatic activity of continental marginal arc occurred when the paleo-South-China Plate subducted northwestwardly in Neoproterozoic, and suggesting that the subducting movement continued to 800 Ma or later.
2020, 45(1): 194-210.
doi: 10.3799/dqkx.2018.325
Abstract:
Carboniferous volcanic rocks are widely distributed in the Jueluotage belt of eastern Tianshan, located in the southern Central Asian Orogenic Belt. The petrogenesis and tectonic history of this region have long been a matter of debate. Data on major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age of zircons are reported for Carboniferous volcanic rocks in the Juluotage area in this paper. The main results are obtained as follows:(1) These volcanic rocks erupted in two episodes, with zircon U-Pb ages of ca. 336 Ma and ca. 320 Ma respectively. The ca. 336 Ma volcanic rocks are dominated by arc-like igneous rocks. They show enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depletion of high field-strength elements (HFSEs) according to the N-MORB-normalized and chondrite-normalized diagrams. However, the ca. 320 Ma volcanic rocks mainly consist of MORB-like igneous rocks that show nearly flat REE patterns. They are also slightly enriched in LILEs and LREEs according to the N-MORB-normalized and chondrite-normalized diagrams. (2) The inherited zircon ages of volcanic rocks from the Jueluotage belt show similar trends and features to those of the Central Tianshan Block, suggesting they shared the same basement, at least before the Early Carboniferous. (3) These Carboniferous volcanic rocks are geochemically similar to the back-arc basin basalt (BABB) from the Mariana and Okinawa troughs. The evolution from arc-like volcanic rocks to MORB-like volcanic rocks reflects the development of the back-arc basin in the northern Central Tianshan Block during the Carboniferous. The subduction of the Tianshan Ocean ended at the end of the Late Carboniferous period. The final closure of the main ocean and back-arc basins is likely to be south of the Central Tianshan mountain.
Carboniferous volcanic rocks are widely distributed in the Jueluotage belt of eastern Tianshan, located in the southern Central Asian Orogenic Belt. The petrogenesis and tectonic history of this region have long been a matter of debate. Data on major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age of zircons are reported for Carboniferous volcanic rocks in the Juluotage area in this paper. The main results are obtained as follows:(1) These volcanic rocks erupted in two episodes, with zircon U-Pb ages of ca. 336 Ma and ca. 320 Ma respectively. The ca. 336 Ma volcanic rocks are dominated by arc-like igneous rocks. They show enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depletion of high field-strength elements (HFSEs) according to the N-MORB-normalized and chondrite-normalized diagrams. However, the ca. 320 Ma volcanic rocks mainly consist of MORB-like igneous rocks that show nearly flat REE patterns. They are also slightly enriched in LILEs and LREEs according to the N-MORB-normalized and chondrite-normalized diagrams. (2) The inherited zircon ages of volcanic rocks from the Jueluotage belt show similar trends and features to those of the Central Tianshan Block, suggesting they shared the same basement, at least before the Early Carboniferous. (3) These Carboniferous volcanic rocks are geochemically similar to the back-arc basin basalt (BABB) from the Mariana and Okinawa troughs. The evolution from arc-like volcanic rocks to MORB-like volcanic rocks reflects the development of the back-arc basin in the northern Central Tianshan Block during the Carboniferous. The subduction of the Tianshan Ocean ended at the end of the Late Carboniferous period. The final closure of the main ocean and back-arc basins is likely to be south of the Central Tianshan mountain.
2020, 45(1): 211-225.
doi: 10.3799/dqkx.2018.342
Abstract:
The fault structure is very complex, with several extension directions including the north-south, east-west, north-east and north-west in Langshan area, Inner Mongolia. In this study, we conducted the geometrical morphology, fault movement property and structural stress field analyses on the N-S trending fault, combining with field investigation and stereographic projection, to reveal the regional structural stress field and tectonic evolution history in Langshan area. Our preliminary results show that the N-S trending fault in the study area has experienced at least two phases of significant tectonic events. The first phase is in the Late Permian, because a collision collage movement due to the northward movement of North China Craton and the southward movement of Siberia plate produced nearly horizontal compressive tectonic stress field with principal stress axis sigma 1 for north by east 10° or so, plunging to the north, and with dip angle of 15°-20°. As aresult the Langshan mountain region developed a rupture system, which consisted of nearly east-west direction compressive tectonic belt and reverse fault, near the north-east direction of the predominantly left strike-slip faults, near north-west direction of the predominantly right lateral strike-slip faults and nearly north-south extensional fault.These first formed fault systems also have controlled and constrained subsequent tectonic activity in the region.The second phase of tectonic stress field was generated by the subduction of the palaeo-pacific plate from the Jurassic to the Asian continent.The tectonic stress field of the principal stress axis sigma 1 is northeast-southwest, plunging to about 150° and with dip angle of 10°-20°.The effect of the second phase of the tectonic stress field made the early fault tectonics move again, so that the early north-south fault was turned from the original tensile fracture plane to the left-lateral strike-slip, early east-west fracture from compressional structural surface to left-lateral strike-slip, early north-east left-lateral strike-slip surface shifted to the compressive surface and the fold extended direction. The early north-west was converted to a rupture property.
The fault structure is very complex, with several extension directions including the north-south, east-west, north-east and north-west in Langshan area, Inner Mongolia. In this study, we conducted the geometrical morphology, fault movement property and structural stress field analyses on the N-S trending fault, combining with field investigation and stereographic projection, to reveal the regional structural stress field and tectonic evolution history in Langshan area. Our preliminary results show that the N-S trending fault in the study area has experienced at least two phases of significant tectonic events. The first phase is in the Late Permian, because a collision collage movement due to the northward movement of North China Craton and the southward movement of Siberia plate produced nearly horizontal compressive tectonic stress field with principal stress axis sigma 1 for north by east 10° or so, plunging to the north, and with dip angle of 15°-20°. As aresult the Langshan mountain region developed a rupture system, which consisted of nearly east-west direction compressive tectonic belt and reverse fault, near the north-east direction of the predominantly left strike-slip faults, near north-west direction of the predominantly right lateral strike-slip faults and nearly north-south extensional fault.These first formed fault systems also have controlled and constrained subsequent tectonic activity in the region.The second phase of tectonic stress field was generated by the subduction of the palaeo-pacific plate from the Jurassic to the Asian continent.The tectonic stress field of the principal stress axis sigma 1 is northeast-southwest, plunging to about 150° and with dip angle of 10°-20°.The effect of the second phase of the tectonic stress field made the early fault tectonics move again, so that the early north-south fault was turned from the original tensile fracture plane to the left-lateral strike-slip, early east-west fracture from compressional structural surface to left-lateral strike-slip, early north-east left-lateral strike-slip surface shifted to the compressive surface and the fold extended direction. The early north-west was converted to a rupture property.
2020, 45(1): 226-237.
doi: 10.3799/dqkx.2018.304
Abstract:
There are abundant oil and gas resources and great exploration potential in the buried hill of Jizhong depression in Bohai bay basin, and it is of great significance to deepen the study of hydrocarbon accumulation in buried hill and establish the quantitative evaluation formula of hydrocarbon filling capacity. Based on the analyses of the distribution of oil and gas accumulation and geological factors, combining previous research results, the buried hill in the study area is divided into high buried hill, middle buried hill and low buried hill. Represented by Renqiu buried hill, Suqiao buried hill and Niudong 1 buried hill, respectively, three models of hydrocarbon accumulation are established, namely high buried hill forming mode, middle buried hill forming mode and low buried hill forming mode. Among them, the high buried hill formation pattern is mostly distributed in the central uplift zone, which has the characteristics of "two-way oil source and source reservoir connection". The middle buried hill forming model is distributed in the slope zone, which has the characteristics of "lateral oil source and complex transmission". The low buried hill forming model is mostly distributed in deep concave zone and has the characteristics of "top oil source and unconformity transmission source rocks from the cover-flow backward filling". Considering the hydrocarbon-generation intensity of source rock, buried hill reservoir physical properties, assemblage type, hydrocarbon supply in buried hill and the influence of hydrocarbon-supplying window, a formula is established for quantitative evaluation of buried hill hydrocarbon filling capacity, and applied to the Jizhong depression, and the evaluation results show that the hydrocarbon filling capacity of three kinds buried hills is obviously different. The hydrocarbon filling capacity of high buried hill is the strongest, which is followed by the low buried hill, while that of the middle buried hill is the weakest.
There are abundant oil and gas resources and great exploration potential in the buried hill of Jizhong depression in Bohai bay basin, and it is of great significance to deepen the study of hydrocarbon accumulation in buried hill and establish the quantitative evaluation formula of hydrocarbon filling capacity. Based on the analyses of the distribution of oil and gas accumulation and geological factors, combining previous research results, the buried hill in the study area is divided into high buried hill, middle buried hill and low buried hill. Represented by Renqiu buried hill, Suqiao buried hill and Niudong 1 buried hill, respectively, three models of hydrocarbon accumulation are established, namely high buried hill forming mode, middle buried hill forming mode and low buried hill forming mode. Among them, the high buried hill formation pattern is mostly distributed in the central uplift zone, which has the characteristics of "two-way oil source and source reservoir connection". The middle buried hill forming model is distributed in the slope zone, which has the characteristics of "lateral oil source and complex transmission". The low buried hill forming model is mostly distributed in deep concave zone and has the characteristics of "top oil source and unconformity transmission source rocks from the cover-flow backward filling". Considering the hydrocarbon-generation intensity of source rock, buried hill reservoir physical properties, assemblage type, hydrocarbon supply in buried hill and the influence of hydrocarbon-supplying window, a formula is established for quantitative evaluation of buried hill hydrocarbon filling capacity, and applied to the Jizhong depression, and the evaluation results show that the hydrocarbon filling capacity of three kinds buried hills is obviously different. The hydrocarbon filling capacity of high buried hill is the strongest, which is followed by the low buried hill, while that of the middle buried hill is the weakest.
2020, 45(1): 238-250.
doi: 10.3799/dqkx.2018.300
Abstract:
The Huhehu depression, located in the southern of Hailar basin, has favorable oil generating conditions. 270 pyrolysis data and 125 vitrinite reflectance data from 7 wells in the study area are used to analyze the geochemical characteristics and thermal history of Lower Cretaceous source rocks. Results show that the organic matter types of the Lower Cretaceous source rocks are mainly kerogen type Ⅲ-Ⅱ2, and the thermal maturity of the source rocks is characterized by high in the middle and low on all sides. The Nantun Formation belongs to the medium-good source rock with high maturity, which entered the threshold of hydrocarbon generation about 128 Ma, now in high mature to gas generation stage. The Damoguaihe Formation belongs to the poor-medium source rock, which entered the hydrocarbon generating threshold about 124 Ma, and now in the medium to low mature stage, the second member hydrocarbon source rock of Damoguaihe Formation has not entered the peak of hydrocarbon generation up to now. The simulation results of thermal history show that the Huhehu depression has reached the maximum paleogeotemperature at the end of the sedimentary period of Yimin Formation, and the geothermal evolution has the characteristics of increase firstly and then decrease. From the beginning of the Nantun Formation, the geothermal gradient gradually increased to 50-55℃/km, and then decreased to 35.4℃/km now. The higher geothermal field since the Early Cretaceous controlled the formation and accumulation of oil and gas. The inclusion homogenization temperatures and thermal history results indicate that 126-87 Ma(the sedimentary period of Yimin Formation)should be the main hydrocarbon accumulation period in Huhehu depression. As the strata were denuded, and the temperature decreased, the hydrocarbon generated and hydrocarbon expulsed process became weak due to the uplift of the Hailar basin since Late Cretaceous.
The Huhehu depression, located in the southern of Hailar basin, has favorable oil generating conditions. 270 pyrolysis data and 125 vitrinite reflectance data from 7 wells in the study area are used to analyze the geochemical characteristics and thermal history of Lower Cretaceous source rocks. Results show that the organic matter types of the Lower Cretaceous source rocks are mainly kerogen type Ⅲ-Ⅱ2, and the thermal maturity of the source rocks is characterized by high in the middle and low on all sides. The Nantun Formation belongs to the medium-good source rock with high maturity, which entered the threshold of hydrocarbon generation about 128 Ma, now in high mature to gas generation stage. The Damoguaihe Formation belongs to the poor-medium source rock, which entered the hydrocarbon generating threshold about 124 Ma, and now in the medium to low mature stage, the second member hydrocarbon source rock of Damoguaihe Formation has not entered the peak of hydrocarbon generation up to now. The simulation results of thermal history show that the Huhehu depression has reached the maximum paleogeotemperature at the end of the sedimentary period of Yimin Formation, and the geothermal evolution has the characteristics of increase firstly and then decrease. From the beginning of the Nantun Formation, the geothermal gradient gradually increased to 50-55℃/km, and then decreased to 35.4℃/km now. The higher geothermal field since the Early Cretaceous controlled the formation and accumulation of oil and gas. The inclusion homogenization temperatures and thermal history results indicate that 126-87 Ma(the sedimentary period of Yimin Formation)should be the main hydrocarbon accumulation period in Huhehu depression. As the strata were denuded, and the temperature decreased, the hydrocarbon generated and hydrocarbon expulsed process became weak due to the uplift of the Hailar basin since Late Cretaceous.
2020, 45(1): 251-262.
doi: 10.3799/dqkx.2018.285
Abstract:
Micropore structure characterization of organic matters in the coal measures due to bioconversion is of great significance in understanding reservoir reformation by microorganism and revealing the storage and enrichment mechanism of biogenic gas in the coal measures. Pore structure evolution of organic matters in the coal measures degraded by microbe was analyzed using field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (MIP), low-pressure N2 gas adsorption pycnometry and fractal dimension FHH theory in this study. Considering the measuring range of pore size distribution (PSD) and combining the characteristics of microbial ecology, the pore structure type of coal and shale in coal measures is divided into three types. They are micropore (PSD>5 μm), micro-nanopore (5 μm-100 nm), and nanopore (2-100 nm). The PSD and micropore pore volume (PV) of coal and shale samples increase, and the specific surface area (SSA) and micro-nanopore and nanopore PV decrease after bioconversion. The surface fractal diameter (D1) and pore structure fractal diameter (D2) of coal and shale samples decrease after bioconversion, showing that the inner surface of pore becomes smooth and pore structure gets simple due to microbial action. The reformation of pore structure due to bioconversion is benefitial to the migration and enrichment of free gas in the coal measures.
Micropore structure characterization of organic matters in the coal measures due to bioconversion is of great significance in understanding reservoir reformation by microorganism and revealing the storage and enrichment mechanism of biogenic gas in the coal measures. Pore structure evolution of organic matters in the coal measures degraded by microbe was analyzed using field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (MIP), low-pressure N2 gas adsorption pycnometry and fractal dimension FHH theory in this study. Considering the measuring range of pore size distribution (PSD) and combining the characteristics of microbial ecology, the pore structure type of coal and shale in coal measures is divided into three types. They are micropore (PSD>5 μm), micro-nanopore (5 μm-100 nm), and nanopore (2-100 nm). The PSD and micropore pore volume (PV) of coal and shale samples increase, and the specific surface area (SSA) and micro-nanopore and nanopore PV decrease after bioconversion. The surface fractal diameter (D1) and pore structure fractal diameter (D2) of coal and shale samples decrease after bioconversion, showing that the inner surface of pore becomes smooth and pore structure gets simple due to microbial action. The reformation of pore structure due to bioconversion is benefitial to the migration and enrichment of free gas in the coal measures.
2020, 45(1): 263-275.
doi: 10.3799/dqkx.2018.374
Abstract:
Raoyang depression has rich oil and gas reserves and varieties of trap types. However, we have a poor understanding of the oil genetic types and sources. By using the biomarker parameter characteristics of source rocks and crude oils, through hierarchical cluster analysis (HCA), we divided the origin genetic type of the oils, setup the oil-rock relationship and discussed the distribution characteristics of various types of oils in this study. The results show that three sets of source rocks display obvious differences in biomarkers, with each presenting different characteristics of biomarker parameter assemblage. The oil discovered in the northern part of Raoyang depression can be identified into 4 types, which include Es3 type, mixed type derived from Es3 and Es1L, mixed type derived from Es1L and Es3U, and Es1L type. The differences on biomarker assemblages among the potential source rocks indicate that they had different parent materials and formed in different sedimentary environments. The crude oils of Es3 type are distributed in Renbei slope in the north of the Renqiu buried hill. The crude oils of Es3 and Es1L type are distributed in the south central region of the Maxi sag. The crude oils of Es1L and Es3U type are distributed in the Balizhuang oil field on the north of the Maxi sag. The crude oils of Es1L type are distributed in the Lixian slope in the west of study area. Each type of oil has its own distribution characteristics, and it is mainly controlled by the areal distribution and the thermal evolution condition of local source rock.
Raoyang depression has rich oil and gas reserves and varieties of trap types. However, we have a poor understanding of the oil genetic types and sources. By using the biomarker parameter characteristics of source rocks and crude oils, through hierarchical cluster analysis (HCA), we divided the origin genetic type of the oils, setup the oil-rock relationship and discussed the distribution characteristics of various types of oils in this study. The results show that three sets of source rocks display obvious differences in biomarkers, with each presenting different characteristics of biomarker parameter assemblage. The oil discovered in the northern part of Raoyang depression can be identified into 4 types, which include Es3 type, mixed type derived from Es3 and Es1L, mixed type derived from Es1L and Es3U, and Es1L type. The differences on biomarker assemblages among the potential source rocks indicate that they had different parent materials and formed in different sedimentary environments. The crude oils of Es3 type are distributed in Renbei slope in the north of the Renqiu buried hill. The crude oils of Es3 and Es1L type are distributed in the south central region of the Maxi sag. The crude oils of Es1L and Es3U type are distributed in the Balizhuang oil field on the north of the Maxi sag. The crude oils of Es1L type are distributed in the Lixian slope in the west of study area. Each type of oil has its own distribution characteristics, and it is mainly controlled by the areal distribution and the thermal evolution condition of local source rock.
2020, 45(1): 276-284.
doi: 10.3799/dqkx.2018.567
Abstract:
As an important type of unconventional reservoirs, tight oil and gas reservoirs are characterized by small pore scale and obvious micro-heterogeneity. The exploration potential is vast despite the major theoretical challenges in greatly improving the recovery rate of resources. In this study, the micro anisotropic characteristics of pores and minerals in continental tight sandstone reservoirs in Ordos basin, China, are quantitatively analyzed by means of "umbrella deconstruction". The case study shows that there is a significant micro anisotropy in the micro pore-throat development in eight directions, and the development characteristics of the anisotropic filler are obviously different. With the change of sampling angle, the micro pores show continuously unsteady distribution. The fractal dimension could characterize the porosity, permeability and pore-throat development probability. The study can provide important theoretical support and practical basis for revealing the mechanism of tight reservoir permeability, "sweet spot" distribution and guiding the effective development of tight oil and gas.
As an important type of unconventional reservoirs, tight oil and gas reservoirs are characterized by small pore scale and obvious micro-heterogeneity. The exploration potential is vast despite the major theoretical challenges in greatly improving the recovery rate of resources. In this study, the micro anisotropic characteristics of pores and minerals in continental tight sandstone reservoirs in Ordos basin, China, are quantitatively analyzed by means of "umbrella deconstruction". The case study shows that there is a significant micro anisotropy in the micro pore-throat development in eight directions, and the development characteristics of the anisotropic filler are obviously different. With the change of sampling angle, the micro pores show continuously unsteady distribution. The fractal dimension could characterize the porosity, permeability and pore-throat development probability. The study can provide important theoretical support and practical basis for revealing the mechanism of tight reservoir permeability, "sweet spot" distribution and guiding the effective development of tight oil and gas.
2020, 45(1): 285-302.
doi: 10.3799/dqkx.2018.128
Abstract:
The lower Cambrian Yuertusi Formation has been confirmed as the best high-quality source rocks of marine rocks in China, and the source rocks of the Cambrian pre-salt play in the Tarim and crucial research field of the Ediacaran-Cambrian transition. However, little work has been published about the paleo-environments. Yutixi section in Keping is intensively investigated by a multi proxy analysis including thin-sections identification, major elements, trace elements and TOC contents, for revealing the accumulation mechanisms of the sources rocks in this study. Results demonstrate that the samples are rich in trace elements comprising V, U, Ni, Ba, Mo, Cu and Zn. Along with the weakly positive Ce anomalies (Ce/Ce*=0.45), extremely positive Eu anomalies (Eu/Eu*=35.32), Y/Ho (39.77) and barite for samples of basal Yuertusi Fm., the REEs patterns of negative Ce anomalies, positive Y anomalies and positive Eu anomalies verified the drastic hydrothermal activities and anoxic conditions of the Early Cambrian. Specifically, many proxies including U/Al, V/Al, Th/U, V/Sc, and Mo-U covariation ascertained the paleocean in the northern Tarim was an open sea. The paleocean was under sulfide in period of Yuertusi Fm. Group A (Th/Uavg=0.070). Then it changed into suboxic in periods of Yuertusi Fm. Group B (Th/Uavg=1.21), Group C (Th/Uavg=0.62) and Group D (Th/Uavg=1.21) with generally increased oxidation level. Finally it was under sulfide conditions in period of Xiaoerbulake Fm. (Th/Uavg=0.13). In addition, the elevated productivity and well preservation are implied by the high TOC contents (TOCmax=17.2%) and ex-Baavg (8 634.85×10-6) for Yuertusi Fm. Group A. Ultimately, development pattern of the source rocks of the Lower Cambrian Yuertusi Fm., northern Tarim platform is proposed, which is characterized by hydrothermal activities, coastal upwelling and anoxic to sulfide conditions. This study will facilitate both the prospect evaluation of deep and super-deep oil and gas resources in Tarim basin, and studies of Ediacaran-Cambrian paleo-environments transition in NW China.
The lower Cambrian Yuertusi Formation has been confirmed as the best high-quality source rocks of marine rocks in China, and the source rocks of the Cambrian pre-salt play in the Tarim and crucial research field of the Ediacaran-Cambrian transition. However, little work has been published about the paleo-environments. Yutixi section in Keping is intensively investigated by a multi proxy analysis including thin-sections identification, major elements, trace elements and TOC contents, for revealing the accumulation mechanisms of the sources rocks in this study. Results demonstrate that the samples are rich in trace elements comprising V, U, Ni, Ba, Mo, Cu and Zn. Along with the weakly positive Ce anomalies (Ce/Ce*=0.45), extremely positive Eu anomalies (Eu/Eu*=35.32), Y/Ho (39.77) and barite for samples of basal Yuertusi Fm., the REEs patterns of negative Ce anomalies, positive Y anomalies and positive Eu anomalies verified the drastic hydrothermal activities and anoxic conditions of the Early Cambrian. Specifically, many proxies including U/Al, V/Al, Th/U, V/Sc, and Mo-U covariation ascertained the paleocean in the northern Tarim was an open sea. The paleocean was under sulfide in period of Yuertusi Fm. Group A (Th/Uavg=0.070). Then it changed into suboxic in periods of Yuertusi Fm. Group B (Th/Uavg=1.21), Group C (Th/Uavg=0.62) and Group D (Th/Uavg=1.21) with generally increased oxidation level. Finally it was under sulfide conditions in period of Xiaoerbulake Fm. (Th/Uavg=0.13). In addition, the elevated productivity and well preservation are implied by the high TOC contents (TOCmax=17.2%) and ex-Baavg (8 634.85×10-6) for Yuertusi Fm. Group A. Ultimately, development pattern of the source rocks of the Lower Cambrian Yuertusi Fm., northern Tarim platform is proposed, which is characterized by hydrothermal activities, coastal upwelling and anoxic to sulfide conditions. This study will facilitate both the prospect evaluation of deep and super-deep oil and gas resources in Tarim basin, and studies of Ediacaran-Cambrian paleo-environments transition in NW China.
2020, 45(1): 303-316.
doi: 10.3799/dqkx.2017.577
Abstract:
A large quantity of hydrocarbon inclusions were found in the gypsum of Cenozoic Qingshuiying Formation in Liupanshan basin, which is of a crucial significance to the understanding of the oil and gas reservoir-forming conditions and the development of exploration concept. Development and composition features of hydrocarbon inclusions in gypsum of Qingshuiying Formation were studied by field geological investigation, sample collection and laboratory analysis, etc. Sources and formation of the oil and gas were analyzed by oil-source correlation, and then the conditions and characteristics of petroleum accumulation in Cenozoic of Liupanshan basin were studied. The results show that the oil in the crystal gypsum was derived from the hydrocarbon source rock of Qingshuiying Formation, while the oil in the filament gypsum was derived from the hydrocarbon source rock of Cretaceous Madongshan Formation. The hydrocarbon source rock of Qingshuiying Formation has certain hydrocarbon generating potential, which is the potential source rock in Liupanshan basin. The oil and gas was derived from the Cretaceous hydrocarbon source rock, which can migrate to the upper formation along the fault, and accumulate in Cenozoic. The reservoir traps near the faults connected to the Cretaceous source rock are the favorable oil accumulation zones in Cenozoic.
A large quantity of hydrocarbon inclusions were found in the gypsum of Cenozoic Qingshuiying Formation in Liupanshan basin, which is of a crucial significance to the understanding of the oil and gas reservoir-forming conditions and the development of exploration concept. Development and composition features of hydrocarbon inclusions in gypsum of Qingshuiying Formation were studied by field geological investigation, sample collection and laboratory analysis, etc. Sources and formation of the oil and gas were analyzed by oil-source correlation, and then the conditions and characteristics of petroleum accumulation in Cenozoic of Liupanshan basin were studied. The results show that the oil in the crystal gypsum was derived from the hydrocarbon source rock of Qingshuiying Formation, while the oil in the filament gypsum was derived from the hydrocarbon source rock of Cretaceous Madongshan Formation. The hydrocarbon source rock of Qingshuiying Formation has certain hydrocarbon generating potential, which is the potential source rock in Liupanshan basin. The oil and gas was derived from the Cretaceous hydrocarbon source rock, which can migrate to the upper formation along the fault, and accumulate in Cenozoic. The reservoir traps near the faults connected to the Cretaceous source rock are the favorable oil accumulation zones in Cenozoic.
2020, 45(1): 317-329.
doi: 10.3799/dqkx.2018.319
Abstract:
The glycerol dialkyl glycerol ether (GDGTs) may be subjected to degradation during the sample storage, which may have an effect on the application of GDGTs. Thus, it is important to understand the resistance of GDGTs compounds to degradation for the accurate application of GDGT derived proxies. In this study, the extractions (GDGTs) of stalagmite sample from the year of 2012 was reanalyzed in 2017. It is found that the absolute concentration of GDGTs has decreased and the relative content of each component has changed significantly. The change of the concentration of bacterial brGDGT is smaller than that of the archaeal isoGDGTs, which corresponds to the decreased Ri/b value and increased BIT values. Therefore, compared to the isoGDGTs, the brGDGTs are more stable during the processes of degradation. Smaller changes of the concentration of archaeal isoGDGTs of less-cyclic-moieties and increasing CBT values indicate that GDGTs with fewer rings tend to be more stable during the degradation processes. The isoGDGTs based TEX86 values also decrease significantly. The increasing of the brGDGTs based MBT values show that GDGTs with more cyclopentyl moieties are more easily to be degraded.
The glycerol dialkyl glycerol ether (GDGTs) may be subjected to degradation during the sample storage, which may have an effect on the application of GDGTs. Thus, it is important to understand the resistance of GDGTs compounds to degradation for the accurate application of GDGT derived proxies. In this study, the extractions (GDGTs) of stalagmite sample from the year of 2012 was reanalyzed in 2017. It is found that the absolute concentration of GDGTs has decreased and the relative content of each component has changed significantly. The change of the concentration of bacterial brGDGT is smaller than that of the archaeal isoGDGTs, which corresponds to the decreased Ri/b value and increased BIT values. Therefore, compared to the isoGDGTs, the brGDGTs are more stable during the processes of degradation. Smaller changes of the concentration of archaeal isoGDGTs of less-cyclic-moieties and increasing CBT values indicate that GDGTs with fewer rings tend to be more stable during the degradation processes. The isoGDGTs based TEX86 values also decrease significantly. The increasing of the brGDGTs based MBT values show that GDGTs with more cyclopentyl moieties are more easily to be degraded.
2020, 45(1): 330-340.
doi: 10.3799/dqkx.2018.336
Abstract:
Quaternary volcanoes are widely distributed in the North Hainan island. However, the stability and the relationship with the Changliu-Xiangou fault remain controversial. 184 magnetotelluric data covering the Lei-Hu-Ling volcanic system were acquired in the Quaternary volcanic region of North Hainan island in this study. The analysis of phase tensors indicates the existence of electrical anisotropy, and consequently the electrical structure was obtained by 1D anisotropic inversion. The inversion results show that the minimum resistivity direction is almost parallel to the Changliu-Xiangou fault in the shallow (~1-5 km) and strikes south-north in the deep (~5-15 km), which is oblique to the fault. It is found that the Changliu-Xiangou fault is not a deep-large fault and does not control the deeper magmas path. We prefer to attribute the deep anisotropy to accumulation of high-salinity fluids in the storage zone with high porosity, which is associated with the transportation of magmatic fluids and volatiles supplied by a partial melting zone at greater depths. It is concluded that the Lei-Hu-Ling volcanic system is dormant at present.
Quaternary volcanoes are widely distributed in the North Hainan island. However, the stability and the relationship with the Changliu-Xiangou fault remain controversial. 184 magnetotelluric data covering the Lei-Hu-Ling volcanic system were acquired in the Quaternary volcanic region of North Hainan island in this study. The analysis of phase tensors indicates the existence of electrical anisotropy, and consequently the electrical structure was obtained by 1D anisotropic inversion. The inversion results show that the minimum resistivity direction is almost parallel to the Changliu-Xiangou fault in the shallow (~1-5 km) and strikes south-north in the deep (~5-15 km), which is oblique to the fault. It is found that the Changliu-Xiangou fault is not a deep-large fault and does not control the deeper magmas path. We prefer to attribute the deep anisotropy to accumulation of high-salinity fluids in the storage zone with high porosity, which is associated with the transportation of magmatic fluids and volatiles supplied by a partial melting zone at greater depths. It is concluded that the Lei-Hu-Ling volcanic system is dormant at present.
2020, 45(1): 341-354.
doi: 10.3799/dqkx.2017.576
Abstract:
There are many wading landslides in Three Gorges reservoir area, however, the spatial distribution and influencing factors of these landslides are not yet clear. The relevant data of 593 landslides along the Yangtze River from Three Gorges Dam to Jiangjin city were collected in this study. The lithology, slope structure, elevation and slopes angle were selected as the key control factors and reservoir water was selected as inducing factor. The reservoir bank slope on both sides of the Yangtze River was divided into several sections according to different factors. The spatial distribution characteristics and its internal mechanism of landslides were analyzed. The results could be summarized as follows:(1) The macroscopic spatial distribution of landslides presents significant regional differences and zonation characteristics because of the engineering geological properties of different rock groups. (2) It also shows obvious local differences in the left and right sides or the upstream and downstream in the same rock group. This may be related to the slope structure. The density of landslides in the consequent slope is much higher than transverse slope or converse slope. (3) Due to the topographic conditions and the influence of the reservoir water, the elevation of trailing and slopes angle of the landslides are gradually reduced from the head to tail region. Landslides front elevation were mainly focused on 100-175 m. The main inducing factor of landslide deformation was the fluctuation of reservoir water level. The effect was most significant when the water level fluctuated in front and middle of the landslide. The effect of the fluctuation of reservoir water level to landslide decreases gradually with the adjustment of deformation. The research results can facilitate the future prevention and controlling of landslides in Three Gorges reservoir area.
There are many wading landslides in Three Gorges reservoir area, however, the spatial distribution and influencing factors of these landslides are not yet clear. The relevant data of 593 landslides along the Yangtze River from Three Gorges Dam to Jiangjin city were collected in this study. The lithology, slope structure, elevation and slopes angle were selected as the key control factors and reservoir water was selected as inducing factor. The reservoir bank slope on both sides of the Yangtze River was divided into several sections according to different factors. The spatial distribution characteristics and its internal mechanism of landslides were analyzed. The results could be summarized as follows:(1) The macroscopic spatial distribution of landslides presents significant regional differences and zonation characteristics because of the engineering geological properties of different rock groups. (2) It also shows obvious local differences in the left and right sides or the upstream and downstream in the same rock group. This may be related to the slope structure. The density of landslides in the consequent slope is much higher than transverse slope or converse slope. (3) Due to the topographic conditions and the influence of the reservoir water, the elevation of trailing and slopes angle of the landslides are gradually reduced from the head to tail region. Landslides front elevation were mainly focused on 100-175 m. The main inducing factor of landslide deformation was the fluctuation of reservoir water level. The effect was most significant when the water level fluctuated in front and middle of the landslide. The effect of the fluctuation of reservoir water level to landslide decreases gradually with the adjustment of deformation. The research results can facilitate the future prevention and controlling of landslides in Three Gorges reservoir area.
