The Hujiayu Cu deposit,representative of the ＂HuBi-type＂ Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton,is primarily hosted in graphitebearing schists and carbonate ro...The Hujiayu Cu deposit,representative of the ＂HuBi-type＂ Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton,is primarily hosted in graphitebearing schists and carbonate rocks.The ore minerals comprise mainly chalcopyrite,with minor sphalerite,siegenite[（Co,Ni）_3S_4],and clausthalite[Pb（S,Se）].The gangue minerals are mainly quartz and dolomite,with minor albite.Four fluid inclusion types were recognized in the chalcopyrite-pyrite-dolomite-quartz veins,including CO_2-rich inclusions（type Ⅰ）,low-salinity,liquid-dominated,biphase aqueous inclusions（type Ⅱ）,solid-bearing aqueous inclusions（type Ⅲ）,and solid-bearing aqueous-carbonic inclusions（type Ⅳ）.Type I inclusion can be further divided into two sub-types,i.e.,monophase CO_2 inclusions（type Ⅰa） and biphase CO_2-rich inclusions（with a visible aqueous phase）,and type Ⅲ inclusion is divided into a subtype with a halite daughter mineral（type Ⅲa） and a subtype with multiple solids（type Ⅲb）.Various fluid inclusion assemblages（FIAs） were identified through petrographic observations,and were classified into four groups.The group-1 FIA,consisting of monophase CO_2 inclusions（type Ⅰa）,homogenized into the liquid phase in a large range of temperatures from-1 to 28℃,suggesting post-entrapment modification.The group-2 FIA consists of type Ⅰb,Ⅲb and Ⅳ inclusions,and is interpreted to reflect fluid immiscibility.The group-3 FIA comprises type Ⅱ and Ⅲa inclusions,and the group-4FIA consists of type Ⅱ inclusions with consistent phase ratios.The group-1 and group-2 FIAs are interpreted to be entrapped during mineralization,whereas group-3 and group-4 FIAs probably represent the post-mineralization fluids.The solid CO_2 melting temperatures range from-60.6 to56.6℃ and from-66.0 to-63.4℃ for type Ⅰa and type Ⅳ inclusions,respectively.The homogenization temperatures for type Ⅱ inclusions range from 132 to 170℃ for group-3 FIAs and115 to 219℃ for group-4 FIAs.The halite melting temperatures range from 530 to 562℃ for typeⅢ b and Ⅳ inclusions,whereas those for type Ⅲa inclusions range from 198 to 398℃.Laser Raman and SEM-EDS results show that the gas species in fluid inclusions are mainly CO_2 with minor CH_4,and the solids are dominated by calcite and halite.The calcite in the hosting marble and dolomite in the hydrothermal veins have δ^（13）C_（V-pdb） values of-0.2 to 1.2‰ and-1.2 to-6.3‰,and δ^（18）O_（v-smow） values of 14.0 to 20.8 ‰ and 13.2 to 14.3‰,respectively.The fluid inclusion and carbon-oxygen isotope data suggest that the ore-forming fluids were probably derived from metamorphic fluids,which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4-1.8 kbar and 230-240℃,after peak metamorphism.It is proposed that the Hujiayu Cu deposit consists of two mineralization stages.The early stage mineralization,characterized by disseminated and veinlet copper sulfides,probably took place in an environment similar to sediment-hosted stratiform copper mineralization.Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage,leading to the formation of thick quartz-dolomite-sulfides veins.展开更多
The North China Craton(NCC) hosts numerous gold deposits and is known as the most gold-productive region of China. The gold deposits were mostly formed within a few million years in the Early Cretaceous(130–120 Ma), ...The North China Craton(NCC) hosts numerous gold deposits and is known as the most gold-productive region of China. The gold deposits were mostly formed within a few million years in the Early Cretaceous(130–120 Ma), coeval with widespread occurrences of bimodal magmatism, rift basins and metamorphic core complexes that marked the peak of lithospheric thinning and destruction of the NCC. Stable isotope data and geological evidence indicate that ore-forming fluids and other components were largely exsolved from cooling magma and/or derived from mantle degassing during the period of lithospheric extension. Gold mineralization in the NCC contrasts strikingly with that of other cratons where gold ore-forming fluids were sourced mostly from metamorphic devolatization in compressional or transpressional regimes. In this paper, we present a summary and discussion on time-space distribution and ore genesis of gold deposits in the NCC in the context of the timing, spatial variation, and decratonic processes. Compared with orogenic gold deposits in other cratonic blocks, the Early Cretaceous gold deposits in the NCC are quite distinct in that they were deposited from magma-derived fluids under extensional settings and associated closely with destruction of cratonic lithosphere. We argue that Early Cretaceous gold deposits in the NCC cannot be classified as orogenic gold deposits as previously suggested, rather, they are a new type of gold deposits, termed as "decratonic gold deposits" in this study. The westward subduction of the paleo-West Pacific plate(the Izanagi plate) beneath the eastern China continent gave rise to an optimal tectonic setting for large-scale gold mineralization in the Early Cretaceous. Dehydration of the subducted and stagnant slab in the mantle transition zone led to continuous hydration and considerable metasomatism of the mantle wedge beneath the NCC. As a consequence, the refractory mantle became oxidized and highly enriched in large ion lithophile elements and chalcophile elements(e.g., Cu, Au, Ag and Te). Partial melting of such a mantle would have produced voluminous hydrous, Au- and S-bearing basaltic magma, which, together with crust-derived melts induced by underplating of basaltic magma, served as an important source for ore-forming fluids. It is suggested that the Eocene Carlin-type gold deposits in Nevada, occurring geologically in the deformed western margin of the North America Craton, are comparable with the Early Cretaceous gold deposits of the NCC because they share similar tectonic settings and auriferous fluids. The NCC gold deposits are characterized by gold-bearing quartz veins in the Archean amphibolite facies rocks, whereas the Nevada gold deposits are featured by fine-grained sulfide dissemination in Paleozoic marine sedimentary rocks. Their main differences in gold mineralization are the different host rocks, ore-controlling structures, and ore-forming depth. The similar tectonic setting and ore-forming fluid source, however, indicate that the Carlin-type gold deposits in Nevada are actually analogous to decratonic gold deposits in the NCC. Gold deposits in both the NCC and Nevada were formed in a relatively short time interval(<10 Myr) and become progressively younger toward the subduction zone. Younging of gold mineralization toward subduction zone might have been attributed to retreat of subduction zone and rollback of subducted slab. According to the ages of gold deposits on inland and marginal zones, the retreat rates of the Izanagi plate in the western Pacific in the Early Cretaceous and the Farallon plate of the eastern Pacific in the Eocene are estimated at 8.8 cm/yr and 3.3 cm/yr, respectively.展开更多
The mineral inclusions in zircon from gneisses in ultra-high pressure (UHP) zone of the Dabie Mountains were identified by using a laser Raman microspectrometer. Coesite occurs as inclusions in zircons from all types ...The mineral inclusions in zircon from gneisses in ultra-high pressure (UHP) zone of the Dabie Mountains were identified by using a laser Raman microspectrometer. Coesite occurs as inclusions in zircons from all types of gneiss. Other important minerals, such as jadeite, omphacite, aragonite, barite, and anhydrite were also found as inclusion minerals. These discoveries indicate that ( i) gneissic country rocks had metamorphosed at the same time as the enclosed eclogites; and (ii) SO4-2 -bearing fluids were presentin the UHP metamorphic process, which is manifested by occurrence of barite and anhydrite coexisting with coesite.展开更多
Zircon U-Pb ages have been determined for basement gneisses in the Bayan Obo REE-Fe-Nb deposit. On the U-Pb concordia diagram data of three samples yield up-per intercept ages of 1948-1917 Ma. Thus, these basement gne...Zircon U-Pb ages have been determined for basement gneisses in the Bayan Obo REE-Fe-Nb deposit. On the U-Pb concordia diagram data of three samples yield up-per intercept ages of 1948-1917 Ma. Thus, these basement gneisses are of Paleoproterozoic. Rock association of these basement gneisses includes tonalite, syenite and paragneiss, not belonging to a sedimentary package. It is more appro-priate to consider this association as a basement complex rather than a stratigraphic 'Group'.展开更多
基金financed by Major State Basic Research Development Program(2012CB416605)Natural Science Foundation of China(41402083)
文摘The Hujiayu Cu deposit,representative of the ＂HuBi-type＂ Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton,is primarily hosted in graphitebearing schists and carbonate rocks.The ore minerals comprise mainly chalcopyrite,with minor sphalerite,siegenite[（Co,Ni）_3S_4],and clausthalite[Pb（S,Se）].The gangue minerals are mainly quartz and dolomite,with minor albite.Four fluid inclusion types were recognized in the chalcopyrite-pyrite-dolomite-quartz veins,including CO_2-rich inclusions（type Ⅰ）,low-salinity,liquid-dominated,biphase aqueous inclusions（type Ⅱ）,solid-bearing aqueous inclusions（type Ⅲ）,and solid-bearing aqueous-carbonic inclusions（type Ⅳ）.Type I inclusion can be further divided into two sub-types,i.e.,monophase CO_2 inclusions（type Ⅰa） and biphase CO_2-rich inclusions（with a visible aqueous phase）,and type Ⅲ inclusion is divided into a subtype with a halite daughter mineral（type Ⅲa） and a subtype with multiple solids（type Ⅲb）.Various fluid inclusion assemblages（FIAs） were identified through petrographic observations,and were classified into four groups.The group-1 FIA,consisting of monophase CO_2 inclusions（type Ⅰa）,homogenized into the liquid phase in a large range of temperatures from-1 to 28℃,suggesting post-entrapment modification.The group-2 FIA consists of type Ⅰb,Ⅲb and Ⅳ inclusions,and is interpreted to reflect fluid immiscibility.The group-3 FIA comprises type Ⅱ and Ⅲa inclusions,and the group-4FIA consists of type Ⅱ inclusions with consistent phase ratios.The group-1 and group-2 FIAs are interpreted to be entrapped during mineralization,whereas group-3 and group-4 FIAs probably represent the post-mineralization fluids.The solid CO_2 melting temperatures range from-60.6 to56.6℃ and from-66.0 to-63.4℃ for type Ⅰa and type Ⅳ inclusions,respectively.The homogenization temperatures for type Ⅱ inclusions range from 132 to 170℃ for group-3 FIAs and115 to 219℃ for group-4 FIAs.The halite melting temperatures range from 530 to 562℃ for typeⅢ b and Ⅳ inclusions,whereas those for type Ⅲa inclusions range from 198 to 398℃.Laser Raman and SEM-EDS results show that the gas species in fluid inclusions are mainly CO_2 with minor CH_4,and the solids are dominated by calcite and halite.The calcite in the hosting marble and dolomite in the hydrothermal veins have δ^（13）C_（V-pdb） values of-0.2 to 1.2‰ and-1.2 to-6.3‰,and δ^（18）O_（v-smow） values of 14.0 to 20.8 ‰ and 13.2 to 14.3‰,respectively.The fluid inclusion and carbon-oxygen isotope data suggest that the ore-forming fluids were probably derived from metamorphic fluids,which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4-1.8 kbar and 230-240℃,after peak metamorphism.It is proposed that the Hujiayu Cu deposit consists of two mineralization stages.The early stage mineralization,characterized by disseminated and veinlet copper sulfides,probably took place in an environment similar to sediment-hosted stratiform copper mineralization.Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage,leading to the formation of thick quartz-dolomite-sulfides veins.
基金financially supported by the National Natural Science Foundation of China(Grant No.91414301)project of the State Key Laboratory of Lithospheric Evolution(Grant No.1303)
文摘The North China Craton(NCC) hosts numerous gold deposits and is known as the most gold-productive region of China. The gold deposits were mostly formed within a few million years in the Early Cretaceous(130–120 Ma), coeval with widespread occurrences of bimodal magmatism, rift basins and metamorphic core complexes that marked the peak of lithospheric thinning and destruction of the NCC. Stable isotope data and geological evidence indicate that ore-forming fluids and other components were largely exsolved from cooling magma and/or derived from mantle degassing during the period of lithospheric extension. Gold mineralization in the NCC contrasts strikingly with that of other cratons where gold ore-forming fluids were sourced mostly from metamorphic devolatization in compressional or transpressional regimes. In this paper, we present a summary and discussion on time-space distribution and ore genesis of gold deposits in the NCC in the context of the timing, spatial variation, and decratonic processes. Compared with orogenic gold deposits in other cratonic blocks, the Early Cretaceous gold deposits in the NCC are quite distinct in that they were deposited from magma-derived fluids under extensional settings and associated closely with destruction of cratonic lithosphere. We argue that Early Cretaceous gold deposits in the NCC cannot be classified as orogenic gold deposits as previously suggested, rather, they are a new type of gold deposits, termed as "decratonic gold deposits" in this study. The westward subduction of the paleo-West Pacific plate(the Izanagi plate) beneath the eastern China continent gave rise to an optimal tectonic setting for large-scale gold mineralization in the Early Cretaceous. Dehydration of the subducted and stagnant slab in the mantle transition zone led to continuous hydration and considerable metasomatism of the mantle wedge beneath the NCC. As a consequence, the refractory mantle became oxidized and highly enriched in large ion lithophile elements and chalcophile elements(e.g., Cu, Au, Ag and Te). Partial melting of such a mantle would have produced voluminous hydrous, Au- and S-bearing basaltic magma, which, together with crust-derived melts induced by underplating of basaltic magma, served as an important source for ore-forming fluids. It is suggested that the Eocene Carlin-type gold deposits in Nevada, occurring geologically in the deformed western margin of the North America Craton, are comparable with the Early Cretaceous gold deposits of the NCC because they share similar tectonic settings and auriferous fluids. The NCC gold deposits are characterized by gold-bearing quartz veins in the Archean amphibolite facies rocks, whereas the Nevada gold deposits are featured by fine-grained sulfide dissemination in Paleozoic marine sedimentary rocks. Their main differences in gold mineralization are the different host rocks, ore-controlling structures, and ore-forming depth. The similar tectonic setting and ore-forming fluid source, however, indicate that the Carlin-type gold deposits in Nevada are actually analogous to decratonic gold deposits in the NCC. Gold deposits in both the NCC and Nevada were formed in a relatively short time interval(<10 Myr) and become progressively younger toward the subduction zone. Younging of gold mineralization toward subduction zone might have been attributed to retreat of subduction zone and rollback of subducted slab. According to the ages of gold deposits on inland and marginal zones, the retreat rates of the Izanagi plate in the western Pacific in the Early Cretaceous and the Farallon plate of the eastern Pacific in the Eocene are estimated at 8.8 cm/yr and 3.3 cm/yr, respectively.
基金This work was sup- ported by the National Natural Science Foundation of China (Grant No.49972029) the Ministry of Science and Technology of China (Grant No. G1999075501).
文摘The mineral inclusions in zircon from gneisses in ultra-high pressure (UHP) zone of the Dabie Mountains were identified by using a laser Raman microspectrometer. Coesite occurs as inclusions in zircons from all types of gneiss. Other important minerals, such as jadeite, omphacite, aragonite, barite, and anhydrite were also found as inclusion minerals. These discoveries indicate that ( i) gneissic country rocks had metamorphosed at the same time as the enclosed eclogites; and (ii) SO4-2 -bearing fluids were presentin the UHP metamorphic process, which is manifested by occurrence of barite and anhydrite coexisting with coesite.
基金This work was supported by the NationalNatural Sciences Foundation of China the Science and Technology of Commission of China.
文摘Zircon U-Pb ages have been determined for basement gneisses in the Bayan Obo REE-Fe-Nb deposit. On the U-Pb concordia diagram data of three samples yield up-per intercept ages of 1948-1917 Ma. Thus, these basement gneisses are of Paleoproterozoic. Rock association of these basement gneisses includes tonalite, syenite and paragneiss, not belonging to a sedimentary package. It is more appro-priate to consider this association as a basement complex rather than a stratigraphic 'Group'.
