Abstract:For the purpose of clarifying the ore-forming fluid and the origin of the lead-zinc-silver-gold deposit, the authors used galena, sphalerite and pyrite for microscopic identification, electron microprobe analysis and ICP-MS analysis. The results show that their consolidation sequence is pyrite, followed by sphalerite (altered pyrite by sphalerite) and galena (altered sphalerite and pyrite by galena). From the spatial distribution of sulfides composition, the authors detected that the migrated pathway of ore-forming fluid was from Diannan-Zhenzigou to Erdaogou-Xiquegou and Dadi-Baiyun, respectively. The formation temperature of sulfides dominated by galena is about 322~327℃ according to the consolidation temperature of galena (327℃) and the uniform temperature of quartz fluid inclusions (322℃). Meanwhile, the sulfide formation temperature of sulfides dominated by sphalerite is about 360~400℃ with reference to the uniform temperature of quartz fluid inclusion (300~360℃), the temperature of chalcopyrite dissolved from sphalerite (>350℃), the temperature of galena-sphalerite formation (344~464℃) using the Cd partition coefficient thermometer in galena-shpalerite and the experimental brittle deformation temperature (about 400℃) of pyrite, but the ore-forming temperature of Ag(Pb-Zn) ore deposits was limited to 390~400℃. Since natural gold mainly occurs in pyrite micro-fractures, the brittle deformation temperature of pyrite (about 400℃) and the temperature of quartz fluid inclusions (230~370℃) constrain the metallogenic temperature of gold to about 370~400℃. The higher metallogenic temperature of Ag(Pb-Zn) and gold deposits may be attributed to the deeper location of the samples collected. At last, the relative relationship between the ore-forming fluid and the Mesozoic (especially the Indosinian period) magmatic rocks was judged by the comparative analysis of trace elements. According to the characteristics of the Indosinian magmatic mixing, it is considered that the magmatic mixing might have contributed to the mineralization in this area.

Abstract:The Jiama superlarge copper-polymetallic deposit is located in the eastern part of the Gangdise metallogenic belt, and mainly includes four types of orebodies: skarn orebody, porphyry orebody, hornfels orebody and independent gold orebody. There are extensive magmatic biotites in the intermediate acid intrusions of the deposit, and hornblende is more common in some intrusions. On the basis of comprehensive geological survey in the mining area and detailed drilling core cataloging, the authors studied mineralogy and mineral chemistry of magmatic biotite in ore-bearing monzogranite porphyry and ore-bearing granodiorite porphyry as well as hornblende in ore-bearing granodiorite porphyry so as to reveal its petrogenic and metallogenic significance. The results show that the magmatic biotite in monzogranite porphyry and granodiorite porphyry is Mg-biotite, which is characterized by higher MgO, TiO₂, Al₂O₃, K₂O contents, and lower SiO₂, Na₂O content. Compared with granodiorite porphyry, magmatic biotite in monzonitic granite porphyry has lower content of TiO₂, FeO^T, MgO, MnO, Na₂O, BaO, and higher content of Al₂O₃, SiO₂. Hornblende in granodiorite porphyry belongs to actinolite, which is characterized by higher SiO₂, MgO and CaO content, and lower Al₂O₃, TiO₂, Na₂O and K₂O content. Biotite and hornblende thermometers show that the crystallization temperature of biotite in ore-bearing monzogranite porphyry is 740.1~783.8℃, with an average of 762.4℃, that of biotite in ore-bearing granodiorite porphyry is 750.3~766.9℃, with an average of 757.2℃, and that of hornblende in ore-bearing granodiorite porphyry is 654.1~698.9℃, with an average of 680.3℃. The mineral chemical characteristics of biotite and hornblende indicate that monzogranite porphyry and granodiorite porphyry are calc-alkaline series and I-type granite in the orogenic belt, which has the characteristics of crust-mantle mixed source. Monzogranite porphyry and granodiorite porphyry had higher oxygen fugacity (above NNO) and formed water-rich magmas, which was beneficial for the entry of copper, molybdenum and other ore-forming materials into ore-forming fluid.

Abstract:Detailed petrographic observation and systematic mineralogical chemistry studies of the Xinzhai granite from the north Guangxi were performed in this paper. Based on electron microprobe analysis (EMPA) data, the authors calculated pressure, temperature, oxygen fugacities (log f_O2) and H₂O in co-existent melts conditions of the crystallization of these granites. According to the petrographic features, there are three types of amphiboles in Xinzhai amphibole granite, i.e., euhedral amphibole, unaltered subhedral/anhedral amphiboles and strongly metasomatized subhedral/anhedral amphiboles, which are products of crystallization or metasomatic alteration of the Xinzhai intrusion formed at different depths and stages in the process of emplacement. The EMPA results show that the major element compositions of amphibole in Xinzhai granites are varied depending on crystal morphology, characterized by the decreasing of Al₂O₃, FeO^T, Na₂O, TiO₂, K₂O but increasing of MgO and SiO₂ during magmatic emplacement. The values of crystallization pressures estimated from aluminium-in-amphibole barometry of the euhedral amphibole and unaltered subhedral/anhedral amphiboles are 0.28~0.30 and 0.19~0.26 GPa respectively, indicating that the emplacement depth of the Xinzhai pluton should be greater than 11.3 km. Furthermore, the corresponding crystallization temperatures of emplacement calculated with the amphibole-plagioclase thermometry and amphibole thermometry of the euhedral amphibole and unaltered subhedral/anhedral amphiboles are 767~783℃ and 740~764℃ respectively. Based on the p-T conditions of the pluton crystallization, it is suggested that the Xinzhai pluton experienced isothermal decompression and polybaric crystallization during the emplacement. The calculated oxygen fugacities of the unaltered amphiboles are in the range of ΔNNO+0.1 (log f_O2=-13.5)～ΔNNO+0.5 (log f_O2=－12.9), indicating a relatively high oxygen fugacity. Moreover, the fractional crystallization enriched the water content of the residual melt (4.9%~6.4%) and experienced exolved aqueous fluids when the evolved magma intruded into the shallower depth (7.0~9.5 km) mainly due to depressurization. Genetic mineralogy studies show that amphibole in the Xinzhai granite belongs to calc-alkaline intermediate and acid magma, which was likely derived from a crust-mantle mixed origin and continuously contaminated by crust materials in the process of emplacement.

Abstract:The gabbros were emplaced in the Dakendaban Group in the northwest of Saishiteng Mountain, west section of the tectonic belt of northern margin of Qaidam Basin. They are characterized by low SiO₂, high Al₂O₃, rich MgO and FeO^T but poor ALK, thus belonging to subalkalic calc-alkaline series. Meanwhile, The rocks are enriched in LILE and depleted in HFSE. The chondrite-normalized REE patterns show right-inclined shape with LREE enrichment and flat HREE. The rocks are characterized by low Zr and Zr/Y ratio. The authors consider that the gabbros emplaced in the Saishiteng Mountain were formed in the island-arc environment of an active continental margin. LA-ICP-MS zircon U-Pb dating of gabbros yielded the formation time of 271±3 Ma. Comprehensive analysis shows that the original rocks of gabbros can be considered as a product of partial melting of spinel lherzolite in the lithospheric mantle, and the magma might have undergone fractional crystallization of strong clinopyroxene and weak plagioclase during its evolution, which however was slightly assimilated and contaminated by mid-upper crust materials in the emplacement process. In combination with the tectonic evolution of the tectonic belt of northern margin of Qaidam Basin and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the gabbros were produced in the process when the southward subduction of Zongwulong oceanic crust toward Oulongbuluke block in early Middle Permian. The authors hold that the subduction-collision event of the western part of Zongwulong structural belt was obviously earlier than that of its eastern segment, and the difference of the subduction-collision time shows that the Zongwulong Ocean were probably characterized by scissor-like closure early in the west and late in the east.

Abstract:The Mesozoic large-scale lithosphere thinning of North China Craton (NCC) was documented by coeval volcanic rocks. Petrogenesis and magma source of the mafic volcanic rocks in the NCC is debated, which therefore restricts the understanding of mechanism and termination time of the NCC Mesozoic lithosphere thinning. In this paper, the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) was used to analyze the trace elements of zircons and rutiles in basaltic porphyry from the Xingcheng area, with the purpose of revealing the nature and evolution of magma source and further providing insights into mechanism and termination time of the NCC Mesozoic lithosphere thinning. t_Ti thermometer of zircon shows that the zircons crystallized at 695~779℃, with an average crystallization temperature at 730℃, indicative of an origin of mantle wedge metasomatic peridotite in the subduction zone. Zircons of basaltic porphyry display geochemical characteristics of high HREE and HREE/LREE ratio, negative Nb-Ti anomalies, relative enrichment of Th, U and Hf, positive Ce anomaly, and negative Eu anomaly. Rutile granules are characterized by high concentrations of Nb, Ti and Hf, and Pb, negative Sr anomaly, without obvious Ce and Eu anomalies. Such characteristics indicate that the basaltic porphyry magmas originated from partial melting of metasomatic lithospheric mantle peridotite, and the basaltic magma erupted after oxidation and fractional crystallization. It is therefore proposed that the termination of the North China Craton Mesozoic lithosphere thinning can be restricted to ca. 105 Ma in the Early Cretaceous, and the Xingcheng basaltic magma shows ancient island arc magma affinity. The slab rollback of the subducted Paleo-Pacific plate with high-angle was the dominant geodynamic mechanism, and the overlying lithospheric mantle was heated, corroded, and delaminated, which seems to have been the secondary mechanism.

Abstract:Wudang Group is an important part of the South Qinling basement, and therefore the study of its age and structural property is of great significance for understanding the early geological evolution history of the South Qinling belt and its relationship with the Yangtze plate in the Neoproterozoic. At present, the age, division and comparison of Wudang Group are still controversial. In this paper, zircon SHRIMP U-Pb isotope dating was performed on 4 metamorphic acid volcanic rocks from Wudang Group and 1 granite porphyry invading the group in the Xixia-Neixiang-Yunxi area. The age of a sample located at the bottom of Wudang Group in the Xixia-Neixiang area is 802±13 Ma, which can represent the age of the bottom strata of Wudang Group. In addition, 3 volcanic samples collected from the middle-upper part of the group were formed at 725±9 Ma, 732±6 Ma and 725±7 Ma respectively, and meanwhile the age of the granite porphyry is 703±5 Ma. Based on the above results and combined with previous research data on Wudang Group, it is determined that the formation time of Wudang Group is 810~720 Ma, equivalent to the age of Banxi Group of the Yangtze plate, while the Yaolinghe Group between the Wudang Group and the Sinian could be compared with the Nanhua System.

Abstract:Lots of diabase and gabbro in the Suli area of the west segment of middle Qilian invaded into the Proterozoic geological bodies in the form of dykes. The previous studies of the basic magmatic rocks in this area were relatively insufficient and hence there are rare relevant reports. The content of SiO₂, Al₂O₃ and TiO₂ in the basic dyke are 46.77%～52.37%, 12.82%～15.86% and 1.16%～3.14%, respectively, characterized by high MgO (3.84%～7.98%) and FeO^T(10.42%～15.53%), and poor K₂O (0.10%～1.60%) and P₂O₅ with Na₂O higher than K₂O. The total rare earth content of the whole rock is low (51.28×10^-6～165.11×10^-6), the ratio of light rare earth to heavy rare earth is between 2.67 and 4.90, and the (La/Yb)_N ratio is between 2.04 and 5.03. The rocks are of light rare earth enrichment type, Eu=0.93～1.12, without obvious Eu anomaly. They are characterized by enrichment of Rb, Ba, Th and U and depletion of Nb and K; in addition, large ionic lithophile element abundance range is relatively wide, high field strength element enrichment degree is not strong, and trace element ratio on the cobweb diagram exhibits the pattern of an uplift on the left and relatively flat on the right, with the geochemical performance exhibiting the obvious characteristics of intraplate basalt. The LA-ICP-MS zircon U-Pb age is 819.5±5.2 Ma, and its formation age is older than that of the global Rodinia supercontinent. The results of rock geochemical analysis show that the magma originated from depleted mantle source area, probably being the product of partial melting of spinel mantle peridotite. The magma was formed in an intracontinental rift environment, indicating that it may have been the product of the breakup of Rodinia supercontinent.

Abstract:The Fangyangshan Cu-Pb-Zn polymetallic deposit in Zhenkang County of western Yunnan Province is the first copper-rich deposit recently discovered in the south of Baoshan block. Fluid inclusions and C-O isotopes were studied in order to reveal the source of ore-forming fluid and the genesis of the deposit. The study shows that stage Ⅱ is dominated by liquid-rich inclusions, whose homogenization temperatures range from 240 to 300℃ and salinities from 8.0% to 18.0%. Stage Ⅲ is characterized by CO₂-rich and daughter minerals-bearing fluid inclusions with homogenization temperatures and salinities both showing two peaks of concentration. Their homogenization temperatures range from 360 to 400℃ with salinities from 16.0% to 24.0%, and temperatures from 240 to 320℃ with salinities from 4.0% to 14.0% respectively. Stage Ⅳ is mainly characterized by liquid-rich and pure-liquid inclusions, whose homogenization temperatures range from 220 to 340℃ and salinities range from 8.0% to 14.0%. The values of δ¹³C_V-PDB and δ¹⁸O_V-SMOW in different stages are plotted in the range of -5.88‰ to -2.37‰ and 8.18‰ to 12.79‰ respectively, indicating that ore-forming fluid of the Fangyangshan deposit was sourced from magmatic hydrothermal solution, with its migration process influenced by the dissolution of wall rocks. According to the above studies, the authors hold that the ore-forming fluid in stage Ⅱ and stage Ⅲ of the Fangyangshan deposit mainly came from the medium-high temperature and salinity and CO₂-rich magmatic hydrothermal solution formed by continental collision, and the ore-forming fluid was influenced by medium-low temperature and low salinity meteoric water in late stage Ⅲ and stage Ⅳ. The authors hold that the Fangyangshan Cu-Pb-Zn polymetallic deposit is a middle-high temperature hypothermal deposit which is controlled by the structure. It is also inferred that skarn type orebodies might be found in the deep part of the mining area.

Abstract:In this paper, the authors studied quartz veins and calcite veins in the Xiangyangping uranium ore deposit located in the middle segment of Miao'er Mountain pluton. The research samples include quartz veins, calcite veins and breecia ores. They have similar REE patterns showing LREE-enrichment and strong negative Eu anomalies. This characteristic is the same as that of Douzhashan granite, suggesting that most of the uranium came from surrounding rocks. ∑REE and HREE from calcite veins and breecia ores grew with uranium-enrichment process. Carbon oxygen isotope analysis from calcite shows that most of the ore-forming fluid came from the mantle. The uranium-enrichment calcite shows lower δ¹³C value and high δ¹⁸O value, which indicates that CO₂ degassing seems to have been the main factor in uranium mineral precipitation processing. From mineralization period to post-mineralization period, δ¹³C value of calcite grew and δ¹⁸O value was reduced; besides, breecia ores had weak negative Ce anomalies. All of these data indicate that meteoric water took part in the process of post-mineralization. The authors built the uranium mineralization model for the middle segment of Miao'er Mountain pluton, and emphasized that multiple phases of mineralization and meteoric water seems to have been the critical factor of forming large orebody in this area.

Abstract:Organic matter and brittle minerals in "sweet spot" shale play an important role in controlling the development of fractures as well as revealing and grasping the mechanism and law related to fracture initiation, development and evolution, and hence they have a very important engineering practice value for the optimization of fractured intervals and the stimulation and stable production of shale gas. Taking Longmaxi Formation shale in northwest Guizhou as an example, the authors analyzed the mechanism of fracture control related to organic matter and brittle minerals in the "sweet spot" layer, and demonstrated a complete process of fracture formation and evolution in the "sweet spot" layer by numerical simulation experiment. The results show that the synchronous enrichment of organic matter and brittle minerals provides an important material basis for the development and evolution of fractures. The fracture tends to propagate along the structural plane formed by organic matter, and the pore structure of organic matter has obvious inducing mechanism for the formation of the fracture. Under the action of external force, the internal accumulated energy is transformed into the deformation and displacement of brittle minerals. The higher the content of brittle minerals, the stronger the fracture development capability and the higher the fracture development degree. The numerical simulation shows a whole process of fracture initiation and evolution due to the stress concentration at the end of organic pores and brittle minerals. This paper is expected to provide some references for further understanding of fracture mechanism and adjustment of fracturing parameters in the process of fracturing construction.