As the important strategic mineral resource, molybdenum is widely utilised by various industries of national economy. Most of the Chinese Mo deposits are classified as porphyry deposits, such as the Nannihu porphyry Mo-W deposit that is one of the biggest Mo deposits in China. Previous studies mostly focused on the geochemistry, geochronology, and isotopic geochemistry of the Nannihu deposit, but its element immigration and occurrence as well as the ore-forming source remain unclear, which limits the development of ore prospecting. Here we document in-situ major and trace element geochemistry of different stages of molybdenite and rutile from the Nannihu deposit, with major aims to determine ore-forming element immigration and occurrence and to reveal ore-forming processes. Based on mineral assemblages and micro-texture, three types of rutile were identified in the Nannihu deposit. The type-1 rutile has stable Y/Ho ratios 16.74–20.82 and similar upper crust rare earth elements (REEs) patterns, which suggest magmatic and crustal origin. The type-2 and 3 rutiles show wide variation Y/Ho ratios (10.69–49.55 for type-2 and 17.85–43.64 for type-3) and belong to hydrothermal rutiles. In addition, two types of molybdenite are also presented, with the Mo1-type for the major ore-forming stage and the Mo2-type for the late ore-forming stage. The fluctuation of elements in the time resolved LA-ICP-MS spectra curve diagram of the Mo1- and Mo2-type molybdenites indicate that the Te, Bi, Pb, Fe and Mn elements are hosted as mineral inclusions in molybdenite lattices. However, the Re and Se elements in these molybdenites display flat signatures and have negative correlations with Mo and S elements, which imply that the Mo and S elements are substituted and occur as isomorphous constituents in molybdenite lattices. The low Re contents (1.60–12.90 ppm) of molybdenites also suggest crustal-mantle mixing metallic sources for the Nannihu deposit. The increasing Fe (4245 to 23943 ppm) and W (1631 to 5922 ppm) contents in hydrothermal type-2 and type-3 rutiles as well as the increasing W contents (113 to 243 ppm) in Mo1- and Mo2-type molybdenites both indicate that the Nannihu ore-forming fluids were under the temperature and oxygen fugacity (fO2) of reduction conditions. The Mo1-type molybdenite is enriched in light REEs and depleted in high field strength elements (HFSEs), and coupled with the late ore-forming stage of F-rich fluorite, which indicate the ore-forming fluid evolution from Cl- to F-rich. Integrating with previous studies, the mineralising fluids of the Nannihu deposit are suggested to be featured by high Cl−, CO2 and metallic contents. The Mo and W ions in mineralising fluids immigrate as Cl-complexes at the early stage of mineralisation, the Mo4+ is then replaced by W4+ in molybdenite lattices at the later stage, and the high temperature and fO2 of the early ore-forming fluids promote the migration of Mo and W elements during ore-forming processes. Eventually, the fluids boiling, and the reduction conditions in temperature and fO2 jointly result in the final precipitation of Mo and W to generate the giant Nannihu deposit under extensional tectonics.
Olivine in deep-seated ultramafic xenoliths beneath the North China Block serves as a crucial proxy for decoding the compositions, properties, and evolution of the lithospheric mantle. Here, we conduct an investigation on olivine (including gem-grade) hosted in ultramafic xenoliths from Damaping basalt in the northern part of the North China Block. This contribution presents the results from petrographic, Raman spectroscopic, and major and trace elemental studies of olivine, with the aim of characterising the formation environment and genetic type of the olivine. The analysed olivine samples are characterised by high Mg# values (close to 91%) possessing refractory to fertile features and doublet bands with unit Raman spectra beams of 822 and 853 cm−1, which are indicative of a forsterite signature. Major and trace geochemistry of olivine indicates the presence of mantle xenolith olivine. All the analytical olivine assays ≤0.1 wt % CaO, ~40 wt % SiO2, and ≤0.05 wt % Al2O3. Furthermore, olivine displays significantly different concentrations of Ti, Y, Sc, V, Co, and Ni. The Ni/Co values in olivine range from 21.21 to 22.98, indicating that the crystallisation differentiation of basic magma relates to oceanic crust recycling. The V/Sc values in mantle/xenolith olivine vary from 0.54 to 2.64, indicating a more oxidised state of the mantle. Rare earth element (REE) patterns show that the LREEs and HREEs of olivine host obviously differentiated characteristics. The HREE enrichments of olivine and the LREE depletion of clinopyroxene further assert that the mantle in the Damaping area underwent partial melting. The wide variations of Mg# values in olivine and the Cr# values in clinopyroxene, along with major element geochemistry indicate transitional characteristics of different peridotite xenoliths. This is possibly indicative of a newly accreted lithospheric mantle interaction with an old lithospheric mantle at the time of the basaltic eruption during the Paleozoic to Cenozoic.
Abstract Water–rock interaction (WRI) is a topic of interest in geology and geotechnical engineering. Many geological hazards and engineering safety problems are severe under the WRI. This study focuses on the water weakening of rock strength and its influencing factors (water content, immersion time, and wetting–drying cycles). The strength of the rock mass decreases to varying degrees with water content, immersion time, and wetting–drying cycles depending on the rock mass type and mineral composition. The corresponding acoustic emission count and intensity and infrared radiation intensity also weaken accordingly. WRI enhances the plasticity of rock mass and reduces its brittleness. Various microscopic methods for studying the pore characterization and weakening mechanism of the WRI were compared and analyzed. Various methods should be adopted to study the pore evolution of WRI comprehensively. Microscopic methods are used to study the weakening mechanism of WRI. In future work, the mechanical parameters of rocks weakened under long-term water immersion (over years) should be considered, and more attention should be paid to how the laboratory scale is applied to the engineering scale.
The theoretical calculation of a counterweight double-row pile supporting structure is deduced and studied in this paper. The derived calculation method is applied to a Midas GTS NX simulation calculation. A case study of a deep foundation pit project in Shenzhen City is used to verify and analyze the simulation results and the field monitoring results. On this basis, the influence law of deformation parameters such as the row distance, pile diameter of back-row piles and load of the pit top on the pile of a double-row pile is further discussed. The results show that both the front- and back-row piles of counterweight double-row piles are overturning deformation, and the characteristics of the horizontal displacement are basically the same. The maximum value of the horizontal displacement of the pile is at the top and the minimum value is at the bottom. With the increase in the row distance and pile diameter, the horizontal displacement of the pile becomes smaller, and the change in the pile horizontal displacement under a top load is contrary to that. Moreover, the change in the row distance has a great influence on the horizontal displacement of the pile, followed by the load of the pit top, and the pile diameter of the back-row piles has the least influence. Due to the connection effect of the horizontal plate of the counterweight platform, the whole supporting structure is in the form of a hyperstatic structure. The back-row piles can withstand most of the lateral earth pressure, which effectively reduces the deformation of the front pile and improves the overall stiffness of the supporting structure, which is conducive to the excavation stability of the deep foundation pit. Therefore, its extensive use in the Linhai soft soil project can not only effectively reduce the number of internal supports and achieve the purpose of cost saving but also increase the construction face, which is beneficial to the development of dry construction organization and management, in line with the construction concept of green environmental protection and sustainable development advocated at present.
Abstract Understanding coal and rock permeability, and the corresponding influence on stress, is important in the field of energy development. In applied engineering, there is a tendency to employ three‐dimensional methods that are simpler, less time‐ and cost‐expensive, less computationally expensive, and larger scale. Thus, a numerical simulation fluid‐solid coupling method is proposed in this paper. The proposed numerical simulation method utilizes the stress‐permeability test results for elastic and plastic coal samples. The permeability models of the elastic and plastic coal samples under loading and unloading were obtained by fitting the experimental results and embedded them into FLAC 3D software by using FISH language. The results of the uniaxial and triaxial flow simulations are consistent with the experimental results, thereby confirming the accuracy and feasibility of the numerical method proposed in this paper. This allows the permeability of the numerical reservoir model to be continuously updated according to the current stress level in the production process.
Abstract The stress sensitivity of coal seams with different fracture structures affects gas migration characteristics in the fracture zone. In this study, permeability stress experiments for intact and persistent fracture coal samples are carried out. A discrete element method numerical simulation that adopts the changing joint stiffness method is proposed. Based on the experimental data, the corresponding parameters of persistent fractures and elastic joints for fluid–solid coupling simulations are determined. Subsequently, the internal influence mechanisms of fracture morphology on axial and confining stress sensitivities are determined. The simulation results show that sectional fractures with different dip angles in the persistent fracture control the sensitivity to axial and confining stresses. The horizontal segment fracture aperture decreases with an increase in axial stress, which reduces its flow capacity. This results in a decrease in the flow rates in the persistent fracture. When the confining stress increases, the vertical fracture aperture is significantly reduced, considerably reducing the flow rate in the persistent fracture.
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