Mechanism of mineralization in the Changjiang uranium ore field,South China: Evidence from fluid inclusions,hydrothermal alteration,and H–O isotopes

The Changjiang uranium ore field, which contains >10,000 tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O₂, and those of the late syn-ore and post-ore stage contain H₂ and CH₄. The fluid inclusions in quartz of the syn-ore stage include H₂O, H₂O–CO₂, and CO₂ types, and they occur in clusters or along trails. Homogenization temperatures (T_h) for the H₂O–CO₂ and two-phase H₂O inclusions range from 106 °C to >350 °C and cluster in two distinct groups for each type; salinities are lower than 10 wt% NaCl equiv. The ore-forming fluids underwent CO₂ effervescence or phase separation at ∼250 °C under a pressure of 1000–1100 bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe₂O₃/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe₂O₃/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe₂O₃/FeO ratios in the alteration zones. The δ¹⁸O_W–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δD_W–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO₂ effervescence.     

Geochemical characteristics of mud volcano fluids in the southern margin of the Junggar basin,NW China: implications for fluid origin and mud volcano formation mechanisms

Mud volcanoes can provide important information about the underlying strata, hydrocarbon accumulation, and recent neotectonic movements in an area. The fluids erupting from mud volcanoes provide important information about their formation and evolution. The ion concentration and the hydrogen and oxygen isotopes of the fluids that were erupted from the three mud volcano groups, Baiyanggou, Aiqigou, and Dushanzi, and nearby rivers in the southern margin of the Junggar basin, northwestern China, are studied. The concentrations of Na and Cl in mud volcano fluids are clearly elevated, displayed as the Na-Cl type. The δD and δ18O values of the fluids are similar between the Baiyanggou and Dushanzi mud volcanoes, which are mainly from ancient sedimentary pore water. However, the Aiqigou mud volcano is depleted in dissolved Cl and shows lower δ18O values with mixed sources, including deep pore and local meteoric water. Two types of mud volcanoes are proposed in this study. One type is low-energy mud volcanoes with a low volume of fluid of deep origin on the hillcrest, which display as mud pool/pie/hole. The other type is high-energy mud volcanoes having mixed fluid origin in the valley and formed in the shape of a mud cone (dome).     

山东谢家沟金矿床流体包裹体研究及成矿机制的探讨

谢家沟金矿床位于胶东隆起西北缘的焦家断裂带和招平断裂带之间。矿床赋存在中生代玲珑花岗岩体中,矿体主要受NNE向和NNW向断裂构造控制,主要呈脉状或透镜状产出。矿石矿物主要有黄铁矿、黄铜矿、方铅矿、闪锌矿、自然金、银金矿以及少量的碲银矿和辉银矿。围岩蚀变主要有钾长石化、硅化、黄铁绢英岩化、绿泥石化和碳酸盐化等。成矿作用从早到晚可划分为4个阶段:石英-黄铁矿阶段(Ⅰ)、石英-绢云母-黄铁矿阶段(Ⅱ)、石英-多金属硫化物阶段(Ⅲ)、碳酸盐-萤石阶段(Ⅳ)。流体包裹体显微测温和激光拉曼成分分析表明,成矿早期流体温度为308~377℃,盐度为6.29%~8.55%,压力为350 MPa,属于H_2O-CO_2-NaCl流体体系;主成矿期的流体温度为226~331℃,盐度为4.87%~10.29%,压力为280~300 MPa,属于H_2O-CO_2-NaCl±CH_4流体体系。包裹体显微测温及岩相学观察发现,主成矿期的成矿流体发生了不混溶作用,这可能是导致金矿化的主要原因之一。硫同位素研究表明,谢家沟金矿床主成矿期黄铁矿的δ34S值接近或略低于胶东典型金矿床成矿期黄铁矿的δ34S值,暗示这些金矿床的成矿物质可能来自相同的源区;氢、氧同位素对比研究表明,谢家沟金矿床成矿流体表现出较明显的岩浆水特征,可能有大气降水的参与,但参与程度较弱。大气降水与岩浆水混合引起的温度降低、挥发分含量的降低可能是导致金矿化的另一原因。     

Hybrid finite-discrete element modelling of dynamic fracture and resultant fragment casting and muck-piling by rock blast

A state-of-the-art review is conducted to highlight the fracture mechanism in rock blast and advantages and limitations of various methods in modelling it. A hybrid finite-discrete element method (FEM-DEM) is implemented to simulate rock fracture and resultant fragment muck-piling in various blasting scenarios. The modelled crushed, cracked and long radial crack zones are compared with those in literatures to calibrate the hybrid FEM-DEM. Moreover, the hybrid modelling reproduces the rock fragmentation process during blasting. It is concluded that the hybrid FEM-DEM is superior to continuous and discontinuous methods in terms of modelling dynamic fracture of rock under blast-induced impact load.     

应用FTIR-SEM研究一类合成欧珀的微结构及其变彩成因机制

近些年来,相关人工合成欧泊的研究工作主要聚焦于天然与合成欧泊的鉴别与筛选,而相关合成欧泊的微结构、变彩机制及其中水的赋存状态的深入研究鲜见报道。本文通过傅里叶变换红外光谱(FTIR)、场发射扫描电镜(FE-SEM)对一类合成欧泊的微结构、结构缺陷进行较系统的研究。同时,结合对样品的热处理就该合成欧泊体色、变彩机制及其中水的赋存状态作了初步探讨。结果表明:该样品的红外光谱中,在约2900 cm~(-1)、1737 cm~(-1)处出现合成欧泊特征吸收,且在4000~6000 cm-1波段,合成欧泊相比天然欧泊的吸收峰更为复杂。合成欧泊的颜色由体色与变彩两者共同构成,该体色与存在于其微结构中准球状、粒径约205 nm的二氧化硅颗粒的间隙填充物直接关联,且在热处理条件下随着间隙物的析出而渐退。同时,该类合成欧泊的变彩归因于结构致色并由其内部SiO_2颗粒周期性排列而构成的三维的光子带隙结构所致。此外,在欧泊的微观结构发生重构前,变彩所呈现的颜色主要由准球状二氧化硅颗粒粒径与间隙填充物决定。     

Active and passive arching stresses in c′-ϕ′ soils: A sensitivity study using computational limit analysis

Vertical soil arching, commonly known as the “trapdoor mechanism,” is a pervasive phenomenon in various geotechnical applications that can be evaluated through a variety of analytical and numerical approaches, most of which exist due to a variety of proposed arching mechanisms, many of which are focused on either purely frictional or cohesive soils. This study investigates the realized arching mechanisms and associated loads for trapdoors under both active and passive arching conditions in c′-ϕ′ soils through a series of dimensionless charts using both upper and lower bound limit analyses. An associated sensitivity analysis demonstrates that arching loads are highly dependent on collapse mechanism, a function of not only geometry, but soil shear strength, with cohesion affecting the realized mechanism and arching loads.     

Influence of single flaw on the failure process and energy mechanics of rock-like material

This paper investigates the influence of a flaw on crack initiation, the failure mode, deformation field and energy mechanism of the rock-like material under uniaxial compression. The results of laboratory test and numerical simulation demonstrate the flaw inclination effect can be classified into three groups: 0–30°, 30–60° and 75–90°. The characteristic stresses increase as the flaw angle increases. The tensile cracks initiate from gentle flaws (α ⩽ 30°) and shear cracks appear at tips of steep flaws (α ⩾ 45°). The input energy, strain energy and dissipation energy of a specimen show approximate increasing trends as the flaw angle increases.     

兰州九州重塑黄土的抗拉变形破坏机理

通过对不同干密度及含水量的重塑黄土进行室内抗拉强度试验,探究轴向压裂法致使试样破坏的机理及一般规律。结果表明,黄土的抗拉变形破坏可分为4种类型：I类,高干密度低含水量（干密度大于1.65 g·cm^-3,含水量小于15%）,抗拉强度介于8~12 kPa;Ⅱ类,低干密度低含水量（干密度小于1.60 g·cm^-3,含水量小于15%）,抗拉强度介于4~8 kPa;Ⅲ类,高干密度高含水量（干密度大于1.65 g·cm^-3,含水量大于17%）,抗拉强度介于4~8 kPa;IV类,低干密度高含水量（干密度小于1.60 g·cm^-3,含水量大于17%）,抗拉强度介于3~4 kPa。I类、Ⅱ类破坏类型属于脆性破坏,Ⅲ类、IV类属于塑性破坏。重塑黄土抵抗变形最弱的含水量为15%。通过对比分析黄土、普通黄黏土、红黏土及膨胀土的抗拉强度发现,在最优含水量处,不同干密度下黄土的抗拉强度均最小。黏性土的持水能力远超过黄土。黏土及膨胀土的抗拉强度均在最优含水量处达到最大值,而黄土的抗拉强度随着含水量的增加持续减小。研究结果对黄土强度特性的理解具有一定的参考意义。     

棘爪机构伸缩式钻孔水力开采装置的设计与试验

钻孔水力采矿由于安全系数高、工程效益和生态环境保护好等优点在国外一直被研究和使用。我国相关的科研院所也正在开展研究。本文在分析研究国外钻孔水力开采装置的基础上,研制了具有自主知识产权的棘爪机构伸缩式钻孔水力开采装置。该装置利用机械式伸缩水枪的方式扩大水力开采矿层的范围,创新性地采用棘爪机构固定水枪的伸出角度,从而提高单孔开采量,降低开采成本。初步野外实验结果表明,该装置结构设计合理、工作可靠性高,可有效进行开采,达到了预期的目标。     

Characteristics of Ore-controlling Structures and Ore Emplacement Mechanism in the Shihu Gold Deposit,Hebei Province - A Case Study of the No.101 VeinEI北大核心CSCD

The Shihu gold deposit lies in the middle-northern Taihang Mountains and is strictly controlled by faults. The ore-controlling structures in the mining district are not well-documented. Detailed field observation and laboratory investigation of the No.101 vein indicated that the ore-controlling structures of the Shihu gold deposit are the pre-Mesozoic shear joints with SN and NW strike that reactivated during the Yanshanian emerged from. The NNW-SSE left-strike-slip stresses during the main mineralization stage led to the dilation of the NW-strike structural segments along the roof and footwall of the early quartz diorite porphyrite veins, which contributed to the extensive wall-rock alteration, intensive gold mineralization and occur as quartz-vein type ores, and consequently the formation of rich ore pillars. On the contrary, the SN-strike compressive structures correspond to the weak wall-rock alteration and gold mineralization and occur as altered-rock type ores. All the rich ore pillars laterally distributed with high-angle (70°±), and wholly pitched southward with 40° angle. According to the structural ore-controlling regularities, the location between the No.47 and the No.61 exploratory line with elevation below 150 m is an excellent ore-prospecting area. Our prospecting proposal based on the ore-controlling structural analyses in the study area has been proven by the underground works in the Shihu gold deposit. © 2017, Science Press. All right reserved.