Solid Earth Sciences最新文献

Several individual particle native golds were observed in two Mesozoic Group II kimberlites dikes in the southeastern margin of North China Craton (NCC). This discovery provides new information of the nature of the Mesozoic lithospheric mantle beneath the southeastern NCC, the metallogenic setting of gold deposits in the Jiaodong peninsula and the metallogenic theory of decratonic gold deposits.

The Fanchang volcanic basin is located between the ore-cluster districts of Tongling and Ningwu，the Middle-Lower Yangtze Metallogenic Belt (MLYMB), regionally. This study extends a set of methods，i.e., petrographic observations, chemical compositions of intrusive rocks and LA-ICP-MS analysis of magmatic zircons, zircon LA-ICP-MS U-Pb geochronology and whole-rock Sr-Nd isotope analysis, on several intrusive rocks, such as Baimashan, Xiangxingdi, Zhuhouling, Chichong, Suishan, Puyingshan and Zishanling. In detail, in this area, monzonite, quartz diorite, granite and syenite are characterized by a series of high-K calc-alkaline and shoshonite, with a positive anomaly in Rb, Th, U, Ce, Pb, Nd, Sm, Gd and a negative anomaly in Ba, Nb, Ta, Sr, P, Ti, plus LREE enrichments and a flattened right-dip of HREE. Moreover, the diagenesis ages of these intrusions mainly overlap the 128–121 Ma time, all but two exceptions, including Baimashan quartz dioritic porphyrite (138.5 ± 4.9 Ma) and Puyingshan diorite (132.1 ± 3.8 Ma). Interestingly, the inherited zircon ages in the Baimashan intrusion are polyphased, primarily in three intervals, 2211.1–2675.9 Ma, 1862.0–2057.1 Ma and 661.1–990.7 Ma, respectively, indicating diachronic magmatic sources and possible existence of the late-Archean and Proterozoic basements. In addition, crystallization temperatures of these intrusive rocks in the Fanchang volcanic basin significantly increase from 802 °C to 931 °C, while the values of (Ce⁴⁺/Ce³⁺)_zircon decrease from 192.34 to 24.84, throughout the period from 132 Ma to 125 Ma, probably due to the enhanced magma assimilation process with the reductive carbonaceous wall rocks. Furthermore, A-type granites in this area, as defined in this paper, would have been derived from the partial melting of the residues of the early I-type granite source area. Values of (^S7Sr/⁸⁶Sr)_i array between 0.70718 and 0.72642 (a.v. 0.70970) and εNd(t) between −14.0 and 2.2 (a.v. −7.1). In brief, the intrusive rocks in Fanchang volcanic basin is considered to have characteristics of both enriched mantle and lower crust and a strong assimilation process had occurred.

The Zhangbaling tectonic belt is located in the middle segment of Dabie-Sulu Orogenic Belt and in the eastern part of the main segment of the Tan-Lu Fault Zone (TLFZ). Based on detailed field work and comprehensive analysis, this study discussed the tectonic evolution mechanics of the Zhangbaling tectonic belt and the Tan-Lu Fault Zone as well as the relationship between them during Indosinian to early Yanshanian period. Three stages of tectonic deformation have been determined in the period from Indosinian to early Yanshanian in Zhangbaling area, i.e., a series of WNW and nearly EW trending tight congruous nearly recumbent folds formed in the early stage; a flatly ductile shearing zone formed mainly between Beijiangjun lithofm. and Xileng lithofm. in the middle stage and the demonstrate NNE trending wide flat folds and strong thrusting or napping from NW to SE formed in the last stage. The superimposition of these three deformation stages formed the basic tectonic framework of this area. The folding and thrusting (sliding in the front margin) was developed in the mid-upper crust (include sedimentary cover) during the period of subduction and collision processes between South China Block and North China Block, shows a progress deformation of the three stages during the period of Indosinian and early Yanshannian due to the conversion of the Tethyan to the Circle Pacific Ocean tectonic regimes. The Tan-Lu Fault Zone can be divided into two big development phases since Hercynian. The first phase is a fault zone sensu lato, as a boundary of a rifted cratonic margin (or basin) that shows a wide arc-shaped along the northeastern margin of the South China Block during Indosinian. The TLFZ of this period is a collision suture between South China Block and North China Block during Indosinian period, a thrust-napping fault zone belong to the Tethyan regimes sensu lato, as well as the southeast margin of the Zhangbaling tectonic belt. This big phase corresponds to the three stages deformation activities in Zhangbaling area. The second phase is a wrench fault system usually mentioned by most researchers, which belongs to the Pacific Ocean Cycle Regime sensu stricto since early Yanshanian to Himalayan periods.

Centimeter to meter-scale repetitive patterns in composition and texture are sometimes observed in igneous systems. Examples are found in layered intrusions and multi-shelled orbicular granites. These patterns may result from the action of nonlinear self-organization processes in which the interplay between crystallization dynamics, diffusion and thermal conduction causes mineral and crystal size segregations. These mechanisms are analogous to the ones underlying the formation of Liesegang bands and lead to comparable features, such as a geometric progression of the band positions and the presence of doublets. We present here a comprehensive one-dimensional numerical model of Liesegang pattern formation process from binary eutectic melts in igneous systems. The model incorporates nucleation, growth and Ostwald ripening and is applied to both the layered intrusion and the orbicular granite configuration with appropriate simple geometries and cooling boundary conditions. The emergence of cyclic layering is described in terms of two key parameters that control the pattern formation: the scaled latent heat of crystallization (Stefan number) and the ratio of the thermal diffusivity to a characteristic diffusion coefficient (Lewis number). It is found that, in intrusions, a banding pattern compatible with the Liesegang spacing law is generated when the Stefan number is large and the Lewis number small, in agreement with previous studies. For orbicular granites with low Lewis number, we show that the band thickness and the crystal size increase with distance from the rim to the core, in agreement with the observations of Zhang and Lee (2020). This suggests that the pattern progresses inwards from the outer boundary, rather than from a colder core, thus supporting the conceptual model reported in Zhang and Lee (2020). Moreover, the results for both geometries indicate that ripening plays an important role in the formation of realistic patterns.

The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, has attracted considerable attention over the past decades. However, its genesis has been highly debated, especially whether the host dolomite is of sedimentary or igneous origin. In this study, we present a comprehensive study on field geology, petrographic, geochemical, and apatite fission track analyses on REE-mineralized dolomite (H8) and its wall-rock (H9), in an effort to provide additional insights into the formation and preservation of this giant ore deposit. According to field geology and petrology analyses, the ore-hosted dolomite of the H8 unit in/around the Bayan Obo deposit displays the characteristics of sedimentary carbonates and demonstrates complicated deformation and hydrothermal events that resulted in the complex REE mineralization. The parameter of w(As) in pyrite ranging from 100 to 1600 ppm suggesting that the pyrite of the Bayan Obo deposit was mainly formed in a hydrothermal environment, and the thermal event from pyrite at ~440 Ma may represent the mineralization event of the Bayan Obo deposit, or it is an important disturbance event after the mineralization of the deposit. The geochemical research depicts that the evolution process of carbonatite was originated from calcium carbonatites and magnesio-carbonatites to ferro-carbonatite, indicating that the different genesis models of the Bayan Obo deposit that ranging from sedimentary carbonate to volcano-sedimentary and igneous carbonatites are reasonable. The two apatite ages obtained from granite on the east side of the Bayan Obo deposit and the dolomite in the Bayan Obo deposit are ~173 Ma and ~54 Ma, respectively. Time–temperature (t–T) histories of rocks yield new information about the timing of deformation of the Bayan Obo deposit, implying that the east side of the deposit experienced a rapid cooling process between ~300 Ma to ~54 Ma (especially ~300–180 Ma), this reflects the granite underwent a rapid cooling process which coeval with the plate subduction during the closure of the Palaeo-Asian Ocean, while the tectonic movement has little effect on the broken of the H8 dolomite in the mining area. Although the slate of the H9 unit from the east of the Bayan Obo deposit experienced a strong fragmentation in the later tectonic movement, it still made a great contribution to the mineralization for better preservation of REE-rich fluids, leading to the formation of the giant Fe-REE-Nb deposit.

Ground settlement has impacted many infrastructures in Lagos. Rapid urbanisation has further compounded this problem. Monitoring and modelling of ground settlement will provide actionable information that can help to mitigate this problem. In this study, space-based retrievals were used to investigate the magnitude and spatial distribution of ground settlement and the role of groundwater depletion in Lagos. Using the SBAS technique, the ground settlement rates were computed based on the Sentinel-1 data acquired between 2015–2019. The GRACE-derived groundwater head changes between 2002–2017 was quantified, and the ground settlement response to groundwater depletion was modelled. The highest recorded groundwater depletion rate was −11 mm/year, while the highest observed ground settlement rate was −94 mm/year, indicating an intense rate of ground settlement up to the end of 2019. The statistical correlation between ground settlement and groundwater head changes is in the order of 0.4, which is a moderate positive correlation indicating a partial correlation between ground settlement and groundwater depletion. To model settlement, a one-dimensional approach was used to simulate the response of ground settlement to changes in groundwater level. The simulated results fairly correspond to the magnitude of observed settlement, confirming that factors other than groundwater depletion partly control ground settlement in Lagos.

The Hehuashan sediment-hosted Pb–Zn deposit is a large-scale (Pb + Zn＞1 Mt) deposit hosted in the Tongling mining district of the Middle-Lower Yangtze River belt (MLYB). The Pb–Zn ore bodies are composed of two main mineralization types, namely breccia type and massive type. The breccia type ore bodies are hosted by the Triassic limestone and dolostone, while the massive ore bodies are hosted by marble dolostone which are close to the diorite intrusions. The previous research showed that the Hehuashan Pb–Zn deposit is a Mississippi Valley-type (MVT) deposit formed in Middle Triassic and overprinted by late mineralization of magmatic-hydrothermal fluids in Yanshanian period. In this research, we aimed to declare the age of the late mineralization and it's relationship with diorite intrusions. The mineralization age (132.8 ± 3.1 Ma) of the massive ore rocks (late mineralization) is similar to the zircon U–Pb age of the diorite (133–130 Ma), showing that this type mineralization is contemporaneous to the Yanshanian intrusive events. Furthermore, similar initial ⁸⁷Sr/⁸⁶Sr values between sphalerite and diorite indicating a magmatic fluid was involved during the mineralization process and the emplacement of diorite also might have provided ore-forming materials. The heavy sulfur compositions (δ³⁴S = +8.6‰ to +12.8‰) of the sphalerite in massive ore rocks indicate that the sedimentary sulfate may have provided reduced sulfur for sulfides precipitation by sulfate thermochemical reduction process. Sr–Nd isotopic compositions of the diorite in the Hehuashan deposit area showed that the massive Pb–Zn mineralization was related to the intrusion of diorite in an extensional tectonic setting. In summary, the Hehuashan Pb–Zn deposit composes a late Yanshanian Pb–Zn mineralization event and represents a younger Pb–Zn mineralization in the Tongling district and MLYB. The Triassic carbonates have the potential for Pb–Zn deposit exploration in the Tongling district and Middle–Lower Yangtze River belt.

The study aims at employing geo-electrically and hydrogeologically derived indices to assess the vulnerability in an alluvial environment of Obot Akara and Nsit Atai counties in the hinterland of Akwa Ibom State, Southern-eastern Nigeria. Vulnerability maps have been generated using seven indices: depth to groundwater (D); net groundwater recharge (R); aquifer media (A); soil media (S); topography (T); impact of vadose zone (I) and hydraulic conductivity acronymized as DRASTIC. The DRASTIC vulnerability index (V) was computed as the sum of the product of weights and ratings. The results indicate that, for Obot Akara site, about 20% of the aquifer is classified as medium vulnerability, about 40% is high vulnerability and very high vulnerability. . For Nsit Atai, about 60% of the aquifer is classified as medium vulnerability while 20% represents high and very high vulnerability respectively. The results of the vulnerability assessment shows that the DRASTIC quality category in percentage is contributory from the seven parameters and hence avoidable anthropogenic activities leading contamination of groundwater should be avoided while the vulnerability maps should be employed to avoid locating boreholes in areas with very high vulnerability. With appropriate software programmes, ground-based geophysical data, hydrogeological and environmental inputs, the vulnerability assessment has been made to cover both the present and future exploitable groundwater resources for the healthy and economic benefits of human existence.

The Tengshan'ao Sn deposit, located at the Dayishan ore field, Nanling Range, is one of the recently explored deposits in this region. In this study, LA-MC-ICP-MS cassiterite U–Pb and muscovite ⁴⁰Ar-³⁹Ar isotopic dating analyses for altered granite-type Sn ores yielded ages of 156.7 ± 4.3 Ma and 151.1 ± 1.5 Ma, respectively. These ages are consistent to the LA-MC-ICP-MS zircon U–Pb age of the fine-grained two-mica monzogranite (158.2 ± 1.6 Ma), indicating a coeval magmatism and mineralization for the Late Jurassic Tengshan'ao Sn deposit. It is further demonstrated that the formation of this deposit is related to the Early Yanshanian (160-150 Ma) large-scale diagenesis and mineralization in Nanling Range. It is suggested that the late-stage fine-grained two-mica (muscovite) granite has an indicative significance for searching the altered granite type Sn polymetallic mineralization.