2018年, 第21卷, 第1期　
刊出日期：2018-03-25

  

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  Study on fluid inclusions and features of ore-forming fluids of Diakha gold deposit, Mali, West Africa

  Yassa Konare, WANG Keyong, WANG Zhigao

  Global Geology. 2018, 21(1): 1-13. https://doi.org/10.3969/j.issn.1673-9736.2018.01.01

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  The Diakha gold deposit is located in the southeastern margin of the highly prospective/productive Paleoproterozoic (Birimian) Kédougou-Kéniéba Inlier. Gold mineralization is closely associated with a narrow, sub-vertical, NE-trending shear corridor. The corridor is to the east of a sinistrally reactivated D1 west-dipping fault, which emanated from SMSZ (D2 reverse-sinistral regional structure). The mineralization is hosted by fine-grained sandstone and breccias units associated with a moderate to strong hydrothermal alteration, which is dominated by albitization, carbonization, silicification, hematization, and chloritization. Diakha field data and laboratory analysis suggest the existence of two main stages of hydrothermal mineralization, namely stage I and stage Ⅱ. Stage I is of a breccias type of mineralization and represents the major gold bearing stage. It isassociated with hematite+calcite+ankerite/dolomite and quartz pervasive alteration, controlled by the predominantly brittle-ductile NNE to NE-shearing structures of the main deformation event D2. Stage Ⅱ is represented by the second minor hydrothermal episode, showing local quartz-tourmaline and tremolite alteration which is structurally controlled by the dipping S3 cleavage formed during D3 deformation. Petrographic and microthermometric studies of fluid inclusions from quartz veins reveal the presence of early dominant carbonic CO₂-pure and aqueous-carbonic CO₂-rich (LCO₂+VCO₂±H₂O) in stage I. The exclusively aqueous H₂O+NaCl±CO₂ fluid inclusions (L-type) are observed in stage Ⅱ. Interpretation shows that the ore-forming fluids originated from a homogeneous H₂O-CO₂ fluid during phase separation, with trapping temperatures of 250℃ to 280℃ and low salinity (<6 wt% NaCl equiv), indicating an estimated trapping pressure for the deposit between 610 to 800 bars.
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  Geochronology and geochemistry of Baicaogou tuff in Yanji, NE China and its tectonic implications for Early Cretaceous

  WANG Yingchao, SUN Fengyue, LI Liang, XIN Wei, YAN Jiaming, TIAN Lidan

  Global Geology. 2018, 21(1): 14-25. https://doi.org/10.3969/j.issn.1673-9736.2018.01.02

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  The authors studied zircon U-Pb age and geochemical data of Baicaogou tuff in Yanji, Jilin Province. The results indicate that the rocks formed in Early Cretaceous (125. 6±2. 3 Ma). Geochemically, these tuffs have high SiO₂ and total Na₂O + K₂O, low MgO and FeO, and they belong to metaluminous series, the rock are enriched in LREEs and LILEs, depleted in HREEs and HFSEs such as Nb, Ta, Ti, and P, exhibiting an affinity to I-type granite. All these characteristics implied that the volcanic rocks were derived from partial melting of lower crust. Combined with the geochronology and geochemical features of the coeval igneous rocks within NE China, it is concluded that Yanji area was in a back-arc extensional setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.
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  Accumulation model and prediction of Miocene heavy oil distribution in Gulf of Suez Basin, Egypt

  GUO Xufei, SHAN Xuanlong, DU Shang

  Global Geology. 2018, 21(1): 26-35. https://doi.org/10.3969/j.issn.1673-9736.2018.01.03

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  The Gulf of Suez Basin is a very mature and extremely prospective petroleum province. Many heavy oil fields have been found in the Basin, and such reserves are abundant. Characteristics and models of heavy oil are analyzed in this study based on tectonic, basin evolution, stratigraphic distribution and geochemical data. The best reservoirs of heavy oil are Miocene sandstone and limestone formations. Source rocks of hydrocarbon include deep limestone and shale of the Brown Limestone, the Thebes Formation and the Rudeis Formation. Thick evaporite rocks with rock salts and anhydrites deposited broadly throughout the basin are the most important regional seals, whereas Miocene shales are intraformational and regional seals that cover small areas. Heavy oil could be directly generated or densified during vertical migration along faults and reservoir accumulation. The heavy oil accumulation model is a mixed model that includes three mechanisms:fault dispersal, sulfocompound reactions and hydrocarbons generated from immature source rock. After analyzing the model and the distribution of source rocks, reservoirs, heavy oil fields and structures, it is concluded that the potential heavy oil area is at the center of the basin.
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  Application of isolation forest to extract multivariate anomalies from geochemical exploration data

  WU Wei, CHEN Yongliang

  Global Geology. 2018, 21(1): 36-47. https://doi.org/10.3969/j.issn.1673-9736.2018.01.04

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  Constructing a statistical model that best fits the background is a key step in geochemical anomaly identification. But the model is hard to be constructed in situations where the sample population has unknown and/or complex distribution. Isolation forest is an outlier detection approach that explicitly isolates anomaly samples rather than models the population distribution. It can extract multivariate anomalies from huge-sized high-dimensional data with unknown population distribution. For this reason, we tentatively applied the method to identify multivariate anomalies from the stream sediment survey data of the Lalingzaohuo district, an area with a complex geological setting, in Qinghai Province in China. The performance of the isolation forest algorithm in anomaly identification was compared with that of a continuous restricted Boltzmann machine. The results show that the isolation forest model performs superiorly to the continuous restricted Boltzmann machine in multivariate anomaly identification in terms of receiver operating characteristic curve, area under the curve, and data-processing efficiency. The anomalies identified by the isolation forest model occupy 19% of the study area and contain 82% of the known mineral deposits, whereas the anomalies identified by the continuous restricted Boltzmann machine occupy 35% of the study area and contain 88% of the known mineral deposits. It takes 4. 07 and 279. 36 seconds respectively handling the dataset using the two models. Therefore, isolation forest is a useful anomaly detection method that can quickly extract multivariate anomalies from geochemical exploration data.
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  Uncertainty analysis of gravity data inversion

  SHI Jianan, MA Guoqing

  Global Geology. 2018, 21(1): 48-54. https://doi.org/10.3969/j.issn.1673-9736.2018.01.05

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  The traditional gravity modelling methods fail to deal with ambiguity zones. These zones are likely to be associated with uncertain boundaries. Source boundaries can be divided into "hard" and "soft" types. "Hard" boundary stands for distinguished margin of discontinuity of source distributions. Traditional modelling method is based on this type of boundary. Uncertain boundaries of ambiguity zones belong to "soft" boundary. The authors put forward the uncertainty analysis because lots of uncertainty factors need to be taken into consideration. Any powerful system designed to interpret gravity data for a real source distribution must be able to deal with uncertain information. Uncertainty analysis (UA) is a useful tool to estimate the ambiguity of the data interpretation. It is often applied in conjunction with a specific modelling or inversion method. In this study, we use an iterative inversion and its results are evaluated using uncertainty analysis and proved to be effective.
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  3D Joint inversion of gravity and gravity tensor data

  ZHAO Simin, GAO Xiuhe, QIAO Zhongkun, JIANG Dandan, ZHOU Fei, LIN Song

  Global Geology. 2018, 21(1): 55-61. https://doi.org/10.3969/j.issn.1673-9736.2018.01.06

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  The processing and interpretation of gravity and gradient data plays an important role in geophysics. The cross gradient joint inversion is usually used for achieving structure coupling of multiple geophysical models. In order to realize the coupling of gravity and gravity tensor data, the authors analyzed each component. The results show that different types of data contain different direction information, and derived the joint inversion based on cross gradient function and applied it to model data. The theoretical model results show that the cross gradient method can reduce the multi solution and significantly improve the resolution of the inversion. The method was also applied to inverse the gravity tensor data in Vinton salt dome, showing that this method can get higher resolution results than the separate linear inversion, and be closer to the real density from drilling data.
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  Full waveform inversion based on initial model built from envelope inversion

  YIN Chang, SUN Jianguo, MIAO He, YAN Hongqun

  Global Geology. 2018, 21(1): 62-67. https://doi.org/10.3969/j.issn.1673-9736.2018.01.07

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  Full waveform inversion is a fitting process based on full seismic wave field simulation data using the full waveform information in seismic records and theoretically it is the ultimate goal of seismic inversion. However, there are many problems to be solved in practical application. Firstly, it is the strong nonlinear problem between the seismic wave field and inversion parameters; secondly, the lack of low-frequency information in seismic records. In this study, the envelope is used as objective function inversion to provide the inversion result for the multi-scale full waveform inversion as the initial model, solving the lack of low-frequency information in seismic records. Taking the envelope of seismic records as the objective function in combination of multi-scale full waveform inversion became a new inversion strategy, which naturally achieved the compensation of shortage of low-frequency information and inversion from low frequency to high frequency, reducing the non-linearity in the inversion process. The comparison of the result of full waveform inversion of the initial model built through envelope inversion with the result of the conventional multi-scale full waveform inversion indicates the effectiveness of envelope inversion for the recovery of low-frequency information in seismic records.
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  Wavelet packet envelope multi-scale full waveform inversion

  ZHANG Tianze, HAN Liguo

  Global Geology. 2018, 21(1): 68-76. https://doi.org/10.3969/j.issn.1673-9736.2018.01.08

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  Full waveform inversion (FWI) is an effective tool for constructing high resolution velocity models, but it is affected by a local minima problem. Without long offsets and low frequency data, it is difficult to apply the conventional multi-scale FWI to actual seismic data. In this study, the large offset and low frequency information are provided by the method of wavelet packet envelope for the conventional FWI. The gradient can be computed efficiently with the adjoint state method without any additional computational cost. Marmousi synthetic data is used to illustrate that, compared with Hilbert envelope-based FWI, wavelet packet envelope FWI can provide an adequately accurate model for the conventional FWI approach even when the initial model is far from the true model and the low-frequency data are missing.