陕西银硐子海底喷气沉积型银铅多金属矿床的主要成矿特征

通过对陕西银硐子沉积型银铅多金属矿床成矿的古构造、古地理环境以及矿床的含矿岩系的岩石、矿石、矿物成分、微量元素、稀土元素、铅同位素等地球化学特征的研究，并与国外某些海底喷气沉积铅锌矿床相比较，发现它们具有极为相似的地质、地球化学特征，表明该矿床是在正常海相沉积与海底喷气沉积相渗合而形成的，是具有较大经济价值的银铅铜锌贱金属－贵金属（特大型）综合矿床     

Riphean stromatolitic formations fringing the East European platform

Riphean stromatolitic formations flank the East European epi-Karelian platform only in the east and northeast. They are traceable as long (over 3600 km) relatively narrow belt consisting of two rectilinear segments, one running along the Urals western flank from southern extremity of the Bashkirian meganticlinorium to the Polyudov Ridge and the other one extending from the southern and central Timan to the Kil’din Island and northern Norway. Within the belt there are known stromatolitic formations of all Riphean erathems: the Lower and Middle Riphean stromatolitic buildups are confined to the eastern segment of its southern part only, while the Upper Riphean occur everywhere. Their distribution conformable to large structural elements of the plaform margin being replaced by carbonate-terrigenous rocks almost lacking stromatolites westward and southwestward in the Kama-Belaya aulacogen system and by substantially siliciclastic succession eastward and northeastward. The distribution area of Upper Riphean stromatolitic formations includes the Karatavian stratotype region, where 12 stromatolite beds ranging in age from ≥900 to 620 Ma are established. Many of the beds are traceable along the strike far beyond the stratotype region. Representing relatively small reference units, the beds facilitate reconstruction of distribution dynamics of the Upper Riphean stromatolites. Distribution area of the latter was always parallel to marginal structures of the platform, though being of changeable size, particularly of length. Originated in the stratotype region eastern part, stromatolites first advanced into northeastern areas never crossing boundaries of the Upper Riphean distribution area during the Early Karatavian. In the initial Late Karatavian, they occupied a longest distribution area that was sharply reduced at the end of that period. According to distribution peculiarities in space and with time, the Upper Riphean stromatolitic formations accumulated likely in peripheral areas of an open sea or oceanic basin adjacent to the East European platform, rather than in closed epiplatform basins.     

An improved model for estimating the TOC in shale formations

Determination of the total organic carbon (TOC) from well logs is an important step in formation evaluation of shale reservoirs. The ΔlogR model is one of the most commonly used methods in determining the TOC of the source rocks. Wang et al. (2016) made three revisions on ΔlogR model to enhance the prediction accuracy, one of which is to use a changeable slope of log₁₀R_t versus porosity logs to replace a fixed value. Based on the original ΔlogR model and Wang's revision, we propose an improved model for a better TOC estimation in this paper. Specifically, the approximate linear baseline is replaced by a theoretical one. Under this condition, both the slope and resistivity and porosity log values of the baselined rocks vary from depth to depth. The determination of theoretical baseline is presented, and the varied baseline log values are obtained by a simple mathematical technique. In addition to the shale play of Western Canada Sedimentary Basin, two study areas of Sichuan Basin and Ordos Basin in China, were used to assess the applications of the proposed model and its improvement over the original model. The analysis and discussions presented in this paper indicate that the improved model offers more reliable results in shale plays with large lithological variations. Also, the TOC content of the three study areas was estimated using the proposed model. The results showed that the TOC values estimated from well logs and core analysis are in good agreement.     

K-bentonite beds and high-resolution integrated stratigraphy of the uppermost Ordovician Wufeng and the lowest Silurian Longmaxi formations in South China

More than 20 K-bentonite beds were discovered from the Wufeng Formation and the lowest Longmaxi Formation in two sections, both adjacent to the Ordovician-Silurian (O-S) boundary and located in Tongzi, Guizhou Province and Yichang, Hubei Province, some 500 km apart from each other in South China. This indicates that many volcanic eruptions occurred near the southeast margin of the Yangtze Platform between the latest Ordovician and the earliest Silurian. Mainly through biostratigraphic and sequence stratigraphic studies, it was found that almost all these far-apart K-bentonite beds may be correlated with each other. This is the first time that a succession of volcanic ash deposits with high potential of correlation was discovered within a strata interval on the main platforms of China. Therefore, these K-bentonite beds may afford excellent event-marker beds helpful to high-resolution research in integrated stratigraphy as well as other lines of research on the O-S boundary in South China.     

Geochronological arguments for a close relationship between surficial formation profiles and environmental crisis (c. 3000–2000 BP) in Gabon (Central Africa)

We present new ¹⁴C data on charcoal fragments recovered from the lower (coarse-grained Stone Line) and upper (fine-grained Cover Horizon) portions of surficial formation profiles in Gabon. These data and others compiled from the literature enable a reconstruction of the Upper Holocene geological regional history of Gabon. The connection between the geological events recorded in the surficial formations and the Upper Holocene environmental crisis is discussed and a scenario connecting geological events with climatic and environmental changes is proposed. Such a scenario suggests that following the climatic crisis, the reconstitution of soils by aeolian sedimentation could have been an important factor of Bantu expansion.     

Influence of contamination on banded iron formations in the Isua supracrustal belt,West Greenland: Reevaluation of the Eoarchean seawater compositions

Banded Iron Formations(BIFs) are chemical sediments, ubiquitously distributed in the Precambrian supracrustal belts; thus their trace element compositions are helpful for deciphering geochemical evolution on the Earth through time. However, it is necessary to elucidate factors controlling the whole-rock compositions in order to decode the ancient seawater compositions because their compositions are highly variable. We analyzed major and trace element contents of the BIFs in the 3.8-3.7 Ga Isua supracrustal belt(ISB), southern West Greenland. The BIFs are petrographically classified into four types:Black-,Gray-, Green-and White-types, respectively. The Green-type BIFs contain more amphiboles, and are significantly enriched in Co, Ni, Cu, Zn, Y, heavy rare earth element(HREE) and U contents. However,their bulk compositions are not suitable for estimate of seawater composition because the enrichment was caused by secondary mobility of metamorphic Mg, Ca and Si-rich fluid, involvement of carbonate minerals and silicate minerals of olivine and pyroxene and/or later silicification or contamination of volcanic and clastic materials. The White-type BIFs are predominant in quartz, and have lower transition element and REE contents. The Gray-type BIFs contain both quartz and magnetite. The Black-type BIFs are dominated by magnetite, and contain moderate to high transition element and REE contents. But,positive correlations of V, Ni, Zn and U contents with Zr contents suggest that involvement of detrital,volcanic and exhalative materials influences on their contents. The evidence for significant influence of the materials on the transition element contents such as Ni in the BIFs indicates the transition element contents in the Archean ocean were much lower than previously estimated. We reconstructed secular variations of V,Co, Zn and U contents of BIFs through time, which show Ni and Co contents decreased whereas V, Zn and U contents increased through time. Especially, the Ni and Co contents drastically decreased in the Mesoarchean rather than around the Great Oxidation Event. On the other hand, the V,Zn and U contents progressively increased from the Mesoarchean to the Proterozoic. Stratigraphical trends of the BIFs show increase in Y/Ho ratios and decrease in positive Eu anomaly upwards, respectively. The stratigraphic changes indicate that a ratio of hydrothermal fluid to seawater component gradually decrease through the deposition, and support the Eoarchean plate tectonics, analogous to the their stratigraphic variations of seafloor metalliferous sediments at present and in the Mesoarchean.     

小秦岭复杂地层液动锤绳索取心钻探试验研究

小秦岭地区是我国主要的矿产资源地,属于严重的破、碎、硬、漏等复杂地层,特别是岩石结构致密、颗粒细、研磨性较弱的坚硬地层,采用传统回转钻进工艺时,钻头经常出现"打滑"现象,从而导致钻头寿命低、钻探效率低、事故率高、钻探成本高等问题。针对这些问题并结合生产,本文选择了3处典型复杂地层进行了液动潜孔锤+绳索取心"二合一"和传统金刚石绳索取心钻探试验。试验结果表明:与传统金刚石绳索取心钻进工艺相比,液动锤平均回次进尺可提高44%,机械钻速提高21.34%~30.3%,平均综合钻探成本降低了21%~28%,其中钻头费用可降低66.5%。同时,由于碎岩方式的改变,液动潜孔锤钻探具有钻孔防斜效果好、事故率低、钻头使用寿命长等特点。     

辽冀地区条带状铁建造地球化学特征：Ⅰ.主量元素特征

辽冀地区(主要包括鞍山-本溪地区和冀东地区)位于华北克拉通东北部,产出有诸多BIFs型大型-特大型铁矿床。鞍山-本溪地区和冀东地区是我国最大的两个铁矿集区,其中鞍本地区铁矿储量占全国的24%左右,冀东地区铁矿资源储量占全国的10%以上。虽然辽冀地区BIFs大多为形成于新太古代绿岩带中的Algoma型BIFs,但不同矿区BIFs形成环境和受后期改造的程度不一致,鞍本地区BIFs变质级别为绿片岩相-角闪岩相,冀东地区BIFs经历了绿片岩相-麻粒岩相的变质作用,且辽冀地区普遍发育混合岩化。本文主要对比研究了辽冀地区28个铁矿床200件铁矿石的主量元素特征,为探讨辽冀地区BIFs的形成提供了更多的信息。BIFs样品主要由SiO2和Fe2O3T组成,其中鞍山-本溪地区SiO2+Fe2O3T平均为95.10%,冀东地区SiO2+Fe2O3T平均为88.06%,CaO和MgO含量仅次于SiO2和Fe2O3T,且大部分矿区具有正相关关系,Al2O3、TiO2、K2O、Na2O、MnO、P2O5含量很低,这暗示BIFs原岩为一种化学沉积岩,主要为含有少量碳酸盐泥的硅质和铁质的胶体沉积;辽冀地区Al2O3和TiO2均可见明显的正相关,这可能是由于BIFs沉积过程中有少量碎屑物质的加入,这种相关性在冀东地区更为明显,且除SiO2+Fe2O3T外,其它氧化物含量明显高于鞍本地区,说明冀东地区BIFs形成时沉积环境更为动荡,有更多的碎屑物质加入;虽然辽冀不同地区BIFs经历了不同级别的变质作用,形成了不同的矿物组合,但是氧化物含量却变化不大,这说明了变质反应主要为等化学反应;鞍本地区和冀东地区碱质含量也存在差异性,前者的Na2O和K2O含量均低于后者,且后者Na2OK2O,结合野外地质特征,可能暗示了混合岩化作用对冀东地区的影响更为显著。     

论四川盆地下中三叠统(T1j-T2L)地下水的形成

四川盆地下中三叠统嘉陵江组—雷口坡组(T1j-T2L)地下水分布广泛,资源丰富,类型各异。其展布规律及水文地球化学景观系在漫长的地质发展历史阶段中,经沉积、掺杂、淋滤、火山活动、生物化学及剥蚀淋滤等多种因素综合作用形成。对其形成分布的研究论述,有助于对地下水资源的开发利用。