Geochronological framework and Pb, Sr isotope geochemistry of the Qingchengzi Pb–Zn–Ag–Au orefield, Northeastern China

The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (²⁰⁶Pb/²⁰⁴Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (²⁰⁶Pb/²⁰⁴Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 17.12 to 17.41, ²⁰⁷Pb/²⁰⁴Pb = 15.47 to 15.54, ²⁰⁸Pb/²⁰⁴Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (²⁰⁶Pb/²⁰⁴Pb = 18.20 to 24.28 and 18.32 to 20.06, ²⁰⁷Pb/²⁰⁴Pb = 15.69 to 16.44 and 15.66 to 15.98, ²⁰⁸Pb/²⁰⁴Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.     

Source-area controls on the composition of beach and fluvial sands on the southern side of the Gibraltar Strait and Western Alboran Sea (Flysch Basin, Internal and External, Domains, Northern Rif Chain)

The southern side of Gibraltar and the Western Alboran Sea of the northern Rif coasts and rivers provide a natural field laboratory for sampling modern sand at different scales: small catchment basins (first order) and rivers draining mountain belts (second order). The Rifian chain represents a deformed and uplifted thrust-belt and related forelands composed of Palaeozoic nappes, metamorphic and plutonic basement, and their sedimentary Mesozoic and Cenozoic siliciclastic and carbonate cover, respectively. The present physiography of the Rif Chain is shaped by a rugged mountainous relief drained by different scale catchment basins that supply the nearby coastal and marine deep-sea environments. The analysis of the composition of modern fluvial and beach sands is useful for the interpretation of transported sediments by surface processes from the continent toward coasts and later to deep-water environments.Modern beach and fluvial sands of the southern side of Gibraltar and the Western Alboran Sea display three distinct petrologic littoral provinces, from the east to the west and from the north to the south, respectively, designated as: (i) the Tangier–Bel Younech Littoral Province with 90% of sand derived from erosion of Flysch Nappes (Flysch Basin Domain); (ii) the Bel Younech–Sebta Littoral Province with 64% of sand fed mainly by the metamorphic Units of Upper Sebtides and (iii) the Sebta–Ras Mazari Littoral Province with 74% of sand supplied from the epimetamorphic Palaeozoic Ghomaride Nappes and Alpine cover rather than Mesozoic and Cenozoic sedimentary successions of the “Dorsale Calcaire” Units. Comparison of detrital modes of fluvial and coastal marine environments highlights their dispersal pathways and drainage patterns of actualistic sand petrofacies.     

Breakdown of hydrous ringwoodite to pyroxene and spinelloid at high P and T and oxidizing conditions

To get deeper insight into the phase relations in the end-member system Fe₂SiO₄ and in the system (Fe, Mg)₂SiO₄ experiments were performed in a multi-anvil apparatus at 7 and 13 GPa and 1,000–1,200°C as a function of oxygen fugacity. The oxygen fugacity was varied using the solid oxygen buffer systems Fe/FeO, quartz–fayalite–magnetite, MtW and Ni/NiO. The run products were characterized by electron microprobe, Raman- and FTIR-spectroscopy, X-ray powder diffraction and transmission electron microscopy. At fO₂ corresponding to Ni/NiO Fe-ringwoodite transforms to ferrosilite and spinelloid according to the reaction: 9 Fe₂SiO₄ + O₂ = 6 FeSiO₃ + 5 Fe_2.40Si_0.60O₄. Refinement of site occupancies in combination with stoichiometric Fe³⁺ calculations show that 32% of the total Fe is incorporated as Fe³⁺ according to From the Rietveld refinement we identified spl as spinelloid III (isostructural with wadsleyite) and/or spinelloid V. As we used water in excess in the experiments the run products were also analyzed for structural water incorporation. Adding Mg to the system increases the stability field of ringwoodite to higher oxygen fugacity and the spinel structure seems to accept higher Fe³⁺ but also water concentrations that may be linked. At oxygen fugacity corresponding to MtW conditions similar phase relations in respect to the breakdown reaction in the Fe-end-member system were observed but with a strong fractionation of Fe into spl and Mg into coexisting cpx. Thus, through this strong fractionation it is possible to stabilize very Fe-rich wadsleyite with considerable Fe³⁺ concentrations even at an intermediate Fe–Mg bulk composition: assuming constant K _D independent on composition and a bulk composition of x _Fe = 0.44 this fractionation would stabilize spl with x _Fe = 0.72. Thus, spl could be a potential Fe³⁺ bearing phase at P–T conditions of the transition zone but because of the oxidizing conditions and the Fe-rich bulk composition needed one would expect it more in subduction zone environments than in the transition zone in senso stricto.

桩-土间剪切应力的传递探讨

对不同情况下桩-土间剪切应力进行了初步的理论分析,并基于ADINA有限元程序,对不同情况下的桩-土间剪切应力进行了计算对比分析。结果表明,同种土体和桩体条件下,各种模式达到极限状态时桩-土间剪切应力不同,堆载作用下负摩擦力值最大,抗拔桩负摩阻力值最小,桩顶受压桩、托桩的摩阻力值介于其间;桩材泊松比在0.15～0.35范围内时,桩侧摩阻力随泊松比的增大而增大。所得结论有助于进一步认识桩-土相互作用规律以及桩-土荷载传递机制。     

小湾拱坝坝基开挖卸荷松弛效应的有限元分析

针对小湾拱坝坝基开挖中出现的卸荷松弛问题,采用三维非线性有限元法对卸荷松弛效应进行了评价。通过松弛方案与未松弛方案计算,对小湾拱坝的应力和位移、屈服区分布以及坝基浅部关键截面的抗剪安全度、点安全度等方面进行了比较分析。研究结果表明,建基面地基松弛后,主要影响低高程局部区域的变形和应力,对拱坝整体变形和应力分布的影响很小;地基松弛后,拱冠梁建基面（953 m高程）坝踵处第一主应力有所增大,坝趾处第三主应力有所减小;拱冠处梁的作用减弱,拱冠梁处距坝踵20 m范围的点安全系数也略有降低;原设计方案在未松弛情况下基本可以满足《混凝土拱坝设计规范》关于建基面抗剪（断）安全系数的要求;坝基松弛后,抗剪（断）安全系数降低明显,与规范规定的要求相差较大,需采取适当的工程措施。该研究成果已为设计决策提供了依据。     

青海玉树东莫扎抓铅锌矿床地质特征及碳氢氧同位素地球化学研究

东莫扎抓铅锌矿床位于青藏高原羌塘地体东北缘,是"三江"北段铅锌铜银多金属成矿带中的典型代表,对该矿床地质特征和成因类型的研究有助于理解区域铅锌铜银多金属成矿规律,对区域找矿具有重要意义.笔者通过详细的矿区地质考察、系统的矿石光薄片显微镜下鉴定和矿石中方解石的碳、氢、氧同位素分析测试,概述了东莫扎抓铅锌矿床的地质特征和成矿流体的碳、氢、氧同位素组成特征.东莫扎抓铅锌矿床矿体呈似层状展布,产状严格受到矿区逆冲断层的控制,赋矿围岩为上三叠统结扎群波里拉组灰岩和下一中二叠统开心岭群尕迪考组灰岩,发育强白云石化和弱硅化,矿物组合简单,主要为闪锌矿+方铅矿+黄铁矿+白云石+方解石+重晶石,矿石结构以皮壳状、草莓状等胶状结构和他形粒状结构为主,矿石构造为浸染状、角砾状、团块状和脉状.矿石中方解石的δ~(13)C_(V-PDB)值、δ~(18)O_(V-SMOW)值分别为δD_(V-SMOW)值分别为-1.8‰～+3.3‰、+6.1‰～+24.6‰和-137‰～-53‰,计算得到成矿流体的δ~(18)O_(流体)值为-0.5‰～+13.8‰.研究结果表明,东莫扎抓铅锌矿床的成矿流体主要来自盆地封存热卤水和大气降水,金属物质可能由区域流体在长距离迁移过程中通过与碳酸盐岩地层相互作用,以及淋滤含矿地层底部的火山岩而得来,成矿过程中伴随着碳酸盐岩的溶解作用,可能存在有机质的参与.据此,笔者将东莫扎抓矿床归为发育在碰撞造山带中受逆冲推覆断裂构造控制的类MVT铅锌矿床,并初步建立了东莫扎抓铅锌矿床的构造控矿模型.     

灰色误差理论在岩矿测试数据处理中的应用

介绍了灰色误差理论,并应用于岩矿测试数据基本处理。结果表明,在误差变化范围小的岩矿化学分析中,测量数据的累加曲线为线性关系,在元素不均匀性分布的岩矿化学分析(例如不均匀性金的测试)和岩石强度试验等分散性大的测试中,测量数据的累加曲线为曲线关系,但灰色标准偏差和传统统计学的标准偏差非常接近。该方法简单易行,具有相当的精度和较强的实用性,对测量数据少或难于寻求统计规律的测量过程尤为实用,是一种新的尝试。     

嘉陵江流域水保治理前后沿程水沙变化研究

嘉陵江为长江上游重要支流,是长江三峡水库重要的洪水和泥沙来源地。20世纪50年代初至80年代末,多年平均径流量701亿m3,多年平均悬移质输沙量近1.4亿t;“长治”工程的实施,对控制流域水土流失起到了较为明显的积极作用。以流域内主要控制站大量的实测水文气象资料为依据,重点研究了全流域水保治理前后的水沙变化情况及规律。     

BP神经网络识别塔北低阻油气层

简要介绍了塔北低阻油气层岩性剖面、低阻油气层地球物理测井曲线特征,分析了塔北地区低阻油气储层成因,重点论述BP人工神经网络识别油气层、油水同层、水层和干层的方法原理.识别实例表明,BP人工神经网络识别低阻油(气)、水层的结果与实际相符,明显地提高了测井的解释精度.     

高精度重磁测量在油气勘探中的应用效果

在某工区进行了高精度重磁油气勘查,推断出断裂体系、密度界面和预测含油气古潜山,取得找油气突出效果,显示出重磁勘查具有的快速高效特点和有效性,对同类或相似地区的油气勘探工作有启迪和借鉴作用。