Mineral Geochemical Compositions of Tourmalines and Their Significance in the Gejiu Tin Polymetallic Deposits, Yunnan, China

Abstract: The Gejiu tin polymetallic deposits are located in the southeastern part of Yunnan Province in China. A detailed electronic microprobe study has been carried out to document geochemical compositions of tourmalines from the deposits. The results indicate a systematic change of mineral geochemical compositions, which might be used as a mineral geochemical tracer for post-magmatic hydrothermal fluid, basin fluid and their mixture. The tourmalines from granite are schorl with Fe/(Fe+Mg) ratios of 0.912-1.00 and Na/(Na+Ca) ratios of 0.892-0.981. Tourmalines as an inclusion in quartz from the ore bodies are dravite with Fe/(Fe+Mg) ratios of 0.212-0.519 and Na/ (Na+Ca) ratios of 0.786-0.997. Tourmalines from the country rocks are dravite with Fe/(Fe+Mg) ratios of 0.313-0.337 and Na/(Na+Ca) ratio of 0.599-0.723. Tourmalines from cassiterite-tourmaline veins that occur in crannies within the country rocks show distinct optical zoning with alternate occurrence of dravite and schorl, Fe/(Fe+Mg)=0.374-0.843, Na/(Na+Ca)=0.538-0.987. It suggests that schorl in granite and dravite in carbonatite are related to magmatic fluid and basin fluid respectively. When magmatic fluid rose up and entered into crannies of the country rocks, consisting mainly of carbonatite, basin fluid would be constantly added to the magmatic fluid. The two types of fluid were mixed in structural crannies of the sedimentary basin accompanied with periodic geochemical oscillations to form material records in chemical composition zonings of tourmalines.     

黔西北青山铅锌矿床主要控矿断裂构造岩-岩相分带模式

青山铅锌矿床是黔西北矿集区内威宁-水城成矿亚带的典型矿床之一,矿体产出严格受构造和岩性双重因素控制。含矿断裂带中构造岩既是构造变形作用的载体,也是相应变形环境的受体,其具有显著的分带特征。针对斜落走滑构造环境下弱蚀变构造岩与热液成矿成生联系研究的薄弱环节,基于构造岩-岩相学填图方法以及不同岩相带内节理、裂隙构造解析,系统采集不同构造岩-岩相带内定向构造岩样品,进行显微构造与地球化学分析,剖析不同岩相带内构造岩类型、物质组成、内部结构、构造及其分带特征,构建了该矿床构造岩-岩相分带模式,即从矿体向外,依次为：张裂岩相带→泥化相带（一、三中段及以上）→扭裂岩相带→压裂岩相带。扭裂岩相带内发育黄铁矿化、铅锌矿化、方解石化、弱白云石化,压裂岩相带内主要发育方解石化,矿化蚀变随着远离矿体呈现出从强变弱的变化规律,成矿环境也随着温度逐渐降低,呈现氧化→弱氧化-弱还原→还原的变化特征。同时,结合宏观和显微构造应力场分析,认为矿体外侧不同类型的构造岩是在统一构造应力场作用下,因不同位置的局部应力场变化而形成的不同类型的构造岩,发育在北西向断裂下盘的次级断裂不仅控制了矿体定位和形态产状,也控制了其外侧构造岩-岩相带。     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.     

Deep structure of the Amur lithospheric Plate border zone

Based on the study of the deep Amur Plate border structure, seismogravimetric (density) and geoelectric sections of the plate lithosphere crossing geologically and seismically defined plate boundaries are compiled to construct a three-dimensional model of its lithosphere. The model demonstrates the almost ring structure of the plate with its inner part dipping for 50 km and uplifted fringing. The plate boundary zone consists of orogenic belts (Mongol-Okhotsk, Selenga-Stanovoi, Transbaikal, and Sikhote-Alin) and margins of the Siberian and North China platforms. The plate boundaries are described by mantle diapirism models with a largely bilateral inclined dip of its bordering belts.     

论金川硫化铜镍矿床中贵金属元素的分带机制

在金川硫化铜镍矿床最大的寓矿体内,贵金属元素存在明显分带:Pt、Pd、Au、Ag含量在边部较低,向中心部位逐渐增高;Os、Ir、Ru、Rh含量在中心部位较低,向边部逐渐增高。通过对实际资料的分析,认为这种分带特征是硫化物液相结晶分异作用造成的。     

A garnet population in Yellowknife schist, Canada

Abstract Data are presented on a garnet population in a specimen of garnet-biotite-plagioclase-quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10-μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe] increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca] remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe] concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located. A garnet-forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic, i.e. Vαα⁵.     

金伯利岩中铬铁矿原生表面特征成因类型及其成分特征

对金伯利岩中铬铁矿原生表面特征的成因做出了合理解释,并提出了４种成因类型;通过探讨铬铁矿原生表面特征,化学成分,含矿性之间的关系,初步建立了铬铁矿原生表面特征与金刚石含矿性之间的关系。     

大井锡多金属矿床矿化中心的探讨

讨论一个大井矿床，矿体，矿物及元素的空间分带性，结合对成矿物流体物理化学条件空间变化的研究，推断了大井矿区矿化中心，认为矿区主要的矿化中心有两个，一为矿区中部，即老区东部－北区南部一带，另一个矿化中心在矿区的西部。这两个矿化中心分别位于北东向的F2和F2断裂附近，表明这两条北东向断裂是矿区的导矿构造。     

Spatial structure analysis of regionalized compositions

Like compositions in general, regionalized compositions present the problem of spurious spatial correlation. To avoid this problem, this paper uses the additive-logratio transformation of regionalized compositions, following techniques introduced over the last few years for the statistical analysis of compositional data. It leads to an appropriate definition of a spatial covariance structure to describe spatial dependence between regionalized variables subject to constant-sum constraints in the case of weak stationarity. To illustrate stated problems, simulated data are used.     

中国林业可持续发展及其关键科学问题

在扼要介绍和分析中国森林资源的状况、演变和存在问题的基础上,着重探讨了中国林业可持续发展的战略和为实现林业可持续发展所必需研究解决的关键科学问题。在林业可持续发展战略部分全面论述了它的前提、目标、方针和六项对策措施。在关键科学问题部分则分别从森林的环境功能、森林的生产功能、林木遗传改良、森林保护和森林资源管理等五个方面列出所需要研究解决的关键科学问题,为制订今后的科学研究规划提供参考。