A Preliminary Study on Fluid Inclusions and Mineralization of Xitieshan Sedimentary-Exhalative (SEDEX) Lead-Zinc Deposit

The Xitieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and had developed a complete marine sedimentary-exhalative system. Our preliminary study of ore-forming fluids shows that fluid inclusions in quartz from altered stockwork rocks that represent the pipe facies have a wide range of temperature and salinity. The intense fluid activities are characteristics of the pipe facies of the exhalative system. Fluid inclusions in carbonates near the unstratified ore bodies hosted in the thick-bedded marble which represents vent-proximal facies are large in size and have moderate to high temperatures. They represent unerupted sub-seafloor fluid activity. Fluids in altered stockwork rocks and carbonates have similar H2O-NaCl-CO2 system, both belonging to the sedimentary-exhalative system. The fluids migrate from the pipe facies to the unstratified ore bodies. Boiling of the fluids causes the separation of CO2 vapor and liquid H2O. When the fluids migrate into the unconsolidated thick-bedded marble, the escape of CO2, decreasing temperature and pressure as well as some involvement of seawater into the fluids result in the unmixing of fluids with high and low salinity and deposition of ore-forming materials. The two unmixed fluids were trapped in unconsolidated carbonates and the ore-forming materials were deposited in the unconsolidated carbonates to form the sedimentary-exhalative type unstratified ore bodies. The ore-forming temperature of unstratified ore bodies is up to high temperature indicating that there is a huge ore-forming potential in its deep.     

A thermobarometer for sphene (titanite)

Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree. In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content (ppm by weight), pressure (GPa) and reciprocal absolute temperature: The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

The fidelity of melt inclusions as records of melt composition

A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10⁻⁹–10⁻⁸ m s⁻¹, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO₂, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

新疆蒙其古尔铀矿床成矿流体特征及其与铀成矿的关系

蒙其古尔铀矿床为大型层间氧化带砂岩型铀矿床,前人对其成矿流体来源的研究相对较少。为探讨其成矿流体的性质和来源,笔者利用显微测温学和激光拉曼光谱方法,对其流体包裹体的温度、盐度、密度和成分进行了系统的分析研究,并对其成矿流体的氢、氧同位素及含矿目的层砂岩方解石胶结物的碳、氧同位素组成特征进行了研究。研究表明,该矿床流体包裹体主要有气烃包裹体、液烃包裹体和盐水包裹体,成群分布于砂岩粒间方解石胶结物中,或沿切穿石英颗粒的微裂隙呈线状或带状分布,均一温度为56~76℃,盐度w(NaCleq)为1.23%~19.84%,密度为0.99~1.12 g/cm~3,气体成分以CH_4为主;成矿流体的δD(H_2O)V_SMOW=-93.0‰~-48.3‰,δ~(18)O(H_2O)V_SMOW=-10.3‰~-5.1‰,方解石胶结物的δ~(13)CV_PDB=-10.9‰~-7.2‰,δ~(18)OV_SMOW=17.6‰~24.9‰。上述特征揭示出蒙其古尔铀矿床成矿流体是由大气降水性质的地表水和煤系地层有机质脱羧基作用产生的有机酸及伴生的CH_4等还原性气体(煤型气)两部分组成,具有低温、盐度跨度大、中等密度及多期叠加等特点,明显有别于深部油气大规模充注。地表含铀含氧水层间渗入与煤系地层产生煤型气等还原性渗出流体的共同作用,形成了蒙其古尔铀矿床。     

塔里木北缘库鲁克塔格地块大金沟金矿流体包裹体特征:对矿床成因的启示

在成矿地质背景和矿床地质特征研究基础上,对大金沟金矿主矿体中含矿石英脉流体包裹体开展了系统的岩相观察和显微测温工作。结果表明:大金沟金矿含矿石英脉中主要发育富液相的气液两相包裹体,偶见纯液相或气相包裹体。这些包裹体可分为低温高盐度(75℃~121℃,16.71 wt%NaCl~19.68 wt%NaCl)、低温低盐度(100℃~172℃,0.7 wt%NaCl~5.71 wt%NaCl)、中低温中低盐度(75℃~253℃,6.16 wt%NaCl~12.51 wt%NaCl)和高温中低盐度(331℃~420℃,4.96 wt%NaCl~12.51 wt%NaCl)等4类。大金沟金矿主成矿期流体包裹体的均一温度(75℃~253℃)和盐度(0.7 wt%NaCl~19.68 wt%NaCl)变化较大,且两者的相关性不明显,属典型的中低盐度流体体系,与造山过程相关的变质流体类似,表明该矿床可能为造山型金矿床。     

辽宁青城子矿集区金银矿成矿流体特征和成矿物质来源示踪

青城子矿集区地处辽东-吉南裂谷带西端,是金银多金属的重要矿集区,其成矿作用复杂,为了深入揭示其成矿流体特征和成矿物质来源,近期作者对矿集区内典型的金银矿床(高家堡子银矿、小佟家堡子金矿、杨树金矿和林家三道沟金矿)的地质特征研究的基础上,开展了流体包裹体测温和激光拉曼成分分析,H、O、C、S、Pb同位素的分析,进行了多元同位素体系的综合示踪。青城子矿集区金银矿床最为发育富液相包裹体,局部发育气相包裹体,个别石英脉中少量发育H_2O-CO_2三相包裹体和CO_2两相包裹体,成矿温度范围大,是多阶段成矿作用发展演化的反映,主成矿温度在120~210℃之间。成矿流体为中温低盐度低密度的水盐流体。激光拉曼成分特征,成矿流体总体属于含H_2的H_2O-NaCl-CH_4-CO_2体系,属于还原性流体,具有深源的特征。氢氧同位素结果显示,成矿热液主要来源于岩浆水和大气降水。碳氧同位素结果显示,成矿流体起源或流经含有大量有机质的地层,地层中的有机质可能参与了金的富集成矿。硫同位素特征表明,矿体中的硫来自海水硫酸盐还原硫与深部岩浆热液来源的混合,可能主要来自海水硫酸盐的还原。铅同位素结果表明,金银矿石中铅是地层与岩浆岩的混合铅。     

贵州晴隆锑矿床成矿流体He-Ar同位素地球化学

晴隆锑矿床是华南中生代低温成矿域右江盆地Au-Sb-As-Hg矿集区内的大型锑矿床,成矿时代约150Ma。本文以该矿床矿石矿物辉锑矿中的流体包裹体为测试对象,研究了成矿流体的He和Ar同位素地球化学。研究表明,成矿流体的~3He/~4He为0.13~0.46Ra(Ra为空气的3He/4He值,1.4×10~(-6)),~(40)Ar/~(36)Ar为305~327,He、Ar同位素组成具有一定程度的正相关;成矿流体由两个端元组成:一是含地壳He的低温饱和空气的雨水,二是含地幔He的高温流体。含地幔He的高温流体可能来自右江盆地下部的侏罗纪壳幔混合成因花岗岩浆,这种岩浆的形成机制与华夏地块侏罗纪与钨锡成矿有关的花岗岩具有相似性。晴隆锑矿床以大气降水为主的成矿流体的加热、循环并浸取矿床围岩"大厂层"下伏地层中的成矿元素而成矿,受深部壳幔混合成因花岗岩浆释放出的含幔源He和岩浆S的热流所驱动。     

安徽庐枞盆地小包庄铁矿的矿床地质特征和成矿机制以及与罗河铁矿的关系

小包庄铁矿位于长江中下游成矿带庐枞火山岩盆地中,是近年发现的又一大型玢岩型铁矿床。本文基于详细的岩相学、矿相学观察,对小包庄铁矿的围岩及典型矿物的主微量元素、硫同位素、流体包裹体进行了分析测试,结果显示,小包庄铁矿磁铁矿矿石中不同成矿阶段形成的黄铁矿、硬石膏的δ34S值异常高,黄铁矿集中在6‰~15‰之间,硬石膏集中在16‰~24‰之间,并且随着成矿期次由早到晚、深度由深到浅,黄铁矿、硬石膏的δ34S值逐渐降低,同时测得小包庄铁矿下部三叠系东马鞍山组膏盐层的δ34S值为30.1‰,反映了小包庄铁矿有膏盐层硫的加入。包裹体测温结果表明,小包庄铁矿透辉石-磁铁矿-硬石膏化蚀变阶段、绿帘石化蚀变阶段、碳酸盐-硬石膏-黄铁矿化蚀变阶段和硬石膏-石英-黄铁矿化蚀变阶段的成矿流体温度分布区间分别为326~496℃、259~317℃、178~280℃和107~189℃,盐度分布区间为14%~21%NaCleqv、11%~20%NaCleqv、6%~16%NaCleqv、2%~13%NaCleqv,成矿流体经历了从高温高盐度向低温低盐度演化的过程。从成矿岩体、矿体矿石特征、蚀变分带以及成矿温度等方面与罗河铁矿进行对比表明,小包庄铁矿与罗河铁矿为同一类型的两个独立矿床,其在空间上上下叠覆的模式可能是由闪长质岩浆在不同深度侵位造成的。     

大兴安岭南段热液脉状矿床的流体包裹体和稳定同位素特征:对矿床成因的启示

大兴安岭南段发育大井、双尖子山、布金黑、拜仁达坝、维拉斯托等多个具有典型后生特征的热液脉状铅锌银锡多金属矿床。为了查明上述矿床在成矿流体、成矿物质等方面的特征与差异,进而总结大兴安岭南段热液脉状矿床成矿作用特点,本次研究在野外地质调研的基础上,对上述矿床进行了流体包裹体、激光拉曼和氢氧硫同位素的研究,并取得了如下主要的认识:(1)双尖子山银多金属矿床成矿流体属简单盐水体系热液,维拉斯托和布金黑矿床成矿流体属富碳质盐水体系热液,而大井铜矿成矿流体则为含子矿物的不均匀盐水体系热液;(2)大井、布金黑、拜仁达坝和维拉斯托等矿床早期成矿流体均来自于岩浆热液,但布金黑、拜仁达坝和维拉斯托矿床成矿流体在运移过程中明显地受到了大气降水和地层中有机质的影响;(3)大兴安岭南段多数热液脉状多金属矿床矿石硫同位素δ34S值具有岩浆来源特点,个别矿床硫同位素δ34S值偏高可能是由复杂的岩浆源区性质及地层硫混入所引起。总的来说,大兴安岭南段不同热液脉状矿床在物质来源和流体演化方面的差异明显,而这也体现了该区中生代热液成矿作用的多样性和复杂性特点。     

Formation of mosaic diamonds from the Zarnitsa kimberlite

Mosaic diamonds from the Zarnitsa kimberlite (Daldyn field, Yakutian diamondiferous province) are morphologicaly and structurally similar to dark gray mosaic diamonds of varieties V and VII found frequently in placers of the northeastern Siberian craton. However, although being similar in microstructure, the two groups of diamonds differ in formation mechanism: splitting of crystals in the case of placer diamonds (V and VII) and growth by geometric selection in the Zarnitsa kimberlite diamonds. Selective growth on originally polycrystalline substrates in the latter has produced radial micro structures with grains coarsening rimward from distinctly polycrystalline cores. Besides the formation mechanisms, diamonds of the two groups differ in origin of mineral inclusions, distribution of defects and nitrogen impurity, and carbon isotope composition. Unlike the placer diamonds of varieties V and VII, the analyzed crystals from the Zarnitsa kimberlite enclose peridotitic minerals (olivines and subcalcic Cr-bearing pyropes) and have total nitrogen contents common to natural kimberlitic diamonds (0 to 1761 ppm) and typical mantle carbon isotope compositions (-1.9 to -6.2%c 5¹³C; -4.2%c on average). The distribution of defect centers in the Zarnitsa diamond samples fits the annealing model implying that nitrogen aggregation decreases from core to rim.