萨尔托海铬铁矿中的矿物包体研究

铬铁矿中矿物包体的研究对于探索铬铁矿的成因具有重要的指示意义。通过薄片扫描电镜背散射结合电子探针分析,在萨尔托海高铝铬铁矿中发现了大量矿物包体,包括:单斜辉石、斜方辉石、橄榄石、钠-金云母、钠-铬韭闪石、出溶的单斜辉石等,这些矿物呈单相或多相产出,在铬尖晶石结晶时被包裹。含水矿物钠-金云母,钠-铬韭闪石的大量出现指示含水环境,表明铬铁矿形成于含水的母岩浆中,可能形成于受俯冲流体/熔体影响的弧后扩张背景。结合之前在萨尔托海铬铁矿中发现的超高压矿物金刚石,认为萨尔托海铬铁矿的形成可能经历了深部地幔和浅部含水大洋岩石圈两个阶段。     

Diamond Discovered in High-Al Chromitites of the Sartohay Ophiolite, Xinjiang Province, China

In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high-Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province,western China,which is characterized by having high-Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds,moissanite and other highly reduced minerals,as well as common crustal minerals. Thus far,more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100-200 μm in size and range in color from pale yellow to reddish-orange to colorless. Most of the grains are anhedral to subhedral octahedra,commonly with elongate forms exhibiting well-developed striations. They all display characteristic Raman spectra with shifts between 1325 cm-1 and 1333 cm-1,mostly 1331.51 cm-1 or 1326.96 cm-1. The moissanite grains are light blue to dark blue,broken crystals,50-150 μm across,commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm-1,785 cm-1,and 966 cm-1. This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high-Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.     

Diamonds, Super-Reduced and Crustal Minerals in Chromitites of the Hegenshan and Sartohay Ophiolites, Central Asian Orogenic Belt, China

The Central Asian Orogenic Belt(CAOB)is a huge tectonic mélange that lies between the North China Craton and the Siberian Block.It is composed of multiple orogenic belts,continental fragments,magmatic and metamorphic rocks,suture zones and discontinuous ophiolite belts.Although the Hegenshan and Sartohay ophiolites are separated by nearly 3000 km and lie in completely different parts of the CAOB,they are remarkably similar in many respects.Both are composed mainly of serpentinized peridotite and dunite,with minor gabbro and sparse basalt.They both host significant podiform chromitites that consist of high-Al,refractory magnesiochromite with Cr#s[100Cr/(Cr+Al)]averaging60.The Sartohay ophiolite has a zircon U-Pb age of ca.300 Ma and has been intruded by granitic plutons of similar age,resulting in intense hydrothermal activity and the formation of gold-bearing listwanites.The age of the Hegenshan is not firmly established but is thought to have formed in the Carboniferous.Like many other ophiolites that we have investigated in other orogenic belts,the chromitites in these two bodieshave abundant diamonds,as well as numerous super-reduced and crustal minerals.The diamonds are mostly,colorless to pale yellow,200-300μm across and have euhedral to anhedral shapes.They all have low carbon isotopes(δ14C=-18 to-29)and some have visible inclusions.These are accompanied by numerous super-reduced minerals such as moissanite,native elements(Fe,Cr,Si,Al,Mn),and alloys(e.g.,Ni-Mn-Fe,Ni-Fe-Al,Ni-Mn-Co,Cr-Ni-Fe,Cr-Fe,Cr-Fe-Mn),as well as a wide range of oxides,sulfides and silicates.Grains of zircon are abundant in the chromitites of both ophiolites and range in age from Precambrian to Cretaceous,reflecting both incorporation of old zircons and modification of grains by hydrothermal alteration.Our investigation confirms that high-Al,refractory chromitites in these two ophiolites have the same range of exotic minerals as high-Cr metallurgical chromitites such as those in the Luobusa ophiolite of Tibet.These collections of exotic minerals in ophiolitic chromitites indicate complex,multi-stage recycling of oceanic and continental crustal material at least to the mantle transition zone,followed by uprise and emplacement of the peridotites into relatively shallow ophiolites.     

新疆萨尔托海铬铁矿中铂族矿物及硫化物特征

新疆萨尔托海高Al型铬铁矿中几乎不含原生的铂族矿物(PGM)和贱金属硫化物(BMS)包体,显示出成矿岩浆贫硫的特征。BMS多产于铬铁矿铬粒间裂隙、基质及蚀变环带中,主要以赫硫镍矿和针镍矿为主,其次为辉铜矿、砷镍矿、硫砷镍矿、毒砂等。PGM以包体产于BMS或铬铁矿粒间缝隙中,以硫钌矿(RuS2)为主,还包括硫锇矿(OsS2)、硫镍锇矿\[(Os,Ni)S2\]、硫钌锇矿\[(Ru,Os)S2\],锑钯矿(Pd5Sb2)和少量Cu、Pt、Au的硫化物。铬铁矿全岩ΣPGE含量50. 64×10-9~92. 00×10-9,较世界范围内蛇绿岩型铬铁矿低,且具有IPGE较PPGE富集的特点,PdN/IrN在0. 1~0. 9之间,具有Os相对Ir富集的特点。铬铁矿主量元素和原位微量元素显示出与菲律宾阿科杰高Al型铬铁矿以及MORB中尖晶石相似的地球化学特征。根据萨尔托海铬铁矿中PGM及BMS的种类、产出特征,结合铬铁矿全岩PGE及单矿物微量元素地球化学特征,认为铬铁矿的形成与贫硫的拉斑玄武质岩浆与地幔橄榄岩的熔体岩石反应有关。铬铁矿形成后的晚期岩浆阶段使得自形程度较高的PGM(如硫锇矿)和BMS(如赫硫镍矿)形成,随后向热液阶段转变的过程中,由于温压条件改变、热液蚀变,形成了萨尔托海铬铁矿中Fe- Ni- As- S和PGM矿物组合。     

蛇绿岩中铬铁矿研究的问题与思考

蛇绿岩中的铬铁矿广泛分布在全球不同时代和地区的造山带中,是工业铬矿的重要来源。金属铬的物理化学性质决定了其耐高温、耐腐蚀和抗氧化等许多特性,是生产不锈钢等特种钢、用于军工、航天等领域的重要战略资源。尽管研究历史悠久,但许多关键的科学问题并没有解决,是地球科学研究的前沿和热点。本文简述了铬铁矿的性质,综述了全球铬铁矿资源的分布与需求现状;通过大数据分析,回顾了铬铁矿的研究历史和进展;概述了蛇绿岩铬铁矿的主要产出特征和地质背景;探讨了研究中存在的关键科学问题,包括蛇绿岩豆荚状铬铁矿成矿母岩浆性质,成矿物质来源和聚集成矿作用以及成矿动力学背景;讨论了铬铁矿成因的不同模式以及相关的高温高压实验岩石学的研究进展;综述了蛇绿岩铬铁矿中新发现的一系列新矿物及其地质意义,提出蛇绿岩铬铁矿是研究俯冲物质深地幔循环和地球深部动力学的重要窗口等新认识。     

岩盐中合成烃包裹体与母油的地球化学特征及其在油源对比中的意义

油气包裹体广泛应用于油气成藏研究,但油气包裹体能否继承母油荧光及地球化学特征等,尚缺乏直接的实验证据。本文在开放体系下用NaCl挥发结晶法对一轻质原油进行合成烃类包裹体实验,通过镜下观察和激光剥蚀色谱-质谱技术分析合成的烃包裹体和母油的荧光特征、成分特征及地化特征,研究两者的异同点。结果表明,在NaCl合成烃类包裹体中共发现三种相态的包裹体:纯液相烃包裹体、气液两相烃包裹体和气油水三相包裹体;共有绿黄色荧光、蓝色荧光两种不同颜色的荧光特征。相对于母油,合成包裹体的饱和烃和芳烃类化合物中的轻质组分含量较低,可以推测在母油被捕获成为包裹体的过程中,不同的化学成分存在差异性捕获。但合成包裹体的Pr/Ph等地化参数和不同系列化合物相对含量与母油相差不大,能很好地反映母油的沉积环境等地化特征。因此,包裹体成分信息可以应用于油源对比分析。     

Ophiolitic Chromitites Originated from Ancient SCLM

<正>Ophiolitic chromitites are traditionally considered as the products of melt-mantle interaction in the supra-subduction zone after partial melting of mantle peridotites during upwelling of asthenosphere at oceanic spreading centers.The occurrence of natural diamonds in situ in chromitites and peridotites in southern Tibet(Yang et al.,2013;Griffin     

Platinum-group Mineral (PGM) and Base-metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

Voluminous platinum-group mineral(PGM) inclusions including erlichmanite(Os,Ru)S_2, laurite(Ru,Os)S_2, and irarsite(Ir,Os,Ru,Rh)As S, as well as native osmium Os(Ir) and inclusions of base metal sulphides(BMS), including millerite(NiS), heazlewoodite(Ni_3S_2), covellite(CuS) and digenite(Cu_3S_2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite(biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides(such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S_2 and ?O_2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S_2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S_2 increased. The abundance of primary BMS in the chromitites suggests that ?S_2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.     

遵化豆荚状铬铁矿岩浆流变学性质

对遵化豆荚状铬铁矿中250个铬铁矿豆体的形态、大小、向椭球体转变的程度等统计分析表明,豆体在上升期间与岩浆作用的过程中伴有由等轴体(近似为球体)向椭球体转变的过程。其主要原因是豆体在相对岩浆运动的过程中,豆体的表面与岩浆之间的粘滞摩擦造成的。粘滞摩擦力的大小、作用于豆体表面的时间又取决于岩浆的粘度、豆体相对于岩浆的沉降速度、豆体半径的大小及岩浆上升的速度,所以这些因素将决定豆体变形(磨蚀、椭球化)的程度。因此,可用豆体与岩浆相互作用的物理过程来模拟豆体的变形特点,而反过来据实测的豆体变形程度数据又可推算岩浆真实的上升速度。     

Millerite-Maucherite-Pentlandite-Heazlewoodite in the Sartohay Chromite Deposit, West Junggar, Xinjiang, China

Please refer to the attachment(s) for more details     

Origin of Peridotites and Chromitites in the Pozanti-kar Santi Ophiolite, Turkey

The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by spinel harzburgites with minor dunites.The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition,suggesting high degrees of partial melting.Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns,implying the crystallization of interstitial sulphides from sulphur-saturated melts(e.g.MORB-like forearc basalt).U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone.Based on the mineral and whole-rock chemical compositions,the PKO peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon(MTZ).Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element(PGE)contents than those of the MTZ chromitites.However,both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt(e.g.boninitic or island arc tholeiitic melt).Dunite enveloping chromitite lenses in the ho*s ting harzburgite resulted from melt-rock reaction.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than200 grains of microdiamond and more than 100 grains of moissanite(Si C)have been separated from podiform chromitites.These minerals have been identified by EDX and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam.In total,61δ13CPDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.31δ13CPDB results for Moissanite vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.Diamond has relatively large variation in nitrogen isotopic composition with 40δ15NAIR results ranging from-19.1‰to 16.6‰.The discovery of diamond,moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PKO chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.     

The Chromitites Associated with the Pan-African Ophiolites in Egypt

正Ophiolites components occur in Pan-African belt in Central Eastern Desert(CED)and South Eastern Desert(SED.The ultramafic components are severely serpentinized and in some areas occur as small fresh     

新疆萨尔托海铬铁矿中的Fe-Ni-As-S矿物研究

新疆萨尔托海铬铁矿是一个典型的与蛇绿岩有关的高铝型豆荚状铬铁矿,其中矿石铬尖晶石发生了明显的热液蚀变,发育了富Cr的蚀变环边,形成高铁铬铁矿,Cr#在蚀变后升高,发生了Cr元素的次生富集。在矿石颗粒间隙中的Fe-Ni-As-S矿物组合主要为镍黄铁矿-赫硫镍矿-针镍矿-砷镍矿。围岩纯橄岩普遍发育强烈的蛇纹石化,其中的Fe-Ni-As-S矿物组合为赫硫镍矿-镍黄铁矿-砷镍矿,还有少量的针镍矿和铜矿物。通过对硫化物的成分对比分析,认为矿石中的镍黄铁矿和赫硫镍矿都属于岩浆演化的产物(600℃),与赫硫镍矿和针镍矿一样,均从贫S的母岩浆中通过岩浆熔离过程形成。围岩和矿石中的含砷矿物以及围岩中的镍黄铁矿都是晚期热液活动的结果,其中砷镍矿具有特殊的蠕虫状-乳滴状结构,与围岩中的赫硫镍矿和镍黄铁矿共生。围岩和矿石中Fe-Ni-As-S矿物组合的形态和成分差异,说明金属矿物的整体演化从岩浆期到热液期经历了从贫S到富As的环境变化,最终形成了现在所观察到的复杂Fe-Ni-As-S矿物组合。     

西昆仑库地蛇绿岩富Al型豆荚状铬铁矿床成因

西昆仑库地蛇绿岩发育小规模的铬铁矿床,矿体呈豆荚状和层状、似层状,均与纯橄岩紧密伴生。这些纯橄岩主要由橄榄石和副矿物尖晶石组成,与方辉橄榄岩相比,橄榄岩中的橄榄石粒径粗(平均2.5mm),Mg#(88~90)低,这与它们全岩低Mg#(90)值,富Al_2O_3、TiO_2、Cr_2O_3、Fe_2O_3相吻合,与熔融残余成因的纯橄岩明显不同,反映了其很可能是由熔体与方辉橄榄岩反应而成。矿体主要由块状、浸染状及脉状铬铁矿石组成;铬铁矿石中的尖晶石具有低而相对稳定的Cr#(43~56),低于富铬型铬铁矿矿床中的铬铁矿(Cr#60)。块状矿石与纯橄岩呈突变接触,矿石中的尖晶石呈浑圆状,包裹有较多橄榄石、辉石等硅酸盐矿物及角闪石等含水硅酸盐矿物;浸染状铬铁矿石中的尖晶石与橄榄石颗粒构成交织结构,或呈云朵状,沿橄榄石颗粒边界相互连接,矿石的结构构造显示了熔/岩反应成因特征。通过计算分析,我们认为该区富铝型铬铁矿石是由拉斑玄武质熔体与地幔橄榄岩反应而成,由于熔体中含有较高的H_2O,参与反应的熔体可能源于弧后扩张脊环境。     

西藏罗布莎蛇绿岩铬铁矿中金刚石的研究

在西藏罗布莎豆荚状铬铁矿中发现过金刚石,但迄今仍有人因为自己所取的小试样中没有分离出金刚石而否定罗布莎等地金刚石的存在。由于金刚石涉及西藏雅鲁藏布江缝合线和蛇绿岩、铬铁矿成因等问题,笔者重新在罗布莎铬铁矿中取样,经过选矿,又在1500kg样品中选出了25粒金刚石和近70种伴生矿物。这就表明西藏金刚石是的确存在的,而且它和蛇绿岩的铬铁矿同是深部成因的产物。     

新疆萨尔托海糜棱岩化石英菱镁岩元素地球化学特征及其对金成矿作用的贡献

新疆萨尔托海石英菱镁岩产在达拉布特蛇绿混杂岩带中,是蛇纹岩在剪切带深部发生热液交代作用的产物。蛇纹岩先转变为滑石片岩,再进一步转变为石英菱镁岩。石英菱镁岩局部发生剪切变形,形成糜棱岩化石英菱镁岩。与蛇纹岩相比,石英菱镁岩的MgO和SiO_2含量降低,Al_2O_3和CaO含量升高;而糜棱岩化石英菱镁岩的MgO含量较石英菱镁岩降低,Al_2O_3和SiO_2含量较石英菱镁岩升高。微量元素地球化学对比研究表明石英菱镁岩继承了蛇纹岩的微量元素特征;而糜棱岩化石英菱镁岩的微量元素含量较蛇纹岩发生了显著变化,稀土元素、高场强元素和自然金的含量明显升高,指示这些元素在剪切变形过程中发生迁移富集。剪切变形伴随强烈的流体/岩石反应,并导致剪切带流体组成和物理化学性质发生改变,从而影响矿物结晶或分解,并控制微量元素的带入迁出。蛇纹岩转变为石英菱镁岩的过程释放Au,可为糜棱岩化石英菱镁岩中的金矿化提供成矿元素。     

西藏罗布莎蛇绿岩豆荚状铬铁矿石中的合金成分

从西藏雅鲁藏布江蛇绿岩带的罗布莎豆荚状铬铁矿床中 ,揭示出包含 70～ 80种矿物的一个地幔矿物群 ,其中特别引人注意的是含有多种合金。本文报道了已发现的合金类型和它们的化学成分。这些合金矿物主要通过人工重砂选矿提取的 ,少数合金在矿石光片中可以见到。本文报道的部分合金系有 :Ni(Fe) - C- Cr系 ,W-Cr- Co系 ,Al- Fe- L a系 ,Fe- Si- Ti系 ,Ag- Sn- Si系 ,Ni- Ir- Fe系 ,Fe- Pd- Pt系 ,Fe- Ni- C系。这些碳化物、金属硅以及铁合金等表明它们形成于还原环境 ,然而主岩铬铁矿石则形成于氧化环境 ,认为罗布莎铬铁矿是从玻安质岩浆中结晶的。这样合金矿物可能是外来晶体 ;或者它们形成于地核被后来上升的地幔柱带到浅部 ,包在铬铁矿中 ;或者是滞留在地幔中的成核物质后来被铬铁矿捕获。     

The Petrology and Geochemistry of Listwaenite in the Sartohay Ophiolitic Melange of West Junggar, Xinjiang, China

The west Junggar,located in the eastern part of Balkash-Junggar tectonic province,is a major component of the core of the Central Asian metallogenic region.This area is characterized by occurrences of ophiolitic mélanges,such as the Sartohay ophiolitic mélange in the NE and the Tangbale ophiolitic mélange in the west.As a hydrothermal alteration product of serpentinite in the Sartohay ophiolitic mélange,listwaenite lenses are gold-mineralized and crop out on surface in the ophiolitic mélange via weathering of exhumated hanging wall of fault zone.Listwaenite is mainly composed of magnesite,quartz,dolomite,and trace amounts of mariposite,chromian spinel,talc and sulfide.A vertical thermal gradient model for the hydrothermal alteration shows that serpentinite would first be transformed to talc schist,then into listwaenite as the ophiolite slices continued to rise along shear zone,with XCO2,oxygen and sulfur fugacity increase and temperature decrease.Both serpentine and magnetite were progressively destroyed during the transformation from serpentinite to talc schist,andcompletely vanished in listwaenite,while mariposite generated in weakly deformed to mylonitized listwaenite.Concentrations of most trace elements including high field strength elements and metallogenic elements,increasing from undeformed,through weakly deformed,to mylonitized listwaenite,show a positive correlation with deformation degree and content of apatite,rutile,monazite,zircon and sulfide in listwaenite.The shear zone served as pathways for percolation and accumulation of fluid and trace elements during the metasomatism from serpentinite to listwaenite.Compared to undeformed listwaenite,mylonitized listwaenite will be more favorable to be fractured and brecciated due to more intense shearing,which caused strong metasomatic reaction and then induced trace element-bearing mylonitized listwaenite.