云南剑川金河岩体中地幔流体交代特征及其成矿意义

云南剑川金河岩体中的硅化霓辉正长斑岩含有较多的镁铁-超镁铁质深源岩石包体.岩相学研究表明,伴随交代蚀变,其主岩和各类包体岩石中普遍发育沿矿物粒间和矿物晶体裂隙或解离纹贯入或穿插的呈网状和细脉浸染状分布的黑色不透明物质.对该物质进行了电子探针及扫描电镜和能谱分析,发现其组成以微晶硅酸盐矿物为主,含少量微晶石英,尤其是含有微晶地幔标型矿物--含钛磁铁矿(或镜铁矿)和含铬自然铁;后两者与硅酸盐矿物构成超显微熔离结构交生.研究认为,这种物质组成和特征,是具超临界和熔浆性质的地幔流体的微观表现,其活动所引发的交代蚀变表现为硅化、钠化及深源包体中的暗色矿物由辉石→角闪石→黑云母的退变序列.这一地幔流体作用微观踪迹的揭示,对于分析和论证滇西地区新生代多金属成矿的深部地质作用规律具有重要的理论和现实意义.     

云南六合富碱斑岩和包体岩石的显微岩相学与地幔流体作用示踪研究

云南六合霓辉正长斑岩含有较多镁铁-超镁铁质包体以及特殊的富铁熔浆包体.岩相学研究表明,伴随交代蚀变,寄主岩石和各类包体中普遍发育沿粒间和矿物晶体裂隙或解理纹贯入或穿插的呈网状和细脉浸染状分布的黑色不透明物质——富铁微晶玻璃,其性质与富铁熔浆包体类似;对该类物质采用电子探针及扫描电镜和能谱分析,发现它们是一种纳米-微米级的超显微隐晶固体,其组成以硅酸盐和石英为主,尤其含有地幔标型矿物碳硅石、自然铁和钛铁矿等;这些矿物间构成超显微熔离结构交生.研究认为,这种物质组成和特征是具超临界和熔浆性质的地幔流体作用的现实微观踪迹,其活动引发的交代蚀变表现为碱性蚀变和硅化并伴随不同程度的金属矿化,以及寄主岩石和包体中暗色矿物由辉石→角闪石→黑云母→绿泥石的退变序列.稀土和微量元素地球化学研究表明,霓辉正长斑岩及其包体和富铁熔浆包体具有相似的富集轻稀土元素和 Ce 负异常以及富集大离子亲石元素和高场强元素的特征.本文综合研究认为,富铁微晶玻璃和富铁熔浆包体是起源于富集地幔,但独立于富碱岩浆并被富碱岩浆裹挟的成分不混溶产物.它们是地幔流体作用现实微观踪迹的两种表现形式,是引发壳幔物质混染并导致寄主岩石和包体中交代蚀变和金属矿化的重要物质源和动力源.     

云南中甸峨眉山玄武岩中超基性—基性岩包体的地幔流体微观踪迹及其熔离成矿意义

通过对中甸峨眉山玄武岩中超基性—基性岩包体的岩相学、电子探针、扫描电镜和能谱分析,发现在光学显微镜下呈黑色不透明的物质,在电子显微镜下表现为硅酸盐和尖晶石族氧化物为主的超显微隐晶—非晶质集合体。结合超微晶矿物晶体化学和元素地球化学分析认为,该物质是一种具熔浆和超临界性质及地幔流体属性的富铁(微晶)玻璃,是深部地质过程中,包含于幔源岩浆并与其同步运移但互不混熔的地幔流体作用遗留的微观踪迹物质。该地幔流体在伴随幔源岩浆的结晶成岩过程中,表现出由硅酸盐→尖晶石→磁铁矿→铬铁矿→钛铁矿的熔离作用,并相应触发主岩和包体岩石的交代蚀变和金属矿化,是推动和引发地壳中成岩成矿的重要动力源和物质源。由此进一步认识到,滇西地区上地幔发育峨眉地幔柱活动,为该区二叠纪之后的不同部位和不同矿种的内生多金属成矿提供了有利的深部地质背景条件。     

云南六合霓辉正长斑岩中特殊包体岩石的发现及其意义

在六合霓辉正长斑岩中广泛发育的各类包体岩石中，新近发现了三类特殊包体。通过岩相学、岩石化学、电子探针和扫描电镜分析研究表明，纯方解石包晶（体）和含石英的黄玉伟晶岩包体是地幔流体在运移过程中发生去气作用的产物；黑色微晶富铁硅酸盐熔融包体是地幔流体伴随去气作用的分离产物，其主要矿物组成为石英、绿泥石和富铁的柱状和片状硅酸盐矿物，特征矿物有自然铁、磷灰石和锆石；据全岩化学成分和矿物组成特征及结晶状态分析，表明该包体岩石不是常规岩浆作用产物，尤其自然铁的存在进一步证明该微晶熔融包体是还原条件或缺氧环境下的地幔流体物质。通过对特殊包体岩石的研究，为该区不同岩性和地层中的多金属成矿提供了重要的深部过程地球化学背景。     

云南六合霓辉正长斑岩中特殊包体岩石的发现及其意义

在六合霓辉正长斑岩中广泛发育的各类包体岩石中,新近发现了三类特殊包体.通过岩相学、岩石化学、电子探针和扫描电镜分析研究表明,纯方解石包晶(体)和含石英的黄玉伟晶岩包体是地幔流体在运移过程中发生去气作用的产物;黑色微晶富铁硅酸盐熔融包体是地幔流体伴随去气作用的分离产物,其主要矿物组成为石英、绿泥石和富铁的柱状和片状硅酸盐矿物,特征矿物有自然铁、磷灰石和锆石;据全岩化学成分和矿物组成特征及结晶状态分析,表明该包体岩石不是常规岩浆作用产物,尤其自然铁的存在进一步证明该微晶熔融包体是还原条件或缺氧环境下的地幔流体物质.通过对特殊包体岩石的研究,为该区不同岩性和地层中的多金属成矿提供了重要的深部过程地球化学背景.     

滇西地区富碱斑岩中地幔流体作用踪迹及其成矿作用意义

滇西地区大量产出的富碱斑岩及其包体岩石的形成和演化与该区新生代陆内变形、构造作用、幔源岩浆和深源流体活动,及其与此有关而广泛发育的内生多金属矿产存在必然的联系。本文通过岩相学、岩石化学、电子探针、扫描电镜和能谱分析,较为系统地分析论证了这一关联的内在统一制约和联系的纽带即深部地质过程和由此相伴的含矿地幔流体作用,初步揭示了这种深部过程和地幔流体作用的微观踪迹和方式可以直接表现为:(1)呈脉状和浸染状穿插于深源包体岩石中的富钠玻璃,透光镜下呈微晶和雏晶,颜色随成分差异而不均匀,化学成分以高硅、铝、钠、铁,低钾、钙、镁为特征,矿物成分以钠长石、角闪石、磁铁矿(镜铁矿)、钛铁矿组合为特征,是富碱岩浆携带包体岩石之前即已存在的上地幔流体;(2)呈脉状、团块状和浸染状穿插于主岩和各类包体岩石的富铁玻璃;(3)呈独立包体产出于霓辉正长斑岩中的富铁熔浆包体。后两者物质在透光镜下无光性,呈黑色不透明,反光镜下不反光,但在电子显微镜下呈显微晶质结构,化学成分以高硅、铝、铁,低钙、镁、钠、钾为特征,矿物组成上以硅酸盐和石英为主,含有碳硅石、含铬自然铁、钛铁矿、磷灰石等地幔标志矿物,其中微晶金属和非金属矿物之间呈熔离结构交生,反映了地幔流体的熔浆性质及其与富碱岩浆不混溶的特征;由地幔流体对岩石的交代浸染作用,引起主岩和包体岩石中普遍发育各种蚀变作用,如角闪石化、硅化和绿泥石化等,并导致矿物组合总体上表现为暗色矿物由辉石→角闪石→黑云母→绿泥石的退变序列。该地幔流体微观踪迹的三种表现形式与富碱岩浆共存,并共同运移,但两者由于组成和性质的差异而互不混溶;结合透岩浆流体成矿作用理论和本文论证的综合分析认为,当富碱岩浆和地幔流体系统封闭较好,地幔流体则伴随富碱岩浆的结晶过程对富碱斑岩进行同步自交代蚀变,在斑岩体或其深部形成矿床,构成正岩浆成矿体系,典型成矿实例如马厂箐斑岩钼矿床;若在此成岩成矿过程中发生构造作用扰动,则地幔流体进入岩体与围岩的接触带,或紧邻接触带的地层围岩中进行交代蚀变成矿,构成接触带成矿体系,典型成矿实例如马厂箐矿区中赋存于夕卡岩—大理岩带中的斑岩型铜矿和主要赋存于地层围岩中的斑岩型金矿,若金矿出现在斑岩体内,则一般受控于穿切斑岩体的成岩后断裂;若岩浆和流体运移通道的深大断裂体系发育,环境相对开放,则地幔流体伴随富碱岩浆的成岩过程而脱离岩浆沿分支断裂通道进入远离岩体的不同地层岩石中进行交代蚀变成矿,构成远程热液成矿系统,典型成矿实例如兰坪金顶超大型铅锌矿床;在这一成矿过程中,地幔流体可以随深度和环境变化引起的物理化学条件变化,其性质由熔浆→超临界流体→液相流体转化,并运载和沿途活化成矿物质至适宜容矿部位集中,促使幔壳物质叠加成矿;进而有利于深部成矿并形成大型和超大型矿床。     

造山带背景铜镍矿床蚀变过程与水的来源——东天山黄山南矿床氢氧同位素的指示

造山带背景铜镍矿床以富水为主要特征,但水(流体)在成矿中的作用以及其对岩石的改造过程仍不明确。本文以黄山南铜镍矿床为例,通过辉橄岩和橄辉岩等超镁铁岩的蚀变矿物组合和H-O同位素变化规律,限定蚀变过程与流体性质及来源。黄山南超镁铁岩原生矿物主要有尖晶石、橄榄石、斜方辉石、单斜辉石和少量填隙状角闪石、云母,蚀变矿物有角闪石(浅闪石、阳起石、透闪石、普通角闪石以及镁闪石)、滑石、绿泥石和蛇纹石等。根据岩石结构与蚀变矿物比例,将超镁铁岩分为弱蚀变、中等蚀变和强蚀变3类。蚀变矿物组合与角闪石成分指示超镁铁岩经历了高温蚀变阶段(>700℃),形成了镁质闪石+滑石+绿泥石;中温蚀变阶段(700～550℃)和低温蚀变阶段(<550℃),分别形成了钙质闪石+滑石+绿泥石+蛇纹石与滑石+碳酸盐+蛇纹石的矿物组合。蚀变岩石普遍以中低温蚀变为主,可能与中低温阶段的叠加-改造作用相关。岩石随蚀变程度增加,Si、Na、K、Mn等主量元素和Rb、Ba等微量元素呈现明显降低趋势,表明大多数元素在流体改造过程中从岩石中迁出,说明蚀变过程为开放体系。中-强蚀变岩石中,硫化物矿物边部的形态呈锯齿状,但主体仍为磁...     

云南老王寨金矿的深部地质过程——来自显微岩相学和元素地球化学的证据

通过老王寨金矿围岩、蚀变岩石、矿石的显微岩相学和元素地球化学的系统研究发现：伴随硅化、碳酸盐化等蚀变和硫化物矿化,岩（矿）石中发育沿粒间、裂隙或角闪石解理纹贯入或穿插的黑色不透明超显微隐晶,经电子探针、扫描电镜和能谱分析鉴定,该固体物质主要由超显微晶石英、硅酸盐、碳酸盐、硫化物以及呈熔离交生关系的金红石、白钨矿和镜铁矿（磁铁矿）组成;熔离交生结构与沉淀共晶结构共存的特有岩相学特征指示成矿流体具备由熔体性质向热液性质转化的特性。结合岩（矿）石稀土微量元素研究,认为这种在透射光显微镜下呈黑色不透明的超显微隐晶固体物质,是具熔浆流体特点和超临界流体性质的地幔流体引发交代蚀变的直接微观表现之一;在交代蚀变过程中其性质由熔浆向地壳热液过渡,同时与地壳物质发生混染而有利于成矿作用的进行。     

滇西富碱斑岩中特殊包体岩石的流体包裹体幔源不混溶特征

滇西地区沿金沙江-哀牢山断裂带产出了一套新生代富碱斑岩,其中发现了与镁铁-超镁铁质深源包体岩石共生的含石英的方解石包晶(体)、石英钠长石伟晶岩包体和含玻璃包裹体的纯石英包晶(体)以及富铁熔浆包体.流体包裹体地球化学研究表明,该四类特殊包体的形成与富含CO2流体持续减压而造成的不混溶作用有关;而玻璃包裹体与水溶液包裹体以...     

钒钛磁铁矿床的透岩浆流体成矿模型——来自马达加斯加扎卡岩体的证据

目前,国内外学术界对赋存于镁铁-超镁铁杂岩体中的钒钛磁铁矿床的成因存在多种不同的解释,在一定程度上对这类矿床的资源勘查造成了困惑。因此,阐明镁铁-超镁铁侵入体中成矿物质大规模聚集的机理依然是矿床学界和勘查学界面临的重要科学前缘问题。马达加斯加中北部Alaotra湖地区的扎卡岩体是一个赋存有超大型钒钛磁铁矿床的典型镁铁质层状侵入体,已探明铁矿石远景资源量8亿t(含333),伴生钛6000万t,伴生钒230万t,为阐明钒钛磁铁矿床的成因提供了良好的条件。本文报道了扎卡岩体的地质学、矿体地质学、岩相学、矿相学和岩石地球化学特征,主要得出以下认识:(1)扎卡岩体发育韵律性火成层理,为一个典型的镁铁质层状岩体,但伟晶岩可构成独立的侵入单元;(2)矿体主要赋存在层状岩体中,可划分为整合型、不整合型和伟晶岩型等3种不同的类型;(3)辉长岩具有橄榄石→斜长石→单斜辉石或橄榄石→斜长石+单斜辉石的结晶顺序,暗示扎卡岩体为一浅成层状岩体,岩浆侵位深度约3 km;(4)矿石矿物与硅酸盐矿物呈热力学不平衡关系,且与含水暗色矿物平衡共生,暗示成矿作用发生在岩浆演化晚期的富含挥发分环境;(5)致密块状铁矿石的REE四分组效应也表明成矿过程伴随着富F、Cl等组分的流体活动。据此,本文提出扎卡岩体中钒钛磁铁矿床的形成与透岩浆流体过程有关。     

Special Xenoliths in an Aegirine–Augite Syenite Porphyry in Liuhe, Yunnan, China: Discovery and Implications

Three special types of xenoliths have recently been found in an aegirine–augite syenite porphyry in Liuhe, Yunnan, China. Petrographical, petrochemical, electron microprobe, and scanning electron microscopy studies indicate that pure calcite xenocrysts and quartz-bearing topaz pegmatite xenoliths result from the degassing of mantle fluids during their migration, and that black microcrystalline iron-rich silicate-melt xenoliths are the product of the extraction of mantle fluids accompanying degassing and are composed dominantly of quartz, chlorite, and iron-rich columnar and sheet silicate minerals with characteristic minerals, such as native iron, apatite, and zircon. According to the bulk-rock chemical and mineral compositions and crystallization states, the microcrystalline melt xenoliths are not the product of conventional magmatism, and especially the existence of native iron further proves that the xenoliths were mantle fluid materials under reduction or anoxic conditions. The study of the special xenoliths furnishes an important deep-process geochemical background of polymetallic mineralization in different rocks and strata in the study area.     

Special Xenoliths in an Aegirine-Augite Syenite Porphyry in Liuhe, Yunnan, China： Discovery and Implications

Three special types of xenoliths have recently been found in an aegirine-augite syenite porphyry in Liuhe,Yunnan,China.Petrographical,petrochemical,electron microprobe,and scanning electron microscopy studies indicate that pure calcite xenocrysts and quartz-bearing topaz pegmatite xenoliths result from the degassing of mantle fluids during their migration,and that black microcrystalline iron-rich silicate-melt xenoliths are the product of the extraction of mantle fluids accompanying degassing and are com...     

Sr and Nd isotope composition of deformed peridotite xenoliths from Udachnaya kimberlite pipe

New results of Rb–Sr and Sm–Nd isotope analyses have been obtained on samples of deformed peridotite xenoliths collected from the Udachnaya kimberlite pipe (Yakutia). The data obtained imply two main stages of metasomatic alteration of the lithospheric mantle base matter in the central part of the Siberian Craton. Elevated ratios of Sr isotopes may be considered as evidence of an ancient stage of metasomatic enrichment by a carbonatite melt. The acquired Nd isotope composition together with the geochemistry of the deformed peridotite xenoliths suggests that the second stage of metasomatic alteration took place shortly before formation of the kimberlite melt. The metasomatic agent of this stage had a silicate character and arrived from an asthenosphere source, common for the normal OIB type (PREMA) and the Group-I kimberlite.

     

The Genesis Mechanism of the Mantle Fluid Action and Evolution in the Ore‐Forming Process: A Case Study of the Laowangzhai Gold Deposit in Yunnan,China

Based on petrological studies of the wall rocks, mineralizing rocks, ores and veins from the Laowangzhai gold deposit, it is discovered that along with the development of silication, carbonation and sulfidation, a kind of black opaque ultra-microlite material runs through the spaces between grains, fissures and cleavages. Under observations of the electron microprobe, scanning electronic microscopy and energy spectrum, this kind of ultra-microlite material is confirmed to consist of ultra microcrystalline quartz, silicate, sulfides and carbonates, as well as rutile, scheelite and specularite (magnetite), showing characters of liquation by the analyses of SEM and energy spectrum. The coexistence of immiscibility and precipitating co-crystallization strongly suggests that the mineralizing fluid changed from the melt to the hydrothermal fluid. Combined with the element geochemical researches, it is realized that the ultra-microlite aggregate is the direct relics of the mantle fluid behaving like a melt and supercritical fluid, which goes along with the mantle-derived magma and will escape from the magma body at a proper time. During the alteration process, the nature of the mantle fluid changed and it is mixed with the crustal fluid, which are favorable for mineralization in the Loawangzhai gold deposit.     

内蒙古阿尔山——柴河地区陆下岩石圈地幔性质研究——第四纪碱性玄武岩中橄榄岩捕掳体中单斜辉石微量元素证据

阿尔山—柴河第四纪碱性玄武岩中地幔捕掳体为尖晶石相的二辉橄榄岩和方辉橄榄岩,方辉橄榄岩数量略多于二辉橄榄岩。采用激光剥蚀等离子体质谱(LA--ICP--MS)对研究区地幔橄榄岩中的单斜辉石和橄榄石等矿物进行了成分分析,结合橄榄岩包体的岩相学、岩石化学的特征,重点探讨了研究区所经历的部分熔融作用和地幔交代作用。结果显示,少数样品的熔融程度5%,大多数样品熔融程度范围为10%~20%,研究区陆下岩石圈地幔性质以难熔、亏损为主要特征。同时也经历了复杂的交代作用改造,交代介质为富挥发组分的硅酸盐熔/流体。与华北克拉通东北缘陆下岩石圈地幔比较,推测研究区遭受破坏和改造的程度较小,并保留有相当量的古老地幔残余。     

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel <101> direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 μm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles. Received: 16 November 1998 / Accepted: 17 May 1999     

Multi-stage metasomatism of diamondiferous eclogite xenoliths from the Udachnaya kimberlite pipe,Yakutia, Siberia

The primary garnet (pyrope-almandine)-omphacite (Cpx 1, 6.5–7 wt% Na₂O)-sulfide (Fe-Ni-Co mss) assemblage of the two diamondiferous eclogite xenoliths studied (U33/1 and UX/1) experienced two mantle metasomatic events. The metasomatic event I is recorded by the formation of platy phlogopite (~ 10 wt% K₂O), prior to incorporation of the xenoliths in the kimberlite. The bulk of the metasomatic alteration, consisting of spongy-textured clinopyroxene (Cpx 2A, 1–3 wt% Na₂O), coarser-grained clinopyroxene (Cpx 2B, 2–5 wt% Na₂O), pargasitic amphibole (~ 0.8 wt% K₂O; 3–3.5 wt% Na₂O), kelyphite (Cpx 3, mostly <1 wt% Na₂O; and zoned Mg-Fe-Al spinel), sodalite, calcite, K-feldspar, djerfisherite (K_5.95Na_0.02Fe_18.72Ni_2.36Co_0.01Cu_4.08S₂₆Cl ) and a small amount of K-Ca-Fe-Mg glass, is ascribed to the metasomatic event II that occurred also in the upper mantle, but after the xenoliths were incorporated in the kimberlite. A pervasive chloritic alteration (mainly clinochlore + magnetite) that overprints earlier assemblages probably took place in the upper crustal environment. The diamonds are invariably associated with secondary clinopyroxene and chlorite, but the diamonds formed before the entrainment of the xenoliths in the Udachnaya kimberlite.Editorial Responsibility: T.L. Grove     

Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes

A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjected to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxene - Cr-spinel increase at the expense of garnet and orthopyroxene with increasing degrees of metasomatism. The most metasomatised xenolith is a wehrlite. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but enriched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemical composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculations show that the modally metasomatised rocks became enriched in incompatible elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Major elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metasomatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poor environment. The data suggest that the xenoliths represent the mantle wallrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and incompatible element-enriched component of this melt percolated into the wallrock along a strong temperature gradient and caused the observed range of metasomatism.