铬铁矿成矿作用与研究进展

【研究目的】铬铁矿的形成与基性—超基性岩有关,对于铬铁矿的成因机制存在诸多争议,深化铬铁矿的成因研究对于实现铬铁矿找矿突破具有重要意义。【研究方法】通过对前人研究成果的梳理,总结了铬铁矿的成矿类型、成矿时代、矿体特征和成因研究进展。【研究结果】铬铁矿资源和产量高度集中,南非、哈萨克斯坦、芬兰和印度4国的铬铁矿储量占全球的95%以上,南非、哈萨克斯坦和土耳其等国主宰了世界铬铁矿的供应市场。铬铁矿类型主要有原生铬铁矿和次生铬铁矿两大类,原生铬铁矿主要是层状和豆荚状2种类型,次生铬铁矿主要是铬铁矿海滨（或海成）砂矿。层状铬铁矿通常形成于稳定克拉通内部的基性—超基性侵入体内,通常规模巨大。豆荚状铬铁矿与蛇绿岩密切共生,分布最为广泛,但矿床规模普遍较小,其产状、结构构造和共生矿物组合等存在较大差异。豆荚状铬铁矿主要形成于显生宙,少量形成于中—新元古代,层状铬铁矿主要形成于元古代,集中于古元古代。【结论】对于层状铬铁矿成因模式争议较小,主要有同化混染硅铝质围岩和岩浆混合两种模式;而豆荚状铬铁矿的成因模式较多,尚未达成共识,未来应重点聚焦铬的富集机制研究。     

对富铝型豆荚状铬铁矿矿床成因的新认识一一以新疆萨尔托海铬铁矿矿床为例

萨尔托海铬铁矿矿床产于萨尔托海蛇绿岩块的地幔橄榄岩中,属于富铝型豆荚状铬铁矿床。该矿床与一套橄长岩和辉长岩类岩石紧密伴生,矿体周围常被一薄的绿泥石壳所包裹。本区铬铁矿的形成包括两个阶段:第一阶段是原始地幔岩经高度熔融形成富铬铬铁矿;第二阶段形成富铝铬铁矿。交代作用伴随着新生单斜辉石和斜长石的形成。原始地幔岩的高度熔融以及基性熔体在地幔橄榄岩中的形成和存在是萨尔托海铬铁矿形成的先决条件。     

全球铬铁矿床成因类型、地质特征及时空分布规律初探

全球铬铁矿资源丰富,但分布极不均衡.铬铁矿的成因类型主要有层状和豆荚状2种,这2种类型的铬铁矿床地质特征、成因模式差异显著.在全球范围内铬铁矿矿床的成矿时代和空间分布具有明显的时空规律性,古元古代是铬铁矿最重要成矿期,该期形成的铬铁矿占总资源量的58.5％,以形成大型-超大型层状铬铁矿为主,中-新生代是全球铬铁矿床形成数量最多、分布范围最广的重要成矿期,该期形成的铬铁矿占总资源量的24.9％,以形成中小型豆荚状铬铁矿为主.铬铁矿在全球的分布可划分为5个重要层状铬铁矿矿田(南非布什维尔德-津巴布韦大岩墙、北美斯蒂尔沃特、南美坎坡福莫索、印度苏金达、芬兰凯米-俄罗斯普拉科夫斯科)和7个豆荚状铬铁矿带(津巴布韦舒鲁圭、东北非、乌拉尔、特提斯、西太平洋岛弧、加勒比岛弧和马达加斯加),并对各重要成矿区带的资源潜力进行了探讨.通过对全球铬铁矿成因类型、地质特征和时空分布规律的探讨,对深入了解地幔的物质组成、物理化学环境、地幔物质的运移、深部地质作用及板块运动的动力学机制、深俯冲和地球深部再循环轨迹有重要的理论意义.     

柴达木盆地北缘开屏沟纯橄岩中铬铁矿环带成因及其构造意义

开屏沟纯橄岩位于柴达木盆地北缘西段,岩石发生强烈的蛇纹石化,因此很难解析其性质和形成环境,本文通过对橄榄岩内发育的铬铁矿进行研究,探讨纯橄岩的性质及形成过程。岩相学观察和电子探针测试表明,铬铁矿具有明显核边结构,核部为铝铬铁矿,具有相对高Al_2O_3,低FeO~T、TiO_2、Cr~#特点,指示寄主原岩形成于俯冲带(SSZ)环境,寄主原岩为俯冲带橄揽岩。核部铝铬铁矿为岩浆型铬铁矿,通过计算得到其结晶温度平均为1372℃,结晶压力平均为2.96GPa,Δlgf_(O2)/sub平均为-1.42,表明其形成于地幔软流圈。边部为高铁铬铁矿,具有低Al_2O_3,高FeO~T、TiO_2、Cr~#特征,指示铬铁矿边部受到蛇纹石化蚀变与富Fe流体的共同作用。综上,推测开屏沟纯橄岩原岩形成于SSZ环境,后期受到流体交代改造发生蛇纹石化,最终暴露地表。     

芬兰科密铬铁矿床研究新进展

芬兰科密铬铁矿床是欧洲规模最大的铬铁矿床。对该矿床的地质背景、矿床特征、矿床成因及找矿标志进行了总结。结果表明,该矿床与古元古代层状超镁铁质杂岩体具有密切的时空分布关系,杂岩体内的铬铁矿床具典型的层状堆积特征,矿层延伸稳定。由于Cr/Fe值较低,该矿床矿石品位在世界同类矿床中属偏低水平。同位素年代学证据表明,其形成于古元古代早期(2.44 Ga),是由卡累利阿造山作用诱发多期次岩浆侵入活动并与新太古代基底岩层发生混染作用形成的。科密铬铁矿床属于层状铬铁矿床,其地质特征与中国目前已发现的豆荚状或似层状铬铁矿床存在一定区别。     

探讨变质作用对铬铁矿床的改造——古老岩层中铬铁矿找矿方向研究

提要：岗上超镁铁质岩主要由纯橄岩和石榴橄榄岩组成,主要组成矿物橄榄石、铬铁矿、石榴子石、单斜辉石和斜方辉石等。铬铁矿的Cr^#[Cr/(Cr+Mg)×100]从51到89变化,铬铁矿矿物表现为4期次演化的特点,反映了从岩浆期向榴辉岩相、角闪岩相和绿片岩相演化特征。随着超镁铁质岩的演化,铬铁矿中Cr^#不断增大,而铬铁矿Mg^#〔Mg×100/(Mg+Fe²⁺)〕不断减少,氧逸度不断增加。在绿片岩相－绿片角闪岩相退变质过程中,铬铁矿中Cr、Mg和Al减少,Fe相对增加,产生富Cr尖晶石变质作用样式。晚期剪切变形等次生变化有利于富铬铬铁矿矿物的形成和铬铁矿的富集。同时,绿片岩相变质作用降低了铬铁矿与其他硅酸盐矿物的结合强度,降低了开采强度和成本,使原本不易于开采的铬铁矿体变得可以开采。这些意味着该地区铬铁矿矿体展布要结合区域构造特征和变质作用进行研究、尤其是结合中晚期脆韧性构造进行分析。     

内蒙古贺根山蛇绿岩中铬铁矿特征及大地构造环境

为探讨内蒙古贺根山蛇绿岩形成的物化条件和大地构造环境,选取贺根山未蚀变豆荚状铬铁矿进行矿物学和矿物地球化学分析。结果表明,铬铁矿成分均一,不发育环带结构;铬铁矿中的Cr_2O_3含量为36.12%~43.77%,Al_2O_3为22.81%~30.14%,TFeO为12.63%~17.56%,Cr~#为46~56,Mg~#为68~78,Yfe为4~7,Fe~(2+#)为22~36,为富铝型铬铁矿,其寄主岩石属于蛇绿岩。铬铁矿结晶温度均为1395℃,结晶压力平均为3.3 GPa,推断其形成深度约为103 km;相对于FMQ缓冲剂的地幔氧逸度为FMQ+1.03~FMQ+1.83 lg单位,均值为FMQ+1.59 lg单位;地幔熔融程度F为20.63%~21.50%,均值为20.93%。推测贺根山铬铁矿原岩橄榄岩单元源区为石榴石二辉橄榄岩,形成于亏损地幔,可能产自俯冲带环境中的洋内弧后盆地环境。     

阿尔巴尼亚布尔齐泽壳-幔过渡带豆荚状铬铁矿成因及其对富Ti熔体交代作用的记录

豆荚状铬铁矿是关键金属铬的重要来源之一,尽管豆荚状铬铁矿的研究取得了诸多进展,但对于发育于蛇绿岩壳-幔过渡带的铬铁矿成因却涉及较少。阿尔巴尼亚布尔齐泽岩体壳-幔过渡带中产出的Cerruja豆荚状铬铁矿矿床,其矿体及纯橄岩围岩普遍被辉石岩脉穿切,辉石岩脉与矿体接触带以及辉石岩脉中的铬尖晶石强烈破碎,在铬尖晶石的裂隙和包裹体中发育大量富Ti矿物相,如金红石、钛铁矿和榍石等,是研究壳-幔过渡带铬铁矿成因的理想对象。Cerruja豆荚状铬铁矿及纯橄岩围岩中铬尖晶石Cr#分别为0.56~0.58和0.52~0.55,属于高铝型铬铁矿。接触带及辉石岩脉中的铬尖晶石Cr#明显升高（分别为0.57~0.67和0.72~0.83）,且Ti、V、Mn、Sc、Co、Zn和Ga含量也升高。本文依据铬尖晶石的结构及矿物化学成分变化特征,提出布尔齐泽壳-幔过渡带铬铁矿经历多阶段演化叠加：首先,Mirdita-Pindos洋盆在侏罗纪（约165 Ma）发生洋内初始俯冲,软流圈物质上涌生成的MORB-like弧前玄武质熔体随着俯冲的进行逐渐向玻安质熔体演变,期间产生的过渡型熔体与地幔橄榄岩反应生成高铝型铬铁矿;然后,部分MORB-like弧前玄武质熔体随着堆晶间隙分离结晶往富Fe和Ti的方向演化,改造早期形成的高铝型铬铁矿并结晶高铬型铬铁矿,同时生成金红石、钛铁矿和榍石等富Ti矿物相。     

太古宙绿岩带中铬铁矿床研究现状与展望

太古宙绿岩带中铬铁矿床不仅具有重要经济价值，而且是研究地球早期物质组成和地幔演化的重要对象。太古宙铬铁矿床形成时代久远，普遍经历了后期复杂的变质、变形和流体改造等一系列地质作用。相较于显生宙豆荚状铬铁矿和元古宙层状铬铁矿，对太古宙绿岩带内产出铬铁矿的研究还较为薄弱，尤其对铬铁矿体形成的地质背景和岩浆作用缺乏深入系统的研究。本文综述了格陵兰、印度、澳大利亚西部、加拿大以及津巴布韦等典型太古宙绿岩带中铬铁矿床的主要地质特征和研究现状，讨论了相关科学问题和研究展望。绿岩带中铬铁矿床主要产在古太古代-新太古代(3.3～2.7Ga)克拉通内的超基性岩石单元中，矿体常与蛇纹石化纯橄岩伴生。铬铁矿体产状复杂，既有层状和似层状，也有透镜状和不规则状，因此不能简单归于层状铬铁矿或豆荚状铬铁矿。铬铁矿岩以块状为主，其铬铁矿Cr^#相对较高(>60),属高铬型，Mg^#中等偏高(21～93)且与Cr^#呈负相关关系。研究认为，绿岩带中铬铁矿的成矿母岩浆来自深部亏损含水地幔分异所形成的科马提质岩浆，且受到了地壳物质混染。在开放的岩浆房内，含矿原...     

中国铬铁矿的铂族元素分布特征

用NiS溶解和Te沉淀方法富集铂族元素（PGE），制成镍扣，再溶解于浓HCl中，使PGE和Te一起沉淀。制备的样品溶液在ELAN－5000型电感耦合等离子质谱仪（ICP－MS）上分析PGE。中国铬铁矿矿石，包括蛇绿岩套中的豆荚状铬铁矿床、非层状侵入体铬铁矿，计13个矿床（化）样品，其PGE模式表明，主要呈RU正异常模式，个别不同模式是由母岩不同以及铂族元素矿化叠加引起的。铬铁矿的PGE模式不取决于铬铁矿的化学成分，而取决于其母岩性质以及形成温度和铂族元素的熔点。     

Thick chromitite of the Jacurici Complex (NE Craton São Francisco,Brazil): Cumulate chromite slurry in a conduit

The Jacurici Complex, located in the NE part of the São Francisco Craton, hosts the largest chromite deposit in Brazil. The mineralized intrusion is considered to be a single N-S elongated layered body, disrupted into many segments by subsequent deformation. The ore is hosted in a thick, massive layer. Two segments, Ipueira and Medrado, have been previously studied. We provide new geological information, and chromite composition results from the Monte Alegre Sul and Várzea do Macaco segments located farther north, and integrate these with previous results. The aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer. All segments exhibit similar stratigraphic successions with an ultramafic zone (250 m thick) hosting a 5–8 m thick main chromitite layer (MCL), and a mafic zone (40 m thick). The chromite composition of the MCL, Mg-numbers (0.48–0.72) and Cr-numbers (0.59–0.68), is similar to chromites from layered intrusions and other thick chromitites. Previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition (up to Fo₉₃) and orthopyroxene composition (up to En₉₄) from harzburgite samples, and that it originated from an old subcontinental lithospheric mantle. We estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite, or low siliceous high-Mg basalt, with a higher Fe/Mg ratio. The petrologic evidence from the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit. Crustal contamination has previously been considered as the trigger for the chromite crystallization, based on isotope studies, as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole (up to 20%). The abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization. Fluids may have played an important role in the chromite formation and/or accumulation. However, the trigger for chromite crystallization remains debatable. The anomalous thickness of the chromitite is a difficult feature to explain. We suggest a combined model where chromite crystallized along the margins of the magma conduit, producing a semi-consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed. Subsequently, the conduit was obstructed and the resident magma fractionated to produce a more evolved composition.     

新疆北部幔源岩浆矿床的类型、时空分布及成矿谱系

新疆北部与幔源岩浆有关的矿床种类齐全,成矿环境复杂,时代和类型繁多,在中国乃至世界颇具特色。主要矿床类型包括铬铁矿矿床、钒钛磁铁矿矿床、铜镍硫化物矿床、铂族元素(PGE)矿床、铜镍-钒钛铁复合型矿床、含钴磁铁矿矿床、玄武岩自然铜矿床、热液型钴-多金属矿床,以及非金属矿床等。按照含矿地质体的类型,可分为6种类型:蛇绿岩型、层状杂岩型、小侵入体型、阿拉斯加型、浅成岩型和喷出岩型。这些幔源岩浆矿床可划分为3个成岩成矿系列:铜镍系列、钛铁系列和铬铁系列。钛铁系列以碱性层状岩体型钒钛磁铁矿、铁磷矿为代表,岩石具有明显的富Fe特征,属于碱性富铁质的高钛玄武岩系列;铜镍系列以小侵入体型铜镍矿、阿拉斯加型铜镍-PGE矿为代表,岩石属于铁质的拉斑玄武岩-钙碱性系列;铬铁系列主要为蛇绿岩型铬铁矿,岩石具富Mg贫Fe特征,属于镁质系列。3个系列的岩浆都具有亏损地幔源特征,可能都与地幔柱活动有关;岩浆源区富含相应的成矿元素,是形成3个系列矿床相应成矿地质体的主要条件。3个系列矿床的成矿机制可分为深部熔离/岩浆分异、就地分凝、矿浆贯入、岩浆热液等过程。根据各系列矿床之间存在的紧密联系,建立了与幔源岩浆作用有关的3个系列矿床综合模式: 亏损地幔部分熔融产生的幔源岩浆在上升过程中发生熔离/分异,分离为3个系列,由于外部物质加入在地壳深部发生分异和熔离,在不同深度富集形成铬铁矿、钒钛磁铁矿和铜镍硫化物矿床,临近地表时流体富集和分离成含矿流体,分别形成浅成岩型磁铁矿和喷出岩型自然铜矿。新疆北部各类幔源岩浆矿床从早到晚主要产于3期构造阶段/构造类型: 大陆裂解期、板块俯冲期、碰撞/后碰撞造山期(又分3个阶段: 碰撞后伸展阶段、幔柱叠加造山阶段、后碰撞结束阶段)。     

Crystallization of Podiform Chromitites from Silicate Magmas and the Formation of Nodular Textures

Abstract: Podiform chromite deposits consist of numerous individual accumulations of chromite in the mantle sequences of ophiolites, suggesting formation in separate, mini-magma conduits in the upper mantle. They may show unique nodular and orbicular textures. Simple mixing of two distinct magmas, invoked for chromite deposits in layered intrusions, is inadequate to explain the formation of podiform chromite deposits. More likely, melt/rock interaction triggers the precipitation of chromite by addition of newly-formed droplets of melt to the main body of magma passing through a conduit, a process similar to that of magma mingling but involving a turbulent, moving magma so that newly-formed melt droplets behave like snowballs. These droplets concentrate chromite to form an outer shell and, while the magma is moving upwards, less dense silicate melts are squeezed out of the droplets as the shell collapses to form a nodule. Upon cooling, both orbicular and nodular textures are preserved in the chromitite.     

Podiform Chromite Ore Bodies: a Genetic Model

This paper presents an attempt to interpret the formation ofpodiform chromite ore found in ophiolitic complexes from magmaticaccumulation inside cavities along the basalt conduits feedingthe main magma chamber. Two processes play a major role in chromite concentration: anactive upward magma flow in narrow dykes; and an active convectioninside the cavity, for which a numerical modelling has beenperformed. The physical conditions imposed by the model compare well withgeophysical and geological data.     

富铝型豆荚状铬铁矿床的成矿模式

富铝型豆荚状铬铁矿床系指产于ＰＴＧ系列蛇绿岩套地幔橄榄岩中的矿石，以富铝（Ａｌ２Ｏ３＞２０％）、低铬（Ｃｒ２Ｏ３＜４５％）为特征的铬铁矿床。该类型矿床以萨尔托海、贺根山及洪古勒楞铬铁矿床为代表。含富铝型铬铁矿床的岩体以伴有丰富的基性分凝体、含长地幔橄榄岩的出现以及矿石四周发育绿泥石薄壳而区别于含富铬型铬铁矿床的岩体。作者的研究表明富铝型铬铁矿床是原始地幔岩不同程度部分熔融再造的产物。富铬矿浆代表高度部分熔融的最终残余。富铝型铬铁矿床的成矿模式有两种：一为原始地幔岩中低度部分熔融再造的产物，以缺失高熔杂岩带为特征，属中低熔再造成矿模式，以洪古勒楞矿床为代表；另一种为原始地幔岩高度熔融再造的产物，以发育高熔杂岩带为特征，基性熔体与富铬矿浆之间曾发生了再平衡作用，属高熔再造－再平衡成矿模式，以萨尔托海及贺根山矿床为代表。     

层状侵入体及其韵律层成因

层状侵入体以其独特的韵律结构记录了岩浆的演化分异过程。围绕韵律层理的成因 ,不同学者提出了很多成层机制 ,归纳起来大体有两类 :一类与岩浆流动和动力成层有关 ,包括流动分异、对流成层、化学成因等 ;另一类为非动力成层机制 ,主要有火成堆积、重力分异、压实作用和固化收缩等观点。影响韵律形成的因素很多 ,通常包括岩浆成分、对流作用、扩散作用、热梯度、氧逸度、温度和压力等。在评述这些成层机制和影响因素的基础上 ,结合攀枝花层状侵入体的特点 ,认为该岩体的韵律结构是在岩浆的固化收缩和压实作用下 ,晶体定向生长和自组织排列的结果 ,是二元 (辉石 ,斜长石 )、固 (岩石 )—液 (岩浆 )相转变系统中动量、能量和物种质量的复杂耦合所形成的分形自组织现象。对层状侵入体的进一步研究应集中在加强实验 (尤其是高温高压实验 )、引入非线性分析以及与成矿研究相结合等方面。     

Experimental study of interaction between fluid-bearing basaltic melts and peridotite: A mantle-crustal source of trap magmas in the Norilsk area

The paper reports data on the geology and tectono-magmatic reactivation of the Norilsk area and on the stratigraphy and geochemistry of its volcanic sequence, with the discussion of the sources and genesis of the ore magmas and the scale of the ore-forming process. According to the geochemistry of the lavas and intrusive rocks (Ti concentration and the La/Sm and Gd/Yb ratios), two types of the parental magmas are recognized: high-Ti magmas of the OIB type (from bottom to top, suites iv, sv, and gd of phase 1) and low-Ti magmas (suites hk, tk, and nd of phase 2 and suites mr-mk of phase 3), which were derived from the lithospheric mantle. The magmatic differentiation of the parental low-Ti magma of the tk type into a magma of the nd type was associated with the derivation of an evolved magma of the nd type, which was depleted in ore elements, and an ore magma, which was a mixture of silicate and sulfide melts, protocrysts of silicate minerals, and chromite. Judging from their geochemical parameters, the intrusions of the lower Norilsk type were comagmatic with the lavas of the upper part of the nd suite, and the ore-bearing intrusions of the upper Norilsk type were comagmatic with the lavas of the mr-mk suites. When the ore-bearing intrusions were emplaced, their magmas entrained droplets of sulfide melt and protocrysts of olivine and chromite and brought them to the modern magmatic chamber. These protocrysts are xenogenic with respect to the magma that formed the intrusions. In certain instances (Talnakh and Kharaelakh intrusions), the moving magma entrained single portions of sulfide magma, which were emplaced as individual subphases. The experimental study of the peridotite-basalt-fluid system shows that mantle reservoirs with protoliths of subducted oceanic crustal material could serve as sources of relatively low-temperature (1250–1350°C) high-Ti magnesian magmas of the rifting stage from an olivine-free source.     

Pressure fluctuations and the formation of the PGE-rich Merensky and chromitite reefs, Bushveld Complex

The Merensky Reef and the underlying Upper Group 2 chromitite layer, in the Critical Zone of the Bushveld Complex, host much of the world’s platinum-group element (PGE) mineralization. The genesis is still debated. A number of features of the Merensky Reef are not consistent with the hypotheses involving mixing of magmas. Uniform mixing between two magmas over an area of 150 by 300 km and a thickness of 3–30 km seems implausible. The Merensky Reef occurs at the interval where Main Zone magma is added, but the relative proportions of the PGE in the Merensky Reef are comparable to those of the Critical Zone magma. Mineral and isotopic evidence in certain profiles through the Merensky Unit suggest either mixing of minerals, not magmas, and in one case, the lack of any chemical evidence for the presence of the second magma. The absence of cumulus sulphides immediately above the Merensky Reef is not predicted by this model. An alternative model is proposed here that depends upon pressure changes, not chemical processes, to produce the mineralization in chromite-rich and sulphide-rich reefs. Magma was added at these levels, but did not mix. This addition caused a temporary increase in the pressure in the extant Critical Zone magma. Immiscible sulphide liquid and/or chromite formed. Sinking sulphide liquid and/or chromite scavenged PGE (as clusters, nanoparticles or platinum-group minerals) from the magma and accumulated at the floor. Rupturing of the roof resulted in a pressure decrease and a return to sulphur-undersaturation of the magma.     

川滇地区铬铁矿找矿前景的一些分析

我国铬铁矿资源奇缺.长期以来我国冶炼合金钢所用的铬铁矿,基本上都是依赖于国外进口.长此下去将严重地影响我国钢铁工业的进一步发展.我国川滇地区,基性和超基性岩广布,具一定的产出铬铁矿的地质条件,而前人作一些工作后多持否定意见.本文作者试图用最新的成矿观点和最新的研究成果,对川滇地区的找铬前景进行一些讨论.铬铁矿的产出只与基性、超基性岩有关.目前国际上对于基性、超基性岩的分类方案繁多.但无论何种方案,都认为与铬铁矿有关的超基性岩主要是其中的两类,即人们所熟悉的