贺根山豆荚状铬铁矿中硅酸盐包体及其地质意义

贺根山豆荚状铬铁矿是典型的高Al型铬铁矿(Cr#=47.8～54.9,Mg#=64.1～73.7),其中的包体以硅酸盐矿物为主(包括橄榄石、斜方辉石、单斜辉石、韭闪石、钠长石).根据包体形状、矿物组合及分布特征可将其划分为三类.第一类包体呈孤立单矿物相,主要包括橄榄石和单斜辉石,第二类包体由平衡共生的单斜辉石和斜方辉石构成,上述两类包体均具有被熔蚀的边,且零星分布在尖晶石中,属于捕虏晶成因.第三类包体属于熔融包体,具有多边形外形,包含复杂的矿物相并密集分布于尖晶石核部.利用尖晶石颗粒内部保存完好的单斜辉石以及单斜辉石和斜方辉石包体估算的温度(1148～1254℃)与压力(490～1290 MPa)表明,贺根山豆荚状铬铁矿矿床的形成深度为16～43 km.熔融包体中含大量钠长石和韭闪石,指示铬铁矿母熔体富集H2 O、Na和Si.与铬铁矿平衡母熔体的Al2 O3含量(15.4％～16.3％)、TiO2含量(0.3％～0.9％)和FeO/MgO比值(0.6～1.1)与低Ti拉斑玄武质熔体的类似.利用尖晶石和橄榄石包体计算获得铬铁矿原始熔体的Mg O含量为～19.8％.贺根山豆荚状铬铁矿经历了深部预富集和浅部成矿两个阶段,其中浅部成矿作用涉及熔体与方辉橄榄岩反应以及演化的熔体与原始熔体混合等过程.     

蛇绿岩地幔岩中自由SiO2的发现及其地质意义

自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。     

Mineral Inclusions in Chromite from the Chromite Deposit in the Kudi Ophiolite, Tibet, Proto-Tethys

High-Al chromite from the Kudi chromitites contains a wide range of mineral inclusions. They include clinopyroxene, amphibole, phlogopite, olivine, orthopyroxene, apatite, base-metal sulfides, calcite and brucite. The modal abundance of inclusions vary greatly among different grains of chromite. The common inclusions are clinopyroxene and amphibole, which occur as monomineral or polymineral associated with other minerals. The shapes of these inclusions tend to follow the growth plane of host chromite. Mineral assemblages and textures demonstrate that some inclusions(olivine, clinopyroxene) are trapped during magmatic stage, and most of the inclusions(e.g., amphibole, phlogopite) are trapped during recrystallization of chromite. Sulfide inclusions are pentlandite, chalcopyrite and cubanite. They occur either as isolated grains or together with silicate minerals, and formed from the separation of sulfide-bearing liquid from silicate magma. The parental magma of chromitites contains Al_2O_3 15.0wt%–16.5wt%, TiO_20.30wt%–1.05wt% based on calculation with the composition of chromite, similar to parental magma of high-Al chromitites from elsewhere and the estimated melt composition is comparable with that of MORB. Considering the high-Mg olivine in disseminated chromitite and abundant hydrous inclusions, we propose that Kudi chromitites formed beneath a volcanic front during the subduction initiation of Proto-Tethys.     

Alteration of Platinum-Group and Base-Metal Mineral Assemblages in Ophiolite Chromitites from the Dobromirtsi Massif,Rhodope Mountains (Bulgaria)

The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS₂-erlichmanite OsS₂, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)₉S₈, followed by heazlewoodite (Ni₃S₂), breithauptite (NiSb), maucherite (Ni₁₁As₈), godlevskite (Ni₇S₆), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni_5-XAs₂), awaruite (Ni₃Fe) and chalcopyrite (CuFeS₂). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS₂). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO₂ states, fS₂ also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe₂O₃) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn.     

来自蛇绿岩地幔的硫（砷）化物矿物组合

近来在西藏雅鲁藏布江蛇绿岩带的罗布莎蛇绿岩块的地幔豆荚状铬铁矿中发现一个包括金刚石、柯石英、自然元素、合金、氧化物以及硫(砷)化物组成的地幔矿物群。该矿物群的硫(砷)化物具有特殊化学成分并呈包裹体分布在贱金属(BM)和铂族元素(PGE)或它们的合金中,大量化学成分分析得知它们主要由下列元素组成:S、As、Te、Fe、Ni、Co、Cu、Pt、Pd、Ru、Rh、Os、Ir、Mn和Ti。根据化学成分可辨别出约30种硫(砷)化物矿物:FeS、NiS、(Ni,Fe)S、Fe3S2、Ni3S2、(Ru,Os,Ir)S2、Rh7As3、Rh5Ni(Cu)As4、Pd4Rh3As3、Pd8As2、Pd3TeAs、Pd7Te3、RuAs、PtAs2、Ni4Rh3As3、Rh(As,S)2、(Rh,Ir)(As,S)2、Ir(As,S)2、MnS、Ti7S3、Ti7N3、Rh3.5Se3.5CuS2、RhS、Ir2S3、(Ir,Cu)2、S3(Co,Ni,Fe)2(As,S)3、(Ir,Pt)(As,S)2、Ru3(As,S)7以及(BM)x(PGE)yS10-(x y)等,其中包括已定名和未定名的矿物。由于矿物粒度小(<25μm),缺乏X射线分析资料,有待进一步研究。     

西藏雅鲁藏布江缝合带东段泽当豆荚状铬铁矿特征

泽当蛇绿岩位于雅鲁藏布江缝合带东段,岩体由地幔橄榄岩、辉长辉绿岩、辉石岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和二辉橄榄岩。在方辉橄榄岩中发现7处豆荚状铬铁矿,矿石类型主要有致密块状和浸染状。出露地表的长度0.5~3m,厚0.2~1m。矿体的延伸方向为北西向,与岩体展布的方向一致,铬铁矿的Cr~#=67.9~88.5,属于高铬型铬铁矿。泽当地幔橄榄岩岩相学特征以及矿物组合、矿物化学成份及岩相学特征,显示岩体至少存在两次的部分熔融过程,即为早期的MOR构造背景,以及后期SSZ环境的改造。铬铁矿的铂族元素(PGE)以富集Os、Rh、Pd,亏损Ir、Ru、Pt的负斜率分布模式,表明其形成过程中经受后期熔体/流体的改造。对比罗布莎岩体的矿物组合,矿物化学和地球化学等特征,显示泽当豆荚状铬铁矿矿体与典型高铬型具相似性,存在较大的找矿空间。     

西藏罗布莎豆荚状铬铁矿床深部找矿突破与成因模式讨论

中国铬铁矿资源的瓶颈状态已持续多年。最近,在西藏罗布莎蛇绿岩地幔橄榄岩的深部勘探发现200万t致密块状铬铁矿床,这是中国近50年来铬铁矿找矿的重大突破,对今后继续寻找同类型的铬铁矿床具有重要指导意义。蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿床是工业需求铬的重要来源。研究豆荚状铬铁矿的成矿作用和矿体围岩地幔橄榄岩地质特征,建立铬铁矿的成矿模型和找矿标志,是开展寻找同类型矿床的重要保证。随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中金刚石等深部矿物的不断发现和深入研究,人们对蛇绿岩型铬铁矿的物质来源和形成过程,有了新的认识,提出了铬铁矿的深部成因模式。研究认为深部成因铬铁矿床主要经历了4个阶段:(1)早期俯冲到地幔过渡带(410~660 km)的陆壳和洋壳物质被脱水和肢解,过渡带产生的热和流体促成了地幔的熔融和Cr的释放和汇聚;(2)铬铁矿浆在地幔柱驱动下,运移到过渡带顶部冷凝固结,并有强还原的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,进入"塑性—半塑性地幔橄榄岩"中;(3)随着物质向上移动,深度降低,早期超高压相矿物发生相变,如斯石英转变成柯石英,高压相的铬铁矿中出溶成柯石英和单斜辉石;(4)在侵位过程和俯冲带环境下,含水熔体与方辉橄榄岩反应形成了不含超高压矿物的规模相对较小的浸染状铬铁矿及纯橄岩岩壳。进一步研究表明,同处雅鲁藏布江缝合带西段的几个大型地幔橄榄岩岩体与罗布莎岩体可以对比,经历了相同的构造背景和豆荚状铬铁矿的成矿作用,存在较大的找矿空间。     

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。     

Textural Evidence for the Chromite‐Oversaturated Character of the Melt Involved in Podiform Chromitite Formation

Well‐preserved oval‐shaped dunite clots occur within the exceptionally fresh massive podiform chromitites from the Coto Block of the Zambales Ophiolite Complex, the Philippines. The dunite/chromitite boundary shows an interlocking texture; olivine inclusions in chromites in the podiform chromitites show the same optical extinction with larger adjacent olivines in the dunite clots. This texture was formed by the reaction between chromite‐oversaturated melt and its dunite inclusions. The existence of such type of melt was previously only hypothesized to explain the origin of layered and podiform chromitites but is now confirmed by this discovery.     

对豆荚状铬铁矿床成因的认识

文章提出豆荚状铬铁矿床不是上地幔局部溶融后的残余物，而是亏损地幔又经部分溶融形成的溶体发生液相不混溶，导致硅酸盐岩浆和富铬矿浆的形成。当比重不同高度分馏的溶体在上升过程发生了分离，轻熔体上升侵入于岩浆房、重熔体和铬铁矿浆，就地堆积或贯入围岩形成豆荚状铬铁矿床。豆荚状铬铁矿的伴生围岩纯橄岩，不是高度亏损的上地幔残余，可能是与铬铁矿浆伴随的一种硅酸盐熔体 。与大量纯橄岩脉伴随的基性岩、超基性岩脉，也属