Podiform chromitite formation in a low-Cr/high-Al system: An example from the Southwest Indian Ridge (SWIR)

Recent reassessment of abyssal peridotites obtained during the dredging of the oblique supersegment and the easternmost subsection of the Southwest Indian Ridge by the R/V Knorr Cruise 162 and the R/V Yokosuka YK98-07 revealed the occurrence of dunites containing podiform chromitites and dunites with variable chromite concentration closely associated with lherzolite and harzburgite. The size of the chromitite pods varies from a few mm to 2 cm in width. Chromites in the podifom chromitites have very low Cr# (=0.22–0.23) and low TiO₂ (<0.17 wt%). They are almost free of silicate inclusions except for a few euhedral sulfide grains which occur far from cracks and lamellae and are considered primary in origin. The lherzolite which possibly represents the wallrock hosting the dunites with podiform chromitites also show low spinel Cr#(=0.16) and low Cr# in the clinopyroxenes (=0.09–0.10) and orthopyroxenes (=0.07–0.09). The small size of the SWIR podiform chromitites is strongly controlled by the low Cr/Al available in the wallrock and the invading melt. The presence of sulfide inclusions and the absence of PGEs further attest to the low Cr/Al (i.e. low refractoriness) in the system involved in the genesis of the SWIR podiform chromitites. Lastly, the discovery of podiform chromitites in the SWIR implies that the formation of podiform chromitite at mid-oceanic ridges, regardless of its spreading rate, is highly possible.     

Petrological characteristics of podiform chromitites and associated peridotites of the Pan African Proterozoic ophiolite complexes of Egypt

Mafic-ultramafic fragments of a dismembered ophiolite complex are abundant in the late Precambrian Pan African belt of the Eastern Desert of Egypt and north-east Sudan. The ultramafic bodies in the Eastern Desert of Egypt are mostly characterised by the harzburgite–dunite–chromitite association. Because of their severe metamorphism, almost all primary silicates were converted to secondary minerals and we use the chrome spinel as a reliable petrogenetic indicator. The podiform chromitite deposits are common as small and irregularly shaped masses in the central and southern parts of the Eastern Desert. They strongly vary in texture, degree of alteration and chemical composition of chrome spinel. The podiform chromitites exhibit a wide range of composition from high Cr to high Al varieties. The Cr of chrome spinel ranges from 0.65 to 0.85 in dunite, quite similar in the high-Cr chromitite, whereas it is around 0.5 in harzburgite. Primary hydrous mineral inclusions, amphibole and phlogopite, in chrome spinel are reported for the first time from the Pan African Proterozoic podiform chromitites. The petrological characteristics of Pan African podiform chromitites and associated peridotites of Egypt are similar to those of Phanerozoic ophiolites. The Proterozoic podiform chromitites may have formed in the same way as the Phanerozoic ones, namely by melt-harzburgite reaction and subsequent melt mixing. The similarity of the mantle section of the late Proterozoic and the Phanerozoic ophiolites suggests that the thermal conditions controlling genesis of the crust–mantle system basically have not changed since the late Proterozoic era. The Pan African harzburgite is very similar to abyssal peridotite at fast-spreading ridges, and the high-Cr, low-Ti character of spinel in chromitite and dunite indicates a genetic link with a supra-subduction zone setting. The late Proterozoic ophiolites of Egypt are possibly a fragment of oceanic lithosphere modified by arc-related magmatic rocks, or a fragment of back-arc basin lithosphere. Received: 26 October 1999 / Accepted: 28 June 2000     

豆荚状铬铁矿:古大洋岩石圈残片的重要证据

豆荚状铬铁矿为蛇绿岩的特征性矿产 ,保留了上地幔岩浆构造作用、高温变形以及岩石成因的重要信息。它们常见于方辉橄榄岩内 ,位于大洋岩石圈莫霍面下 1～ 2km的古深度范围内。豆荚状铬铁矿常被纯橄岩薄壳围限 ,保留特征的豆状、豆壳状等构造。豆荚状铬铁矿的TiO2 含量较低 ,铂族元素 (PGE)的分布模式显示特征的负斜率。普遍认为 ,豆荚状铬铁矿形成于部分熔融条件下 ,涉及原始地幔熔体与亏损地幔橄榄岩的相互作用 ,伴随复杂的岩浆混合及结晶过程。狭窄的上地幔岩浆通道或孔穴为豆荚状铬铁矿理想的堆积部位。超俯冲带 (弧后盆地、岛弧、弧前 )、大洋中脊、转换断层均可能是豆荚状铬铁矿形成的理想环境。其中 ,洋脊扩张模式及大洋上俯冲带模式较好地解释了豆荚状铬铁矿成因。对于经历高级变质及多期变形的华北大陆基底 ,豆荚状铬铁矿是研究古老蛇绿岩最直接而有效的地质标志 ,对于研究古大洋岩石圈增生过程 ,上地幔演化 ,探索早期板块构造意义重大。     

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。     

Diversity of platinum-group mineral assemblages in banded and podiform chromitite from the Kraubath ultramafic massif,Austria: evidence for an ophiolitic transition zone?

We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-Pd-Bi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh)₂S₄, braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.     

Upper crustal podiform chromitite from the northern Oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for Cr concentration

A new type of podiform chromitite was found at Wadi Hilti in the northern Oman ophiolite. It is within a late-intrusive dunite body, possibly derived from olivine-rich crystal mush, between the sheeted dike complex and upper gabbro. This chromitite forms small (<30 cm in thickness) pods with irregular to lenticular shapes. Neither layering nor graded bedding is observed within the pods. The chromitite is in the upper crust, by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the Moho transition zone to the upper mantle. It is distributed along a small felsic to gabbroic melt pool within the dunite body, which was formed by melting of gabbroic blocks captured by the mush. Chromian spinel was precipitated due to mixing of two kinds of melt, a basaltic interstitial melt from the mush and an evolved, possibly felsic, melt formed by the melting of gabbro blocks. The podiform chromitite reported here is strikingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions. The former contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions. Chromian spinel of the upper crustal podiform chromitite from Oman has relatively low content of (Cr₂O₃ + Al₂O₃), the Cr/(Cr + Al) atomic ratio of around 0.6, and the relatively high TiO₂ content ranging from 1 to 3 wt%. We conclude that assimilation of relatively Si-rich materials (crustal rocks or mantle orthopyroxene) by olivine-spinel saturated melts can explain the genesis of any type of chromitite.Editorial responsibility: V. Trommsdorff     

Palladium and gold mineralization in podiform chromitite at Kraubath, Austria

Summary ?We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGE-sulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly laurite-erlichmanite) and alloys, are typical only of the unaltered podiform chromitite (assemblage I). Euhedral mono- and polyphase PGM grains in the submicron to 100 μm range show features of primary magmatic assemblages. The diversity of PGM in these assemblages is unusual for ophiolitic environments. In assemblage II, laurite-erlichmanite is intergrown with and overgrown by PGE-sulpharsenides; other minerals of assemblage I are missing. Potarite, stibiopalladinite, mertieite II, native gold and Ag-Au alloys, as well as PGE-sulpharsenides, sperrylite and base metal arsenides and sulphides are characteristic for the highly altered chromitite (assemblage III). They occur either interstitial to chromite in association with metamorphic silicates, in chromite rims or along cracks, and are thus interpreted as having formed by remobilization of PGE by hydrothermal processes during polyphase regional metamorphism. Received August 3, 2000;/revised version accepted December 28, 2000     

大洋地幔橄榄岩-铬铁矿中的金刚石和深地幔再循环

全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成（δ13C-18‰~-28‰）,并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。      

豆荚状铬铁矿铂族元素地球化学研究进展

豆荚状铬铁矿是蛇绿岩中特有的一类矿产，按其化学成分可分为高Cr型和高Al型两种。其中的PGE主要以RuS2和Os、Ir、Ru合金等包体形式存在，或以类质同像形式进入铬铁矿晶格。两种类型的铬铁矿均表现出负倾斜型PGE配分模式，其Pt、Pd含量相近；与高Al型铬铁矿相比，高Cr型铬铁矿有更高的Os、Ir、Ru含量，部分豆荚状铬铁矿表现出Pt、Pd相对富集的平坦到正倾斜型PGE配分模式。目前对豆荚状铬铁矿PGE含量及配分模式还缺少一个统一的解释，但其PGE地球化学可为豆荚状铬铁矿的成因及构造背景解释提供更多的信息。     

豆荚状铬铁矿床研究回顾与展望

豆荚状铬铁矿是蛇绿岩的特征性矿产,对其成因的认识还存在较大的分歧,包括:（1）早期岩浆熔离;（2）地幔熔融残余;（3）熔体-岩石反应.豆荚状铬铁矿及其围岩地幔橄榄岩中大量异常地幔矿物群的发现,引起了地质学家对其形成过程的重新思考.回顾了铬铁矿的研究,借助pMELTS热力学软件模拟浅部地幔过程,使用定量化的方法限定这些过程对豆荚状铬铁矿形成的贡献,通过一个新的角度讨论其形成.初步模拟结果显示,单独的地幔部分熔融、熔体分离结晶以及拉斑质熔体与亏损地幔的反应等过程形成的铬铁矿,无论在数量还是品位上都难以达到矿床水平,暗示豆荚状铬铁矿的形成可能为多种作用耦合的结果,或与深部地幔作用有关.      

西藏雅鲁藏布江缝合带西段发现高铬型和高铝型豆荚状铬铁矿体

豆英状铬铁矿按其矿物化学组分分为高铝型(Cr#值为20～ 60)和高铬型(Cr#值为60～80)两类(Thayer,1970),在全球已报道的豆英状铬铁矿中普遍为在一岩体内只存一种类型的矿体,而在同一岩体内发现两种类型的铬铁矿体较少见.位于雅鲁藏布江缝合带西段普兰岩体中首次发现同时存在高铬型和高铝型铬铁矿,岩体由地幔橄榄岩、辉长辉绿岩、火山岩等组成.地幔橄榄岩主要为方辉橄榄岩、纯橄岩和少量二辉橄榄岩.在方辉橄榄岩中发现7处透镜状的铬铁矿矿体露头,矿石类型主要有致密块状、稠密浸染状和稀疏浸染状等.矿体长2～6m,厚0.5～2m,矿体的最大延伸方向为北西-南东向,与岩体的展布方向一致,矿石的Cr#=52～88,高铬型铬铁矿包括Cr-2～5矿体,Cr#值为63～89,高铝型铬铁矿有Cr-1和Cr-6矿体,Cr#=52 ～55.矿石中脉石矿物主要为橄榄石、角闪石、蛇纹石等.普兰地幔橄榄岩的矿物结构显示,岩体经历了强烈的部分熔融以及塑性变形作用,地幔橄榄岩的地球化学特征显示岩体形成于MOR,后受到SSZ环境的改造.并且依据铬尖晶石-橄榄石/单斜辉石的矿物化学成分,识别出普兰地幔橄榄岩至少经历了3次不同的部分熔融,包括早期部分熔融(～10％)、晚期部分熔融(20％～30％)和局部的减压部分熔融作用(～15％).对比其他铬铁矿矿体和地幔橄榄岩的矿物组合,矿物化学和地球化学等,显示普兰豆荚状铬铁矿矿体与典型高铬型、高铝型铬铁矿具相似性,并存在较大的找矿空间.     

雅鲁藏布江缝合带西段达机翁地幔橄榄岩及铬铁矿的铂族元素特征

达机翁蛇绿岩位于雅鲁藏布江缝合带的西段北亚带,该蛇绿岩主要由地幔橄榄岩、玄武岩以及硅质岩组成,其中地幔橄榄岩以方辉橄榄岩为主,同时含有少量的纯橄榄岩,纯橄岩主要呈不规则透镜状或团块状分布于方辉橄榄岩中。在达机翁地幔橄榄岩中产出有3种不同类型的铬铁矿,分别为块状,豆状以及浸染状铬铁矿。文章主要对达机翁地幔橄榄岩的方辉橄榄岩及豆荚状铬铁矿进行了研究,结合岩石的主量元素和铂族元素,对地幔橄榄岩和豆荚状铬铁矿的成因以及雅鲁藏布江缝合带的找矿远景进行了探讨。达机翁地幔橄榄岩具有较高的Mg O含量以及较低的Al2O3和Ca O等含量,这种亏损的全岩成分指示了达机翁地幔橄榄岩经历了较高的部分熔融作用,同时方辉橄榄岩的PGEs的总量为23.68×10-9~31.02×10-9,高于原始地幔的值,Pd和Cu 2个元素的含量较为分散明显偏离部分熔融曲线,指示了达机翁方辉橄榄岩可能遭受了熔体的改造,在熔体-岩石反应的过程中,导致了富含PPGE的硫化物的加入。达机翁豆荚状铬铁矿为高Cr型铬铁矿,具有IPGE和Rh明显富集以及Pt,Pd明显亏损的特征,不同类型的铬铁矿之间具有一致的PGEs的分配模式。雅鲁藏布江缝合带内大量分布的超镁铁岩体在岩石组合、地球化学特征、成因以及形成时代等方面,均具有相似性,是中国铬铁矿找矿的有利远景区。     

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

In recent years diamonds and other unusual minerals(carbides,nitrides,metal alloys and native elements) have been recovered from mantle peridotites and chromitites(both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings.Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite,west Albania.So far,more than 20 grains of microdiamonds and 30 grains of moissanites(SiC) have been separated from the podiform chromitite.The diamonds are mostly light yellow,transparent,euhedral crystals,200~300 μm across,with a range of morphologies;some are octahedral and cuboctahedron and others are elongate and irregular.Secondary electron images show that some grains have well-developed striatums.All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ~1325 cm~(-1).The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue,but some are yellow to light yeUow.All the analyzed grains have typical Raman spectra with shifts at 766 cm~(-1),787 cm~(-1),and 967 cm~(-1).The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon.This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys.Our new findings suggest that diamonds and moissanites are present,and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.     

蛇绿岩中金刚石和异常矿物的全球记录

蛇绿岩型金刚石产在地幔橄榄岩和铬铁矿中,是新建立的金刚石产出类型,不同于产在金伯利岩和超高压变质岩中的金刚石。全球已在21个蛇绿岩中发现了该类金刚石,含金刚石的蛇绿岩主要分布在特提斯造山带、乌拉尔- 中亚造山带、日高变质岩带和北美克拉马斯- 阿卡特兰造山带。本文梳理了含金刚石蛇绿岩的全球分布和地质背景以及蛇绿岩中超高压－强还原矿物与其它壳幔矿物组合的特征,讨论了已有的含金刚石铬铁矿和地幔橄榄岩的四种成因机制。金刚石和伴生的超高压－强还原矿物组合产在不同时代的造山带蛇绿岩中,不仅仅揭示了金刚石在蛇绿岩中普遍存在,需要重新思考蛇绿岩和铬铁矿的成因以及它们形成的地质构造背景,还证实了蛇绿岩地幔橄榄岩和铬铁矿是地球深部矿物重要的储存库,为认识地球深部的物质组成和物理化学环境,以及深部物质运移的轨迹和动力学过程等提供了天然样品。     

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.     

Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).     

Unexpectedly high-PGE chromitite from the deeper mantle section of the northern Oman ophiolite and its tectonic implications

Unusually high, platinum-group element (PGE) enrichments are reported for the first time in a podiform chromitite of the northern Oman ophiolite. The chromitite contains Б.5 ppm of total PGE, being highly enriched in the IPGE subgroup (Ir, Os and Ru) and strongly depleted in the PPGE subgroup (Rh, Pt and Pd). Its platinum-group minerals (PGMs) are classified into three types arranged in order of abundance: (1) sulphides (Os-rich laurite, laurite-erlishmanite solid solution and an unnamed Ir sulphide), (2) alloys (Os-Ir alloy and Ir-Rh alloy), and (3) sulpharsenides (irarsite and hollingworthite). The high PGE concentrations are observed only in a discordant chromitite deep in the mantle section, which has high-Cr# (>0.7) spinel with an olivine matrix. All the other types of chromitite (in the Moho transition zone (MTZ) and concordant pods in the deeper mantle section) are poor in PGEs and tend to have spinels with lower Cr# (up to 0.6). This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low-Cr#, PGE-poor chromitites at the Moho transition zone and, to a lesser extent, within the mantle, possibly beneath a fast-spreading mid-ocean ridge. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr#, PGE-rich discordant chromitite in the upper mantle, possibly in a supra-subduction zone setting.     

Chondritic osmium isotopic composition of Archean ophiolitic mantle, North China craton

Podiform chromitites are diagnostic but rare features of Phanerozoic ophiolites, and often contain the most pristine textural, chemical and isotopic record of convective upper mantle conditions extant during ophiolite genesis. Ophiolitic podiform chromitites, owing to their high Os concentrations and low Re/Os ratios provide the best evidence for the Os-isotopic evolution of oceanic mantle, but established records of ophiolitic chromites from bona fide Archean ophiolites are still lacking. We report Re–Os isotopic compositions of the world's oldest known ophiolitic podiform chromites from the 2.5 billion year old Dongwanzi–Zunhua ophiolite, North China. This provides the oldest Os isotope composition for the convective upper mantle yet obtained from ophiolitic podiform chromitites, and reveals a chondritic Os isotopic composition of the Archean convective upper mantle.     

Platinum‐Group Elements Geochemistry and Chromian Spinel Composition in Podiform Chromitites and Associated Peridotites from the Cheshmeh‐Bid Deposit,Neyriz, Southern Iran: Implications for Geotectonic Setting

Dunite and serpentinized harzburgite in the Cheshmeh-Bid area, northwest of the Neyriz ophiolite in Iran, host podiform chromitite that occur as schlieren-type, tabular and aligned massive lenses of various sizes. The most important chromitite ore textures in the Cheshmeh-Bid deposit are massive, nodular and disseminated. Massive chromitite, dunite, and harzburgite host rocks were analyzed for trace and platinum-group elements geochemistry. Chromian spinel in chromitite is characterized by high Cr~#(0.72-0.78), high Mg~#(0.62–0.68) and low TiO_2(0.12 wt%-0.2 wt%) content. These data are similar to those of chromitites deposited from high degrees of mantle partial melting. The Cr~# of chromian spinel ranges from 0.73 to 0.8 in dunite, similar to the high-Cr chromitite, whereas it ranges from 0.56 to 0.65 in harzburgite. The calculated melt composition of the high-Cr chromitites of the Cheshmeh-Bid is 11.53 wt%–12.94 wt% Al_2O_3, 0.21 wt%–0.33 wt% TiO_2 with FeO/MgO ratios of 0.69-0.97, which are interpreted as more refractory melts akin to boninitic compositions. The total PGE content of the Cheshmeh-Bid chromitite, dunite and harzburgite are very low(average of 220.4, 34.5 and 47.3 ppb, respectively). The Pd/Ir ratio, which is an indicator of PGE fractionation, is very low(0.05–0.18) in the Cheshmeh-Bid chromitites and show that these rocks derived from a depleted mantle. The chromitites are characterized by high-Cr~#, low Pd + Pt(4–14 ppb) and high IPGE/PPGE ratios(8.2–22.25), resulting in a general negatively patterns, suggesting a high-degree of partial melting is responsible for the formation of the Cheshmeh-Bid chromitites. Therefore parent magma probably experiences a very low fractionation and was derived by an increasing partial melting. These geochemical characteristics show that the Cheshmeh-Bid chromitites have been probably derived from a boninitic melts in a supra-subduction setting that reacted with depleted peridotites. The high-Cr chromitite has relatively uniform mantle-normalized PGE patterns, with a steep slope, positive Ru and negative Pt, Pd anomalies, and enrichment of PGE relative to the chondrite. The dunite(total PGE = 47.25 ppb) and harzburgite(total PGE =3 4.5 ppb) are highly depleted in PGE and show slightly positive slopes PGE spidergrams, accompanied by a small positive Ru, Pt and Pd anomalies and their Pdn/Irn ratio ranges between 1.55–1.7 and 1.36-1.94, respectively. Trace element contents of the Cheshmeh-Bid chromitites, such as Ga, V, Zn, Co, Ni, and Mn, are low and vary between 13–26, 466–842, 22-84, 115–179, 826–-1210, and 697–1136 ppm, respectively. These contents are compatible with other boninitic chromitites worldwide. The chromian spinel and bulk PGE geochemistry for the Cheshmeh-Bid chromitites suggest that high-Cr chromitites were generated from Cr-rich and, Ti-and Al-poor boninitic melts, most probably in a fore-arc tectonic setting related with a supra-subduction zone, similarly to other ophiolites in the outer Zagros ophiolitic belt.     

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。