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.     

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO₂ content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.     

The Concentration of the Platinum-Group Elements in South African Komatiites: Implications for Mantle Sources, Melting Regime and PGE Fractionation during Crystallization

We have analysed 18 samples of komatiite from five consecutivelava flows of the Komati Formation at Spinifex Creek, BarbertonMountain Land. Our samples include massive komatiite, varioustypes of spinifex-textured komatiite, and flow-top breccias.The rocks have low platinum-group element (PGE) contents andPd/Ir ratios relative to komatiites from elsewhere, at 0·45–2ppb Os, 1–1·4 ppb Ir, <1–5 ppb Ru, 0·33–0·79ppb Rh, 1·7–6 ppb Pt, 1·6–6·1ppb Pd, and Pd/Ir 3·3. Pt/Pd ratios are c. 1·1.Platinum-group elements are depleted relative to Cu (Cu/Pd =15 300). They display a tendency to increase in the less magnesiansamples, suggesting that the magmas were S-undersaturated uponeruption and that all PGE were incompatible with respect tocrystallizing olivine. Komatiites from the Westonaria Formationof the Ventersdorp Supergroup and the Roodekrans Complex nearJohannesburg have broadly similar PGE patterns and concentrationsto the Komati rocks, suggesting that the PGE contents of SouthAfrican ultrabasic magmas are controlled by similar processesduring partial mantle melting and low-P magmatic crystallization.Most workers believe that the Barberton komatiites formed byrelatively moderate-degree batch melting of the mantle at highpressure. Based on the concentration of Zr in the Komati samples,we estimate that the degree of partial melting was between 26and 33%. We suggest that the low PGE contents and Pd/Ir ratiosof all analysed South African komatiites are the result of sulphideshaving been retained in the mantle source during partial melting.The difference in Pd/Ir between our samples and Al-undepletedkomatiites from elsewhere further suggests that the PGE arefractionated during progressive partial melting of the mantle.Thus, our data are in agreement with other recent studies showingthat the PGE are hosted by different phases in the mantle, withPd being concentrated by interstitial Cu-rich sulphide, andthe IPGE (Os, Ir, Ru) and Rh resting in monosulphide solid solutionincluded within silicates. Pt is possibly controlled by a discreterefractory phase, as Pt/Pd ratios of most komatiites worldwideare sub-chondritic. KEY WORDS: platinum-group elements; komatiites; Barberton; mantle melting; South Africa     

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。     

Platinum group elements zoning and mineralogy of chromitites from the cumulate sequence of the Nurali massif (Southern Urals, Russia)

The Nurali lherzolite massif is one of the dismembered ophiolite bodies associated with the Main Uralian Fault (Southern Urals, Russia). It comprises a mainly lherzolitic mantle section, an ultramafic clinopyroxene-rich cumulate sequence (Transition Zone), and an amphibole gabbro unit.The cumulate section hosts small chromitite bodies at different stratigraphic heights within the sequence. Chromitite bodies from three different levels along a full section of the cumulate sequence and two from other localities were investigated. They differ in the host lithology, chromitite texture and composition, and PGE content and mineralogy. Chromitites at the lowest level, which are hosted by clinopyroxenite, form cm-scale flattened lenses. They have high Cr# and low Mg# chromites and are enriched in Pt and Pd relative to Os and Ir. At a higher, intermediate level, the chromitites are hosted by dunite. They form meter thick lenses, contain low Cr# and high Mg# chromites, have high PGE contents (up to 26,700 ppb), and are enriched in Os, Ir and Ru relative to Pt and Pd, reflecting a mineralogy dominated by laurite–erlichmanite and PGE–Fe alloys. At the highest level are chromitites hosted by olivine–enstatite rocks. These chromitites have high Cr# and relatively low Mg# chromites and very low PGE content, with laurite as the dominant PGE mineral.The platinum group minerals (PGMs) show extreme zoning, with compositions ranging from erlichmanite to almost pure laurite and from Os-rich to Ru-rich alloys, with variable and irregular zoning patterns.Two chromitite bodies up to 6 km from the main sequence can be correlated with the latter based on geochemistry and mineralogy, implying that the variations in chromitite geochemistry are due to processes that operated on the scale of the massif rather than those that operated on the scale of the outcrop.Pertsev et al. [Pertsev, A.N., Spadea, P., Savelieva, G.N., Gaggero, L., 1997. Nature of the transition zone in the Nurali ophiolite, Southern Urals. Tectonophysics 276, 163–180.] propose that the Transition Zone formed by solidification of a series of small magma bodies that partially overlapped in time and space. The magmas formed by successive partial melting of the underlying mantle. We suggest that this process determined the changing PGE geochemistry of the successive batches of magma. The PGE distribution fits a model of selected extraction from the mantle, where monosulphide solid solution–sulphide liquid equilibrium was attained until complete melting of the monosulphide solid solution. Later and localized variations in fS₂ resulted in the formation of different PGMs with complex zoning patterns.     

Heterogeneously depleted Precambrian lithosphere deduced from mantle peridotites and associated chromitite deposits of Al'Ays ophiolite,Northwestern Arabian Shield,Saudi Arabia

The mantle section of Al'Ays ophiolite consists of heterogeneously depleted harzburgites, dunites and large-sized chromitite pods. Two chromitite-bearing sites (Site1 and Site2), about 10 km apart horizontally from one another, were examined for their upper mantle rocks. Cr-spinels from the two sites have different chemistry; Cr-rich in Site1 and Al-rich in Site2. The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, dunites and chromitites is remarkably high 0.78, 0.77 and 0.87, respectively, in Site1, compared with those of Site2 which have intermediate ratio averages 0.5, 0.56 and 0.6, respectively. The platinum-group elements (PGE) in chromitites also show contrasting patterns from Site1 to Site2; having elevated IPGE (Os, Ir, Ru) and strongly depleted in PPGE (Rh, Pt, Pd) with steep negative slopes in the former, and gentle negative slopes in the latter. The oxygen fugacity (Δlog fO₂) values deduced from harzburgites and dunites of Site1 show a wide variation under reducing conditions, mostly below the FMQ buffer. The Site2 harzburgites and dunites, on the other hand are mostly above the FMQ buffer. Two magmatic stages are suggested for the lithospheric evolution of Al'Ays ophiolite in response to a switch of tectonic setting. The first stage produced a peridotites–chromitites suite with Al-rich Cr-spinels, possibly beneath a mid-ocean ridge setting, or most likely in back-arc rift of a supra-subduction zone setting. The second stage involved higher degrees of partial melting, produced a peridotites–chromitites suite with Cr-rich Cr-spinels, possibly in a fore-arc setting. The coexistence of compositionally different mantle suites with different melting histories in a restricted area of an ophiolite complex may be attributable to a mechanically juxtaposed by mantle convection during recycling. The mantle harzburgites and dunites are apt to be compositionally modified during recycling process; being highly depleted (Site1 case) than their original composition (Site2 case).     

Polychronous formation of mantle complexes in ophiolites

The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0?1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.     

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。     

塔里木大火成岩省瓦吉里塔格霞石岩铂族元素地球化学特征

作为塔里木大火成岩省形成最晚的火成岩,新疆巴楚瓦吉里塔格霞石岩的岩浆源区性质的确定对于揭示塔里木大火成岩省的深部地质过程具有重要的约束作用。对瓦吉里塔格霞石岩的铂族元素地球化学特征进行了研究,铂族元素(PGE)分析结果显示,原始地幔标准化的PGE呈正斜率型分布,且Pd/Ir值高于原始地幔比值,说明霞石岩的铂族元素发生了分异。霞石岩全岩的PGE与Mg O呈正相关,Pd/Ir、Cu/Pd与Mg O则呈负相关,说明PGE的分异主要受到橄榄石的结晶分异作用控制,也是其Cu/Pd值极高及岩浆S饱和的因素之一,同时Cu/Pd值说明霞石岩岩浆为硫饱和岩浆,但是没有因素导致岩浆S过饱和进而发生硫化物的熔离。与其他大火成岩省岩石相比,瓦吉里塔格霞石岩极度亏损PGE,SCSS(硫承载量)计算结果表明母岩浆在形成之初就发生S过饱和,主要是地幔低程度部分熔融造成的,据此认为地幔源区的部分熔融程度在塔里木大火成岩省Cu-Ni硫化物铂族元素矿床形成过程中起着至关重要的作用。     

阜新中生代火山岩的铂族元素特征——以碱锅和乌拉哈达为例

采用镍锍火试金法结合ICP—MS分析了碱锅玄武岩和乌拉哈达高镁安山岩样品中的Ir．Ru、Rh、Pt和Pd的含量。原始地幔标准化后的PGE分布模式呈正斜率型，Pd／Ir值高于相应的地幔比值，表明铂族元素发生了分异，这是由于在部分熔融过程中，Ir存在于地幔矿物相尖晶石和合金中，而Pd赋存于硫化物中造成的，乌拉哈达高镁安山岩中的铂族元素还可能在结晶分异过程中受到先期结晶的矿物相和合金的影响。阜新火山岩Pt的负异常可能是包含Pt的金属合金残留在地幔中造成。     

西藏罗布莎不同类型铬铁矿的特征及成因模式讨论

蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿是铬的主要来源。已有的研究表明,豆荚状铬铁矿形成于洋中脊或俯冲带的浅部地幔环境。但随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中不断发现金刚石等深部矿物,人们也开始质疑豆荚状铬铁矿的浅部成因理论。本文系统研究了西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床,识别出两类铬铁矿,一类以方辉橄榄岩为围岩的致密块状铬铁矿(Cr1#),另一类是以纯橄岩壳为围岩的浸染状铬铁矿(Cr2#)。两类铬铁矿在铬尖晶石的矿物化学成分、PGE和Re-Os同位素特征上存在较大差别,属不同演化过程的结果。地幔橄榄岩的地球化学特征指示罗布莎橄榄岩中存在由低铬且轻稀土亏损和高铬且轻稀土富集的两类方辉橄榄岩。在此基础上,提出豆荚状铬铁矿为多阶段形成的新认识,经历了早期俯冲至地幔过渡带(410~660km)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;铬铁矿浆在地幔柱/地幔对流驱动下,运移至过渡带顶部冷凝固结,并有强还原性的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,并进入"塑性-半塑性地幔橄榄岩"中;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;在侵位过程和俯冲带环境,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(Cr2#)及纯橄岩壳。     

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     

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.     

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).     

Chemical homogeneity of high-Cr chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of Bir Tuluha ophiolite,Northern Arabian Shield,Saudi Arabia

The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.     

Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia)

The Massif du Sud is a large ophiolitic complex that crops out in the southern region of New Caledonia (SW Pacific). It is dominated by harzburgite tectonite that locally shows a transitional gradation to massive dunite up section. Clinopyroxene, orthopyroxene and plagioclase progressively appear in dunite up to the transition to layered wehrlite and orthopyroxene–gabbro. The dunite–wehrlite and wehrlite–gabbro contacts are parallel and the latter defines the paleo-Moho.Highly depleted modal, mineral and bulk rock compositions indicate that harzburgites are residues after high degrees (20–30%) of partial melting mainly in the spinel-stability field. Their relative enrichment in HFSE, LREE and MREE is due to re-equilibration of melting residues with percolating melts. Dunite formed in the Moho transition zone by reaction between residual mantle harzburgite and olivine-saturated melts that led to pyroxene dissolution and olivine precipitation. Rare clinopyroxene and plagioclase crystallized in interstitial melt pores of dunite from primitive, low-TiO₂, ultra-depleted liquids with a geochemical signature transitional between those of island arc tholeiites and boninites.Ascending batches of relatively high-SiO₂, ultra-depleted melts migrated through the Moho transition zone and generated wehrlite by olivine dissolution and crystallization of clinopyroxene, orthopyroxene and plagioclase in variable amounts. These liquids were more evolved and were produced by higher degrees of melting or from a more depleted source compared with melts that locally crystallized clinopyroxene in dunite. Ultra-depleted magmas, non-cogenetic with those that formed the Moho transition zone, ascended to the lower crust and generated gabbroic cumulates with subduction-related affinity. Thus, the ultramafic and mafic rocks in the Moho transition zone and lower crust of the Massif du Sud ophiolite are not products of fractional crystallization from a single magma-type but are the result of migration and accumulation of different melts in a multi-stage evolution.The record of high partial melting in the mantle section, and migration and accumulation of ultra-depleted subduction-related melts in the Moho transition zone and lower crust support that the Massif du Sud ophiolite is a portion of forearc lithosphere generated in an extensional regime during the early phases of the subduction zone evolution. Our results show the existence of different types of ultra-depleted melt compositions arriving at the Moho transition zone and lower crust of an infant intraoceanic paleo-arc. Ultra-depleted melts may thus be a significant component of the melt budget generated in oceanic spreading forearcs prior to aggregation and mixing of a large range of melt compositions in the crust.     

The Acoje Block Platiniferous Dunite Horizon, Zambales Ophiolite Complex, Philippines: Melt Type and Associated Geochemical Controls

Abstract: The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is made up of the mid-ocean ridge-related Coto block and the island arc-related Acoje block. This crust-mantle sequence hosts platinum-group elements (PGE) in the Acoje block. The melts responsible for the PGE-bearing nickel sulfide and chromitite deposits are of magmatic origin characterized by high-MgO basalt to boninitic composition which, being second or third-stage melts, carry higher PGE budgets. Metal ratio diagrams, utilizing base and precious metals, reveal that the distribution and deposition of the PGE in the Acoje block are affected by olivine, chromite and sulfide crystallization. The generation, accumulation and segregation of the PGE, oxide and sulfide minerals from the melts are governed by the combined factors of high degrees of partial melting, multiple melt replenishment with concomitant magma mixing and fractional crystallization. Although previous sulfide segregation events could have occurred below the PGE-bearing nickel sulfide horizon as shown by the Ni/Cu (>1), the Cu/Pd and Ni/Pd strongly suggest that the main platiniferous zone is confined within the Acoje block transition zone dunite.     

Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S₂) of 10^?0.39–10^?0.07.     

雅鲁藏布江缝合带蛇绿岩中铬铁矿的前景讨论

蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿是铬的主要来源,是中国极缺的重要战略资源。开展豆荚状铬铁矿成矿作用及围岩地幔橄榄岩的研究,是进一步寻找铬铁矿床和缓解中国铬铁矿资源的瓶颈状态的必要手段。本文以西藏雅鲁藏布江蛇绿岩带内几个主要的地幔橄榄岩体及其中的铬铁矿体为研究主体。在野外地质调查的基础上,系统总结了蛇绿岩的组成、矿物成分、岩石地球化学成分和Re-Os同位素等特征,探讨铬铁矿和地幔橄榄岩的形成过程,取得以下进展和认识:(1)雅鲁藏布江缝合带各段的岩石组合存在较大差异,构造背景的演化过程也不同,佐证了特提斯洋演化过程的不均一性;(2)在雅江西段存在高铝型和高铬型两类铬铁矿矿体,其余都为高铬型铬铁矿,铬尖晶石的矿物化学特征记录了不同构造背景的痕迹;(3)地幔橄榄岩的矿物学和地球化学表明地幔橄榄岩及铬铁矿具有深海地幔橄榄岩和岛弧地幔橄榄岩两者的特点,是岩石/熔体反应和部分熔融作用叠加的结果;(4)提出豆荚状铬铁矿为多阶段形成的认识,经历了早期俯冲到地幔过渡带,在地幔柱/地幔对流驱动下,运移到过渡带顶部冷凝固结,在侵位过程和俯冲带环境,含水熔体与方辉橄榄岩反应的过程;(5)在雅鲁藏布江缝合带中金刚石等超高压矿物的普遍存在,西段的几个大型岩体与罗布莎存在较多相似之处,均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。     

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