Platinum-Group Mineral Characterization in Concentrates from High-Grade PGE Al-rich Chromitites of Korydallos Area in the Pindos Ophiolite Complex (NW Greece)

The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly non-stoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu₄), Pd-bearing tetra-auricupride [(Au,Pd)Cu], nielsenite (PdCu₃), sperrylite (PtAs₂), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd)Cu₃], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd₁₁Sb₂As₂), zvyagintsevite (Pd₃Pb), native Au, keithconnite (Pd₂₀Te₇), naldrettite (Pd₂Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 µm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphur-bearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian spinel had already started to crystallize. The implemented technique has led to the recovery of more, as well as noble, PGM grains than the in situ mineralogical examination of single chromitite samples. Although, the majority of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. In summary, this research provides information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing.     

新疆萨尔托海高铝铬铁矿中异常矿物群的发现及意义

在萨尔托海高铝型铬铁矿中发现20余种矿物,包括金刚石、单质铬、自然铁和单质硅等自然元素类;碳化物碳硅石;铁镍、铁镍铬合金等金属合金;方铁矿、金红石、赤铁矿、磁铁矿、钛铁矿、石英和铬尖晶石等氧化物类;方铅矿、闪锌矿、针镍矿、赫硫镍矿和毒砂等硫化物类;镁橄榄石、顽火辉石、透辉石、蛇纹石、锆石和长石等硅酸岩类。这些超高压、强还原性和壳源矿物与俄罗斯极地乌拉尔以及西藏罗布莎铬铁矿可以对比,暗示萨尔托海高铝铬铁矿和高铬铬铁矿一样,可能存在深部地幔成矿阶段。深部地幔矿物以及浅部壳源矿物的发现,暗示萨尔托海铬铁矿的形成可能经历了深部地幔预富集和浅部再造富集成矿两个阶段。     

X射线荧光光谱法测定高铝粘土中主成分方法研究

使用日本理学ZSX PrimusⅡ型X射线荧光光谱仪,采用熔融玻璃片和经验系数进行基体校正的方法,对我室6件国家标准物质、4件监控样以及用标样和纯值试剂配制的21件混合标准进行组分测定。分析结果表明,此方法能快速、准确测定高铝粘土中主成分Al2O3、SiO2、Fe2O3、TiO2,其准确度和精密度均能达到国家标准方法规定的要求。     

Remnants of Early Continental Crust in the Amgaon Gneisses, Central India: Geochemical Evidence

The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than 2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.     

P–T modelling of the andalusite–kyanite–andalusite sequence and related assemblages in high-Al graphitic pelites. Prograde and retrograde paths in a late kyanite belt in the Variscan Iberia

The exceptional andalusite–kyanite–andalusite sequence occurs in Al‐rich graphitic slates in a narrow pelite belt on the hangingwall of a ductile normal fault in NW Variscan Iberia. Early chiastolite is replaced by Ky–Ms–Pg aggregates, which are overgrown by pleochroic andalusite near granites intruded along the fault. Slates plot in AKFM above the chloritoid‐chlorite tie‐line. Their P–T grids are modelled with Thermocalc v2.7 and the 1998 databases in the NaKFMASH and KFMASH systems. The univariant reaction Ctd + And/Ky = St + Chl + Qtz + H₂O ends at progressively lower pressure as F/FM increases and A/AFM decreases, shrinking the assemblage Cld–Ky–Chl, and opening a chlorite‐free Cld–Ky trivariant field on the low temperature reaction side. This modelling matches the observed absence of chlorite in high F/FM rocks, which is restricted to low pressure in the andalusite stability field. The P–T path deduced from modelling shows a first prograde event in the andalusite field followed by retrogression into the kyanite field, most likely coupled with a slight pressure increase. The final prograde evolution into the andalusite field can be explained by two different prograde paths. Granite intrusion caused the first prograde part of the path with andalusite growth. The subsequent thermal relaxation, together with a_H2O decrease, generated the retrograde andalusite–kyanite transformation, plus chlorite consumption and chloritoid growth. This transformation could have been related to folding in the beginning, and aided later by downthrowing due to normal faulting. Heat supplied by syntectonic granite intrusion explains the isobaric part of the path in the late stages of evolution, causing the prograde andalusite growth after the assemblage St–Ky–Chl. Near postectonic granites, a prograde path with pressure decrease originated the assemblage St–And–Chl.     

航空高光谱识别的高、中、低铝绢云母矿物成因学研究

国内外学者在航空高光谱矿物填图时,根据Al-OH光谱特征吸收波长位置不同将浅色云母类矿物分为2~3类,分别命名为高铝绢云母、中铝绢云母、低铝绢云母,但对它们的成因特点却只有零星的讨论,没有比较系统的有针对性的研究。本文在新疆雪米斯坦铀多金属成矿带获取CASI/SASI航空高光谱数据,并开展矿物填图的基础上,针对高铝绢云母、中铝绢云母、低铝绢云母等三种矿物的成因学专门开展了比较系统的研究。采用的方法主要是在开展详细的野外地质观察、采样的基础上,对采集的上述三类矿物蚀变岩样品进行详细的室内光谱测量、Al-OH吸收波长位置统计、显微薄片观察、全岩X衍射分析、粘土X衍射分析等研究,并结流体成矿理论和已有矿物成因学知识,进行了详细深入的分析讨论。研究表明,高铝绢云母蚀变岩蚀变强烈,云母类蚀变矿物以绢云母、伊利石为主,且主要与叶腊石和微晶石英等矿物伴生,而低铝绢云母蚀变岩蚀变强度中等,以伊利石、伊蒙混层为主,绿泥石增加,主要与方解石、浊沸石等矿物伴生;高铝绢云母蚀变岩具有相对高的石英含量和粘土矿物总量,具有相对较低的钾长石、斜长石以及方解石含量;低铝绢云母蚀变岩则具有相对较低的石英和粘土含量,具有相对较高的斜长石、钾长石和方解石含量。通过深入分析讨论,本文提出了在研究区航空高光谱识别的高铝绢云母形成于相对高温、偏酸性的热液流体环境,低铝形成于相对低温、偏碱性的热液流体环境。这一新认识对航空高光谱遥感矿物填图结果的深入应用和深层次找矿信息反演具有重要意义。     

Alteration of chromitites from the Voidolakkos and Xerolivado mines,Vourinos ophiolite complex,Greece: implications for deformation‐induced metamorphism

Chromitites, associated with upper mantle spinel peridotites from the Voidolakkos and Xerolivado districts, located in the Vourinos ophiolite complex, northwestern continental Greece, were re‐investigated with respect to their structural and textural mode of occurrence, as well as their compositional signatures. They include variably deformed banded and podiform chromitite bodies made up of massive, semi‐massive, nodular, anti‐nodular, schlieren and disseminated chromian spinel. Chromitites have suffered intense shearing that was more severe in all but disseminated textured ore. Deformation has partly produced elongation of chromian spinel nodules and widespread protocataclastic zones within chromitites. The examined deposits are composed of magnesiochromite that shows a quite restricted range of Cr# [Cr/(Cr + Al)] values varying between 0.76 and 0.83, whereas Mg# [Mg/(Mg + Fe²⁺)] ranges from 0.55 to 0.67 accompanied by relatively low content in TiO₂ (<0.15 wt.% on average). Compositional data indicate that these high‐Cr chromitite bodies crystallized from melts of boninitic affinities that have been compositionally modified after reaction with depleted harzburgite, followed by interaction with relatively undifferentiated low SiO₂ melts within an intertwined system of dunite channels in the mantle wedge below the fore‐arc region of a supra‐subduction zone (SSZ). Magnesiochromite displays limited textural modification, being scarcely transformed to an opaque spinel phase along grain boundaries and fracture walls. The opaque spinel phase is characterized by elevated Cr# (0.76–0.97), relatively low Mg# (0.33–0.63) and low Fe³⁺# (≤0.14) and corresponds to ferrian chromite. Microscopic studies revealed that ferrian chromite is paragenetically associated with clinochlore even in unaltered chromitite specimens and the degree of serpentinization does not correlate with the frequency and abundance of alteration effects on magnesiochromite. Therefore, it is supported that regional metamorphism prior to serpentinization was responsible for the formation of the ferrian chromite–clinochlore association. In addition, magnesiochromite alteration was systematically recorded only in variably sheared chromitites displaying protocataclastic brecciation, thus it can be claimed that metamorphism was mainly governed by deformation mechanisms, which took place during the transition from ductile to semi‐brittle conditions. Ferrian chromite can be locally erratically enriched in MnO and ZnO, which is attributed to a Mn‐ and Zn‐bearing, slightly acid post‐magmatic fluid that invaded the chromitites across weakness zones. Overall, the data suggest that after magnesiochromite equilibration with the intergranular olivine, both phases were partly replaced by ferrian chromite and clinochlore, respectively, during a brief, fluid assisted episode of retrogade metamorphism that took place within a temperature interval between 700 and 300 °C, before ocean‐floor alteration. Copyright © 2014 John Wiley & Sons, Ltd.     

Neoproterozoic podiform chromitites in serpentinites of the Abu Meriewa–Hagar Dungash district, Eastern Desert, Egypt: Geotectonic implications and metamorphism

Podiform chromitites hosted in serpentinites (after harzburgite and dunite) and talc‐carbonate rocks from the Abu Meriewa–Hagar Dungash district (MHD), Eastern Desert of Egypt, together with metagabbros, pillow metavolcanics, and metasediments, form an ophiolitic mélange formed during the Neoproterozoic Pan‐African Orogeny. The chromitites show massive, disseminated, and nodular textures. Chromite cores in chromitites have high and restricted ranges of Cr# (0.65–0.75) and Mg# (0.64–0.83), implying primary compositions not affected by metamorphism. Therefore, they are used as reliable indicators of parent magma composition and tectonic affinities of these highly metamorphosed rocks. On the contrary, the altered rims are high‐Cr, low‐Fe³⁺ spinel (rather than ferritchromit) enriched in Cr, Fe, and Mn, and depleted in Al and Mg (Cr# = 0.75–0.97, Mg# = 0.29–0.79), due to equilibration with interstitial silicates during regional metamorphism up to transitional greenschist–amphibolite facies at about 500–550°C. The primary chromite compositions suggest derivation from a high‐Mg tholeiitic, to possibly boninitic, parental magma in a supra‐subduction zone (arc–marginal basin) environment, similar to the spatially associated metavolcanic rocks. The MHD chromitites are most probably formed by melt–rock interaction mechanisms. The high Cr# of the investigated chromites suggests high degrees of partial melting of a depleted harzburgite source by interaction with primitive basaltic melt of deeper origin followed by mixing. Such Cr‐rich chromites are common in chromitites from the Eastern Desert of Egypt, implying broad thermal anomalies, possibly linked to an important geodynamic feature of the Arabian–Nubian Shield (ANS) evolution. This could revive interest in models that involve asthenospheric uprise, related to plume interaction or most probably due to oblique convergence of arc terranes during early evolution of the ANS.     

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.     

蛇绿岩型铬铁矿床包壳纯橄榄岩中的流体过程印记:来自西藏雅鲁藏布江缝合带罗布莎和泽当岩体的地质学、岩石学和橄榄石晶体化学证据

岩浆型矿床一般认为是岩浆分异的产物,因为这类矿床通常缺乏强烈的近矿围岩蚀变。蛇绿岩中的豆荚状铬铁矿被认为是一种典型的岩浆型矿床,流行的成因模型包括岩浆通道模型和熔体-岩石反应模型。深部晶体群的大量发现,表明铬铁矿成矿系统不是一种理想系统,而是至少由两类子系统组成的复杂性动力系统。因此,流行模型不再适用,必须构建能够整合新证据的成因模型。这类矿床的典型地质特征是具有从方辉橄榄岩围岩经包壳纯橄榄岩到铬铁矿石的分带,且包壳纯橄榄岩与铬铁矿之间为渐变接触关系,包壳纯橄榄岩与方辉橄榄岩之间既可以为渐变接触关系,也可以为截然接触关系或侵入接触关系。因此,阐明纯橄榄岩的成因是理解豆荚状铬铁矿形成机制的关键。西藏雅鲁藏布江缝合带中罗布莎和泽当两个代表性超镁铁质杂岩体的新观察揭示:(1)包壳纯橄榄岩的出露宽度变化于厘米级到百米级,但岩石具有均匀的细粒结构,流行模型难以解释;(2)包壳纯橄榄岩可以划分为至少两种构造类型:块状纯橄榄岩和片理化纯橄榄岩,暗示了纯橄榄岩形成过程的多阶段特点;(3)包壳纯橄榄岩主要由变晶橄榄石组成,仅含有少量由熔体或流体析出的橄榄石晶体;(4)与方辉橄榄岩相比,包壳纯橄榄岩中的橄榄石具有高MgO、Cr2O3、CaO和低MnO、Al2O3的特点,展示了矛盾的晶体化学特征;(5)邻近铬铁矿体的纯橄榄岩中常见反豆状结构,类似于多相稀释流体流体制中紊流产生的中尺度结构。上述看似矛盾的证据表明包壳纯橄榄岩的形成过程有大量深部流体的参与,因而流体过程可以作为构建一个新的整合模型的基础。据此,文中提出一个熔体-流体流模型,其基本机制是溶解-沉淀反应Opx+Fluid→Ol±Sp±Cpx±Pl±SiO2(fluid),而基本前提则是深部还原流体的持续供给和熔体-流体流的快速上升。此外,文中还表明,依据火成岩地质学、岩石学和名义无水矿物晶体化学证据也可以再造岩浆系统的流体过程。