Distribution of platinum-group elements in magmatic and altered ores in the Jinchuan intrusion, China: an example of selenium remobilization by postmagmatic fluids

The division of platinum-group elements (PGE) between those hosted in platinum-group minerals (PGM) versus those in solid solution in base metal sulfides (BMS) has been determined for ores from the PGE-bearing Ni-Cu-rich Jinchuan intrusion in northwest China. All the BMS are devoid of Pt and Ir, and magmatic BMS are also barren of Rh. These PGE may have been scavenged by arsenic to form PGM during magmatic crystallization of the BMS. Pd, Os, and Ru are recorded in BMS and Pd is predominantly in solid solution in pentlandite. Unlike the fresh magmatic ores, in altered or serpentinized ores, Pd-PGM are present. Froodite is hosted in magnetite, formed during alteration of BMS, accompanied by sulfur loss and liberation of Pd. Michenerite ([Pd,Pt]BiTe), sperrylite (PtAs₂), and Au-bearing PGM are located in altered silicates. Irarsite (IrAsS) occurs mainly enclosed in BMS. Padmaite (PdBiSe), identified at the junctions of magnetite and BMS, was the last PGM to form and locally partially replaces earlier non-Se-bearing PGM. We propose that padmaite formed under oxidizing conditions during late local remobilization of Se from the BMS. Se-bearing PGM are rare and our review shows they are frequently associated with carbonate, suggesting that Pd and Se can be mobilized great distances in low pH oxidizing fluids and may be precipitated on contact with carbonate. S/Se ratios are used by researchers of magmatic Ni-Cu-PGE ores to determine sulfur loss, assuming Se is immobile and representative of magmatic sulfur content. This study shows that Se as well as S is potentially mobile and this should be considered in the use of S/Se ratios.     

High-Al and high-Cr podiform chromitites from the western Yarlung-Zangbo suture zone,Tibet: Implications from mineralogy and geochemistry of chromian spinel,and platinum-group elements

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 1 Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.     

Geochemical structure of the Kivakka layered olivinite-norite-gabbronorite intrusion,North Karelia: Distribution of chalcophile elements

The distribution of elements and their correlations were analyzed throughout the section of the Kivakka intrusion to formulate the geochemical tendencies in the behavior of elements of different groups during formation of layered basic-ultrabasic complexes. (1) It was shown that the distribution of trace elements in the layered series is controlled by their ability or inability to enter isomorphically the cumulus minerals. The distribution of trace elements that occur as isomorphic admixtures in cumulus minerals (Cr, Mn, Zn, Co, Ni, and Ti), regardless of their geochemical type, is correlated with the crystallization order and distribution of minerals in the silicate matrix of the rocks. Elements that practically are not involved into silicates (S, Cu, Ag) show no any systematic variations; i.e., they are not controlled by fractionation of major rock-forming mineral. Their behavior is driven by the appearance of individual phase—sulfide melt, whose localization, formation time, and scales of fractionation are determined by saturation of parental melt in sulfide sulfur and general degree of its fractionation. (2) The comparison of cross-section variations of elements that isomorphically substitute for major elements in the Fe-Mg silicates, but differ in chalcophile affinity (in order of increase of chalcophile affinity: Mn → Zn → Co → Ni) reflects the contribution of unmixing and fractionation of sulfide melt during intrusion solidification. This is quantitatively defined primarily by partitioning coefficients of elements between cumulus silicates and magmatic melt, on the one hand, and between magmatic and sulfide melts, on the other hand. (3) The absence of simple correlation between local sulfide-rich horizons and silicate matrix of the rocks and signs of independent fractionation of sulfide melt prevent any attempts to predict the localization and scales of local sulfide mineralization within the layered series on the basis of petrochemical and related criteria. Only tendencies in the distribution of ore elements and sulfur across the section can be used for these purposes.     

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.     

Genesis and tectonic setting of ophiolitic chromitites from the Dehsheikh ultramafic complex (Kerman,southeastern Iran): Inferences from platinum-group elements and chromite compositions

Chromitite bodies of various sizes associated with dunite envelopes have been found in the Dehsheikh ultramafic massif, in the southeastern part of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies, and show both magmatic and deformational textures, including massive, disseminated, banded and nodular types. The Dehsheikh chromitites display a variation in Cr# [100 × Cr / (Cr + Al)] from 69 to 78, which is typical of high-Cr chromitites. The Al₂O₃ and TiO₂ contents of chromites range from 10.3 wt.% to 16.9 wt.% and 0.12 wt.% to 0.35 wt.%, respectively. The Al₂O₃, TiO₂, and FeO/MgO values calculated for parental melts of Dehsheikh chromitites are within the range of boninitic melts. Chondrite-normalized distribution patterns of platinum-group elements show relative enrichments in Ru, Ir, and Os, and depletions in Rh, Pd, and Pt that are typical of chromitites associated with ophiolites formed by high degrees of mantle partial melting. The presence of Na-rich amphibole inclusions in chromite grains, together with the mineralogical and chemical composition of the chromitites and estimates of their parental melt compositions are used to help establish the tectono-magmatic setting. It is shown that the Dehsheikh massif is an ophiolite formed in a suprasubduction zone setting. We suggest that the composition of the rocks in this section was influenced by hydrous partial melts which might be formed in the subduction zone. Variable melt/rock interaction produced melt channel networks in the dunite which allowed the parental melt of the chromitite to percolate through them. Similar characteristics have been observed in other ophiolite complexes from the outer Zagros Iranian ophiolitic belt; these are believed to be the product of magmatism in a fore-arc environment.     

Rhenium-osmium isotope and platinum-group elements in the Xinjie layered intrusion, SW China: Implications for source mantle composition, mantle evolution, PGE fractionation and mineralization

The Xinjie mafic-ultramafic layered intrusion in the Emeishan large igneous province (ELIP) hosts Cu-Ni-platinum group element (PGE) sulfide ore layers within the lower part and Fe-Ti-V oxide-bearing horizons within the middle part. The major magmatic Cu-Ni-PGE sulfide ores and spatially associated cumulate rocks are examined for their PGE contents and Re-Os isotopic systematics. The samples yielded a Re-Os isochron with an age of 262 ± 27 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os of 0.12460 ± 0.00011 (γ_Os(t) = −0.5 ± 0.1). The age is in good agreement with the previously reported U-Pb zircon age, indicating that the Re-Os system remained closed for most samples since the intrusion emplacement. They have near-chondritic γ_Os(t) values ranging from −0.7 to −0.2, similar to those of the Lijiang picrites and Song Da komatiites. Exceptionally, two samples from the roof zone and one from upper sequence exhibit radiogenic γ_Os(t) values (+0.6 to +8.6), showing minor contamination by the overlying Emeishan basalts.The PGE-rich ores contain relatively high PGE and small amounts of sulfides (generally less than 2%) and the abundance of Cu and PGE correlate well with S, implying that the distribution of these elements is controlled by the segregation and accumulation of a sulfide liquid. Some ore samples are poor in S (mostly <800 ppm), which may due to late-stage S loss caused by the dissolution of FeS from pre-existing sulfides through their interaction with sulfide-unsaturated flowing magma. The combined study shows that the Xinjie intrusion may be derived from ferropicritic magmas. The sharp reversals in Mg#, Cr/FeO_T and Cr/TiO₂ ratios immediately below Units 2-4, together with high Cu/Zr ratios decreasing from each PGE ore layer within these cyclic units, are consistent with multiple magma replenishment episodes. The sulfides in the cumulate rocks show little evidence of PGE depletion with height and thus appear to have segregated from successive inputs of fertile magma. This suggests that the Xinjie intrusion crystallized from in an open magma system, e.g., a magma conduit. The compositions of the disseminated sulfides in most samples can be modeled by applying an R factor (silicate-sulfide mass ratio) of between 1000 and 8000, indicating the segregation of only small amounts of sulfide liquid in the parental ferropicritic magmas. Thus, continuous mixing between primitive ferropicritic magma and differentiated resident magma could lead to crystallization of chromite, Cr-bearing magnetite and subsequently abundant Fe-Ti oxides, thereby the segregation of PGE-rich Cu-sulfide.When considered in the light of previous studies on plume-derived komatiites and picrites worldwide, the close-to-chondritic Os isotopic composition for most Xinjie samples, Lijiang picrites and Song Da komatiites suggest that the ferropicritic magma in the ELIP were generated from a plume. This comprised recycled Neoproterozic oceanic lithosphere, including depleted peridotite mantle embedded with geochemically enriched domains. The ascending magmas thereafter interacted with minor (possibly <10%) subducted/altered oceanic crust. This comparison suggests that the komatiitic melts in the ELIP originated from a greater-than normal degree of melting of incompatible trace element depleted, refractory mantle components in the plume source.     

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex,Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [= Cr / (Cr + Al)] ranging from 0.76 to 0.77, Mg# [= Mg/(Mg + Fe^2 +)] from 0.63 to 0.65 and TiO₂ < 0.2 wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO₂ contents (averaging 0.07 wt.%). The Fe^3 +# is very low, (< 0.08 wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os + Ir + Ru)/(Rh + Pt + Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (< 0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The Pd_N/Ir_N ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their Pd_N/Ir_N ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.     

西藏达巴—休古嘎布绿岩带铂族元素的分布特征

区内各岩体的地幔橄榄岩中PGE含量及分布型式有所差异.相对于原始地幔,拉昂错和东坡岩体的PGE含量没有显著异常,标准化分布曲线大致平坦,PPGE与IPGE之间分馏不明显,总体特征与中国日喀则地区以及阿尔卑斯和东地中海地区的地幔橄榄岩相似.而当穷岩体的PPGE含量变化范围较大,出现IPGE与PPGE强分馏.单斜辉石岩脉的PGE含量及分布型式与地幔橄榄岩不同:Os、Ir、Ru、Rh亏损而Pt、Pd富集,模式曲线向左倾斜,可能反映其较高程度部分熔融作用和熔体分离作用的产物.铬铁矿石的PGE含量较高,而且变化幅度大(19.7×10-9～5526×10-9),各样品∑PGE平均值(729.33×10-9)是原始地幔的25.6倍.与原始地幔相比,Os、Ir、Ru、Rh总体上表现为较强富集,Pt、Pd富集与亏损并存.其标准化模式曲线较分散,曲线向右倾斜,IPGE与PPGE分馏较强,多数样品呈现Pt负异常.这些特点与中国罗布莎、希腊、南斯拉夫以及塞浦路斯等世界各地蛇绿岩中的铬铁矿床类似.铬铁矿石的PGE含量与相应样品的BMS含量无相关性,PGE不是主要赋存于BMs内.推测PGE主要以PGM的形式存在,具体矿物名称有待进一步发现与研究.     

Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt–rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35–85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15–20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.     

梅田矿区岩浆热变煤中微量元素分布特征

梅田矿区主采煤层－龙谭组１２号煤层主要是受矿区北部骑田岭花岗岩侵入体影响的典型岩浆热液变质无烟煤,煤的变质指标如挥发分和镜质组反射率Ｒ＾０（ｍａｘ）等与岩体有密切关系。本次研究按距离岩体由近而远采集了一系列煤样,运用中子活化（ＩＮＡＡ）和微区分析等测试手段研究了样品中近４０种微量元素的分布特征,结果表明：（１）Ａｓ,ｄ,Ｈｇ,Ｐｂ,Ｓｅ等对人体有毒元素在煤中的质量分数较地壳克拉克值高数倍至数十倍,     

Late veins of C3 carbonatite intrusion from Jacupiranga complex (Southern Brazil): fluid and melt inclusions and mineralogy

At Jacupiranga (Brazil) the latest manifestations of C3 carbonatite are veins and impregnations of s?vite composition containing carbonates, Fe-oxides, apatite, phlogopite, olivine, clinohumite, pyrochlore, zirconolite. Apatite contains fluid inclusions l.s. of two types: (i) two-phase fluid inclusions of complex composition (H₂O±CO₂±NaCl±KCl±NaHCO₃) with 17–50?vol.% of gas bubble, (ii) multi-phase melt inclusions of two types, carbonate-H₂O rich and silicate-carbonate-H₂O rich with very high homogenization temperatures up to 584°C and up to 1194°C, respectively. The coeval origin of fluid and melt inclusions allows to calculate the trapping pressures which indicate approximate depths of 30 to 60?km for the origin of the veins. Carbonatite veins contain Ba-poor tetra-ferriphlogopite of extreme composition which extends the compositional trend of the micas of Jacupiranga. The occurrence of zirconolite, Nb-rich pyrochlore and Ta-rich U-pyrochlore is mutually exclusive and related, probably, to the variation of fluorine content in the magmatic fluids.     

Contrasting platinum-group mineral assemblages from chromitites of the Nizhny Tagil and Guli massifs (Russia): Implications for composition, sources and age

The Nizhny Tagil and Guli clinopyroxenite-dunite massifs, located in the Middle Urals and Maimecha-Kotui Province, respectively, are associated with world-class platinum-group elements (PGE) placer deposits. Both massifs contain small bodies of schlieren to massive chromitite associated with dunite. The predominance of Pt-Fe alloys at Nizhny Tagil is consistnt with the whole-rock “M”-shaped mantle-normalized PGE pattern of the chromitite. In contrast, the preponderance of laurite and Os-Ir alloys at Guli is consistent with a negatively sloped PGE pattern, the latter being characteristic of ophiolite-type podiform chromitites. The ‘unradiogenic’ ¹⁸⁷Os/¹⁸⁸Os values obtained for both platinum-group minerals (PGM) and chromitite are indicative of a common near-to-chondritic source for the PGE and implies that the osmium isotope budget of chromitite is largely controlled by laurite and Os-rich alloy. Average model ¹⁸⁷Os/¹⁸⁸Os ages calculated for the Nizhny Tagil and Guli massifs correspond to the late Riphean (e.g., 862 ± 48 Ma and 616 ± 8 Ma, respectively). The compositional and isotope-geochemical results provide new constraints on the temporal evolution of ultramafic rocks of the Uralian Platinum Belt and northern segment of the Siberian Platform.     

Geochemical evolution of laterites from two areas of the semiarid region in Bahia State, Brazil

Lateritic covers developed during the Pleistocene in two distinct areas (Gentio do Ouro District and Sento Sé) of the State of Bahia, Brazil, have been investigated geochemically. Samples of the bedrock (gabbros, from the Gentio do Ouro District, and silicate facies iron formation and associated iron-rich banded carbonates, from the Sento Sé Area) and the lateritic products (latosols, cuirasses and pisolites) were analyzed for major and selected trace elements (Co, Cu, Cr, Ni, V, Pb, Ba, Au and ree). The purpose of the research was to study the geochemical evolution of the laterites and the relationship of weathering processes to secondary gold mineralization.Four types of element mobilities are recognized, defined by progressive depletion (type A) or enrichment (type C) in the sequence latosol-cuirasse-pisolite or by extreme enrichment (type B) or depletion (type D) in the cuirasse. Cr and V have similar behavior (type C), whatever the type of parent rock: however, when the original rocks are gabbros, Co, Cu and Ni demonstrate behavior A, B and A, respectively. Also Ti has the same type of mobility (type A), whatever the type of parent rock.The concentrations of some residual elements, e.g., Ti, V, Mn, Fe and Au, are inherited from the original rocks, that is, they exhibit higher values in the lateritic materials on parent rocks in which their contents are high.The enrichment of ree in the lateritic materials on iron-rich carbonates is attributed to relatively basic and reducing conditions at depth. This environment allowed the ions of these elements (Ce, Sm, Eu and Yb in the reduced ionic form) to stay in solution along with Fe²⁺. At shallow depht, under oxidizing conditions, the iron oxyhydroxides adsorbed the ree ions. This adsorption was more effective for La, Ce and Yb ions.The ree contents in the lateritic products from gabbros indicate a depletion of hree plus Eu, Nd and Sm are also depleted in cuirasses and Ce is depleted in pisolites. The loss of these ree and the slight concentration of lree in the red latosol of the Gentio do Ouro District are attributed to partial leaching of ree by a relatively acidic solution. The adsorption of ree by iron oxyhydroxides was less intense here than in the weathering of the iron-rich carbonates at Sento Sé Area. It was also more effective for La and Ce (ions). This type of weathering solution may be generated from oxidation of the accessory pyrite present in the gabbros of the Gentio do Ouro District. It may dissolve primary gold which then precipitates on encountering basic conditions at depth.     

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P–T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg#_cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.     

Metasomatic processes in the Lukkulaisvaara layered intrusion,Russia, and formation of low-sulfide PGE mineralization

The Nadezhda ore-controlling structure and related rocks of the Lukkulaisvaara layered intrusion from the Oulanka plutonic group have been studied. Tensile deformation arising at the cooling stage at interfaces of rocks dissimilar in composition with different compressibility coefficients controlled the intensity of secondary processes and the high concentration of sulfide and platinum group minerals (PGM) along the perimeter of the fine-grained gabbronorite body in contact zones with country rocks. These zones were specific geochemical barriers. Magmas of supplementary injections could have been one of the sources of fluid affecting the rocks. Fe-Ni-Cu sulfides and PGM crystallized within the temperature range T = 800-350°C. Regional metamorphism did not exceed conditions of greenschist facies. Results of modeling of metasomatic processes indicate that decompression played the leading role in formation of the major secondary mineral assemblages: Czo-An₁₀-Chl-Tr(Act) and Qtz-An₇₀-Amph-Bt. The decreasing pressure under isothermal conditions resulted in a rise in solubility of metals in chloride solution and their transport into dilatable zones. The cooling model leading to the enrichment of rocks in potassium and then to its leaching is realized more seldom, resulting in a Qtz-Ms-Chl-Act-Czo mineral assemblage and eventually in Qtz-Chl veins. The change of P and T during pluton cooling resulted in changing of the direction of metasomatic processes and telescoping of different superimposed mineral assemblages: Amph(±Act)-Bt ± Chl 1 ± Qtz as an early one and Qtz-Czo-Chl₂-Ab-Ms as a late one. The model mineral assemblages that match natural counterparts are as follows: (1) Qtz-Pl-Amph-Bt and Qtz-Pl-Chl-Bt(Ms) and (2) Tr(Act)-Chl-An₁₀-Czo, Qtz-An₁₀-Chl-Ms(Bt), and Qtz-Czo-Chl-An₁₀-Ms. At the same time, the An_40–50-Amph-Bt, Qtz-An_40–50-Amph-Bt, Qtz-An_10–20-Chl-Bt(Ms), An_30–40-Chl-Tr(Act)-Bt, An₃₀-Chl-Tr(Act), and An₃₀-Tr (Act)-Bt assemblages are suggested to be intermediate and reflect the direction of metasomatic processes.     

Physicochemical parameters of magmatic and hydrothermal processes at the Yaman-Kasy massive sulfide deposit, the southern Urals

Melt and fluid inclusions in minerals have been studied and physicochemical parameters of magmatic processes and hydrothermal systems estimated at the Yaman-Kasy copper massive sulfide deposit in the southern Urals. It was established that relatively low-temperature (910–945°C) rhyodacitic melts belonging to the tholeiitic series and containing 2.7–5.2 wt % water participated in the formation of the igneous complexes that host the Yaman-Kasy deposit. As follows from ion microprobe results, these silicic magmas had a primitive character. In the distribution of trace elements, including REE, the rhyodacites are closer to basaltic rather than silicic volcanic rocks, and they are distinguished in this respect from the igneous rocks from other massive sulfide deposits of the Urals and the Rudny Altai. Two types of solutions actively took part in the formation of hydrothermal systems: (1) solutions with a moderate salinity (5–10 wt % dissolved salts) and (2) solutions with a low salinity (a value close to that of seawater or even lower). Concentrated fluids with more than 11.5 wt % dissolved salts were much less abundant. Hydrothermal solutions heated to 130–160, 160–270, or occasionally 280–310°C predominated in ore formation. The sequence of mineral-forming processes at the Yaman-Kasy deposit is demonstrated. Mineral assemblages were formed with an inversion of the parameters characterizing ore-forming solutions. An increase in the temperature and salinity of solutions at the early stages was followed by a decrease at the final stages. The evolution of the hydrothermal system at the Yaman-Kasy deposit has much in common with the parameters of black smokers in the present-day Pacific backarc basins.     

Mineralogy and geochemistry of platinum-group elements in the Aguablanca Ni-Cu deposit (SW Spain)

Summary The Aguablanca Ni-Cu-(PGE) magmatic sulphide deposit is associated with a magmatic breccia located in the northern part of the Aguablanca gabbro (SW, Iberia). Three types of ores are present: semi-massive, disseminated, and chalcopyrite-rich veined ore. The principal ore minerals are pyrrhotite, pentlandite and chalcopyrite. A relatively abundant platinum-group mineral (PGM) assemblage is present and includes merenskyite, melonite, michenerite, moncheite and sperrylite. Moreover, concentrations of base and precious metals and micro-PIXE analyses were obtained for the three ore-types. The mineralogy and the mantle-normalised chalcophile element profiles strongly suggest that semi-massive ore represents mss crystallisation, whereas the disseminated ore represents an unfractionated sulphide liquid and the chalcopyrite-rich veined ore a Cu-rich sulphide liquid. Palladium-bearing minerals occur commonly enclosed within sulphides, indicating a magmatic origin rather than hydrothermal. The occurrences and the composition of these minerals suggest that Pd was initially dissolved in the sulphides and subsequently exsolved at low temperatures to form bismutotellurides. Negative Pt and Au anomalies in the mantle-normalised chalcophile element profiles, a lack of Cu-S correlation and textural observations (such as sperrylite losing its euhedral shape when in contact with altered minerals) suggest partial remobilisation of Pt, Au and Cu by postmagmatic hydrothermal fluids after the sulphide crystallisation. Authors’ addresses: R. Pi?a, L. Ortega, R. Lunar, Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, ES-28040 Madrid, Spain; F. Gervilla, Facultad de Ciencias, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, Avda. Fuentenueva, s/n, ES-18002 Granada, Spain     

陕西太白金矿含金角砾岩中铂族元素特征

采用硫镍火法试金(NiS-FA)结合电感耦合等离子质谱(ICP-MS)分析了太白金矿硫化物和含金角砾岩中铂族元素的含量,结果显示,与秦岭地区八卦庙相比铂族元素含量较高,而低于原始地幔,其中铂(Pt)、钯(Pd)、钌(Ru)富集,并结合前人研究资料对铂族元素的来源和迁移机制进行探讨。铂族元素可能受深源的影响,IPGE(Ir、Os、Ru)可能主要以硫化物形式存在而PPGE(Rh、Pt、Pd)可能主要以单质存在。