铂族元素矿物共生组合(英文)

由于铂族元素能有效地降低汽车尾气的污染 ,其需求量日益增加 ,对铂族元素矿床的寻找已是当务之急。着重从矿物矿床学角度对铂族元素的矿物共生特点进行了探讨。铂族元素可呈独立矿床产出 ,主要产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中。铂族元素也伴生于铜镍矿床中 ,该类铜镍矿床主要与苏长岩侵入体、溢流玄武岩及科马提岩有关。产于基性超基性层状侵入体中的铂族矿物有铂钯硫化物、铂铁合金、钌硫化物、铑硫化物、铂钯碲化物、钯砷化物及钯的合金。这些铂族矿物可与硫化物矿物共生 ,也可与硅酸盐矿物共生 ,还可与铬铁矿及其他氧化物矿物共生。产于蛇绿岩套中的铂族矿物主要是钌铱锇的矿物 ,而铂钯铑的矿物则较少出现 ,这些铂族矿物可呈合金、硫化物、硫砷化物以及砷化物 4种形式出现。产于阿拉斯加式侵入体中的铂族矿物主要有铂铁合金、锑铂矿、硫铂矿、砷铂矿、硫锇矿及马兰矿等少数几种 ,其中铂铁合金与铬铁矿及与其同时结晶的高温硅酸盐矿物共生 ,而其他的铂族矿物则与后来的变质作用及蛇纹岩化作用中形成的多金属硫化物及砷化物共生。产于铜镍矿床中的铂族矿物主要是铂和钯的矿物。产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中的铂族矿物的共同特点是它们均与铬铁矿?     

A comparison of selected Precambrian Brazilian chromitites: Chromite, PGE-PGM, and Re/Os as parental source indicators

Mineralogical and geochemical studies were carried out in chromitites belonging to the mafic–ultramafic bodies of Niquelândia, Luanga, and Campo Formoso, which are, respectively, included in the Goiás Massif and the Amazon and São Francisco cratons. The main platinum-group minerals (PGM) included or associated with chromite grains are laurite in Niquelândia and Campo Formoso and sperrylite and braggite in Luanga. The most common primary base metal sulfides (BMS) are pentlandite, chalcopyrite, and minor pyrrhotite. Also present are base metal alloys (BMA), such as awaruite, and the BMS millerite, pyrite, and copper as secondary mineral phases linked to later alteration process. The Luanga chromites display the lowest Cr₂O₃/Al₂O₃ and Cr₂O₃/FeO_t ratios. The chondrite-normalized profiles are strongly enriched in the platinum PGE subgroup (PPGE, Pt, Pd, Rh). The average Pd/Ir ratio (24.2) and ¹⁸⁷Os/¹⁸⁸Os values (0.17869–0.18584) are very high. Niquelândia chromites have higher Cr₂O₃/Al₂O₃ and Cr₂O₃/FeO_t ratios than Luanga. Its PGE contents are low and chondrite-normalized profiles depleted, mainly in the PPGE subgroup. The average Pd/Ir ratio (0.45) and ¹⁸⁷Os/¹⁸⁸Os values (0.12598–0.12777) are low. Campo Formoso chromites have the highest Cr₂O₃/Al₂O₃ and Cr₂O₃/FeO_t ratios; its average Pd/Ir ratio (0.72) and chondrite-normalized profiles (except the pronounced Ru spike) are closer to those of Niquelândia. The remarkable differences in terms of chromite bulk-composition, PGE contents and patterns, Pd/Ir ratios, and ¹⁸⁷Os/¹⁸⁸Os values associated with probable distinctions in the inferred geochemical compositions of the respective parental magmas indicate that the Luanga and Niquelândia complexes originated from distinct parental sources. Geochemical and isotopic features indicate that Luanga chromitites and associated rocks are consistent with a parental magma, either originated from an enriched mantle reservoir or strongly contaminated, whereas Niquelândia derives from a depleted mantle source, which displays a slightly positive Os anomaly.     

Enrichment of PGE through interaction of evolved boninitic magmas with early formed cumulates in a gabbro–breccia zone of the Mesoarchean Nuasahi massif (eastern India)

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 10³–3.2 × 10³) are lower than mantle (6.2 × 10³) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 10³; komatiite: Cu/Pd ratio ∼8 × 10³) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.     

Platinum-group-mineral inclusions in chromite from the bird river sill,Manitoba

Platinum-group minerals (PGM) have been identified as inclusions in chromite from the Bird River Sill, Manitoba. The inclusions are small (<20 microns) and are commonly euhedral. The PGM inclusions are (Ru, Os, Ir) S₂, laurite, and (Os, Ir, Ru alloy), rutheniridosmine: Laurites contain up to 2.99 wt. % palladium. Arsenic content is negligible and no platinum or rhodium has been detected. One platinum-group element alloy contains 0.96 wt. % rhodium but neither platinum nor palladium has been detected. Laurite inclusions in chromite from the ultramafic zone record two compositional trends; first increasing and then decreasing Ru/(Ru+Os+Ir) up section. PGM inclusions and other solid inclusions occur as discrete phases in chromite and are part of the chromite precipitation event. Increasing oxygen fugacity by wall rock assimilation or new magma injection initiates chromite precipitation, locally increasing the sulphur content of the magma to convert PGE alloys to sulphides.     

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     

Platinum-group minerals from the Jinbaoshan Pd–Pt deposit,SW China: evidence for magmatic origin and hydrothermal alteration

The Jinbaoshan Pt–Pd deposit in Yunnan, SW China, is hosted in a wehrlite body, which is a member of the Permian (∼260 Ma) Emeishan Large Igneous Province (ELIP). The deposit is reported to contain one million tonnes of Pt–Pd ore grading 0.21% Ni and 0.16% Cu with 3.0 g/t (Pd + Pt). Platinum-group minerals (PGM) mostly are ∼10 μm in diameter, and are commonly Te-, Sn- and As-bearing, including moncheite (PtTe₂), atokite (Pd₃Sn), kotulskite (PdTe), sperrylite (PtAs₂), irarsite (IrAsS), cooperite (PtS), sudburyite (PdSb), and Pt–Fe alloy. Primary rock-forming minerals are olivine and clinopyroxene, with clinopyroxene forming anhedral poikilitic crystals surrounding olivine. Primary chromite occurs either as euhedral grains enclosed within olivine or as an interstitial phase to the olivine. However, the intrusion has undergone extensive hydrothermal alteration. Most olivine grains have been altered to serpentine, and interstitial clinopyroxene is often altered to actinolite/tremolite and locally biotite. Interstitial chromite grains are either partially or totally replaced by secondary magnetite. Base-metal sulfides (BMS), such as pentlandite and chalcopyrite, are usually interstitial to the altered olivine. PGM are located with the BMS and are therefore also interstitial to the serpentinized olivine grains, occurring within altered interstitial clinopyroxene and chromite, or along the edges of these minerals, which predominantly altered to actinolite/tremolite, serpentine and magnetite. Hydrothermal fluids were responsible for the release of the platinum-group elements (PGE) from the BMS to precipitate the PGM at low temperature during pervasive alteration. A sequence of alteration of the PGM has been recognized. Initially moncheite and atokite have been corroded and recrystallized during the formation of actinolite/tremolite, and then, cooperite and moncheite were altered to Pt–Fe alloy where they are in contact with serpentine. Sudburyite occurs in veins indicating late Pd mobility. However, textural evidence shows that the PGM are still in close proximity to the BMS. They occur in PGE-rich layers located at specific igneous horizons in the intrusion, suggesting that PGE were originally magmatic concentrations that, within a PGE-rich horizon, crystallized with BMS late in the olivine/clinopyroxene crystallization sequence and have not been significantly transported during serpentinization and alteration.     

Dispersion Pattern of PGE and Ag in Chromiferous Ultramafic Suite of Rocks in Sukinda Valley,India

Potential chromite ore deposits of India are situated in Sukinda, Odisha, which may also be considered as a potential resource for platinum group elements (PGEs). This paper reports on PGE geochemistry in twenty six samples covering chromite ores, chromitites and associated ultramafic rocks of the Sukinda ultramafic complex. Platinum group element contents range from 213 to 487 ppb in the chromite ore body, from 63 to 538 ppb in rocks that have chromite dendrites or dissemination and from 38 to 389 ppb in associated olivine–peridotite, serpentinite, pyroxenite and brecciated rocks. The PGEs are divided into two sub‐groups: IPGE (Ir, Os, and Ru) and PPGE (Pd, Pt, and Rh) based on their chemical behaviour. The IPGE and PPGE in these three litho‐members show a contrasting relationship e.g. average IPGE content decreases from chromite to chromitite and associated rocks while PPGE increases in the same order. Appreciable Ag in chromitite (270–842 ppb) is recorded. Positive correlation between IPGE with Cr₂O₃ and with Al₂O₃ is observed while these are negatively correlated with MgO. Covariant relationships between Au and Mg in rocks devoid of chromite and between Ag and Fe in chromitite sample are observed. Chromite in all seams and some chromitite samples exhibit an IPGE‐enriched chondrite normalized pattern while PPGE are highly fractionated and show a steep negative slope, thereby indicating that PGE in the parental melt fractionates and IPGE‐compatible elements prefer to settle with chromite. The rocks devoid of chromite and rocks containing accessory chromite exhibit a nearly flat pattern in chondrite‐normalized PGE plots and this suggests a limited fractionation of PGE in these rocks. Variation in the distribution pattern of PGE and Ag in three typical litho‐members of the Sukinda Valley may be related to multiple intrusion of ultramafic magma, containing variable volume percentage of chromite.     

Solid inclusions in chrome-spinels and platinum group element concentrations from the hochgrössen and kraubath ultramafic massifs (Austria)

A great variety of platinum group mineral, sulfide and silicate inclusions in chrome spinel from Hochgrössen and Kraubath ultramafic massifs, and platinum group element contents of three different rock types have been investigated. Both ultramafic massifs are tectonically isolated bodies, variably serpentinized and metamorphosed (greenschist to lower amphibolite facies), and show ophiolitic geochemical affinities. The chromite from massive chromitites and disseminated in serpentinized dunites and serpentinites, exhibits compositional zonation as the result of alteration during serpentinization and metamorphism. Three distinctive alteration stages are indicated in the chrome-spinels from the Hochgrössen, whereas alteration is less significant in chromites from Kraubath: The core of chrome spinel represents the least altered part, surrounded by an inner rim characterized by slight compositional differences in Cr, Mn, Fe²⁺ and Al with respect to the core. The outer rim is formed by ferritchromite with a sharp boundary to the inner rim and shows a significant decrease of Al, Mg, Cr and increase of Fe²⁺, Fe³⁺ and Ni compared to the core. Two different groups of inclusions in chrome-spinel are present: the first group occurs within the chromite core, and comprises olivine, orthopyroxene, amphibole, sulfides and platinum-group minerals, i.e. dominated by Ru-Os-Ir-sulfides. The second group is formed by chlorite, serpentine, galena, pyrite, arsenopyrite, Pt-Pd-Rh-dominated sulfarsenides and sperrylite. In particular the abundance of Pt-Pd-Rh-sulfarsenides and arsenides is typical of both ultramafic massifs and is very unusual for chromitites from ophiolites. Morphology, paragenesis and chemical composition indicate a different origin for these two groups of inclusions. The first group is intimately related to the crystallisation of the chromite host. The second group of inclusions clearly displays a secondary formation during serpentinization and metamorphism, closely related to the alteration of chrome-spinel and the development of ferritchromite. The distribution patterns of the platinum group elements from massive chromitites, disseminated chrome-spinel bearing serpentinites and serpentinites exhibit variable enrichment of Rh, Pt and Pd, Rh, Pt for the Hochgrössen and Kraubath massifs, respectively. These results are in accordance with the occurrence and distribution of platinum-group mineral phases. A remobilisation of Pt, Pd, and Rh, together with Ni, Cu and possibly Fe as bisulfide and/or hydroxide complexes and deposition of metals by the reaction of the metal bearing hydrothermal fluid with chromite is proposed.     

Platinum-group mineral assemblage of the Prizhimny Creek (Koryak Highland)

In the alluvial deposits of the Prizhlimny Creek (southern part of the Koryak Highland), grains of platinum-group minerals are found along with gold. We have established that the grains are native platinum (Pt, Fe) containing Cu (up to 5 wt.%), Os (up to 8 wt.%), and Rh (up to 2 wt.%). Inclusions in the platinum are native osmium (the content of Ir impurity reaches 12 wt.%, the average content being 0.2–4 wt.%), an unnamed intermetallic compound of composition PtRh, sulfides and arsenides of PGE (cooperite, laurite, malanite, cuproiridsite, cuprorhodsite, sperrylite, hollingworthite, unnamed compounds PdS, (Ir,Ru)S₂, (Ir,Pt)S₂, Cu, and Fe (bornite, chalcopyrite), chromite, and Cr-magnetite. Replacement of native-osmium crystals by compound IrO₂ is described. It has been established that this compound formed during oxidation accompanied by the replacement of isoferroplatinum by native platinum. The data obtained agree with the results of study of platinum-group mineral assemblages from placers localized in weakly eroded Ural–Alaskan-type massifs whose apical parts formed under high oxygen activity conditions. Clinopyroxenites of the Prizhimny massif are considered to be the potential source of PGE.     

Platinoids in the Kaalamo Differentiated Massif in the Northern Ladoga Region,Karelia, Russia

The Kaalamo massif is located in the Northern Ladoga region, Karelia, on the extension of the Kotalahti Belt of Ni-bearing ultramafic intrusions in Finland. The massif, 1.89 Ga in age, is differentiated from pyroxenite to diorite. Nickel–copper sulfide mineralization with platinoids is related to the pyroxenite phase. The ore consists of two mineral types: (i) pentlandite–chalcopyrite–pyrrhotite and (ii) chalcopyrite, both enriched in PGE. Pd and Pt bismuthotellurides, as well as Pd and Pt tellurobismuthides, are represented by the following mineral species: kotulskite, sobolevskite, merenskyite, michenerite, moncheite, keithconnite, telluropalladinite; Pt and Pd sulfides comprise vysotskite, cooperite, braggite, palladium pentlandite, and some other rare phases. High-palladium minerals are contained in pentlandite–chalcopyrite–pyrrhotite ore. Native gold intergrown with kotulskite commonly contains microinclusions (1–3 μm) of Pd stannides: paolovite and atokite. Ore with 20–60% copper sulfides (0.2–6.0% Cu) contains 5.1–6.6 gpt PGE and up to 0.13–2.3 gpt Au. Pd minerals, arsenides and sulfoarsenides of Pt, Rh, Ir, Os, and Ru are identified as well. These are sperrylite, ruthenium platarsite, hollingworthite, and irarsite; silvery gold and paolovite have also been noted. All these minerals have been revealed in the massif for the first time. The paper also presents data on the compositions of 25 PGE minerals (PGM) from Kaalamo ores.     

攀西裂谷地区层状镁铁岩的PGE矿化作用

攀西裂谷位于四川西部,裂谷经历了元古宙和海西期二次地幔柱活动,形成多处穹窿构造和层状镁铁质岩体的侵入。后一期的层状岩体赋存著名的超大型钒钛磁铁矿床。中国和南非合作研究认为,层状岩体PGE矿化应进一步研究。以新街岩体为代表,经钻探工程建立了岩体剖面;岩石学、矿物学和地球化学研究证实,岩体有三个岩浆旋回和许多韵律层,层厚仅2~3cm。自上而下岩相为辉长岩、橄辉岩、辉石岩和橄榄岩,造岩矿物为贵橄榄石、普通辉石、钛普通辉石和中长石。岩体下部旋回,硫化物较富集,多在高镁质岩相。硅酸盐、氧化物和硫化物三系列矿物共生而不混熔。硫化物呈浸染状,主要有三层,产在橄榄岩、辉石岩和下辉长岩内。铂族矿物有砷铂矿、自然铂、硫锇矿、铋碲钯矿、碲铋矿、碲银矿、自然银等。PGE富集可能有三个阶段:岩浆早期,岩浆中"S"不饱和,PGE易进入硅酸盐;岩浆晚期"S"逸度增高,硫化物富集,为PGE富集阶段;热液阶段PGE再分配富集。PGE和Ni、Cu、S为正相关关系,和Fe、Ti相辅相成,无明显关系。岩石中PGE背景值为(0.166~0.411)×10-6,PGE矿化体的品位变化较大,为(0.94~0.976)×10-6。有的钻孔样品Pt+Pd含量大于1×10-6,可做进一步找矿的依据。     

Platinum group element mineralization of the Svetly Bor and Veresovy Bor clinopyroxenite–dunite massifs,Middle Urals,Russia

The new data for the geology and mineralogy of the platinum group element (PGE) mineralization related to the chromite–platinum ore zones within the dunite of the Svetly Bor and Veresovy Bor massifs in the Middle Urals are discussed. The geological setting of the chromite–platinum ore zones, their platinum content, compositional and morphological features of the platinum group minerals (PGM) are compared to those within the Nizhny Tagil massif, the world standard of the zonal complexes in the Platinum Ural belt. The chromite–platinum orebodies are spatially related to the contacts between differently granular dunites. Majority of PGM are formed by Pt–Fe alloys that are close in terms of stoichiometry to isoferroplatinum (Pt₃Fe), and associated with Os–Ir alloys, Ru–Os and Ir–Rh sulfides, and Ir–Rh thiospinels of the cuproiridsite–cuprorhodsite–ferrorhodsite solid solution. The tetraferroplatinum (PtFe)–tulameenite (PtFe_0.5Cu_0.5) solid solution and Pt–Cu alloys belong to the later PGM assemblage. The established features of the chromite–platinum ore zones testify to the highly probable identification of the PGE mineralization within the dunite of the Svetly Bor and Vesesovy Bor massifs and could be used in prospecting and exploration for platinum.     

Geochemical Characteristics and Metallogenesis of the Qingkuangshan Ni‐Cu‐PGE Mineralized Mafic‐Ultramafic Intrusion in Huili County,Sichuan Province,SW China

The Qingkuangshan Ni-Cu-PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni-Cu-PGE deposits in the Emeishan Large Igneous Province (ELIP). The ore-bearing intrusion is a mafic-ultramafic body. This paper reports major elements, trace elements and platinum-group elements in different types of rocks and sulfide-mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore-forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle-normalized patterns of Ni-Cu-PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni-Cu-PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni-Cu-PGE deposits in the ELIP, the Qingkuangshan Ni-Cu-PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni-Cu deposit, and has similar PGE contents to the Yangliuping Ni-Cu-PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide-bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.     

Chromitites from the Luanga Complex,Carajás,Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75 Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150 meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr + Al + Fe³⁺)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20 cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al₂O₃ activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe³⁺ and Fe²⁺ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe²⁺) cations and minor to none exchange between trivalent cations (Cr³⁺, Al³⁺ and Fe³⁺). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe²⁺ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100 Mg/(Mg + Fe²⁺)) and Al# (100Al/(Cr + Al + Fe³⁺)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.     

Platinum-group minerals and tellurides from the PGE-bearing Xinjie layered intrusion in the Emeishan Large Igneous Province, SW China

The Xinjie layered intrusion is one of a number of major ultramafic-mafic bodies hosting Fe-Ti-V deposits and Cu-Ni-PGE sulfide deposits in the Pan-Xi (Panzhihua-Xichang) area of the Sichuan Province, SW China. The Xinjie ultramafic-mafic layered intrusion, genetically related to the Permian plume-related Emeishan flood basalts, consists of three lithological cycles, each representing a sequence from ultramafic to mafic-felsic composition. The basal part of the intrusion is composed of three lithological units, namely, the Marginal Unit (MU), Peridotite Unit (PeU) and Pyroxenite Unit (PyU). In the present study, three major PGE-mineralised Cu-Ni sulfide layers were discovered within the Marginal and Pyroxenite Units. The major base-metal sulfides (BMS) comprise chalcopyrite, pyrrhotite, and pentlandite. Detailed microscopic and microprobe analyses revealed the presence of the sperrylite and Pd-Pt-Bi-Te minerals (merenskyite, moncheite, and michenerite). These PGMs are commonly associated with the BMS, or magnetite coexisting with BMS in the PGE-enriched layers. The 1:1 substitution between Pt and Pd, as well as between Te and Bi, confirms the complete solid-solution series between moncheite and merenskyite. The textural association of the PGMs with BMS and Fe-Ti oxides (magnetite) suggests that the PGMs may have crystallised slightly later than the hosting magnetite and BMS. The formation of magnetite may have played an important role in producing the sulfur-saturated melt and the PGEs thus concentrated in the sulfide liquid during the crystallisation history. It is therefore suggested that the Cu-Ni-PGE-bearing layers in the basal part of the Xinjie intrusion were generated by magma evolution processes.     

Genesis of the Jinchuan PGE deposit, China: evidence from fluid inclusions, mineralogy and geochemistry of precious elements

Summary The Jinchuan deposit is a platinum group element (PGE)-rich sulfide deposit in China. Drilling and surface sampling show that three categories of platinum group element (PGE) mineralization occur; type I formed at magmatic temperatures, type II occurs in hydrothermally altered zones of the intrusion, and type III in sheared dunite and lherzolite. All ore types were analyzed for Os, Ir, Ru, Rh, Pd, Pt and Au, as well as for Cu, Ni, Co and S. Type I ore has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios of <7 and relatively flat chondrite-normalized noble metal patterns; the platinum group minerals (PGM) are dominated by sperrylite and moncheite associated with chalcopyrite, pyrrhotite and pentlandite. Type II has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 40 to 330 and noble metal distribution patterns with a positive slope; the most common PGM are sperrylite and Pd bismuthotelluride phases concentrated mostly at the margins of base metal sulfides. Type III ores have the highest (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 240 to 710; the most abundant PGM are sperrylite and phases of the Pt–Pd–Te–Bi–As–Cl system. It is concluded that the Jinchuan deposit formed as a result of primary magmatic crystallization followed by hydrothermal remobilization, transport, and deposition of the PGE.     

Controls on disseminated PGE–Cu–Ni sulfide mineralization within the Rietfontein deposit,Eastern Limb,Bushveld Complex,South Africa: Implications for the formation of contact-type magmatic sulfide deposits

The Rietfontein platinum group element (PGE)–Cu–Ni sulfide deposit of the Eastern Limb of the Bushveld Complex hosts disseminated contact-style mineralization that is similar to other economic magmatic sulfide deposits in marginal settings within the complex. The mineralization at Rietfontein consists of disseminated PGE-bearing base metal sulfides that are preferentially located at the contact between a distinct package of marginal norites overlain by a thick heterogeneous unit dominated by gabbronorites with lesser norites and ultramafic rocks. Down-hole composite data and metal scatterplots indicate that the PGE correlate well with Ni, Cu and S and that only minor metal remobilization has taken place within the basal norite sequence. Plots of (Nb/Th)_PM vs. (Th/Yb)_PM indicate that the melts that formed the Rietfontein intrusive sequence were strongly crustally contaminated prior to emplacement at Rietfontein, whereas inverse relationships between PGE tenors and S/Se ratios indicate that these magmas assimilated crustal S, causing S-saturation and the formation of immiscible sulfides under high R-factor conditions that generated high PGE tenor sulfides. Reverse zoning of cumulus minerals at Rietfontein suggests that fresh primitive melts were introduced to a partially fractionated staging chamber. The introduction of new magmas into the chamber caused overpressure and the forced evacuation of the contents of the chamber, leading to the emplacement of the existing magmas within the staging chamber at Rietfontein in two separate pulses. The first pulse of magma contained late-formed cumulus phases, including low Mg# orthopyroxene and plagioclase, was emplaced between footwall unreactive and S-poor Pretoria Group quartzites and a hangingwall sequence of Rooiberg Group felsites, and was rapidly chilled to form the basal norite sequence at Rietfontein. The second pulse of magma contained early formed cumulus phases, including olivine, chromite, and high Mg# orthopyroxene, and was emplaced above the chilled norite sequence as a crystal mush to form gabbronorites and ultramafic rocks. This second pulse of magma also contained PGE-bearing base metal sulfides that accumulated at the contact between this second batch of magma and the already chilled basal norite sequence. The formation of Platreef-type mineralization outside of the Northern Limb of the Bushveld Complex confirms there are a number of areas within the Bushveld Complex that are prospective for this style of mineralization.     

Towards a model for the in situ origin of PGE reefs in layered intrusions: insights from chromitite seams of the Rum Eastern Layered Intrusion, Scotland

The current debate on the origin of platinum-group element (PGE) reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt/fluids migrating upward from the underlying cumulate pile. Here, we show that PGE-rich chromitite seams of the Rum Eastern Layered Intrusion provide evidence for an alternative origin of such deposits in layered intrusions. These laterally extensive 2-mm-thick chromitite seams occur at the bases of several cyclic mafic–ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal–liquid interface. This follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries, even where these are vertically oriented or overhanging. High concentrations of PGE (up to 2–3 ppm Pd + Pt) are controlled by fine-grained base-metal sulphides, which are closely associated with chromitite seams. The following sequence of events explains the origin of the PGE-rich chromitite seams: (a) emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate, followed by in situ growth of chromite against the base, (b) precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation, (c) the scavenging of PGE by sulphide droplets from fresh magma continuously brought towards the base by convection. Since the rate of magma convection is 10⁵–10⁷ times higher than that of the solidification (km/year to km/day versus 0.5–1.0 cm/year), the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume. As a result, the sulphide-bearing rocks are able to reach economic concentrations of PGE (several ppm). We tentatively suggest that the basic principles of our model may be used to explain the origin of PGE-rich chromitites and classical PGE reefs in other layered mafic–ultramafic intrusions.     

磁铁矿中微量元素和铂族元素的组成特征

文章分析了磁铁矿中铂族元素(PGE)含量的特征,发现磁铁矿中铂族元素富集为10-9级,主要为PPGE(即Pd,Pt,Rh)。富集PPGE的主要原因是其在热液活动的晚期阶段可以被轻微的再分配和富集,其含量的高低严格受岩浆结晶过程中氧逸度的控制。磁铁矿的结晶程度直接影响岩浆体系的氧逸度,控制了PGE的含量分布,进而影响成矿。     

Platinum-group element geochemistry of the Zhuqing Fe-Ti-V oxide ore-bearing mafic intrusions in western Yangtze Block,SW China: control of platinum-group elements by magnetite

Platinum-group element (PGE) geochemistry combined with elemental geochemistry and magnetite compositions are reported for the Mesoproterozoic Zhuqing Fe–Ti–V oxide ore-bearing mafic intrusions in the western Yangtze Block, SW China. All the Zhuqing gabbros display extremely low concentrations of chalcophile elements and PGEs. The oxide-rich gabbros contain relatively higher contents of Cr, Ni, Ir, Ru, Rh, and lower contents of Pt and Pd than the oxide-poor gabbros. The abundances of whole-rock concentrations of Ni, Ir, Ru, and Rh correlate well with V contents in the Zhuqing gabbros, implying that the distributions of these elements are controlled by magnetite. The fractionation between Ir–Ru–Rh and Pt–Pd in the Zhuqing gabbros is mainly attributed to fractional crystallization of chromite and magnetite, whereas Ru anomalies are mainly due to variable degrees of compatibility of PGE in magnetite. The order of relative incompatibility of PGEs is calculated to be Pd?<?Pt?<?Rh?<?Ir?<?Ru. The very low PGE contents and Cu/Zr ratios and high Cu/Pd ratios suggest initially S-saturated magma parents that were highly depleted in PGE, which mainly formed due to low degrees of partial melting leaving sulfides concentrating PGEs behind in the mantle. Moreover, the low MgO, Ni, Ir and Ru contents and high Cu/Ni and Pd/Ir ratios for the gabbros suggest a highly evolved parental magma. Fe–Ti oxides fractionally crystallized from the highly evolved magma and subsequently settled in the lower sections of the magma chamber, where they concentrated and formed Fe–Ti–V oxide ore layers at the base of the lower and upper cycles. Multiple episodes of magma replenishment in the magma chamber may have been involved in the formation of the Zhuqing intrusions.