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1.
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  相似文献   

2.
The Merensky Reef of the Bushveld Complex contains one of theworld’s largest concentrations of platinum-group elements(PGE). We have investigated ‘normal’ reef, its footwalland its hanging wall at Impala Platinum Mines. The Reef is 46cm thick and consists from bottom to top of leuconorite, anorthosite,chromitite and a very coarse-grained melanorite. The footwallis leuconorite and the hanging wall is melanorite. The onlyhydrous mineral present is biotite, which amounts to 1%, orless, of the rock. All of the rocks contain 0·1–5%interstitial sulphides (pyrrhotite, pentlandite and chalcopyrite),with the Reef rocks containing the most sulphides (1–5%).Lithophile inter-element ratios suggest that the magma fromwhich the rocks formed was a mixture of the two parental magmasof the Bushveld Complex (a high-Mg basaltic andesite and a tholeiiticbasalt). The Reef rocks have low incompatible element contentsindicating that they contain 10% or less melt fraction. Nickel,Cu, Se, Ag, Au and the PGE show good correlations with S inthe silicate rocks, suggesting control of the abundance of thesemetals by sulphides. The concentration of the chalcophile elementsand PGE in the silicate rocks may be modelled by assuming thatthe rocks contain sulphide liquid formed in equilibrium withthe evolving silicate magma. It is, however, difficult to modelthe Os, Ir, Ru, Rh and Pt concentrations in the chromititesby sulphide liquid collection alone, as the rocks contain 3–4times more Os, Ir, Ru, Rh and Pt than the sulphide-collectionmodel would predict. Two possible solutions to this are: (1)platinum-group minerals (PGM) crystallize from the sulphideliquid in the chromitites; (2) PGM crystallize directly fromthe silicate magma. To model the concentrations of Os, Ir, Ru,Rh and Pt in the chromitites it is necessary to postulate thatin addition to the 1% sulphides in the chromitites there isa small quantity (0·005%) of cumulus PGM (laurite, cooperiteand malanite) present. Sulphide liquids do crystallize PGM atlow fS2. Possibly the sulphide liquid that was trapped betweenthe chromite grains lost some Fe and S by reaction with thechromite and this provoked the crystallization of PGM from thesulphide liquid. Alternatively, the PGM could have crystallizeddirectly from the silicate magma when it became saturated inchromite. A weakness of this model is that at present the exactmechanism of how and why the magma becomes saturated in PGMand chromite synchronously is not understood. A third modelfor the concentration of PGE in the Reef is that the PGE arecollected from the underlying cumulus pile by Cl-rich hydrousfluids and concentrated in the Reef at a reaction front. Althoughthere is ample evidence of compaction and intercumulus meltmigration in the Impala rocks, we do not think that the PGEwere introduced into the Reef from below, because the rocksunderlying the Reef are not depleted in PGE, whereas those overlyingthe Reef are depleted. This distribution pattern is inconsistentwith a model that requires introduction of PGE by intercumulusfluid percolation from below. KEY WORDS: Merensky Reef; platinum-group elements; chalcophile elements; microstructures  相似文献   

3.
Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust.Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (∼75% loss). In contrast, basaltic eclogites display significant depletion of Pt (?60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide.Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled 186Os-187Os enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of 187Os-186Os enrichment.  相似文献   

4.
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.  相似文献   

5.
The nature of PGE-Re (PGE = Pt, Pd, Os, Ir, Ru) behavior in subcontinental lithospheric mantle was investigated using new, high precision PGE-Re abundance measurements and previously published Re-Os isotopic analyses of peridotite xenoliths from the Sierra Nevada and Mojave Province, California. Ru/Ir ratios and Ir concentrations are constant over a wide range in S content and major-element fertility indices (e.g., Mg/(Mg+Fe)), indicating that Ru and Ir are not only compatible during partial melting, but also that their partitioning behaviors may not be controlled entirely by sulfide. Pt/Ir, Pd/Ir, Os/Ir, and Re/Ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample (1026V), which is characterized by radiogenic 187Os/188Os, low Re/Os ratio, and large enrichments in Cu, Os, Pt, Pd, and S relative to Ir (COPPS metasomatism). Assuming chondritic initial relative abundances, the magnitudes of some of the depletions in Pt, Pd, Os, and Re relative to Ir and Ru require incompatible behavior or substantial secondary loss. In detail, some samples, which are otherwise characterized by fertile major-element indices, exhibit low S contents and subchondritic Os/Ir and Pd/Ir ratios, indicating that depletions in Pd and Os relative to Ir are not simple functions of the degree of melting as inferred from major elements. Possible mechanisms for depleting Pt, Pd, Os, and Re relative to Ir and Ru include partitioning into chromian spinels and alloys, partitioning between sulfide and sulfide liquids, mobilization by aqueous fluids, or secondary loss associated with late-stage sulfide breakdown. However, it is not possible to explain all of the depletions in Pt, Pd, Os, and Re by any single mechanism.The preferential enrichment in Os over Re and Ir in sample 1026V is somewhat paradoxical because this sample’s radiogenic 187Os/188Os requires a metasomatic agent, originating from a source with a high time-integrated Re/Os ratio. The abundant garnet websterite xenoliths may be a suitable source because they have high Re/Os ratios, radiogenic Os, and abundant garnet, which may sequester Re over Os during partial melting. However, their extremely low Os contents require the processing of large amounts of garnet websterite to concentrate enough Os into the metasomatic sulfides needed to enrich sample 1026V in Os. The homogeneity in 187Os/188Os ratio in the remaining xenoliths suggest that their Os isotopic compositions were not significantly affected by PGE metasomatism. The singular nature of 1026V’s composition emphasizes the rarity of COPPS metasomatism.  相似文献   

6.
The peridotites of the Manipur Ophiolite Complex (MOC) have been examined based on mineral chemistry, major elements and PGE contents. They represent high-magnesian cumulates with Mg# > 0.90 (Mg/Mg+Fe) in olivine and Cr# > 0.12 (Cr/Cr+Al) in spinel. High Mg* contents of the olivine show that these rocks are most likely derived from partial melting of the residual upper mantle. The peridotites contain higher concentration of Palladium Group PGE (PPGE) (Rh=4.4−6.6ppb; Pd=336−458ppb and Pt=14.6−36.4ppb) than the Iridium Group PGE (IPGE) (Os=2.4−5.8ppb; Ir=3.2−4.16ppb and Ru=5.2−7ppb). These are characterized by overall enrichment of PGE concentration (σPGE=365.8 − 516.6 ppb) and high ratio of (Pt+Pd)/(Os+Ir+Ru). This suggests that the rocks are formed by partial melting and crystal fractionation of olivine-rich (picritic) magma.  相似文献   

7.
Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur (Kharanur) massifs). The new and published data are used to consider similarities and differences between ore chrome-spinel from the chromitites of the northern and southern ophiolite sheets as well as the species diversity of PGE minerals and the evolution of PGE mineralization. Previously unknown PGE minerals have been found in the studied chromitites.Ore chrome-spinel in the chromitites from the northern sheet occurs in medium- and low-alumina forms, whereas the chromitites from the southern sheet contain only medium-alumina chrome-spinel. The PGE minerals in the chromitites from the southern sheet are Os–Ir–Ru solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse Rh–Pt–Pd mineralization: Pt–Ir–Ru–Os and isoferroplatinum with Ir and Os–Ir–Ru lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: Pt–Cu, Pt–Pd–Cu, PdHg, Rh2SnCu, RhNiAs, PtAs2, and PtSb2. The speciation of the PGE minerals is described along with multiphase intergrowths. The relations of Os–Ir–Ru solid solutions with laurite and irarsite are considered along with the microstructure of irarsite–osarsite–ruarsite solid solutions. Zoned Os–Ir–Ru crystals have been found. Zone Os82–99 in these crystals contains Ni3S2 inclusions, which mark off crystal growth zones. Different sources of PGE mineralization are presumed for the chromitites from the northern and southern sheets.The stages of PGE mineralization have been defined for the chromitites from the Il’chir ophiolite belt. The Pt–Ir–Ru–Os and (Os, Ru)S2 inclusions in Os–Ir–Ru solid solutions might be relics of primitive-mantle PGE minerals. During the partial melting of the upper mantle, Os–Ir–Ru and Pt–Fe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, Os–Ir–Ru solid solutions were replaced by sulfides and sulfarsenides of these metals. Mantle metasomatism under the effect of reduced mantle fluids was accompanied by PGE remobilization and redeposition with the formation of the following assemblage: garutiite (Ni,Fe,Ir), zaccariniite (RhNiAs), (Ir,Ni,Cu)S3, Pt–Cu, Pt–Cu–Fe–Ni, Cu–Pt–Pd, and Rh–Cu–Sn–Sb. The zoned Os–Ir–Ru crystals in the chromitites from the northern sheet suggest dissolution and redeposition of Os–Ir–Ru primary-mantle solid solutions by bisulfide complexes. Most likely, the PGE remobilization took place during early serpentinization at 450–600 ºC and 13–16 kbar.During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chrome-spinel into ferrichromite–chrome-magnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelite–maucherite–ferrichromite–sperrylite assemblage formed in epidote-amphibolitic facies settings during this stage.The PGE mineral assemblage reflects different stages in the formation of the chromitites and dunite-harzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.  相似文献   

8.
The Lanzo lherzolite massif shows that the top of asthenosphericdiapirs is a zone of preferential S, Cu, Au, and platinum-groupelement (PGE) enrichment. Residual plagioclase lherzolites whichunderwent a limited extraction of mid-ocean ridge basalt (MORB)melt during diapiric uprise are enriched in Ru, PPGE (Pt andPd), and Au (3–10 times the pristine asthenospheric mantle),whereas they are moderately enriched in Cu (up to 38 ppm), depletedin Ir (Ir–Nimn=1–0.1), and have S contents rangingbetween 95 and 215 ppm. The behaviour of chalcophile elementsin the lherzolites cannot be modelled by equilibrium batch melting.The precious metals vary independently of lithophile elementcontent and modal rock composition. It is suggested that theexcess of PPGE, Au, and Cu was introduced either by a plume-typemagma which cross-cut the Lanzo massif before rifting or bydownward percolation of sulphides segregated from the MORB magmasextracted from the asthenospheric diapir. Calculated chalcophile element compositions of the extractedmelts show high Cu/Pd ratios (27–145) typical of meltsthat have experienced early S saturation. This adds straightforwardevidence to theoretical modelling and indirect assumptions basedon extruded MORB, i.e., it is possible to produce high D-elementdepleted magma compositions simply by low degrees of mantlemelting under S-saturated conditions without fractionation inmagma chambers. The MORB magma has circulated via ariegite andgabbro dykes as well as dunitic bands. All of these rocks arestrongly depleted in Ir. Their Ir/Nimnratios range between 0192in the gabbros and 0068–0168 in the dunites. The depletionin Ir (and probably Os) is attributed to segregation of Ir-bearingalloys from the MORB melt before it cross-cut the peridotites. The dunitic bands are distinguished from the harzburgites anddunites analysed so far by a large range of total precious metalcontent (17–77 ppb), positive Pd/Irmn ratios, and an excessof S (up to 210 ppm) and/or Cu (up to 87 ppm). Chalcophile elementdata support a model whereby the dunitic bands have formed fromreaction with percolating S-saturated melts. The progressiveenrichments in Cu and PPGEs observed in the latest percolationstages (Pd/Irmn=16–28; Cu/S=0–4) may be modelledby increasing the degree of melting that produced the percolatingmelt(s). A similar model may account for strong Cu and PPGEenrichments in the dunitic transition zone of some ophioliticcomplexes. * Present address; Laurentian University, Ramsey Lake Road, Sudbury, Ontario, P3E 2C6, Canada  相似文献   

9.
金川铜镍硫化物矿床岩浆通道型矿体地质地球化学特征   总被引：7，自引：0，他引：7  
田毓龙  包国忠  汤中立  王玉山 《地质学报》2009,83(10):1515-1525
金川铜镍硫化物矿床6行富铜（铂族）矿体曾因Cu、Pt、Pd等含量明显高于相邻其它矿体而被认为是岩浆期后热液叠加作用的产物,研究发现,空间上该矿体受断层构造控制,在矿石组构、矿物组成和硫同位素组成方面与相邻岩浆融离型1号矿体一致,显示了该矿体岩浆成矿作用的特征。在元素地球化学方面,6行富铜（铂族）矿体的Cu、Ni、Pt、Pd含量及Cu/Ni比值明显高于1号主矿体,而Os、Ir、Rh、Ru却明显低于后者,同时,前者相对富含LREE,轻、重稀土分异程度高于后者。根据硫化物结晶分异过程中金属元素分配规律及稀土元素特征,阐明了6行富铜（铂族）矿体为岩浆通道型矿体,是岩浆硫化物晚期结晶的产物。矿区中西部存在的Cu、Ni、Pt、Pd、Au等含量高,而Os、Ir、Rh、Ru含量低的部位,是寻找岩浆通道型矿体的有利部位。  相似文献   

10.
Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex   总被引：21，自引：7，他引：14  
We have analysed sulphides, silicates, and chromites of theMerensky Reef for platinum-group elements (PGEs), Re and Auusing laser ablation-inductively coupled plasma mass spectrometryand synthetic pyrrhotite standards annealed with known quantitiesof noble metals. Os, Ir and Ru reside in solid solution in pyrrhotiteand pentlandite, Rh and part of the Reef’s Pd in pentlandite,whereas Pt, Au, Re and some Pd form discrete phases. Olivineand chromite, often suspected to carry Os, Ir and Ru, are PGEfree. All phases analysed contain noble metals as discrete micro-inclusionswith diameters typically <100 nm. Inclusions in sulphidescommonly have the element combinations Os–Ir–Ptand Pt–Pd–Au. Inclusions in olivine and chromiteare dominated by Pt ± Au–Pd. Few inclusion spectracan be related to discrete noble metal phases, and few inclusionshave formed by sub-solidus exsolution. Rather, some PGE inclusions,notably those in olivine and chromite, are early-magmatic nuggetstrapped when their host phases crystallized. We suggest thatthe silicate melt layer that preceded the Merensky Reef wasPGE oversaturated at early cumulus times. Experiments combinedwith available sulphide–silicate partition coefficientssuggest that a silicate melt in equilibrium with a sulphidemelt containing the PGE spectrum of the Merensky ore would indeedbe oversaturated with respect to the least soluble noble metals.Sulphide melt apparently played little role in enriching thenoble metals in the Merensky Reef; rather, its role was to immobilizea pre-existing in situ stratiform PGE anomaly in the liquid-stratifiedmagma chamber. KEY WORDS: Bushveld Complex; Merensky Reef; laser-ablation ICP-MS; platinum-group mineralization  相似文献   

11.
The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block.In the present study,the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts,with minor picrites in the lower part and andesites,trachytes,and rhyolites in the upper part.The picrites have relatively higher platinum-group element（PGE） contents（ΣPGE=16.3-28.2 ppb）,with high Cu/Zr and Pd/Zr ratios,and low S contents（5.03-16.9 ppm）,indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province（ELIP） mantle source.The slightly high Cu/Pd ratios（11 000-24 000） relative to that of the primitive mantle suggest that 0.007%sulfides have been retained in the mantle source.The PGE contents of the high-Ti basalts exhibit a wider range（ΣPGE=0.517-30.8 ppb）.The samples in the middle and upper parts are depleted in PGE and haveεNd（260 Ma） ratios ranging from -2.8 to -2.2,suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides,and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts.The samples in this area show Pt-Pd type primitive mantle-normalized PGE patterns,and the Pd/Ir ratios are higher than that of the primitive mantle（Pd/Ir=1）,indicating that the obvious differentiation between Ir-group platinum-group elements（IPGE） and Pd-group platinum-group elements（PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution.The Pd/Pt ratios of most samples are higher than the average ratio of mantle（Pd/Pt=0.55）,showing that the differentiation happened between Pt and Pd.The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle;whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys,coprecipitating with Ir-Ru-Os alloys.Some high-Ti basalt samples exhibit negative Ru anomalies,possibly due to removal of laurite collected by the early crystallized chromites.  相似文献   

12.
The Permian Hulu intrusion is one of several sulphide-bearing Permian mafic–ultramafic intrusions in the eastern part of the eastern Tianshan located at the southern margin of the Central Asian Orogenic Belt (CAOB) in Xinjiang, NW China. The intrusion is composed of lherzolite, olivine websterite, gabbro, and gabbro-diorite. Disseminated and net-textured Ni-Cu sulphide ores are located at the bottom of the lopolith complex. Negative Zr, Hf, Nb, and Ta anomalies, whole-rock εNd(t) values of +5.7 to +8.8, and variable (Th/Nb)PM values (from 1.06 to 8.13) suggest that the source of the Hulu complexes is depleted mantle metasomatized by subducted slab-derived fluid and/or melt (~5% global subducted sediment and 15% slab fluid) that has experienced approximately 3% lower crustal and 10% upper crustal contamination. The Hulu intrusion is characterized by low PGE abundances i.e. 0.03–1.08 ppb Ir, 0.04–0.69 ppb Ru, 0.02–2.15 ppb Rh, 0.30–48.71 ppb Pt, and 0.21–344 ppb Pd. Our calculations indicate that if the Pd, Os, Ir, and Cu contents of the primary magma were 2.1 ppb, 0.03 ppb, 0.05 ppb, and 200 ppm, respectively, a variable R-factor between 200 and 1600 with residual magma that had experienced 0.01% early-sulphide segregation can explain the variation in Pd, Os, and Ir contents of sulphide-poor and disseminated sulphide samples of the Hulu deposit. Basaltic magma fractionation and assimilation and/or contamination of sulphur-bearing crustal materials might have triggered sulphur saturation to form Cu-Ni sulphide ores. Tarim basaltic PGE contents cannot be used as the mineralized parent magma for the Hulu intrusion because of the differing evolutionary trends of the Ni/Pd and Cu/Ir values. However, similar Cu/Ni and Pd/Ir values in Tarim basalts and Hulu Cu-Ni sulphide ores, as well as the same early sulphide segregation process, show that certain genetic relationships between them and magma sources are probably similar to each other.  相似文献   

13.
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.  相似文献   

14.
The emission of platinum group elements (PGE) from automobile catalytic converters has led to enrichment of PGE in road dusts and roadside soils in urban areas that are well above the natural background levels. This paper evaluates the source of contamination of all the PGE and Au in road dusts and roadside soils in the Pearl River Delta region, including three major cities, Shenzhen, Guangzhou and Hong Kong, South China. Samples were digested using Carius tube and analyzed by isotope dilution ICP-MS; Os was separated by distillation and other PGE by Te-coprecipitation. All samples have elevated PGE concentrations above the background values of uncontaminated soils and contain higher Pt, Pd and Rh than other PGE. The maximum values are 181 ng/g Pt, 514 ng/g Pd, 53 ng/g Rh and 1345 ng/g Au. There are clear positive correlations between Pt and Pd, Pt and Rh, and Pd and Rh, indicating that the main emitted of PGE from automobile catalyst are Pt, Pd and Rh. High concentrations of Au were also found in road dust samples from Hong Kong and Shenzhen. Dust samples with higher Os contents have lower 187Os/188Os ratios. Samples from Hong Kong show relatively high Pt/Rh ratios. Positive correlations between Pt and Ru, and Pt and Ir were found in Shenzhen and Hong Kong, but only positive correlations between Pt and Ir were found in Guangzhou. These different characteristics reflect different automobile catalytic systems used in Hong Kong and mainland China.  相似文献   

15.
The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation, a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast, Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals, including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals. We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite.  相似文献   

16.
The concentrations of platinum-group elements (PGE), Co, Re,Au and Ag have been determined in the base-metal sulphide (BMS)of a section of the Merensky Reef. In addition we performeddetailed image analysis of the platinum-group minerals (PGM).The aims of the study were to establish: (1) whether the BMSare the principal host of these elements; (2) whether individualelements preferentially partition into a specific BMS; (3) whetherthe concentration of the elements varies with stratigraphy orlithology; (4) what is the proportion of PGE hosted by PGM;(5) whether the PGM and the PGE found in BMS could account forthe complete PGE budget of the whole-rocks. In all lithologies,most of the PGE (65 up to 85%) are hosted by PGM (essentiallyPt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride).Lesser amounts of PGE occur in solid solution within the BMS.In most cases, the PGM occur at the contact between the BMSand silicates or oxides, or are included within the BMS. Pentlanditeis the principal BMS host of all of the PGE, except Pt, andcontains up to 600 ppm combined PGE. It is preferentially enrichedin Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, butexcludes both Pt and Pd. Chalcopyrite contains very little ofthe PGE, but does concentrate Ag and Cd. Platinum and Au donot partition into any of the BMS. Instead, they occur in theform of PGM and electrum. In the chromitite layers the whole-rockconcentrations of all the PGE except Pd are enriched by a factorof five relative to S, Ni, Cu and Au. This enrichment couldbe attributed to BMS in these layers being richer in PGE thanthe BMS in the silicate layers. However, the PGE content inthe BMS varies only slightly as a function of the stratigraphy.The BMS in the chromitites contain twice as much PGE as theBMS in the silicate rocks, but this is not sufficient to explainthe strong enrichment of PGE in the chromitites. In the lightof our results, we propose that the collection of the PGE occurredin two steps in the chromitites: some PGM formed before sulphidesaturation during chromitite layer formation. The remainingPGE were collected by an immiscible sulphide liquid that percolateddownward until it encountered the chromitite layers. In thesilicate rocks, PGE were collected by only the sulphide liquid. KEY WORDS: Merensky Reef; Rustenburg Platinum Mine; sulphide; platinum-group elements; image analysis; laser ablation ICP-MS  相似文献   

17.
Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth.  相似文献   

18.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 μm. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, ∼1480 to 1468 K at Ptot = 10−3 bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 × CI that likely represent mixing of grains that condensed at various temperatures.  相似文献   

19.
Platinum group elements (PGE) enrichment occurs in Zn–Cu and Ni-rich ophiolities in a number of geological settings. Platinum group elements (PGE) mineralization in Pyroxenite from the Faryab ophiolities of Zagros belt in south Iran was studied. The ophiolite rocks represent blocks of Tethyan oceanic crust that were emplaced on the continental margin during the late Cretaceous period. Much of lower ophiolitic section is composed of homogeneous harzburgite, while upper sections harzburgite interlayer with dunite and pyroxenite are included. This study focused on pyroxenite that includes most of sulfide mineralization in Faryab. More than 500 samples were investigated from polished thin sections; that cover all area of Faryab. The sulfide phases include pyrrhotite, pentlandite, millerite, violarite, smythite, and heazlewoodite. The results show that in almost all the samples Os is below the 2 ppb detection limit, Platinum values vary from <5 to 91 ppb and the light PGE (Ru, Rh, and Pd) relative to the heavy PGE (Os, Ir, and Pt) are more concentrated. Calculation showed that in pyroxenites Pd–Pt is occurring with orthopyroxenite and Rh–Os is occurring in clinopyroxenite. Ni/Pd ratios in Faryab vary between 7 and 356 and Pd/Ir ratio is 0.1–27. This indicates that in Faryab area partial melt of mantle occurred. Pd/Rh ratio in Faryab is 0.1–11, and Pd/Pt varies between 0.2 and 1.5. Pd/Ir ratio in Faryab decreases and shows that PGE in Faryab occurred.  相似文献   

20.
Platinum-group elements （PGE） in PGE-rich porphyry copper （gold） deposits are mainly Pt and Pd, whereas the concentrations of other PGE （Ru, Rh, Os, Ir） are significantly low. Moreover, Pt and Pd mainly exist in sulfides in the forms of crystal lattice or tiny platinum-group mineral （PGM） inclusions. The present data show that there is a positive relationship between Pt and Pd concentrations and Cu （Au） in porphyry copper （gold） deposits. The comparison of chondrite-normalized PGE distribution patterns between the ore-bearing porphyry intrusions and ore-barren porphyry intrusions in arc setting, 187^Os/188^Os, 87^Sr/86^Sr and S isotopes for porphyry copper （gold） deposits shows that PGEs were mainly derived from the mantle, and fluids from subduction zones devoted trivial PGE to the magma. The porphyry copper （gold） deposits associated with subducted events are most probably enriched in PGE, whereas those related to crustal thickening, lithospheric delamination or underplating rarely concentrate PGE. The osmium isotopic compositions in porphyry copper （gold） deposits reveal that （187^Os/188^Os）i values are highly variable and not lower than those of primitive upper mantle （PUM） and mantle peridotite, however, osmium concentrations are commonly lower than mantle peridotite, suggesting that parental magmas of some porphyry intrusions had experienced crustal contamination during magma evolution. Experimental investigations have proved that PGE exist in the forms of Cl^- and HS^- complexes during transportation and migration of the oreforming fluids. This paper summarizes previous studies including crucial controlling factors and mechanisms for PGE enrichment, and points out that the mantle-derived magmas parental to porphyry intrusions are the prerequisite for PGE enrichment in porphyry copper （gold） deposits. Favorable physical and chemical conditions （including salinity, temperature, pressure, pH, and oxygen fugacity） in hydrothermal fluids crucially control the  相似文献   

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