云南鸡街碱性超基性岩铂族元素地球化学特征

本文首次报道了攀西裂谷南段云南省鸡街碱性超基性岩中铂族元素(PGE)的地球化学特征。采用改进的Carius管法测定了霞霓钠辉岩、霓霞岩和磷霞岩中的低含量PGE。PPGE与IPGE呈现强烈分馏,推断幔源岩在低程度部分熔融过程中Pt、Pd表现为相似的不相容性,而Ir、Ru表现为相容性,这种分馏效应随着结晶分异作用的进行而逐渐增强。3种岩石均出现Ru的负异常及Pt、Pd的解耦,说明母岩浆经历了早期的橄榄石晶出,在结晶分异过程中Pd较Pt更不相容。由于岩浆上升过程中的压力减小和结晶分异作用导致的成份变化使岩浆可以达到硫的局部饱和而熔离,表现为部分样品中Cu/Pd远高于原始地幔值。本文通过碱性超基性岩与金伯利岩、煌斑岩和邻区碱性、过碱性玄武岩PGE特征的对比,探讨了其岩浆源区及演化特征。     

Chalcophile element constraints on magma differentiation of Quaternary volcanoes in Tengchong,SW China

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.     

Precambrian mafic magmatism in the Bastar craton,central India

The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.     

阜新中生代火山岩的铂族元素特征——以碱锅和乌拉哈达为例

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

塔里木大火成岩省瓦吉里塔格霞石岩铂族元素地球化学特征

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

新疆北山红石山含铜镍镁铁-超镁铁质岩体PGE和Re-Os同位素地球化学特征及成矿意义

对新疆北山地区红石山镁铁-超镁铁质岩体中含铜镍的硫化物矿石和岩石进行了铂族元素和Re-Os同位素地球化学特征研究,结果表明,矿石及岩石的铂族元素(PGE)总量较低,变化于0.54×10-9~15.84×10-9之间。较低的Pd/Ir比值表明岩石主要受岩浆作用控制,后期热液作用影响不明显。较高的Cu/Pd和Ti/Pd比值表明岩浆在演化过程中发生了硫化物的熔离。岩体的母岩浆为有早期结晶橄榄石加入的高镁的玄武质岩浆。γOs(t)的变化较大,变化于-282~+282之间,表明有较多的地壳物质混入。地壳物质混染和橄榄石等矿物的分离结晶可能是引起岩浆中的S达到饱和进而熔离的重要因素。红石山岩体是经历了结晶分异和硫化物熔离后橄榄石的堆积体与残余岩浆演化的混合体。     

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

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

Platinum-Group Element Geochemical Characteristics of the Picrites and High-Ti Basalts in the Binchuan Area,Yunnan Province

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 haveε_Nd（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.     

Geochemical study of peridotites from the Manipur Ophiolite Complex,Northeast India with special reference to their PGE concentration

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.     

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.     

新疆北山地区两个含铜镍镁铁—超镁铁质岩体铂族元素地球化学研究

对北山地区坡一和罗东含铜镍的镁铁-超镁铁质岩体铂族元素研究表明,两个岩体的铂族元素(PGE)总量较低,PPGE较IPGE富集,原始地幔标准化模式呈正斜率,均较原始地幔亏损,具Ir和Rh的弱负异常。较低的Pd/Ir比值表明岩石主要受岩浆作用控制,后期热液作用影响不明显。两个岩体的原生岩浆均为MgO 含量较高的PGE不亏损的拉斑玄武质岩浆,较高的Cu/Pd、Ti/Pd比值表明岩浆在演化过程中发生了硫化物的熔离。罗东岩体早期矿物相（橄榄石、铬铁矿）的分离结晶作用对岩浆中的硫达到饱和具有重要的贡献,而坡一岩体该作用对硫化物熔离的贡献不明显。坡一和罗东岩体的R值表明两岩体均具有达到中型Ni矿床的潜力。     

Platinum-group elemental chemistry of the Baima and Taihe Fe–Ti oxide bearing gabbroic intrusions of the Emeishan large igneous province,SW China

Nickel-, copper-, and platinum group element (PGE)-enriched sulphide mineralization in large igneous provinces has attracted numerous PGE studies. However, the distribution and behavior of PGEs as well as the history of sulphide saturation are less clear in oxide-dominated mineralization. Platinum group elements of oxide-bearing layered mafic intrusions from the Emeishan large igneous province are examined in this study. Samples collected from the Baima and Taihe oxide-bearing layered gabbroic intrusions reveal contrasting results. The samples from Baima gabbroic rocks have low total PGE abundances (ΣPGE < 4 ppb) whereas the Taihe gabbroic rocks, on average, have more than double the concentration but are variable ranging from ΣPGE < 2 ppb to ΣPGE ∼300 ppb. The Baima gabbro is platinum-subgroup PGE (PPGE = Rh, Pt and Pd) enriched and iridium-subgroup PGE (IPGE = Os, Ir and Ru) depleted, with a distinct positive Ru anomaly on a primitive mantle normalized multi-element plot. The Taihe gabbros are also PPGE enriched but with negative Ru and Pd anomalies on a primitive mantle normalized multi-element plot. The PGE concentrations of Baima rocks are indicative of fractionation of a relatively evolved, mafic, S-undersaturated parental magma that was affected by earlier sulphide segregation. In contrast, the Taihe rocks record evidence of both S-saturated and S-undersaturated conditions and that the parental magma was likely emplaced very close to S-saturation. Comparisons of the platinum group element contents in the Emeishan flood basalts and the Emeishan oxide-bearing intrusions suggest that the PGE budget in a magma is not controlled by magma series (high-Ti vs. low-Ti), but very much by crustal contamination. The unlikelihood of substantial crustal contamination in the Taihe magma allowed the magma to remain S-undersaturated for a longer duration. PGE and sulphide mineralization was not identified in the Taihe intrusion but the presence of one PGE-enriched sample (Pt + Pd = ∼300 ppb) suggests that the parental magma likely did not experience sulphide segregation and is a potential target for further prospecting.     

喀拉通克铜镍矿床铂族元素地球化学特征及其成矿作用意义

喀拉通克铜镍矿床位于准噶尔板块北缘,矿区主要矿体赋存于Y1-Y3号岩体中。矿石构造类型为致密块状和浸染状两大类,其中前者与后者呈贯入接触,不同浸染状类型之间为过渡关系。岩石和矿石的PGE总量偏低,且以PPGE为主,IPGE含量较低。整体上岩石中的PGE含量显示随基性程度降低而变小。矿石中的PGE含量随硫化物含量增加增大,显示PGE主要分布于硫化物熔离形成的物相中。100%硫化物计算后,矿石PGE含量平均仅为573×10^-9。各岩体中浸染状矿石PGE组成并无明显差异;岩石和矿石具有相似的PGE分配模式,均属于Pt-Pd配分型。岩石Ni/Cu-Pd/Ir关系以及岩石地球化学资料显示,形成喀拉通克岩体的初始岩浆为MgO含量较高的玄武质岩浆,属于PGE不亏损的岩浆。基于PGE不亏损的大陆拉斑玄武岩初始岩浆推算,喀拉通克矿床母岩浆明显亏损PGE,而深部硫化物熔离可能是导致母岩浆PGE亏损的主要原因。岩石和矿石Pd/Pt比值总体特征,岩石Cr与Ni、Ir、Ru和Rh相关性,以及硫同位素和岩石学资料分析表明,初始岩浆在地壳深部发生的橄榄石、铬铁矿等矿物的分离结晶作用,可能是促使硫过饱和与深部熔离的主要因素。IPGE与PPGE分异特征及其相关分析,结合矿床宏观地质特征,推断该矿床浸染状矿的成矿作用经历了初始岩浆（PGE不亏损）→橄榄石等矿物分离结晶→硫化物深部熔离→成矿母岩浆（PGE亏损）→上侵并结晶分异的成矿过程。块状矿则可能是这一过程中PGE亏损的成矿母岩浆相对滞后熔离形成的硫化物熔体贯入的结果。     

大别造山带祝家铺辉长岩的铂族元素特征

采用镍锍火试金法结合ICP-MS分析了祝家铺14个辉长岩样品中的Ir，Ru，Rh，Pt和Pd的含量。结果显示其PGE的含量较低，原始地幔标准化后的PGE分布模式呈正斜率型，PPGE相对原始地幔略微亏损，而IPGE强烈亏损，Pd／Ir值(22—138)远高于相应的地幔比值，表明铂族元素发生了分异。对祝家铺辉长岩的铂族元素研究表明，在其源区发生过硫化物的分异作用，地壳的混入可能促进了硫化物的饱和。祝家铺辉长岩中铂族元素的分异是因为在地幔部分熔融和岩浆演化过程中，PPGE主要受硫化物控制，而Ir则存在于非硫化物相如尖晶石、可能还有合金之中。     

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo_93-94. The emplaced lavas are characterized by (Pd/Ir)_N = 4.0 to 4.6, (Os/Ir)_N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk D_Os,Ir = 2.4-7.1) and that Pt and Pd were incompatible (bulk D_Pt,Pd < 0.2) during lava differentiation, whereas bulk D_Ru was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (D_PGEs^Ol-Liq = 0.04-0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: D_Os,Ir = 2.3, D_Ru = 1.0, D_Pt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.     

Geochemical characteristics of the platinum-group elements in the Abulangdang ultramafic intrusion,Sichuan Province,China

The contents of the platinum-group elements (PGEs: Os, Ir, Ru, Rh, Pt, Pd) in the Abulangdang ultramafic intrusion have been determined using ICP-MS after nickel sulfide fire assay preconcentration. Different samples show significant differences in absolute PGE abundance. They display a pronounced negative incline in mantle-normalized patterns which are characterized by strong enrichment in IPGEs (Os, Ir, Ru) and depleting to slight enrichment in PPGEs (Rh, Pt, Pd). The characteristics of PGE distribution in the Abulangdang rocks are due to the combined action of sulfide and non-sulfide (spinel/chromite or alloy or micro-granular aggregation of metals). In comparison with the mafic-ultramafic rocks which host Ni-Cu-PGE deposits in the Emeishan Large Igneous Province (ELIP), it is assumed that the Abulangdang ultramafic intrusion may be the product of early-stage magma activity in the ELIP.     

攀西小关河地区核桃树富铂岩浆硫化物矿床岩石地球化学特征及成矿机制研究

核桃树富铂岩浆硫化物矿床位于四川会理县小关河地区,是峨眉山大火成岩省中含较高铂族元素含量的岩浆硫化物矿床之一。本文通过对核桃树岩体及部分硫化物矿石主量元素、微量元素及铂族元素的系统分析,讨论了该岩体的岩浆源区及母岩浆性质、地幔部分熔融程度,并探讨了其成因机制。研究认为,核桃树含矿岩体属拉斑玄武岩成因系列,具有与峨眉山玄武岩相似的微量元素组成特征,是峨眉山大火成岩省构造-岩浆活动的产物;铂族元素的原始地幔标准化配分型式与金宝山铂钯矿相似,没有PGE相对于Ni和Cu的明显亏损,Pt和Pd相对Os、Ir、Ru和Rh富集,为PPGE富集的左倾型式,Pd/Ir=1.5~13.1,低于一般大陆拉斑玄武岩,与原始地幔接近。通过岩石地球化学及模拟分析表明,成矿母岩浆MgO约为11.93%、SiO_2约为49.88%、FeOT约为13.71%、TiO_2约为2.61%,为高Mg拉斑玄武质岩浆,是由类似于洋岛玄武岩岩浆源区成分的地幔经过较高程度(约20.17%)的部分熔融形成的苦橄质岩浆演化而来。与小关河地区主要的几类岩浆硫化物矿床的镍铜铂族元素组成及硫化物熔离模式对比分析发现,核桃树高的PGE含量和低的Cu/Pd比值说明了该矿床的硫化物是从PGE不亏损的玄武质岩浆中熔离出来的,类似金宝山矿床。成岩成矿机制分析认为,部分熔融形成的苦橄质岩浆在上升的过程中,发生了以橄榄石(约12.7%)为代表的镁铁质矿物堆积,并形成残余髙镁玄武质岩浆;部分残余髙镁玄武质岩浆向浅部运移过程中,由于温度降低、混染等因素的影响,导致岩浆S饱和,触发硫化物熔离作用的发生(R值为2000~50000),熔离出硫化物熔体与岩浆通道内晶出的橄榄石构成含矿"晶粥",在构造挤压作用下,在浅部岩浆房中由于重力影响发生堆积作用形成具有较富PGE的含矿岩体,R值较大变化正好与PGE含量较大变化相对应。     

北大别白垩纪镁铁－超镁铁岩的铂族元素地球化学研究

采用镍锍火试金法结合ICP-MS分析了12个北大别白垩纪镁铁-超镁铁岩样品的Ir,Ru,Rh,Pt和Pd的含量,结果显示铂族元素（PGE）的含量较低,原始地幔标准化后的PGE分布模式呈正斜率型,PPGE相对原始地幔略微亏损,而IPGE强烈亏损,Pd/Ir值远高于相应的地幔比值。这些镁铁-超镁铁岩中PGE的强烈分异是地幔低程度的部分熔融过程中,PPGE主要受硫化物控制,而Ir则存在于非硫化物相如尖晶石,可能还有合金之中造成的。同时,铂族元素的分布特征表明这些镁铁-超镁铁岩是岩浆结晶分异的产物。     

Platinum-group element contents of chromites from mafic–ultramafic layered flows, Abitibi greenstone belt, Ontario: implications for geochemical fractionation and mineral exploration

Summary ?The PGE contents of chromite separated from peridotite layers of Archaean mafic–ultramafic flows, Abitibi belt (Canada), indicate enrichment in Os–Ir–Ru (600 ppb) relative to Pd–Au (<5 ppb). Evidently, chromite was a sink for Ir–Os–Ru during melt-chromite fractionation in each of the flows. However, an additional phase, probably olivine, is required to explain the bulk Ir content of the sulphide-poor peridotites. In contrast, the chromite Pt contents range from <10 ppb to 400 ppb, with large variation in Pt/Ru (0.02–2.76) and Pt/Pd (5–400) ratios. The Pt enrichment may be related to the presence of Pt spinel structure compounds in oxidised melt, reflecting Fe–Ti spinel-related mineralisation in higher pyroxenite-gabbro layers. Received December 5, 2002; revised version accepted January 7, 2003     

大别造山带毛屋超镁铁岩的铂族元素研究

采用镍锍火试金法结合ICP-MS分析了毛屋斜方辉石岩和石榴二辉岩样品中的Ir、Ru、Rh、Pt和Pd的含量,结果显示其铂族元素(PGE)的含量随岩石类型无规律性的变化,原始地幔标准化后的铂族元素分布模式呈负斜率,Pd、Ir发生了分异。毛屋超镁铁岩铂族元素特征的形成受岩石中铂族元素的存在相制约,PPGE富集在富Cu硫化物,而IPGE以类似残留相、不熔的单硫化物固熔体形式存在,其中地壳混染也起了一定的作用;同时,成岩过程中流体的存在造成了Pt和Pd的活化。因此,单硫化物固熔体和流体的共同作用形成了毛屋超镁铁岩类似残留地幔岩的铂族元素分布特征。