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1.
The Merensky Reef of the Bushveld Complex is one of the world'slargest resources of platinum group elements (PGE); however,mechanisms for its formation remain poorly understood, and manycontradictory theories have been proposed. We present precisecompositional data [major elements, trace elements, and platinumgroup elements (PGE)] for 370 samples from four borehole coresections of the Merensky Reef in one area of the western BushveldComplex. Trace element patterns (incompatible elements and rareearth elements) exhibit systematic variations, including small-scalecyclic changes indicative of the presence of cumulus crystalsand intercumulus liquid derived from different magmas. Ratiosof highly incompatible elements for the different sections areintermediate to those of the proposed parental magmas (CriticalZone and Main Zone types) that gave rise to the Bushveld Complex.Mingling, but not complete mixing of different magmas is suggestedto have occurred during the formation of the Merensky Reef.The trace element patterns are indicative of transient associationsbetween distinct magma layers. The porosity of the cumulatesis shown to affect significantly the distribution of sulphidesand PGE. A genetic link is made between the thickness of theMerensky pyroxenite, the total PGE and sulphide content, petrologicaland textural features, and the trace element signatures in thesections studied. The rare earth elements reveal the importantrole of plagioclase in the formation of the Merensky pyroxenite,and the distribution of sulphide. KEY WORDS: Merensky Reef; platinum group elements; trace elements  相似文献   

2.
The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration. This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone.  相似文献   

3.
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   

4.
Summary Unusual facies of the Merensky Reef, the UG-2 and the UG-1 chromitite layers are developed in the western sector of the eastern Bushveld Complex. Within the basal pyroxenite of the Merensky unit, mineralization can be developed at up to four levels. Some of these contain significant mineralization with an increase in the Pt/Pd ratio upward in the succession.The UG-2 chromitite layer consists of a lower, sulphide-rich layer and an upper, sulphide-poor layer. Although these two layers are separated by a pyroxenite parting in places, both contain high platinum-group element (PGE) values. Textural features such as inclusions of base metal sulphides in chromite grains, and the moulding of sintered chromite grains around sulphides, indicates that immiscible sulphide liquid separated prior to or simultaneously with chromite crystallization. The presence of platinum minerals within the sulphides of the inclusions and enclosed in all the base metal sulphides interstitial to chromite, indicates that the PGE were extracted from the magma by the sulphide liquid.Textural and compositional evidence suggests that the sulphide enrichment in the UG-1 chromitite layer is also of magmatic origin, but that these sulphides underwent remobilization at high temperatures.Magma mixing processes are considered to have produced the chromitite layers. The high sulphide content associated with the chromitite layers in the upper critical zone in this sector is ascribed to favourable compositions and proportions of the magmas involved in the mixing process.
PGE-Vererzung im westlichen Sektor des östlichen Bushveld-Komplexes
Zusammenfassung Ungewöhnliche Fazies des Merensky-Reefes sowie der UG-2 und der UG-1 Chromitite kommen im westlichen Sektor des östlichen Bushveld Komplexes vor. In den basalen Pyroxeniten der Merensky-Einheit liegt Vererzung in bis zu vier verschiedenen Niveaus vor. Einige von diesen enthalten signifikante Metallgehalte, wobei das Pt/Pd Verhältnis gegen das Hangende hin zunimmt.Der UG-2 Chromitit besteht aus einer unteren, Sulfid-reichen, und einer oberen, Sulfid-armen Lage. Obwohl diese beiden Lagen stellenweise durch eine pyroxenitische Zwischenschicht getrennt sind, enthalten beide hohe Platin-Gruppen-Elementgehalte (PGE). Texturen wie z.B. Einschlüsse von Buntmetallsulfiden in Chromitkörnern, und die Anordnung von gesinterten Chromitkörnern um Sulfide herum weisen darauf hin, daß eine unmischbare Sulfidschmelze vor oder gleichzeitig mit der Chromitkristallisation abgetrennt wurde. Das Vorkommen von Platin-Mineralen in den Sulfiden der Einschlüsse, und in allen Buntmetallsulfiden die zwischen Chromitkörnern vorkommen, zeigen, daß die PGE durch eine Sulfidschmelze aus dem Magma entfernt worden sind.Texturelle und chemische Parameter zeigen, daß die Sulfidanreicherung in den UG-1 Chromititen auch einen magmatischen Ursprung hat, jedoch waren diese Sulfide später von einer Hochtemperatur-Mobilisation betroffen.Die Chromitit-Lagen werden durch Magmen-Mischung, der hohe Sulfid-Gehalt in den Chromitit-Lagen der oberen Kritischen Zone in diesem Sektor durch günstige Zusammensetzungen und Verhältnisse der Magmen, die an diesem Mischungsprozess teilgenommen haben erklärt.


With 7 Figures  相似文献   

5.
Large mafic–ultramafic layered intrusions may containlayers enriched in platinum-group elements (PGE). In many cases,the PGE are hosted by disseminated sulphides. We have investigatedthe distribution of the sulphides in three dimensions in twooriented samples of the Merensky Reef and the J-M Reef. Theaim of the study was to test the hypothesis that the sulphidescrystallized from a base metal sulphide liquid that percolatedthrough the cumulate pile during compaction. The distributionof sulphides was quantified using: (1) X-ray computed tomography;(2) microstructural analysis of polished thin sections orientedparallel to the paleovertical; (3) measurement of dihedral anglesbetween sulphides and silicates or oxides. In the Merensky Reefand the J-M Reef, sulphides are connected in three dimensionsand fill paleovertical dilatancies formed during compaction,which facilitated the downward migration of sulphide liquidin the cumulate. In the melanorite of the Merensky Reef, thesulphide content increases from top to bottom, reaching a maximumvalue above the underlying chromitite layer. In the chromititelayers sulphide melt connectivity is negligible. Thus, the chromititemay have acted as a filter, preventing extensive migration ofsulphide melt downwards into the footwall. This could partiallyexplain the enrichment in PGE of the chromitite layer and theobserved paucity of sulphide in the footwall. KEY WORDS: X-ray computed tomography; microstructures; sulphides; Merensky Reef; J-M Reef  相似文献   

6.
铂族元素在地壳中的富集:以布什维尔德杂岩为例(英文)   总被引:1,自引:0,他引:1  
地幔是地壳铂族元素富集的主要源库。铂族元素迁移主要有两个途径:(1)地幔部分熔融物质侵入地壳;(2)地幔板片就位于俯冲/碰撞带。前一途径比后一途径重要得多。地幔物质进入地壳造成铂族元素富集并成为可供开采的主矿产而非副产品,这一过程可包含许多成矿作用机制:(i)基性侵入体中Ni-Cu硫化物矿浆的发育,岩浆冷却与分离结晶作用导致富含Cu,Pt,Pd的硫化物矿浆的形成;(ii)层状侵入体一定层位形成高品位的铂族元素硫化物层,伴生或不伴生铬铁岩;(iii)富铂族元素及硫化物的岩浆沿着层状侵入体的边缘就位;(iv)直至层状侵入体结晶分异作用晚期的硫化物不混溶滞后分离;(v)不发育硫化物不混溶作用的铬铁矿结晶作用;(vi)低程度硫化物浸染带中的热液作用与铂族元素富集;(vii)乌拉尔-阿拉斯加型侵入体重结晶过程中的铂族元素与铬铁矿的次生富集作用,岩体在风化过程中形成砂矿床;(viii)黑色页岩形成过程中Pt的富集。南非布什维尔德火成杂岩蕴藏世界Pt资源的75%,Pd资源的54%,Rh资源的82%,并具有(ii)、(iii)、(iv)、(v)、(vi)成矿作用的实例。在这些作用中,作用(ii)形成的现有经济储量和资源量占90%,作用(iii)占9%。Merensky矿层(占总资源量30%)是一个铂族元素富集层位,它含1~3铬铁矿薄层,在可采宽度内硫化物平均含量为1%~3%(质量分数)。硫化物一般被认为是铂族元素的主要聚集体。该矿层由两个或两个以上含硫化物的基性热岩浆上升汇聚而成。这些岩浆的汇聚造成超镁铁质堆晶岩的厚度(主要是斜方辉石岩,某些地区包括橄榄岩)变化于50cm至数米之间。开采通常集中在厚度不到1m的地带。矿层的成因至今仍存在争议,一些观点认为铂族元素来自下部上升的热液流体,另一些观点认为铂族元素来自上部岩浆的硫化物沉降作用,并形成了Merensky辉石岩。已经知道矿层上覆的辉石岩、苏长岩和斜长岩中矿物来自两种岩浆类型:一种富含MgO(12%,质量分数)和Cr,而贫Al2O3(12%);另一种含典型的粒玄岩成分。UG-2铬铁岩含有全部经济资源量的58%,由一0.6~1m厚的铬铁岩层(有时见辉石岩夹层)和上覆的1~3层由铬铁矿所构成的薄层。虽然硫化物被认为至少是某些情况下对铂族元素的富集起作用,但UG-2的硫化物含量(0.5%~1.5%)显著低于Merensky矿层。UG-2层之下共有13个铬铁岩层位,所有的都含铂族元素,虽然铂族元素总含量和(Pt+Pd)/(Ru+Ir+Os)比值远低于UG-2。UG-2内所含的辉石岩"夹层"具高的87Sr/86Sr比值,说明与顶部熔融岩石的混合促进了铬铁岩和硫化物的形成。作用(iii)的主要实例是Platreef。目前它占总资源量的9%。不过,沿该带正积极开展找矿勘探工作,这一比例将来还会提高。这一矿层的厚度比Merensky和UG-2都要大,目前开采厚度达50多米。Platreef呈带状,上部为斜方辉石岩的堆晶岩;下部为辉石岩、长石辉石岩和苏长岩,它们与页岩、铁矿层和白云岩强烈相互作用,直接形成了底盘岩石。笔者认为Platreef是不同期次岩浆作用的结果,这些作用形成了不同的单元产物,包括布什维尔德主岩浆房的UG-2和Merensky矿层。新的岩浆进入主岩浆房会造成先存岩浆移位、岩浆错动并会冲破岩浆房的壁。圆筒状、带状岩管中的超镁铁岩含极高的Pt品位,在布什维尔德杂岩的下部切穿堆晶层,被认为是热液再活化的产物。它们现在未被开采,只是构成存封的铂族元素资源,对整个杂岩体资源没有重要的贡献。  相似文献   

7.
总结南非布什维尔德杂岩体中Merensky Reef(简称MR矿层)和Platreef(简称PR接触带)两类铂族元素矿床的矿床地质、矿化特征以及铂族元素的赋存状态。MR矿床是典型的层状铂族元素(PGE)矿床,在杂岩体东部和西部发育,PGE总含量稳定,赋存在堆晶间隙硫化物中,常以PGE硫化物的形式产出。PR接触带型矿化集中在杂岩体北段,整体上不连续,各个矿床的具体特征由于底盘岩性的多变而不同,PGE主要赋存在碲化物和砷化物等半金属化合物中,可以脱离硫化物产在硅酸盐矿物中。相关的实验研究显示,PGE在岩浆结晶过程中发生分异,Pd/Ir比值体现了硫化物的分异程度;Pd比Pt更容易被氧化以及在热液中迁移,Pt/Pd比值体现了混染和热液的作用,这些因素造成了PR接触带与MR矿层中PGE赋存状态的差异。岩浆可能在侵入之前已经达到了硫饱和,岩浆房的压力变化和岩浆通道对于PGE的富集有重要意义,热液流体可以对已经形成的PGE矿化进行改造。  相似文献   

8.
In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization.  相似文献   

9.
This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl2O3 and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl2O3. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H2O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr2O3 content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.  相似文献   

10.
A new geological map of the Rustenburg Layered Suite south of the Ysterberg–Planknek fault of the northern/Potgietersrus limb of the Bushveld Complex is presented, displaying features that were not available for publication in the past and are considered contributing to the complexity of this region. The northern limb is known for the Platreef, atypical mafic lithologies in sections of the layered sequence and the unusual development of the ultramafic Lower Zone as satellite bodies or offshoots at the base of the intrusion. The outcrop and suboutcrop pattern of Lower Zone Grasvally body and its relation to the surrounding geology of Main Zone, Critical Zone, and floor rocks is described. The extent of the base metal sulfide (BMS) and platinum-group element (PGE)-mineralized cyclic unit 11 of the Drummonlea harzburgite–chromitite sub zone is shown. Only that which is considered to be the equivalents of the mafic Upper Critical Zone has thus far been traced south of Potgietersrus/Mokopane. The Platreef is traced from the farm Townlands and further northwards. The presence of Platreef proper south of Potgietersrus/Mokopane appears to be speculative. However, Merensky Reef, UG 2, and equivalent layers outcrop or were intersected to the south of the town. The Kleinmeid Syncline comprising Main Zone/Critical Zone layers and its structure is discussed. The lateral lithological transfomation of the Merensky Reef/UG 2 and equivalent layers south of the Ysterberg–Planknek fault to Platreef north of this fault is recorded. Attenuation of both the Main Zone and Upper Zone is observed from the northwest towards the town and resulted in only the lower units being developed. The lateral change of Main Zone and Upper Zone lithologies from the northwest towards the town is described. The PGE and BMS economic potential south of the town are briefly tabulated.  相似文献   

11.
Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein, where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel, the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks, but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation, the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas with the crustal rocks that underlie the Complex.  相似文献   

12.
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   

13.
Northwest of Pretoria, the UG2-Merensky Reef interval overlies a Critical Zone-Lower Zone sequence that contains numerous large blocks of floor material. Nevertheless, individual layers can be correlated with equivalent units at Crocodile River mine, the Rustenburg, Impala, Union, and Amandelbult sections. Concentrations of platinum-group elements in two borehole intersections of the UG2 chromitite are 4 ppm over 1.2 m and 2.4 ppm over 2.2 m. Therefore, bulk PGE levels appear to be only moderately lower than those at Western Platinum mine. This renders models explaining PGE enrichment by upward percolating melt or fluids problematic. The Merensky Reef, although containing sulphides, is only weakly mineralized with PGE (0.6 ppm). The UG2 pyroxenite is separated from the UG2 chromitite by a 15 m noritic layer. The introduction of feldspathic cumulates between two units that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts of the chamber distally located with regard to a major feeder zone at Union Section. It also suggests that the UG2 unit is a multiple rather than a single cyclic unit.  相似文献   

14.
Halogen-bearing minerals, especially apatite, are minor butubiquitous phases throughout the Bushveld Complex. Interstitialapatite is near end-member chlorapatite below the Merensky reef(Lower and Critical Zones) and has increasingly fluorian compositionswith increasing structural height above the reef (Main and UpperZones). Cl/F variations in biotite are more limited owing tocrystal-chemical controls on halogen substitution, but are alsoconsistent with a decrease in the Cl/F ratio with structuralheight in the complex. A detailed section of the upper LowerZone to the Critical Zone is characterized by an upward decreasein sulfide mode from 0·01–0·1% to trace–0·001%.Cu tends to correlate with other incompatible elements in mostsamples, whereas the platinum-group elements (PGE) can behaveindependently, particularly in the Critical Zone. The decreasein the Cl/F ratio of apatite in the Main Zone is associatedwith a shift to more radiogenic Sr isotopic signature, implyingthat the unusually Cl-rich Lower and Critical Zones are notdue to assimilation of crustal rocks. Nor is the Main Zone moreCl rich where it onlaps the country rocks of the floor, suggestinglittle if any Cl was introduced by infiltrating country rockfluids. Instead, the results are consistent with other studiesthat suggest Bushveld volatile components are largely magmatic.This is also supported by apatite–biotite geothermometry,which gives typical equilibrium temperatures of 750°C. Theincreasingly fluorian apatite with height in the Upper Zonecan be explained by volatile saturation and exsolved a Cl-richvolatile phase. The high Cl/F ratio inferred for the Lower andCritical Zone magma(s) and the evidence for volatile saturationduring crystallization of the Upper Zone indicate the Lowerand Critical Zones magma(s) were unusually volatile rich andcould easily have separated a Cl-rich fluid phase during solidificationof the interstitial liquid. The stratigraphic distribution ofS, Cu and the PGE in the Critical Zone cannot readily be explainedeither by precipitation of sulfide as a cotectic phase or asa function of trapped liquid abundance. Evidence from potholesand the PGE-rich Driekop pipe of the Bushveld Complex implythat migrating Cl-rich fluids mobilized the base and preciousmetal sulfides. We suggest that the distribution of sulfideminerals and the chalcophile elements in the Lower and CriticalZones reflects a general process of vapor refining and chromatographicseparation of these elements during the evolution and migrationof a metalliferous, Cl-rich fluid phase. KEY WORDS: Bushveld Complex; chlorine; platinum-group elements; layered intrusions  相似文献   

15.
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   

16.
The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2 reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall.  相似文献   

17.
Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.  相似文献   

18.
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.  相似文献   

19.
The Sudbury Igneous Complex (SIC) contains abundant sulphides, especially near the base, and hosts one of the worlds largest nickel and copper deposits. The Bushveld Complex (BC) contains relatively little sulphide, but hosts the worlds largest platinum-group element deposits. The most recent calculations of the sulphur solubility in magmas that produced the BC are based on the sulphur solubility of mid-ocean ridge basalts that have less SiO2 than Bushveld magmas. Such a difference may lead to an overestimation of sulphur solubility by as much as 25%. The revised sulphur solubility curve presented here for Bushveld magmas may also have relevance to the SIC in view of its siliceous nature. Sulphur solubility curves can be used to determine the proportion of sulphide expected in cumulate rocks once sulphur saturation is attained. These models are tested using observed sulphide contents in both intrusions. The observed decreasing sulphur contents (>0.3–0.05% S) from the base of the SIC upward are broadly consistent with these sulphur solubility curves, and are consistent with sulphide saturation through the entire mafic portion. In contrast, the lower half of the BC contains extremely little sulphur (generally <0.02% S), except for two thin layers, which is not consistent with sustained sulphide saturation at any level. Previous interpretations of the sulphur content of Bushveld rocks have suggested that the Lower and Critical Zones were sulphide saturated, but that they had then lost some of the sulphide due to various processes. The present sulphide content of the cumulates of the BC is so low that, if they had once been saturated, over 90% of all the sulphide must have been removed. Mass balance calculations indicate that these large amounts of displaced sulphur remain unaccounted for in such models. Instead, the observed sulphur contents are in reasonable agreement with that expected in a cumulate sequence forming from a sulphur-undersaturated magma. Whereas the Merensky Reef and Bastard pyroxenite contain minor sulphides, the compositions of the immediate hanging wall rocks indicate sulphide undersaturation. Such an abrupt return to sulphide undersaturation is not consistent with models involving sulphide formation from large volumes of magma. One possible explanation for these two observations is that intermittent sulphur degassing occurred through a fractured roof of the BC, so that the magma was never continuously sulphur-saturated with respect to an immiscible sulphide liquid.  相似文献   

20.
The petrology of base metal sulfides and associated accessory minerals in rocks away from economically significant ore zones such as the Merensky Reef of the Bushveld Complex has previously received only scant attention, yet this information is critical in the evaluation of models for the formation of Bushveld-type platinum-group element (PGE) deposits. Trace sulfide minerals, primarily pyrite, pyrrhotite, pentlandite, and chalcopyrite are generally less than 100 microns in size, and occur as disseminated interstitial individual grains, as polyphase assemblages, and less commonly as inclusions in pyroxene, plagioclase, and olivine. Pyrite after pyrrhotite is commonly associated with low temperature greenschist alteration haloes around sulfide grains. Pyrrhotite hosted by Cr- and Ti-poor magnetite (Fe3O4) occurs in several samples from the Marginal to Lower Critical Zones below the platiniferous Merensky Reef. These grains occur with calcite that is in textural equilibrium with the igneous silicate minerals, occur with Cl-rich apatite, and are interpreted as resulting from high temperature sulfur loss during degassing of interstitial liquid. A quantitative model demonstrates how many of the first-order features of the Bushveld ore metal distribution could have developed by vapor refining of the crystal pile by chloride–carbonate-rich fluids during which sulfur and sulfide are continuously recycled, with sulfur moving from the interior of the crystal pile to the top during vapor degassing.  相似文献   

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