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1.
The Aurora Project is a Cu-Ni-PGE magmatic sulphide deposit in the northern limb of the Bushveld Complex of South Africa. Since 1992 mining in the northern limb has focussed on the Platreef deposit, located along the margin of the complex. Aurora has previously been suggested to represent a far-northern facies of the Platreef located along the basal margin of the complex and this study provides new data with which to test this assertion. In contrast to the Platreef, the base metal sulphide mineralisation at Aurora is both Cu-rich (Ni/Cu < 1) and Au-rich. The sulphides are hosted predominantly in leucocratic rocks (gabbronorites and leucogabbronorites) with low Cr/MgO (< 30) where pigeonite and orthopyroxene co-exist as low-Ca pyroxenes without cumulus magnetite. This mineral association is found in the Upper Main Zone and the Aurora mineral chemistry is consistent with this stratigraphic interval. Pigeonite gabbronorites above the Aurora mineralisation have high Cu/Pd ratios (> 50,000) reflecting the preferential removal of Pd over Cu in the sulphides below. Similarly high Cu/Pd ratios characterise the Upper Main Zone in the northern limb above the pigeonite + orthopyroxene interval and suggest that Aurora-style sulphide mineralisation may be developed here as well. The same mineralogy and geochemical features also appear to be present in the T Zone of the Waterberg PGE deposit, located under younger cover rocks to the north of Aurora. If these links are proved they indicate the potential for a previously unsuspected zone of Cu-Ni-PGE mineralisation extending for over 40 km along strike through the Upper Main Zone of the northern Bushveld.  相似文献   

2.
The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long, are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections. These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides, bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE grade may be decoupled from S and base metal abundance.  相似文献   

3.
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   

4.
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   

5.
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   

6.
南非西布什维尔德杂岩体铂、钯、金区域地球化学测量   总被引：5，自引：1，他引：4  
张洪  陈方伦  D. De Bruin 《物探与化探》1998,22(5):360-370,359
利用南非勒斯滕堡地区区域地球化学测量水系沉积物和土壤样品,采用化学光谱法测定Pt、Pd和Au三种元素,对该区布什维尔德杂岩体铂、钯和金进行区域地球化学勘查,发现新的铂族矿(带)异常,在主带上部和上部带底部发现新的Pt、Pd和Au的异常带。  相似文献   

7.
The Merensky Reef and the underlying Upper Group 2 chromitite layer, in the Critical Zone of the Bushveld Complex, host much of the world’s platinum-group element (PGE) mineralization. The genesis is still debated. A number of features of the Merensky Reef are not consistent with the hypotheses involving mixing of magmas. Uniform mixing between two magmas over an area of 150 by 300 km and a thickness of 3–30 km seems implausible. The Merensky Reef occurs at the interval where Main Zone magma is added, but the relative proportions of the PGE in the Merensky Reef are comparable to those of the Critical Zone magma. Mineral and isotopic evidence in certain profiles through the Merensky Unit suggest either mixing of minerals, not magmas, and in one case, the lack of any chemical evidence for the presence of the second magma. The absence of cumulus sulphides immediately above the Merensky Reef is not predicted by this model. An alternative model is proposed here that depends upon pressure changes, not chemical processes, to produce the mineralization in chromite-rich and sulphide-rich reefs. Magma was added at these levels, but did not mix. This addition caused a temporary increase in the pressure in the extant Critical Zone magma. Immiscible sulphide liquid and/or chromite formed. Sinking sulphide liquid and/or chromite scavenged PGE (as clusters, nanoparticles or platinum-group minerals) from the magma and accumulated at the floor. Rupturing of the roof resulted in a pressure decrease and a return to sulphur-undersaturation of the magma.  相似文献   

8.
Analyses of stream sediment and soil samples from the Bushveld Complex, South Africa have revealed enhanced precious metal concentrations, which can be related both to mining activities and the presence of hidden concentrations of platinum-group elements (PGEs) and gold. The economically important PGE deposits hosted by the Upper Critical Zone of the Rustenburg Layered Suite are revealed by a high PGE and Au content in the overlying soils. A second zone of elevated precious metal concentrations straddles the boundary between the Main and Upper Zones and has to date been traced for more than 100 km. This zone follows the igneous layering of the Rustenburg Layered Suite and is offset by the Brits Graben. It is therefore thought to be the reflection of a magmatic PGE-Au mineralisation. Received: 31 May 1996 / Accepted: 7 January 1997  相似文献   

9.
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   

10.
The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr2O3 content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.  相似文献   

11.
Sulphide separates from mineralisation in Archaean and Palaeoproterozoic host rocks of the Disko Bugt area, central West Greenland, have been analysed for trace elements and Pb isotopes. Isotopic compositions of lead from sulphide separates of Archaean supracrustal rocks show wide variations. Archaean semi-massive sulphides and sulphides separated from felsic metavolcanites yield an errorchron age of 2821 +77/−82, with a model μ1 value of 7.36; this is comparable to the estimated age of the supracrustal rocks. The two most prominent mineralised sites, the Andersen and Eqip prospects, have their own unique Pb isotope pattern; the Andersen prospect is considered to represent the result of an upper crust of Palaeoproterozoic process. The sulphide separates of Palaeoproterozoic epigenetic mineralisation hosted in shear and fault zones in the supracrustal rocks has a common origin, e.g. linked to a metamorphic peak and/or hydrothermal alteration. Gold-bearing samples indicate a local origin for associated sulphides; no regional processes seem to be involved in the formation of the gold occurrences. Received: 17 March 1997 / Accepted: 8 July 1997  相似文献   

12.
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   

13.
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   

14.
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   

15.
The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.  相似文献   

16.
Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discovery of ore-grade mineralisation to the southwest of the deposit, much of which occurs within the Upper Pale Beds, a horizon that is only weakly mineralised above the main orebody. Within this new U lens, mineralisation preferentially occurs within bioclastic carbonate grainstones and calcareous quartz sandstones, and is dominated by sulphide replacement of the carbonate component of the host sequence. Much of the replacive mineralisation is spatially associated with hydrothermal cavities, which are filled by a variable mixture of brecciated replacement sulphide minerals, space-filling sulphide and gangue cements, and internal sediments. Mineralisation also occurs within veins and dissolution seams, and as disseminated sulphide minerals. Massive mineralisation is typically a complex, chaotic, combination of replacement, cavity, and fracture-filling sulphides. Fluid inclusion analyses of ore-stage saddle dolomite indicate temperatures at the time of sulphide precipitation of ~90–150 °C, with a maximum of 175 °C. These temperatures are lower than those typically proposed for Irish-type deposits.  相似文献   

17.
We report the first Nd isotopic data on the cumulate rocks of the Bushveld Complex, South Africa. We analysed 17 whole-rock samples covering 4700 m of stratigraphy through the Lower, Critical and Main Zones of the intrusion at Union Section, north-western Bushveld Complex. The basal ultramafic portions of the complex have markedly higher ɛNd(T) (−5.3 to −6.0) than the gabbronoritic Main Zone (ɛNd(T) −6.4 to −7.9). The rocks of the Upper Critical Zone have intermediate values. These results are in agreement with new Nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas, and with published Sr isotopic data, all of which show a larger crustal component in the upper part of the intrusion. In contrast, the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures. The basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions. The variations of isotopic and trace-element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated Bushveld magmas. Those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace-element-rich partial melt of upper crust, whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace-element-poor residue of partial melting. Received: 5 October 1999 / Accepted: 7 July 2000  相似文献   

18.
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   

19.
“His mind was like a soup dish—wide and shallow; ...” - Irving Stone on William Jennings Bryan
A compilation of the Sr-isotopic stratigraphy of the Bushveld Complex, shows that the evolution of the magma chamber occurred in two major stages. During the lower open-system Integration Stage (Lower, Critical and Lower Main Zone), there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing, crystallisation and deposition of cumulates. Larger influxes correspond to the boundaries of the zones and sub-zones and are marked by sustained isotopic shifts, major changes in mineral assemblages and development of unconformities. During the upper, closed system Differentiation Stage (Upper Main Zone and Upper Zone), there were no major magma additions (other than that which initiated the Upper Zone), and the thick magma layers evolved by fractional crystallisation. The Lower and Lower Critical Zones are restricted to a belt that runs from Steelpoort and Burgersfort in the northeast, to Rustenburg and Northam in the west and an outlier of the Lower and Lower Critical Zone, up to the LG4 chromitite layer, in the far western extension north of Zeerust. It is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the Lower Critical Zone occur. These chromitites include the economically important c. 1 m thick LG6 and MG1 layers exposed around both the Eastern and Western lobes of the Bushveld Complex. The Upper Critical Zone has a greater lateral extent than the Lower Critical Zone and overlies but also onlaps the floor-rocks to the south of the Steelpoort area . The source of the magmas also appears to have been towards the south as the MG chromitite layers degrade and thin northward whereas the LG layers are very well represented in the North and degrade southward. Sr and Os isotope data indicate that the major chromitite layers including the LG6, MG1 and UG2 originated in a similar way. Extremely abrupt and stratigraphically restricted increases in the Sr isotope ratio imply that there was massive contamination of intruding melt which “hit the roof” of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite (Kruger 1999; Kinnaird et al. 2002). Therefore, each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor. Nevertheless, this was achieved by in situ contamination by roof-rock melt of the intruding Critical Zone liquids that had an orthopyroxenitic to noritic lineage. The Main Zone is present in the Eastern and Western lobes of the Bushveld Complex where it overlies the Critical Zone, and onlaps the floor-rocks to the south, and the north where it is also the basal zone in the Northern lobe. The new magma first intruded the Northern lobe north of the Thabazimbi–Murchison Lineament, interacted with the floor-rocks, incorporated sulphur and precipitated the “Platreef” along the floor-rock contact before flowing south into the main chamber. This exceptionally large influx of new magma then eroded an unconformity on the Critical Zone cumulate pile, and initiated the Main Zone in the main chamber by precipitating the Merensky Reef on the unconformity. The Upper Zone magma flowed into the chamber from the southern “Bethal” lobe as well as the TML. This gigantic influx eroded the Main Zone rocks and caused very large-scale unconformable relationships, clearly evident as the “Gap” areas in the Western Bushveld Complex. The base of this influx, which is also coincident with the Pyroxenite Marker and a troctolitic layer in the Northern lobe, is the petrological and stratigraphic base of the Upper Zone. Sr-isotope data show that all the PGE rich ores (including chromitites) are related to influxes of magma, and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion; with associated transgressive disconformities onto the floor-rocks coincident with major zone changes. These positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped. The outcrop patterns and the concordance of geochemical, isotopic and mineralogical stratigraphy, indicate that during crystallisation, the Bushveld Complex was a wide and shallow, lobate, sill-like sheet, and the rock-strata and mineral deposits are quasi-continuous over the whole intrusion.
F. Johan KrugerEmail:
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20.
Summary Migration of residual liquid can potentially affect the textures and mineral compositions in layered intrusions, but is difficult to conclusively demonstrate. In the Upper Zone of the Bushveld Complex a metabasaltic xenolith forms a locally impermeable horizon, which acted as a barrier to vertically migrating residua. Increased Ab content in plagioclase, and K2O and Zr in whole-rock analyses in the anorthosite directly below the xenolith, compared to the same horizon along strike of the xenolith, demonstrate trapping of residual liquid and/or fluid beneath the xenolith.Comparison of Cu/Ni and Cu/S ratios of the mineralised anorthosite in the normal sequence and below the xenolith suggests that these are primary magmatic sulphides which crystallised within the anorthosite and are not derived by sinking of dense interstitial sulphide liquid originally associated with the overlying magnetite layer or introduced hydrothermally.
Vertikale Migration von Restmagma in der Upper Zone des Bushveld-Komplexes
Zusammenfassung Die Migration von Restschmelzen kann Texturen und Mineralzusammensetzungen in geschichteten Intrusionen beeinflussen, jedoch ist es schwierig, dies eindeutig nachzuweisen. In der Upper Zone des Bushveld-Komplexes bildet ein metabasaltischer Xenolith einen lokal undurchlässigen Horizont, der für vertikal migrierte Residuen als Barriere fungierte. Erhöhte Ab-Gehalte in Plagioklas, sowie erhöhte K2O und Zr-Werte in Gesamtgesteins-Analysen in Anorthosit direkt unterhalb des Xenolithen-verglichen mit der Zusammensetzung des gleichen Horizontes in Streichen des Xenolithen-weisen auf Konzentration von Restschmelzen und/oder Fluiden im Liegenden des Xenolithen hin. Der Vergleich von Cu/Ni und Cu/S Verhältnissen der mineralisierten Anorthosite in der normalen Abfolge mit denen unterhalb des Xenolithen zeigt, daß es sich hier um primäre magmatische Sulfide handelt, die innerhalb des Anorthosits kristallisierten. Diese Sulfide lassen sich nicht auf das Absinken von dichter Sulfidschmelze, die ursprünglich mit den überlagernden Magnetit-Bändern in Zusammenhang standen, und auch nicht auf hydrothermale Zufuhr zurückführen.
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