mapping the 3d extent of the northern lobe of the bushveld layered mafic intrusion from geophysical data abstract geophysical models image the 3d geometry of the mafic portion of the bushveld complex north of the thabazimbi murchison lineament tml critical for understanding the origin of the world s largest layered mafic intrusion and platinum group element deposits the combination of the gravity and magnetic data with recent seismic mt borehole and rock property measurements powerfully constrains the models the intrusion north of the tml is generally shallowly buried generally 12 000 m averaging 4000 m a feeder suggested by a large modeled thickness 10 000 m and funnel shape for lower zone magmas could have originated near the intersection of ns and ne trending tml faults under mokopane the tml has been thought to be the feeder zone for the entire bushveld complex but the identification of local feeders and or dikes in the tml in the models is complicated by uncertainties on the syn and post bushveld deformation history however modeled moderately thick high density material near the intersection of faults within the central and western tml may represent feeders for parts of the bushveld complex if deformation was minimal the correspondence of flat high resistivity and density regions reflect the sill like geometry of the bushveld complex without evidence for feeders north of mokopane magnetotelluric models indicate that the transvaal sedimentary basin underlies much of the bushveld complex north of the tml further than previously thought and important because the degree of reaction and assimilation of the transvaal rocks with the mafic magmas resulted in a variety of mineralization zones

stability assessment and slope design at sandsloot open pit south africa sandsloot open pit is located on the northern limb of the bushveld igneous complex it is the largest open pit platinum mine in the world three major joint sets have been recognized at sandsloot which are related to the regional tectonic history they have an important influence on slope stability in the open pit notably in terms of planar and wedge failures detailed geological and geotechnical data are often a notable unknown factor in the design and operation of an open pit the lack of which may pose a significant risk to the mining venture as data are accumulated and used effectively so the risk of unforeseen conditions is reduced and accordingly safety and productivity is increased usually the geotechnical work undertaken at an open pit mine is in connection with improving slope stability at sandsloot open pit geological and geotechnical data have been obtained by face mapping scanline surveys from exploration drillholes and from laboratory tests such data have been used to delineate different geotechnical zones in which different types of slope failure have occurred these are the usual types of slope failure associated with rock masses namely planar wedge toppling and circular failures analysis of the data has allowed optimum design parameters to be developed for these zones which has led to improved slope stability in other words this has allowed slope management programmes to be initiated as well as slope optimization of the hangingwall the latter resulted in an improved slope configuration and an increase in the ultimate angle of the wall by 7 this has resulted in substantial savings as well as an improvement in safety

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits 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

pge enrichment in chromitite layers and the merensky reef of the western bushveld complex a re os and rb sr isotope study abstract four poikilitic pyroxenites of the bastard unit all being relatively low in osmium concentrations define a re os isochron age of 2043 11 ma with initial 187os 188os 0 151 which is in accordance with the normally cited crystallization age of the bushveld complex the isochron fit demonstrates an excellent radiogenic os isotopic homogeneity of this portion of the complex at the time of crystallization and excludes any later disturbance of the re os isotope system initial 187os 188os ratios of platinum group element pge enriched sulfides and whole rocks of the directly underlying merensky reef are highly radiogenic and variable with values between 0 168 and 0 181 separates of chromite grains and interstitial phases of critical zone chromitite layers show initial 187os 188os signatures ranging from the near chondritic mantle values of lower group chromitites i e 0 120 to 0 137 for middle group chromitites and to 0 150 for the upper group 2 chromitite layer osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold showing the presence of highly pge enriched interstitial sulfides equal or very similar initial 187os 188os signatures of the interstitial minerals and the cumulus chromite grains together with their coupled increasing and decreasing trends in this ratio clearly demonstrate that interstitial sulfides and associated cumulus chromite of the different chromitite layers scavenged their osmium from the same or similar sources therefore a pge enrichment of the interstitial sulfides by a primary magmatic process such as precipitation of immiscible sulfide droplets during magma mixing appears more likely than a scenario involving fluid or melt transport of pge into these layers such a primary magmatic pge enrichment process is also preferred for the formation of the merensky reef

discordant iron rich ultramafic pegmatites in the bushveld complex and their relationship to iron rich intercumulus and residual liquids the iron rich ultramafic pegmatites comprise a suite of coarse grained rocks that form discordant bodies within the layered sequence of the bushveld complex these pegmatites which are considerably more abundant than is generally realized provide evidence for the differentiation of iron rich residual melts the pegmatites are composed largely of iron rich olivine and clinopyroxene together with ti magnetite and ilmenite feldspar is characteristically absent but paradoxically the pegmatites preferentially replace anorthositic cumulates two subgroups are recognized olivine clinopyroxene pegmatite and fe ti oxide pegmatite with increased stratigraphic height the pegmatites become richer in fe ti oxides thus olivine clinopyroxene pegmatite is prevalent in the upper critical and lower main zones whereas fe ti oxide pegmatite is restricted to the upper main and upper zones zoned pegmatite with a core of fe ti oxide pegmatite is transitional between the two subgroups new whole rock and electron microprobe analyses of olivine clinopyroxene pegmatite from the upper critical and lower main zones provide convincing evidence that their composition is directly related to height cryptic compositional variations are analogous to those displayed by the layered cumulates but for a given height the pegmatites are always more evolved compositions of clinopyroxene in the pegmatites reflect a near linear relationship with height whereas cumulus pyroxenes display upward iron enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid the olivine clinopyroxene pegmatite formed by magmatic replacement of earlier formed cumulates in response to infiltration of iron rich melts suitably differentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers the absence of feldspar although not fully understood is attributed to an immiscible relationship between dense iron rich melts and light silica alkali rich liquids the latter infiltrated upward to be reincorporated into the resident magma the iron rich melts however drained down into the crystallizing cumulate pile channelling along early formed fractures and joints was significant locally resulting in huge pipe like bodies of pegmatite the iron rich melts became increasingly differentiated with height partly in response to the fractional crystallization of more evolved cumulates the olivine clinopyroxene pegmatites are related to infiltration of fe ti oxide rich silicate melt whereas fe ti oxide pegmatite is ascribed to fe ti oxide liquid as originally argued by bateman 1951 the bushveld complex followed the fenner trend of almost uninterrupted iron enrichment evidence of pronounced iron enrichment is however manifested in the discordant iron rich ultramafic pegmatites several thousands of metres below the height at which iron rich cumulates are observed

cumulative deformation and original geometry of the bushveld complex abstract a structural review and re interpretation of the bushveld complex based on 254 georeferenced maps from research technical reports and geophysical surveys suggests that its current geometry differs markedly from its original geometry broadly n s flattening left lateral transpressional shearing along an interconnected fault network steepening of cumulate layering and 10 km of uplift to the north of the northern margin of the complex s main chamber were accompanied by an eastward verging escape structure burgersfort bulge and syn cooling gap areas on its western limb southward verging deformation was transmitted from the villa nora fragment via thrusting in the rooiberg area generating 50 70 km of southward displacement uplift and erosional loss of the northern limb s western portion a string of anomalous occurrences of bushveld complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber the southern limb underwent predominantly right lateral ductile offset from the western limb along the crocodile river and rustenburg faults expressed by an in line series of nnw trending magnetic highs to the south of their mapped terminations left lateral shearing and progressive splaying of a nne trending dyke swarm to the west of an inflection point provides strain markers for this offset and 35 anticlockwise rotation of the burgersfort bulge and central parts of the main chamber although recent work suggests that the complex cooled within 1 myr late intrusion syn cooling sse wards movement of its deeper ductile central parts within at least 5 10 myr after its initial intrusion the transvaalide fold and thrust event and the expulsion of fluids to form the high ti igneous suite hitis and related bodies appear to have closely post dated the complex s intrusion removal of the effects of cumulative strain produces a more equant complex shortly after its intrusion occupying an orthogonal network of normal faults and perpendicular strike slip transverse faults

post deposition structures in the bushveld complex mafic sequence the stratigraphy of the bushveld complex is characterized by laterally continuous horizontal layering at the interface of some of these layers there are phenomena analogous to flame load cast ball and pillow and liquid escape structures of sedimentary rocks three plastic deformation styles recognized in the complex are i vertical ii vertical followed by horizontal and iii vertical fluid movement with brittle failure these post deposition structures are used to estimate viscosity and density of adjacent layers during consolidation the distribution of the structures through the mafic sequence provides some information on magma movement and rates of crystal accumulation

the conformable emplacement of the bushveld mafic rocks along a regional unconformity in the transvaal succession of south africa abstract a previously unrecognized regional unconformity occurs in the proterozoic transvaal succession of the kaapvaal province of southern africa the predominantly marine and siliciclastic rocks of the lower two thirds of the pretoria group are unconformably overlain by a terrestrial and largely volcanic sequence the most voluminous portion of which is the rooiberg felsite if the bushveld complex is mentally subtracted from map patterns the rooiberg felsite also locally overlies pre pretoria sequences and the archean crystalline basement the bushveld complex is not lopolithic as commonly supposed instead regionally condordant emplacement of the complex along the unconformity explains both the seemingly discordant footwall features and the lack of pretoria and older strata other than a few xenoliths in the hanging wall the lack of any geochemical evidence for major crustal contamination of bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation the bushveld complex the unconformity and the enclosing strata are synformally folded making the bushveld appear to be lopolithic due to this folding and multiple periods of erosion the original extents of the bushveld complex the unconformity and the enclosing strata are now much reduced other smaller mafic intrusions along or immediately below the same regional unconformity probably are bushveld related

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa 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 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

overview of the magnetic signatures of the palaeoproterozoic rustenburg layered suite bushveld complex south africa abstract aeromagnetic data clearly delineate the mafic rocks of the economically significant bushveld igneous complex this is mainly due to the abundance of magnetite in the upper zone of the rustenburg layered suite of the bushveld but strongly remanently magnetised rocks in the main zone also contribute significantly in places in addition to delineating the extent of the magnetic rocks in the complex the magnetic anomalies also provide information about the dip and depth of these units the presence of varying degrees of remanent magnetisation in most of the magnetic lithologies of the rustenburg layered suite complicates the interpretation of the data the combination of available regional and high resolution airborne magnetic data with published palaeomagnetic data reveals characteristic magnetic signatures associated with the different magnetic lithologies in the rustenburg layered suite as expected the ferrogabbros of the upper zone cause the highest amplitude magnetic anomalies but in places subtle features within the main zone can also be detected a marker with strong remanent magnetisation located in the main zone close to the contact with the upper zone is responsible for very high amplitude negative anomalies in the southern parts of both the eastern and western lobes of the bushveld complex prominent anomalies are not necessarily related to a specific lithology but can result from the interaction between anomalies caused by differently magnetised bodies the magnetic data provided substantial information at different levels of detail ranging from contacts between zones and layering within zones to magnetite pipes dykes and faults that can have an impact on mine planning finally simple modelling of the magnetic data supports the concept of continuous mafic rocks between the western and eastern lobes

bushveld aged fluid flow peak metamorphism and gold mobilization in the witwatersrand basin south africa constraints from in situ shrimp u pb dating of monazite and xenotime in situ u pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the witwatersrand basin south africa reveals two episodes of tectonothermal activity a major event between 2 06 and 2 03 ga is recorded in the wit watersrand and transvaal supergroups in the northwestern and central basin and broadly coincides with the ca 2 06 ga bushveld event in the central and southern basin a previously unrecognized event has been dated between 2 14 and 2 12 ga the widespread geographic and stratigraphic occurrence of bushveld aged monazite and xenotime including both auriferous reefs and unmineralized strata indicates that metamorphism and fluid flow associated with magmatism was pervasive affecting most of the succession 10 km thick in the central and northern parts of the basin the metamorphic phosphate dates which are younger away from the complex indicate a lag of 20 30 m y between emplacement and phosphate growth in the central basin 100 km south suggesting that heat related to magmatism was transferred southward at an average rate of 3 5 mm yr 1 the absence of 2 06 2 03 ga phosphates in the welkom goldfield at the southern end of the basin implies that bushveld related heating and fluid flow did not affect this part of the basin the intergrowth of ca 2 045 ga monazite with gold in quartz pebble conglomerate from the west rand goldfield indicates that fluid flow related to the bushveld event caused mobilization of gold in the witwatersrand basin

continuity between eastern and western bushveld complex south africa confirmed by xenoliths from kimberlite the three dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops in some cases aided by drilling and or geophysical data however geophysical models are often equivocal for the 2 06 ga bushveld complex of south africa early geological models proposed a shape of a single gently inward dipping lopolith subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete dipping wedge shaped intrusions separated by 150 km a more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model distinguishing between these possibilities is difficult from surface based studies because the central regions of the complex are obscured by large volumes of younger granites and sedimentary volcanic cover rocks here we describe xenoliths from the cretaceous palmietgat kimberlite pipe located mid way between the exposed western and eastern lobes of the complex they are chromite bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping critical zone of the bushveld complex this result provides strong support for a regionally interconnected bushveld complex implying its emplacement as a single sill like body confirming the continuity of the bushveld complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry

structure of the bushveld complex from resistivity measurements knowledge of the deep structure of the arc like bodies constituting the bushveld complex south africa is essential not only to understand the emplacement history of this huge layered intrusion but also for success in the exploration for the platiniferous merensky reef and chromitite layers occurring near its base despite many studies the structure of the complex has remained obscure here we describe new insights into the structure and additional geophysical boundary conditions based on a geoelectrical study this has provided a geoelectrical stratigraphy according to which a conductive layer below the granitic rocks of both the eastern and western compartments of the complex is correlated with sedimentary rocks of the transvaal sequence over large areas rather than with the mafic sequence as conventionally assumed a new feature of this model is that the mafic sequence in both compartments occurs as segments of an inward dipping sheet transgressing the layering of the sedimentary rocks into which it intruded the geoelectrical model supports earlier gravity models in that mafic rocks are absent from the central part of the complex but shows that here the bushveld granite directly overlies archaean basement rather than transvaal sequence rocks as previously accepted

the role of fluid pressure in the formation of bedding parallel thrust hosted gold deposits sabie pilgrim s rest goldfield eastern transvaal abstract shallowly dipping thrust hosted gold veins occur within the paleoproterozoic transvaal supergroup rocks in the eastern transvaal of south africa internally the veins record episodic events of fluid infiltration and subsequent mineral deposition within discrete local scale thrust faults evidence also exists for subvertical dilation within the individual veins and textures which record open space filling are preserved the probable depth of formation suggests lithostatic pressures in excess of 2 2 kbar and the vein textures indicate numerous reactivation events associated with individual thrust faults occurring in a terrain characterised by low levels of preserved strain fault reactivation may have occurred under conditions of low differential stress in the presence of high pressure fluids with p fluid approaching or exceeding lithostatic levels

8000 yr of black carbon accumulation in a colluvial soil from nw spain analytical pyrolysis gc ms and solid state 13c nmr nuclear magnetic resonance were applied to the naoh extractable organic matter fraction of a colluvial soil from galicia nw spain that represents more than 8500 yr of accumulation while molecular indicators of vegetation change were looked for it seemed likely that any such signal was disturbed by the intense fire regime of the area this conclusion was drawn from 1 the presence of three charcoal layers 2 the high proportion of aryl c in nmr spectra non quantitative and 3 the prevalence of benzenes and polycyclic aromatic hydrocarbons pahs in the chromatograms 38 6 of total identified peak area also in charcoal poor samples if this conclusion is accurate the area has been subjected to burning episodes for at least 8000 yr additionally the results indicate that biomass burning residues black carbon bc may become naoh extractable after long periods of degradation in mineral soil these results add to our knowledge of the long term fate of bc in soil which is a potential agent in the global c cycle

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

flexural slip structures in the bushveld complex south africa this paper investigates structures in the layered critical zone of the bushveld complex in south africa that have detrimentally affected mining operations the common structures are layer parallel faults with reverse dip slip and ramp faults with curved slip planes and prominent striations geometric variations include duplex ramps contained within floor and roof layer parallel faults and linking ramps connecting separate layer parallel faults hidden in the footwall and hangingwall the orientations geometries displacements and shear senses of the layer parallel faults and ramp faults are interpreted to be flexural slip structures formed during bending of the originally horizontal bushveld complex into a basin fold geometry during crustal loading

palaeomagnetism of the 2054 ma bushveld complex south africa implications for emplacement and cooling summary the kaapvaal craton south africa was the host of several major magmatic events around 2000 ma including the bushveld complex the world s largest known layered mafic intrusion 0 5 1 106 km3 the bushveld complex has been the subject of numerous palaeomagnetic studies which yielded a large spread in palaeomagnetic pole positions for the different zones and interpreted to indicate that the bushveld complex was emplaced and cooled over a time span of 50 million years new palaeomagnetic data collected from 100 sites 996 drill cores from all zones of the rustenburg layered suite of the bushveld complex yield exceptional palaeomagnetic results with high unblocking hb components carried by magnetite comparable palaeomagnetic poles from all zones mean pole latitude 19 2 n longitude 030 8 e a95 5 8 correction made after online publication 22 september 2009 the mean pole values have been corrected eliminates the previously noted large spread in poles and this observation concurs with precise age data that constrain the time period of emplacement of the bushveld complex to a few million years at around 2054 ma bedding corrected hb components from all zones produced better directional groupings which together with at least seven reversals strongly points to a primary magnetic signature this imply that cooling of the bushveld complex below the blocking temperature of magnetite 580 c occurred in a near horizontal position and based on the maximum observed reversal rates in the geological history 5 ma 1 we estimate a minimum cooling interval for the bushveld complex of 1 4 million years

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

stress field and active faults in the orogenic front of the andes in the malargue fold and thrust belt 35 36 s abstract we integrate field and wellbore data to discuss the stress field in the frontal sector of the malargue fold and thrust belt andes of argentina surface observations indicate n s thrusts and nw to wnw and ese strike slip faults are active in the study area inversion of fault kinematic indicators combined with borehole breakout data and a mini frac test within the study area constrain the quaternary to recent stress state which is characterized by a subhorizontal e w oriented maximum stress and by intermediate and minimum stresses with similar magnitudes that are locally interchanged producing a setting in which reverse and strike slip faults are alternatively active the implications of the recognized structures for earthquake hazard are examined

palaeomagnetism and 40ar 39ar geochronology of mafic dykes from the eastern bushveld complex south africa summary we report palaeomagnetic and 40ar 39ar age data for dykes that intrude the 2 ga eastern bushveld igneous complex bic the dykes were previously assumed to be of karroo age jurassic 180 ma based on their ne sw orientation palaeomagnetic data pole position 8 7 n 22 e dp dm 18 20 6 however clearly demonstrate that these dykes are precambrian in age either 1 9 ga and close to the early proterozoic bushveld age or 1649 10 ma based on 40ar 39ar plagioclase laser fusion ages from one of the dykes both normal and reverse polarity dykes are identified and a positive reversal test together with a semi conclusive contact test attests to a primary magnetization if the 40ar 39ar age represents a primary cooling age then palaeomagnetic poles from south africa kalahari at 1 9 to 2 ga and 1650 ma are virtually identical and suggest an apparent polar wander loop alternatively the kalahari craton drifted from high southerly 50 to high northerly latitudes or vice versa during this interval conversely if we assign a 1 9 ga age for dyke emplacement as suggested from a comparison with kalahari palaeomagnetic poles e g waterberg soutpansberg pole the 1650 ma 40ar 39ar age must relate to a thermal disturbance that did not erase the primary magnetic signature

the geology and structure of the rustenburg layered suite in the potgietersrus mokopane area of the bushveld complex south africa 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

stratigraphic and structural controls on gold mineralization in the pilgrim s rest goldfield eastern transvaal south africa abstract since 1872 approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the pilgrim s rest goldfield the epigenetic stratiform ore bodies are found primarily in the malmani dolomite subgroup a laterally extensive lower proterozoic carbonate platform succession a strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation analysis of the host sediments indicates deposition in a retrogradational followed by an aggradational sequence that comprises supratidal sabkha siliciclastic sediments to midshelf carbonates the retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles mineralization is with few exceptions restricted to shallow water siliciclastic sediments deeper water carbonates are barren more importantly siliciclastic mudstones and sandstones have acted as zones of detachment for thin skinned thrust fault deformation resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes evidence of shallow hinterland dipping duplex antiformal stack and imbricate thrust systems abounds in several underground and field exposures the gold silver emplacement model for the pilgrim s rest goldfield infers fluids derived from a deep seated magmatic source oxygen isotopic compositions of the mineralizing fluids recalculated from 18o quartz 12 1 to 19 5 point to a magmatic source and subsequent mixing with saline evolved formation water a magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions 100 c km the bushveld igneous complex rustenburg layered suite to the west of pilgrim s rest is thought to be the source of the auriferous hydrothermal solutions and its emplacement was the driving mechanism of thrust fault deformation in the pilgrim s rest goldfield

diapirism initiated by the bushveld complex south africa the bushveld complex induced the development of a unique type of diapir during its intrusion these diapirs are preserved as large domal structures exposed on the margin and within the northeastern mafic layered sequence they were produced by gravitational loading and heating of argillaceous and arenaceous floor rocks gravity induced deformation is restricted to rocks above a detachment level that corresponds approximately with the 550 c peak metamorphic isotherm and with lithological contacts of contrasting competency strongly foliated and boudinaged contact metamorphosed sedimentary rocks between domes were deformed by lateral extension along flow lines directed toward dome culminations at the highest structural levels domes have bulbous geometries and the enclosing layered igneous rocks are deformed into series of outward verging folds that define a broad rim syncline deformation within the domal cores is represented by constriction that produced radial curtain type folds that have steeply plunging lineations and concentrically oriented folds in the outer shell diapirism is closely linked to magma emplacement mechanisms floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion with the coalescence of intrusion fingers into a single sheet interfinger folds matured into large diapiric domes that rose to the upper levels of the magma chamber strain rates estimated from strain analyses dome geometry and model cooling calculations are on the order of 10 14 s 1 corresponding to diapiric uplift rates of 0 6 cm yr

plagioclase content of cyclic units in the bushveld complex south africa the upper critical zone of the bushveld complex south africa has been divided into so called cyclic units ideally they should consist of from the base upward layers of chromitite pyroxenite norite and anorthosite some hypotheses for their origin suggest that they result from the addition and mixing of a magnesian magma having orthopyroxene or olivine as liquidus mineral these models predict that the bulk cumulate composition in a cyclic unit formed from such magma ought to contain excess orthopyroxene to plagioclase relative to their cotectic proportions an hypothesis tested here all rocks investigated here from the upper critical zone at impala platinum mine are mixtures of cumulus plagioclase and orthopyroxene except for the chromitites with very minor intercumulus minerals mainly clinopyroxene hence the proportion of plagioclase can be determined by measurement of the density of a sample the density of every piece of continuous bore core from the upper group 2 ug2 and merensky cyclic units has been determined and the proportion of plagioclase calculated for the bastard cyclic unit estimates of the proportion of plagioclase have been made based on published modal and chemical analyses vertical summation of the proportion of plagioclase indicates that these three cyclic units ug2 merensky and bastard respectively contain 64 74 and over 80 plagioclase by weight for the merensky and bastard cyclic units such proportions exceed that of the plagioclase orthopyroxene cotectic and could not have been produced by processes involving magnesian magma

strontium isotope evidence for preserved density stratification in the main zone of the bushveld complex south africa the bushveld complex formed from two isotopically distinct magmas magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates rocks with ratios of 0 7065 lie below the lower 2 2 km of the main zone ratio 0 7085 whereas those with ratios of 0 7073 lie above it this unit represents a liquid layer intruded into the density stratified magma pile near the base of the chamber the merensky reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals

connectivity between the western and eastern limbs of the bushveld complex abstract the mafic layered rocks of the bushveld complex are 6 8 km thick and crop out over an area of 65 000 km 2 previous interpretations of the bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies however the mafic sequences in these arcuate western and eastern limbs are remarkably similar with at least six petrologically distinctive layers and sequences being recognisable in both limbs such similarity of sequences in two totally discrete bodies 200 300 km apart is petrologically implausible and it is suggested that they formed within a single lopolithic intrusion all previous bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km with this revised whole crustal model it becomes possible to construct a gravity model consistent with observed data which includes a 6 km thick sequence of mafic rocks connecting the western and eastern limbs of the bushveld complex the exact depth at which the mafic rocks of the bushveld complex lie in the centre of the structure cannot be constrained by the gravity data such a first order model is an approximation because there have been subsequent deformation and structural readjustments in the crust some of them probably related to the emplacement of the bushveld complex specifically the observed geometry of the rocks around the crocodile river dennilton marble hall and malope domes suggests that major upwarping of the crust occurred on a variety of scales triggered by emplacement of the bushveld complex

thrust controlled gold mineralisation at the elandshoogte mine sabie pilgrim s rest goldfield south africa stratiform quartz sulphide gold veins locally termed reefs are hosted within the proterozoic transvaal sequence sedimentary succession in the sabie pilgrim s rest goldfield eastern transvaal these deposits have produced about 180 tonnes of gold and share many characteristics with those of telfer western australia detailed examination of the elandshoogte mine shows that gold deposition occurred in two stages both linked to bedding parallel thrust faulting within the sedimentary pile deformation being concentrated within incompetent shale beds interlayered within more competent units the majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early formed sulphide minerals deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone from early shearing in shales to later duplex faulting of the quartz reef fluids accompanying faulting are implied to have transported gold and a magmatic source of mineralisation is suggested

a new occurrence of merensky reef on the flanks of the zaaikloof dome northeastern bushveld complex relationship between diapirism and magma replenishment exploration for platinum group elements has resulted in the discovery of an occurrence of merensky reef in the eastern limb of the bushveld complex south africa the new discovery was made on the dwarsrand property in the poorly known boundary area between the intensively explored western and central sectors of the eastern limb this boundary area is largely composed of domes and basins that may in part be aligned along a regional lineament the wonderkop fault transvaal supergroup metasediments in the core parts of the domes form prominent hills whereas the bushveld complex in the intervening basins is restricted to areas of poor outcrop the formation of domes is attributed to diapirism triggered by gravitational loading and heating of the transvaal supergroup in response to intrusion of the bushveld complex merensky reef was located beneath cover rocks in an area where the critical and main zones are uplifted and attenuated against the southern flank of the zaaikloof dome drill holes although collared in upper zone intersected the critical and main zones at relatively shallow depths as a result of the uplift the critical and main zones including the merensky reef are steeply dipping the base of the upper zone forms an intrusive unconformity and the layering in this zone has a much shallower dip economically significant grades were obtained on the dwarsrand property but exploration was terminated because of poor strike continuity the new findings suggest that uplift and attenuation of the bushveld complex on the flanks of domes in the northeastern part of the complex are both more extensive and more severe than previously realized a complex history of intrusion and uplift is implied some domes even penetrate into the overlying bushveld granite the occurrence of markedly angular unconformities may be contrasted with the layercake sequences typically associated with the bushveld complex diapirism in response to crustal loading and heating by ultramafic sequences occurred in the early stages of magma emplacement episodic intrusion and uplift are consistent with multiple replenishment models in which each of the many dozens of cycles that comprise the critical and main zones corresponds to a new magma pulse with the oldest layers being dragged to higher structural levels in response to the uplift periodic reactivation of doming is indicated by the lowermost layers within the critical and main zones abutting against the southern flank of the zaaikloof dome at progressively higher levels the transgressive nature of the upper zone and bushveld granite relative to the critical and main zones may indicate that the upper zone formed by new intrusions of magma rather than the generally accepted model of differentiation within the chamber the discovery of hidden merensky reef at relatively shallow depths beneath transgressive upper zone and granite could provide the impetus for further exploration possibilities within the bushveld complex particularly on the flanks of regionally extensive domal structures

platinum group minerals in the middle group of chromitite layers at marikana western bushveld complex indications for collection mechanisms and postmagmatic modification the platinum group minerals in a drill core taken through the middle group of chromitite layers in the critical zone at marikana in the western bushveld complex were found to consist mainly of laurite as inclusions in chromite grains the platinum group minerals containing pt pd and rh are concentrated in the intercumulus silicates and frequently associated with base metal sulphides up to about 20 of all platinum group minerals in the investigated chromitite layers contain sub stantial amounts of as the base metal sulphides are strongly modified in the postmagmatic stage which led to a significant loss of fe and s in this way concentrating cu ni and the platinum group elements by factors of up to 10 interaction between chromite and base metal sulphides cannot account for all the fe lost in chromite poor samples and the importance of additional processes is indicated inclusions in chromite and orthopyroxene indicate the formation of discrete platinum group minerals and as rich phases before the

exploration geophysics of the bushveld complex in south africa the bushveld complex consists of three roughly coeval components namely the lebowa granite suite the rashoop granophyre and the rustenburrg layered suite rls the latter being the only economically important unit the rls underlies an area of 65 000 km2 in the northeast sector of south africa and is the oldest and largest mafic layered complex in the world figure 1 it is also the largest repository of magmatic ore deposits and hosts inter alia 80 of the world s platinum group metal pgm resources and most of the chromitite resources over an east west distance of roughly 350 km the rls outcrops as three apparently discrete crescent shaped lobes the western eastern and northern limbs and lies below cover rocks along the southeastern limb the far western limb is currently not of economic significance lithologies range across mafic and ultramafic cumulates plus anorthosites pseudo stratigraphic layering exhibits dips of 8 20 toward the center of the complex the ug2 and merensky reefs constitute the major pgm bearing horizons of mining interest and fall close to the top of the 1500 m thick critical zone an assemblage of alternating leuconorite anorthosite and pyroxene rocks outcropping midway through the rls succession

the tectonic setting of the bushveld complex in southern africa part 1 structural deformation and distribution abstract structural and stratigraphic aspects of the lebowa granite suite the granites and rustenburg layered suite the basic rocks are examined and compared in various compartments of the bushveld complex south africa the coeval molopo farms complex located largely in botswana is included in this examination structural features are evaluated in terms of relationships to major structural features of the kaapvaal craton such as the thabazimbi murchison lineament and its extension in botswana the kgomodikae lineament the melinda abbotspoort rustenburg and steelpoort faults and the barberton lineament in the south histories of structural development in the eastern and western compartments indicate a pre bushveld age of some structural features and models are proposed of recurring movements along pre existing fundamental lines of weakness it is concluded that features such as the thabazimbi murchison lineament are lithospheric in extent and that they played an active role controlling the emplacement of the bushveld and molopo farms complexes

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

the thabazimbi murchison lineament of the kaapvaal craton south africa 2700 ma of episodic deformation the thabazimbi murchison lineament is a prominent tectonic feature extending for approximately 500 km in a nne ssw direction across the kaapvaal craton the lineament has a tectonic history of more than 2 5 ga because it appears to form and or influence the boundary between the archaean granite greenstone terrain of the kaapvaal craton and the southern margin of the high grade terrain of the limpopo mobile belt the northern margin of the archaean proterozoic transvaal basin the southern margin of the mesoproterozoic waterberg basin and has probably also influenced the emplacement of the mesoproterozoic bushveld complex mesozoic karoo basalts and sediments formed the springbok flats these lavas erupted into half graben structures formed by the normal reactivation of the previously strike slip zebedelia fault part of the thabazimbi murchison relay system the lineament is cut by a mesozoic dyke swarm associated with the break up of gondwana

southern african crustal evolution and composition constraints from receiver function studies 1 stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern africa reveals systematic spatial variations in the ratio of crustal p and s wave velocities crustal thickness h and the amplitude of the converted moho phases r the eastern zimbabwe and the southern kaapvaal cratons are characterized by small h 38 km small 1 73 and large r 0 15 values suggesting that the relatively undisturbed archean crust beneath southern africa is separated from the mantle by a sharp moho and is felsic in composition the limpopo belt which was created by a collisional event at 2 7 ga displays large h 43 km but similar and r values relative to the cratonic areas the bushveld mafic intrusion complex and its surrounding areas show large 1 78 large h 43 km and small r 0 11 values reflecting the intrusion of mafic material into the original crust as a result of the bushveld event at 2 05 ga excluding the bushveld the spatially consistent and age independent low accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material within such an island arc model our data combined with xenolith data excluding mantle delamination in cratonic environments suggest that the modification to a felsic composition e g by the partial melting of basalt and removal of residue by delamination is restricted to have occurred during the collision between the arcs and the continent