sills associated with the bushveld complex south africa an estimate of the parental magma composition abstract four distinct ages of sills intrude the floor rocks under the bushveld complex the first is a metadolerite and pre dates the bushveld complex the middle two are generally related to the complex itself one has quench textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the bushveld complex the second suite is a hypersthene microgabbro representative of the second major injection of magma into the complex the last is doleritic and does not obviously correlate with any portion of the layered sequence the sills genetically related to the complex display several features 1 lack of chilled margins coarse grain size ability to melt country rock xenoliths extreme heterogeneity 2 atypical of normal sills but explicable in their present context

the merensky reef bushveld complex mixing of minerals not mixing of magmas a major geochemical hiatus occurs in the bushveld complex at the level of the platiniferous merensky reef close to the critical main zone boundary the origin of this hiatus and its relationship to mineralization has not been fully resolved initial sr isotope ratios for plagioclase and cr mgo values for orthopyroxene from critical and main zone rocks are significantly different geochemical data through the merensky and bastard cyclic units indicate that orthopyroxene in the merensky and bastard units originated from magma with a composition like that of the critical zone magma it sometimes occurs together with plagioclase that originated from main zone magma in a detailed study of the eastern limb the pyroxenite at the base of the merensky unit contains plagioclase and orthopyroxene that have compositions typical of the critical zone but in the overlying part of the merensky unit plagioclase compositions are more typical of the main zone whereas orthopyroxene retains a critical zone signature similarly in the bastard pyroxenite plagioclase has a range of sr isotope values between that of the main and critical zones but orthopyroxene has values typical of the critical zone these observations of mineral disequilibrium clearly show that the two major minerals in the merensky and bastard units were formed from two different but coexisting magmas a model that accounts for this disequilibrium is proposed here it invokes the influx of main zone magma at the base of the merensky unit during which the critical zone magma was displaced upward but the two magmas did not mix the elevated critical zone liquid continued to crystallize orthopyroxene which sank through the influx of magma of the main zone due to the density contrast these grains collected on the crystal pile to form the merensky pyroxenite the main zone magma into which the orthopyroxene accumulated crystallized interstitial plagioclase that had an sr isotope ratio typical of the main zone whole rock major element geochemical data show that a variable proportion of the plagioclase in both the merensky and bastard pyroxenites is cumulus it is inferred to have accumulated with orthopyroxene and has an initial sr isotope ratio typical of the critical zone thus the two pyroxenites now have a mixed sr isotope signature of cumulus critical zone and intercumulus main zone plagioclase that varies along strike above the pyroxenites the sr signature of the norites and anorthosites of both cyclic units is dominated by cumulus plagioclase from the main zone magma but some plagioclase with a critical zone signature may be present we conclude that the variations in initial sr isotope ratios do not result from mixing of magmas but from variable accumulation of orthopyroxene and plagioclase from a higher isotopically distinct layer of magma into an underlying layer the merensky and bastard cyclic units therefore may have features of either critical or main zone magma depending on which chemical parameter cr mgo and sr isotopes is considered they are therefore classified as a transitional unit between the critical and main zones models that suggest magma mixing was responsible for sulfide saturation and the creation of an immiscible sulfide liquid are not supported by this study and alternative models must be sought to explain the sulfide saturation event in the merensky reef

two magmatic series with island arc affinities within the vourinos ophiolite within the vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks which constitute the earlier krapa sequence and the younger asprokambo sequence the asprokambo dyke basic magmas which are poor in incompatible elements and relatively ni and cr rich bear some resemblance to very low ti basalts transitional to boninites found in subduction related arcs or interarc basins krapa series magmas from sills massive and pillow lavas are best equated with low k tholeiites of island arc suites compositions of al and ti poor cpx in lavas from both series are comparable to those in island arc basalts the asprokambo cpx being richer in ca and cr than those from krapa the large volume of cumulates from the krapa sequence displays the following crystallization order ol sp cpx pl opx mt periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates wehrlite ol clinopyroxenite and melagabbro the upper cumulates gabbronorite and leucogabbronorite with minor mt bearing gabbronorite crystallized in the upper levels of a magma chamber which became progressively smaller with time in the asprokambo sequence ol sp opx cpx pi and amph are the successively crystallizing phases the ortho to heteradcumulates consist of websterite pl websterite gabbronorite amphibole bearing leuconorite diorite and granophyre in cumulates especially in the lower krapa sequence significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism petrological features indicate that the evolution of the krapa series is more compatible with an intermediate fractional equilibrium crystallization history in an initially open system whereas nearly perfect fractional crystallization in closed system may have occurred in the small asprokambo magma chambers chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates significantly the asprokambo intrusives have igneous mg hornblende and vanadium bearing chromian aluminous titaniferous magnetite crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks major and trace element modelling necessitates a two stage model for the petrogenesis of the vourinos parental melts involving high degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids the primary magmas parental to the krapa and asprokambo series could have been derived respectively by 20 and 30 equilibrium partial fusion of variably depleted lherzolitic sources leaving residua having a harzburgitic to dunitic composition

the magnetic stratification of layered mafic intrusions natural examples and numerical models abstract mafic ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber this layering occurs at scales from several hundreds of meters in some large intrusions down to scales of a few millimeters layering forming processes are numerous and include both internal processes such as crystal settling double diffusive convection flow segregation and compaction and external processes such as magma recharge extraction or mixing mafic magma chambers may operate as a closed system or as an open system several recent investigations have been conducted on borehole cores through the insizwa sill south africa the great dyke zimbabwe and the bushveld complex south africa we present an overview of magnetic results on these intrusions the magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers pyrrhotite the only ferrimagnetic sulfide is generally restricted to sulfide rich reefs merensky j m bohmke or to massive sulfide deposits at the base of the intrusion all intrusions display distinct variations of their magnetic properties anisotropy of magnetic susceptibility magnetic remanence magnetic saturation high field magnetic susceptibility across the layering the significance of these magnetic zones varies depending on the nature of variations from zone to zone for example a drop in magnetic susceptibility does not necessarily correspond to a new magma batch unless it is also associated with a change in magnetic anisotropy parameters in the studied cases all open systems the magnetic susceptibility and the total iron content estimated from high field measurements increase towards the top of these intrusions suggesting a general pattern of iron enrichment a numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated closed system intrusion no convection the effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view conversely these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

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

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

experimental data at 3 kbars pressure on parental magma to the bushveld complex experimental studies mainly under 3 kbars pressure have been undertaken on representative samples to determine if any of these compositions could be parental magma to the bushveld complex one such composition with 12 5 mgo mg mg fe of 0 72 and quartz normative crystallizes olivine fo88 as liquidus mineral at about 1 300 c followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure there is a temperature drop of over 100 c before the appearance of plagioclase and finally clinopyroxene this crystallization sequence is in excellent agreement with the observed sequence in the lower part of the bushveld complex

sill intrusion into pyroxenitic mush and the development of the lower upper critical zone boundary of the bushveld complex implications for the origin of stratiform anorthosites and chromitites in layered intrusions

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

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

multiple sulfur isotope evidence for surface derived sulfur in the bushveld complex abstract the rustenberg layered suite of the bushveld complex originated in the mantle but is characterized by variably radiogenic strontium neodymium lead and osmium initial isotope ratios and more elevated 18 o values than expected from a partial melt of primitive mantle it is not clear whether bushveld magmas assimilated material from the crust or whether they were instead generated from an eclogitic mantle source that contained crustal material here we report the 33 s of fifteen whole rock and sulfide separate samples from both the main and critical zones of the bushveld including the ug2 chromitite and the merensky reef all samples have positive 33 s values 0 11 0 02 1 and 34 s values 2 2 0 6 1 non zero 33 s is fundamentally a surface process since high temperature processes are not known to produce significant change in 33 s the positive 33 s indicates that the bushveld sulfur contains a sedimentary or surface derived component the homogeneity of 33 s values for this suite of samples contrasts with the heterogeneity displayed by strontium neodymium lead and osmium initial isotope ratios of the bushveld and requires that the sulfur contamination occurred prior to the emplacement of the bushveld in the upper crust possible sources of contamination include lower crust and sub continental lithospheric mantle limited multiple sulfur isotopic measurements of these reservoirs exist more work needs to be done to constrain whether they contain surface derived sulfur with non zero 33 s and if they do whether the concentrations and isotopic compositions of these reservoirs can explain the measured 33 s of the bushveld

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

distribution of mafic sills in the transvaal supergroup northeastern south africa systematic measured profiles on a regional scale are used to document the distribution of noritic and amphibolitic sills 787 in total in the largely sedimentary latest archaean early proterozoic succession of the transvaal supergroup in northeastern south africa an aggregate thickness of over 2 2 km of sills intruded this 12 km thick succession the proportion of sill to sill plus sedimentary rock increases upward average 16 towards the overlying mafic phase of the bushveld complex the average thickness of sills is 34 m but increases in the same sense the emplacement level of the bushveld complex transgresses across some 5 km of the stratigraphy of the upper formations of the transvaal supergroup a node of maximum intensity of noritic sill intrusion is developed in these uppermost strata adjacent to the location where the transgressive relationship is most pronounced the structural styles of the transvaal supergroup strata influence intensity of sill intrusion other factors being equal maximum intrusion intensity is associated with areas of shallow homoclinal dip supplementary material a spreadsheet including all 440 formation traverses including grid references formation thicknesses and number and thicknesses of every sill therein is available at http www geolsoc org uk sup18807

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

gold mineralization within the witwatersrand basin south africa evidence for a modified placer origin and the role of the vredefort impact event the chemical composition of gold within the archaean metasedimentary rocks of the witwatersrand supergroup displays significant heterogeneity at the micro meso and regional scales a detailed electron microbeam analytical and petrological study of the main auriferous horizons in the central rand group throughout the witwatersrand basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres contemporaneous chlorite formation was strongly rock buffered gold mobilization occurred under fluid poor conditions at temperatures that did not exceed 350 c widespread circulation of mineralizing fluids within the central rand group is not supported by the gold and chlorite chemical data brittle deformation that affects most of the paragenetic sequence of the central rand group late in its post depositional history is followed by sequences of mineral growth and dissolution that appear throughout the central rand group and have consistent textural relationships with gold the consistent location within the paragenetic sequence the wide regional and stratigraphic extent of the brittle deformation together with mineral chemical and petrological data suggest that the vredefort impact event 2 02 ga was the cause of this late deformation and that post impact fluid poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event

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

origin and tectonic interpretation of multiple fault patterns abstract it is shown that in two dimensional and three dimensional deformation accommodated by fracture the symmetry of the fault patterns is an intrinsic attribute because it reflects the symmetry of either stress or strain tensors the deformation accommodated by sliding along pre existing planes when there is kinematic interaction between that planes forms multiple fault pattern and multiple slickenline sets during a single deformation event these fault patterns have no restrictions with respect to symmetry number of fault sets or fault orientation the kinematic analysis developed here shows that an interacting system is formed by two cross cutting faults and three slickenlines one slickenline must be parallel to the intersection line between the planes also it is demonstrated that the slickenlines generally do not correspond to the shear stress solution on the planes thus the interaction between planes does not satisfy the assumption of parallelism between shear stress and slip vector we conclude that the inversion methods to calculate paleostress tensors can lead to erroneous interpretations in structurally complex zones with many pre existing planes of weakness we propose four possibilities to form multiple fault patterns 1 two or more events of faulting obeying coulomb s law with a change of orientation of the principal stresses in each event 2 reactivation of non interacting planes according to the bott 1959 model 3 one three dimensional strain event that obeys the slip model this mechanism will form an orthorhombic four fault pattern and two slickenline sets in a single strain event and 4 one or more events obeying the interacting block model proposed here with or without rotation of the principal stresses we propose the last origin as the most common in continental regions

tectonic nature and seismic response of top basement detachment faults in magma poor rifted margins 1 top basement detachment faults tbdf represent exhumed segments of downward concave detachments accommodating tens of kilometers of offset tbdfs are key structures of magma poor rifted margins but are difficult to identify in seismic profiles because they coincide with top basement reflections we have constructed realistic seismic models of an alpine example which were used to simulate full marine seismic surveys and to identify the seismic fingerprint of tbdfs in cmp stacked and time migrated sections minor extensional allochthons which locally cap tbdfs give rise to pronounced diffractions which are in stark contrast to continuous reflections from uncapped segments of tbdfs in time migrated synthetic sections we find a continuous and constant amplitude reflection alternating with a laterally discontinuous reflection of variable amplitude these diagnostics were also identified in the lusigal 12 seismic profile imaging the hobby high detachment a tbdf in the iberia abyssal plain using these diagnostics small extensional allochthons and high angle faults cutting the hobby high detachment are observed in lusigal 12 and the entire sedimentary cover can be shown to be postrift

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

the petrology of high mg dikes from the beartooth mountains montana a search for the parent magma of the stillwater complex six geochemically distinct groups of precambrian high mg dikes have been examined to determine if any could have been a sample of or closely related to the parent magma of the stillwater complex only two groups have sufficiently magnesian olivine and orthopyroxene but neither of these has sufficiently calcic plagioclase in terms of major elements there appear to be only two distinct magma types one has a high orthopyroxene component and is petrographically similar to noritic dikes and sills near the great dyke of zimbabwe and the bushveld complex of south africa the other is similar to primitive tholeiites with higher plagioclase and diopside components than the first all of the groups are enriched in light ree and have unusually high k2o concentrations model calculations indicate that crustal contamination 30 50 of fractionating komatiitic magmas is consistent with the major and trace element characteristics of the first magma type less extensive contamination of komatiitic magmas may produce the parental magmas of the large layered intrusions

crystallization of titaniferous chromite magnesian ilmenite and armalcolite in tholeiitic suites in the karoo igneous province titaniferous chromite up to 8 wt tio2 and magnesian ilmenite up to 10 wt mgo coexist at the base of the differentiated tholeiitic mount ayliff intrusion in the karoo province of southern africa suggesting that the original magma was tio2 rich picritic lavas with 3 tio2 from the lebombo monocline of the karoo province also contain microphenocrysts of magnesian ilmenite up to 6 wt mgo and armalcolite up to 7 wt mgo these oxide mineral associations and compositions are atypical of tholeiitic magmas in which chromite usually has less than 1 wt tio2 ilmenite less than 3 wt mgo and armalcolite is rarely a primary mineral experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of tio2 on the crystallization and composition of chromite ilmenite and armalcolite the results indicate that increasing the tio2 content of picritic magmas increases the tio2 content of the spinel mainly at the expense of al2o3 whereas cr2o3 is not affected spinel compositions in the mount ayliff intrusion with over 45 wt cr2o3 less than 10 wt al2o3 and 8 wt tio2 were duplicated in experiments on a picrite at temperatures of about 1 200 c at the ni nio buffer increasing fo2 from fayalite magnetite quartz to ni nio buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite the total feo content of the liquid has little influence on the crystallization temperature of these phases the tio2 content of the liquid when either ilmenite or armalcolite crystallizes varies inversely with sio2 content the mgo content of the liquid at which ilmenite or armalcolite crystallizes depends upon the tio2 content of the starting composition with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt mgo and 5 5 wt tio2 the partition coefficent for mgo between armalcolite or ilmenite and liquid is about 1 5 observed magnesian armalcolite and ilmenite compositions in picrite lavas both minerals and in the mount ayliff intrusion ilmenite only are consistent with crystallization from a tio2 rich magma with approximately 5 wt mgo the fe23 tio5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1 150 1 110 c and fo2 of the ni nio to ni nio 1 log unit similarities also exist between the compositions of chromite ilmenite and armalcolite and liquid fraction ation trends of some hawaiian high tio2 lavas and the experimental studies presented here

platinum group element distribution in the main zone and upper zone of the bushveld complex south africa abstract the platinum group element pge contents of the upper portions of the bushveld complex were investigated with three questions in mind a in natural systems does magnetite concentrate os ir ru and rh ipge as has been observed in experimental systems b is there a au pd enriched layer present such as observed in the upper parts of the skaergaard intrusion c can changes in metal ratios be used for prospecting for pge deposits in the sulfide poor main magnetite layer of the eastern bushveld ir and rh are enriched relative to pt pd and au and this could be because magnetite preferentially concentrated ir and rh over pt pd and au however in most other magnetite layers no enrichment was observed this could be because most magnetite layers contain approximately 1 sulfides and the pge budget is dominated by the sulfides these sulfides obscured the effects of magnetite collecting ipge because sulfides collect all the pge and the partition coefficients for the pge into a sulfide liquid are much greater than the partition coefficient for ipge into magnetite the weighted average of the platinum group elements pge and au over the 2000 m of sampled stratigraphy is au 2 1 ppb pd 1 7 ppb pt 1 7 ppb rh 0 18 ppb ru comparison of the composition of the marginal rocks of the bushveld with a weighted average for the complete 6 km of the bushveld cumulates shows that the cumulate pile is much richer in compatible elements ir rh cr and poorer in incompatible elements sm and hf than the marginal rocks two possible solutions to this are a the magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or b fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

strontium isotopic evidence against magma addition in the upper zone of the bushveld complex abstract sr isotopic data from the main and upper zones of the bushveld complex show that the evolution of the upper zone started with a large influx of magma close to the level of the pyroxenite marker a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites whole rock samples which span the complete stratigraphic succession ca 2100 m above this layer fall on a single rb sr isochron 2066 58ma and hence have a common initial ratio of 0 7073 1 this ratio is significantly lower than those of the main zone ca 0 7085 below the level of the pyroxenite marker the entire upper zone crystallized from a mixed magma which was thoroughly blended before crystallization this magma had an isotopic ratio intermediate between that of the main zone and the added magma which had an initial ratio of ca 0 7067 further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma which is considered improbable hence the layering and mineralogical diversity of the upper zone was produced by internal processes and not produced by magma influxes during crystallization the lithological compositional and isotopic changes at the pyroxenite marker and the petrological coherence of all rocks above this horizon support the placing of the upper zone boundary at this point in the stratigraphy

cr and sr keys to parental magmas and processes in the bushveld complex south africa abstract large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization the exact composition s of the replenishing magma s in the case of the bushveld complex south africa has been debated mainly on the basis of major element composition and likely crystallization sequences the intrusion is dominated by orthopyroxene and plagioclase and so their cr and sr contents and likely partition coefficient values can be used to re investigate the appropriateness of the various proposed parental magmas one magma type with about 12 mgo 1000 ppm cr and 180 ppm sr can explain the genesis of the entire lower and critical zones a number of other magma compositions proposed to produce the critical zone fail to match these trace element constraints by being too poor in cr a fundamentally different magma type was added at the base of the main zone but none of the proposed compositions is consistent with the trace element requirements specifically the cr contents are higher than predicted from pyroxene compositions a further geological constraint is demonstrated from a consideration of the cr budget at this level there is an abrupt decrease from about 0 4 to 0 1 cr 2 o 3 in orthopyroxene across this critical zone main zone transition no realistic proportions of mixing between the residual magma at the top of the critical zone and any proposed added magma composition can have produced a composition that could have crystallized these low cr orthopyroxenes instead it is suggested that the resident magma from the upper critical zone was expelled from the chamber possibly as sills into the country rocks during influx of a dense differentiated magma near the level of the pyroxenite marker in the main zone there is further addition of a ferrobasaltic magma with 6 mgo 111 ppm cr and 350 ppm sr that is consistent with the geochemical requirements

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time