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

controls on the disseminated ni cu pge sulfide mineralization at the tubane section northern molopo farms complex botswana implications for the formation of conduit style magmatic sulfide ores abstract the molopo farms complex mfc is located near the western margin of the archean kaapvaal craton in southern botswana the complex hosts low grade disseminated ni cu and platinum group elements pge mineralization restricted to the lower ultramafic cumulates notably in the feldspathic orthopyroxenites the mineralized feldspathic orthopyroxenites contain up to 0 47 ppm pt pd au manifested in the form of platinum group mineral pgm assemblages dominated by pge bismuthotellurides asernides and minor au and ag alloys the pgm encountered are froodite pdbi2 moncheite ptte2 michenerite pdbite sperrylite ptas2 largely associated with altered silicates and sulfide minerals the association of pgm with sulfides suggests that they were originally concentrated by sulfide melt while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of pgm with altered silicates the cu pd ratios of most of the rocks fall within the primitive mantle range however the cu pd ratios of the feldspathic orthopyroxenites are more variable with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides this is supported by the cu zr ratios of these rocks which are also variable above and below unity trace element ratios on the th yb pm versus nb th pm plot indicate strong crustal contamination across all units including those not in proximity to the country rocks the 34s of the magmatic rocks are variable between 6 3 and 11 4 in contrast the 34s of dolomitic siltstones of the transvaal supergroup which constitute the immediate country rocks vary from 4 1 to 0 6 these data indicate that crustal contamination largely occurred prior to magma emplacement it is inferred that the most important contaminant and source of crustal s were the ramotswa and ditlhojana formations that include sulfidic shales at depth we consider that magnesian basaltic magmas of b1 bushveld magma composition underwent crustal contamination assimilation of crustal s and segregation of sulfide melt in the feeder conduit or staging chamber prior to final magma emplacement the feeder conduit or staging chamber was subsequently invaded by further b1 magma this magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final mfc chamber studied here after the staging chamber had already partially solidified it was invaded by yet another influx of magma possibly even more primitive than the b1 bushveld magma being a crystal mush this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final mfc chamber the presence of centimeter sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas therefore it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging chamber

satellite gravity and the mass balance of the antarctic ice sheet changes in the earth s gravity field with time have important applications to a broad range of disciplines any process that involves a large enough horizontal redistribution of mass either within the earth or on or above its surface is potentially detectable in particular when ice sheets grow or shrink gravity changes as mass is redistributed in the solid earth and between the oceans and the ice sheets the sources of global sea level rise about 2 mm a 1 over the last century and in particular the contribution of the antarctic ice sheet thereto are not well understood gravity measurements can help to diminish this uncertainty the technology currently exists to measure gravity with high accuracy by a dual satellite mission in which the distance between the satellites is precisely monitored we estimate from recent studies that temporal changes in the gravity field as determined by a satellite gravity mission lasting 5 years at an orbital height of 400 km would be sensitive to changes in the overall mass of the antarctic ice sheet to a precision corresponding to better than 0 01 mm a 1 of sea level change however the effects of three other phenomena that could each produce a temporally varying gravity signal with characteristics comparable to that caused by a change in antarctic ice postglacial rebound inter annual variability in snowfall and atmospheric pressure trends also need to be evaluated postglacial rebound could be partly separated from ice mass changes with the aid of global positioning system campaigns and numerical models of rebound that use improved determinations of mantle viscosity also provided by the gravity mission determination of inter annual ice mass changes will be aided by measurements of moisture flux divergence around the perimeters of the ice sheets and direct observations of inter annual changes by the gravity satellite itself the removal of pressure effects over antarctica will become more effective as the number of automatic weather stations in the interior of the continent increases even after corrections are made for these factors the uncertainties they cause limit the accuracy in the determination of the contribution of the antarctic ice sheet to sea level change to about 0 5 mm a 1 however there is a strong complementarity between gravity measurements and the surface height measurements that will be produced by nasa s laser altimeter mission early next century together they should be able to determine that contribution to an accuracy of about 0 1 mm a 1

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

increased ice loading in the antarctic peninsula since the 1850s and its effect on glacial isostatic adjustment antarctic peninsula ap ice core records indicate significant accumulation increase since 1855 and any resultant ice mass increase has the potential to contribute substantially to present day glacial isostatic adjustment gia we derive empirical orthogonal functions from climate model output to infer typical spatial patterns of accumulation over the ap and by combining with ice core records estimate annual accumulation for the period 1855 2010 in response to this accumulation history high resolution ice sheet modeling predicts ice thickness increases of up to 45 m with the greatest thickening in the northern and western ap whilst this thickening is predicted to affect grace estimates by no more than 6 2 gt yr it may contribute up to 7 mm yr to the present day gia uplift rate depending on the chosen earth model with a strong east west gradient across the ap its consideration is therefore critical to the interpretation of observed gps velocities in the ap

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

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 tin zone sediment hosted hydrothermal tin mineralization at rooiberg south africa the rooiberg tin field also known as the rooiberg fragment is located within the western lobe of the bushveld complex the fragment is triangular shaped consists of early proterozoic transvaal sequence volcano sedimentary rocks and is surrounded by granitoid intrusives of the lebowa granite suite practically all the significant tin deposits are hosted by arkoses located towards the transition with shaly arkoses at the stratigraphic top of the boschoffsberg quartzite member this stratabound distribution of individual deposits gave rise to the concept of a regionally developed continuous stanniferous zone on regional scale the individual deposits are broadly similar and are collectively classified as replacement and open space filling type particular styles of mineralization such as tourmaline pockets orbicules disseminated cassiterite steep and flat dipping sulphidic fractures and hydrothermal carbonate breccias appear to dominate each mine petrological and geochemical investigations of four mineralized centres indicated the presence of a 500 600 m thick stratified zone of pervasively altered clastic sedimentary rocks of possible arkosic precursor composition alteration displays a distinct zonal distribution consisting of a grey green sodic albitized foot wall an approximately 80 m transitional sericitized tourmalinized sulphidic tin zone and red hematitic potassic hanging wall the observed zonal pattern and localization of economically significant cassiterite is explained by the evolutionary path of magmatic stanniferous hydrothermal fluids genetically related to the surrounding acid phase of the bushveld complex areas of high fracture density the tectonic focal points acted as conduits for the ascending reactive fluids factors such as fluid lithostatic pressure equilibration limited fracture evolution and or impermeable shaly arkose retarded this ascent as a result accumulation lateral spreading of fluid and pervasive alteration occurred over a broad stratified front lateral overlapping of these fronts creates the impression of a continuous tin zone abundance of geochemical common denominators between the various focal points mines supports a shared composition and source for these contemporaneous fluids

understanding subsidence in the mississippi delta region due to sediment ice and ocean loading insights from geophysical modeling the processes responsible for land surface subsidence in the mississippi delta md have been vigorously debated numerous studies have postulated a dominant role for isostatic subsidence associated with sediment loading previous computational modeling of present day vertical land motion has been carried out in order to understand geodetic data while the magnitudes of these measured rates have been reproduced the model parameter values required have often been extreme and in some cases unrealistic in contrast subsidence rates in the md on the 103 year timescale due to delta loading estimated from relative sea level reconstructions are an order of magnitude lower in an attempt to resolve this conflict a sensitivity analysis was carried out using a spherically symmetric viscoelastic solid earth deformation model with sediment ice and ocean load histories the model results were compared with geologic and geodetic observations that provide a record of vertical land motion over three distinctly different timescales past 80 kyr past 7 kyr and past 15 years it was found that glacial isostatic adjustment is likely to be the dominant contributor to vertical motion of the pleistocene and underlying basement present day basement subsidence rates solely due to sediment loading are found to be less than 0 5 mm yr 1 the analysis supports previous suggestions in the literature that earth rheology parameters are time dependent specifically the effective elastic thickness of the lithosphere may be 50 km on a 105 year timescale but closer to 100 km over 103 to 104 year timescales

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

pre transvaal wrench tectonics along the northern margin of the witwatersrand basin south africa abstract the pre transvaal tectonic history of the northern margin of the witwatersrand basin is re evaluated in terms of wrench fault tectonics three major east northeast trending left lateral wrench fault systems are recognized the sugarbush rietfontein and kromdraai systems they are characterized by pull apart basins e g bezuidenhout valley and kromdraai grabens narrow slivers of displaced stratigraphy e g langermanskop rietfontein block and marginal convex upward thrusts changes in fault orientation result in areas of compression passing laterally into areas of extension over short distances splays terminate individual fault systems regions of wrench fault overlap are characterized by fold domains e g west rand syncline east rand basin with folds characteristically trending nw se the left lateral strike slip system appears to be of craton wide scale and probably extends at least as far as the barberton greenstone belt the wrench system appears to have come into existence during witwatersrand times but experienced its major movement during middle ventersdorp times the left lateral strike slip system is believed to be related to rifting along the ventersdorp trough and to right lateral faulting along the southwestern margin of the witwatersrand basin these tectonics occurred prior to a post transvaal period of folding and thrusting related to the emplacement of the vredefort dome the latter is responsible for the penetrative regional cleavage and bedding sub parallel faults throughout the area concerned the structural model proposed here has implications for the existence of possible outliers of mineralized upper witwatersrand stratigraphy

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

correction to ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

sublimation and aeolian sand movement from a frozen surface experimental results from presqu ile beach ontario abstract recent aeolian literature has indicated that sand movement can and does take place on frozen surfaces the winter aeolian process is influenced by the pore ice which binds the sand grains together and increases the surface resistance to wind action sublimation of the frozen moisture provides loose grains of sand which subsequently can be entrained by the wind field experiments carried out on the presqu ile beach ontario in the 1992 1993 winter provide some data on variables which affect sand loss from frozen surfaces because of sublimation sand moisture content determines the resistivity of the surface to sublimation induced sand loss frozen surfaces with higher moisture contents experience less sand loss the existing literature has indicated that temperature plays an important role in the occurrence of sublimation higher temperatures i e temperatures which are closer to 0 c result in higher rates of sublimation however the presqu ile experiments show that wind speed is more strongly correlated with sublimation induced particle loss than temperature or relative humidity higher wind speeds allow sand from the surface and sublimation produced water vapour to be removed from the area thus providing an incentive for further sublimation and sand loss to take place information on the variables which affect sublimation induced sand loss from frozen surfaces can be used to increase understanding of the processes involved in sand movement on frozen surfaces under subzero temperatures

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

the geological setting of the platinoid and base metal sulfide mineralization in the platreef of the bushveld complex in drenthe north of potgietersrus on the farm drenthe the platreef consists of a complex assemblage of norites melanorites pyroxenites serpentinites and xenoliths of dolomite it is a zone of up to 250 m thick between the basal contact of the complex and the overlying main zone norites the composition of the orthopyroxene shows a rapid increase from en 60 to en 70 in the lower 30 m of the sequence and a sharp drop from en 70 to en 60 at the top of the platreef this compositional break at the upper contact is due to a change from primary bronzite in the platreef to inverted pigeonitc in the main zone high temperature metamorphic assemblages are preserved in the larger xenoliths whereas their outer margins and the smaller xenoliths contain retrograde metamorphic mineral assemblages the igneous rocks display irregularly distributed zones of alteration to bronzite bastitites and serpentinites the alteration process was probably dominated by degassing of the xenoliths the distribution and composition of the major sulfide minerals indicate a change from a sulfur rich to a metal rich environment during the crystallization of the platreef pyrite monoclinic pyrrhotite pentlandite and chalcopyrite dominate the lower part of the platreef and hexagonal pyrrhotite pentlandite and chalcopyrite or cubanite prevail in the upper portion whole rock analyses show an upward decrease in the ni cu ratio from 1 95 to 1 70 large variations occur in the distribution and grade of ni cu and platinum group elements although highest values are associated with a zone of serpentinization the ni and cu values are unreliable indicators in predicting pt and pd concentrations in zones of major metal enrichment ni and cu correlate strongly while pt and pd have more significant correlations with ni than cu although the minerals of the platinum group elements are primarily associated with chalcopyrite the pt pd ratio decreases upward in the sequence the mineralization is considered to have been caused by the breakup of dolomite xenoliths that added co 2 h sub 2 o and s to the magma thus simultaneously lowering the solubility of sulfur in the magma and adding sulfur from an external source which resulted in the precipitation of an immiscible sulfide liquid

structures related to the antimony line murchison schist belt kaapvaal craton south africa abstract sporadic epigenetic antimony gold mineralisation along the ene trending antimony line of the murchison schist belt is structurally controlled and spatially related to carbonated rock types with quartz carbonate veins in tension fractures the antimony line is a discontinuous zone of brittle ductile deformation and is locally oblique to lithological layering within a broad ductile shear zone this tectonically early d 1 shear zone has an oblique to dip slip reverse north side up movement sense with downward facing fold structures in the hangingwall later d 2 fold structures deformed the antimony line and its wallrocks into steep structures the antimony line is intruded by granitoids which post date d 1 and are not directly related to mineralisation the source of which may have been metamorphic fluids

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the