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

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

re and os distribution and os isotope composition of the platreef at the sandsloot mogolakwena mine bushveld complex south africa abstract rhenium and os concentrations and os isotopic compositions were determined for a suite of previously characterized samples from a traverse across the platreef at the sandsloot mogolakwena mine of the northern limb of the bushveld complex the samples comprise five pyroxenites and five parapyroxenites contact rocks representing variable mixtures of calc silicate country rocks and magmatic rocks the isotopic data from the pyroxenites define a re os regression line with an age 2011 51 ma that agrees within uncertainty with the accepted age for the bushveld complex whereas the parapyroxenite data scatter about this trend the initial 187os 188os ratios of the pyroxenites 0 1820 0 1848 are similar to but slightly more radiogenic than previously published values for the merensky reef our preferred interpretation is that this section of the platreef formed from a well mixed crustally contaminated magma genetically related to that which gave rise to the merensky reef the platreef parental magma may also have experienced minor additional contamination by wall rock interaction during emplacement

kinetic controls on the partitioning of trace elements between silicate and sulfide liquids mass transfer between sulfide and silicate liquids is often discussed elements pge has resulted from liquation immiscible by reference to a mass ratio between hypothetical liquid reservoirs liquid phase separation of sulfide liquid from mafic or at equilibrium defined as r in an open system not at equilibrium ultramafic silicate magma a central assumption of modthe mass of silicate melt from which a growing sulfide droplet els of magmatic sulfide deposit formation is that the base appears to have scavenged chalcophile elements will depend on metals and pge are strongly partitioned into the sulfide diffusivities di and dfes of the element and of the fes component melt previous discussions of the process by which sulfide in the silicate melt respectively on the degree of supersaturation of liquids become enriched in chalcophile metals have fothe melt with fes c o on the radius a of the droplet and on the cused almost exclusively on models in which coexisting velocity v with which the droplet is advected through the melt so bodies of sulfide and silicate melt are considered to be that in equilibrium at all times the intent of this paper is to demonstrate that natural sulfide deposits preserved in igneous rocks will in many r 1 c fes o di 1 2 adiv dfes 1 2 adfesv instances record metal distributions that never attained equilibrium as a result of the kinetic constraints on metal chalcophile elements are expected to show inter element variations partitioning between phases the consequences are that in d spanning several orders of magnitude in some melts leading different elements will seem to show evidence for equito large inter element variations in effective r in a given sulfide libration at different silicate sulfide mass ratios within body these variations in effective r will lead to spatial separations single sulfide bodies i describe the time dependent evolubetween zones of maximum enrichment of chalcophile elements tion of metal distributions in silicate sulfide melt systems produced during fractional segregation of sulfide in layered intrusions starting with analytical solutions for simple boundary whereas it has proven impossible to model the generation of spatially conditions and then adapting them to more complex offset stratiform horizons in the australian munni munni layered systems as numerical models intrusion using equilibrium partition coefficients the present kinetic model reproduces the observed patterns with great fidelity

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

absence of shock metamorphic effects in the bushveld complex south africa results of an intensive search abstract an extensive search was made for distinctive shock metamorphic effects in order to test the theory that the bushveld complex bc was formed by the impact of large extraterrestrial objects field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock metamorphic effects as shatter cones unusual fragmental or melt breccias and shock lamellae in quartz these are 1 pre bc quartzite xenoliths in the rooiberg group a thick sequence of acid lavas rhyolites and rhyodacites and related rocks proposed by earlier workers as a possible crater filling impact melt and 2 the locally observable contact between the rooiberg group and the underlying pre bc basement pretoria group a contact possibly corresponding to the original crater floor in established impact structures more than 320 samples of pre bc quartzite were collected and examined petrographically for shock metamorphic effects no megascopic shock metamorphic effects shatter cones or unusual breccias were observed in any outcrop in thin section no petrographic shock effects e g shock lamellae hi quartz and kink bands in biotite were observed in any sample the quartzites from both the basement 80 samples and the rooiberg xenoliths 245 samples are virtually undeformed cataclastic textures are rare and individual quartz grains show only normal metamorphic deformation lamellae bohm lamellae and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures although some of these parallel sets superficially resemble so called decorated shock lamellae universal stage measurements of their orientation demonstrate that they are totally different the absence of both megascopic and microscopic shock metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the bc are not correct and that the bc is not an impact structure this conclusion also agrees with 1 the complex stratigraphy now recognized in the lower rooiberg group which is not consistent with an impact melt origin 2 the absence in the rooiberg group of any of the distinctive melt rich fragmentai breccias which dominate the crater filling deposits in established large impact structures and 3 the undeformed character of the basement rocks below the rooiberg group and the apparently concordant contact between them

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

origin of phlogopite orthopyroxene inclusions in chromites from the merensky reef of the bushveld complex south africa about 30 of the chromite grains of variable sizes in a chromitite seam at the base of the merensky reef of the bushveld complex on the farm vlakfontein contain abundant composite mineral inclusions the inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite they vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains na phlogopite and orthopyroxene are most abundant in the inclusions edenitic hornblende k phlogopite oligoclase and quartz are less abundant cl rich apatite rutile zircon and chalcopyrite are present at trace levels na phlogopite is unique to the inclusions it has not been found elsewhere in the bushveld complex other minerals in the inclusions are also present in the matrix of the chromitite seam but their compositions are different the mg mg fe2 ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix k phlogopite in the inclusions contains more na than in the matrix the average compositions of the inclusions are characterized by high mgo 26 wt na2o 2 4 wt and h2o 2 6 wt and low cao 1 1 wt and feo 4 4 wt the 18o value of the trapped melt estimated by analysis of inclusion rich and inclusion poor chromites is 7 this value is consistent with the previous estimates for the bushveld magma and with the 18o values of silicate minerals throughout the reef the textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile rich melts during chromite crystallization the volatile rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and na k rich fluids expelled from the underlying crystal pile the addition of fluid to the magma is thought to have caused dissolution of orthpyroxene leaving the system saturated only in chromite both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution most of the cr and al in the inclusions was contributed through wall dissolution of the host chromite dissolution of minor rutile trapped along with orthopyroxene provided most of the ti in the inclusions the na and k rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile rich melts

concentrations and burial fluxes of organic and inorganic carbon on the eastern margins of the arabian sea abstract eleven sediment cores were analysed from the continental margins of the eastern arabian sea 22 n to 10 n for organic carbon c org nitrogen n and caco 3 contents similar variations are observed in both lateral and the vertical org n and caco 3 ranging from 1 6 0 10 0 82 and 10 81 respectively the c n ratios varying from 6 15 averaging 9 3 indicate that c org is mostly of marine origin the burial fluxes bf of c org and caco 3 are in the range of 0 8 21 and 39 194 g m 2 a 1 respectively the upper limits of burial efficiency be for caco 3 compared to its rain rate of 25 g m 2 a 1 in sediment trap at a depth of 2500 m from a nearby region ranges from 39 to 780 be values greater than 100 in the northern 15 n could be most likely due to redeposition of material from shelf and slope regions as well as influx of detrital carbonates from coastal areas the inverse correlation observed in c org and caco 3 distribution is indicative of post depositional effects due to sulphate reduction associated processes this can lead to dissolution of caco 3 and a better preservation of c org due to faster deposition rate and higher detrital clay content the bf and paleoproductivity calculated using the transfer function of sarnthein et al miner soc spec publ 64 1992 411 show increasing trend between 10 and 2 ka with a significant periodicity of 3000 a corresponding to that observed for monsoon induced productivity

volcanism of the palaeoproterozoic bushveld large igneous province the rooiberg group kaapvaal craton south africa abstract the volcanic rocks of the rooiberg group represent the uppermost unit in the palaeoproterozoic transvaal supergroup and form one of the largest provinces of silicic volcanic rocks in the world although stratigraphically associated with the transvaal basin fill the rooiberg group is petrogenetically linked with the larger bushveld magmatic event for which emplacement was preceded by the extrusion of the vast rooiberg lava flows in the northern part of the kaapvaal craton like many silicic dominated large igneous provinces lips which are increasingly recognised in the rock record the rooiberg group volcanics are intracontinental subaerial and are dominated by voluminous lava flows originally the rooiberg group is inferred to have covered an area of more than 200 000 km2 of which after erosion an area of 50 000 67 000 km2 remains the lava flows form a stratigraphic succession up to 6 km thick and are divided into four formations in ascending order dullstroom damwal kwaggasnek and schrikkloof due to a scarcity of reliable geochronological data the temporal span of the rooiberg group is poorly understood the rooiberg group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 c minor explosive eruptions are represented by pyroclastic rocks and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes the rocks are essentially undeformed and have not been buried so that their original textures are well preserved the bushveld complex and the associated rooiberg group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume this thorough review of the geochronology physical volcanology and geochemistry of the rooiberg group enables construction of a geodynamic model

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

the origin of geochemical diversity of lunar mantle sources inferred from the combined u pb rb sr and sm nd isotope systematics of mare basalt 10017 the results of our combined u pb rb sr and sm nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources as well as the u pb isotope systematics of the moon the rb sr sm nd and 238 u 206 pb isotope systems yield concordant crystallization ages of 3 633 0 057 ga 3 678 0 069 ga and 3 616 0 098 ga respectively the 235 u 207 pb isochron yields an older though still concordant age of 3 80 0 12 ga neither the 206 pb 207 pb system nor u pb concordia system yields an age for 10017 that is concordant with the age determined from the sm nd rb sr and 238 u 206 pb systems the initial 87 sr 86 sr of 10017 is 0 69941 7 and the initial end is 3 2 0 4 initial pb isotopic compositions determined from the u pb isochrons are 206 pb 204 pbi 31 11 and 207 pb 204 pbi 34 15 together these initial pb compositions constrain the l value of the 10017 source to be 70 30 assuming a single stage pb growth model this is considerably lower than l values typically estimated for mare basalt sources 100 600 regardless the l values calculated for the sources of mare basalts as well as other lunar samples show a range that is larger than can be explained by fractionation of u from pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources the u pb isotope systematics may reflect late stage formation of a sulfide phase which strongly fractionates pb from u but has minimal effect on rb sr or sm nd compositions during crystallization of the lunar magma ocean 2007 elsevier ltd all rights reserved

diffusive isotopic contamination of mafic magma by coexisting silicic liquid in the muskox intrusion shifts in 87 sr 86 sr and 143 nd 144 nd ratios measured in cumulates from the upper levels of the muskox mafic intrusion indicate that isotopic and bulk chemical exchange were decoupled across a mafic silicic liquid interface during crystallization of the intrusion modeling of diffusive exchange between liquid layers demonstrates that isotopic compositions of silicate liquids in layered magma chambers may be strongly affected by this process on time scales of 10 3 to 10 4 years diffusive contamination can be used to place constraints on the physical processes and time scales of magmatic systems

olivine and sulfur isotopic compositions of the uitkomst ni cu sulfide ore bearing complex south africa evidence for sulfur contamination and multiple magma emplacements the uitkomst complex in northern south africa is host to a ni cu deposit containing a minerable reserve of 2 9 mt massive ore with grades of 2 ni 1 cu and 6 ppm pt pd and an indicated plus inferred resource of 98 mt disseminated ore with grades of 0 6 ni 0 2 cu and 1 ppm pt pd based on similar age and composition to that of the bushveld complex the uitkomst complex is believed by some investigators to be a satellite body of the bushveld complex it has a tubular shape with an exposed surface area of about 0 8 8 km and a thickness of up to 1000 m apparently exploiting the bedding planes of gently dipping footwall sediments that include shale quartzite and dolomite the uikomst complex comprised of stratiform mafic and ultramafic rock units that from base to top include gabbro sulfide mineralized harzburgite unmineralized harzburgite pyroxenite gabbronorite and gabbro the compositions of olivine from different rock units are significantly different olivines from the gabbronorite unit are highly fractionated containing 30 mol fo forsterite mg2sio4 and 300 ppm ni olivines from the underlying pyroxenite and harzburgite units are much more primitive containing 86 91 mol fo abrupt changes in the contents of ni in olivine occur within the pyroxenite unit and between the mineralized and unmineralized harzburgite units the contents of ni in olivine from the unmineralized harzburgite unit are between 2900 and 3600 ppm the contents of ni in olivine from the underlying mineralized harzburgite units are 1500 ppm lower olivines from the lower part of the pyroxenite unit are similar to olivines from the underlying harzburgite unit whereas olivines from the upper part of the pyroxenite unit contain much lower ni 600 ppm numerical modelling suggests that the strikingly different compositions of olivine are related to different parental magmas with different mgo feo ratios and or ni contents the 34s values of the basal gabbro and unmineralized rock units range from 0 9 to 2 6 which are similar to typical mantle values 0 3 in contrast the 34s values of the sulfide ores in the harzburgite units are significantly lower varying between 2 6 and 7 1 the low 34s values of the sulfide ores indicate addition of 34s depleted crustal sulfur the obvious sources of such crustal sulfur are the malmani dolomite and the timebal hill shale that contain pyrite with negative 34s values up to 11 and 18 respectively sulfide saturation in the magma of the mineralized harzburgite units is thought to have resulted from addition of the crustal sulfur the resulting immiscible sulfide liquid droplets were then concentrated in the base of the magma channel some of the sulfide liquid was expelled into the basal gabbro that separated the active magma channel from the footwall quartzite new magma then entered the channel displacing most of the resident magma to form the unmineralized harzburgite the overlying pyroxenite gabbronorite and gabbro units formed either by in situ differentiation of the same magma giving rise to the underlying unmineralized harzburgite or by subsequent emplacement of a more evolved magma

layering and precious metals mineralization in the rincon del tigre complex eastern bolivia geologic investigations supporting recent technically successful platinum exploration of the rincon del tigre complex a ca 4 6 km thick layered sill have provided significant new understandings of the layering development and precious metals mineralization of one of south america s largest mafic ultramafic intrusions three macrocyclic units are recognized from the base up basal unit unit 1 and unit 2 each most likely the product of a major replenishment with mafic magma of one compositional type the ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite granular harzburgite olivine bronzitite each a few meters thick with occasional development of bronzitite at the top mafic rocks absent from unit 1 and present only locally in the basal unit form a well developed norite gabbro magnetite gabbro sequence in the upper half of unit 2 associated with the base of the magnetite gabbro is the precious metals zone a persistent zone of low grade subeconomic sulfide and precious metals mineralization 80 to 185 m thick the precious metals zone comprises an upper cu sulfide rich portion its base the sulfide phase boundary about 23 to 70 m above the magnetite phase boundary and a lower precious metals rich portion the lower part straddling the magnetite phase boundary the individual precious metals are concentrated in separate sulfide poor subzones each up to many tens of meters thick in the stratigraphic order rh pt pd au with peak offsets at the same scale modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic precious metals layering of the sulfides in the precious metals zone all at three different superimposed scales from about 3 m to several tens of meters a possible interpretive framework for the cryptic layering and perhaps for much of the other layering including the meter scale layering of the ultramafic rocks is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and in the magnetite gabbro by repeated convective overturns caused by the onset of magnetite precipitation the characteristics of the precious metals zone fit an orthomagmatic model the composition stratigraphic relations and modal correlation with magnetite of the sulfides all imply that s saturation was induced by loss of feo to magnetite precipitation this took place after prolonged closed system fractionation following the last major magma injection at the base of unit 2 during which crystallization of olivine pyroxene plagioclase and finally magnetite precipitation led to overall s cu and precious metals enrichment and ni and finally fe depletion of the magma the development of the mineralization and its complex layering was controlled by sulfide segregation rayleigh fractionation the different but generally very high partition coefficients dsilicate sulfide for each precious metal and repeated convective overturns at three different superimposed scales the decoupled metals distribution is analogous and similar in origin to that of the main sulfide zone of the great dyke zimbabwe and of the main sulfide layer offset portion of the munni munni complex western australia but the stratigraphic order of metal enrichment peaks is different pt pd au vs pd pt au this suggests a reversal in the relative magnitudes of the partition coefficients dpt and dpd that was possibly brought about by changes in the temperature and o s and fe activities of the magmas between the middle and late stages of crystallization constancy of d values during fractionation of mafic magma should not be assumed the precious metals zone is one of several skaergaard type strata bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions and the origin of the skaergaard mineralized zone itself including the potentially economic platinova reefs can be explained in terms of the precious metals zone model presented here the very low grade great thickness and wide separation of individual metal subzones in the precious metals zone are similar to those of the subeconomic axial facies of the main sulfide zone in the wider parts of the great dyke cf the economic more compact marginal facies on the basis of the lateral and vertical thermal model of the great dyke magma chamber the subeconomic status of the precious metals zone may be in large part a function of the specific fluid dynamics of the rincon del tigre magma chamber

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

trace elements in the merensky reef and adjacent norites bushveld complex south africa trace elements were analysed in rocks and minerals from three sections across the merensky reef in the rustenburg platinum mine in the bushveld complex of south africa whole rocks and separated minerals were analysed by inductively coupled plasma mass spectrometer icp ms and in situ analyses were carried out by ion microprobe and by laser source icp ms merensky reef pyroxenites contain extremely high concentrations of a wide range of trace elements these include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements for major elements and compatible trace elements the measured concentrations in cumulus phases and the bulk rock compositions are similar for highly incompatible elements however concentrations in bulk rocks are far higher than those measured in the cumulus phases in situ analyses of plagioclase have far lower concentrations of th zr and rare earth elements than icp ms analyses of bulk separates of plagioclase a difference that is attributed to the presence of trace element rich accessory phases in the bulk mineral separates we used these data to calculate the trace element composition of the magmas parental to the merensky unit and adjacent norites we argue that there is no reason to assume that the amount of trapped liquid in the merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites we propose that the bushveld complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks the magma that gave rise to the merensky unit was the more contaminated and unusually rich in incompatible trace elements and when it entered the main bushveld chamber it precipitated the unusual phases that characterize the merensky reef the hybrid magma segregated sulphides or platinum group element rich phases during the course of the contamination in the lower chamber these phases accumulated following irruption into the main bushveld chamber to form the merensky ore deposits

concentrations of rare earth elements in silicate rocks of the lower critical and main zones of the bushveld complex concentrations of rare earth elements in the lower critical and main zones of the bushveld complex at union section show distinct variation with stratigraphic height in that lower and lower critical zone cumulates are lree and th enriched ce sm 10 25 ce smn 3 65 th smn 6 17 over main zone cumulates ce sm 4 10 ce smn approximately 2 th smn 1 57 the upper critical zone constitutes a transitional interval in terms of ree concentrations ce sm 9 17 this pattern is broadly coupled albeit in a contraposed manner to that in initial sr isotopic ratio and confirms earlier studies that the bushveld complex crystallized from at least two compositionally distinct parental magmas mixing between the two magmas occurred as early as in the lower zone and is sensitively reflected by variation in ce sm th sm modal plagioclase and chromite while ree contents of the lower and critical zones are approximately 15 those of the corresponding b1 and b2 marginal rocks the main zone cumulates have similar ree contents to the b3 marginal rocks this suggests that the main zone rocks either crystallized from a highly viscous crystal mush without appreciable fractionation or from magma strongly ree enriched over the marginal rocks which would imply that the main zone is not represented in the suite of marginal rocks

multiple magma evolution and ore forming processes of the hongge layered intrusion sw china insights from sr nd isotopes trace elements and platinum group elements abstract the hongge layered intrusion 259 ma which is located in the inner zone of the emeishan large igneous province elip is one of the most typical fe ti v ore deposits in the pan xi area mafic ultramafic layered intrusions of the elip have attracted a lot of attention lately because these intrusions host world class fe ti v oxide deposits plus interesting cu ni pge mineralization which may have economic potential this paper reports new whole rock major and trace element compositions pge abundances and sr nd isotopic data for selected cumulate rocks and basalts we use these data to investigate the nature of parental magmas and the controls on its evolution from the source mantle en route to the surface involving the hongge ore bearing intrusion two abrupt changes in mt ilm and trace element ratios such as ba th with depths in the hongge layered intrusion indicate that this intrusion formed by at least two pluses of relatively primitive magma the whole rock sr nd isotopic data of basaltic and intrusive rocks plot in the region of emeishan low ti basalts and the compositions of residual liquid at 1260 c and 1155 c calculated by melts are similar to our actual high ti bfq 2 and low ti bc 1 basltic samples indicate they are co magmatic rather than derivation from a distinct source total pge abundances in the hongge samples are extremely low ranging from 0 5 to 10 ppb sulfide bearing rocks in the hongge intrusion and the nearby coeval banfangqing and baicao basalts have similar mantle like pd pt ratios 2 6 and extremely high cu pd ratios 3 104 to 4 105 indicating that sulfide segregation took place at depth prior to emplacement at hongge and eruption in this region sulfide saturation in the hongge magma may have resulted from such crustal contamination event crystallization of silicate minerals under the anhydrous magma magma hydration plus fe ti enrichments in the parental magma are three critical factors for the formation of fe ti oxide ore layers in the hongge intrusion

ultramafic rocks in the centre of the vredefort structure south africa geochemical affinity to bushveld rocks abstract the geochemistry and mineral chemistry of ultramafic rocks in the centre of the vredefort structure have been reinterpreted a detailed examination of previously published major element trace and ree analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material a strong geochemical affinity to ultramafic rocks from the bushveld complex is evident the magmatic nature of the ultramafic rocks in the centre of the vredefort structure has implications for the discussion of the metamorphic history of the vredefprt structure which shows evidence of unusually high and as yet not satisfactorily explained geothermal gradients during peak metamorphism

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

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

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

strontium isotope disequilibrium of plagioclase in the upper critical zone of the bushveld complex evidence for mixing of crystal slurries we report in situ sr isotope data for plagioclase of the bushveld complex we found disequilibrium sr isotopic compositions on several scales 1 between cores and rims of plagioclase grains in the merensky pyroxenite the bastard anorthosite and the ug1 unit and its noritic footwall 2 between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the merensky unit the footwall anorthosite of the merensky reef and the footwall norite of the ug1 chromitite the data are consistent with a model of co accumulation of cumulus plagioclase grains that had crystallized from different magmas followed by late stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile we propose that the rocks formed through slumping of semi consolidated crystal slurries at the top of the critical zone during subsidence of the center of the intrusion slumping led to sorting of crystals based on density differences resulting in a layered interval of pyroxenites norites and anorthosites

isotopic evolution of the middle to late proterozoic awasib mountain terrain in southern namibia abstract the middle to late proterozoic awasib mountain terrain amt is situated at the approximate contact between the supracrustal sinclair sequence and the namaqualand metamorphic complex in southern namibia tholeiitic to alkaline metabasalts from the kairab metamorphic complex which forms part of the basement to the sinclair sequence in the amt yield an rb sr isochron age of 1461 169 ma with an initial 87sr 86sr r0 of 0 70269 12 a preliminary pb pb isochron age of 1467 215 201 ma for these metabasalts appears to confirm the rb sr age these metabasalts have been intruded by the calc alkaline aunis tonalite gneiss which yields an rb sr isochron age of 1271 62 ma and r0 of 0 7029 3 the low r0 obtained for both of these basement units implies that the ages are primary and hence that the isotope systematics have not been disturbed by subsequent deformation and metamorphism andesitic and rhyolitic volcanics of the haiber flats formation hff are correlated with the sinclair sequence basaltic andesites from the hff have shoshonitic affinities and may be classified as medium to high k and orogenic in character one basaltic andesite unit yields an age of 1086 44 ma and r0 of 0 70305 17 the low r0 indicating derivation from a mantle source region that is slightly depleted relative to a bulk earth or uniform reservoir composition the errorchron age estimate of 1038 74 ma and r0 of 0 718 15 obtained for a rhyolite porphyry from the hff is within error of that deduced for the basaltic andesite however the high r0 of this largely ignimbritic unit is probably due to a significant component of melted crust the younger possibly subvolcanic awasib granite yields an errorchron with an apparent age of 934 70 ma and r0 of 0 719 12 scatter of these data may be attributed to compositional variation in a postulated crustal source the a type characteristics exhibited by both the hff rhyolite and awasib granite are compatible with their derivation by crustal anatexis available isotopic and geochemical data together with field evidence suggest an extensional palaeoenvironment for much of the amt and sinclair sequence while isotopic data alone are insufficient to establish the nature of the rifting the broadly contemporaneous namaqua orogeny suggests that rifting may be collision induced furthermore the mantle source of abundant orogenic lavas in the amt and sinclair sequence appears to owe its imprint of subduction related metasomatism to a more ancient event

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