zoning of platinum group mineral assemblages in the ug2 chromitite determined through in situ sem eds based image analysis in situ scanning electron microscopy energy dispersive x ray spectrometry analysis of platinum group minerals pgm and base metal sulfides in the ug2 chromitite shows that this ore body is zoned along at least 6 km of strike the uppermost part of the ug2 chromitite referred to as the leader seam is 16 cm thick and has a pgm assemblage that is dominated by pge arsenides sulpho arsenides and alloys 70 vol of all pgm which are typical secondary pgm assemblages in other segments of ug2 this is the first time such laterally persistent secondary assemblages have been identified in the ug2 chromitite as previously they were only known to occur adjacent to transgressive fluid bearing structures e g pipes faults the underlying main seam is thicker one to nine seams totaling 130 cm and has a pgm assemblage that consists mostly of pt sulfide pt pd sulfide pt rh cu sulfide laurite and fe pt alloys 85 vol of all pgm typically regarded as primary magmatic constituents of ug2 chromitite there are however some subtle vertical changes in the pgm assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts the origin of these secondary pgm assemblages is related to alteration by hydrothermal fluids and or fluid rich melts that infiltrated during crystallization of the ug2 and may possibly have been derived from the ug2 chromitite itself

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

bubble migration in a compacting crystal liquid mush recent theoretical models have suggested that bubbles are unlikely to undergo significant migration in a compaction crystal mush by capillary invasion while the system remains partly molten to test this experiments of bubble migration during compaction in a crystal liquid mush were modeled using deformable foam crystals in corn syrup in a volumetric burette compacted with rods of varying weights a bubble source was provided by sodium bicarbonate alka seltzer large bubbles several crystal sizes are pinched by the compacting matrix and become overpressured and deformed as the bubbles experience a load change from hydrostatic to lithostatic once they begin to move they move much faster than the compaction driven liquid bubbles that are about the same size as the crystals but larger than the narrower pore throats move by deformation or breaking into smaller bubbles as they are forced through pore restrictions bubbles that are less than the typical pore diameter generally move with the liquid the liquid bubble mixture behaves as a single phase with a lower density than the bubble free liquid and as a consequence it rises faster than bubble free liquid and allows for faster compaction the overpressure required to force a bubble through the matrix max grain size 5 mm is modest about 5 and it is estimated that for a grain size of 1 mm the required overpressure would be about 25 using apatite distribution in a stillwater olivine gabbro as an analog for bubble nucleation and growth it is suggested that relatively large bubbles initially nucleate and grow in liquid rich channels that develop late in the compaction history overpressure from compaction allows bubbles to rise higher into hotter parts of the crystal pile where they redissolve and increase the volatile content of the liquid over what it would have without the bubble migration leading to progressively earlier vapor saturation during crystallization of the interstitial liquid bubbles can also move rapidly by surfing on porosity waves that can develop in a compacting mush

re os and s isotope evidence for the origin of platreef mineralization bushveld complex abstract the bushveld complex contains the largest platinum group element pge deposits of the world that are represented by persistent stratiform reefs highly enriched with pge with respect to underlying and overlying rocks new re os isotope and elemental la mc icpms data on platinum group minerals pgm from the mineralized reefs are presented with implications to correlation between the different segments of the bushveld complex and a role of superimposed processes at the reef formation we analyzed laurite rus 2 hollingworthite rhass sperrylite ptas 2 and pt fe alloys from the merensky reef pseudoreef and the pge reef of the platreef the measured 187 os 188 os value for platreef laurite is 0 1751 0 0004 whereas the ratios for sperrylite and hollingworthite range to slightly higher values 0 1713 0 1818 and 0 1744 0 1835 respectively the observed textures of the analyzed pgm such as pt fe symplectites in base metal sulfides bms laurite inclusions in chromite and sperrylite rims around sulfide silicate aggregates are interpreted as features of primary magmatic crystallization whereas hollingworthite overgrowths and exsolutions in sperrylite are likely to have originated from later solid state transformation or metasomatic processes the platreef is a composite sill like body in the northern limb correlative to the critical zone in terms of stratigraphic position whole rock geochemistry and isotope characteristics the pristine magmatic character of sulfides and pgm in the stratiform reefs at the top of the platreef strongly resembles the style of merensky reef mineralization however the basal part of the platreef pyroxenitic sequence is variably contaminated and mineralized with a significant hydrothermal overprint sulfides from underlying lower zone peridotite yield 34 s values varying from 9 to 14 2 that are much higher than the values for the overlying platreef and are a consequence of sulfur assimilation from sedimentary sulfates the same homogeneous mantle like s isotope compositions in the high grade pge reef of the platreef and the merensky reef can be explained by enhanced s isotope exchange through the orthomagmatic process of sulfide upgrading the similar values and the limited variations of the initial os isotopic ratios of all pgms in both reefs also support their magmatic crystallization and origin from a common source

occurrence and petrological significance of graphite in the upper critical zone western bushveld complex south africa abstract graphite occurs as a major rock forming constituent in pyroxenitic pegmatites near the platiniferous merensky reef in the western bushveld it is associated with amphibole biotite low k phyllosilicates chlorite sulphides and platinum group minerals rhass irass locally rocks with up to 80 graphite occur chlorine is a significant constituent in both hydrous silicates 0 1 0 3 cl and graphite 0 2 1 9 magnetite and quartz also occur with the above association this facilitates estimation oft 500 600 c andfo2 10 21 to 10 23 bar during graphite deposition which took place from c o h s fluids at an oxygen fugacity in the vicinity of qmf in equilibrium with maximum h2o mole fraction the latter accounts for the widespread association of graphite with hydrous silicates there is evidence for buffering offo2 of the melt by fluid phase this process may be more widespread than hitherto assumed c o h s fluids are considered instrumental not only in the formation of graphite rich pegmatites and associated mineralization but also in the genesis of pothole depressions and in the general development of stratiform pegmatites such as the merensky reef in layered igneous complexes

replacement of base metal sulfides by actinolite epidote calcite and magnetite in the ug2 and merensky reef of the bushveld complex south africa interaction of base metal sulfides pyrrhotite pentlandite and chalcopyrite with aqueous fluids in the ug2 chromitite layer and the merensky reef in the western bushveld complex has produced four common replacements 1 replacement of sulfides and orthopyroxene by actinolite or tremolite 2 replacement of sulfides and plagioclase by epidote 3 replacement of sulfides by calcite and 4 replacement of sulfides by magnetite the first three replacements are directly related to the presence of sulfides in the rocks and occur at the grain scale silicate minerals that are not in direct contact with sulfides are usually not affected the replacement of sulfides by magnetite accompanies regional serpentinization superimposed on the union section platinum group minerals including ru pt and pd sulfides pt and pd tellurides and pd arsenides occur in replacement aureoles around sulfide aggregates as well as within the aggregates suggesting that the platinum group minerals are more resistant to hydrothermal alteration than the associated base metal sulfides redistribution of chalcopyrite at least at the millimeter scale variable losses of s and ni and perhaps decoupling of pt and pd at the centimeter scale took place during secondary hydrothermal alteration these effects should be considered before models of primary mineralization processes can be meaningfully applied

investigations of the stillwater complex part v apatites as indicators of evolving fluid composition variations in the f cl and oh contents of apatite are not constrained by crystal chemical factors in contrast to micas and amphiboles and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks microprobe analysis of intercumulus apatites from the stillwater complex reveal that cl rich apatites typically containing 6 0 wt cl occur throughout the lower 1 3 of the complex excluding the basal series a change from cl rich to more f rich apatite occurs within olivine bearing zone i ob i of the banded series the host zone of the platiniferous j m reef although apatite compositions are somewhat variable above the j m reef more f rich apatites predominante and typically contain 1 2 wt f and 3 0 wt cl the most f rich apatites occur in the uppermost exposed cumulates pristine apatites from coeval sills and dikes from below the complex and from the basal series are similarly f rich in all apatites the cl and f contents are lower in rocks affected by later metamorphic fluids rare earth element ree concentrations in chlorapatites show a marked peak in the olivine rich rocks of the j m reef and contain up to 2 wt ce2o3 la2o3 the trend of first increasing then decreasing cl f ratios with stratigraphic height is modeled by a vapor driven zone refining process occurring within the cumulate pile causing cl enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of cl from the magma as crystallization proceeded the abrupt change from cl rich to more f rich apatites within ob i is interpreted as the result of a breakdown of the cl rich zone refining front and mixing with cl poor supernatant melt any high temperature fluids that exsolved and circulated through the lower 1 3 of the complex must have been enriched in cl and could have transported ree and trace metals

three dimensional seismic velocity structure of the earth s mantle using body wave travel times from intra plate and deep focus earthquakes this paper combines the recent measurements of p and s wave travel times from intra plate earthquakes with travel times from deep focus earthquakes reported earlier to infer the velocity structure of the mantle it is reasonable that the source bias for intra plate earthquakes is less pronounced than that for other shallow focus earthquakes such as those occurring in island arcs the inclusion of intra plate earthquakes compensates for the lack of uniform areal distribution exhibited by the deep focus earthquake data and augments the number of high quality travel time observations the features observed in the variation of teleseismic travel times from intra plate earthquakes are consistent with the travel time variation determined from deep focus earthquakes especially the p wave travel time variation the combined intra plate and deep focus earthquake travel time data were converted to determine an average spherically symmetric compressional and shear velocity model and subsequently a three dimensional compressional velocity model for the earth s mantle lateral variation in the mantle accounted for most of the scatter in the observed p wave travel times the average compressional velocity model suggests two rapid velocity increases at 500 and 670 km depths and a small velocity gradient in the depth interval of 100 450 km the 3 d model determined from the augmented travel time data set corroborated the earlier findings 1 that the lateral heterogeneities are more pronounced near the upper mantle and near the core mantle boundary and 2 that the surface tectonic features are correlated with compressional velocity anomalies near the upper mantle but not with velocity anomalies below the upper mantle

petrology geochemistry and the mechanisms determining the distribution of platinum group element and base metal sulphide mineralisation in the platreef at overysel northern bushveld complex south africa platinum group element pge mineralisation within the platreef at overysel is controlled by the presence of base metal sulphides bms the floor rocks at overysel are archean basement gneisses and unlike other localities along the strike of the platreef where the floor is comprised of transvaal supergroup sediments the intimate pge bms relationship holds strong into the footwall rocks decoupling of pge from bms is rare and the bms and platinum group mineral assemblages in the platreef and the footwall are almost identical there is minimal overprinting by hydrothermal fluids therefore the mineralisation style present at overysel may represent the most primary style of platreef mineralisation preserved anywhere along the strike chondrite normalised pge profiles reveal a progressive fractionation of the pge with depth into the footwall with ir ru and rh dramatically depleted with depth compared to pt pd and au this feature is not observed at sandsloot and zwartfontein to the south of overysel where the footwall rocks are carbonates there is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable inter granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the platreef magma pge were originally concentrated within a sulphide liquid in the platreef magma and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the ipge and rh from it whilst still within the mafic platreef transport of pge into the gneisses via downward migration of the residual sulphide liquid fractionated out the remaining ipge and rh in the upper parts of the gneisses leaving a slick of disseminated sulphides in the gneiss with the residual liquid becoming progressively more depleted in these elements relative to pt pd and au highly sulphide rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall this study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of pge from the platreef into the footwall rocks where the floor rocks are sediments fluid activity related to metamorphism assimilation and later serpentinisation has decoupled pge from bms in places and transport of pge into the footwall is via hydrothermal fluids in contrast where the floor is comprised of anhydrous gneiss such as at overysel there is limited fluid activity and pge behaviour is controlled by the behaviour of sulphide liquids producing an intimate pge bms association xenoliths and irregular bands of chromitite within the platreef are described in detail for the first time these are rich in the ipge and rh and evidence from laurite inclusions indicates they must have crystallised from a pge saturated magma the disturbed and xenolithic nature of the chromitites would suggest they are rip up clasts either disturbed by later pulses of platreef magma in a multi phase emplacement or transported into the platreef from a pre existing source in a deeper staging chamber or conduit

cooperite and braggite from the bushveld complex implications for the miscibility gap and identification analyses of the pt pd ni sulphides cooperite braggite and vysotskite reported from worldwide occurrences seem to imply a continuum of compositions between vysotskite and cooperite with no obvious miscibility gap this is contrary to the experimentally confirmed miscibility gap between cooperite and braggite and the established compositional gap between co existing cooperite and braggite from the merensky reef although the only unambiguous way of distinguishing between cooperite and braggite is to obtain structural information through x ray diffraction or equivalent techniques most identification of pt pd ni sulphides is based on microanalytical techniques due to the small grain size of most platinum group minerals ni contents also have to be considered because a classification based on the pt pd ratio alone can be very misleading naming of pt pd ni sulphide compositions with high pt contents based on qualitative or semi quantitative analyses should be avoided natural pt pd ni sulphides which project into the compositional gap established by experimental work cannot be named without supporting structural information compositions of grains which plot inside the gap are considered to be metastable and to result from the loss of pd through interaction with hydrothermal fluids

platinum group element distribution in base metal sulfides of the merensky reef from the eastern and western bushveld complex south africa base metal sulfides in magmatic ni cu pge deposits are important carriers of platinum group elements pge the distribution and concentrations of pge in pentlandite pyrrhotite chalcopyrite and pyrite were determined in samples from the mineralized portion of four merensky reef intersections from the eastern and western bushveld complex electron microprobe analysis was used for major elements and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms for trace elements pge ag and au whole rock trace element analyses were performed on representative samples to obtain mineralogical balances in merensky reef samples from the western bushveld both pt and pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity samples from the eastern bushveld reveal more complex distribution patterns in situ la icp ms analyses of pge in sulfides reveal that pentlandite carries distinctly elevated pge contents whereas pyrrhotite and chalcopyrite only contain very low pge concentrations pentlandite is the principal host of pd and rh in the ores palladium and rh concentrations in pentlandite reach up to 700 and 130 ppm respectively in the samples from the eastern bushveld and up to 1 750 ppm pd and up to 1 000 ppm rh in samples from the western bushveld only traces of pt are present in the base metal sulfides bms pyrrhotite contains significant though generally low amounts of ru os and ir but hardly any pd or rh chalcopyrite contains most of the ag but carries only extremely low pge concentrations mass balance calculations performed on the merensky reef samples reveal that in general pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 of the pd and rh and smaller amounts 10 40 of the os ir and ru chalcopyrite and pyrrhotite usually contain less than 10 of the whole rock pge the remaining pge concentrations and especially most of the pt up to 100 are present in the form of discrete platinum group minerals such as cooperite braggite sperrylite moncheite and isoferroplatinum distribution patterns of whole rock cu ni and s versus whole rock pd and pt show commonly distinct offsets the general sequence of offset patterns of pge and bms maxima in the order from bottom to top is pd in pentlandite pd in whole rock cu ni and s the relationship is not that straightforward in general some of the reef sequences studied only partially show similar trends or are more complex in general however the highest pd concentrations in pentlandite appear to be related to the earliest volumetrically rather small sulfide liquids at the base of the merensky reef sequence a possible explanation for the offset patterns may be rayleigh fractionation

platinum group mineralization in the rum layered intrusion scottish hebrides uk abundant and diverse platinum group minerals occur in the rum layered intrusion inner hebrides scotland platinum group minerals are associated with chromite rich horizons in both the eastern and western layered series and are enclosed by ni and cu rich sulphides or silicates the most common phases are pd cu alloys pt fe alloys laurite ru ir os s2 sperrylite ptas2 and pt pd bearing bismuthides tellurides and antimonides the mineralization on rum is due to 1 initial concentration in the parental magma linked to a high degree of melting associated with mantle plume activity and 2 localized concentration due to magma mixing and subsequent crystallization within an open system magma chamber

chromitites platiniferes des complexes ophiolitiques de l estrie beauce appalaches du sud du quebec des chromitites contenant du platine du palladium et du rhodium viennent d etre mises a jour dans les appalaches du sud du quebec les complexes ophiolitiques hotes de ces mineralisations sont d ge ordovicien inferieur et d affinite boninitique de plus ils sont etroitement lies a un arc insulaire les chromitites a pt pd rh se situent dans les dunites de la sequence a cumulat a leur passage aux pyroxenites le platine le palladium et le rhodium sont exprimes soit sous forme d alliages et de sulfures complexes automorphes ou soit sous forme d alliages a pt pd et d arseniures rhodiferes xenomorphes cette derniere paragenese accompagne la serpentinisation des cumulats ultramafiques

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

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

pge in the 3 5 jamestown ophiolite complex barberton greenstone belt with implications for pge distribution in simatic lithosphere the 3 5 ga mafic ultramafic rocks of the barberton greenstone belt south africa form a pseudostratigraphy comparable to that of phanerozoic ophiolites this archaean complex referred to here as the jamestown ophiolite complex consists of high temperature tectonometamorphic peridotites overlain by an intrusive extrusive igneous section which in turn is capped by a chert shale sequence elevated pge concentrations with distinctly different chondrite normalized patterns have been found at three pseudostratigraphic levels of this ophiolite from the top down high pge values are first encountered in an undeformed chromitite interlayered with meta pyroxenites the pge abundances and normalized patterns are similar to those reported from pge mineralized zones in large mafic layered complexes chromitemagnetite pods occur stratigraphically below the chromitite their pge patterns resemble those reported from chromite pods in phanerozoic ophiolites and those of the overlying chromitite the host rocks of the pods deformed talc carbonate rocks are interpreted as a mixture of deformed high temperature cumulates and depleted upper mantle residue within a ductile shear zone in all the above occurrences the pge were concentrated by igneous upper mantle crustal fractionation processes of mantle derived liquids the third type of pge concentration is found in an unusual sulphur poor nife silicate spinel pod called the bon accord ba deposit and an associated hematite magnetite chromite hmc pod both of which occur in coarse grained meta dunites at the lowest levels in the ophiolite these host rocks are interpreted as high temperature tectonites strongly depleted in ree they probably represent depleted upper mantle residue from which the overlying rocks were derived by partial melting ba can be divided into two groups group a has pat chondritelike pge patterns group b has a lower concentration of all the pge but the relative depletion is most severe for os and ir which are the most refractory thus the patterns have a positive slope both ba groups have negative pt anomalies in their pge patterns more pronounced for group a while the hmc pod which is depleted in the other pge relative to ba has a strong positive pt anomaly it is difficult to explain these fractionations in terms of known upper mantle crustal processes especially in the absence of sulphur the pge differentiation of ba is interpreted as an indication of chemical fractionation which started in the lower mantle we conclude that the body represents a metal silicate sample derived from an almost chondrite like lower mantle where it resided as a left over from inefficient core formation this siderophile enriched mantle fractionated during ascent and some of its residue was incorporated into the lower portion of the lithosphere during formation of the archaean oceanic crust tectonic and chemical data suggest that the thick and old lithospheric keel beneath the south african craton has a large component of archaean residual sima and is therefore likely to be relatively enriched in pge

anoxic sediments an important environment for pge an overview anoxic sediments and metasediments have been recently recognized as important host environments for the platinum group elements pge economically significant pge concentrations have been reported from marine metal rich black shales in china yukon canada and poland these ore bearing horizons are characterized by extremely restricted vertical and sometimes larger horizontal extents they are associated with the evolution of intracontinental rifting structures without any magmatic activity suggesting a possible driving mechanism for mineralization processes complex pge associations including a significant relationship of pd and pt with ni bi se y and also with sb cu co au u and f are suggested using the results of a new statistical evaluation of available geochemical data these occurrences show more common features with other hydrothermal pge deposits the rift volcanic related marine metal rich black shales of the czech republic and finland however display lower pge values the mineralized layers are much thicker with a typically limited horizontal extent showing a close association with volcanic centres pd reveals a strong correlation with ni zn au and v pt exhibits a highly significant link with the cu and mo phases low average nicu ratios and pge au chondrite normalized patterns are similar to those of some magmatic pge deposits hosted in mafic and ultramafic rocks it is very likely that organic matter played an important role in the genesis of pge accumulations in metal rich black shales in both rift and rift volcanic related environments

chemostratigraphy across the cretaceous tertiary boundary and a critical assessment of the iridium anomaly the elevated concentration of iridium one of the platinum group elements pge at the cretaceous tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end cretaceous mass extinctions new chemostratigraphic data for cross boundary sections from both hemispheres are not easily explained in terms of such an impact event for example the observation that the pge patterns show marked differences between the hemispheres the new constraints indicate that models of mantle derived pge should be seriously considered and that pge anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth s history

highly siderophile and strongly chalcophile elements in magmatic ore deposits an ore deposit by definition must be economically viable that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit bates and jackson 1987 this requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth at the oxygen fugacities found in the crust native fe is not normally stable and thus the highly siderophile elements defined as ru rh pd re os ir pt and au cannot behave as siderophile elements except in rare cases such as on disko island klock et al 1986 where the magma is sufficiently reduced for native fe to be present however if mafic magmas become saturated in a base metal sulfide liquid the highly siderophile elements behave as highly chalcophile elements table 1 thus these elements are generally found in association with base metal sulfide minerals which crystallized from a magmatic sulfide liquid namely pyrrhotite pentlandite chalcopyrite cubanite pyrite an exception to this is au although au is strongly chalcophile and is produced as a by product from many platinum group element pge deposits table 2 most primary au deposits consist of native au groves et al 1998 these will not be discussed in this chapter there are many pge deposits i e accumulations of pge minerals and base metal sulfides containing pge bates and jackson 1987 around the world but most of these do not constitute pge ore deposits because they are either too small or their grade is too low or other political or infrastructure factors prevent the economic exploitation of the deposit bates and jackson 1987 for the purpose of this work we have defined pge ore deposits as those which have significant production 2 of the annual world

platinum ore in three dimensions insights from high resolution x ray computed tomography platinum group elements referred to as pge comprising pt pd rh ru ir and os are strategic metals with a wide variety of industrial applications most of the world s pge production is mined from large mafic ultramafic intrusions such as the bushveld complex in south africa which currently provides 75 of the world s pt production the pge mineralization is found within distinctive layers tens to hundred of centimeters thick but extending laterally for many tens of kilometers where the pge occur at low parts per million levels as platinum group minerals pgm and in solid solution within disseminated base metal sulfides there is still heated debate at the most fundamental level about the mode of formation of this class of deposit genetic models range from primary magmatic sulfide collection to concentration by migrating halogen rich fluids a crucial line of evidence is the spatial relationship between the pgm which are the most important pge bearing phases and the base metal sulfide aggregates or blebs so far all observations have been carried out using two dimensional mineralogical studies where textural relationships with other minerals are ambiguous and with statistical limitations owing to sampling of trace phases intersecting random surfaces we present the first detailed three dimensional in situ analysis of the pgm at the sample scale using high resolution x ray computed tomography coupled with conventional microscopic and mineralogical study we find a striking and highly consistent relationship of pgm grains with the edges of complex shaped magmatic sulfide blebs and the intersection of these blebs with chromite silicate grain boundaries these new three dimensional observations strongly support an orthomagmatic model coupled with nucleation and growth of pgm at the margins of sulfide liquid droplets

recrystallization and the origin of layering in the bushveld complex abstract three rock types are distinguished in this study a the layered rocks b the reef pegmatites which are parallel to the layering and form prominent marker horizons and c the iron rich ultramafic irum pegmatites which are transgressive to the layered rocks reaction textures are observed in the layered rocks adjacent to reef pegmatites and are compared with similar reaction textures from alteration zones adjacent to and caused by the iron rich ultramafic irum pegmatites these include a clinopyroxene coronas around orthopyroxene b hornblende lamellae along 100 of orthopyroxene c orthopyroxene altered to olivine serpentine talc and or chlorite in the alteration zones adjacent to irum pegmatites the textures were generated by interaction between the pegmatite forming fluid and the wallrocks a similar origin is proposed for reaction textures in the layered rocks in the latter case they were formed by the fluid responsible for the formation of reef pegmatites in the layered rocks the reaction textures were modified to variable degrees during and after the emplacement of reef pegmatites these modifications include in leuconorites a clinopyroxene coronas overgrown by feldspar b orthopyroxene overgrown by feldspar and hornblende lamellae transforming to biotite and in olivine pyroxenites c olivine serpentine textures overgrown by a second generation of orthopyroxene the modifications suggest the subsolidus dissolution of orthopyroxene and precipitation of feldspar in leuconorites and the reverse process in pyroxenites the textures suggest considerable transfer of matter in the solid rock due to a fluid gradient orthopyroxene recrystallized in areas with a high ah2o plagioclase in regions of low ah2o this process is believed to be responsible for the stratification of the rock into pyroxene and plagioclase rich layers spherical mineral inclusions are relict textures in the three main minerals chromite orthopyroxene and plagioclase a number of similarities between the different inclusions suggest a similar origin the high content of hydrous minerals in all three suggests their formation during the emplacement of reef pegmatites these hydrous minerals were then trapped in the recrystallizing plagioclase orthopyroxene and chromite also spherical mineral inclusions witness the late crystallization of pyroxene in melanorite pyroxenite and the crystallization of plagioclase in leuconorite anorthosites after the emplacement of reef pegmatites

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

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

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

the first occurrence of platinum group minerals pgm in a chromite deposit in the eastern desert egypt the platinum group mineralogy pgm of the chromitite from gebel lawi in the southeastern desert has been investigated the most abundant base metal sulfides bms associated with the lawi chromite are pentlandite millerite and heazlewoodite the major platinum group minerals identified were as follows laurite irosru s2 osmian iridium osir hollingworthite rhass tellurian arsenopalladinite pdtesbas polarite pdhg besides cuprian palladian gold cupdau a pd sb hg and hgte phases laurite and osmian iridium occur preferentially in chromite os ir commonly forms composite pgm with laurite hollingworthite and tellurian arsenopalladinite are included within serpentine and close to the base metal sulfides the cuprian palladian gold shares boundaries with chromite polarite together with the pd sb hg and hgte phases are embedded in serpentine palladium is the most abundant pge in the gebel lawi chromite

cu ni pge mineralization at rometolvas koillismaa layered igneous complex finland sulphides tellurides and sulpharsenides with special reference to the platinum group minerals pgm have been studied from a subeconomic cu ni pge mineralization encountered within the syote section of the lower proterozoic 2 44 ga koillismaa layered igneous complex klic in northern finland using electron microprobe and ore microscopical methods the ore minerals occur partly as strata bound patches and spots associated with spots of light coloured secondary low temperature silicates in the gabbronorite iv of the general igneous stratigraphic column of the complex and partly as a fine grained impregnation in the penecontemporaneous basic sills and dykes among the pgm sperrylite michenerite and a palladian bismuthian melonite have been encountered the chemical composition is reported for these minerals as well as for the rest of the ore minerals chalcopyrite pentlandite pyrrhotite pyrite sphalerite cobaltite and hessite it is concluded that volatile components played a significant role in the solution transport and the final deposition of the sulphides and the pgm

shape and distribution analysis of merensky reef potholing northam platinum mine western bushveld complex implications for pothole formation and growth syn magmatic removal of the cumulate pile during the formation of the bushveld complex resulted in potholes erosion progressed downward in the cumulate pile resulting in a series of steep transgressive contacts between locally conformable potholed reefs in the regional pothole sub facies of the swartklip facies in the western limb of the bushveld complex the deepest of these potholes third order or fwp2 potholing occurs where the base of the merensky cyclic unit transgresses the upper pseudo reef chromitite marker horizon the base of a fwp2 pothole on northam platinum mine consists of an unconformable stringer merensky chromitite overlain by a medium grained poikilitic orthopyroxenite and underlain by either a pegmatitic harzburgite or the medium grained lower pseudo reef anorthosite detailed shape and distribution analysis of fwp2 potholes reveals underlying patterns in their shape and distribution which in turn suggest a structural control the ratio between pothole short vs long axes is 0 624 n 1 385 although the ratio increases from 0 48 to 0 61 in the long axis range 10 to 60 m then decreases from 0 61 to 0 57 from 61 to 100 m increasing again from 0 57 to 0 61 from 101 to 400 m suggesting that there is not a simple relationship between pothole shape and size shape circularity eccentricity and dendricity analysis of a subset of 638 potholes indicates that potholes with long axes 100 m have an average normalized shape that is bilobate and elongate on a 120 orientation the average aspect ratio short axis length divided by long axis length of potholes is highest for potholes with long axis lengths 100 m and lowest for potholes with long axis lengths between 35 and 60 m the most common long axis orientation for potholes with long axis lengths 100 m fractal analysis indicates that the distribution of pothole centers is controlled neither by a single nor several interacting fractal dimensions autocorrelation fry analysis of the distribution of pothole centers shows recurring pothole distribution trends at 038 070 and 110 for potholes over the full range of long axis lengths while the trends of 008 and 152 occur in potholes with long axes lengths between 60 and 100 m chi squared x2 analysis of the locations of pothole centers suggests that the distribution of small potholes is highly non uniform but becomes exponentially more uniform with increasing pothole size the model which best fits the observed shape and distribution analysis is a combination of protracted independent growth and nearest neighbor merging along specific orientations for instance the clustered distribution of original pothole centers resulted in merged potholes with long axes lengths of up to 60 m exhibiting short vs long axes ratios of 0 61 preferred orientations of 150 and alignment along 010 and 150 trends further independent growth allowed for merging of similar sized and smaller neighboring potholes generating potholes with long axes of up to 100 m in length a preferred long axis orientation of 150 and alignment along 010 040 075 and 150 subsequent preferential merging occurred along a 120 trend thereby preserving a bilobate form this implies that while pothole initiation and enlargement may be driven by a top down i e possibly thermomechanical process an underlying linear or structural catalyst control is revealed in changes in pothole shape during enlargement and furthermore in the preferred trends along which potholes merged over a considerable period possibly concomitant with adjustment of major structures in the footwall to the bushveld complex and pulses into the magma chamber

laser ablation icp ms study of platinum group elements in sulphides from the platreef at turfspruit northern limb of the bushveld complex south africa the platreef unit of the northern bushveld complex comprises a diverse package of pyroxenites peridotites and mafic lithologies with associated ni cu platinum group element pge mineralisation base metal sulphides bms are generally more abundant in the platreef than in other bushveld pge deposits such as the merensky reef and the ug2 chromitite but the platreef though thicker has lower overall pge grades despite a commonly held belief that pges are closely associated with sulphide mineralisation a detailed study by laser ablation icp ms la icp ms on a core through the platreef at turfspruit suggests that this is not strictly the case while a significant proportion of the pd os and ir were found to be hosted by bms pt irrespective of its whole rock concentration was not only at the top of the platreef is pt directly associated with sulphide minerals where pt pd sb te bi bearing inclusions were detected in the chalcopyrite portions of large composite sulphides in contrast pd os and ir occur in solid solution and as discrete inclusions within the bms throughout the core for os and ir this is usually in the form of os ir alloys whereas pd forms a range of pd te bi sb phases scanning electron microscope observations on samples from the top of the core revealed the presence of 0 2 mm long ptpd 2 sb te bi 2 michenerite maslovite laths within the chalcopyrite portions of large composite sulphides additional pt bearing minerals including sperrylite and geversite and a number of pd te bi sb minerals were observed in or close to the alteration rims of these sulphides this textural association was observed throughout the core similar platinum group minerals pgms were observed within the felsic assemblages composed of quartz plagioclase alkali feldspar and clinopyroxene produced by late stage felsic melts that permeated the platreef many of these pgms occur a significant distance away from any sulphide minerals we believe these features can all be linked to the introduction of as sb te and bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths where the degree of contamination was high all of the pt and some of the pd formed as and sb bearing pgm that were expelled to the edges of the sulphide droplets many of these were redistributed where they came into contact with late stage felsic melts where no felsic melt interactions occurred the expelled pt and pd arsenides and antimonides remained along the margins of the sulphides at the top of the platreef where the effects of contamination were relatively low some of the pt remained within the sulphide liquids on cooling this formed the micro inclusions and blade like laths of pt pd sb bi te in the chalcopyrite

evidence for as contamination and the partitioning of pd into pentlandite and co platinum group elements into pyrite in the fazenda mirabela intrusion brazil the fazenda mirabela intrusion in brazil hosts two zones of mineralization the santa rita ni cu sulfide ore zone occurring predominantly in an orthopyroxenite layer and an underlying basal platinum group element pge anomaly hosted in s poor dunite we show that in the northern and southern marginal zones of the intrusion pt pd ni cu fe bi te 2 minerals are accompanied by as bearing platinum group minerals pgm sperrylite ptas2 and irarsite rhirpt ass these as bearing pgm are extremely rare in the central zone of the intrusion suggesting that as has been introduced into the margins of the intrusion from the country rocks via crustal assimilation or synmagmatic hydrothermal processes other pgm and precious metal minerals pmm including hessite ag2te pd ag tellurides electrum and native au are observed in the santa rita ore zone and do not show any lateral variation in the semimetal and s poor dunite hosting the basal pge anomaly pge are predominantly retained in the base metal sulfides and only minor pge alloys are present in this s poor dunite pt au and semimetals partitioned into a fractionated cu rich liquid which formed during the crystallization of monosulfide solid solution mss and was largely removed by high temperature magmatic fluids palladium was not affected by this process and significant concentrations of pd are identified in pentlandite suggesting that pd preferentially partitioned into mss as it crystallized and subsequently pentlandite as it exsolved pyrite in the santa rita ore zone is co rich and contains higher concentrations of ipge os ir ru and rh than pyrrhotite and pentlandite suggesting that pyrite has not replaced and inherited the pge concentrations of these sulfides the pd poor nature of pyrite also indicates that it has not replaced pd rich pentlandite it is proposed that the ipge and co preferentially partitioned into pyrite as it exsolved from mss

the merensky cyclic unit and its impact on footwall cumulates below normal and regional pothole reef types in the western bushveld complex the merensky reef of the bushveld complex occurs in its highest stratigraphic position as a heterogeneous pegmatitic feldspathic melanorite bounded by two narrow chromitite stringers at the base of the merensky cyclic unit mcu in the swartklip facies of the rustenburg layered suite the occurrence of widespread thermal and mechanical erosion termed potholing has led to the subdivision of the merensky reef into normal reef and regional pothole reef sub facies the transition between the two sub facies occurs where the mcu transgresses the lower chromitite stringer of the normal merensky reef and cuts down into the underlying cumulate lithologies in the regional pothole reef at the northam platinum mine several economic reef types are identified where the merensky reef becomes conformable to cumulate layering in particular to the footwall marker np2 reef type and the upper pseudoreef p2 reef type the normal merensky reef as well as the p2 and np2 reefs contains economic platinum group element pge grades and includes the lower portion of the mcu melanorite and the merensky chromitite whole rock geochemistry indicates that this package is compositionally identical in normal p2 and np2 reefs suggesting that the base of the mcu is a relatively homogeneous drape over both normal and regional pothole reef regions however the lower sections of the three reefs are variables depending on the depth of transgression of the mcu in the normal and p2 reef types transgression by the mcu was arrested within harzburgites melanorites and norites resulting in coarse pegmatitic textures in the immediate footwall units for the np2 reef transgression by the mcu was arrested within leucocratic rocks and resulted in the formation of troctolites below the merensky chromitite these troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and pge contents relative to equivalent units in the footwall of the normal reef along with micro textural relationships these features suggest that troctolization of leucocratic cumulates in the np2 reef beneath the merensky chromitite was a result of a reactive infiltration of a chromite saturated melt and an immiscible sulphide liquid from the overlying mcu rather than a significant fluid flux from below in all reef types the concentration of s defines symmetrical peaks centred on the merensky chromitite and chromitites from pre existing cyclic units in normal and p2 reefs whereas pge concentrations define asymmetrical peaks with higher pge contents in reconstituted footwall rocks relative to the mcu melanorite this signature is attributable to a magmatic model of pge collection followed by deposition towards the base of the mcu and within reconstituted footwall rocks the continuity of the asymmetrical magmatic pge signature between the normal reef and regional pothole reef sub facies indicates that pge mineralization inherent to the merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall

platinum group elements in sulphide minerals platinum group minerals and whole rocks of the merensky reef bushveld complex south africa implications for the formation of the reef the concentrations of platinum group elements pge co re au and ag have been determined in the base metal sulphide bms of a section of the merensky reef in addition we performed detailed image analysis of the platinum group minerals pgm the aims of the study were to establish 1 whether the bms are the principal host of these elements 2 whether individual elements preferentially partition into a specific bms 3 whether the concentration of the elements varies with stratigraphy or lithology 4 what is the proportion of pge hosted by pgm 5 whether the pgm and the pge found in bms could account for the complete pge budget of the whole rocks in all lithologies most of the pge 65 up to 85 are hosted by pgm essentially pt fe alloy pt pd sulphide pt pd bismuthotelluride lesser amounts of pge occur in solid solution within the bms in most cases the pgm occur at the contact between the bms and silicates or oxides or are included within the bms pentlandite is the principal bms host of all of the pge except pt and contains up to 600 ppm combined pge it is preferentially enriched in pd rh and co pyrrhotite contains rh os ir and ru but excludes both pt and pd chalcopyrite contains very little of the pge but does concentrate ag and cd platinum and au do not partition into any of the bms instead they occur in the form of pgm and electrum in the chromitite layers the whole rock concentrations of all the pge except pd are enriched by a factor of five relative to s ni cu and au this enrichment could be attributed to bms in these layers being richer in pge than the bms in the silicate layers however the pge content in the bms varies only slightly as a function of the stratigraphy the bms in the chromitites contain twice as much pge as the bms in the silicate rocks but this is not sufficient to explain the strong enrichment of pge in the chromitites in the light of our results we propose that the collection of the pge occurred in two steps in the chromitites some pgm formed before sulphide saturation during chromitite layer formation the remaining pge were collected by an immiscible sulphide liquid that percolated downward until it encountered the chromitite layers in the silicate rocks pge were collected by only the sulphide liquid

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

the tectonic significance of pgm bearing chromitites at the ranomena mine toamasina chromite district madagascar this paper presents new data on platinum group mineral pgm bearing chromitites from the ranomena mine the only past producing chromite mine in the north toamasina chromite district of eastern madagascar such data can provide an insight on tectonic setting that led to emplacement of their host ultramafic body and can help to highlight the differences with other chromite deposits of madagascar chromitite host rocks are harzburgite and pyroxenite and gangue minerals are orthopyroxene and minor olivine chromite mineral chemistry shows a strong affinity with chromitites from layered intrusions and pge patterns are similar to those from the stratigraphically lowest chromitites in the bushveld complex pgms are dominated by laurite and are free of alloys pge mineralogy is consistent with early crystallization of laurite erlichmanite at low fs2 previous studies have suggested that the north toamasina ultramafic bodies are fragments of dismembered ophiolites and that their presence within the so called suture zone supports the suggestion that they mark the closure of an ocean between east and west gondwana even though these ultramafic bodies have been interpreted as ophiolitic our data better support an origin as small layered intrusions rather than as fragments of oceanic crust

an overview of the pge concentrations in the shetland ophiolite complex extremely anomalous platinum group element pge concentrations are known at one site cliff in the shetland ophiolite complex systematic pge analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous pge occurrences throughout the cumulate ultramafic sequence and their distribution is described here more sites with pge enrichments as anomalous as those at cliff have not been found at cliff the anomalous values of over 60 ppm pt pd are restricted to a chromite rich sulphide bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite lower values of up to 6 4 ppm pt pd occur in chromite poor sulphide rich dunites also within this dunite lens the only other chromite rich dunite lens in the cliff area also has detected pt and pd but with low pt pd values of 200 ppb in contrast harzburgites metasediments underlying the basal thrust serpentinites from the basal thrust and internal faults are all barren of pt and pd detection limit 20 ppb in the cliff area therefore detected pge values are restricted to magmatic chromite rich dunite lenses pge analyses of drill core from beneath the disused chromite quarries at cliff are low 227 ppb pt pd indicating that the extremely anomalous pge values are very restricted this supports the idea that pge concentrated magmatically within the dunite lens at cliff may have been remobilized locally to produce extremely anomalous pge values in an alteration zone only a few metres in diameter elsewhere in the ophiolite anomalous pt pd values of up to 4 ppm occur in chromite rich sulphide bearing dunite within the dunite unit which stratigraphically overlies the harzburgite pt pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences high level wehrlites and pyroxenites within the gabbro unit representing late ultramafic differentiates contain lower levels of pge of up to 310 ppb pt pd the gabbros all have less than 20 ppb pt and pd this distribution indicates that the pge concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with ni cu au and cr pge have not been found above detection limits in fault zones away from primary pge mineralization sites and it is concluded that pge are not significantly hydrothermally reconcentrated at cliff remobilization may have occurred over short distances of only a few metres at most

platinum group elements in a 3 5 ga nickel iron occurrence possible evidence of a deep mantle origin the bon accord ba ni fe deposit occurs in chemically depleted ultramafic rocks of the circa 3 5 ga jamestown ophiolite complex in the barberton greenstone belt of the kaapvaal craton south africa the host rocks of the ba body are high temperature ultramafic tectonites which probably represent the upper mantle residue from which the overlying magmatic rocks were separated during partial melting ba is unusual both mineralogically and chemically it consists of a rare ni rich assemblage ni oxide bunsenite spinels trevorite nichromite and silicates e g liebenbergite the ni end member olivine and their altered equivalents nio average 38 and feo fe2o3 average 34 are the major chemical constituents the cu and s contents are very low both 2000 c lower mantle high ni fe and ni co ratios relative to c 1 chondrite suggest that ba might have been derived from siderophile rich material that remained in the lower mantle after inefficient core formation a model is presented wherein such a metal silicate heterogeneity is fractionated and oxidized during ascent through the mantle in a thermal plume which originates in the lowermost mantle the d layer some of its residue is finally incorporated as pods into the lithospheric mantle during formation of the archean oceanic crust inclusion of such fractionated pods in the old subcontinental keel of the kaapvaal craton might constitute a potential pge source which could have been tapped by subsequent magmatic activity

au ir ni and co in some chromitites of the eastern bushveld complex south africa abstract instrumental neutron activation analyses of the lower group middle group and upper group chromitite layers of the eastern bushveld complex are reported the elements analysed were ir 300 ppb au 240 ppb ni 540 to 2240 ppm co 101 to 370 ppm ir and au are higher in the upper critical zone compared to the lower critical zone the relationship of the element abundance to the amount and composition of the chromite in the sample and the possible controls on the distribution of the elements are discussed at low levels of concentration ir is most likely controlled by discrete primary ir bearing platinum group minerals and au is related to small amounts of base metal sulphide both being sympathetic with the volume percent chromite in the sample higher ir and au levels are possibly related to sulphides present in even greater amounts

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

origin of the pegmatitic pyroxenite in the merensky unit bushveld complex south africa the genesis of the pegmatitic pyroxenite that often forms the base of the merensky unit in the bushveld complex is re examined large 1 cm orthopyroxene grains contain tricuspidate inclusions of plagioclase and chains and rings of chromite grains which are interpreted to have grown by reaction between small primary orthopyroxene grains and superheated liquid this superheated liquid may have been an added magma or be due to a pressure reduction as a result of lateral expansion of the chamber there would then have been a period of non accumulation of grains permitting prolonged interaction with the crystal mush at the crystal liquid interface crystal ageing and grain enlargement of original orthopyroxene grains would ensue only after the pegmatitic pyroxenite had developed did another layer of chromite and pyroxenite with normal grain size accumulate above it immiscible sulphide liquids formed with the second pyroxenite but percolated down as a result of their density contrast even as far as the footwall anorthosite in some cases whole rock abundances of incompatible trace elements in the pegmatitic pyroxenite are comparable with or lower than those of the overlying pyroxenite and so there is no evidence for addition and or trapping of large proportions of interstitial liquid or of an incompatible element enriched liquid or fluid in the production of the pegmatitic rock because of the coarse grained nature of the rock modal analysis especially for minor minerals is unreliable annealing has destroyed primary textures such that petrographic studies should not be used in isolation to distinguish cumulus and intercumulus components geochemical data suggest that the merensky pyroxenite both pegmatitic and non pegmatitic typically consists of about 70 80 cumulus orthopyroxene and 10 20 cumulus plagioclase with a further 10 of intercumulus minerals and could be considered to be a heteradcumulate

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

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

distribution of platinum group elements in the platreef at overysel northern bushveld complex a combined pgm and la icp ms study detailed mineralogical and laser ablation inductively coupled plasma mass spectrometry studies have revealed the physical manifestation of the platinum group elements pge within the platreef at overysel northern bushveld complex south africa the pge in the platreef were originally concentrated in an immiscible sulfide liquid along with semi metals such as bi and te as the sulfide liquid began to crystallize virtually all the os ir ru and rh partitioned into monosulfide solid solution mss which on further cooling exsolved to form pyrrhotite and pentlandite with os ir and ru remaining in solid solution in both phases with rh prefentially partitioning into pentlandite platinum some pd and au were concentrated in the residual sulfide liquid after mss crystallization and were then concentrated in an immiscible late stage melt along with semi metals which was expelled to the grain boundaries during crystallization of intermediate solid solution iss to form pt and pd tellurides and electrum around the margins of the sulfide grains tiny droplets of this melt trapped in the crystallizing mss and iss cooled to form pt bi te microinclusions in all sulfide phases whilst the excess pd was accommodated in solid solution in pentlandite minor redistribution and recrystallization by hydrothermal fluids occurred around xenoliths and at the very base of the mineralized zone within the footwall however the overall lack of secondary alteration coupled with the volatile poor nature of the gneissic footwall have allowed the preservation of what may be the most primary style of platreef mineralization the lack of pgm inclusions within early liquidus phases suggests very early sulfur saturation in the platreef lending support to theories involving s saturation occurring prior to intrusion of the platreef possibly within a staging chamber

platinum group elements in silicate rocks of the lower critical and main zones at union section western bushveld complex concentration patterns of platinum group elements pge in the lower critical and main zones of the bushveld complex are modelled by a process of metal collection by segregating sulphide melt followed by fractional crystallization of monosulphide solid solution mss separation of os ir ru rh enriched mss cumulate ore and pt pd enriched residual sulphide melt is inferred to have occurred by upward percolation of interstitial silicate melt entraining sulphide melt whereas mss crystals are entrapped by early crystallizing chromite this process resulted in os ir ru rh enrichment of the chromitites and an upward increasing trend of pt pd os ir ru crystallization of platinum group minerals pgm from s undersaturated sulphide melt or from coalescing noble metal clusters are alternative possibilities to explain the observed pge patterns but remain difficult to model at present remobilization of pt and pd by percolating late magmatic hydrous fluids could have resulted in metal enrichment of pre existing magmatic sulphides but this model is not supported by the metal budget of the reefs and the field data there is no evidence for solid substitution of pge into oxides and silicates

zonation of merensky style platinum group element mineralization in turfspruit thick reef facies northern limb of the bushveld complex the stratigraphy whole rock composition and mineralogy from a thick undisturbed section of the platreef at depth on turfspruit in the northern limb of the bushveld complex have been examined in detail this section has much more in common with the critical zone than with the platreef structure updip although the high grade platinum group element pge reefs located close to the boundary with the main zone are much thicker than any known merensky reef facies a key feature that marks the conversion of the platreefstyle mineralization into the merensky style is the regular appearance of chromite seams which become more persistent with decreasing contamination westward and downdip the similar zonal distribution of platinum group minerals pgm in two consecutive thick pge reefs correlates with changes in silicate compositions and vertical zonation of pge concentrations in whole rocks and base metal sulfides this supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes the typical merensky style ore assemblages of predominant pt sulfides and alloys are developed within 1 m thick intervals which correspond to the highest temperature mixing zones between two magmas whereas the rest of the reefs host pt pd bismuthotellurides and arsenides pge content and tenor as well as cu pd in major base metal sulfides in the turfspruit reefs are within the range of the typical high grade merensky values suggesting the same enrichment process