anomalous sulfur poor platinum group element mineralization in komatiitic cumulates mount clifford western australia an unusual occurrence of sulfur poor platinum group element pge mineralization has been discovered at mount clifford in the agnew wiluna belt western australia within the lower part of a 1 000 m thick komatiitic dunite body the host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite high precision whole rock icp ms data show that the pge mineralization extends over tens of meters whereas the lateral extent is unknown palladium and pt abundances are closely associated with base metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide poor base of a disseminated sulfide bearing layer whole rock concentrations range up to 4 100 ppb pd and 920 ppb pt iridium rh and ru follow the trends in pd and pt but are strongly depleted ir 0 4 ppb ru 0 5 ppb rh 0 1 ppb platinum group minerals pgm were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups pd antimonides pt dominated pgm and pt bearing ni antimonides the pge mineralization zone is underlain by an interval of strong ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between ni and the mg number of the rock this relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide however there is no direct evidence of this sulfide mass anywhere in the entire mount clifford dunite body the ni depletion is accompanied by strong depletion in ipge ir ru and rh the association of the pge zone with a reversal in the mg fe ratio of the host cumulates and a distinctive offset relationship of multiple pt pd and cu s peaks is suggestive of a primary magmatic origin however this is inconsistent with current models for offset reef pge concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated pge undepleted magma the association of enriched pt and pd with highly depleted ni and ipge in immediately underlying cumulates conflicts with this model we suggest a combination of processes early sulfide segregation at low r values producing ni and ipge depleted olivine partial redissolution and removal of most of this sulfide component during a major replenishment event and subsequent overprint by a migrating pt pd cu rich fluid

facies outcrop gamma ray and c o isotopic signature of exposed miocene subtropical continental shelf carbonates north west cape western australia abstract exposed uplifted miocene carbonate sequences of the cape range north west cape western australia provide outcrop analogues of seismic sequences from offshore parts of the shelf facies include deep shelf marls very fine and fine packstone larger foram wackestone floatstone and muddy rudstone foram coralline algal skeletal fragment packstone wackestone shallow seagrass facies lagoonal wackestone mudstone with scattered corals and tidal flat laminites the exposed early miocene units include the mandu highstand a sequence in the tulki and one in the middle miocene trealla limestone sequences contain decameter scale 5 20 m thick 4th order parasequences evident on gamma ray logs and by facies stacking that shallow and coarsen up they appear to be due to eccentricity driven sea level changes which may have been up to 50 m higher frequency meter scale parasequences of deep water marl up into larger foram rudstone floatstone perhaps precession obliquity are evident at the base of the exposed mandu section these parasequences are not merely random storm deposits this is indicated by the covariance of c and o isotopes with the lighter values associated with deepening and deposition of deep shelf marls and the heavier values being associated with shallowing and deposition of larger foram facies the uplifted miocene continental shelf sediments of the north west cape preserve a record of eustasy paleoclimate and paleoceanography and thus provide a window into factors affecting the shelf that can be compared with coeval better studied deep sea cores

the os and nd isotopic systematics of c 2 44 ga akanvaara and koitelainen mafic layered intrusions in northern finland abstract we have determined os and nd isotopic compositions of various rock types in and adjacent to the c 2 44 ga akanvaara and koitelainen layered intrusions which are part of the extensive paleoproterozoic ultramafic to felsic mantle plume related magmatism in the fennoscandian shield the initial e nd in the layered sequences of both intrusions fall within a limited range around 2 0 being compatible with results from other coeval layered intrusion in the shield the initial os isotopic ratios in chromitites are near chondritic the total range of os t is from 1 to 6 with the highest values reported for low os chromites no systematic differences are found in the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample the studied intrusions share similar isotopic and geochemical features with rocks from younger flood basalt provinces and also the same problem of the origin of the lithospheric source component viz crustal assimilation versus melt generation within lithospheric mantle the observed nd os isotopic systematics of the intrusions require that the magmas had negative e nd values already before emplacement and can best be accounted for by 15 25 deep crustal contamination of a komatiitic melt initially having a near chondritic os isotopic composition coupled with additional minor in situ assimilation of country rock material this hypothesis is supported by the existence of coeval crustally contaminated komatiitic volcanism on the fennoscandian shield

re os isotopic investigation of the stillwater complex montana abstract samples from the j m reef and the g chromitite horizons of the stillwater complex were analyzed for their re and os concentrations and os isotopic compositions there is large variability in the 187 os 186 os ratio both within the g chromitite and between the horizons that is unsupported by in situ re decay since the time at which the stillwater complex formed the lowest initial 187 os 186 os of 0 895 0 014 determined on a sample from the g chromitite indicates a normal mantle source region for the ultramafic series parent magma other samples from the g chromitite are intermediate in 187 os 186 os between this value and the j m reef average initial 187 os 186 os of 1 32 0 10 the os concentration of the g chromitite samples is also quite variable and is probably caused by heterogeneous distribution of early crystallizing platinum group element bearing phases predominantly laurite ru os ir s 2 oxygen isotopic analyses of the j m reef indicate that the variability of initial os isotopic composition is the result of magmatic processes a model is presented in which a normal mantle melt was contaminated with mafic to intermediate crust at the time of the ultramafic banded series transition to produce the j m reef os isotopic enrichment the g chromitite horizon recorded two events with os isotopes first the magmas that mixed to become saturated in chromite also precipitated laurite which carried the mantle os isotopic signature and second intercumulus liquid that migrated to the g chromitite level by convection in the cumulus pile precipitated late stage intercumulus sulfides which carried radiogenic os

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

sulfur isotope systematics and platinum group element behavior in ree enriched metasomatic fluids a study of mantle xenoliths from dish hill california usa the sulfur isotopic compositions trace and rare earth element ree and platinum group element pge contents were determined for a suite of anhydrous spinel lherzolite xenoliths and their metasomatized equivalents from dish hill california usa to evaluate the mobility of these elements in metasomatic fluids anhydrous xenoliths have ree patterns that vary from being slightly lree depleted to flat as well as variable enrichments of lile ba and sr and hfse th and nb that indicate they were affected by cryptic metasomatism the formation of amphibole in xenoliths modal metasomatism is accompanied by pronounced enrichments in hfses and rees particularly lrees with the exception of a single xenolith with slightly elevated pge contents the metasomatic fluids did not produce any appreciable changes in the platinum group element pge contents suggesting these elements are relatively immobile under these mantle conditions typical concentrations of pges in xenoliths from dish hill are 0 8 ppb os 1 ppb ir 0 6 ppb rh 2 5 ppb pt 1 8 ppb pd and 1 7 ppb au modal metasomatism has resulted in a variable increase in sulfur content of the xenoliths from a value of 10 ppm for anhydrous xenoliths to between 25 and 120 ppm for amphibole rich xenoliths the 34s values of sulfides in anhydrous xenoliths from dish hill are near 7 which are unusually high for the mantle and indicate that the lithospheric mantle under california was most likely affected by subduction of crustal sulfur sulfides in amphibole bearing xenoliths have variable but generally lower 34s values of 3 to 8 7 suggesting that some portion of the metasomatic sulfur was derived from an asthenospheric mantle source with a more primordial sulfur isotopic composition

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

role of late magmatic fluids in merensky type platinum deposits a discussion a number of features of platinum group element pge rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids the presence of pegmatitic textures intergrowth of sulfides with intercumulus volatile rich phases high cl content in volatile bearing phases and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of pge by a fluid phase of late magmatic derivation we argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive uniform pge concentrations were formed by magmatic cumulus processes the association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers which cause these layers to act as traps for fluids migrating through the crystal pile these fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in ci redissolution of these fluids into vapor undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites immiscible sulfide oxide liquid solidifies at near solidus temperatures in the interstitial pore space between early crystallizing silicate minerals and it is forced into areas occupied by late crystallizing ci bearing silicates this accounts for intergrowths of cu and pge rich sulfides with amphiboles and micas a ubiquitous feature of sulfide bearing gabbros further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration formation of fluid inclusions and possible local redistribution of sulfides and pge cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus graphite deposition may be catalyzed by sulfides and pge

an epeiric ramp low energy cool water carbonate facies in a tertiary inland sea murray basin south australia the murray supergroup records temperate water carbonate deposition within a shallow mesotrophic oligo miocene inland sea protected from high energy waves and swells of the open ocean by a granitic archipelago at its southern margin rocks are very well preserved and exposed in nearly continuous outcrop along the river murray in south australia most facies are rich in carbonate silt contain a background assemblage of gastropods especially turritellids and infaunal bivalves and are packaged on a decimetre scale defined by firmground and hardground omission surfaces bioturbation is pervasive and overprinted resulting in rare preservation of physical sedimentary structures facies are grouped into four associations large foraminiferan bryozoan echinoid bryozoan mollusc and clay facies interpreted to represent shallow water 50 m deposition under progressively higher trophic resource levels from low mesotrophy to eutrophy and restricted marine conditions from relatively offshore to nearshore regions a large scale shift from high to low mesotrophic conditions within lower miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow land locked sea overall low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity inshore heterozoan facies became progressively muddier and restricted towards the shoreline in contrast periods of wet climate led to rising trophic resource levels resulting in a shallowing of the euphotic zone a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids rates of carbonate production and accumulation were relatively low the murray basin is best described as an epeiric ramp wide facies belts developed in a shallow sea on a low angled slope reaching many hundreds of kilometres in length grainy shoal and back barrier facies were absent internally generated waves impinged the sea floor in offshore regions and because of friction along a wide and shallow sea floor created a low energy expanse of waters across the proximal ramp storms were the dominating depositional process capable of disrupting the entire sea floor

on the reconstruction of the transfer function for atmospherically generated seiches a method recently proposed to separate source and topographic effects in observed tsunami spectra is revised and applied to atmospherically generated seiches the method is used to investigate the origin of abnormal seiche events rissaga waves recorded in the region of ciutadella balearic islands the reconstructed open sea source spectra and the observed atmospheric pressure spectra are combined to estimate the transfer function between the atmosphere and the sea surface this function is used to predict sea level spectra for given locations near to the coast theoretically computed spectra for two rissaga events are in good agreement with observations

metal redistribution during fluid exsolution and migration in the middle banded series of the stillwater complex montana abstract olivine bearing zones iii and iv ob iii and iv of the stillwater complex mt preserve evidence of fluid migration particularly in the variation of apatite compositions this feature coupled with the lack of evidence of the addition of substantial amounts of primitive magma allows these zones to be used to test the importance of volatile fluids in redistributing metals the concentrations of cu au ag and as all show marked increases at and or below the levels inferred to represent the top of the fluid migration front in the crystal pile however no significant sulfide enrichment is associated with the maximum metal enrichments the au concentration peak is much sharper and offset stratigraphically above that of cu and corresponds with the maximum cl content of interstitial apatite these concentration profiles are modeled numerically assuming fluid transport and chromatographic concentration of the metals mass balance calculations indicate that cumulates significantly below the level of the inferred fluid saturation front lost cu while those at or immediately below gained cu we conclude that cl rich fluids redistributed the background metal concentrations although no dramatic enrichments are observed enrichment factors range from two to three the results demonstrate the potential importance of fluids in redistributing and concentrating metals in layered intrusions

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

warm temperate marine carbonate sedimentation in an early miocene tide influenced incised valley provence south east france the saumane venasque compound palaeovalley succession accumulated in a strongly tide influenced embayment or estuary warm temperate normal marine to brackish conditions led to deposition of extensive cross bedded biofragmental calcarenites echinoids bryozoans coralline algae barnacles and benthic foraminifera were produced in seagrass meadows on rocky substrates colonized by macroalgae and within subaqueous dune fields there are two sequences s1 and s2 the first of which contains three high frequency sequences s1a s1b and s1c sequence 1 is largely confined to the palaeovalley with its upper part covering interfluves each of these has a similar upward succession of deposits that includes i a basal erosional surface that is bored and glauconitized ii a discontinuous lagoonal lime mudstone or wackestone iii a thin conglomerate generated by tidal ravinement iv a transgressive systems tract series of cross bedded calcarenites v a maximum flooding interval of argillaceous muddy quartzose open marine limestones and vi a thin highstand systems tract of fine grained calcarenite tidal currents during stages s1a s1b and s1c were accentuated by the constricted valley topography whereas basin scale factors enhanced tidal currents during the deposition of s2 the upper part of the succession in all but s1c has been removed by later erosion there is an overall upward temporal change with quartz barnacles encrusting corallines and epifaunal echinoids decreasing but bryozoans articulated corallines and infaunal echinoids increasing this trend is interpreted to be the result of changing oceanographic conditions as the valley was filled bathymetric relief was reduced rocky substrates were replaced as carbonate factories by seagrass meadows and subaqueous dunes and the setting became progressively less confined and more open marine these limestones are characteristic of a suite of similar cool water calcareous sand bodies in environments with little siliciclastic or fresh water input during times of high amplitude sea level change wherein complex inboard antecedent topography was flooded by a rising ocean

widespread seagrass meadows during the early miocene burdigalian in southwestern australia paralleled modern seagrass distributions abstract the trealla limestone which forms a veneer of foraminiferal rich limestone and covers over 4000 km2 of the tectonically stable southern carnarvon platform contains the oldest evidence for seagrass meadows known in western australia it belongs to lower tf1 letter stage based on larger benthic foraminifers of the burdigalian within interval 19 16 ma and is the only miocene unit on the platform the trealla limestone here is coeval to the deeper water lower part of the much thicker trealla limestone in the exmouth sub basin to the northwest a seagrass meadow depositional environment is indicated by most of the studied microfacies based on comparisons with modern sediment composition and foraminiferal assemblages found in innermost shelf seagrass banks between 23 s and 32 s on the present western australian coast some levels in the trealla limestone on the platform e g with abundant miliolid foraminifers and absence of coral debris probably were deposited in and around dense seagrass meadows similar to those present today in the metahaline 40 55 ppt salinity reaches of shark bay whereas other levels e g coral floatstone were deposited under normal marine conditions based on comparisons with modern carbonate sediment and biota in the region the biogenic composition of the trealla limestone on the platform suggests winter minimum sea surface temperatures within the range of 17 22 c the nullabor limestone in the eucla basin at 10 further south on the southern australian margin is similar at least in part in composition and age to the trealla limestone on the southern carnarvon platform both may have been deposited during a burdigalian sea level rise and during part of the miocene climate optimum recognized elsewhere sediment composition and foraminiferal assemblages including epiphytic types found in the trealla and nullabor limestones are similar to those in the modern seagrass banks of the region this suggests that seagrasses similar to the extensive posidonia and amphibolus stands which are dominant today between 25 s and 35 s along the western australian coast were widespread during the burdigalian

shallowing upward sequences in purbeckian peritidal carbonates lowermost cretaceous swiss and french jura mountains purbeckian carbonates in the swiss and french jura goldberg formation lower berriasian comprise shallow subtidal intertidal supratidal low energy high energy marine brackish freshwater and hypersaline facies these facies are arranged in small 0 2 1 5 m thick sequences which display a dominant shallowing upward component and which form the fundamental units of the highly structured purbeckian sedimentary record six types of small scale sequences can be recognized a intertidal to supratidal overprinting of shallow lagoonal facies b algal marsh sequence with frequent dolomitization c sabkha sequence often associated with collapse breccia d tidal flat sequence with desiccation features e lacustrine sequence f terrestrial overprinting of subtidal or intertidal facies episodic event deposits such as tempestites are superimposed thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences green marls and black pebbles are common at the top and indicate long subaerial exposure the sequences are often incomplete as subtidal facies may be absent or their upper part can be eroded lateral facies changes are common which is due to the very shallow and partly emergent purbeckian platform where various depositional environments were juxtaposed however many sequence boundaries are well developed and can be correlated over large parts of the study area the purbeckian shallowing upward sequences were generated by climatically controlled sea level changes autocyclic processes occurred locally but were overprinted by drops of sea level affecting the entire platform the small scale sequences are most probably related to the 20 000 year cycle of the precession of the equinoxes larger sequences with usually well developed emersion surfaces are attributed to the 100 000 and 400 000 year eccentricity cycles of the earth s orbit identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces which often cut across facies boundaries and thus to interpret in detail the palaeogeographic sedimentological and diagenetic evolution of the purbeckian peritidal carbonate environments

on the diversity of the pge content in chromitites hosted in ophiolites and in porphyry cu systems controlling factors abstract the present study on the diversity of the platinum group elements pge content in chromitites and porphyry cu deposits from greece and everywhere literature data aims to define factors controlling precious metal enrichment in ophiolite complexes and the unknown pd and pt potential in porphyry cu systems through an integrated set of mineralogical and geochemical approaches the pge content in chromitites associated with ophiolite complexes is commonly low a few 100 s of ppb however a significant pge enrichment up to 10 s of ppm is a feature of relatively small chromite occurrences regardless their major element composition plots of the pd ir versus normalized pt pt ratios defined separate fields corresponding to various chromitite types large chromite deposits exhibit low pd ir values reflecting low degree of fractionation primitive magmas negative pt pt partial melting of already depleted mantle and increasing partial melting trend from high al towards to high cr deposits the association with hydrous silicates cr bearing hydrogrossular chlorite and serpentine and texture features of os ir ru rich compatible or ipge minerals coupled with very low pd ir and negative pt pt values were attributed to an origin by partial melting of already depleted mantle and increased partial melting degree during a subsequent crustal mantle recycling stage via fluid driven processes assuming that the increasing pd ir reflects a fractionation trend of parent magma a significant enrichment in pt and pd incompatible or ppge occurring in small occurrences either high cr or high al type may be a sensitive fingerprint for the orientation in the mantle sequence of the ophiolites and exploration for chromite commonly the pge content in porphyry cu mo au deposits is less than 10 ppb however elevated pd pt contents over 6 ppm in sulfide rich samples are a characteristic feature of certain porphyry cu au alkaline intrusions evidence of a favorable tectonic setting post collisional for the origin of porphyry cu au pd pt elevated porphyry deposits include elevated up to hundreds of ppm contents in cr co ni and re and elevated 187 os 188 os ratios reflecting mixing and contribution to the magma composition the oxidized nature of parent magmas abundant magnetite and the evolved mineralized fluid system in porphyry deposits are considered to be critical requirements for the transportation and deposition of sufficient au and pge elevated pd pt and au contents in porphyry deposits combined with the association of the major pd mineral merenskyite with bornite and chalcopyrite major cu minerals are encouraging economic factors for the contribution to the global pd pt production

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

fluid fluxing of cumulates the j m reef and associated rocks of the stillwater complex montana olivine bearing rocks in the lower half of ob i the olivinerich rocks also define pegmatoidal pothole structures 3 rock composition texture and grain size vary considerably in olivinebearing units characterized by segregation between coarser mafic regions and medium grained felsic regions 4 primary plagioclase in olivine rich rocks has an eroded texture 5 modal variations key words layered intrusions partial melting platinum group elements are consistent with progressive incongruent melt reactions and hydrous minerals are most abundant in olivine rich rocks 6 hydrous melt inclusions now crystallized to polyphase hydrous mineral bearing assemblages are present in olivine and chromite

oceanographic controls on shallow water temperate carbonate sedimentation spencer gulf south australia spencer gulf is a large ca 22 000 km2 shallow 60 m water depth embayment with active heterozoan carbonate sedimentation gulf waters are metahaline salinities 39 to 47 and warm temperate ca 12 to 28 c with inverse estuarine circulation the integrated approach of facies analysis paired with high resolution monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems biofragments mainly bivalves benthic foraminifera bryozoans coralline algae and echinoids accumulate in five benthic environments luxuriant seagrass meadows patchy seagrass sand flats rhodolith pavements open gravel sand plains and muddy seafloors the biotic diversity of spencer gulf is remarkably high considering the elevated seawater salinities echinoids and coralline algae traditionally considered stenohaline organisms are ubiquitous euphotic zone depth is interpreted as the primary control on environmental distribution whereas seawater salinity temperature hydrodynamics and nutrient availability are viewed as secondary controls luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations ca 0 to 1 mg chl a m 3 low diversity bivalve dominated deposits occur in meadows with highest seawater salinities and temperatures 43 to 47 up to 28 c patchy seagrass sand flats cover less illuminated seafloors open gravel sand plains contain coarse bivalve bryozoan sediments interpreted as subphotic deposits in waters with near normal marine salinities and moderate trophic resources 0 5 to 1 6 mg chl a m 3 to support diverse suspension feeders rhodolith pavements coralline algal gravels form where seagrass growth is arrested either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth 0 6 to 2 mg chl a m 3 or bottom waters that are too energetic for seagrasses currents up to 2 m sec 1 muddy seafloors occur in low energy areas below the euphotic zone the relationships between oceanographic influences and depositional patterns outlined in spencer gulf are valuable for environmental interpretations of other recent and ancient particularly neogene high salinity and temperate carbonate systems worldwide

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

crustal contamination and fluid rock interaction during the formation of the platreef northern limb of the bushveld complex south africa the platreef is the main platinum group element pge bearing and contains some of the most important magmatic ore deposits yet discovered the intrusion covers an area of facies of the northern limb of the bushveld complex but unlike the merensky reef of the eastern and western limbs it is in direct roughly 65 000 km e g tankard et al 1982 and lies almost entirely within the bounds of the transvaal basin contact with the country rock mineral separate o values for samples from the upper zone and main zone of the northern limb fig 1 the layered igneous rocks the rustenburg layered suite sacs 1980 can be divided into eastern indicate crystallization from a well mixed already contaminated magma having a o value of 7 5 pyroxenes from the pyroxenites and western limbs of approximately the same size and a smaller northern limb the northern limb differs in facies of the platreef at sandsloot mine have o values that are up to 2 4 higher than pyroxenes from the upper and main several important aspects from the more well known eastern and western limbs e g van der merwe 1976 zones these differences can be explained by additional assimilation of up to 18 dolomite which is in contact with the intrusion at buchanan et al 1981 cawthorn et al 1985 eales cawthorn 1996 first the layered rocks show a this locality samples from the platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted pronounced transgression across the floor rocks from a with fluids during slow cooling quartz veins with granophyric level above the magaliesberg quartzite formation south margins have o values between 10 1 and 12 2 which of the town of potgietersrus across the whole pretoria suggest that the fluid that passed through the cooling platreef had group and the banded iron formations and dolomites a o value of 7 9 these data together with hydrogen isotope of the older chuniespoort group onto archaean granitic data from minerals and whole rock samples d of biotite 60 rocks fig 1 second the igneous layering shows an to 88 of the platreef suggest interaction with magmatic fluid apparent transgressive relationship with respect to the at low water rock ratios interaction of this fluid with the calcsilicate contact such that ultramafic rocks of the lower zone footwall rocks lowered their o values abut against the contact south of potgietersrus whereas to the north rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the

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

dynamics of evolving magma chambers textural and chemical evolution of cumulates at the arrival of new liquidus phases abstract basaltic magma chambers are best exemplified by layered intrusions fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5 10 liquidus minerals most minerals appear in the stratigraphy of layered intrusions via two distinct stages they first emerge as oikocrysts large irregularly shaped and interstitial crystals that form in a crystal liquid framework and then as primocrysts relatively small idiomorphic and cumulus crystals that crystallize from a main magma body the transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features a a similarity in chemical composition of the last forming oikocrysts and the first forming primocrysts b slow arrival modal overabundance and finer than normal size of the first primocrysts c a low abundance of trapped liquid in rocks with the first forming primocrysts d a step like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival and e the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section the phase equilibria control on the cumulus stratigraphy of layered intrusions require large long lived and largely molten magma chambers and therefore their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the earth s crust we therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts the entire magma body during its internal evolution is thought to be kinetically supercooled i e kept slightly below its liquidus temperature with respect to cumulus phases that crystallize at the margins each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre existing crystals at the floor roof and sidewalls of basaltic magma chambers this implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body

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

experimental study of the solubility of platinum and iridium in basic silicate melts implications for the differentiation of platinum group elements during magmatic processes this study presents evidence for the dissolution of two platinum group elements pge s pt and ir in silicate melts of basic composition in the absence of any associated fluid phase the dissolution of these pge s is attributed to the formation of a complex ion in which the noble metal is uncharged an increase in oxygen fugacity in equilibrium with the melt brings about a considerable decrease in ir solubility in this way we can explain the early enrichment in ir group pge s ipge s ir ru os observed during the crystallization of ultrabasic cumulates by contrast however pt remains soluble in the melt an increase in the fs2 results in a pronounced increase in the solubility of pt in the silicate melt the sulfur saturation which leads to the precipitation of the base metal sulfides bms s also brings about the fixation of pt group pge s ppge s pt pd rh which are dissolved in bms s this accounts for the presence of ppge s in the nickel sulfide horizons of stratiform deposits the occurrence of platiniferous fe crystallizing with chromite in alaskan type complexes at oxygen fugacities of 10 6 bar can be explained by the low activity of fe in fe pt alloys such alloys have been formed in our laboratory at such fo2

multiple s isotope studies of the stillwater complex and country rocks an assessment of the role of crustal s in the origin of pge enrichment found in the j m reef and related rocks abstract the j m reef in the stillwater complex of montana is the highest grade pd deposit in the world averaging approximately 14 ppm pd one suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal s because 34s values alone are often insufficient to document in igneous rocks the assimilation of s from archean sedimentary rocks a combined study of both 33s and 34s values and computed 33s values was undertaken 34s and 33s values of cordierite bearing hornfels as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations one is characterized by anomalous positive 33s values from 0 to 0 32 and 34s values from 0 5 to 1 6 whereas the other is characterized by anomalous negative 33s values from 0 to 0 22 and 34s values from 1 5 to 5 8 34s and 33s values for rocks from the basal series of the complex range from 0 9 to 3 0 and 0 01 to 0 18 respectively the anomalous s isotope ratios indicate that for some samples at least 50 of the their s was of crustal origin in contrast most of the rocks from the j m reef are characterized by 34s and 33s values between 1 3 and 1 8 and 0 06 to 0 06 respectively these values deviate only slightly if at all from those thought to be representative of mantle s and suggest that i contamination was not an important process for pge enrichment in the complex ii the contaminant was also characterized by near 0 33s values or iii s isotopic exchange in the staging chambers was an important process upgrading of pge values via reaction between sulfide in the chamber and uncontaminated mantle derived magma may also have resulted in the conversion of s isotopic ratios that may have been indicative of crustal s addition to mantle like values rare 34s values up to 7 2 in the j m reef and 8 2 in chromitite from the ultramafic series provide strong evidence for localized assimilation of crustal s which did not experience isotopic homogenization the s isotopic ratios of the j m reef are non unique in terms of a specific genesis as they may be interpreted in either a downward sulfide collection model or an upward fluid transport model

solubility of pt in sulphide mattes implications for the genesis of pge rich horizons in layered intrusions the partitioning of pt in sulphide melt matte has been studied as a function of fs2 and fo2 at 1200 and 1300 c the results show that the solubility of pt in mattes increases strongly with increasing fs2 and decreases weakly with increasing fo2 the increase in pt solubility with increasing fs2 is attributed to pt dissolving in the melt as a sulphide species and the weak inverse dependence of pt solubility on fo2 to the diluting effect of increasing o in the melt at high fo2 these results coupled with measurements of pt solubility in silicate melts taken from the literature allow the calculation of pt matte silicate melt partition coefficients d pt matte sil for a range of conditions pertinent to the formation of pt rich horizons in layered intrusions the calculated values range between 107 and 1011 depending on fo2 and fs2 several orders of magnitude higher than previously published values our preferred value for d pt matte sil for conditions appropriate to the merensky reef is 107 and for the stillwater pt rich horizon 108 the new results are consistent with the magmatic hypothesis for pt rich horizons in layered intrusions