platinum group minerals in the skouries cu au pd pt te porphyry deposit the skouries deposit is a platinum group element pge enriched cu au porphyry system located in the chalkidiki peninsula greece with associated ag bi and te enrichment the deposit is hosted by multiple porphyritic monzonite and syenite intrusions which originated from a magma chamber at depth an initial quartz monzonite porphyritic intrusion contains a quartz magnetite chalcopyrite pyrite vein stockwork with intense potassic alteration the quartz monzonite intrusion is cross cut by a set of syenite and mafic porphyry dykes and quartz chalcopyrite bornite magnetite veins which host the majority of the cu and au mineralisation late stage quartz pyrite veins with associated phyllic alteration crosscut all previous vein generations electron microprobe and scanning electron microscopy shows that the pge are hosted by platinum group minerals pgm in the quartz chalcopyrite bornite magnetite veins and within potassic alteration assemblages the pge mineralisation in skouries is therefore part of the main high temperature hypogene mineralisation event platinum group minerals at skouries include sopcheite ag4pd3te4 merenskyite pd pt te bi 2 and kotulskite pd te bi with rare telargpalite pd ag 3te isomertieite pd11sb2as2 naldrettite pd2sb testibiopalladite pdte sb te and sobolevskite pdbi the most common platinum group mineral is sopcheite the pgm in skouries are small 52 m2 on average and occur as spherical grains on the boundaries between sulphides and silicates and as inclusions within hydrothermal quartz and sulphides these observations support a semi metal collector model whereby an immiscible bi te melt acts as a collector for pge and other precious metals in high temperature hydrothermal fluids this mechanism would allow the formation of pgm in porphyries without pt and pd fluid saturation

self destructive sulfide segregation systems and the formation of high grade magmatic ore deposits the metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals some high grade magmatic ni cu pge deposits and many magmatic platinum group element pge ni cu deposits demand very high magma sulfide ratios that seem improbable in the light of physical and kinetic constraints recent models for some high grade deposits invoke multistage upgrading processes in which an early formed sulfide liquid reacts with multiple later batches of silicate magma quantitative models of this open system process demonstrate that it is indeed more efficient than a closed system however it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids further increasing metal concentrations in the remaining sulfide liquids this combined process is termed multistage dissolution upgrading and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude furthermore if sulfide liquids are extensively dissolved during enrichment their base metal concentrations will generally stabilize at a limiting value whereas their pge concentrations will generally increase without limit this divergence could account for unusually high pge concentrations and high pge base metal ratios in many pge dominated deposits the multistage dissolution upgrading model is tested using reasonable degrees of dissolution in the context of natural deposits in the noril sk area and the bushveld intrusion the models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits thus dynamic sulfide forming magmatic systems may be intrinsically self destructive but this apparently undesirable attribute could play an important role in forming high grade deposits however dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches in this case no sulfide deposit would remain but metal depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits

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

continental tectonics and continental kinetics abstract we present a model of continental growth which combines the results of geochemical studies and tectonic ideas about the evolution of continents through geological time the process of continental growth is mainly controlled by surface phenomena continental material is extracted from the mantle along subduction zones at the periphery of oceans and is destroyed in collision zones where it is remobilized and made available for subduction we derive an equation for s the portion of the earth s surface occupied by continents which reads as follows ds dt a 1 s b s coefficients a and b depend on the geometry of plates on their number and on their velocities we assume that they decrease exponentially with time with the same time scale this model satisfies both geochemical and tectonic constraints and allows the integration of several current observations in a single framework

a record of assimilation preserved by exotic minerals in the lowermost platinum group element deposit of the bushveld complex the volspruit sulphide zone abstract low gradeplatinum group element mineralisation in the volspruit sulphide zone is sulphide poor here we present a petrological investigation on the distribution of pges and chalcophile metals in mineralised pyroxenite cumulates from the volspruit sulphide zone to determine the origin of the pge mineralisation in ultramafic cumulates and evaluate whether volspruit style mineralisation could occur in the stratigraphically lowest ultramafic portions of other layered intrusions electron microscopy of pyroxenite cumulates revealed 1 chromite inclusions containing dolomite albite monazite pb chlorides base metal sulphides and pt as minerals 2 the presence of exotic microxenocrysts systematic mapping of high density mineral assemblages in pyroxenite cumulates across the volspruit sulphide zone identified 196 precious metal mineral grains pt pd rh au or ag minerals 98 pb sulphide grains se cl 27 pb chloride grains k se te s as well as 1 grain of pb telluride 1 monazite grain and 1 grain of u pb th oxide trace element analyses of base metal sulphides reveal the highest s se values in pyrrhotite and chalcopyrite yet recorded in the bushveld complex while some base metal sulphides are enriched in pges the overall low grade of the deposit and inferred fertile ultramafic magma s require relatively low r factors mass of silicate to sulphide melt compared to other sulphide poor pge deposits with a calculated r factor of 500 3000 we consider that the presence of exotic inclusions in chromite exotic microxenocrysts and pb zn cl grains enclosed within primary base metal sulphide assemblages provide strong evidence for crustal contamination in the volspruit sulphide zone the malmani dolomite and the black reef quartzite within the lower chuniespoort group 2 2 2 4 ga are the most likely source of xenocrysts assimilated in a staging chamber beneath the main grasvally chamber in which the volspruit sulphide zone developed it is possible that the malmani dolomite contained an enrichment of pb zn cl and s minerals prior to assimilation the assimilation of dolomite and limestone would locally increase the fo2 of the magma triggering chromite crystallisation the sudden removal of fe from the melt coupled with the addition of external sulfur triggered saturation of an immiscible sulphide melt in the ultramafic volspruit magma chromite and base metal sulphides were subsequently emplaced into the main grasvally magma chamber as a crystal bearing slurry therefore we consider it is possible for pge mineralisation to occur in the ultramafic portion of any layered intrusion intruding in the vicinity of carbonate units even if this style of mineralisation in the lowermost portions of layered intrusions is sub economic it may reduce the grade or opportunity for pge mineralisation higher up in the local magmatic stratigraphy or in later magma emplacement events sourced from the same reservoir the technique of specifically searching for microxenocrysts could be applied beyond layered intrusion research to identify the range of crustal contaminants in other magmatic systems where macro scale xenoliths are neither sampled nor preserved

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

platinum group minerals in the limoeiro ni cu pge sulfide deposit brazil the effect of magmatic and upper amphibolite to granulite metamorphic processes on pgm formation the limoeiro ni cu platinum group elements pge deposit is a recent discovery associated with an igneous tubular conduit system in northeastern brazil representative ores from the deposit have been used for platinum group minerals pgm identification and for pge in base metal sulfides bms quantification ninety eighty percent of the pgm in the massive sulfide ores is homogeneous pt ni bi bearing merenskyite pdte2 enclosed primarily by pyrrhotite suggesting that it is formed by exsolution from monosulfide solid solution mss merenskyite gradually but systematically becomes poorer in pt and ni with increasing fractionation which is interpreted to reflect a transition to a more evolved sulfide liquid that segregated in the eastern parts of the intrusion in massive sulfide ores merenskyite forms unusually large up to 5000 m2 euhedral grains commonly in contact with spherical silicate inclusions bms hosts 12 16 of the pd with the remainder hosted by pgm which is interpreted to indicate that merenskyite recrystallized from a pge bearing bismuthotelluride metamorphic melt formed during high grade metamorphism sperrylite ptas2 is the second most abundant pgm 18 of pgm in disseminated ore and in contrast to merenskyite occurs mainly as very small median of 25 m2 inclusions in high temperature silicates and oxides interpreted to have crystallized at high temperatures directly from sulfide blebs that formed and were transported within the limoeiro magma conduit

geochronological evidence for an extension of the northern lobe of the bushveld complex limpopo province south africa abstract we have conducted the first geochronological study of the ultramafic mafic succession the waterberg project located in the southern marginal zone of the limpopo belt north of the northern lobe of the bushveld complex using u pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the waterberg group unconformably overlying the mafics this contribution is the first in a series of publications describing the well mineralized succession and will therefore give an overview of the local geology u pb baddeleyite ages for the main mafic sill intruding the area supplement the data the succession is extremely well mineralized and therefore of interest to the wider geological community the intrusion ages for the lobe are 2059 3 and 2053 5 ma and within error coeval with the intrusion ages of the bushveld complex detrital zircon age probability density distributions indicate a maximum age of deposition of c 2045 ma for the waterberg group and show abundant peaks of archean age paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe while the archean zircons are most likely sourced from the limpopo complex to the northeast the major dolerite sill intruding the project area has been confirmed to belong the umkondo igneous province with a baddeleyite age of 1125 5 ma the combination of these new age dates local geology and existing literature is used to argue that i the ultramafic mafic succession is related to the bushveld complex ii a significant unconformity is present between igneous rocks and the overlying sediments and iii the area was exposed to a level of paleoproterozoic tectonism not previously observed in the bushveld complex

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa the platreef the putative local analogue of the merensky reef forms the floor to the mafic succession in the northern limb of the bushveld complex we define the platreef as the lithologically variable unit dominated by pyroxenite which is irregularly mineralised with pge cu and ni between the transvaal metasedimentary footwall or archaean basement and the overlying main zone gabbronorite we define the mineralisation around calcsilicate xenoliths within the main zone in the far north of the limb as a platreef style mineralisation the platreef ss has a strike extent of 30 km whereas platreef style mineralisation occurs over a strike length of 110 km the platreef varies from 400 m thick in the s to 30 modal in some intersections these are dominated by pyrrhotite with lesser pentlandite and chalcopyrite minor pyrite and traces of a wide compositional range of sulphides in the southern sector mineralised zones have cu grades of 0 1 0 25 and ni 0 15 0 36 massive sulphides are localised commonly but not exclusively towards the contact with footwall metasedimentary rocks magmatic sulphides are disseminated or net textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite much of the sulphide is associated with intergranular plagioclase or quartz feldspar symplectites along the margins of rounded cumulus orthopyroxenes the pges in the southern sector occur as tellurides bismuthides arsenides antimonides bismuthoantimonides and complex bismuthotellurides pgm are rarely included in the sulphides but occur as micron sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones the pt pd ratio 1 and pge grade may be decoupled from s and base metal abundance

precious metals in magmatic fe ni cu sulfides from the potosi chromitite deposit eastern cuba abstract the moa baracoa ophiolite in eastern cuba is one of the few known ophiolites that display sulfide mineralization attributable to a magmatic origin in association with podiform chromite ores hosted in the mantle crust transition these sulfide ores chiefly consist of fe ni cu sulfides namely pyrrhotite pentlandite chalcopyrite and cubanite partly altered to valleriite the sulfide mineralization is located along the contact between the podiform like chromite ores and intruding pegmatitic gabroic dykes the detailed mineralogical study of the sulfide mineralization coupled with the first ever laser ablation icp ms analysis reveals that this sulfide mineralization show contents of the precious metals os ir ru pt re au ag and other semi metals co ni cu se te bi pb as sb comparable to those sulfides from the magmatic sulfide deposits associated with mafic complexes hosted in the continental crust the results obtained from this study confirm that fe ni cu sulfides at potosi are magmatic in origin and very likely derived from the solidification of droplets of sulfide melt segregated by immiscibility from the intruding mafic melts once they interacted with the pre existing chromitite at the mantle crust transition zone of the ophiolite the immiscibility of sulfide melt was achieved as a result of a progressive increase of fs2 very likely triggered by a set of circumstances including the progressive fractionation of the intruding mafic melt leading to increase of asio2 and accumulation of volatiles as well as the crystallization of oxides two main generations of pentlandite were observed one generation is primary in origin and it was locally exsolved along with pyrrhotite from monosulfide solid solution mss during low temperature cooling the second type of pentlandite resulted from the reaction of mss with coexisting droplets of cu and ni rich sulfide melt la icp ms analysis reveals that most precious metals ru os ir re au ag were concentrated along with the base metal sulfides bms although their distribution among the different bms pyrrhotite pentlandite chalcopyrite and cubanite does not strictly follow the expected distribution according to the known melt solid and solid solid partition coefficients unlike the other analyzed pges pt was not preferentially concentrated in bms but as discrete micrometer sized sperrylite grains the crystallization of sperrylite took place before and contemporaneous to sulfide segregation and pt as nanoparticles probably played an important role in the pt uptake as nucleation seeds for the formation of micron sized sperrylite grains these observations highlight the open system nature of the ore forming system as well as the important role of arsenic in concentrating pge in high temperature silicate and sulfide melts

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

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

continuity between eastern and western bushveld complex south africa confirmed by xenoliths from kimberlite the three dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops in some cases aided by drilling and or geophysical data however geophysical models are often equivocal for the 2 06 ga bushveld complex of south africa early geological models proposed a shape of a single gently inward dipping lopolith subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete dipping wedge shaped intrusions separated by 150 km a more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model distinguishing between these possibilities is difficult from surface based studies because the central regions of the complex are obscured by large volumes of younger granites and sedimentary volcanic cover rocks here we describe xenoliths from the cretaceous palmietgat kimberlite pipe located mid way between the exposed western and eastern lobes of the complex they are chromite bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping critical zone of the bushveld complex this result provides strong support for a regionally interconnected bushveld complex implying its emplacement as a single sill like body confirming the continuity of the bushveld complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry

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

palaeoenvironmental changes at the frasnian famennian boundary in key european sections chemostratigraphic constraints abstract the study aims to put constraints on global environmental changes which took place during the frasnian famennian f f transition by using a geochemical multiproxy approach for this several key european f f boundary sections deposited in different palaeogeographic settings were selected including in addition to the stratotype and parastratotype sections of coumiac montagne noire france and steinbruch schmidt branau germany respectively also some other sections thoroughly characterized by other authors the drilling budesheimer bach prumer mulde germany kowala section holy cross mountains poland la serre france changes in the detrital flux along the sections were evaluated on the basis of both variations in mineral composition and in some diagnostic element ratios like ti al the biogenic flux was assessed as a function of the corg content the input of nutrients or the palaeoproductivity was determined by the p2o5 and ba in excess the degree of anoxia during deposition was constrained by determining the concentration of redox sensitive metals e g fe mn in the bulk sediment of specific element ratios e g v cr as well as the size of pyrite grains together with the degree of pyritization dop in the budesheimer bach core the hydrothermal and volcanic influence in the sedimentary record was detected using e g the al al fe mn and zr al2o3 ratios the kellwasser horizons are characterized by a general decrease in the detrital input together with the increase of al normalized concentration of some of the transition metals these changes are interpreted as reflecting a relative sea level rise and a higher hydrothermal influence in contrast to these the establishment of dysoxic anoxic conditions in the depositional environment as well as the increase of organic matter burial are not always restricted to the kellwasser horizons kwh moreover the hydrothermal influence can lead to misinterpretation with respect to redox conditions and productivity due to the additional input of some specific elements our results indicate the coincidence of sea level rise with hydrothermalism and volcanism during the deposition of the kwh in the area of south laurussia and north gondwana the resulting enrichment in metals and nutrients is coupled in several f f boundary sections with a higher microbial activity as already observed by some authors these effects coupled with a general lowering of the degree of oxygenation of the water bodies could have played a significant role in the demise of the biota

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

bacterial mediation red matrices diagenesis devonian montagne noire southern france abstract two devonian red carbonate rock sections are studied in the montagne noire at coumiac frasnian famennian and at the pic de vissou eifelian givetian the sediments are grey red mudstones and wackestones rich in pelagic fossils they are characteristic of an outer ramp the coumiac sequence is condensed with numerous hardgrounds and hiatuses the pic de vissou succession is more complete and of shallower origin in both cases the origin of the red coloration of the micritic matrix is probably linked to bacterial activity which produced submicronic hematite both iron and manganese concentrations are low average 0 2 bacteria form ferruginous microstromatolites blisters microtufts hedgehogs filling sponge perforations and thin continuous mineralized films probably biofilms hardgrounds are underlined by ferruginous microstromatolites the origin of the matrix color is probably related to the destruction of these bacterial constructions the submicronic hematite ultimately coating the crystal faces of the calcite mosaic during early lithification microfissures appeared and were invaded by microbial colonies scanning electron microscopy sem shows that these colonies are composed of spheroidal beads this suggests continuity of the bacterial activity during early diagenesis later on these early fissures were cut by burrows subsequently a secondary fissure network transected all the previously mentioned sedimentary structures this late fissure network is characterized by diagenetically remobilized hematite and or calcite the latest alterations are stylolites and ultimate tectonic fractures

a volcanic production rate for the mariana island arc abstract despite the importance of volcanic production rates along subducting plate boundaries few reliable values are available a well constrained rate has here been determined for the past 5 m y in the mariana island arc the calculation covers the period during the opening of the mariana trough an active back arc basin in which the volcanic centers are located and most volcanic products are contained about half the extruded volcanic rocks occur in central cones with the other half in a large volcanicclastic apron after correction of volumes for porosity and for changes in mineral densities the average extrusion rate of 12 4 km 3 km m y was calculated the local rate varies along the arc roughly as does the subduction rate before quantification of that correlation can be made however apparent temporal fluctuations of volcanism on a global scale which do not appear to be correlated with subduction rates must be examined and explained

multi stage hydrothermal processes involved in low sulfide cu ni pge mineralization in the footwall of the sudbury igneous complex canada amy lake pge zone east range the amy lake pge zone is a low sulfide type cu ni pge mineralization in the east range footwall of the 1 85 ga sudbury igneous complex occurring in a 100 m wide sudbury breccia belt that coincides with an impact related major fracture zone bay fault zone detailed hydrothermal alteration mapping fluid inclusion trace element and stable isotope studies revealed a complex alteration and mineralization history in a multi source multi stage sudbury related hydrothermal system the two major stages of syn sudbury hydrothermal activity are characterized by similarly high salinity high temperature fluids that are 1 locally derived from footwall granophyre bodies and typified with high ni cu and pge s ratios and high ree contents magmatic hydrothermal stage and 2 a more voluminous cu ni pge rich fluid flux probably originated from the sudbury igneous complex footwall contact hydrothermal stage the second hydrothermal flux was introduced by brittle fractures in the area and resulted in a complex zonation of alteration assemblages and mineralization governed by local footwall composition the sudbury related hydrothermal event was overprinted by shear related epidote veining and calcite chlorite replacement both regionally present in the sudbury structure based on analogies the most important factors involved in the formation of hydrothermal low sulfide mineralization are proposed to be 1 accumulation of pge enriched fluids 2 large scale brittle structures as conduits to these fluids and 3 adequate host rock composition as a chemical trap resulting in sulfide and pgm precipitation in environments meeting these criteria hydrothermal pge mineralization is known to have formed not only in the sudbury footwall but also from mafic ultramafic intrusions associated with primary magmatic pge from several locations around the world

a hydrothermal ni as pge geochemical halo around the miitel komatiite hosted nickel sulfide deposit yilgarn craton western australia the remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes the recognition of which could potentially enlarge the detectable footprint of this ore type the miitel komatiite hosted nickel sulfide deposit in western australia was used as a case study to investigate the nature and 3 d geometry of the geochemical halo created by the remobilization of base metals gold and platinum group elements pge into the rocks surrounding the mineralization at miitel anomalous metal enrichment is found in the country rocks surrounding the massive sulfides up to 250 m away from the ore this enrichment detected using portable x ray fluorescence pxrf and backed up by laboratory analyses occurs in the mount edwards footwall basalt within decimeters of the contact with the overlying widgiemooltha komatiites it is associated with the presence of nickel arsenides gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets and are interpreted to form during the circulation of arsenic rich hydrothermal fluids results of lead fire assay analyses and in situ laser ablation inductively coupled plasma mass spectrometer la icp ms analyses indicate high pge concentrations pd and pt and minor gold associated with the observed nickel and arsenic enrichment results from a larger regional scale study combined with this pge enrichment suggest that the massive nickel sulfides from the miitel ore are the source of the remobilized nickel in the country rocks the presence of pd and pt enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes

the mineralogy and mineral associations of platinum group elements and precious metals in the aurora cu ni au pge deposit northern limb bushveld complex abstract aurora is a platinum group element pge prospect hosted in the northern limb of the bushveld complex south africa it is one of only three deposits discovered in the northern limb so far to be hosted in the melanocratic leucocratic gabbroic cumulates of the main zone of the rustenberg layered suite aurora moorddrift and waterberg t zone deposits rather than in predominantly ultramafic rocks e g platreef the host cumulates at aurora have been divided into three principal units and they intrude the dolomites of the lower transvaal supergroup base metal sulphide bms mineralisation with pge is present in the leucogabbronorites and gabbronorites of unit 2 and in coarse grained gabbronorite veins which intrude the peridotites of unit 1 these veins contain up to 50 interstitial pyrrhotite pentlandite chalcopyrite pyrite unit 2 contains 1 3 pentlandite pyrrhotite chalcopyrite assemblages and 1 5 chalcopyrite pyrite pyrrhotite associated with hydrothermal alteration the pge content of aurora however is predominantly controlled by the presence of platinum group minerals pgm not bms la icp ms analysis of sulphides shows the bms in aurora have lower pge concentrations than other bushveld magmatic sulphides with pentlandite carrying much lower concentrations of pd average 23 ppm than the platreef or the merensky reef sem eds analysis of 26 sections characterised 995 platinum group minerals pgm and precious metal bearing minerals pmm with a total area of 27850 m2 and an average size of 28 2 m2 of the pgm and pmm identified in aurora 85 by area are pd te bi minerals with 6 pd te minerals 4 electrum and 3 ag te minerals along with minor pd bi pd as pt te bi pt as and pt s minerals that collectively comprise 2 of total area only 25 of the pgm and pmm in aurora are bms hosted with the rest hosted in silicates of the total pgm and pmm area 22 are hosted in alteration silicates quartz chlorite or actinolite in an alteration halo around sulphides unusually 52 of the pgm and pmm are spatially removed from bms instead hosted in alteration silicates and within cracks in primary silicates away from any bms this indicates a multi stage ore genesis model with hydrothermal remobilisation of pge important for ore formation the style and host rocks for mineralisation in the aurora deposit are fundamentally different from other deposits in the northern limb of the bushveld hosted in ultramafic rocks such as the platreef gnpa member deposits and the f zone of the waterberg deposit all of which contain a greater diversity of pgm and bms with higher precious metal contents the mineralisation most similar to aurora is the t zone of the waterberg deposit located to the north of aurora which been suggested to be an along strike equivalent of the aurora main zone mineralisation however despite strong similarities in pgm mineralogy and s isotope signatures there are significant differences in bms mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically at present it seems more likely that aurora and the waterberg t zone reflect similar fluid influenced processes operating in different parts of the main zone perhaps at different times and in different structural basins rather than a continuous mineralised zone along strike

evolution of backarc rifting mariana trough 20 24 n the mariana trough is an actively opening backarc basin in the western pacific the trough formed by extension which longitudinally split an earlier arc massif creating a crescent shaped basin between the remnant west mariana ridge and eastern active volcanoes of the mariana arc opening increases southward from the volcano islands near 24 n where the two arcs join at 18 n in the central mariana trough the basin is widest and may be opening primarily by seafloor spreading we present a synthesis of closely spaced shipboard gravity magnetic and bathymetry measurements from the northern basin 20 24 n an area that undergoes a significant progressive southward increase in extension we have identified three stages in the evolution of rifting in this area 1 asymmetric rifting from 24 n to 22 15 n where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin 2 a localization of rifting from 22 15 n to 21 n where the primary zone of rifting separates from the active volcanic arc and 3 a further concentration of rifting from 21 n to 20 n leading to the formation of deep tectonic grabens near the center of the basin this last stage may be a precursor to or incipient seafloor spreading we describe a new type of magnetic lineation resolved in a three dimensional seafloor magnetization inversion of the trough between 20 and 24 n the detailed character of these lineations their association with tectonic structures and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting less magnetic rifted arc crust the development of these well defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough although tectonic deformation appears to be more widely distributed in addition the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward

new mineralogical and isotopic constraints on main zone hosted pge mineralisation at moorddrift northern bushveld complex the northern limb of the bushveld complex south africa contains a number of occurrences of platinum group element pge mineralisation within main zone rocks whereas the rest of the complex has pge depleted main zone units on the farm moorddrift cu ni pge sulphide mineralisation is hosted within the upper main zone in a layered package of gabbronorites mottled anorthosites and thin pyroxenites our observations indicate that a 10 m thick reef style package of mineralisation has been extensively disturbed forming a mega breccia which in some localities may distribute mineralised rocks over intersections of over 300 m the sulphides are made up of pyrrhotite pentlandite and chalcopyrite heavily altered around their margins and overprinted by secondary pyrite platinum group mineral assemblages typical of primary magmatic deposits with pt and pd tellurides and sperrylite are present in the reef style package whereas there is a decrease in tellurides and an increase in antimonides in the disturbed package interpreted to be related to hydrothermal recrystallization during veining and brecciation sulphur isotopes show that all sulphides within the mineralised package on moorddrift have a crustal signature consistent with local country rock sediments of the transvaal supergroup we interpret the mineralisation at moorddrift as a primary sulphide reef likely produced as a result of the mixing of crustally contaminated magmas in the upper main zone which has been locally disrupted post crystallisation at present there are no firm links between moorddrift and the other known pge occurrences in the main zone at the aurora and waterberg projects although the stratigraphic position of all may be similar and thus intriguing nonetheless they do demonstrate that the main zone of the northern limb of the bushveld complex unlike the eastern and western limbs can be considered a fertile unit for potential pge mineralisation

geochemistry and pge of the lower mineralized zone of the waterberg project south africa the waterberg deposit is located north of the northern lobe of the bushveld complex in south africa and represents a large high grade new platinum group element pge discovery the first comprehensive study of the lower ultramafic section of the area has been completed and whole rock chemical analyses and pge data are presented the section studied comprises mineralized harzburgite and marginal orthopyroxenite overlain by troctolite grading into gabbroic rocks whole rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine plagioclase and pyroxene normalized trace element data display hree depletion strong positive eu and sr anomalies and lree enrichment the negative anomalies for th rb nb and ta are typical for rocks of the bushveld complex normalized pge distributions are fractionated pd ir 177 pdenriched and au poor emplacement of the magmas is believed to have commenced with west east trending finger like intrusions followed by lateral dilation and emplacement of sulfide droplet bearing ultramafic magmas this was followed by a second phase of intrusions characterized by sheet like bodies of troctolite fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession the waterberg project is located in the southern marginal zone of the limpopo belt this position in a structurally active area may have facilitated the creation of space for initial magmas it is argued that the mafic to ultramafic succession of the waterberg project does not represent a simple marginal extension of the northern lobe nor does it directly correlate with the platreef it shares geological features but represents a separate magmatic basin

accumulation of a ferric mineral in the biofilm of montacuta ferruginosa mollusca bivalvia biomineralization bioaccumulation and inference of paleoenvironments abstract infrared absorption spectra of the amorphous and phosphorus rich ferric mineral associated with the biofilm of montacuta ferruginosa a marine bivalve were obtained by applying the kbr disc method phosphate absorption bands due to p o stretching vibrations were observed at 1100 and 1020 cm 1 this result as well as the similarity of spectra with other well known bio minerals suggests that the mineral is an amorphous iron oxyhydroxide gel with phosphate sorbed on its surface rather than a pure ferric phosphate it is suggested that phosphate ions are of microbial origin because phosphate producing enzymes were detected in vivo apart from iron the mineral phase is devoid of other heavy metals despite its similarity with other abiotically precipitated phases it is argued that the mineral is the result of in situ microbial biomineralization processes in the biofilm and not the result of a simple bioaccumulation process this is supported by microscopic observations a geological implication is that the simple presence of such iron minerals in ancient sedimentary environments where microbes have not been fossilized might be considered as an indicator of microorganisms performing biomineralization by comparison with the present day environment of m ferruginosa precise paleoenvironmental conditions may be inferred

sulfur isotope variations within the platreef ni cu pge deposit genetic implications for the origin of sulfide mineralization the platreef the world s third largest platinum group element pge deposit is a 10 400 m thick pyroxenitic unit at the base of the northern limb of the mafic bushveld complex hosting pge mineralization in association with base metal sulfides the sulfide mineralization is thought to have been either largely magmatic in origin with contamination by assimilation of local floor rocks considered an ore modifying process or a product of assimilation of country rock s and silicic contamination which caused s saturation in the magma with the contamination being fundamental to the ore forming process we have performed an extensive and detailed sulfur isotope study of the platreef which indicates that magmatic signatures 34s 0 to 2 are preserved in early formed sulfide droplets within the platreef pyroxenites in the area from sandsloot to witrivier these values are comparable to sulfide inclusions in diamonds in the nearby klipspringer kimberlite and are considered to have a primary mantle origin there is no indication of any significant external s in any of the primary sulfides in the sandsloot witrivier area however later sulfides found in calc silicate floor rocks at sandsloot and zwartfontein and in xenoliths of calc silicate throughout the section indicate an input of country rock s anhydrite bearing horizons in the malmani dolomites may have exchanged s with magmatic platreef sulfide during platreef intrusion imparting a heavier s isotope signature in the fluid affected rocks than those in the early sulfides the archean basement although containing minor amounts of sulfide with a negative 34s value is not a significant contributor to the s budget of the platreef previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides rather than sulfates such as at turfspruit the platreef sulfides are extensively contaminated with country rock s assimilation of pyrite bearing shales in the turfspruit area has locally upgraded the s content of the platreef and given the basal platreef sulfides a heavier isotopic signature sulfur saturation in the platreef magma took place before contamination probably in a staging chamber prior to intrusion a major pulse of magma entrained the preformed pge rich sulfides and was injected to form the platreef where assimilation of country rock sulfides upgraded the s content on a strictly local scale and hydrothermal leaching introduced s from country rock sulfates into later stage sulfides again on a local scale south of zwartfontein it is proposed that the platreef was intruded into sediments of the transvaal supergroup which formed the floor and the roof of the platreef north of zwartfontein the platreef intruded the boundary between the malmani supergroup sediments and the archean basement this could have potentially mineralized the roof calc silicates north of zwartfontein in a manner similar to the floor in the sandsloot area the later intrusion of the main zone magma formed a magmatic unconformity on top of the cold platreef and entrained large rafts of mineralized calc silicate for tens of kilometers north of the last footwall outcrop of calc silicate

iron bacterial and fungal mats bajocian stratotype mid jurassic northern normandy france abstract the oolithe ferrugineuse de bayeux formation is located at the historical bajocian stratotype of sainte honorine des pertes north of bayeux normandy the condensed formation ranges from the base of the humphriesianum zone to the parkinsoni zone and is divided into four beds of decimetric scale three main microfacies are present 1 oncoid rudstones 2 ooid bioclastic packstones and 3 silty burrowed wackestones packstones sedimentation took place in a very quiet environment below the photic zone and below or near the storm wave base the general setting is a distal carbonate ramp its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers the inferred depth is around 100 m free oxygen concentration was low dysaerobic conditions are indicated by a scarcity of benthic macrofauna ferruginous structures are numerous in the first two microfacies and absent in the last hematite staining is not uniform and follows many sedimentary patterns among the more widespread fe structures are perforation infillings with endolithic micro organisms microstromatolites oncoids ooids blisters coatings and hardgrounds these structures can be associated and none are mutually exclusive hematite coated filaments of different sizes and shapes are observed in the micrite matrix the walls of various organisms the calcite crystals associated with the fe cortical laminations the perforations and burrow the hardgrounds and microstromatolites petrographical and sem examinations suggest that the laminated crusts oncoids and hardgrounds are formed by microbial iron mats dominated by filamentous bacteria and fungi seven types of microbes are recognized filaments five morphotypes spheroidal bodies and stalked bodies filamentous microfossils of type 1 to 4 resemble the present day filamentous bacteria beggiatoales and cytophagaceae because of their large diameter and their branching nature filaments of type 5 are possibly filamentous fungi another argument in favor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some oomycota in deep calm and dysaerobic waters many interfaces e g between aerobic and dysaerobic waters are present in the sediments the stability of the soluble reduced state of iron fe 2 is higher at such interfaces and many ferric iron encrusted microbial fossils are observed iron could thus serve as an electron donor for microbial iron oxidation processes other microbial iron deposition pathways are also possible it appears that regardless of geological age paleozoic mesozoic and geographical location the same microbiological mechanisms are probably responsible for the red color in calcareous stratified or unstratified bodies the presence of fossilized iron encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in various paleo micro environments

distribution of platinum group elements in magmatic and altered ores in the jinchuan intrusion china an example of selenium remobilization by postmagmatic fluids the division of platinum group elements pge between those hosted in platinum group minerals pgm versus those in solid solution in base metal sulfides bms has been determined for ores from the pge bearing ni cu rich jinchuan intrusion in northwest china all the bms are devoid of pt and ir and magmatic bms are also barren of rh these pge may have been scavenged by arsenic to form pgm during magmatic crystallization of the bms pd os and ru are recorded in bms and pd is predominantly in solid solution in pentlandite unlike the fresh magmatic ores in altered or serpentinized ores pd pgm are present froodite is hosted in magnetite formed during alteration of bms accompanied by sulfur loss and liberation of pd michenerite pd pt bite sperrylite ptas2 and au bearing pgm are located in altered silicates irarsite irass occurs mainly enclosed in bms padmaite pdbise identified at the junctions of magnetite and bms was the last pgm to form and locally partially replaces earlier non se bearing pgm we propose that padmaite formed under oxidizing conditions during late local remobilization of se from the bms se bearing pgm are rare and our review shows they are frequently associated with carbonate suggesting that pd and se can be mobilized great distances in low ph oxidizing fluids and may be precipitated on contact with carbonate s se ratios are used by researchers of magmatic ni cu pge ores to determine sulfur loss assuming se is immobile and representative of magmatic sulfur content this study shows that se as well as s is potentially mobile and this should be considered in the use of s se ratios

the fate of subducted oceanic slabs in the shallow mantle insights from boron isotopes and light element composition of metasomatized blueschists from the mariana forearc abstract serpentine muds from south chamorro seamount scs drilled during odp leg 195 at site 1200 contain metamafic clasts that experienced blueschist facies metamorphism including the critical mineral assemblage pumpellyite na amphibole epidote these schists represent fragments from the actual slab mantle interface at 27 km depth their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the mariana forearc a region of melange formation as it can also be observed in onland exposures such as the catalina schist as the mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer during the subduction processes at an active convergent margin this study presents the first data of detailed b isotope 11 b and light element variations in blueschist facies minerals from the mariana arc system the primary foci are b and li concentrations and 11 b values analyzed by sims and tof sims techniques minerals such as na rich amphibole phengite and chlorite are found to be strongly enriched in li up to 70 g g be up to 8 g g and b up to 35 g g and with 11 b values of 6 4 these new data are consistent with isotopically heavy b being released into the mariana forearc mantle wedge serpentinization of dry mantle peridotite after interaction with b rich slab released high ph fluids and confirm models of significant b loss and b isotope fractionation during forearc shallow slab dehydration the elevated li be and b concentrations in minerals that comprise the bulk of the rocks namely amphibole phengite and chlorite bear a strong potential to further transport li and b as well as the isotopically light component of b to greater depths in the mantle where ongoing metamorphism is responsible for further isotope and elemental fractionation and the formation of distinct mantle reservoirs e g volcanic arc and oceanic intra plate oib magmas

trace element stratigraphy of the bellevue core northern bushveld multiple magma injections obscured by diffusive processes the 3 km long bellevue core was recovered from the upper zone uz and main zone mz of the northern limb of the bushveld complex the largest known layered mafic intrusion we present abundance data for 57 trace elements hosted in plagioclase clinopyroxene low ca pyroxene and olivine obtained by laser ablation inductively coupled plasma mass spectrometry these data permit the first comprehensive investigation of trace element distribution in the bushveld complex on a mineral by mineral basis and show that the cr content of pyroxene is the only proxy that records three cryptic magma injections coincident with two pyroxenite horizons and the uz mz boundary on the basis of cr abundance we infer that an anomalous troctolitic horizon at the base of the bellevue core was related to injection of relatively primitive magma analogous to the lower or critical zone magmas in the eastern and western limbs of the bushveld complex this horizon defines the greatest inconsistency of magmatic stratigraphy between the northern limb and the eastern and western limbs of the complex and is prospective for platinum group element exploration however the most significant finding of this study is that plagioclase pyroxene and olivine have undergone extensive diffusive modification the major and trace element geochemistry of these minerals records sub solidus equilibration during the prolonged cooling of the bushveld complex controlled by diffusion rates and local partitioning between adjacent minerals by analogy texturally mature cumulates in any layered intrusion are also likely to be chemically re equilibrated

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

from black and white to colour in the silurian abstract ironstones and iron rich limestones regularly occur as components of time specific intervals of the palaeozoic as well as in younger times brett et al 2012 this issue silurian sediments deposited at high latitudes along the peri gondwana border are characterized by black and white limestone and graptolitic shale sequences those in the carnic alps southern austria additionally contain colourful pink to red limestones and ironstones laminated structures such as the ferruginous coatings around skeletal fragments mostly trilobites and some cephalopods and echinoderms and stromatolitic features along discontinuity surfaces display dark red green white and brownish colours due to the presence of goethite magnetite hematite chamosite calcite and subordinate apatite confocal laser raman microscopy and complementary microscopic analysis of these ferruginous laminated structures document the presence of carbonaceous matter associated with fossilized microbial structures in the form of stromatolites filaments and coccoids suggesting a microbial role in the colouring of the silurian world of the carnic alps iron concentrations up to 30 that of matrix and surrounding non ferruginous rocks suggest blooms of iron microbe activity in response to the time specific occurrence of chemically charged sea water during global biotic events

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

hydrothermal remobilisation around a deformed and remobilised komatiite hosted ni cu pge deposit sarah s find agnew wiluna greenstone belt yilgarn craton western australia the sarah s find nickel deposit located 4 5 km north of the mount keith nickel mine western australia was chosen as a case study to investigate the nature and three dimensional geometry of a geochemical halo created by the hydrothermal remobilisation of base metals and platinum group elements into the country rock surrounding a small massive ni sulphide orebody portable and laboratory based xrf analyses were carried out on samples from a shear zone localised along the basal komatiite dacite contact that hosts the orebody a geochemical halo was identified that extends along the shear zone up to 1780 m away from the massive sulphides parallel to a prominent stretching lineation elevated ni and pd are associated with high as co cu and s palladium and pt concentrations increase with proximity to massive sulphides from 6 to 1190 ppb pd these anomalous concentrations reflect the presence of sulfarsenides and sulphides either physically remobilised and forming veinlets close to the massive sulphides or hydrothermally transported and redeposited within the foliation in situ laser ablation icp ms indicates that pd and pt are hosted within these nickel sulfarsenides this ni co as pd geochemical halo observed around the sarah s find ore body is interpreted as forming syn deformation by the circulation of as rich hydrothermal fluids dissolving base metals pd and pt from the orebody and redepositing them along the sheared footwall contact similar ni co pd pt as geochemical haloes could potentially exist around any magmatic nickel sulphide mineral system that has undergone a phase of arsenic metasomatism and may be a generally applicable proximity indicator for nickel sulphides in hydrothermally altered terranes

the ug2 merensky reef interval of the bushveld complex northwest of pretoria northwest of pretoria the ug2 merensky reef interval overlies a critical zone lower zone sequence that contains numerous large blocks of floor material nevertheless individual layers can be correlated with equivalent units at crocodile river mine the rustenburg impala union and amandelbult sections concentrations of platinum group elements in two borehole intersections of the ug2 chromitite are 4 ppm over 1 2 m and 2 4 ppm over 2 2 m therefore bulk pge levels appear to be only moderately lower than those at western platinum mine this renders models explaining pge enrichment by upward percolating melt or fluids problematic the merensky reef although containing sulphides is only weakly mineralized with pge 0 6 ppm the ug2 pyroxenite is separated from the ug2 chromitite by a 15 m noritic layer the introduction of feldspathic cumulates between two units that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts of the chamber distally located with regard to a major feeder zone at union section it also suggests that the ug2 unit is a multiple rather than a single cyclic unit

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

palladium platinum and gold concentration in porphyry copper systems of greece and their genetic significance compilation of new and published data on precious metal and associated trace element contents in mineralized porphyry copper intrusions of greece indicates that the pd content in the potassic propylitic zone of the skouries deposit is relatively high ranging between 60 and 200 ppb average 110 ppb whereas in the porphyry copper intrusions of gerakario and pontokerasia it is very low 3 ppb pd the oxidized zone of fissoka op 65 exhibits low pd and cu contents but higher au te as pb and zn contents the pt content is much lower than pd in the porphyry copper intrusions studied the pd pt ratio in the skouries deposit ranges from 9 to 60 while it is much lower 2 in the mineralized samples from fissoka gerakario and pontokerasia a decrease of the pd content and pd pt ratio with increasing te as pb and zn content may indicate that the pd distribution in porphyry copper systems is effected by the evolution of the magma hydrothermal system also mineralogical and geochemical data on the skouries deposit coupled with textural relations between base metal sulphides pgm and au ag tellurides indicate that the main pd bearing mineral merenskyite is associated with the vein type chalcopyrite or bornite and was deposited during the main stage of mineralization the high magmatic oxidation state and the magmatic features of the hydrothermal system limited participation of meteoric portion in the skouries intrusion may be a strong control of its pd potential assuming that pd is mainly associated with chalcopyrite in the porphyry copper deposits the calculated pd from measured pd contents in chalcopyrite average 3300 ppb pd are relatively high and comparable to that in the chalcopyrite concentrate made after the processing of large composite drill hole samples 2400 ppb pd to 21 wt cu it is considered to be an encouraging economic factor for pd as a by product probably along with au although the main product is cu

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

litho and chemostratigraphy of the flatreef pge deposit northern bushveld complex the flatreef is a world class platinum group element pge deposit recently discovered down dip from existing mining and exploration operations on the northern limb of the bushveld complex current indicated resources stand at 42 moz pge 346 mt with 3 8 g t pt pd rh au 0 32 ni and 0 16 cu which in the case of pt is equivalent to 10 years global annual production making it one of the largest pge deposits on earth the grade and thickness of the flatreef mineralised interval is highly unusual with some drill core intersections containing up to 4 5 g t pt pd rh au over 90 m in drill core here we document the down dip and along strike litho and chemostratigraphy of the flatreef and its footwall and hanging wall rocks based on a diamond drill core database totalling 720 km at the base of the sequence intersected in the drill cores are up to 700 m thick sills of ultramafic rocks dunite harzburgite pyroxenite emplaced into pelitic dolomitic and locally quartzitic and evaporitic rocks belonging to the duitschland formation of the transvaal supergroup next is an approximately 100 200 m sequence of low grade sulphide mineralised layered mafic ultramafic rocks containing abundant sedimentary xenoliths and in places several chromite seams or stringers this is overlain by a 100 m thick sequence of well mineralised mafic ultramafic rocks the flatreef sensu strictu overlain by a laterally persistent mottled anorthosite layer at the base of 1 km of homogenous main zone gabbronorite based on stratigraphic lithological and compositional analogies to the layered rocks in the eastern and western bushveld complex we correlate the flatreef and its chromite bearing footwall rocks with the upper critical zone notably the interval between the ug2 chromitite and the bastard reef as found elsewhere in the bushveld complex this includes recognition of a merensky reef correlative the ultramafic rocks below the main chromitite seam ug2 correlative in the flatreef footwall are correlated with the lower critical and lower zones however compared to the western and eastern bushveld limbs the studied sequence is strongly enriched in sulphide and pge many of the rocks show elevated cao k2o rb and zr contents and lateral continuity of layers between drill cores is less pronounced than elsewhere in the bushveld whereas ultramafic units are locally considerably thickened these compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels intrusion of granitic magmas and the influence of multiple structural events such as pre to syn emplacement regional scale open folding and growth faults evidence for the existence of potholes also exists in the shallow up dip portions of the project area the entire magmatic sequence below the main zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores this package represents the platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the bushveld complex the new data presented here thus indicate that the platreef is a relatively more contaminated up dip extension of parts of the critical and lower zones

holocene eruptions of mt popa myanmar volcanological evidence of the ongoing subduction of indian plate along arakan trench in the western part of myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of indian plate under eurasian plate volcanoes of burma arc however did not produce eruptions in the recorded history and are considered extinct such perception questions the ongoing subduction as well as keeping local officials unaware of possibility of future volcanic eruptions in the country we have investigated the youngest lava flows and pyroclasts of mt popa which is the best preserved polygenetic volcanic edifice in myanmar the ar ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately the radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that mt popa produced several eruptions in the beginning of holocene the youngest eruption occurred 8000 bp and included the 1 3 km3 gravitational collapse of the volcanic cone with deposition of the 11 km long debris avalanche immediately followed by emplacement of 0 1 km3 of pyroclastic flow of calc alkaline basaltic andesite composition the collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault which was formed during the pre collapse asymmetric gravitational spreading of the volcanic cone the recently erupted products are geochemically similar to products of young monogenetic volcanoes of monywa area located 150 km north mt popa and both display patterns consistent with magma generation at an active subduction system low magma production rate of mt popa 3 10 5 km3 year averaged for the 1 ma of the volcano history is in agreement with the highly oblique angle and slow rate of the subduction the fact of the holocene eruptions of calc alkaline composition in burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between india and asia

hydrothermal alteration in a modern suprasubduction zone the tonga forearc crust an extensive suite of hydrothermally altered basalts gabbros and plagiogranites was recovered from the trench facing slope of the tonga forearc the tectonic setting lithology and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone ssz ophiolites petrography mineral chemistry and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern ssz setting seawater derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures 800 c as the plutonic sequence cooled amphibole progressively replaced the mafic phases followed by chlorite and epidote below 550 c basalts record peak alteration temperatures up to 773 c however most were altered at lower temperatures typical of mid ocean ridge mor volcanic sequences kelman 1998 epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water rock ratios pervasively altered gabbros and basalts display evidence of late cataclastic deformation and or contain veins that is likely related to a later tectonic event such as trench rollback the range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both ssz ophiolites and mors however the degree of alteration in basalts and the presence of epidosites in the tonga collection are most similar to alteration characteristics in ssz ophiolites the initiation of high temperature brittle deformation in the absence of ductile deformation suggests that the tonga forearc crust was constructed in a magma rich environment similar to fast spreading mors