karstification beneath dam sites from conceptual models to realistic scenarios summary dam sites and reservoirs located above soluble rock are often damaged by increased leakage through the sub surface within the life time of the structure the high hydraulic gradients driving the water through the fracture and fissure system of the bedrock have a strong impact on the aquifer evolution the increased permeability if not prevented leads to an imminent danger of high leakage rates breakthrough as well as a result the structural safety of the dam site itself is at risk past experience has shown that this may have large environmental and economical consequences for a better understanding of the evolution of karst aquifer systems in the vicinity of dam sites a three dimensional conceptual model is presented we show the evolution of the karst aquifer for simple three dimensional dam site setups keeping the symmetry and simplicity of the models we can relate our results to the two and one dimensional scenarios presented in the past implementing a statistical fracture network and topographic information to this basic setup we show that these complex three dimensional properties of the real aquifers have a significant influence on the karstification and cannot always be addressed by two and one dimensional models

the merensky reef bushveld complex mixing of minerals not mixing of magmas a major geochemical hiatus occurs in the bushveld complex at the level of the platiniferous merensky reef close to the critical main zone boundary the origin of this hiatus and its relationship to mineralization has not been fully resolved initial sr isotope ratios for plagioclase and cr mgo values for orthopyroxene from critical and main zone rocks are significantly different geochemical data through the merensky and bastard cyclic units indicate that orthopyroxene in the merensky and bastard units originated from magma with a composition like that of the critical zone magma it sometimes occurs together with plagioclase that originated from main zone magma in a detailed study of the eastern limb the pyroxenite at the base of the merensky unit contains plagioclase and orthopyroxene that have compositions typical of the critical zone but in the overlying part of the merensky unit plagioclase compositions are more typical of the main zone whereas orthopyroxene retains a critical zone signature similarly in the bastard pyroxenite plagioclase has a range of sr isotope values between that of the main and critical zones but orthopyroxene has values typical of the critical zone these observations of mineral disequilibrium clearly show that the two major minerals in the merensky and bastard units were formed from two different but coexisting magmas a model that accounts for this disequilibrium is proposed here it invokes the influx of main zone magma at the base of the merensky unit during which the critical zone magma was displaced upward but the two magmas did not mix the elevated critical zone liquid continued to crystallize orthopyroxene which sank through the influx of magma of the main zone due to the density contrast these grains collected on the crystal pile to form the merensky pyroxenite the main zone magma into which the orthopyroxene accumulated crystallized interstitial plagioclase that had an sr isotope ratio typical of the main zone whole rock major element geochemical data show that a variable proportion of the plagioclase in both the merensky and bastard pyroxenites is cumulus it is inferred to have accumulated with orthopyroxene and has an initial sr isotope ratio typical of the critical zone thus the two pyroxenites now have a mixed sr isotope signature of cumulus critical zone and intercumulus main zone plagioclase that varies along strike above the pyroxenites the sr signature of the norites and anorthosites of both cyclic units is dominated by cumulus plagioclase from the main zone magma but some plagioclase with a critical zone signature may be present we conclude that the variations in initial sr isotope ratios do not result from mixing of magmas but from variable accumulation of orthopyroxene and plagioclase from a higher isotopically distinct layer of magma into an underlying layer the merensky and bastard cyclic units therefore may have features of either critical or main zone magma depending on which chemical parameter cr mgo and sr isotopes is considered they are therefore classified as a transitional unit between the critical and main zones models that suggest magma mixing was responsible for sulfide saturation and the creation of an immiscible sulfide liquid are not supported by this study and alternative models must be sought to explain the sulfide saturation event in the merensky reef

fossil site prediction using geographic information systems gis and suitability analysis the two medicine formation mt a test case abstract fossil site discovery has traditionally been the result of educated guesswork followed by systematic searching of terrain this study approached the issues of fossil site identification by looking at key variables in a gis setting the data were analyzed to create a predictive model for finding fossils thus facilitating the process of fossil discovery and saving time and money geospatial variables believed to be most useful for finding fossils were examined and ranked on a scale from 1 to 4 with 4 being the most advantageous score for finding fossils weighted sum addition combined the layers to create a suitability surface field testing and subsequent analysis showed the model accurately predicted areas of high medium and low fossil likelihood field observations and additional site data led to model refinements and increased resolution of fossil density distribution the final model explained a statistically significant 90 of fossil density variation in the two medicine formation

the mineralogy petrology and origin of the merensky cyclic unit in the western bushveld complex the merensky cyclic unit of the bushveld complex represents the first products to crystallize after the influx and mixing of a large new batch of magma in the chamber excluding the merensky pegmatoid the merensky cyclic unit grades upward from an orthopyroxenite at the base through norite to anorthosite at the top of the sequence it is followed by the very similar bastard cyclic unit the mg mg fe 1 ratio of the pyroxenes changes relatively smoothly from about 0 80 at the base of the pyroxenite to 0 70 at the top of the anorthosite with no abrupt changes between rock types there is a suggestion of an upward increase in the average an content of the plagioclase from midpoint values of an 73 in the norite to an 76 at the top of the anorthosite there also appears to be a decrease in the range of an compositions 10 mole in the norite to 4 5 mole in the anorthosite upward in the sequence as determined by electron microprobe the merensky pegmatoid situated at the base of the cyclic unit commonly but not everywhere contains the nickel copper and platinum group minerals the pegmatoid shows evidence of high fluid activity such as pegmatoidal texture high biotite content redistribution of sulfide phases and the presence of unusual minor phases normally associated with hydrothermal activity variations in the mg fe ratio of the orthopyroxenes from the merensky cyclic unit indicate that this unit formed from a limited thickness of magma between 10 and 30 m whereas mass balance requires that the platinum group element ni and cu mineralization be derived from at least 4 to 5 times this volume of magma to explain these data several processes are inferred to have occurred initially a new influx of slightly more siliceous magma mixed with that already in the chamber by turbulent convection penecontemporaneous warping of the crystal pile and some erosion and solution of the footwall crystal mush occurred resulting in a disconformity the mixing process also led to the formation of dense immiscible sulfide droplets which scavenged platinum group elements ni and cu from the liquid and were then deposited on the floor of the chamber after the mixing event a stable diffusive regime i e a double diffusive convection system became operative as heat was lost from the top of the magma chamber the basal double diffusive convection layer about 10 30 m thick then started bottom crystallization of orthopyroxene resulting in the upward mg fe fractionation observed plagioclase subsequently joined orthopyroxene on the liquidus but this phase nucleated and grew within the convecting liquid which caused convection in the layer to slow as the viscosity sharply increased orthopyroxene continued to crystallize from the bottom up and enclosed small plagioclase crystals in large sievelike orthopyroxene oikocrysts or mottles the plagioclase crystals not enclosed by pyroxene started to impinge mutually and sinter to form large polygonal grains penecontemporaneous infiltration of material from below the merensky unit modified the isotopic and chemical character of the crystal pile by mixing with and displacing the preexisting interstitial liquid the merensky pegmatoid is inferred to postdate both the initial mineralization and formation of the other units in the merensky cyclic unit we conclude that the pegmatoid is the result of trapping of the final late fluids expelled from the footwall rocks this fluid reacted with the basal part of the merensky pyroxenite and also redistributed the platinum group elements ni and cu to form the coarse pegmatoid and its assemblage of platinum group minerals and other sulfide minerals

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

magmatic metasomatism and formation of the merensky reef bushveld complex the rare earth element ree contents of pyroxenes and other minerals from the merensky reef and stratigraphically adjacent rocks of the atok section bushveld complex have been determined with the ion microprobe merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the ree than their cumulus counterparts in rocks several meters below the reef chondrite normalized merensky clinopyroxene ce contents vary from 10 to 90 for ce and from 4 to 17 for yb they also possess deep negative eu anomalies the eu anomalics being deeper for crystals having high ree contents and relatively shallow for pyroxenes with low ree contents similar compositional characteristics are displayed by cl rich apatite which is an accessory phase in the rocks interstitial pyroxene in cumulates above and below the reef also tends to have elevated ree contents and in general is not in equilibrium with coexisting cumulus minerals the melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts that from which merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in ree than any normal tholeiitic or alkalic basalt despite their highly evolved nature in terms of the ree the merensky reef pyroxenes are not evolved in terms of major elements the decoupling of incompatible trace and major elements is best explained by a metasomatic process it is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile the fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

porous sidewall and sloping floor crystallization experiments using a reactive mush implications for the self channelization of residual melts in cumulates abstract tank crystallization experiments with aqueous solutions were used to explore interactions between a buoyant residual fluid and a porous cumulate at a sloping floor and a vertical wall the fluid flow path in a non reactive mush glass beads was used as a control for a vertical non reactive mush the rise of the buoyant solute depleted fluid was confined within it fluid flow paths were curved in the case of a sloping mushy floor with the upper part of the mush being principally a plume discharge zone and the lower part being a recharge zone more realistic reactive mushes were created by supercooling a nh 4 cl solution cooling against a vertical surface produced an initial mat of dendrites that were subsequently overgrown by a vertically grooved dendrite structure the grooves contained upwardly flowing residual melt while the ridges extended beyond the buoyant boundary layer into the thermally convecting solute rich part of the tank with time the grooves were covered over by a dendrite mat but remained as open channels within the mush focussing and accelerating the upflow of residual melt a buoyant boundary layer on the outside of the mush was not observed in the experiment with a sloping floor a thin mat of homogeneous mush was succeeded by a more organized deposit characterized by chimneys residual melt escaped as plumes through the chimneys chimneys formed because buoyant residual melt dissolved the mush as it rose and warmed chimney spacing increased and the number of chimneys decreased as the mush thickened dissection of the mush showed that individual chimney channels were inclined upslope so that melt residual from crystallization at the foot of the slope emerged from chimneys near the top of the slope formation of channels in cumulates would greatly increase fractionation efficiency by increasing the effective volume from which residual melts could be extracted and by increasing the rate at which residual melts could be transferred to the upper part of the chamber

conduit evolution in deep seated settings conceptual and numerical models based on field observations 1 to examine the interrelation between hydrogeological environment and conduit development in deep seated settings a conceptual model is tested by numerical modeling based on field observations simplified model settings are designed and crucial parameters are varied a coupled continuum pipe flow model is employed for simulating conduit development within the soluble unit of a multilayer aquifer system in agreement with field observations the evolving cave patterns are characterized by pronounced horizontal passages and multiple vertical conduits at the bottom of the soluble unit but only few at the top the frequency distribution of conduit diameters is found to be bimodal if the permeability of the rock formation is sufficiently high to allow competitive conduit development governed by the feedback between increasing flow and dissolution rates this feedback however is suppressed in low permeability formations as a consequence conduit development is uniform rather than competitive

geochemistry of the merensky reef rustenburg section bushveld complex controls on the silicate framework and distribution of trace elements this whole rock and silicate mineral study focuses on the genesis of the merensky reef sequence as well as the footwall and hanging wall norites at an area of rustenburg platinum mines in a demonstrably normal undisturbed environment continuous sampling provides major and trace element variations and mineral compositions and allows an evaluation of the post liquidus processes which affected the sequence following the formation of liquidus phases three stages are envisaged to have modified the rocks these are a migration of fluid during early compaction of cumulates b circulation of fluids within the crystal mush and c reaction and solidification of trapped liquid liquidus compositions are nowhere preserved in the sequence a strong link is demonstrated between orthopyroxene compositions e g mg and tio2 and the incompatible trace element content of the whole rocks the final amount of trapped liquid is shown to have been variable but never exceeded 10 calculated liquidus pre equilibration orthopyroxene compositions show an up sequence progression of evolving compositions from the footwall norite to the hanging wall norite initial sr isotopic values do not support a simple magma mixing model by which radiogenic main zone magma mixes with that of the critical zone at the level of the merensky reef there is evidence that the hanging wall norite formed from a much more evolved magma these conclusions have implications for the distribution and origin of the pge enriched merensky reef package

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

towards a model for the in situ origin of pge reefs in layered intrusions insights from chromitite seams of the rum eastern layered intrusion scotland the current debate on the origin of platinum group element pge reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt fluids migrating upward from the underlying cumulate pile here we show that pge rich chromitite seams of the rum eastern layered intrusion provide evidence for an alternative origin of such deposits in layered intrusions these laterally extensive 2 mm thick chromitite seams occur at the bases of several cyclic mafic ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal liquid interface this follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries even where these are vertically oriented or overhanging high concentrations of pge up to 2 3 ppm pd pt are controlled by fine grained base metal sulphides which are closely associated with chromitite seams the following sequence of events explains the origin of the pge rich chromitite seams a emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate followed by in situ growth of chromite against the base b precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation c the scavenging of pge by sulphide droplets from fresh magma continuously brought towards the base by convection since the rate of magma convection is 105 107 times higher than that of the solidification km year to km day versus 0 5 1 0 cm year the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume as a result the sulphide bearing rocks are able to reach economic concentrations of pge several ppm we tentatively suggest that the basic principles of our model may be used to explain the origin of pge rich chromitites and classical pge reefs in other layered mafic ultramafic intrusions

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

leopards as taphonomic agents in dolomitic caves implications for bone accumulations in the hominid bearing deposits of south africa it has been hypothesised that leopards were significant contributors to the bone accumulations of the plio pleistocene hominid bearing caves of south africa interpretations of leopard activity in these fossil caves were previously based upon reports of modern leopard behaviour in areas of southern africa that were lacking in caves in 1991 a leopard lair with an accompanying bone accumulation was discovered in a dolomitic cave on the john nash nature reserve south africa all of the bones in this cave could be unambiguously attributed to the activity of one individual leopard over a 1 year period the resulting bone assemblage indicates that when available leopards will preferentially utilise the deep recesses of caves to the exclusion of trees when feeding and that the size of prey leopards are capable of capturing killing and transporting has previously been underestimated the implications this may have for understanding the accumulation of fossils in the hominid bearing caves of south africa are that bones derived form leopards consuming prey in trees probably did not contribute significantly to the assemblages and further that it is not necessary to invoke sabre tooth cat involvement for the larger animals found in these assemblages this modern cave probably represents a more appropriate model for the accumulation of bones in the fossil caves of the sterkfontein valley and the assemblage is being continually monitored to view any and all taphonomic alterations that are occurring

modeling the merensky reef bushveld complex republic of south africa the merensky pegmatoid normal reef in the western bushveld complex is commonly characterized as a pyroxene rich pegmatoidal unit with a base that is enriched in chromite and platinum group element bearing sulfides overlying a leuconorite footwall models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile induced remelting the consequences of the latter interpretation are investigated using the numerical modeling program iridium which links diffusive and advective mass and heat transport with a phase equilibration routine based on the melts program the initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite both initially at 1 190 c and 2 kbar 2 wt of a volatile fluid composed of 75 mol h2o 20 mol co2 and 5 mol h2s is then added to the lower 20 cm of the pyroxenite the system is then allowed to evolve under conditions of chemical diffusion in the liquid the addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite however chemical diffusion across the leuconorite pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite preferential re precipitation of sulfide and chromite and concentration of the pge at this boundary these results mimic actual mineral and compositional profiles across the merensky pegmatoid and illustrate that long term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage

infiltration metasomatism in layered intrusions an example from the stillwater complex montana abstract the contact between anorthosite zone ii an ii and the overlying troctolite of olivine bearing zone v ob v of the stillwater complex is characterized by a 5 m thick adcumulate anorthosite this adcumulate anorthosite separates rocks of an ii characterized by the apparent crystallization sequence plagioclase followed by orthopyroxene clinopyroxene quartz fe ti oxide assemblages from those of ob v where the apparent crystallization sequence is olivine plagioclase followed by orthopyroxene the grain size and compositional zoning of plagioclase in this transition zone is similar to that in the overlying troctolite and distinct from the coarser complexly zoned plagioclase that makes up the underlying anorthosite in contrast the minor interstitial pyroxene in the transition zone compositionally overlaps that in the underlying anorthosite and both have a lower molar mg mg fe ratio than the pyroxene of the troctolite these relationships suggest that the intercumulus liquids of an ii and ob v were compositionally distinct model phase diagram considerations suggest that infiltration of any plagioclase saturated liquids into a troctolite assemblage will result in the assimilation of olivine and depending on the actual composition of the anorthositic liquid will also either assimilate or crystallize plagioclase as well hence the monomineralic anorthosite zone at the top of an ii can be attributed to infiltration of interstitial liquids from an ii that were displaced upward into the troctolite during compaction of an ii this reaction can be either exothermic or endothermic depending on the actual composition of the infiltrating liquid and the bulk solids composition numerical models of the thermal effects of infiltration suggest that heat of reaction can significantly affect the liquid solid ratio away from the infiltration boundary

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

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

modeling three dimensional karst aquifer evolution using different matrix flow contributions summary we have developed the program package karstaquifer which simulates flow and transport in a three dimensional transient karst aquifer consisting of fractures and matrix elements the fractures can be enlarged by chemical dissolution of calcite increasing the secondary permeability of the karst aquifer boundary conditions for our model setup are guided by published benchmark models for karst aquifer evolution in two dimensions length and width with a 100 m high hydraulic head difference across the model domain responsible for relatively fast evolution of the aquifer we are interested in the effect of the third dimension on the evolution of the karst aquifer especially the effect of matrix flow with a matrix defined either as porous medium or as fine fracture system or both we first discuss our model results using a pseudo 3d setup to be able to directly compare results to the 2d benchmark scenarios published we then discuss real 3d models with a prominent fault located in the central part of the aquifer connecting input and output regions finally we replace the prominent fault with a statistical fracture diameter distribution our results compare well with existing 2d scenarios and the additional third dimension offers new insights into the evolution of karst aquifers especially the role of matrix flow under fixed head boundary conditions in their early phases can be studied in more detail

cave development in the swabian alb south west germany a numerical perspective summary the formation of phreatic cave conduits in the carbonate aquifer of the swabian alb south west germany is discussed by means of numerical simulations three active cave conduit systems the blautopf cave the aachtopf cave and the wimsener cave have been partially explored by divers and exhibit a continuous conduit flow path with phreatic loops down to 50 m below the average water table these three systems drain catchments between 50 and 280 km2 and largely follow the dip of the jurassic limestone beds we use the numerical model karst karst aquifer simulation tool to explore the possibility of structural control of the conduit evolution we show that both the dip of the bedding and the frequency of major bedding partings is likely to result in numerical drainage paths which are similar to the observed cave systems

dam sites in soluble rocks a model of increasing leakage by dissolutional widening of fractures beneath a dam abstract water flowing through narrow fissures and fractures in soluble rock e g limestone and gypsum widens these by chemical dissolution this process called karstification sculptures subterranean river systems which drain most of their catchment close to dam sites unnaturally high hydraulic gradients are present to drive the water impounded in the reservoir downstream through fractures reaching below the dam under such conditions the natural process of karstification is accelerated to such an extent that high leakage rates may arise which endanger the operation of the hydraulic structure model simulations of karstification below dams by coupling equations of dissolutional widening to hydrodynamic flow are presented the model scenario is a dam 100 m wide in limestone or gypsum the modelling domain is a two dimensional slice 1 m wide directed perpendicular to the dam it extends 375 m vertically and 750 m horizontally the dam is located in its center this domain is divided by fractures and fissures into blocks of 7 5 7 5 1 m the average aperture width of the fractures is 0 02 cm we performed model runs on standard scenarios for a dam site in limestone with the height h of impounded water 150 m a horizontal impermeable apron of width w 262 m and a grouting curtain reaching down to a depth of g 97 m in a second scenario we changed these construction features to g 187 m and w 82 m to calculate widening of the fractures well established experimental data on the dissolution of limestone and gypsum have been used as they occur in such geochemical settings all model runs show similar characteristic behaviour shortly after filling the reservoir exhibits a small leakage of about 0 01 m 3 s 1 which increases steadily until a breakthrough event occurs after several decades with an abrupt increase of leakage to about 1 m 3 s 1 within the short time of a few years then flow in the fractures becomes turbulent and the leakage increases to 10 m 3 s 1 in a further time span of about 10 years the widths of the fractures are visualized in various time steps small channels propagate downstream and leakage rises slowly until the first channel reaches the surface downstream then breakthrough occurs the laminar flow changes to turbulent and a dense net of fractures which carry flow is established we performed a sensitivity analysis on the dependence of breakthrough times on various parameters determining breakthrough these are the height of impounded water h the depth g of grouting the average aperture width a 0 of the fractures and the chemical parameters which are c eq the equilibrium concentration of ca with respect to calcite and the ca concentration c in of the inflowing water the results show that the most critical parameter is a 0 at fracture aperture widths of 0 01 cm breakthrough times are above 500 years for values of a 0 0 02 cm however breakthrough times are within the lifetime of the structure we have also modelled dam sites in gypsum which exhibit similar breakthrough times however after breakthrough owing to the much larger dissolution rates of gypsum the time until unbearable leakage is obtained is only a few years the modelling can be applied to complex geological settings as phreatic cave conduits below the dam or a complex stratigraphy with varying properties of the rock with respect to hydraulic conductivity and solubility a few examples are given in conclusion our results support the assumption that increasing leakage of dam sites may be caused by dissolutional widening of fractures

silicate liquid immiscibility within the crystal mush late stage magmatic microstructures in the skaergaard intrusion east greenland late stage microstructures developed during the last stages of solidification of the skaergaard intrusion comprise a wide array of reactive and non reactive intergrowths reactive microstructures ascribed to open system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts fish hook pyroxenes and mafic symplectites they form by the addition of fe and ca and removal of alkalis and silica other microstructures include those formed by internally generated redox reactions between olivine fe ti oxides and pyroxene non reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids the open system reactive microstructures occur predominantly in the cumulates on the chamber floor appearing in lzb and disappearing to be replaced by a granophyre ilmenite rich intergrowth association in the upper zone they are not common in the marginal border series their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a si rich immiscible component from the interstitial liquid the loss of the si rich component causes the remaining fe rich liquid to react with the primocrysts bulk rock major element data are consistent with little or no preferential loss of the si rich liquid from the mush but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma the first appearance of the paired conjugate non reactive intergrowths in mz marginal border series points to the early onset of immiscibility in the bulk liquid

the southeastern border of the upper rhine graben a 3d geological model and its importance for tectonics and groundwater flow a 3d geological model of the area east of basel on the southeastern border of the upper rhine graben consisting of 47 faults and six stratigraphic horizons relevant for groundwater flow was developed using borehole data geological maps geological cross sections and outcrop data this model provides new insight into the discussions about the kinematics of the area between the southeastern border of the upper rhine graben and the tabular jura east of basel a 3d analysis showed that both thin skinned and thick skinned tectonic elements occur in the modeled area and that the anticline and a series of narrow graben structures developed simultaneously during an extensional stress field varying from e w to sse nnw which lasted from the middle eocene to late oligocene in a new approach the faults and horizons of the 3d geological model were transferred into discrete elements with distributed hydrogeological properties in order to simulate the 3d groundwater flow regime within the modeled aquifers a three layer approach with a horizontal regularly spaced grid combined with an irregular property distribution of transmissivity in depth permitted the piezometric head of the steady state model to be automatically calibrated to corresponding measurements using more than 200 piezometers groundwater modeling results demonstrated that large scale industrial pumping affected the groundwater flow field in the upper muschelkalk aquifer at distances of up to 2 km to the south the results of this research will act as the basis for further model developments including salt dissolution and solute transport in the area and may ultimately help to provide predictions for widespread land subsidence risks

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

the significance of magmatic erosion for bifurcation of ug1 chromitite layers in the bushveld complex abstract one of the most puzzling features of the ug1 chromitite layers in the famous exposures at dwars river eastern bushveld complex is the bifurcation i e convergence and divergence of layers along strike that isolate lenses of anorthosite the bifurcations have been variously interpreted as resulting from 1 the intermittent accumulation of plagioclase on the chamber floor as lenses terminated by crystallization of continuous chromitite layers the depositional model 2 late stage injections of chromite mush or chromite saturated melt along anastomosing fractures that dismembered semi consolidated plagioclase cumulates the intrusive model 3 post depositional deformation of alternating plagioclase and chromite cumulates resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally the deformational model none of these hypotheses account satisfactorily for the following field observations a wavy and scalloped contacts between anorthosite and chromitite layers b abrupt lateral terminations of thin anorthosite layers within chromitite c in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite many of these inclusions are contiguous with footwall and hanging wall cumulates d transported anorthosite fragments enclosed by chromitite e disrupted anorthosite and chromitite layers overlain by planar chromitite f protrusions of chromitite into underlying anorthosite g merging of chromitite layers around anorthosite domes we propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo chemical erosion of the temporary floor of the chamber the melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers resulting in lens like remnants of anorthosite resting on continuous layers of chromitite on cooling the magma crystallized chromite on the irregular chamber floor draping the remnants of anorthosite and merging with pre existing chromitite layers excavated by erosion with further cooling the magma crystallized chromite bearing anorthosite emplacement of multiple pulses of magma led to repetition of this sequence of events resulting in a complex package of anorthosite lenses and bifurcating chromitite layers this hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the bushveld complex

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

karstification in unconfined limestone aquifers by mixing of phreatic water with surface water from a local input a model summary when water from the surface of a limestone plain seeps down through the fractured rock to the water table of an unconfined aquifer with low hydraulic gradient containing water saturated with respect to calcite mixing of these waters causes renewed aggressivity a model is presented which describes the evolution of karstification by dissolutional widening of the fractures downgradient from the local input of surface water the model couples flow in the fractures with dissolution rates dissolution rates are given by f k 1 c x ceq 4 where c x is the calcium concentration at distance x from the entrance of the fracture ceq is the equilibrium calcium concentration of the h2o caco3 co2 solution in the fracture and k is a rate constant the model describes two domains of waters saturated with respect to calcite at different partial pressure of co2 at the borders of these domains the waters mix and create dissolutional widening of the fractures by mixing corrosion a channel evolves along the border in the downgradient direction by about 100 m in 100 ky below this channel a zone of fractures with aperture widths up to 1 cm has originated the change of the hydraulic conductivity in the mixing zone shifts the border of the domains allowing the channel to grow in the downgradient direction below it the zone of widened fractures is invaded by saturated phreatic water and dissolution stops this process continues at the downgradient part of the conduit in summary we find cave conduits evolving close to the water table leaving significant cavernous structures below them a variety of modelling scenarios with different choices of parameters show that this evolution is typical and changes only in details but not in its basic behaviour

platinum group element mineralization in the critical zone of the western bushveld complex i sulfide poor chromitites below the ug 2 primary stratiform platinum group element pge mineralizations associated with chromitites in the critical zone of the bushveld complex south africa are confined to cumulate layers that are regionally persistent over tens of kilometers four categories ofchromitite are recognized layers at the bases of cycles in the lower critical zone type i layers at the bases of cycles in the upper critical zone type ii thin layers in the intermediate parts of cycles type iii and stringers associated with orthopyroxene pegmatoids type iv these latter may not be of primary magmatic origin nevertheless probably each type of chromitite contains anomalous concentrations of pge chromitites at the bases of cycles comprising the lower group lg middle group mg and upper group ug are further categorized into types ia lg 1 lg 4 ib lg 5 mg 1 iia mg 2 ug 1 and iib ug 2 and above on the basis of lithostratigraphy chromite and pge chemistry and sulfide content type iib chromitites contain significant quantities of sulfides and are not investigated here new whole rock and electron microprobe chemical analyses of the sulfide poor type ia ib and iia chromitites cu contents 1 8 low pge contents 2 type ib chromitites occur in pyroxenite cycles and form an intermediate group these regular variations with increased height are interpreted as a differentiation trend pge are concentrated in sulfide poor chromitites as a result of chromitite control a process not fully understood but thought to involve a combination of direct nucleation of platinum group minerals and localized s saturation it is the crystallization of copious quantities of chromite from hybridized melts close to the crystal liquid interface that triggers this process but magma mixing essential to drive the melt into the chromite field is the overall cause replenishment of the magma chamber was from a single magma batch because the chromitites below the ug 2 are not associated with major isotopic inflections huge volumes of dense hot magma were fed as basal flows along the crystal liquid interface the magma chamber was compartmentalized at an early stage possibly in response to basement tectonics and lateral mixing over distances of 150 km is an alternative mechanism for achieving sufficiently high r values repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height type ia chromitites are ascribed to replenishment and mixing with the relatively primitive lower zone residue type ib crystallized as a result of mixing with lower critical zone resident liquid and type iia due to mixing with more evolved upper critical zone resident liquid differentiation of pge with height is a critical part of the process which culminates in the occurrence of higher grade pge mineralization in sulfide bearing chromitites ug 2 merensky reef in the uppermost part of the upper critical zone

plagioclase content of cyclic units in the bushveld complex south africa the upper critical zone of the bushveld complex south africa has been divided into so called cyclic units ideally they should consist of from the base upward layers of chromitite pyroxenite norite and anorthosite some hypotheses for their origin suggest that they result from the addition and mixing of a magnesian magma having orthopyroxene or olivine as liquidus mineral these models predict that the bulk cumulate composition in a cyclic unit formed from such magma ought to contain excess orthopyroxene to plagioclase relative to their cotectic proportions an hypothesis tested here all rocks investigated here from the upper critical zone at impala platinum mine are mixtures of cumulus plagioclase and orthopyroxene except for the chromitites with very minor intercumulus minerals mainly clinopyroxene hence the proportion of plagioclase can be determined by measurement of the density of a sample the density of every piece of continuous bore core from the upper group 2 ug2 and merensky cyclic units has been determined and the proportion of plagioclase calculated for the bastard cyclic unit estimates of the proportion of plagioclase have been made based on published modal and chemical analyses vertical summation of the proportion of plagioclase indicates that these three cyclic units ug2 merensky and bastard respectively contain 64 74 and over 80 plagioclase by weight for the merensky and bastard cyclic units such proportions exceed that of the plagioclase orthopyroxene cotectic and could not have been produced by processes involving magnesian magma

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

karstification beneath the birs weir in basel switzerland a 3d modeling approach we present a three dimensional model describing the evolution of a gypsum karst aquifer along the birs river near basel in switzerland here a small dam site was built in 1890 to generate hydro electric power after subsidence of a nearby highway geophysical and geological field studies have been carried out and a detailed hydrogeological model has been developed these observations identified a karstified weathered zone as the reason for the subsidence to prevent any further damage remedial construction measures have been carried out we employ our numerical model karstaquifer which simulates the temporal evolution of a karst system by dissolution increasing the porosity in the aquifer we used results from geological and geophysical investigations as a priori information for implementing the initial boundary conditions into our three dimensional numerical model our model accounts for topography precipitation and geology and especially for the anisotropy of the local bedrock it can successfully reproduce the weathered zone in its current horizontal and vertical extent we cm also simulate the possible evolution of the aquifer following a remedial construction phase therefore taken together with the geophysical and geological observations and the groundwater model our three dimensional karst aquifer evolution model enhances the understanding of the development of this heterogeneous karst aquifer system c 2012 elsevier b v all rights reserved hydrogeologie der nordwestschweiz grundwasser

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

morphology and microstructure of chromite crystals in chromitites from the merensky reef bushveld complex south africa the merensky reef of the bushveld complex consists of two chromitite layers separated by coarse grained melanorite microstructural analysis of the chromitite layers using electron backscatter diffraction analysis ebsd high resolution x ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals fine grained idiomorphic and large silicate inclusion bearing crystals the lower chromitite layer contains both populations whereas the upper contains only fine idiomorphic grains most of the inclusion bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre existing smaller idiomorphic crystals two possible mechanisms have been proposed for sintering of chromite crystals 1 amalgamation of a cluster of grains with the same original crystallographic orientation and 2 sintering of randomly orientated crystals followed by annealing into a single grain the ebsd data show no evidence for clusters of similarly oriented grains among the idiomorphic population nor for earlier presence of idiomorphic subgrains spatially related to inclusions and therefore are evidence against both of the proposed sintering mechanisms electron backscatter diffraction analysis maps show deformation related misorientations and curved subgrain boundaries within the large amoeboidal crystals and absence of such features in the fine grained population microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation an alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the merensky reef the upper chromite layer formed from a subsequent magma influx and hence lacked a mechanism to form dendritic chromite this accounts for the difference between the two layers

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

experimental quantification of plagioclase crystal size distribution during cooling of a basaltic liquid an experimental study has been conducted to constrain how thermal history controls crystal size distribution csd of plagioclase in cooling basalts data from all experiments are dominated by a log linear segment of decreasing number density with increasing crystal size consistent with observations in many natural rocks the slope of the csd is found to be a function of cooling rate faster cooling leading to greater slopes at constant cooling rate the csds flatten as temperature decreases and are sometimes kinked characteristics consistent with independent textural observations that crystal agglomeration contributes significantly to crystal growth a downturn is observed toward small size which suggests ostwald ripening furthermore we find that thermal history above the liquidus has a major influence on csds and on the temperatures of phase appearance we conclude that near the liquidus heterogeneous nucleation typically dominates

karstification of aquifers interspersed with non soluble rocks from basic principles towards case studies abstract we have developed a numerical model able to describe the karstification of aquifers in fractured rocks containing soluble limestone or gypsum and insoluble layers when water is flowing along fractures crossing the soluble layers it is able to dissolve the material there to increase the aperture width of the conduit and consequently to increase the local hydraulic conductivity depending on the thickness and the distribution of these layers the dissolution can be active only for limited periods or during the whole evolution time fractures located in insoluble layers do not change at all we are interested in the integral effect of these local processes and study four simplified scenarios of karstification along a prominent wide conduit crossing a fractured limestone block we keep the initial and the boundary conditions the same for all scenarios and vary only in the amount and the distribution of the soluble material we demonstrate that aquifers in 100 limestone without any insoluble layers develop along areas with high hydraulic conductivities and high hydraulic gradients creating channel like pathways on the other hand aquifers containing soluble layers with limited thickness develop faster and exhibit diffuse patterns determined by the chemical properties of the rock the second part of the paper is a step towards modeling of real karst systems we present the evolution of an aquifer located in the vicinity of a large hydraulic structure all initial and boundary conditions except the amount and the distribution of the soluble rock remain the same for all scenarios as a material example for the bedrock we chose gipskeuper from an aquifer along the birs river in switzerland this rock consists of soluble gypsum layers and insoluble clays and marls with typical layer thickness in the range of millimeters to centimeters the basic processes discussed in the first part of the paper remain valid we demonstrate that large insoluble zones can impair the karstification process and even completely block it while areas with thin soluble layers can provide a preferential pathway and decrease the evolution times considerably finally we show that the evolution of the leakage rates and the head distribution within the aquifer can sometimes reveal misleading information about the stage of karstification and the safeness of the dam our model can be used not only to study simplified geological settings and basic processes but also to address some of the complications arising when modeling real aquifers

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

gypsum karst problems in constructing dams in the usa gypsum is a highly soluble rock and is dissolved readily to form caves sinkholes disappearing streams and other karst features that typically are also present in limestones and dolomites gypsum karst is widespread in the usa and has caused problems at several sites where dams were built or where dam construction was considered gypsum karst is present at least locally in most areas where gypsum crops out or is less than 30 60 m below the land surface these karst features can compromise on the ability of a dam to hold water in a reservoir and can even cause collapse of a dam gypsum karst in the abutments or foundation of a dam can allow water to pass through around or under a dam and solution channels can enlarge quickly once water starts flowing through such a karst system the common procedure for controlling gypsum karst beneath the dam is a deep cut off trench backfilled with impermeable material or a close spaced grout curtain that hopefully will fill all cavities in oklahoma the proposed upper mangum dam was abandoned before construction because of extensive gypsum karst in the abutments and impoundment area catastrophic failure of the quail creek dike in southwest utah in 1989 was due to flow of water through an undetected karstified gypsum unit beneath the earth fill embankment the dike was rebuilt at a cost of us 12 million with construction of a cut off trench 600 m long and 25 m deep other dams in the usa with severe gypsum karst leakage problems in recent years are horsetooth and carter lake dams in colorado and anchor dam in wyoming

developing a geographic information system gis for mapping and analysing fossil deposits at swartkrans gauteng province south africa the accumulation of fossil remains bone tools and stone tools at the plio pleistocene site of swartkrans has been attributed to a variety of depositional agents these agents include hominin activity carnivore activity alluvial deposition and gravitation based on recent studies it is highly probable that the accumulations found at swartkrans and other plio pleistocene cave sites in south africa have resulted from a combination of these processes in order to explore further the taphonomic nature of swartkrans a geographic information system gis was developed combining all existing information produced from the final seven year period of c k brain s excavations and a recent survey of the site the amalgamation of this information into a gis has resulted in a digital archive of swartkrans information that allows the user to simultaneously visualize and analyse fossil artifact and geological materials within their original spatial contexts allowing a more comprehensive investigation of the site post excavation three dimensional reconstruction and gis development for sites such as swartkrans present many obstacles when using traditional gis approaches due to the presence of overhanging features certain limitations of conventional gis software yield erroneous results these limitations are based on the fact that conventional gis packages interpret z values as attributes rather than as true spatial coordinates a brief overview of gis and the challenges involved in the mapping and three dimensional reconstruction of sites such as swartkrans using traditional gis approaches are discussed as well as potential applications of the system

the petrogenesis of merensky reef potholes at the western platinum mine bushveld complex sr isotopic evidence for synmagmatic deformation potholes represent areas where the normally planar pge rich merensky reef of the upper critical zone of the bushveld complex transgresses its footwall such geometric relationships being unusual in layered intrusions the recognition of vertical dykes of merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull apart sites resulting from tensional deformation due to the loading effects of major new magma additions in contrast crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement the merensky reef and its footwall have distinct initial sr isotope ratios r 0 0 7066 and 0 7066 respectively which may be used to constrain these processes related to pothole formation merensky reef in potholes r 0 0 7069 0 7078 shows no isotopic evidence of assimilation of or reaction with footwall material discrete discordant replacement bodies of anorthosite extend from the footwall lithologies to cross cut the merensky reef and its hanging wall the initial sr isotope ratio in these replaced rocks is totally reset to footwall values r 0 0 7066 and immediately adjacent stratiform lithologies are slightly modified towards footwall values in contrast neptunian pyroxenitic merensky dykes cross cutting the footwall lithologies with a large surface area to volume ratio and low sr content do not display footwall like sr isotope initial ratios r 0 0 7077 and thus show no evidence for assimilation of or reaction with footwall material furthermore pegmatoidal replacement pyroxenite replacement pegmatoid at the base of the merensky reef within potholes has a high initial ratio r 0 0 7071 and so models of pervasive metasomatism by footwall material are not applicable this isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during or after the formation of merensky reef potholes a basic tenet of existing pothole formation hypotheses involving footwall mass transfer in contrast the isotopic data are entirely consistent with an extensional model for pothole formation with the more radiogenic merensky magma migrating laterally to fill extensional zones in the footwall layers

evaluation and rehabilitation of dam seepage problems a case study kafrein dam abstract the intention of this study is to locate the probable sources of the kafrein dam seepage problem a methodology known as excitation response analysis will be considered analytical expressions depicting the groundwater level response in the underlying aquifer as a result of changes in the reservoir water level were derived from a linearized groundwater flow equation for one dimensional semi infinite isotropic and homogeneous porous media the results revealed that a large proportion of the seepage water originates from a site at a distance about 1 km from the observation point m1 and streamlines along the existing faults emphasis in this work was also placed on controling and mitigating the seepage problem

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

reevaluation of the role of external sulfur addition in the genesis of ni cu pge deposits evidence from the nebo babel ni cu pge deposit west musgrave western australia the nebo babel ni cu pge sulfide deposit located in the west musgrave block western australia is hosted within a tube like concentrically zoned gabbronorite magma conduit emplaced at 1068 ma the nebo babel deposit is associated with the ca 1078 ma giles complex which is part of the warakurna large igneous province previous geologic and geochemical studies have shown that the nebo babel consists of chemically related but temporally distinct magma pulses the initial magma pulses which were the most primitive and the most contaminated formed the marginal units of the conduit the later pulses were emplaced into the core of the conduit the chilled margin rocks contain sulfide globules mineralization is confined to the early magma pulses and consists of massive sulfide breccias and stringers and disseminated gabbronorite hosted sulfides the ni tenor of the massive sulfides which have undergone sulfide liquid fractionation and the disseminated sulfides are almost the same indicating that both formed at similar r factors the whole rock massive and disseminated sulfides contain 650 ppb and 60 to 500 ppb total pge respectively all of the nebo babel rock units are slightly pge depleted with cu pd ratios higher than primitive mantle indicating early saturation and segregation of a sulfide and or pgm rocks with trace cumulus sulfides have the lowest cu zr and highest cu pd ratios and the most fractionated metal patterns indicating that these units have undergone the highest degrees of ni cu and pge depletion the average 34s value of all nebo babel disseminated and massive sulfides is 0 4 0 2 per mil which is consistent with the s originating in the mantle sulfur saturation did not occur in situ and is inferred to have been triggered by assimilation of felsic orthogneiss perhaps in the parental magma chamber this initial magma pulse with entrained sulfides formed marginal units of the nebo babel intrusion some of the entrained sulfides accumulated within physical traps on the way into the conduit and or at the exit from the chamber these sulfides were later emplaced along the footwall contact as massive and or marginal fragment laden sulfide liquid distribution of the disseminated sulfides was influenced by the conduit geometry and magma flow dynamics the final and volumetrically dominant barren magma pulse underwent crystal fractionation and oxide precipitation in the core of the nebo babel intrusion which in turn caused sulfide saturation and formed trace but pge depleted sulfides in these units the nebo babel deposit has a number of features in common with other ni cu pge deposits hosted in dynamic magma conduits e g voisey s bay such as multiple magma pulses and sulfide entrainment from depth rather than in situ sulfide segregation however si addition was the primary cause of sulfide saturation and external s addition did not play a role in the genesis of the nebo babel deposit

heterogeneous nucleation as the predominant mode of crystallization in natural magmas numerical model and implications for crystal melt interaction crystallization of natural magmas is inherently a disequilibrium process which involves nucleation and growth kinetics melt crystal mechanical interactions and subsolidus modifications which are all recorded in the resulting rock texture we use a new high resolution three dimensional numerical model to address the significance and consequences of homogeneous versus heterogeneous crystal nucleation in silicate magmas with increasing amount of heterogeneous nuclei during crystallization initially equigranular textures evolve to porphyritic bimodal and spherulitic types the corresponding crystal size distributions csds become concave up curved the clustering index progressively decreases and the grain contact relationships record increased clustering concave up curved csds previously interpreted as resulting from multistage crystallization mixing of crystal populations grain agglomeration or size dependent growth are now predicted consistently with other size spatial and clustering parameters to form by heterogeneous crystal nucleation correlation relationships between various textural parameters and the fraction of heterogeneous nuclei are calibrated and used on representative volcanic and plutonic rocks including cumulate rocks to deduce the fraction of heterogeneous nuclei the results indicate that 60 to 99 of all nuclei are heterogeneous for plutonic and cumulate rocks the estimate of the heterogeneous nuclei fraction based on the clustering index is significantly lower than other estimates such discrepancies in general point to the occurrence of other processes and here the results imply that crystal mush compaction and interstitial melt extraction were involved during the magma solidification formation of crystals in clusters implicit for heterogeneous nucleation implies that greater efficiency of crystal melt separation is expected in these situations

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

heterogeneous nucleation and epitaxial crystal growth of magmatic minerals electron backscatter diffraction analysis of dendritic clinopyroxene cpx forming in rapidly cooled basalt reveals two features that are unexpected for phases growing from a liquid 1 helical growth about 010 cpx the crystallographic b axis with incremental rotation up to 0 4 m 1 within branch segments and large rotational jumps 10 46 between closely spaced branch segments and 2 strong crystallographic preferred orientation cpo between cpx and titanomagnetite timt decorating branch tips such that 010 cpx aligns with one of the six symmetrically equivalent 110 timt face poles more than 80 of timt crystals occur within 5 of the cpo with cpx substrate as do 22 of timt crystals in contact with euhedral cpx in natural etna basalt the probability distribution of distances between an arbitrary unit vector and the nearest 110 timt normal unit vector was found by numerical simulation and indicates that the observed alignment to within 5 occurs with frequency 0 024 thus the cpo occurs 9 33 times more commonly than expected from randomly distributed crystals the cpo matches previous observations of epitaxial relationship between spinel and host pyroxene during subsolidus exsolution but has not previously been reported among magmatic cpx and timt conspicuous contiguity among phenocryst phases is inferred to result from heterogeneous nucleation facilitated by epitaxy associated with cpo an inference supported by high resolution electron microscopy observations of 150 nm timt crystals adhering preferentially to silicate substrates epitaxial relationships among phases nucleating from a melt may contribute to fabrics in magnetic properties dictate the textures of igneous rocks by promoting development of crystal clusters and even influence magma transport and eruption styles through a control over magma rheology

the importance of colloids for the behavior of uranium isotopes in the low salinity zone of a stable estuary particle mediated removal processes of u isotopes were investigated during spring flood discharge in the low salinity zone lsz up to 3 practical salinity units psu of a stable estuary a shipboard ultrafiltration cross flow filtration cff technique was used to separate particles 0 2 m and colloids between 3000 daltons 3 kd and 0 2 m from ultrafiltered water 3 kd containing dissolved species sediment traps were used to collect sinking material concentration of fe and organic c which are indicators of the major u carrier phases were used to interpret the behavior of 234 u 238 u during estuarine mixing colloids dominated the river water transport of u carrying 90 of the u on entering the estuary colloids accounted for the dominant fraction of u to about a salinity of 1 psu but only a minor fraction 1 psu there is a general correlation between u and salinity in all filtered fractions the 234 u 238 u ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in 234 u compared with secular equilibrium 234 u 266 567 this clearly shows that all size fractions are dominated by nondetrital u consideration of u isotope systematics across the estuary reveals that substantial u exchange must occur involving larger particles at least to 1 psu and involving colloids at least to 1 5 psu further exchange at higher salinities may also occur as the proportion of u on colloids decreases with increasing salinity this may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing in the estuary the results show that although u is a soluble element that shows generally conservative mixing in estuaries removal occurs in the very low salinity zone and this zone represents a significant sink of u variation in composition and concentration of colloidal particles between different estuaries might thus be an important factor for determining the varying behavior of u between estuaries

strontium isotopic evidence against magma addition in the upper zone of the bushveld complex abstract sr isotopic data from the main and upper zones of the bushveld complex show that the evolution of the upper zone started with a large influx of magma close to the level of the pyroxenite marker a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites whole rock samples which span the complete stratigraphic succession ca 2100 m above this layer fall on a single rb sr isochron 2066 58ma and hence have a common initial ratio of 0 7073 1 this ratio is significantly lower than those of the main zone ca 0 7085 below the level of the pyroxenite marker the entire upper zone crystallized from a mixed magma which was thoroughly blended before crystallization this magma had an isotopic ratio intermediate between that of the main zone and the added magma which had an initial ratio of ca 0 7067 further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma which is considered improbable hence the layering and mineralogical diversity of the upper zone was produced by internal processes and not produced by magma influxes during crystallization the lithological compositional and isotopic changes at the pyroxenite marker and the petrological coherence of all rocks above this horizon support the placing of the upper zone boundary at this point in the stratigraphy

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

northwest africa 773 lunar origin and iron enrichment trend the meteorite northwest africa 773 nwa 773 is a lunar sample with implications for the evolution of mafic magmas on the moon a combination of key parameters including whole rock oxygen isotopic composition fe mn ratios in mafic silicates noble gas concentrations a kreep like rare earth element pattern and the presence of regolith agglutinate fragments indicate a lunar origin for nwa 773 partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation terrestrial weathering has caused fracturing and precipitation of carich carbonates and sulfates in the fractures but lunar minerals appear fresh and unoxidized the meteorite is composed of two distinct lithologies a two pyroxene olivine gabbro with cumulate texture and a polymict fragmental regolith breccia the olivine gabbro is dominated by cumulate olivine with pigeonite augite and interstitial plagioclase feldspar the breccia consists of several types of clasts but is dominated by clasts from the gabbro and more feo rich derivatives variations in clast mineral assemblage and pyroxene mg mg fe and ti ti cr record an igneous fe enrichment trend that culminated in crystallization of fayalite silica hedenbergitebearing symplectites the fe enrichment trend and cumulate textures observed in nwa 773 are similar to features of terrestrial ponded lava flows and shallow level mafic intrusives indicating that nwa 773 may be from a layered mafic intrusion or a thick differentiated lava flow nwa 773 and several other mafic lunar meteorites have lree enriched patters distinct from apollo and luna mare basalts which tend to be lree depleted this is somewhat surprising in light of remote sensing data that indicates that the apollo and luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements

predictive modelling of fossil bearing locality distributions in the elliot formation upper triassic lower jurassic south africa using a combined multivariate and spatial statistical analyses of present day environmental data abstract traditional methods of palaeontogical fieldwork often involve costly and inefficient searching for suitable fossil bearing sites the advent of digital spatial data offers an opportunity to improve fieldwork search efficiency in this paper we develop a model that identifies potential fossil localities at the reconnaissance stage of prospecting using a criterion of present day environmental suitability for fossil discovery this model employs techniques from analytical biology geographical information systems gis and remote sensing the model is designed to be flexible with no limit on the number of variables and can use a variety of data types both continuous and categorical by means of a principle coordinate analysis pco we tested the model on outcrops of the elliot formation upper triassic lower jurassic in the free state south africa our model correctly predicted 95 of the known occurrences and reduced the potential prospective area from approximately 2300 km 2 of mapped total outcrop to 597 km 2 of prospectable outcrop initial field testing in 2015 identified two new sites based on model output

karstification of an aquifer along the birs river switzerland a modeling approach abstract the impact caused by a construction of a dam site on the birs river close to basel switzerland on the evolution of a gypsum karst aquifer in the vicinity of the hydraulic building is studied several sinkholes provoked subsidence of the dam and the highway nearby extensive technical measures had to be conducted in order to prevent further karstification the numerous geophysical and geological field studies executed in the area together with a 3d hydrogeological model of the aquifer provide a very detailed information about the boundary conditions and the local properties determining the karstification in this work we present a 2d karst evolution model of the aquifer in the vicinity of the dam structure in contrast to older studies this time the focus of the research is not the basic processes governing the karst evolution but modeling the temporal development of the real aquifer using the large amount of information about the location we demonstrate that a detailed knowledge of the local properties of the rock hydraulic conductivity solubility is of crucial importance when modeling real aquifers from a wide range of possible evolution scenarios we deduce a warning that solutional features such as sinkholes can develop far away from the hydraulic structure and endanger facilities at the surface our model is able to reproduce and successfully explain the main geological features revealed by field studies we suggest a workflow to combine the data from field observations groundwater modeling and karst evolution modeling and to study the karstification of real aquifers we propose a scenario for the evolution of the aquifer and a reasonable range for the values of the basic parameters governing the karstification

water losses from a reservoir built in karst the example of the boljuncica reservoir istria croatia the paper presents the case of the boljuncica reservoir which began operation in 1973 it is situated on the istria peninsula croatia this is a multipurpose reservoir which was built in order to protect the downstream area from flood to store water for irrigation and to control sediment transport the reservoir is situated on the contact zone between water impermeable eocene flysch and deep eocene and cretaceous limestone the bottom of the reservoir is covered partly by both flysch and quaternary deposits water losses from the reservoir bottom are so large that the main service intended for the reservoir the storage of water for irrigation is impossible after every intensive precipitation which occurs often in this region the reservoir fills very quickly the problem is that its retention of water is very short and lasts only a few days the water volume of the reservoir at the spillway altitude of 93 00 m a s l is about 6 5 106 m3 because of water losses from the reservoir bottom the mean annual volume of water stored in it during the period of 1977 2005 was only 0 5 106 m3 which is less than 8 of the full reservoir volume on the reservoir bottom many new swallow holes opened through the sediment cover after each time it filled and emptied with water special attention is paid to the groundwater level analyses interdisciplinary analyses and investigations of hydrological and hydrogeological factors causing the formation of swallow holes and water losses from the boljuncica reservoir are discussed the example given in this paper explains one unsuccessful case of building a reservoir in dinaric karst caused mainly due to insufficient geological hydrogeological and hydrological investigations in order to prevent water losses from the boljuncica reservoir very complex and expensive work needs to be done but its success regarding the reasonable reduction of water losses from the reservoir is in question

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

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

integrated investigations of karst phenomena in urban environments abstract theories that describe karst systems are often limited to conceptual models however engineering projects within complex karst systems demand the development of tools that allow site specific descriptions of the hydrogeologic settings and calibrating the processes of karst evolution subsidence of a river dam and an adjacent highway both constructed on gypsum containing rock southeast of basel switzerland required remedial construction measures a monitoring network was set up to safeguard surface and subsurface water resources during the construction measures the primary project goal was to develop tools that enable a continuous characterization of the groundwater flow regime and that facilitate the evaluation of the long term performance of the infrastructures investigative methods included high resolution 3d hydrogeological modeling and the integration of geological hydrometrical and hydrogeophysical field data of varying quality particular focus was placed on the hydraulic behavior of the complex conduit system results help to understand the evolution of distinct karst features and zones of preferential flow the location of fracture zones and parts of the old meandering river course playing a major role in the karst evolution process could be identified together with the hydrometrical investigations and hydrogeological modeling the evolution of the karst system and its dynamics can be interpreted in relation to the groundwater flow regime

mineral disequilibrium in the merensky reef western bushveld complex south africa new sm nd isotopic evidence the sm nd isotopic data from the merensky reef and its immediate footwall suggest that orthopyroxene and plagioclase are in isotopic disequilibrium with one another such that plagioclase is enriched in radiogenic nd relative to a bushveld aged reference isochron passing through the pyroxene and whole rock data the whole rocks and orthopyroxenes give end values at 2 06 ga between 7 46 and 8 46 whereas the plagioclases have end values of 1 13 to 3 37 orthopyroxene was derived from a liquid affected by pre emplacement crustal contamination and settled by density driven accumulation into a liquid crystal mush dominated by plagioclase derived from a relatively uncontaminated previous liquid isotopic constraints on the potential contaminant favour pre emplacement contamination by late archaean granitoids and also require that orthopyroxene preceded plagioclase on the liquidus of the incoming magma the presence of isotopic disequilibrium suggests that isotopic compositions of whole rock samples in cumulus rocks must be interpreted with caution

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

the impact of hydrochemical boundary conditions on the evolution of limestone karst aquifers abstract the early evolution of karst aquifers depends on a manifold of initial and boundary conditions such as geological setting hydrologic properties of the initial aquifer and petrologic properties of the rock when all water entering at various inputs into the aquifer has equal chemical composition with respect to the system h2o co2 caco3 early evolution under conditions of constant head exhibits breakthrough bt behaviour if the chemical compositions of the input waters are different deep in the aquifer where the saturated solutions mix renewed aggressiveness occurs and additional dissolutional widening of fractures by mixing corrosion mc changes the hydrologic properties of the aquifer to study the impact of mc on the evolution of karst we have modelled a simple karst aquifer consisting of a confined limestone bed with two symmetrically located inputs at constant head and open flow conditions along the entire width at base level to calculate dissolutional widening of the fractures the well known dissolution kinetics of limestone was used which is linear up to 90 of saturation with respect to calcite and then switches to a nonlinear fourth order rate law first two extremes are modelled a both inputs receive aggressive water of equal chemical composition with ca2 0 75 ca2 eq in this case two channels migrate downstream with that from one input more competitive and reaching base level first causing bt b water at both inputs is saturated with respect to calcite but in equilibrium with different partial pressures of co2 therefore dissolution widening can occur only where these waters mix a central channel starts to grow extending down head until base level is reached flow rates through the aquifer first rise and become constant after the channel has reached base level in the following runs these two extreme modes of karstification are combined the waters entering have different chemical compositions and therefore different equilibrium concentrations ca2 eq this allows mc to be active they are also undersaturated with the inflowing solutions at concentration ca2 in f ca2 eq where f is the ratio of saturation in comparison to the extreme limit a the action of mc now creates permeability where the solutions mix and diverts the evolution of conduits into this region finally one conduit reaches base level and causes bt this behaviour is found for f 0 7 0 9 and 0 96 for solutions more close to equilibrium with respect to calcite f 0 99 0 9925 and 0 995 bt behaviour is replaced by a steady increase in flow rates in the early state as in the case of mc controlled evolution case b a central channel not connected to the input is created by mc and reaches base level after this event further increase in flow rates is caused by slow dissolutional widening by the slightly undersaturated input solutions flowing towards the central channel comparison of the various model aquifers at termination of the computer runs reveals significant differences in their properties caused solely by changes of the hydrochemical boundary conditions

a model of the early evolution of karst aquifers in limestone in the dimensions of length and depth a new model of the early evolution of limestone karst aquifers in the dimensions of length and depth is presented in its initial state the aquifer consists of a rock massive with evenly spaced fractures of about 50 mm aperture widths with an hydraulic conductivity of 10 27 ms 21 in addition to this a coarser network of prominent fractures with aperture widths of several 100 mm is also present boundary conditions of constant recharge 450 mm year or constant head from the input of allogenic streams are imposed first the position of the water table in the aquifer is calculated then dissolutional widening during a time step in all the fractures below the water table is found by use of the well known nonlinear dissolution kinetics of limestone this is iterated and the position of the water table as well as the fracture widths are found as a function of time in the case of constant recharge to a karst plateau the water table in any case drops to base level and conduits there propagate from the spring headwards if constant head conditions are valid the position of the water table remains almost stable and conduits propagate along the water table from the input towards the spring there is competition between conduit evolution along prominent fractures and along tight fissures close to the water table in any case under constant head conditions one of these pathways wins and early karst evolution is terminated by a breakthrough event with an explosive increase of the flow through the aquifer until constant head conditions break down depending on the boundary conditions of constant head or constant recharge or a combination of both it is possible to describe models of cave genesis which have been derived from field evidence such as the water table models of swinnerton geol soc am bull 34 1932 662 and rhoades and sinacori j geol 49 1941 785 as well as the four state model by ford and ewers can j earth sci 15 1978 1783 q 2001 elsevier science b v all rights reserved

postmagmatic sulphur loss in the skaergaard intrusion implications for the formation of the platinova reef abstract the stratabound platinova au pd reef in the skaergaard intrusion is intimately linked to cu fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the cu s ratio these features suggest that the reef is related to silicate sulphide liquid immiscibility in the skaergaard magma however although the evidence is strong present day sulphur concentrations are too low to support such a model the detailed knowledge of the shape volume and compositional relations of the skaergaard lithologies allows for numerical modeling of sulphur and copper concentrations during fractionation of the magma these data can be evaluated with the current models for sulphur saturation in iron rich mafic magmas to assess the original sulphur concentrations the extent of fractionation the composition of sulphide liquids and the extent of postmagmatic sulphur loss mass balance modeling yields initial copper and sulphur concentrations of 250 and 894 ppm for the parental magma to the lower zone a prior to sulphur saturation cu behaves as an incompatible element with a bulk partition coefficient in the cumulus material dcumuluscu of 0 13 indicating some incorporation of cu into mafic silicates probably augite sulphur can be assumed to be perfectly incompatible in the cumulus minerals although it will be included as a component of the interstitial liquid these values yield sulphur saturation at around 635 ppm cu and 2607 ppm s assuming the sulphur to originally be present as a mixture of monosulphide mss and intermediate solid solution sulphide iss the primary sulphides in the cumulates would amount to 0 07 modal through lower and middle zones and 0 53 modal on average through upper zone the expected weight proportion of primary chalcopyrite pyrrhotite evolves from 30 70 at the base of lza through 26 74 at the level of the platinova reef to 18 82 at the sandwich horizon the present day bornite magnetite mineral assemblage is likely to have been derived from a primary pyrrhotite chalcopyrite assemblage through reequilibration and postmagmatic oxidation significant sulphur appears to have been lost from the lower and middle parts of the layered series lower zone a to upper zone b whereas the sulphides of the upper zone c can be explained in the context of a sulphur gain local scale reequilibration of cu from mafic silicates into sulphides may have contributed to the sulphide mineralogy of the lower and middle zones the structure of the platinova reef probably formed from an original single stratabound reef sulphur loss through oxidation in upward migrating deuteric or hydrothermal fluids could have caused dissolution and reprecipitation of pd and au in a cumulate with several stratiform redox barriers this process can thus explain the successive pd peaks of the reef as well as the variable stratigraphic offset of pd and au across the intrusion