revised age of the late neogene terror bird titanis in north america during the great american interchange the giant flightless terror bird titanis walleri is known from florida and texas during the late neogene the age of t walleri is problematic because this taxon co occurs with temporally mixed i e time averaged faunas at two key sites thus prior to this study t walleri from the santa fe river florida type locality was either as old as late pliocene ca 2 2 ma or as young as latest pleistocene ca 15 ka likewise t walleri from the nueces river texas was either early pliocene ca 5 ma or latest pleistocene ca 15 ka in order to better resolve this age range the rare earth element ree patterns of t walleri from the santa fe river florida were compared to two biochronologically distinctive groups late pliocene versus late pleistocene of fossil mammals from the same locality similarly the ree patterns of t walleri from texas were compared to two groups early pliocene versus latest pleistocene of fossil mammals from the same locality the ree patterns of t walleri from florida are indistinguishable from those of the co occurring late pliocene mammals likewise the ree pattern of t walleri from texas is indistinguishable from those of the co occurring early pliocene mammals given these ree constraints the revised age of t walleri is early pliocene in texas ca 5 ma and late pliocene ca 2 2 1 8 ma in florida as such t walleri is interpreted as an early immigrant during the great american interchange prior to the formation of the isthmian land bridge no evidence currently exists for pleistocene t walleri in north america

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

geochemical and fluid dynamic modeling of compositional variations in archean komatiite hosted nickel sulfide ores in western australia stratigraphic volcanological petrological and fluid dynamic studies suggest that archean komatiite associated nickel sulfide deposits in western australia formed by thermal erosion of unconsolidated sulfidic sediments beneath channelized lava flows geochemical and nd isotope data for the type area at kambalda suggest that mineralized lava channels are less contaminated than barren flanking segments of the flow owing to high discharge rates and continuous replenishment within the channel this interpretation has several implications for the mode of emplacement of sulfides their degree of equilibration with the host komatiite and the compositions of sulfide ores 1 the solubility of sulfur in komatiite is relatively low so most of the sulfide in eroded sediments will be melted and remain as a dense layer at the base of the flow or be incorporated into the turbulently flowing komatiite as immiscible sulfide droplets 2 sulfide ores will not necessarily be in equilibrium with overlying komatiites but will record equilibration with variably contaminated lavas during initial stages of emplacement and crystallization and 3 ore compositions will vary from channel to channel shoot to shoot depending on i the composition of the hybridized komatiite ii the fluid dynamics of the system and the effective magma sulfide ratio r factor and iii the sulfide silicate partition coefficients under the prevailing conditions of temperature and f sub o 2 f sub s 2 the size of immiscible droplets and the interface between two immiscible layers are affected by inertial forces during turbulent flow turbulence causes coalescence of small droplets and breakup of large droplets leading to an equilibrium droplet size if the average size of the droplets is small and or the flowage rate is high they may be carried in suspension in the flowing komatiite droplets have a large surface area to volume ratio and will be effective in scavenging chalcophile elements since they are carried by the turbulent motion of the flowing komatiite if the average size of the droplets is large or if the flowage rate is low they will settle and collect at the base of the komatiite komatiite and sulfide liquids may continue to flow but the sulfide layer will have a low surface area to volume ratio and will be ineffective in extracting chalcophile elements from the overlying komatiite layer thus the initial stages of emplacement when sulfides are initially melted and are most likely to be in suspension if at all control the effective r factor and therefore the tenor of the sulfide ores numerical modeling of the observed variations in ore compositions constrained by known komatiite compositions and experimentally or empirically determined distribution constants indicates that the observed range of ore compositions in western australian komatiite associated nickel sulfide deposits can be accounted for primarily by variations in the effective sulfide silicate ratio 500 r 100 although required variations in the partition coefficient for ni 200 d ni 100 suggest that f sub o 2 f sub s 2 may also have varied systematically assimilation of h 2 o fes rich sediments should increase f sub o 2 f sub s 2 because of the relative abundances of the two components in the magma and unconsolidated sediments if low tenor ores represent equilibration at lower r and higher f sub o 2 f sub s 2 and high tenor ores represent equilibration at higher r and lower f sub o 2 f sub s 2 as suggested by the modeling then d ni appears to decrease with increasing f sub o 2 f sub s 2 as suggested by doyle and naldrett 1987 the range of calculated r values is relatively low suggesting that low tenor sulfides were not entrained into the komatiite lava but remained as a segregated layer at the base of the flow and that high tenor sulfides were not transported long distances in suspension but settled rapidly to the base of the flow magma compositions vary only slightly from an inferred parental komatiite low tenor ores appear to have equilibrated with only slightly more evolved contaminated and fractionated lavas

geochemistry of the dundonald komatiite basalt suite and genesis of dundeal ni deposit abitibi subprovince canada the genesis of konatiites basalts and associated ninineralization in the dundonald township area on tario is critical to understanding the metallogenic evolution of the kidd munro assemblage one of the most primitive volcanic assemblages in the world the 2 5 kin thick stratigraphic succession has a basal calc alkalic basalt dacite rhyolite sequence 2716 8 2 1 ma u pb zircon cut by the dundonald peridotite gabbro sill 27077 ma u pb zircon overlain by komatiitic and basaltic flows containing magmatic ni sulfide mineral ization which are in turn overlain by chemically distinctive low ti basalt flows the dundonald komatiites correlate with komatiites in the footwall of the giant kidd creek volcanic associated massive sulfide deposit 40 km to the west but are slightly older than the komatiites of munro township which are 40 km to the east the dundonald komatiites comprise a munro type al undepleted komatiite suite with liquid compositions having a120 ffio o 18 to 22 and mgo up to 30 3 vet percent anhydrous n 24 most samples in the ko matiite suite have slightly depleted light bee contents with lan smn 0 6 to 1 1 and lan ybn 0 4 to 0 9 n 21 and negligible ti ti and zr zr anomalies their major and trace element geochemistry is consistent with a derivation by sigfificant partial melting of a chemically primitive mantle with little or no influence of majoritc garnet flows that are host to the dundeal and alexo ni deposits have distinctively high thpm nbpm ratios 1 7 n 3 suggesting crustal contamination and peperite textures that are consistent with contami nation from nearby graphitic argillite the chemically distinctive lmv ti basalts have 49 to 53 percent sios 4 5 to 9 5 percent mgo 18 8 to 21 5 percent a1203 0 30 to 0 37 percent tios moderately depleted light ree con tents with lan smn 0 9 to 1 2 and la n ybn 0 4 to 0 6 high ni cr and ba contents 570 6 800 390 630 90 320 ppm respectively n 6 their high ai203 contents place them in the calc alkalic field on a jensen cation plot although their trace element ratios zr y la yb are more consistent with a tholeiitic affinity they are compositionally identical to and they correlate with low ti basalts in the kidd creek volcanic complex their geochemistry is consistent with a derivation by partial melting of a refractory harzburgitic mantle that has undergone a previous melt extraction possibly influenced by a large ion lithophile enriched hydrous phase the empire flow is the thickest and most primitive komatiite flow and is host to the dundeal ni deposit re source of 0 4 mt at 2 0 ni the empire flow contains an elongate dunite peridotite basal unit interpreted as the core of a flow that thermally eroded into footwall heterolithic breccias and pillowed andesites the main dundeal ni horizon overlies part of the basal dunite peridotite unit within the flow channel and extends over the cutbank wall the dundeal deposit and the nearby dundonald south ni deposit have lo v total sulfide con tents and high ni s ratios 0 9 and 1 0 respectively in comparison to most komatiite hosted magmatic sulfide deposits the high ni tenor in sulfide is believed to be due to assimilation of reducing carbonaceous sediment which causes greater ni partitioning into the magmatic sulfide phase

provenancing of hominid and mammalian fossils from kanjera kenya using edxrf abstract we describe a rapid non destructive energy dispersive x ray fluorescence edxrf technique for provenancing fossil bone and apply it to surface collected fossils from kanjera kenya the locality is best known for its controversial anatomically modern human fossils potentially among the earliest from africa excavations of the 1987 1988 smithsonian expedition provided the in situ fossil sample necessary to provenance the previous finds relative sr y zr u rb rn and th concentrations were used to characterize 301 fossils from six stratigraphic units the sources of the surface fossils important fossils from past kanjera expeditions including the hominid and theropithecus oswaldi samples were analysed and their relative elemental values used to assign them to a reference sample the hominids post date kanjera formation deposition the t oswaldi fossils are largely attributable to the early pleistocene kanjera bed kn 2a secondary deposition of kanjera formation fossils into later strata was noted

oldowan behavior and raw material transport perspectives from the kanjera formation the archaeological record of oldowan hominins represents a diverse behavioral system it has been suggested that exploitation of lithic resources by oldowan hominins was simplistic and represented mostly use of local sources of stone here we investigate the raw material selection and transport behaviors of oldowan hominins reflected in the stone artifact assemblages from the kanjera south formation south rachuonyo district kenya using geochemical methods ed xrf artifacts are linked to primary and secondary source outcrops throughout southwestern kenya these data show that hominins selected raw materials for transport at frequencies that are significantly different from their availability on ancient landscapes furthermore a substantial proportion of the assemblage represents transport over relatively long distances 10 km our study further suggests that in the early stages of stone tool use hominins used a wide variety of raw materials and selected these materials at some distance from their eventual discard locations early hominin behavior may have incorporated an understanding of raw material source distributions across a more extensive landscape than has been previously documented this supports the growing perspective that oldowan technology represents a more complex behavioral pattern than is usually associated with the beginnings of hominin tool use

paleosea floor volcanic associated massive sulfide mineralization related to a cooling komatiite flow abitibi subprovince canada paleosea floor volcanic associated massive sulfide mineralization has been discovered 50 to 150 m upsection from a late archean peridotitic komatiite flow where the flow is thickest in a thermal erosion channel the terminus massive sulfide occurrence is within carbonaceous argillites that are intercalated with basaltic komatiite flows and flow breccias it contains appreciable zn and cu contents and is associated with silicification and chlorite epidote alteration ultramafic magmas are ideal heat sources for hydrothermal convective systems because of their extremely high liquidus temperatures and their tendency to become less permeable during serpentinization which impedes advective cooling heuristic calculations indicate that there is sufficient enthalpy in the peridotitic komatiite flow to drive the hydrothermal system responsible for the terminus mineralization this hypothesis is tested using two dimensional finite element heat and fluid flow simulations the results suggest that under the right circumstances 1 relatively thick komatiite flows may take hundreds of years to cool 2 komatiite flow driven hydrothermal systems can form sea floor or subsea floor deposits comparable in mass to those along the mid atlantic ridge 3 much of the metal leaching by hydrothermal fluids occurs below and adjacent to the flow and 4 the shape of heat source komatiite flows or subsea floor magma chambers affects the locus and distribution of hydrothermal upwelling similar base metal precipitating paleohydrothermal systems are present above ponded komatiite flows broadly along strike at the potter zn cu deposit in munro township ontario these paleohydrothermal systems and the calculations presented here suggest that thick ponded basaltic flows on the modern sea floor have sufficient enthalpy to form significant hydrothermal sulfide accumulations

transient response of an atmospheric gcm to north atlantic sst anomalies abstract a high resolution atmospheric general circulation model gcm is used to evaluate the atmospheric response to north atlantic sea surface temperature sst anomalies the transient evolution of the response is studied in detail the linear and nonlinear effects can thus be contrasted and separated by their different time scales baroclinic patterns related directly to the surface anomalies quickly reach their maximum manifestation however barotropic patterns related to the mid and upper troposphere eddy vorticity fluxes have longer time scales with much more important amplitude able to gradually replace the initial baroclinic response this study thus provides an evolutional picture of the two types of response in a gcm

tectonic controls on rift basin morphology evolution of the northern malawi nyasa rift radiometric k ar and ar 40 ar 39 age determinations of volcanic and volcaniclastic rocks combined with structural gravity and seismic reflection data are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern malawi nyasa rift and to characterize changes in basin and flank morphologies with time faulting and volcanism within the tukuyu karonga basin began at approximately 8 6 ma when sediments were deposited in abroad initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief extensions primarily by a slip along the border fault and subsequent regional isostatic compensation led to the development of a 5 km deep basin bounded by broad uplifted flanks along the low relief basin margin opposite border fault younger stratigraphic sequences commonly onlap older wedge shaped sequences although their internal geometry is often progradational intrabasinal faulting flankuplift and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2 5 ma when cross rift transfer faults developed to link the tukuyu karonga basin to the rukwa basin local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin s depocenter sequence boundaries are commonly erosional along this low relief hanging wall margin and conformable in the deep lake basin the geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake irrespective of paleoclimatic fluctuations

quaternary mammals and the end villafranchian dispersal event a turning point in the history of eurasia the history of land mammals is characterized by dispersal events i e short periods of rapid intercontinental migrations and faunal replacements one of these the elephant equus event of lindsay et al may be dated somewhere between 3 and 2 5 ma a second event the wolf event may fall near 1 7 ma the end villafranchian event is discussed in detail it may be dated between 1 0 and 0 9 ma and is marked by a practically total rejuvenation of the fauna with the appearance of new types of adaptation unknown in the villafranchian this faunal revolution is easily recognized all over eurasia and is accompanied by wide ranging changes in climate and vegetation this whole sequence of phenomena may have been triggered by extensive tectonic movements in the mountain belts of central and southern asia

mountain ranges climate and weathering do orogens strengthen or weaken the silicate weathering carbon sink abstract the role of mountains in the geological evolution of the carbon cycle has been intensively debated for the last decades mountains are thought to increase the local physical erosion which in turns promotes silicate weathering organic carbon transport and burial and release of sulfuric acid by dissolution of sulfides in this contribution we explore the impact of mountain ranges on silicate weathering mountains modify the global pattern of atmospheric circulation as well as the local erosion conditions using an ipcc class climate model we first estimate the climatic impact of mountains by comparing the present day climate with the climate when all the continents are assumed to be flat we then use these climate output to calculate weathering changes when mountains are present or absent using standard expression for physical erosion and a 1d vertical model for rock weathering we found that large scale climate changes and enhanced rock supply by erosion due to mountain uplift have opposite effect with similar orders of magnitude a thorough testing of the weathering model parameters by data model comparison shows that best fit parameterizations lead to a decrease of weathering rate in the absence of mountain by about 20 however we demonstrate that solutions predicting an increase in weathering in the absence of mountain cannot be excluded a clear discrimination between the solutions predicting an increase or a decrease in global weathering is pending on the improvement of the existing global databases for silicate weathering nevertheless imposing a constant and homogeneous erosion rate for models without relief we found that weathering decrease becomes unequivocal for very low erosion rates below 10 t km2 yr we conclude that further monitoring of continental silicate weathering should be performed with a spatial distribution allowing to discriminate between the various continental landscapes mountains plains

characterization of gold mineralization in the northern cariboo gold district british columbia canada through integration of compositional studies of lode and detrital gold with historical placer production a template for evaluation of orogenic gold districts there is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold although in many cases the nature of the source mineralization for these placer deposits remains unclear this study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy followed by synthesis of this information with records of both mineral occurrences and historical placer mining the northern cariboo gold district in east central british columbia canada was chosen as a location for the study because of the gold endowment of the area 1 2 million ounces moz of lode gold and between 0 5 and 3 moz of placer gold and the information available from both placer and lode gold mining compositional analysis of 533 gold grains from 21 lode localities and 1 914 gold grains from 30 placer localities from throughout the cariboo gold district has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section a distinctive low 4 7 ag gold that exhibits a strong bi association in the mineral inclusion suite is geographically limited to the wells area where it is recorded in both lode mineralization and its placer expression regionally pervasive mineralization yields gold of binary au ag alloy and a simple inclusion suite of sulfides and sulfarsenides gold in most large placers in trunk drainages was derived from multiple mostly small occurrences of this type the nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities while others are dominated by gold deposited in a single stage the new information from gold grain analysis has been considered in the context of other information classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types the compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness au ag ratio to reconstruct the size and distribution of hypogene sources prior to erosion synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources

high temperature environments of human evolution in east africa based on bond ordering in paleosol carbonates many important hominid bearing fossil localities in east africa are in regions that are extremely hot and dry although humans are well adapted to such conditions it has been inferred that east african environments were cooler or more wooded during the pliocene and pleistocene when this region was a central stage of human evolution here we show that the turkana basin kenya today one of the hottest places on earth has been continually hot during the past 4 million years the distribution of 13c 18o bonds in paleosol carbonates indicates that soil temperatures during periods of carbonate formation were typically above 30 c and often in excess of 35 c similar soil temperatures are observed today in the turkana basin and reflect high air temperatures combined with solar heating of the soil surface these results are specific to periods of soil carbonate formation and we suggest that such periods composed a large fraction of integrated time in the turkana basin if correct this interpretation has implications for human thermophysiology and implies a long standing human association with marginal environments

quaternary fossil fauna from the luangwa valley zambia this paper describes a large collection of quaternary fossil fauna from the luangwa rift valley zambia stone age artefacts have been recovered from stratified fluvial contexts but no in situ fossil faunas have yet been recovered we report on 500 fossil specimens collected from the surface of point bars exposed seasonally along the banks of the main luangwa river channel we used non destructive x ray fluorescence analysis of the fossils chemical signatures to determine whether they derive from one or many primary contexts and the relationship between chemical signature and state of preservation specimens are identified to taxon genus to reconstruct palaeoenvironments and biochronology a relatively wide range of taxa is identified including a fossil hominin talus described here none of the fossils is positively attributable to extinct species except a femur of an extinct theropithecus reported in 2003 although no additional extinct taxa were identified some of the remains were attributable to genera that are not currently found in this region the results suggest that most of the assemblage derives from sediments which are middle pleistocene or later and that past environments in the luangwa valley may have differed from the habitat availability found today

high resolution records of location and stratigraphic provenance from the rare earth element composition of fossil bones abstract bone apatite acts as a natural timed sampling device scavenging trace elements from local pore waters over timescales of ca 1 50 ka the rare earth element ree and u th composition of fossil bones reflects associated pore water compositions during the period of recrystallisation the ree composition of fossil bones is controlled by partitioning of ree between pore waters and particle surfaces and the ree composition of fossil bones reflects the ree composition of pore waters which vary spatially and temporally light ree are preferentially sorped onto particle surfaces thus the high la yb values seen in many bones from coastal marine and aeolian environments are best explained by release of ree from light ree enriched particles to local pore waters and subsequent immobilisation in recrystallising bones the ree compositions of bones recovered from pedogenically altered diatomite sediments of the olorgesailie formation of southern kenya vary over spatial scales of less than 10 m location accounts for 48 of the observed variation in bone chemistry and bones recovered from eight discrete excavations within the same time equivalent stratigraphic layer can be assigned to their excavation location with 70 accuracy based on a discriminant analysis of ree u and th composition despite this within layer variation bones recovered from different stratigraphic horizons within the olorgesailie formation can also be distinguished on the basis of their trace element composition bones recovered from four stratigraphic horizons spanning ca 0 5 million years were assigned to their correct stratigraphic layer with 90 accuracy where sedimentological conditions are favourable the trace element composition of fossil bone may be used to test stratigraphic provenance and burial location in excavated bone with a temporal resolution of

fission track dating of volcanically derived sedimentary rocks depositional ages of sedimentary rocks can be determined using fission track single grain ages on zircons from layers of volcanic ash or bentonite even when the layers have been contaminated by older grains this is done by compiling an age probability distribution or age spectrum for a sample from individual grain ages an age spectrum is a simple and unambiguous way of testing for contamination and extracting useful age information the youngest peak in the age spectrum approximates the time of deposition in most contaminated samples 30 or more grains should be counted to produce a reliable spectrum however useful reproducible ages can be obtained by counting less than 10 grains in samples where most of the older contaminating grains can be removed the few older grains that remain after removing the obviously abraded ones may then be eliminated by examining the age spectrum although ages determined in this way are probably not precise enough for use in defining stratigraphic boundaries they still provide a means of obtaining an isotopic age in sediments that cannot be dated by other radiometric methods 18 references 3 figures 3 tables

variation in peperite textures associated with differing host sediment properties peperites formed by mixing of magma and wet sediment are well exposed along punta china baja california mexico where two sills intrude a section of lava flows limestones and volcaniclastic rocks irregular lobes and dikes extend from the sills several meters into host sediments including highly comminuted flow top breccias lithic lapilli tuff breccias and shelly micrites whereas intrusive contacts with lava flows are sharp and planar where one sill intruded both coarse grained volcaniclastic rock and fine grained limestone textural differences between the hosts produced strikingly different styles of peperite blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias the blocks consequently underwent in situ fragmentation as they were rapidly quenched the high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse grained permeable host elutriation of fine grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter scale fluidization of the host the contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous immiscible fluids fluidal peperite intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host thin section scale elutriation pipes formed microglobular peperite represents a frozen example of a fuel coolant interaction fci between basaltic magma and fluidized micrite host the intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma vapor host interface such intimate intermixing of magma and water bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism

gold bearing fault zones related to late archean orogenic folding of upper and middle crust in the abitibi granite greenstone belt ontario abstract this work investigates the structures of the abitibi granite greenstone belt ontario with a focus on regional scale folds and on regional scale auriferous fault zones the folds and the fault zones resulted from late archean orogenesis the analysis is based on studies of surface geology and on modelling of the gravity field amphibolite grade orthogneiss was involved in the regional folding that also deformed the overlying greenstones folding resulted in uplift of the orthogneiss that preserves the metamorphic mineral assemblage ferropargasite garnet biotite andesine quartz geothermobarometric calculations indicate recrystallization of a metatonalite sample under peak paleopressures of 7 0 1 2 to 7 9 1 1 kb providing evidence that folding involved crust to depths of at least 18 25 km the regional bouger anomaly map reveals previously unrecognized first order folds interpretation of the bouger anomaly map also confirms that granitic batholiths are hosted by anticlines a wavelet based inversion method was used to determine the locations orientations and continuations at depth of the maxima of density contrasts the maximum density contrasts correspond to steeply dipping geological contacts on the limbs of the open to tight regional scale folds regional scale fault zones only have prominent gravity signatures where they correspond to sheared rocks on the limbs of folds modelled gravity profiles were used to produce profiles that visualize the shapes of batholiths and greenstone bodies within the present day upper crust the granitic batholiths have tabular shapes which are locally folded with preserved main body thicknesses of 1 3 km the greenstones form keels corresponding to synclines some of which reach 5 6 km depth the results of this study confirm that the principal deformational response of the abitibi granite greenstone belt to late archean collisional tectonics was large half wavelength 10 40 km folding of the upper and middle crust our results and interpretations also strongly suggest the regional scale fault zones which are loci for orogenic gold deposits are fold related structures they represent quasi continuous lineaments of shearing and faulting on the limbs of regional folds and locally in the cores of detachment anticlines as shown in a previous paper

tectonic uplift and eastern africa aridification the history of eastern african hominids has been linked to a progressive increase of open grassland during the past 8 million years this trend was explained by global climatic processes which do not account for the massive uplift of eastern african topography that occurred during this period atmosphere and biosphere simulations quantify the role played by these tectonic events the reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation engendering the strong aridification and paleoenvironmental changes suggested by the data

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

nickel sulfide deposits in australia characteristics resources and potential abstract australia s nickel sulfide industry has had a fluctuating history since the discovery in 1966 of massive sulfides at kambalda in the eastern goldfields of western australia periods of buoyant nickel prices and high demand speculative exploration and frenetic investment the nickel boom years have been interspersed by protracted periods of relatively depressed metal prices exploration inactivity and low discovery rates despite this unpredictable evolution the industry has had a significant impact on the world nickel scene with australia having a global resource of nickel metal from sulfide ores of 12 9 mt five world class deposits 1 mt contained ni and a production status of number three after russia and canada more than 90 of the nation s known global resources of nickel metal from sulfide sources were discovered during the relative short period of 1966 to 1973 australia s nickel sulfide deposits are associated with ultramafic and or mafic igneous rocks in three major geotectonic settings 1 archean komatiites emplaced in rift zones of granite greenstone belts 2 precambrian tholeiitic mafic ultramafic intrusions emplaced in rift zones of archean cratons and proterozoic orogens and 3 hydrothermal remobilized deposits of various ages and settings the komatiitic association is economically by far the most important accounting for more than 95 of the nation s identified nickel sulfide resources the ages of australian komatiitic and tholeiitic hosted deposits generally correlate with three major global scale nickel metallogenic events at 3000 ma 2700 ma and 1900 ma these events are interpreted to correspond to periods of juvenile crustal growth and the development of large volumes of primitive komatiitic and tholeiitic magmas caused by large scale mantle overturn and mantle plume activities there is considerable potential for the further discovery of komatiite hosted deposits in archean granite greenstone terranes including both large and smaller high grade 5 to 9 ni deposits that may be enriched in pges 2 to 5 g t especially where the host ultramafic sequences are poorly exposed analysis of the major komatiite provinces of the world reveals that fertile komatiitic sequences are generally of late archean 2700 ma or paleoproterozoic 1900 ma age have dominantly al undepleted al 2 o 3 tio 2 15 to 25 chemical affinities and often occur with sulfur bearing country rocks in dynamic high magma flux environments such as compound sheet flows with internal pathways facies kambalda type or dunitic compound sheet flow facies mt keith type most precambrian provinces in australia particularly the proterozoic orogenic belts contain an abundance of sulfur saturated tholeiitic mafic ultramafic intrusions that have not been fully investigated for their potential to host basal ni cu sulfides voisey s bay type mineralization the major exploration challenges for finding these deposits are to determine the pre deformational geometries and younging directions of the intrusions and to locate structural depressions in the basal contacts and feeder conduits under cover stratabound pge ni cu cr deposits hosted by large archean proterozoic layered mafic ultramafic intrusions munni munni panton of tholeiitic affinity have comparable global nickel resources to many komatiite deposits but low grades

the late cenozoic uplift climate change paradox the geologic evidence for worldwide uplift of mountain ranges in the neogene is ambiguous estimates of paleoelevation vary according to whether they are based on the characteristics of fossil floras on the masses and grain sizes of eroded sediments or on calculations of increased thickness of the lithosphere as a result of faulting detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns the geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift we suggest that the spread of c4 plants in the late miocene may have altered both the erosion and climate systems these changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past

freshwater fish δ18o indicates a messinian change of the precipitation regime in central africa tracking terrestrial environmental change throughout the neogene is a challenge notably in areas such as central africa where the few available data consist of a few vertebrate fossil assemblages here we aim to quantify the evolution of the 18 o of the main water body between four neogene wet episodes in the chad basin ranging from the late miocene to the early pliocene the 18 o of the open water body was inferred from oxygen isotope measurements of phosphate in the apatite of open water fish tooth enamel the more open the fish habitat the lower the 18 o as revealed by the teeth of the large tiger fish hydrocynus sampled in the four available chadian vertebrate fossiliferous areas i e toros menalla anthracotherid unit kossom bougoudi kolle and koro toro all located in the djurab desert chad and dated at 7 04 0 18 ma 5 26 0 23 ma 3 96 0 48 ma and 3 58 0 27 ma respectively the 18 o values increased by 2 between the two sites having ages that bracket the messinian time period and there was a slight increase of 0 6 difference between the three pliocene sites these results reflect unambiguously change in the water cycle in central africa during the late neogene interpreted as a constant drying trend between the four successive wet episodes registered in the djurab and a shift during the messinian

mobility of gold and silver in lateritic weathering profiles some observations from western australia experimental evidence suggests that very acid chloride solutions generated in lateritic profiles by ferrolysis oxidation and hydrolysis of iron are responsible for the dissolution of gold and silver au is subsequently reprecipitated by reduction of the aucl 4 super ion with fe super 2 reduction of the agcl complex does not similarly occur because of the relative redox potentials of fe super 2 fe super 3 and ag agcl thus chemical refinement of gold occurs during lateritization the development of low ph and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences modified journal abstract

biomarker records of late neogene changes in northeast african vegetation open savannah grasslands dominated by c4 plants became a significant component of northeast african vegetation during the late neogene we present molecule specific carbon isotopic mea surements of terrestrial plant biomarkers preserved in marine sed iments off northeast africa that allow reconstruction of orbital scale vegetation changes in short time windows over the past 9 4 m y the biomarker data show large amplitude vegetation vari ability as early as 3 8 ma with the greatest c4 expansion occurring after 3 4 ma we sampled orbital scale oscillations of up to 5 almost as large as the observed late neogene range of 7 sug gesting that large and repeated oscillations between more open and more closed landscapes were an important aspect of northeast af rican vegetation change during the past 4 m y

fractionation of the platinum group elements and gold in some komatiites of the abitibi greenstone belt northern ontario the noble element patterns from four nickel sulfide deposits in komatiites of the abitibi greenstone belt exhibit various degrees of fractionation pd ir ratios for dundonald texmont hart and alexo are 10 5 12 and 116 respectively the noble element patterns could have been affected by metamorphism including hydrothermal alteration contamination of the magma by a sulfide rich sediment crystal fractionation of monosulfide solid solution solubility of os ir alloys and rus2 and timing of sulfur saturation noble element patterns of samples from the deposits dundonald and alexo of the lowest metamorphic grade prehnite pumpellyite facies are smooth and exhibit no or very small au and pt anomalies however some samples from texmont mid greenschist facies metamorphism contain pt and au anomalies further all samples from the hart deposit which has experienced carbonate alteration have large negative pt and au anomalies contamination of the komatiites with fe sulfide rich sediments found in their vicinity will not change the noble element patterns appreciably a 10 percent contamination of the komatiites will change the pd ir ratio of the sulfides from approximately 5 to 7 5 the massive sulfide at the base of the alexo deposit has a pd ir ratio of 44 the overlying net textured ore has a pd ir ratio of 171 this change in pd ir ratios could be due to the fractionation of monosulfide solid solution from the sulfide liquid the difference in noble element fractionation between the four deposits does not appear to be related to differences in the lithophile element composition of the silicate liquid from which the sulfides separated since all four deposits have a similar lithophile chemistry it is proposed that the differences in the fractionation of the noble element between the deposits is the combined result of the timing of sulfur saturation and the lower solubility of os ir and rus2 in the silicate melt compared to rh pt and pd

high resolution modelling closes the gap between data and model simulations for mid holocene and present day biomes of east africa abstract east africa hosts a striking diversity of terrestrial ecosystems which vary both in space and time due to complex regional topography and a dynamic climate the structure and functioning of these ecosystems under this environmental setting can be studied with dynamic vegetation models dvms in a spatially explicit way yet regional applications of dvms to east africa are rare and a comprehensive validation of such applications is missing here we simulated the present day and mid holocene vegetation of east africa with the dvm lpj guess and we conducted an exhaustive comparison of model outputs with maps of potential modern vegetation distribution and with pollen records of local change through time overall the model was able to reproduce the observed spatial patterns of east african vegetation to see whether running the model at higher spatial resolutions 10 10 contribute to resolve the vegetation distribution better and have a better comparison scale with the observational data i e pollen data we run the model with coarser spatial resolution 0 5 0 5 for the present day as well both the area and point wise comparison showed that a higher spatial resolution allows to better describe spatial vegetation changes induced by the complex topography of east africa our analysis of the difference between modelled mid holocene and modern day vegetation showed that whether a biome shifts to another is best explained by both the amount of change in precipitation it experiences and the amount of precipitation it received originally we also confirmed that tropical forest biomes were more sensitive to a decrease in precipitation compared to woodland and savanna biomes and that holocene vegetation changes in east africa were driven not only by changes in annual precipitation but also by changes in its seasonality

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

two and three dimensional electrical resistivity imaging at a heterogeneous remediation site geometrically complex heterogeneities at a decommissioned sour gas plant could not be adequately characterized with drilling and 2d electrical resistivity surveys alone in addition 2d electrical resistivity imaging profiles produced misleading images as a result of out of plane resistivity anomalies and violation of the 2d assumption accurate amplitude and positioning of electrical conductivity anomalies associated with the subsurface geochemical distribution were required to effectively analyze remediation alternatives forward and inverse modeling and field examples demonstrated that 3d resistivity images were needed to properly reconstruct the amplitude and geometry of the complex resistivity anomalies problematic 3d artifacts in 2d images led to poor inversion fits and spurious conductivity values in the images at depths close to the horizontal offset of the off line anomaly three dimensional surveys were conducted with orthogonal sets of wenner and dipole dipole 2d resistivity survey lines the

excavation of buried late acheulean mid quaternary land surfaces at duinefontein 2 western cape province south africa abstract duinefontein 2 dft2 preserves at least two buried land surfaces within a 10 m thick dune plume on the atlantic coast of south africa about 35 km north of cape town optically stimulated luminescence dating indicates that the sands enclosing the upper surface accumulated around 270 ky ago while the sands between the two surfaces were accumulating about 290 ky ago excavation so far has focused on the upper buried surface which is now exposed over 480 m 2 historically there was no body of fresh water nearby and the surrounding vegetation was a variant of the regional fine leafed shrub or fynbos pedogenic alteration of the sands and bones of water loving mammals and amphibians indicate however that the bones and associated acheulean artifacts accumulated near the edge of a large pond or marsh in addition the principal mammal species buffalo wildebeest and kudu imply a sharply different regional vegetation in which grass and broad leafed bush were much more common the artifacts are distributed across the upper buried surface in no apparent pattern but the large mammal bones tend to occur as clusters of skulls vertebrae ribs and other axial elements often in near anatomical order limb bones are mostly missing and the clusters appear to mark carcasses from which the limb bones were selectively removed the bones rarely show marks from stone tools but marks from carnivore teeth are common together with numerous hyena coprolites the abundant tooth marks suggest that hyenas and perhaps other carnivores were largely responsible for carcass disarticulation the human role appears to have been insignificant which suggests that local acheulean people obtained few large mammals whether by hunting or scavenging among the small number of other acheulean carcass sites for which bone damage observations are available the best documented sites suggest the same limited human ability to acquire large mammals but many additional sites will be necessary to determine if this was the acheulean norm greatly expanded excavation of the lower buried surface at dft2 can provide an additional high quality data point

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

practical techniques for 3d resistivity surveys and data inversion1 techniques to reduce the time needed to carry out 3d resistivity surveys with a moderate number 25 to 100 of electrodes and the computing time required to interpret the data have been developed the electrodes in a 3d survey are normally arranged in a square grid and the pole pole array is used to make the potential measurements the number of measurements required can be reduced to about one third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal vertical and 45 diagonal rows of electrodes passing through the current electrode the smoothness constrained least squares inversion method is used for the data interpretation the computing time required by this technique can be greatly reduced by using a homogeneous half space as the starting model so that the jacobian matrix of partial derivatives can be calculated analytically a quasi newton updating method is then used to estimate the partial derivatives for subsequent iterations this inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time on an 80486dx2 66 microcomputer it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid using the techniques described below 3d resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive microcomputer

late miocene atmospheric co2 concentrations and the expansion of c4 grasses the global expansion of c4 grasslands in the late miocene has been attributed to a large scale decrease in atmospheric carbon dioxide co2 concentrations this triggering mechanism is controversial in part because of a lack of direct evidence for change in the partial pressure of co2 pco2 and because other factors are also important determinants in controlling plant type distributions alkenone based pco2 estimates for the late miocene indicate that pco2 increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago the estimates presented here provide no evidence for major changes in pco2 during the late miocene thus c4 plant expansion was likely driven by additional factors possibly a tectonically related episode of enhanced low latitude aridity or changes in seasonal precipitation patterns on a global scale or both

mid pleistocene change in large mammal faunas of east africa abstract single crystal 40ar 39ar age estimates of 392 000 4000 to 330 000 6000 yr from lainyamok a middle pleistocene fossil locality in the southern kenya rift document the oldest evidence from sub saharan africa of a diverse large mammal fauna consisting entirely of extant species the inferred age of this fauna implies an upper limit for extinction of species that characterize well calibrated mid pleistocene fossil assemblages in east africa for its age and species richness the lainyamok fauna is surprising for its lack of extinct forms e g the bovine pelorovis well documented in later faunal assemblages of east and south africa definitive presence of the south african blesbok damaliscus dorcas is also unexpected especially as this alcelaphine bovid is the dominant large mammal in the lainyamok fauna these age estimates and the faunal composition at lainyamok indicate that geographic ranges and taxonomic associations of extant largebodied mammals were susceptible to wide fluctuations in sub saharan africa over the past 330 000 yr this inference is consistent with the hypothesis of nonanalogue or ephemeral biotas believed to characterize late quaternary ecosystems of northern continents

three and half million year history of moisture availability of south west africa evidence from odp site 1085 biomarker records ocean drilling program site 1085 provides a continuous marine sediment record off southern south west africa for at least the last three and half million years the n alkane 13 c record from this site records changes in past vegetation and provides an indication of the moisture availability of sw africa during this time period very little variation and no apparent trend is observed in the n alkane 13c record suggesting stable long term conditions despite significant changes in east african tectonics and global climate slightly higher n alkane 13c values occur between 3 5 and 2 7 ma suggesting slightly drier conditions than today between 2 5 and 2 7 ma there is a shift to more negative n alkane 13c values suggesting slightly wetter conditions during a 0 2 ma episode that coincides with the intensification of northern hemisphere glaciation inhg from 2 5 to 0 4 ma the n alkane 13c values are very consistent varying by less than 0 5 and suggesting little or no long term change in the moisture availability of south west africa over the last 2 5 million years this is in contrast to the long term drying trend observed further north offshore from the namib desert and in east africa a comparison of the climate history of these regions suggests that southern africa may have been an area of long term stability over the last 3 5 myrs

emplacement origins of coarsely crystalline mafic rocks hosted in greenstone belts examples from the 2 7 ga yilgarn craton western australia coarsely crystalline mafic rocks commonly referred to as dolerite and diabase make up a substantial proportion of archean greenstone belts and are hosts to major orogenic gold deposits yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence we present results from drill core logging of several late archean coarsely crystalline packages from the eastern goldfields superterrane of the yilgarn craton western australia including the golden mile dolerite these large mafic bodies comprise mostly medium to coarsely crystalline mafic rocks but also include a variety of other products of differentiation the bodies range from 75 to 460 m thick and extend for tens of kilometers are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy as such this study excludes mafic dykes and extremely thick and internally complex bodies such as the bushveld complex from base to top the internal stratigraphy generally consists of a lower unit of aphyric basalt dolerite gabbro and peridotite pyroxenite then a voluminous middle dolerite gabbro and an upper quartz dolerite gabbro and a topmost aphyric basalt granophyric veins and domains are common in the upper third of the mafic body contacts are gradational between all lithofacies with the exception of granophric veins which have sharp contacts within this overall stratigraphic framework there is variability in the relative thickness of lithofacies number of lithofacies as well as repetition of some lithofacies several examples also include basalt in the interior in well exposed examples the key evidence for distinguishing intrusions from extrusions is provided by the top contact however the top contact from these examples is ambiguous to overcome this gap we review the character of well studied thick ponded lavas from the literature most aspects of the internal character are not unique to intrusions although the maximum groundmass grain size excluding granophyric veins of the archean examples 3 mm is significantly coarser than that for any known ponded lavas 1 mm the ability to distinguish intrusions from extrusions has important implications for reconstructing archean greenstone stratigraphy and mineral exploration

geophysical prospecting over a meta ultramafic sequence with indicators of gold mineralization in rio grande do sul state southernmost brazil this work investigates the occurrence of gold on meta ultramafic rocks at funcho farm sao sepe rio grande do sul state southernmost brazil the presence of base metals in meta ultramafic rocks whose origin is related to volcanic massive sulfide vms deposits has only rarely been explored in the scientific literature the gold was identified in geochemical prospecting of current sediments in previous works and suggested that the host rock of the mineralization is the serpentinite of funcho farm the methodology used to achieve the study objectives included geological field surveys surveys of structural measurements and acquisition of geophysical data utilizing electrical resistivity dc and induced polarization ip methods using electrical tomography with the execution of dipole dipole arrays in this study nine electrical tomography lines each 400 m were acquired the data were processed in 2d electrical inversion and then interpolated for 3d visualization the information acquired for interpretations during field studies revealed that the probable mineralization is hosted in structures such as fractures and hinges and is also associated with milky quartz veins the data analysis allowed the 3d delimitation of possible deep sulfide mineralizations from hydrothermal fluids probably from the intrusion of the sao sepe granite the results presented in this work are very useful because they can serve as an appropriate analogy of geological reality associated with a reliable deposit architecture helping to outline a strategy for a future drilling campaign directed to the targets found

likely mantle plume activity in the skellefte district northern sweden a reexamination of mafic ultramafic magmatic activity its possible association with vms and gold mineralization abstract most attention has been given to the geology of the extensive vms and subordinate precious metals mineralization in the skellefte district less attention has been given to indications of deep seated origins of felsic and mafic ultramafic volcanic rocks of vms and precious metals mineralizing fluids and the primary origins of these metals a holistic view of the significance of mafic ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented these subjects are discussed in this investigation primitive mantle normalized spider diagrams of rare earth elements ree distinguish two groups of mafic ultramafic volcanic rocks each with distinct geochemical characteristics a mid ocean ridge morb type and a geochemically unusual and problematic calc alkaline basalt cab type which is the main subject of this investigation the morb type mafic volcanic rocks are mostly older than the skellefte group felsic volcanic rocks hosting the vms deposits whereas the more primitive cab type mafic ultramafic volcanic rocks are mostly younger a common source for these cab type mafic mgo wt 14 volcanic rocks is suggested by their similar and distinctive geochemical features these are near chondritic al undepleted al 2 o 3 tio 2 ratios moderate to strong high field strength element hfse depletion light rare earth element lree enrichment and moderate heavy rare earth element hree depletion they outcrop throughout an area of at least 100 100 km gold mineralization is spatially associated with ultramafic volcanic rocks zr and hf depletion has been shown to be associated with al depletion in mafic ultramafic volcanic rocks elsewhere and has been attributed to deep seated partial melting in ascending mantle plumes zr and hf depletion in cab type al undepleted mafic ultramafic volcanic rocks is therefore unusual the solution to this dilemma is suggested to be contamination of an al depleted mantle plume by felsic crustal rocks whereby al depleted ultramafic magmas become al undepleted it will be argued that this model has the potential to explain previous observations of deep seated origins the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization and even the primary origin of metals in vms deposits

hominin evolution in settings of strong environmental variability abstract investigations into how climate change shaped human evolution have begun to focus on environmental dynamics i e the nature and tempo of climate and landscape variability an approach that de emphasizes static reconstructions of early hominin habitats the interaction among insolation cycles is especially apparent in the paleoenvironmental records of the east african rift system where the longest records of human evolution are preserved however environmental indicators such as deep sea oxygen isotopes terrestrial dust flux paleosol carbon isotopes and lake sediments do not point consistently to any simple trend or climate driver of evolutionary change comparison of environmental indicators cautions against an exclusive focus on any given end member of environmental fluctuation driest or wettest warmest or coolest and argues for the impact of the entire range of variability in shaping evolutionary change a model of alternating high and low climate variability for tropical africa further implies that specific environmental indicators reflect different aspects of east african environmental dynamics the model may thus help reconcile some of the conflicting interpretations about the environmental drivers of hominin evolution first and last appearances of hominin lineages benchmark biogeographic events and the emergence of key adaptations and capacities to alter the surroundings are consistently concentrated in the predicted longest intervals of high climate variability the view that emerges is that important changes in stone technology sociality and other aspects of hominin behavior can now be understood as adaptive responses to heightened habitat instability

estimating the age of formation of lakes an example from lake tanganyika east african rift system age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution in the absence of dated core material from the lake s sedimentary basement several techniques have been used to generate such age estimates the most common of these herein called the reflection seismic radiocarbon method rsrm combines estimates of short term sediment accumulation rates derived from radiocarbon dated cores and depth to basement estimates derived from reflection seismic data at or near the same locality to estimate an age to basement age estimates form the rsrm suggest that the structural basins of central lake tanganyika began to form between 9 and 12 ma estimates for the northern and southern basins are younger 7 to 8 ma and 2 to 4 ma respectively the diachroneity of estimates for different segments of the lake is equivocal and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift the rsrm age estimates for lake tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the east african rift system 31 refs 3 figs 1 tab

majorite fractionation and genesis of komatiites in the deep mantle abstract melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone a model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid alumina depleted komatiite generally observed in the early archean can be generated by any degree of partial melting at depths from 450 to 650 km this type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km alumina enriched komatiite can be generated by remelting of the residual mantle after separation of alumina depleted komatiite alumina undepleted komatiite generally observed in the late archean can not be formed by a low degree of partial melting of the primitive mantle at any depths this type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km as an alternative model the alumina depleted komatiite can be formed by a high degree of partial melting of the archean upper mantle depleted in majorite such heterogeneity in the archean upper mantle was generated by a global melting event succeeding the accretion of the earth and erased by convection by the later archean hf isotope data seem to favour the former model of the petrogenesis of komatiite i e single stage melting of the primitive mantle at various depths

paleo environmental cyclicity in the early silurian yangtze sea south china tectonic or glacio eustatic control abstract the yangtze sea of the south china craton experienced strong environmental perturbations during the first 5 6 myr of the silurian the lower silurian lungmachi formation in the jiaoye 1 drillcore chongqing china records three sea level cycles stratigraphic sequences characterized by rapid deepening at the base of each cycle followed by slow shallowing each deepening event was associated with shifts toward higher marine productivity more intensely reducing bottomwater conditions reduced watermass restriction lower detrital input and enhanced siliciclastic weathering intensity we infer control by glacio eustasy in which deglacial sea level rises and concurrent climatic warming triggered the observed environmental changes shallowing was associated with renewed glaciation and characterized by the opposite set of conditions the results of this study thus provide evidence in support of continued continental glaciation during the early silurian i e following the termination of the major end ordovician hirnantian glaciation the study core also exhibits a long term shallowing trend through the entire lower silurian the origin of which may have been tectono epeirogenic uplift of the south china craton during the kwangsian orogeny

komatiites i eruption and flow because of their high eruption temperatures and ultrabasic composition komatiite lavas had low viscosities which typically ranged from 0 1 to 10 pa s a major consequence of this low viscosity is that most lavas erupted as turbulent flows an analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms 1 and reynolds numbers much greater than the critical value of 2000 the lavas would have remained turbulent for most or all of their subsequent flow and emplacement typical horizontal flow rates are estimated to range from 0 5 to 100 m2 s per unit width of flow such turbulent lava flows would have lost their heat by convection to the surroundings at rates which are orders of magnitude greater than the rates for laminar flows which cool by conduction a quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 c hr to a few c hr 1 while the flows remained turbulent these rates are in an appropriate range to cause phenomena such as high nucleation rates strong supersaturation of the lava delayed nucleation of olivine and skeletal or dendritic crystal morphologies komatiites often flowed over ground composed of rocks with lower melting temperatures it is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels melting rates at the lava source are calculated at several metres per day and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream the experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel some sinuous rilles observed in the mare regions of the moon are thought to have formed by thermal erosion hulme 1973 they provide analogues of the channels postulated to form in komatiite eruptions where conditions were in fact more favourable for thermal erosion an assessment of the role of olivine crystals precipitated in the cooling turbulent flows indicates that they will remain in suspension until the lava has come to rest contamination of komatiite lava by underlying rock can be as much as 10 per cent some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios such as ti zr and y zr and the rare earth patterns of derivative lavas to vary substantially some of the variations in such geochemical parameters which are often ascribed to mantle heterogeneity also could have resulted from assimilation of the ground assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age the thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows it is proposed that ores can form by assimilation of sulphur rich sediment which combines with ni from the komatiite to form an immiscible liquid

komatiitic sills and multigenerational peperite at dundonald beach abitibi greenstone belt ontario volcanic architecture and nickel sulfide distribution the density and the tectonic stresses in the deep crust and the physical properties of komatiitic magmas determine the level to which they will rise but once in the near surface environment the density and rheology of the near surface rocks consolidated and dense lava flows and sedimentary rocks versus unconsolidated sedimentary or volcaniclastic deposits govern whether they will be emplaced as lava flows invasive flows or sills where near surface strata are competent i e flow dominated or consolidated sedimentary and or volcaniclastic successions komatiitic magma erupts as flows that form extensive lava shields e g kambalda however where near surface strata are not competent i e unconsolidated volcaniclastic and or sediment dominated successions komatiitic magmas typically are emplaced as high level sills that increase the bulk density of the volcano sedimentary pile and eventually allow the eruption of lava and the construction of complex sub volcanic volcanic lava shields e g dundonald and shaw dome abitibi belt raglan cape smith belt pechenga kola peninsula thompson ni belt manitoba the latter environment is illustrated in the volcano sedimentary succession in dundonald township which is only weakly metamorphosed and deformed and superbly exposed in glacially polished outcrops the volcano sedimentary succession in this area comprises from base to top 1 the mcintosh formation composed of a succession of pillowed and massive intermediate volcanic flows 2 the dundonald formation composed of a lower section of komatiite sills argillites and felsic volcaniclastic deposits and an upper section of komatiite flows komatiitic sills and pillowed intermediate volcanic flows and 3 the frederick house lake formation composed of massive and pillowed mafic flows the distribution and thickness of argillites and felsic volcaniclastic rocks define a synvolcanic graben in which the dundonald south and alexo ni cu pge deposits occur within the center and the margin respectively sills and peperites in the lower komatiitic succession at dundonald beach exhibit a multigenerational emplacement history recording progressive lithification and increases in the bulk density and rheological strength of the unconsolidated argillites which ultimately permitted the eruption of lavas at alexo importantly the nature of the near surface rocks also influences the localization of ni cu pge deposits in lava shields e g kambalda the initial eruptions are typically most voluminous and if erupted at sufficient flow rates form channelized flows conducive to thermomechanical erosion of sulfur rich footwall rocks in sub volcanic volcanic lava shields however channelized units may occur within the subvolcanic plumbing system and or within overlying lavas where only sills are channelized e g thompson mineralization will occur only within the subvolcanic environment where only the flows are channelized e g damba silwane zimbabwe mineralization will occur only within the volcanic environment where both sills and lava flows are channelized e g dundonald shaw dome raglan the distribution of the mineralization is more diverse and may occur as subsea or sea floor ni cu pge deposits

a late cenozoic age for long wavelength surface uplift of the atlas mountains of morocco the atlas mountains have been uplifted by two mechanisms cenozoic thickening of the crust and thinning of the mantle lithosphere due to a buoyant thermal anomaly previously inferred by indirect criteria to have started some 15 ma because crustal shortening related uplift and mantle related uplift affect the topography at different spatial scales we use scattered direct surface evidence to clarify the palaeoelevation dynamics uplifted messinian shallow marine sediments in the

fractionation of rare earth elements within bone mineral a natural cation exchange system the distribution of rare earth elements rees within fossil bones is controlled by their partition coefficients between apatite and pore waters and the rate of diffusion through the bone using simple theoretical models we show that rees are strongly fractionated from one another during diffusive transport and adsorption fractionation occurs due to the relative ease of substituting ree ions of differing ionic radius into the ca sites in the apatite lattice and the degree of fractionation is dependent on the rate of diffusion of ions within bone and therefore the rate of recrystallisation variations in bone thickness recrystallisation rate and potentially pore water composition may all influence the relative distribution of rees and thus ree ratios within bones increases in bone thickness and reductions in either diffusion coefficients or the duration of ree uptake lead to enhanced fractionation of rees in our model simulations interpretations of ree ratios in fossil bones either for palaeoenvironmental or taphonomic applications must consider how fractionation will influence ree ratios within bones particularly when interpreting spatially resolved analyses within single bones

uplift of africa as a potential cause for neogene intensification of the benguela upwelling system the benguela current cooled over the past 12 million years numerical modelling suggests that uplift of parts of africa during this time enhanced coastal low level winds and promoted greater upwelling of cool subsurface waters

pleistocene sea level fluctuations and human evolution on the southern coastal plain of south africa abstract humans evolved in africa but where and how remain unclear here it is proposed that the southern coastal plain scp of south africa may have served as a geographical point of origin through periodic expansion and contraction isolation in response to glacial interglacial changes in sea level and climate during pleistocene interglacial highstands when sea level was above 75 m human populations were isolated for periods of 360 3400 25 yr generations on the scp by the rugged mountains of the cape fold belt climate and vegetation barriers the scp expands five fold as sea level falls from 75 to 120 m during glacial maxima to form a continuous unobstructed coastal plain accessible to the interior an expanded and wet glacial scp may have served as a refuge to humans and large migratory herds and resulted in the mixing of previously isolated groups the expansive glacial scp habitat abruptly contracts by as much as one third in 300 yr during the rapid rise in sea level associated with glacial terminations rapid flooding may have increased population density and competition on the scp to select for humans who expanded their diet to include marine resources or hunted large animals successful adaptations developed on an isolated scp are predicted to widely disperse during glacial terminations when the scp rapidly contracts or during the initial opening of the scp in the transition to glacial maxima the hypothesis that periodic expansion and contraction of the scp as well as the coastal plain of north africa contributed to the stepwise origin of our species over the last 800 thousand years kyr is evaluated by comparing the archeological dna and sea level records these records generally support the hypothesis but more complete and well dated records are required to resolve the extent to which sea level fluctuations influenced the complex history of human evolution

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

geodynamic significance of the trm segment in the east african rift w tanzania active tectonics and paleostress in the ufipa plateau and rukwa basin abstract the tanganyika rukwa malawi trm rift segment in western tanzania is a key sector for understanding the opening dynamics of the east african rift system ears in an oblique opening model it is considered as a dextral transfer fault zone that accommodates the general opening of the ears in an nw se direction in an orthogonal opening model it accommodates pure dip slip normal faulting with extension orthogonal to the rift segments and a general e w extension for the entire ears the central part of the trm rift segment is well exposed in the ufipa plateau and rukwa basin within the paleoproterozoic ubende belt it is also one of the most seismically active regions of the ears we investigated the active tectonic architecture and paleostress evolution of the ufipa plateau and adjacent rukwa basin and in order to define their geodynamic role in the development of the ears and highlight their pre rift brittle tectonic history the active fault architecture fault kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the trm segment two pre rift brittle events are evidenced an older brittle thrusting is related to the interaction between the bangweulu block and the tanzanian craton during the late pan african early paleozoic it was followed by a transpressional inversion during the early mesozoic this inversion stage is the best expressed in the field and caused dextral strike slip faulting along the fault systems that now control the major rift structures it has been erroneously interpreted as related to the late cenozoic ears which instead is characterized by pure normal faulting our third and last stress stage

analysis of gold in surface waters from australian goldfields an investigation into direct hydrogeochemical prospecting for gold 70 water samples dissolved gold discriminated from particulate gold by filtration through a 1 2 mu filter the particulate gold concentrations showed excessive scatter which made their use for locating gold anomalies unpromising dissolved gold occurred in concentrations ranging from or 0 3 to 2 5 parts per trillion to as high as 130 parts per trillion the limit of detection of the analytic method was 0 3 parts per trillon modified journal abstract

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

geochemistry of mineralised and barren komatiites from the perseverance nickel deposit western australia the perseverance formerly known as agnew and the neighbouring rocky s reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs the deposits are overlain by the perseverance ultramafic complex which consists of a central dunite lens 700 m thick by 3 km wide flanked by olivine orthocumulates and spinifex textured komatiites a suite of samples of a zones of komatiite flows from the area has been analysed for major and trace elements rare earth elements and platinum group elements the samples fall into two distinct geochemical groups on the basis of major and rare earth elements 1 1 samples from a zones of spinifex textured flows flanking the central dunite show well constrained linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation 2 2 samples from the perseverance mine mineralised and basal flows from rocky s reward and from unmineralised flows at the base of the perseverance ultramafic complex all show evidence for lree enrichment relative to the first group they are also generally higher in sio2 and lower in feo for a given mgo content geochemical features of the second group are consistent with the effects of crustal assimilation this is in line with current models for nickel sulphide genesis based largely on kambalda which emphasise the role of thermal erosion of sulphidic footwall sediments the chalcophile element ni ir ru pt pd au and cu concentrations in a zone samples from the study area are typical of komatiites komatiites from the perseverance mineralised units and from rocky s reward show evidence of depletion of the more strongly chalcophile pge relative to ni and cu this feature is also evident in massive and matrix sulphide ore samples although less evident in the main perseverance cloud sulphides quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma the parental composition is represented by the spinifex textured units flanking the dunite lens compositions of komatiite liquids sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma olivine and assimilated sulphidic sediment models involving combined assimilation and fractional crystallisation in which olivine and sulphide are fractionally segregated as they form are much less successful at matching the data in physical terms this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0 5 to 1 in order to generate the observed degrees of lree enrichment in flowtops ni depletion in olivine and pge depletion in sulphides and flow tops

early middle pleistocene transitions an overview and recommendation for the defining boundary the early middle pleistocene transition c 1 2 0 5 ma sometimes known as the mid pleistocene revolution represents a major episode in earth history low amplitude 41 ka obliq uity forced climate cycles of the earlier pleistocene were replaced progressively in the later pleistocene by high amplitude 100 ka cycles these later cycles are indicative of slow ice build up and subsequent rapid melting and imply a transition to a strongly non linear forced climate system changes were accompanied by substantially increased global ice volume at 940 ka these climate transformations particularly the increasing severity and duration of cold stages have had a pro found effect on the biota and the physical landscape especially in the northern hemisphere this review assesses and integrates the marine and terrestrial evidence for change across this transition based on the literature and especially the following 17 chapters in the present volume orbital and non orbital climate forcing palaeoceanography stable isotopes organic geochemistry marine micropalaeontology glacial history loess palaeosol sequences pollen analysis large and small mammal palaeoecology and stratigraphy and human evolution and dispersal are all considered and a series of discrete events is identified from marine isotope stage mis 36 c 1 2 ma to mis 13 c 540 460 ma of these the cold mis 22 c 880 870 ka is perhaps the most profound however we here endorse earlier views that on practical grounds the matuyama brunhes palaeomagnetic chron boundary mid point at 773 ka with an estimated duration of 7 ka would serve as the best overall guide for establishing the early middle pleistocene subseries boundary

mineralogical and compositional changes in bones exposed on soil surfaces in amboseli national park kenya diagenetic mechanisms and the role of sediment pore fluids abstract bones exposed on tropical savannah grasslands of amboseli national park kenya undergo extensive post mortem alteration within 40 years a combined analytical approach involving tem microscopy trace metal analysis ftir spectroscopy and petrographic analysis has revealed a complex dynamic diagenetic environment operating within exposed bones driven by evaporative transport of soil water from the bone soil interface to the upper exposed surface of the bone this process results in extensive bone soil water interaction and is responsible for increases in the concentrations of trace elements such as ba and la of 100 1000 within 15 years the maximum and mean size of bone crystallites increases with continued exposure this change in mean crystallite length is correlated positively with increases in bone crystallinity which in turn is associated with degradation of the bone protein microbial decomposition is rarely observed in the amboseli bones but where present resulted in severe dissolution reprecipitation of bone mineral many bones showed extensive permineralization of the larger vascular spaces with calcite and barite and to a lesser extent crandallite permineralization of unburied bones may account for 95 reduction in macro micron millimeter scale porosity in the bone within 2 years of death we produce a model for pre burial diagenesis of bone in arid tropical environments that highlights extensive alteration of bone chemistry within 1 40 years post mortem

geochemistry of volcanic rocks associated with cu zn and ni cu deposits in the abitibi subprovince volcanogenic massive sulfide vhms deposits in the abitibi subprovince are preferentially associated with volcanic successions containing 150 m thicknesses of felsic volcanic rocks approximately 50 by area of volcanic terranes and are found within volcanic sequences of at least three distinct affinities group i which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes is composed of bimodal tholeiitic basalt basaltic andesite and high silica rhyolite the basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element ree contents low light to heavy ree ratios most with la n yb n 0 8 3 and strong negative eu anomalies the kamiskotia matagami and chibougamau lower cycle volcanogenic massive sulfide areas all of which are also underlain by large synvolcanic gabbroic complexes are associated with group i volcanic sequences the kidd creek potter normetal and horne deposits are also included in this category group ii which is host to one third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes is composed of bimodal transitional tholeiitic to calc alkalic andesitc and rhyolite characterized by intermediate high field strength element contents and slightly higher ree ratios la n yb n 1 4 noranda camp excluding the horne deposit and val d or camp group iii is host to only one known deposit the selbaie mine which is unusual in that much of its mineralization cuts stratigraphy the selbaie mine sequence contains calc alkalic andesite rhyolite with relatively low high field strength element and ree contents and higher ree ratios la n yb n 3 9 the vast majority of volcanogenic massive sulfide bearing mafic and felsic volcanic rocks in the abitibi subprovince have la n yb n 5 barren volcanic sequences are group iv calc alkalic basaltic andesite to rhyodacite with low high field strength element and relatively high ree ratios la n yb n 8 20 represented by the upper skead group the quebec hunter mine group and the upper cycle chibougamau rocks and group v mafic to felsic alkalic volcanic rocks with high ree ratios la n yb n 12 62 represented by the timiskaming opemisca and ridout series rocks group i is most similar to thickened oceanic rift suites e g galapagos spreading center iceland east rift group ii is similar to suites in rifled island arcs e g hokuroku district japan and group iii is comparable to continental arc suites e g southern volcanic zone central chile groups iv and v are comparable to arc related suites derived from metasomatized mantle with variable amounts of crustal contamination e g setouchi area japan roman province italy known ni cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills represented by chill compositions with high mgo contents 20 35 wt anhydrous very low incompatible element contents and depleted signatures la n sm n 0 5 0 8 zr y 2 5 alexo marbridge and shaw dome deposits in comparison to most barren komatiites in the abitibi subprovince their depleted trace element signatures indicate minimal crustal assimilation which would appear contrary to the widely held view that crustal contamination e g addition of silica sulfur oxygen of a mantle derived melt is necessary for sulfur saturation and magmatic sulfide segregation this problem may be reconciled by considering the dynamics of komatiite flows lesher and arndt 1990 where contaminated present address 23 euclid avenue ottawa ontario canada kis 2w2 komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled when considered together with geologic features such as evidence for thermal erosion in the footwall langmuir deposits and the presence of sulfidic footwall rocks shaw dome deposits the distinctive depleted geochemical signature of mineralized komatiites is useful in exploration for ni cu deposits

large mammal turnover in africa and the levant between 1 0 and 0 5 ma f aunal change at the early middle pleistocene boundary in europe has long been a topic for discussion however analyses of large mammal turnover at this time in africa have been lacking largely because of the low number of sites dated to this interval recent work particularly in the last 10 years has resulted in a much larger published sample of sites and we synthesize these data in this paper in our multivariate twinspan analyses of african and levantine large mammal faunas we found that localities were subdivided by geographic regions not by age there were some small scale changes with the appearance or extinction of particular taxa but there was no large scale turnover such as that seen in europe the levant was included as a possible route for faunal interchange with east africa but no similarities were found between these areas it therefore appears that the modern zoogeographic separation of the levant and north africa into the palaearctic region and sub saharan africa into the african region can be traced back to at least the early middle pleistocene boundary

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

stratigraphy and u pb zircon geochronology of kidd creek implications for the formation of giant volcanogenic massive sulphide deposits and the tectonic history of the abitibi greenstone belt within the context of a structural stratigraphic analysis of the giant kidd creek cu zn ag deposit precise u pb zircon ages have been obtained for five critical rock units associated with the deposit massive flow banded footwall rhyolite is dated at 2714 3 1 2 ma slightly younger than a previously published age of 2717 2 ma on nearby rhyolite breccia extrusive rhyolite in the immediate hanging wall to the main ore lenses is dated at 2711 5 1 2 ma leaving an interval of one to several million years for the formation of the deposit we consider the demonstrated longevity of the kidd creek hydrothermal system as a major control on its giant size age constraints together with the size of the deposit suggest a time averaged rate of base metal sulphide deposition of about 10 100 t year peak rates were likely one to possibly two orders of magnitude above time averaged rates hence 100 1000 t year or higher detrital zircon ages on greywackes in the structural footwall to the deposit support structu

african climate change and faunal evolution during the pliocene pleistocene abstract environmental theories of african faunal evolution state that important evolutionary changes during the pliocene pleistocene interval the last ca 5 3 million years were mediated by changes in african climate or shifts in climate variability marine sediment sequences demonstrate that subtropical african climate periodically oscillated between markedly wetter and drier conditions paced by earth orbital variations with evidence for step like 0 2 ma increases in african climate variability and aridity near 2 8 ma 1 7 ma and 1 0 ma coincident with the onset and intensification of high latitude glacial cycles analysis of the best dated and most complete african mammal fossil databases indicates african faunal assemblage and perhaps speciation changes during the pliocene pleistocene suggesting more varied and open habitats at 2 9 2 4 ma and after 1 8 ma these intervals correspond to key junctures in early hominid evolution including the emergence of our genus homo pliocene pleistocene shifts in african climate vegetation and faunal assemblages thus appear to be roughly contemporary although detailed comparisons are hampered by sampling gaps dating uncertainties and preservational biases in the fossil record further study of possible relations between african faunal and climatic change will benefit from the accelerating pace of important new fossil discoveries emerging molecular biomarker methods for reconstructing african paleovegetation changes tephra correlations between terrestrial and marine sequences as well as continuing collaborations between the paleoclimatic and paleoanthropological communities

application of 2d and 3d geoelectrical resistivity imaging for engineering site investigation in a crystalline basement terrain southwestern nigeria orthogonal set of 2d geoelectrical resistivity field data consisting of six parallel and five perpendicular profiles were collected in an investigation site using the conventional wenner array seven schlumberger soundings were also conducted on the site to provide id layering information and supplement the orthogonal 2d profiles the observed 2d apparent resistivity data were first processed individually and then collated into 3d data set which was processed using a 3d inversion code the 3d model resistivity images obtained from the inversion are presented as horizontal depth slices some distortions observed in the 2d images from the inversion of the 2d profiles are not observed in the 2d images extracted from the 3d inversion the survey was conducted with the aim of investigating the degree of weathering and fracturing in the weathered profile and thereby ascertaining the suitability of the site for engineering constructions as well as determining its groundwater potential

the langmuir volcanic peridotite associated nickel deposits canadian equivalents of the western australian occurrences the 2 7 b y old abitibi greenstone belt is a crudely s shaped zone of metamorphosed volcanic subvolcanic and sedimentary rocks and associated intrusions through it are scattered a dozen small nickel deposits in volcanic peridotites their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous mgo with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows at least one deposit dumont is hosted by a dunitic intrusion others e g dundonald langmuir 2 are partially hosted in sulfidic iron formations mining of the langmuir 2 deposit permitted extensive underground mapping core logging and sample collecting the other deposits are known mainly from poor surface exposures and drill core the langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in western australia a magmatic origin is supported by the localization of the main ore zone in a paleotrough by the presence of a possible volcanic feeder beneath the ore and by the regular sequence through the basal ultramafic flow of massive ore overlain by net textured ore then by barren peridotite and lastly by a spinilex textured cap to the flow an unusual spinilex textured and sulfide bearing rock and skeletal chromite also support a magmatic origin as do the base and precious metal contents of the ores the high s se ratios averge 23 000 and slightly lower sulfur isotopic ratios average 0 5 per mil delta 34 s compared to most magmatic nickel sulfide deposits are similar to the ratios for windarra western australia and may be explained by assimilation of sulfide rich cherty iron formation other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore and the high sulfur content of some ores these features result from deformation and greenschist facies metamorphism but surviving features of an earlier magmatic history argue against the ores being formed metamorphically evidence of contemporaneous fumarolic activity was found at the south end of the mine but its importance as an ore forming process was not supported by detailed petrological and geochemical studies limited data from the other deposits in the abitibi belt suggest a common origin for the langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced the volume of contained nickel sulfides the presence of sulfidic iron formations and the severity of alteration and metamorphism the deposits in western australia probably were formed by similar processes in particular the langmuir deposits display strikingly similar sulfide chemistries to the windarra deposits which are probably dae to assimilation of sulfide rich cherty iron formation