spinels and mg ilmenites from the noril sk 1 and talnakh intrusions and other mafic rocks of the siberian flood basalt province spinels from the mineralized noril sk 1 and talnakh intrusions show a broad and distinctive compositional spectrum from normal chromite to ti enriched aluminous magnetite in the talnakh intrusion in particular the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase spinels enclosed in olivines are less ti enriched than those outside olivine grains compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid the advanced nature of the process is attributed to the open nature of the intrusions continuous prolonged flow through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness the barren chibacheka river intrusion shows some of the same features but differs from talnakh in that spinel olivine pairs record equilibration temperatures higher than those at talnakh temperatures recorded by spinel olivine pairs at talnakh are anomalously low for such a small intrusion again indicating prolonged slow cooling consistent with a flow through model for the sill spinel compositions support a hypothesized comagmatic relationship between the chibacheka river intrusion and the ti cr rich gudchichinsky picritic basalts spinels in the noril sk and talnakh intrusions imply a parent magma with a lower cr al ratio than the gudchichinsky picritic basalts spinels from the siberian intrusions show a similar range in compositions to the feeder intrusions of the karoo flood basalt province jinchuan spinels share the features of high ti contents and anomalously low equilibration temperatures with the noril sk talnakh spinels mg ilmenites are a feature of the siberian and jinchuan intrusions they are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding mg rich cumulus phases their existence implies moderately but not unusually ti rich parent magmas consistent with compositions of the siberian traps lavas

iron oxide copper gold iocg deposits through earth history implications for origin lithospheric setting and distinction from other epigenetic iron oxide deposits the iron oxide copper gold iocg group of deposits initially defined following discovery of the giant olympic dam cu u au deposit has progressively become too embracing when associated deposits and potential end members or analogs are included the broader group includes several low ti iron oxide associated deposits that include iron oxide p rich iron oxide f and ree rich fe or cu au skarn high grade iron oxide hosted au cu carbonatite hosted cu ree and f rich and iocg sensu stricto deposits consideration of this broad group as a whole obscures the critical features of the iocg sensu stricto deposits such as their temporal distribution and tectonic environment thus leading to difficulties in developing a robust exploration model the iocg sensu stricto deposits are magmatic hydrothermal deposits that contain economic cu and au grades are structurally controlled commonly contain significant volumes of breccia are commonly associated with presulfide sodic or sodic calcic alteration have alteration and or brecciation zones on a large commonly regional scale relative to economic mineralization have abundant low ti iron oxides and or iron silicates intimately associated with but generally paragenetically older than fe cu sulfides have lree enrichment and low s sulfides lack of abundant pyrite lack widespread quartz veins or silicification and show a clear temporal but not close spatial relationship to major magmatic intrusions these intrusions where identified are commonly alkaline to subalkaline mixed mafic even ultramafic to felsic in composition with evidence for mantle derivation of at least the mafic end members of the suite the giant size of many of the deposits and surrounding alteration zones the highly saline ore fluids and the available stable and radiogenic isotope data indicate release of deep volatile rich magmatic fluids through devolatization of causative mantle derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional scale fluid flow paths precambrian deposits are the dominant members of the iocg group in terms of both copper and gold resources the 12 iocg deposits with 100 tonnes t resources are located in intracratonic settings within about 100 km of the margins of archean or paleoproterozoic cratons or other lithospheric boundaries and formed 100 to 200 m y after supercontinent assembly their tectonic setting at formation was most likely anorogenic with magmatism and associated hydrothermal activity driven by mantle underplating and or plumes limited amounts of partial melting of volatile rich and possibly metal enriched metasomatized early precambrian subcontinental lithospheric mantle sclm fertilized during earlier subduction probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the iocg deposits preservation of near surface deposits such as olympic dam is probably due to their formation above buoyant and refractory sclm which resisted delamination and associated uplift most precambrian iron oxide p rich or magnetite apatite kiruna type deposits have a different temporal distribution apparently forming in convergent margin settings prior to or following supercontinent assembly it is only in the phanerozoic that iocg and magnetite apatite deposits are roughly penecontemporaneous in convergent margin settings the phanerozoic iocg deposits such as candelaria chile occur in anomalous extensional to transtensional zones in the coastal cordillera which are also the sites of mantle derived mafic to felsic intrusions that are anomalous in an andean context this implies that special conditions possibly detached slabs of metasomatized sclm may be required in convergent margin settings to generate world class iocg deposits it is likely that formation of giant iocg deposits was mainly a precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized sclm generally smaller and rarer phanerozoic iocg deposits formed in tectonic settings where conditions similar to those in the precambrian were replicated

seismic velocity model of the crust and uppermost mantle around the mirnyi kimberlite field in siberia abstract we present the first detailed seismic velocity models of the crust and uppermost mantle around the mirnyi kimberlite field in yakutia siberia we have digitized vintage seismograms that were acquired in 1981 and 1983 by use of taiga analogue seismographs along two perpendicular seismic profiles the 370 km long northwest striking profile i across the kimberlite pipe was covered by 41 seismographs which recorded seismic signals from 21 chemical shots along the line including one off end shot the perpendicular 340 km long profile ii across profile i ca 30 km to the south of the mirnyi kimberlite field was covered by 45 seismographs which recorded seismic signals from 22 chemical shots including four off end shots each shot involved detonation of between 1 5 and 6 0 tons of tnt distributed in individual charges of 100 200 kg in shallow water both velocity models show normal cratonic structure of the ca 45 km thick crust with only slight undulation of the moho however relatively small seismic velocity is detected to 25 km depth in a ca 60 km wide zone around the kimberlite pipe surrounded by elevated velocity 6 3 km s in the upper crust the lower crust has a relatively constant velocity of 6 8 6 9 km s it appears relatively unaffected by the presence of the kimberlite field extremely large p wave velocity 8 7 km s of the sub moho mantle is interpreted along profile i except for a 70 km wide zone with a normal pn velocity of 8 1 km s below the kimberlite profile ii mainly shows pn velocities of 8 0 8 2 km s with unusually large velocity 8 5 km s in two ca 100 km wide zones at its southwestern end one zone being close to the kimberlite field the nature of these exceptionally large sub moho mantle velocities is not yet understood the difference in velocity in the two profile directions indicates anisotropy but the effect of unusual rock composition e g from a high concentration of garnet cannot be excluded

mire development and environmental change barrington tops new south wales australia abstract sediments began accumulating in nine mires on barrington tops on the northern tablelands of new south wales before 11 000 yr b p and peat became common in the region by 8600 yr b p sedimentation rates were low but increased markedly about 3000 yr b p and again around 500 yr b p as a result of regional climatic change a comparison of the results with other environmental data from the region suggests that conditions in the early holocene were warmer and moister than at present but that cooler and drier environments developed about 3000 yr b p in the last 500 yr a slight warming and either increased precipitation or cloudiness has become evident

climatic controls on holocene lake level changes in europe sensitivity experiments with a simple water balance model were used to constrain the possible climatic causes of distinct holocene patterns of lake level variation in different regions of europe lakes in s sweden were low at 9 ka high around 6 5 ka low again around 4 ka and are high now lakes in estonia show similar but weaker trends lakes in s france were highest around 9 ka lowest around 4 ka intermediate now lakes in greece were also high around 9 ka but continued rising until 7 5 ka then fell gradually from 5 ka with a brief high phase around 3 ka and are low now the model was forced with insolation anomalies deduced from orbital variations temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone inferred from reconstructed changes in the equator to pole temperature gradient in order to evaluate the effects of resultant evaporation changes on catchment water balance the resulting simulated changes in runoff precipitation minus actual evapotranspiration were slight and frequently opposite to the observed trends larger changes in precipitation are plausible and are required to explain the data the required precipitation increase in n europe from 9 ka low to 6 ka high is suggested by gcm experiments to have been a consequence of interacting insolation and residual ice sheet effects on the atmospheric circulation over the north atlantic the explanation of other observed changes including the drying trend during the holocene in s europe has not been provided by gcm experiments to date explanations may lie in changes in mesoscale circulation sea surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way

boninite like rocks from the palaeoproterozoic greenstone belt of bogoin central african republic geochemistry and petrogenesis the upper pillowed amphibolites of bogoin upab are part of the supracrustal series of the bogoin boali greenstone belt western central african republic they are underlain by al depleted komatiites back arc tholeiites and arc related greywackes they are interbedded with undepleted komatiites and overlain by banded iron formations bifs pb pb and sm nd isotopic data yield an imprecise isochron age of about 2 3 ga which is believed to represent the time of greenstone deposition no magmatic minerals are preserved in the upab but pseudomorphs after olivine ca poor and ca rich pyroxenes are present the secondary paragenesis is related to upper greenschist facies metamorphism immobile element geochemistry suggests that upab are not siliceous high magnesium basalts but are very similar to modern boninites they are depleted in p and ti enriched in sr and zr and show high zr sm sr sm al2o3 tio2 and low ti v and ti sc values the ree patterns are u shaped or lree depleted hree enriched the overprint of a magma mixing with the interbedded komatiites on a partial melting event can explain most of the differences noted between upab and modern boninites upab genesis is explained by mixing depleted and enriched components sm nd isotopic data and major element contents suggest participation of a trondhjemite like melt rather than sediments as the enriched component the depleted component can be produced by remelting of a peridotitic residue following extraction of tholeiites similar to the tholeiites of the lower unit the ascending diapir that yielded the upper komatiites may have furnished the necessary heat input to the remelting event the presence of back arc tholeiites arc related greywackes and boninite like upab argues strongly for a compressive margin plate boundary setting for the bogoin greenstone belt

estimating carbon accumulation rates of undrained mires in finland application to boreal and subarctic regions equations based on empirical relationships of peat physical properties were used to estimate the average long term apparent rate of carbon accumulation lorca in finnish mire vegetation regions the results were generalized to the boreal and subarctic regions analyses of 1302 dated peat cores were used to infer carbon accumulation for each mire vegetation region of finland the area weighted lorca for finnish undrained mire areas was 18 5 g m 2 yr 1 and the total carbon sink 0 79 tg yr 1 1 tg 1012g the total carbon pool of finnish undrained mires was estimated as 2257 tg the aapa mire region included 80 of the total net accumulation rate of carbon and 85 of the total carbon reservoirs of finnish undrained mires lorca was signi cantly higher in the raised bog region 26 1 g m 2 yr 1 compared with the aapa mire region 17 3 g m 2 yr 1 and bogs generally had a higher lorca 20 8 g m 2 yr 1 than fens 16 9 g m 2 yr 1 the total c sink for boreal and subarctic mires was estimated at 66 tg yr 1 whi

rip current type spacing and persistence narrabeen beach australia abstract long term daily rip observations on narrabeen beach australia were made together with daily measurements of wave height period direction surf zone width and beach state a total of 3513 rips were observed on 270 days over a 19 month period results show that rips accompany moderate to high waves and are associated with intermediate beach types rip spacing follows wave conditions increasing in spacing size and intensity as waves rise and conversely as they fall rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions rips are classified into three types erosion mega and accretion erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states they accompany general beach erosion they are widely spaced narrabeen ys 300 500 m 100 200 m increasing in size and intensity with the waves until the fully dissipative ripless state is reached they are both temporally and spatially highly variable persisting in one location for only hours to a day or so except where topographically controlled they are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone mega rips are large scale 1 km topographically controlled erosion rips that persist when nearshore and or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf zone dynamics that in turn drive the rip circulation accretion rips prevail during stable or falling wave conditions usually following erosion rip formation they are associated with general beach accretion are more closely spaced narrabeen ys 170 250 m 100 m less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar beach morphology of the longshore bar trough rhythmic bar and beach and transverse bar and rip beach states they are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable they are fair weather rips which may disappear by infilling in the low tide terrace beach state or be destroyed by rising seas rips generally are absent from the reflective low tide terrace and fully dissipative beach states

radiocarbon evidence for rates of lateral expansion in raised mires in southern finland radiocarbon ages from the bottom peat deposits and detailed morphometric measurements at five peatlands in the boreal region of southern finland were used to reconstruct the holocene patterns of raised mire development the results suggest that the areal expansion of the mires has been extremely rapid at times up to several hundreds hectares per millennium but the periods of more rapid expansion were not coincident among the mires the rate of lateral extension was shown to depend mainly on the topography of the mineral substrate horizontal expansion did not always correlate with vertical growth mires expanded slowly between ca 6000 and 4000 yr b p probably as a consequence of drier climatic conditions at that time the mires also expanded slowly during the two to three recent millennia the reason for this most probably lying in steepening of the bottom gradient toward the marginal areas of the present mires the results point to a substantial measure of local environmental control over the lateral extension in raised mire ecosystems

the motloutse complex and the zimbabwe craton limpopo belt transition in botswana abstract the archaean rocks of northeastern botswana are divided into named lithostratigraphic complexes each a different assemblage of stratiform lithologies granitoids and gneisses and each with differences in metamorphic grade structural style and metallic mineralisation the motloutse complex lies between four other complexes it includes granitoids and orthogneisses typical of the zimbabwe craton to the east detached segments of the quartzite carbonate pelite association of the anomalous matsitama schist belt to the west and migmatites similar to those of the limpopo belt to the south but is itself a distinctive lithological assemblage it shows a higher grade of metamorphism than the adjacent cratonic terranes but lacks the granulites of the limpopo belt it is delineated by relatively abrupt changes in structural style and fabric orientation of which most if not all coincide with shear zones or faults it is partly bounded against the limpopo belt by the magogaphate shear zone but also flanks the western boundary of the limpopo belt central zone west of 27 35 e the motloutse complex was formed in the late archaean between 2 7 ga and 2 5 ga ago in a zone of plutonism which separated a crustal fragment bearing the depositional basin of the matsitama shelf type sequences from the developing greenstone belts to the east during regional deformation elements of the matsitama belt and associated paragneisses were thrust from the southwest over the active magmatic zone and were then infolded with and underplated by migmatites orthogneisses and granitoids of the developing motloutse complex the terranes which were thus accreted to the southwestern margin of the zimbabwe craton during this episode of deformation and crustal thickening d1 d3 were then cut across by dextral movement on the magogaphate shear zone during d4 so juxtaposing the limpopo belt with the motloutse complex the area of the motloutse complex was previously regarded as a transitional boundary between the craton and the mobile belt but is here seen instead as an intracratonic accretionary assemblage on which the ene wsw limpopo trend was partly superimposed during d4 shearing delineation of lithostratigraphic complexes elsewhere in the zimbabwe craton and the limpopo belt may be possible and can be expected to clarify the pattern to which these provinces were assembled during the archaean prior to d4 deformation

holocene climatic variations in southern finland reconstructed from peat initiation data radiocarbon and pollen analytical dates of 71 basal peat samples from 23 sites in the boreal region of southern finland were used to reconstruct patterns of peat initiation and the subsequent sprea

a revised chronology for aeolian activity in subarctic fennoscandia during the holocene detailed sedimentological studies and parallel sampling for 14c and infra red stimulated luminescence age assays were undertaken at six sites lying beyond and below the tree line with the objective of establishing the historical development of aeolian landforms in lapland the main issues were a the timing of dune activity and processes of stabilization b the specific processes responsible for the past and contemporary activity new data indicate that many of the primary dunes were active for several millennia after deglaciation diachronous stabilization at both local and regional scales appears likely initial stabilization occurred in local interdune areas and where regional dune fields were colonized by pine generally before 7 ka local large dunes and some regional dunefields only became stable around 4 3 ka when the regional groundwater table rose and late pine colonization was possible the latest episode of aeolian activity dates from ad 1100 1650 across the area a unique deflation trigg

archean cratons diamond and platinum evidence for coupled long lived crust mantle systems diamondiferous intrusions and magmatic pt pd deposits are both concentrated on or adjacent to the oldest cratons those with 3 0 ga high grade gneiss terranes and or greenstone belts given the old age 3 0 ga of peridotitic inclusions both in diamonds and in kimberlites diamonds presumably grew in mantle depleted in basaltic major elements near the base of thickened lithosphere and below early sialic nuclei most genetic models for magmatic pt pd deposits require a pt pd enriched high mg si melt generated from analogously depleted mantle the depleted mantle was most likely formed by removal of basaltic melts that contributed to archean intracratonic greenstone belts extensive melting also decreased density and increased rigidity beneath ancient cratons favoring stabilization and preservation of thick archean continental lithosphere overall these considerations suggest that localized thick sialic crust and rigid lithosphere developed before 3 0 ga forming enduring coupled crust mantle systems below ancient sialic nuclei the sites of selectively preserved greenstone belts the data confirm that early archean terranes are highly prospective for post archean magmatic ore deposits

a sensitive high resolution record of late holocene climatic change from a raised bog in northern england proxy climatic data in the form of plant macrofossils have been analysed from a 5 m core from bolton fell moss cumbria uk detailed analysis of peat from the upper 50 cm of this core is used to demonstrate a strong correlation between changes in the relative proportion of taxa and known climatic changes over the last 1000 years the record of changes in bog vegetation contained within the peat profile is used to reconstruct changes in bog surface wetness for the latter half of the holocene as bog surface wetness is directly controlled by the prevailing climatic conditions this reconstruction can be viewed as a proxy climate record twelve radiocarbon age estimates on the 5m core suggest that between 50 and 500 cm peat accumulated at a relatively constant rate of 12 4 yr cm 1 the regular sampling intervals thus provide a time series of past bog surface wetness spectral analyses of this series indicates that wetness changes are cyclic with a ca 800 year periodicity

crustal evolution and the granulite to eclogite transition in xenoliths from kimberlites in the west african craton a petrographic mineral and bulk chemical study of a xenolith suite of granulites and eclogites from sample creek liberia and koidu sierra leone has been undertaken with a view to determining the nature of the crust upper mantle interface a broad range of xenolith compositions is present from high mgo eclogites to garnet anorthosites and a systematic afm trend is established consistent with mafic and ultramafic melt fractionation at moderate pressures 10 20 kbar a trend is established for the entire xenolith suite among bulk chemistry seismic p wave velocity and a crust mantle c m bulk chemical ratio defined as na2o k2o sio2 feo mgo mole three populations are present a granulitic crustal group sg 3 0 a granulite and eclogite transitional group sg 3 0 3 3 vp 7 2 8 0 km s c m 1 5 3 0 andanexclusivelyuppermantleeclogiticgroup ifsg 3 3 vp 8 2 8 7 km s c m 1 5 from these data and coupled with garnet clinopyroxene mineral thermometry and accessory phases e g diamond graphite coesite kyanite or the presence of plagioclase a xenolith geotherm is established based on stratigraphic sequencing and phase transition boundaries diamond and coesite bearing eclogites conform to the 40 mw m2 standard cratonic low heat flow geotherm whereas the plagioclase granulites at lower pressures correspond to an average rift geotherm of 90 mw m2 the latter is ascribed to igneous underplating onto the lower crust or to thermal perturbations from an earlier tectonic event graphite and kyanite eclogites and the transitional group in sg vp and c m ratio of eclogites and granulites fall between the 40 and 90 mw m2 reference geotherms the xenoliths are meta igneous the lower crust and uppermost mantle are mafic in composition and the petrologic moho is an intercalated interstratified horizon of eclogite and garnet granulite growth of the early crust was largely a consequence of asthenospheric depletion in which underplating rather than lateral accretion was dominant

chemostratigraphy and vegetation in two boreal mires during the holocene two peatlands a fen and a bog in southern finland were studied for peat and carbon accumulation rates chemical and physical properties of the peat profile and past vegetation to assess what stage of succession these peatlands were at the time of human impacts drainage and restoration paludification started at both sites soon after the glacial retreat around 9000 radiocarbon years bp peat accumulated at the fen site at an average rate of 0 28mm yr 1 range 0 16 0 58mm yr during development the peat was composed mostly of carex with remains of pin us sylvestris and betula spp the bog site had an average accumulation rate of 0 47mm yr range 0 25 1 19mm yr 1 and a minerotrophic sedge stage of about 2000 years in its early development after which the peat was composed mostly of sphagna the long term rates of carbon accumulation lorca varied from 6 6 to 15 3 g m 2 yr average 1l 1 g m 2 yr for the fen and from 6 1 to 21 1 g m 2 yr average 13 2g m 2 yr for the bog the main trend in pea

how the growth and freeboard of continents may relate to geometric and kinematic parameters of mid ocean spreading ridges abstract if the volume of continents has been growing since 4 ga then the area of the ocean basins must have been shrinking therefore by inferring a constant continental freeboard in addition to constant continental crustal thicknesses and seawater volume it is possible to calculate the necessary combinations of increased ridge lengths and spreading rates required to displace the seawater in the larger oceans of the past in order to maintain the constant freeboard a reasonable choice from the various possibilities is that at 4 ga ago the ridge length and spreading rates were ca 2 5 times greater than the averages of these parameters during the past 200 ma by 2 5 ga ago the ridge length and spreading rate decreased to about 1 8 times the recent average and by 1 ga ago these features became reduced to approximately 1 4 times recent averages

distribution patterns of rip frequency and intensity in algoa bay south africa a new technique of rip current recording has been employed to provide quantitative data on rip frequency and intensity during twelve aerial surveys along the shores of algoa bay south africa distinct alongshore gradients in surfzone width and beach morphodynamic state were recorded these gradients themselves related to the geography of the bay wind direction and the angle of deep sea swells result in a gradient of rip frequency and intensity in the bay rip currents always appear to be present along the sundays river beach central and northern shores but their presence along the southwest shore was restricted to easterly wind wave conditions both rip frequency and intensity were positively and significantly correlated with surfzone width and beach state over the range of sea conditions encountered beach state accounted for 62 of the alongshore variability in rip frequency and 35 of the variability in intensity

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

peatland development in relation to holocene climatic change in manitoba and saskatchewan canada two peat cores from the southern boreal forest in south central manitoba and central saskatchewan canada were analyzed for microfossils macrofossils and physicochemical properties regional vegetation and local peatland development at these sites show the influence of warm and dry climatic conditions during the middle holocene pollen assemblages at the base of the manitoba site indicate the presence of grassland parkland vegetation in the area prior to 4230 bp at the saskatchewan site boreal forest was present throughout the development of the peatland from before 5020 bp to present both the manitoba and saskatchewan records suggest a more open deciduous character of the regional vegetation cover during the middle holocene in the southernmost part of the expanding boreal forest in comparison with the modern southern mixed wood boreal forest the presence of a typha latifolia domimted marsh phase in the initial stages of development of both peatlands is indicative of marked water level fluctuati

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

geodynamic settings of mineral deposit systems mineral deposits represent extraordinary metal concentrations that form by magmatic magmatic hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and or mechanical energy near plate boundaries as they require the conjunction of specific environmental conditions to form particular mineral deposit types tend to occupy specific geodynamic niches the temporal distributions of mineral deposit types reflect both formational and preservational processes in the archaean and palaeoproterozoic these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle sclm but were decoupled by the neoproterozoic and phanerozoic as a result of evolution to thinner increasingly dense sclm the transition marks a change from mantle plume influenced plate tectonics to modern style plate tectonics with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types as mineral deposits represent an integral part of tectonic process they are essential indicators of that process and geodynamic settings and should be incorporated into any holistic tectonic terrane analysis their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data conversely such reconstructions provide a first order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover

quest for palladium application of the groves hypothesis to precambrian terranes of northeast north america a compilation of all types of magmatic deposits in precambrian terranes of northeast north america revealed a continental scale metallic zonation nickel is homogeneously distributed palladium and chromite are centered on the archean craton as predicted by the groves hypothesis whereas low ti cu u rare earth element iron oxide deposits occur just south and east of the archean and paleoproterozoic margin within the grenville province ti is concentrated along this margin in association with anorthosite nickel is still abundant there but palladium and chromite are remarkably absent this grenvillian ni pd cr paradox suggests that the fractionation process invoked in the classical plume model is inadequate to explain the geochemical characteristics of the anorthosite parental magmas the crustal tongue melting model seems to better explain the metallic zonation observed at the margin of the archean and proterozoic terranes

rip currents as a geological tool this paper considers the nature and sedimentary significance of rip currents rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths such sediments deposited outside the zone of wave deformation may be identified as storm rip current increments within sediments deposited during fair weather composite beds deposited during a given wave cycle by storm rip currents are closely similar to turbidites and many so called fluxo turbidites described from flysch deposits using data gathered in studies conducted on the modern baltic coast supplemented by experimental work and theoretical considerations a sedimentary model is proposed the model may be used to interpret possible rip current deposits among shallow water turbidites and both modern storm sediments and ancient tempestites language en