basal platinum group element mineralization in the federov pansky layered mafic intrusion kola peninsula russia the 2501 to 2446 ma federov pansky layered mafic intrusion in the central part of the kola peninsula russia is situated along the northern contact of the early proterozoic imandra varzuga rift and archean granite gneiss the federov pansky intrusion has eight major zones from bottom to top 1 marginal zone mafic schists 50 to 100 m 2 taxitic gabbronorite zone bpca 150 to 300 m 3 norite zone bcpa pbc 200 m 4 main gabbronorite zone pacb 1 000 m 5 lower layered horizon zone bc pc pbac 100 m 6 gabbro zone pacb pbac 1 500 m 7 upper layered horizon zone pocab 250 m 8 upper gabbro zone pacb 400 m the taxitic gabbronorite zone is restricted to the federov and east lastjavr blocks this zone is distinctive due to its pegmatitic taxitic textures and abundant norite xenoliths concentrated near its upper contact with the norite zone chemically the taxitic gabbronorite is more feldspathic and iron enriched than the overlying mgo and cr 2 o 3 rich norite zone and norite xenoliths the zone hosts cu ni pge mineralization with disseminated sulfides which is continuous for over 2 5 km along strike and 150 to 200 m downdip 98 percent of the sulfides consist of varying amounts of pyrrhotite chalcopyrite and pentlandite the mineralization is divided into an upper mineralized section with two higher grade lenses a and b and the restricted lower mineralized section with six narrower lenses the lower mineralized section is less continuous and less uniform in terms of cu ni pge distribution the norite xenoliths and blocks occurring within the taxitic gabbronorite zone are practically barren of sulfide and can dilute higher pge and base metal grades in the gabbronorite matrix average total pge grades in rocks do not exceed a few grams per tonne the mineralization is palladium rich with a pd pt ratio from 4 22 to 4 34 rh and au grades are 0 1 to 0 5 g t os and ir grades are the most common pge bearing minerals are merenskyite moncheite kotulskite sobolevskite and michenerite less common are sperrylite and vysotskite braggite the cu ni pge sulfides are primarily magmatic in origin this is supported by s isotope compositions with a narrow range of 34 s of 0 2 to 1 4 percent and s se ratios of 1 050 to 3 300 a three stage process is postulated for the genesis of the federov mineralization this involves the intrusion of a more fractionated and sulfur saturated gabbronorite magma and its brecciation and mixing with a residual primitive magma forming immiscible sulfides that scavenged cu ni pge the federov resembles other contact type mineralization such as in the portimo intrusion in finland and the lac des iles and east bull lake intrusions in ontario canada

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

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

effects of magmatic processes serpentinization and talc carbonate alteration on sulfide mineralogy and ore textures in the black swan disseminated nickel sulfide deposit yilgarn craton the black swan disseminated orebody occurs predominantly within the partially carbonated serpentinite core of a zoned serpentinite talc carbonate ultramafic cumulate body of komatiitic parentage the black swan disseminated sulfides depart significantly from the characteristic pattern observed in otherwise similar deposits in western australia and canada which display magnetite bearing pentlandite rich sulfide aggregates blebs in serpentinites and s rich millerite bearing assemblages in talc carbonates the black swan disseminated ores in contrast are dominated by millerite pyrite polydymite assemblages in both serpentinites and talc carbonates with no evident relationship to the progression of carbonation sulfides show three types of intergrowth with gangue minerals which can occur separately or all together intergrowth with magnetite at least some of which appear to be primary magmatic in origin fine intergrowth with interlocking lath textured antigorite and intergrowth with carbonate minerals carbonate commonly replaces the core of sulfide blebs while retaining the primary magmatic outline carbonate replacement of sulfide roughly parallels the overall progression of carbonation of the whole rock but no systematic relationship of the other intergrowth types to carbonate content is observed in the serpentinites serpentine mineralogy determined by microbeam raman spectroscopy comprises lizardite which occurs in pseudomorphic mesh textures after olivine and antigorite formed by variable degrees of overprinting of original lizardite mesh textures ranging to a completely overprinting interlocking texture with no relic igneous texture this progression is interpreted as the result of mild thermal metamorphism of original lizardite serpentinite this progression is completely independent of carbonate content and is also apparently independent of extent of sulfide antigorite intergrowth which is interpreted as being early and related to volume expansion during initial sea floor serpentinization talc is almost completely absent from serpentinites while carbonate content varies widely whole rock ni and s data are very strongly correlated in millerite poor pentlandite rich samples and poorly correlated in samples where millerite is the dominant ni bearing sulfide on this basis we conclude that the pentlandite assemblages are slightly modified survivors of the original magmatic event while the millerite pyrite assemblages formed during the earliest stages of serpentinization overprinting of the early serpentinization event by talc carbonate had essentially no effect on sulfide mineralogy other than inducing intergrowth with carbonate a number of distinctive features such as magnetite rich bleb cores the association of sulfides with segregation vesicles and chromite shells the subspherical morphology of the blebs and preferential replacement of bleb cores are considered to be inherited from the magmatic phase and are attributed to an original high but variable oxygen and halogen content of the sulfide melt component exsolution of this component during solidification of the blebs in some cases produced sulfide centered bubbles now preserved as segregation vesicles while in other cases oxygen was retained within the bleb forming magnetite sulfide intergrowths chromite shells formed as a result of exsolved oxygen interacting with komatiite magma volatile rich blebs solidified in some cases with hollow or porous cores which were subsequently exploited by carbonate replacement

lift off moraines markers of last ice flow directions on the scotian shelf lift off moraines are acoustically incoherent subparallel ridges observed on sidescan sonograms and high resolution reflection seismic profiles on the southeastern continental margin of canada they are up to 3 m high 20 80 m wide and are commonly overlain by stratified proglacial sediments although little is known about them detailed study of high resolution seismic profiles from the emerald basin and the lahave basin on the scotian shelf show that their height width ratio varies with the sounder seabed separation suggesting that the ridges may be narrower than they appear their morphology is similar to degeer moraines or cross valley moraines which form perpendicular to ice flow direction as their orientations can be estimated at the intersection of seismic lines they can be used to estimate ice flow directions since proglacial sediments are draped directly over top of them they are assumed to record the direction of last ice flow this directional data suggests that ice retreated not only

ice keel scour marks in the geological record evidence from carboniferous soft sediment striated surfaces in the parana basin southern brazil abstract soft sediment glacially striated surfaces are common structures in ancient glacial successions such as the gondwanic paleozoic record and document ice erosion on non lithified beds in most cases they have been interpreted as the product of subglacial erosion beneath advancing marine or terrestrial glaciers although direct evidence for glacial sedimentation in associated strata is infrequent in this paper we document soft sediment striated surfaces of carboniferous age occurring in cross laminated sandstones of the lower itarare group southern parana basin brazil and interpret them as scour marks generated by keels of floating ice striated surfaces occur in closely spaced multiple stratigraphic horizons in the lower half of a deglacial sequence whose facies characteristics indicate sedimentation in subaqueous outwash and deltaic settings surfaces are laterally discontinuous commonly bordered by marginal berms and in some cases capped by mudstones with dropstones they have orientations that deviate from the regional paleo ice flow indicated by glacial grooves carved on the pre glacial basement the characteristics observed are consistent with modern and pleistocene ice keel scour marks in addition soft sediment striated surfaces from other gondwanic localities have similar attributes suggesting that ice keel scour marks may be more common in the geological record than previously considered and that vertically repeated striated surfaces do not necessarily indicate multiple ice sheet advances therefore paleo ice flow reconstructions based on soft sediment striated surfaces need to be revaluated considering that the movement of free floating ice masses is controlled by phenomena other than glacier advance

micromorphology of iceberg scour in clays glacial lake agassiz manitoba canada abstract icebergs plough through unconsolidated lake sea sediments gouging out kilometre long grooves 100s of metres wide and tens of metres deep although the surface morphology of iceberg scours is well documented little is known about what scours look like in stratigraphic section particularly where surface characteristics are absent e g through decay or burial this investigation establishes a definitive suite of diagnostic criteria for identifying iceberg scours in clays by macroscopically and microscopically 2d thin sections examining sediment deformation below iceberg scours in former glacial lake agassiz manitoba canada structures unequivocally attributed to iceberg scour in this investigation include sub horizontal microfabrics folds sheath folds augen shapes normal faults reverse thrust faults discrete shears partially destroyed clasts intraclasts type iii multiple domains water escape structures flow structures unistrial plasmic fabric dropstones realigned bedding and specific structural sequences this research will be particularly valuable in palaeoenvironmental reconstruction and in predicting glacial dynamics in addition it may eventually aid structural engineering on polar shelves which could be of great value to oil and gas companies

using fire scars and growth release in subfossil scots pine to reconstruct prehistoric fires abstract fire scars indicating low to moderate intensity fires on peat deposits have been sampled from subfossil scots pine pinus sylvestris l at sites in england wales and south west ireland analysis of ring width responses to one fire event in 2800 bc illustrates its rejuvenating effect on pinus sylvestris woodland supporting a growing body of modern and palaeoecological data that illustrates the regenerative role played by fire in persistence of pinus sylvestris woodland both the scale and timing of these fires suggest that infrequent low to moderate intensity fires are sufficient to stimulate pinus sylvestris growth this effect is shown by average increases in ring width following the fire of between 0 62 and 1 16 mm in non scarred trees and between 0 92 and 2 74 mm in fire scarred individuals growth release in non scarred trees may prove to be a more reliable method of detecting fire than using the relatively rare fire scars alone radii at time of scarring varied between 1 85 and 11 2 cm much smaller than is predicted to survive from modern studies

seismic and bathymetric evidence for four different episodes of iceberg scouring on the new jersey outer shelf possible correlation to heinrich events abstract re analysis of dense chirp seismic surveys on the outer new jersey shelf reveals two sets of shallowly buried incisions the incisions are linear to slightly sinuous oriented ne sw and exhibit cross cutting relationships we interpret both sets as paleo iceberg scours one set cuts into the r horizon a regional unconformity that formed 45 40 ka and which was subsequently buried by the finer grained outer shelf sediment wedge osw the second set is part of an erose boundary between a layered lower unit of the osw and an upper more transparent unit the erose boundary scours are deeper and have steeper slopes than the r horizon scours the latter often exhibit bermed edges whereas the former do not these differences in cross sectional morphology are likely determined by the material properties of the sediments being scoured sand at the r horizon and clay at the erose boundary the orientation of both sets of scours is consistent with previously recognized iceberg scours in stratigraphically younger seafloor on the outer new jersey shelf in 110 150 m water depths located 40 km to the northeast we argue that heinrich events recurring periods of massive iceberg flux into the north atlantic represent the most likely times for transporting icebergs to these latitudes based on stratigraphic relationships a recently published ambient sea level curve and radiocarbon age constraints we attribute the r horizon scours to heinrich event h4 37 cal ka the erose boundary scours to h3 30 cal ka and the seafloor scours to h2 23 cal ka and or h1 17 cal ka facies changes immediately following the r horizon and erose boundary scours may be related to heinrich event forced climate changes perhaps the onset of interstadial conditions and associated melt water pulses floods these pulses floods repeatedly carried fine grained rock flour produced by widespread glacial erosion to the new jersey shelf

the behaviour of platinum group elements during partial melting crystal fractionation and sulphide segregation an example from the cape smith fold belt northern quebec abstract the behaviour of the pt group elements pge during both normal igneous processes and during the formation of pge deposits is poorly understood this is in part because of the limited data set available for nonmineralized rocks accordingly pge concentrations have been determined in komatiitic basalts associated with ni cu sulphide deposits rich in pge from the proterozoic cape smith fold belt of northern quebec the lavas have been divided into olivine phyric pyroxene phyric and plagioclase phyric on the basis of the phenocrysts present olivine phyric lavas have mg s greater than 0 66 and are thought to be primary partial melts the bulk partition coefficients of the pge during partial melting calculated from the average of olivine phyric basalts are 6 2 0 6 and 0 2 for ir rh pt and pd respectively which indicates a decrease in compatibility of the pge in this order the pyroxene and plagioclase phyric lavas are thought to have formed from the olivine phyric lavas by crystal fractionation of olivine and chromite in the pyroxene phyric lavas and olivine chromite and pyroxene in the plagioclase phyric lavas ir shows a strong positive correlation with mg cr and ni and this is attributed to ir having partitioned into olivine or chromite if olivine chromite and clinopyroxene were the only phases to have crystallized then pd should have behaved as an incompatible element however it does not correlate with the lithophile incompatible elements it does however correlate with rh and pt this is interpreted to suggest that rh pt and pd were controlled by sulphide segregation the removal of a small amount of sulphide along with the olivine and chromite during crystal fractionation could have resulted in the mildly compatible behavior exhibited by rh pt and pd the composition of the sulphides in the sulphide deposits may be modelled by assuming that the silicate magma from which they segregated was similar in composition to the spinifex textured komatiites containing 15 mgo

the disturbance regime of an early holocene swamp forest in the czech republic as revealed by dendroecological pollen and macrofossil data abstract a unique remnant of forest dating back to the period 9733 7897 yr bc and consisting of hundreds of tree bases was discovered in the czech republic we aimed to reveal the complex disturbance history of this sub fossil forest using dendrochronology and to describe its detailed plant species composition changes using palaeobotanical techniques analysing such early holocene forest dynamics should help us understand the ability of the forest community to actively adapt to climate change and generally to understand the role of dynamic instability in ecosystem evolution we anatomically identified woody species in 488 samples and determined the ages growth suppressions releases and fire scars in 116 well preserved tree ring series using a modern boundary line approach this image of the forest structure and dynamics was supplemented with analyses of pollen spectra and plant macrofossils in excavated profiles in order to achieve accurate dating we dated 87 samples using 14c and synchronized tree ring series and compared them with an existing pinus sylvestris chronology the developmental trajectory of the forest was unique and did not match the general trend of postglacial pine growth in central europe palaeobotanical proxies indicated that during the circa 2000 years the forest persisted this early holocene ecosystem passed through several phases reflected in the species composition of the vegetation as well as in habitat conditions nevertheless the dominance of pine and the complex fine scale disturbance regime were relatively robust and did not change fundamentally low severity fires and short term changes in soil moisture regime were crucial disturbance agents in the ecosystem stand replacing disturbances were not found up to the gradual collapse of the forest around 8300 yr bc replaced by a swamp community the disturbance regime was relatively stable suggesting a mitigating effect of changing climate due to the predominance of pine in the forest

stratigraphy petrology and platinum group element mineralization of the early proterozoic penikat layered intrusion northern finland the penikat layered intrusion is part of an early proterozoic belt of layered intrusions 2 440 m y in age which extends across northern finland the intrusion is located between late archean granitoids and early proterozoic supracrustal rocks the marginal series with slightly contaminated chilled margin separates the penikat intrusion from the underlying basement complex the layered series is divided into five megacyclic units i to v from bottom to top each unit comprising an ultramafic lower part and a gabbroic upper part three major mineralized zones enriched in platinum group elements pge the sompujaervi sj ala penikka ap and paasivaara pv mineralizations have been delineated along almost 23 km of strike and occur in megacyclic unit iv the sompujaervi mineralization about 1 m thick is mostly associated with pyroxenitic rocks at the lower contact of the unit whereas the ala penikka zone about 0 35 m thick is associated with gabbronorite and poikilitic textured anorthosite in the middle part and the paasivaara zone about 1 m thick with leucogabbro and anorthosite very close to the upper contact the ratios of platinum group elements are relatively constant for each mineralization the paasivaara mineralization represents the most platinum rich variety and the ala penikka the most palladium rich one the ratio of pd to pt being typically about 2 for the sulfide free type and 1 5 3 5 and 0 7 for the sulfide bearing type for the sompujaervi ala penikka and paasivaara mineralizations respectively the sulfur free platinum group minerals pgm sperrylite and isomertieite are the dominant species encountered in the sompujaervi mineralization whereas sulfur bearing platinum group minerals together with sperrylite dominate in the ala penikka and paasivaara mineralizations these are represented by braggite and vysotskite in the ala penikka one and by braggite and cooperite in the more platinum rich paasivaara mineralization according to the new classification of platinum group element deposits proposed by cabri and naldrett 1984 the typically sulfide free sompujaervi mineralization may be included in the oxide silicate association as a possible new type whereas the ala penikka and paasivaara mineralizations may be classified as merensky reef type of the sulfide associations

holocene peatland development and hydrological variability inferred from bog pine dendrochronology and peat stratigraphy a case study from southern sweden dendrochronological analysis was applied to subfossil remains of scots pine pinus sylvestris l buried in a south swedish peat deposit in combination with peat stratigraphy this approach was explored for its potential to provide information on the local hydrological and depositional history at the site forming the basis for a regional palaeohydrological analysis a 726 year ring width chronology was developed and assigned an absolute age of 7233 6508cal a bp 5284 4559 bc through cross dating with german bog pine chronologies whereas two short additional records of older ages were radiocarbon dated registration of growth positions of individual trees allowed assessment of the spatial dynamics of the pine population in response to hydrological changes and peatland ontogeny annually resolved growth variability patterns in the pine population reveal several establishment and degeneration phases probably reflecting fluctuations in bog surface wetness a major establishment phase at 7200 6900 cal a bp reflects the onset of a period of lowered groundwater level also indicated by increased peat humification and a development consistent with regional temperature and lake level reconstructions revealed from other proxies this study demonstrates that subfossil bog pine populations may provide annually to decadally resolved reconstructions of local groundwater variability which are highly relevant in a long term palaeoclimatic context copyright c 2012 john wiley sons ltd

platinum group minerals from the kambalda nickel deposits western australia the relative abundance compositional variability and geologic occurrence of platinum group minerals have been determined by a study of samples from a number of geologic environments in the kambalda nickel deposits these results have been integrated with findings from a previous study by hudson and donaldson 1984 of platinum group minerals in amalgamation residues from the gold recovery circuit of the kambalda nickel mill to give an overall picture of the nature and distribution of platinum group minerals at kambalda the major platinum minerals are sperrylite and moncheite and the major palladium minerals are sudburyite merenskyite stibiopalladinite palladoarsenide michenerite and testibiopalladite palladium also occurs in solid solution in palladian melonite irarsite is the only iridium mineral recognized and no discrete phases have been observed for the other platinum group elements sperrylite occurs within massive pyrrhotite pentlandite ores and is particularly abundant in ores that are rich in chalcopyrite a high proportion of the pt in the kambalda nickel deposits is believed to occur as sperrylite but not all sperrylite is sufficiently coarse in grain size to enable gravity concentration nevertheless sperrylite is the dominant platinum group mineral recovered in gravity concentrates discrete palladium minerals are most abundant in stringers of sulfide in the footwall to the ore in crosscutting sulfide veins or in reaction zones associated with hydrothermal veins and porphyries within the massive and matrix ores coarse discrete palladium minerals are rare and it is concluded that palladium occurs in solid solution or as finely dispersed submicroscopic grains in sulfide minerals predominantly pentlandite the occurrence within the ore zones of pt as sperrylite and pd dispersed in pentlandite is believed to reflect a primary magmatic distribution however the presence of sudburyite moncheite merenskyite michenerite testibiopalladite and palladian melonite in stringers and reaction zones indicates that their formation may be related to postmagmatic processes in particular metamorphic segregation of sulfides and the interaction of ore sulfides with younger hydrothermal fluids

internal stratigraphic architecture of the komatiitic dunite hosted mkd5 disseminated nickel sulfide deposit mount keith domain agnew wiluna greenstone belt western australia the mkd5 nickel deposit is hosted by the mount keith ultramafic complex mkuc a dunite body of komatiitic affinity located in the agnew wiluna greenstone belt western australia the internal architecture of the mkuc comprises seven distinct internal units that range from extreme adcumulate dunite to relatively fractionated pyroxenitic and gabbroic horizons the mkuc is divided into three packages of units the main adcumulate domain mad which is situated in the lower portion of the complex contains the bulk of disseminated nickel sulfide and is dominated by coarse adcumulate olivine textures overlying the mad is an upper fractionated zone which is dominated by mesocumulate to orthocumulate peridotite with domains containing oikocrystic pyroxenite and gabbroic lenses an aerially restricted unit comprising texturally and chemically distinct olivine cumulate rocks is known as the western mineralized zone wmz the fractionation trend between the mad and upz indicates a westerly facing for the mkuc conformable with the bounding stratigraphy in the mkuc truncation of some of the uppermost internal stratigraphic units by the hangingwall contact indicates that the unit has undergone structural modification since its emplacement and suggests that a proportion of the upper section has been removed by faulting furthermore vertical and lateral textural transitions within the internal stratigraphy suggest that the mkuc excluding the wmz was emplaced from an essentially continuous magma flow with the mad representing the period of highest magma flux and the major constructional period of emplacement conversely the wmz is interpreted to represent a later pulse of ultramafic magma emplaced stratigraphically above the main mkuc comparison of the mkuc with other komatiitic dunites from both within the agnew wiluna greenstone belt and worldwide indicates that despite apparent geometrical differences between lens and sheet komatiitic dunites a broad facies architecture can be defined we suggest that the differences in geometry are related to differing degrees of flow localization within dunitic units and are a function of both differences in the thermal characteristics and the degree of litho facies heterogeneity of the enclosing lithologies

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

constraints on the formation of platinum group element deposits in igneous rocks mechanisms of concentration of the platinmn group and other chalcophile elements iu several platinum group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts the similarity of the partition coefficieut d super sulfide melt silicate melt of ir to that of pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility in experiments designed to test the possibility that this fractionation was due to the presence of os ir alloy sulfide and silicate melts were equilibrated with ir metal at 1 450 degrees c 8 kbars total pressure and under sulfur and oxygen fugacity conditions appropriate for nature the ir content of the sulfide liquid was found to be or 3 7 wt percent and that of the coexisting silicate melt inferred to be or 1 000 ppb these concentrations are far in excess of the ir contents of natural sulfides or rocks and indicate that os ir alloys are not stable in the presence of sulfide or silicate melts in nature using our preferred values of d super sulfide melt silicate melt for the chalcophile elements and their estimated concentrations in the mantle limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle the ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits including the j m reef stillwater complex the ug 2 bushveld complex and the lac des isles complex are inconsistent with concentration solely by segregation of sulfide liquid in contrast the merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide stratigraphic variations in chalcophile element concentrations through the ug 2 and the main sulfide zone of the great dyke also indicate that metals must have been redistributed on at least the meter scale our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism

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

comparison of trends of iceberg scour marks with iceberg trajectories and evidence of paleocurrent trends on saglek bank northern labrador shelf high resolution seismic and sidescan sonar surveys on saglek bank indicate extensive areas of iceberg scoured sea bottom and provide data for an analysis of 6075 iceberg scour marks in 537 samples each of 0 25 km by 2 0 km area the general trends of iceberg scours measured on sidescan sonographs are comparable to iceberg trajectories which are mostly controlled by currents on saglek bank iceberg scour and iceberg drift directions are generally northwest southeast straight scours predominate on the shelf edge reflecting the influence of unidirectional oceanic labrador currents bank top scours are commonly curved reflecting the influence of slower moving water more subject to the effects of rotary tidal currents and storms this difference in dispersion of scour trends between the bank top and shelf edge is due to the relative strengths of variable tidal and storm driven currents and unidirectional oceanic currents pack ice forces may also influence iceberg movement modern icebergs scour the she

progressive mixing of meteoritic veneer into the early earth s deep mantle komatiites are ancient volcanic rocks mostly over 2 7 billion years old from the archaean era that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1 in particular the platinum group element pge contents of komatiites provide a unique source of information on core formation mantle differentiation and possibly core mantle interaction2 3 4 5 6 7 8 most of the available pge data on komatiites are from late archaean 2 7 2 9 gyr old or early proterozoic 2 0 2 5 gyr old samples here we show that most early archaean 3 5 3 2 gyr old komatiites from the barberton greenstone belt of south africa and the pilbara craton of western australia are depleted in pge relative to late archaean and younger komatiites early archaean komatiites record a signal of pge depletion in the lower mantle resulting from core formation this signal diminishes with time owing to progressive mixing in to the deep mantle of pge enriched cosmic material that the earth accreted as the late veneer during the early archaean 4 5 3 8 gyr ago meteorite bombardment

late holocene palaeoclimate variability the significance of bog pine dendrochronology related to peat stratigraphy the puścizna wielka raised bog case study orawa nowy targ basin polish inner carpathians abstract the results of dendrochronological and palynological analyses of subfossil pine trees occurring in the peat deposits of the pu cizna wielka raised bog polish carpathians southern poland the only site with numerous subfossil pine trees in the mountainous regions of central europe presently known indicate that the majority of the tree populations grew in the peat bog during the periods ca 5415 3940 cal bp and 3050 2560 cal bp several forestless episodes dated to 5245 5155 cal bp 4525 4395 cal bp and 3940 3050 cal bp were preceded by tree dying off phases caused by an extreme periodical increase in humidity and general climate cooling trends these events are documented based on analyses of pollen and non pollen palynomorph assemblages dendrochronological analyses of the trees as well as numerous radiocarbon datings of the sediment horizons occurring within the peat bog profile the phases of germinations and in turn of tree and shrub invasions of the peat bog areas have been closely connected to drying and occasional warming of the regional climate the last of the forestless periods began about 2600 years ago and continued up to the very recent times currently as a result of desiccation of the peat bog and the lowering of the groundwater level due to improved water drainage system pine trees have returned the peat bog again these results demonstrate that studies of subfossil bog pine trees are quite effective in documenting and reconstructing periods of humidity fluctuation that occurred within the carpathian region over the last several millennia

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

the formation of pt ir alloys and cu pd rich sulfide melts by partial desulfurization of fe ni cu sulfides results of experiments and implications for natural systems abstract we propose a model of high temperature formation of platinum group element pge alloys from base metal sulfides subjected to decreasing sulfur fugacity f s 2 for this purpose we experimentally investigated the effect of partial desulfurization of monosulfide solid solution mss with variable ni and cu contents on the distribution behaviour of pt ir and pd at 1000 c we partially removed sulfur from the pge bearing mss using the tube in tube technique and pyrrhotite as an f s 2 buffer we found that mss undergoing s loss can produce 1 pt and ir exsolution from the mss matrix in the form of pge bearing alloys 2 partial melting of the cu ni bearing mss to form fe rich mss cu ni pd rich sulfide liquid and fe ir pt alloy both of these processes are geologically important the partial melting of the s depleted sulfides could explain the presence of two types of sulfides found in the mantle mss dominated sulfides and s poor sulfides consisting mainly of pentlandite and chalcopyrite we also suggest that the cu ni pd rich liquid formed by partial melting of the sulfides could migrate away from the mss and fe ir pt alloys thus spatially decoupling ir pt and cu pd as observed in reefs and mantle nodules in addition the formation of pge alloys in response to the desulfurization processes potentially could occur 1 after pressure falls during transport of the basalt magma with entrained sulfide droplets 2 after sulfur removal by s undersaturated melts or fluids from pge enriched sulfide proto ore 3 after degassing of the sulfide droplets occurring in a sub volcanic chamber and 4 as a result of sulfide interaction with chromite

primary cumulus platinum minerals in the monts de cristal complex gabon magmatic microenvironments inferred from high definition x ray fluorescence microscopy an unusual occurrence of pt enriched pyroxenites in the monts de cristal igneous complex is characterized by unusually high ratios of pt to other platinum group elements pges and very low cu and sulfide contents synchrotron x ray fluorescence microscopy was used to identify over a hundred discrete grains of platinum minerals and relate their occurrence to textural associations in the host heteradcumulate orthopyroxenites element associations backed up by fib sem and pixe probe observations indicate that most of the pt is associated with either as or trace cu ni rich sulfides or both some of the pt as grains can be identified as sperrylite and most are likely to be pt fe alloy the relative abundances and volumes of pt minerals to sulfide minerals are very large compared with typical magmatic sulfides almost all of the grains observed lie at or within a few tens of m of cumulus orthopyroxene grain boundaries and there is no significant difference between the populations of grains located inside or outside plagioclase oikocrysts these oikocrysts are inferred to have crystallized either at the cumulus stage or very shortly thereafter on the basis of their relationship to ti enrichment in the margins of pyroxene grains not enclosed in oikocrysts this relationship precludes a significant role of trapped intercumulus liquid in pt deposition or mobilization and also allows a confident inference that pt rich and pt as enriched phases precipitated directly from the magma at the cumulus stage these observations lead to the conclusion that fractionation of pt from other pges in this magmatic system is a consequence of a solubility limit for solid pt metal and or pt arsenide

relict iceberg keel marks on the new jersey outer shelf southern hudson apron swath sonar bathymetry reveals sinuous furrows 400 m wide kilometers long and 4 m deep inscribed in semilithified clays on the southern hudson apron we interpret these as keel marks created by floating icebergs detached from the retreating laurentide ice sheet since ca 25 ka keel mark orientations suggest two phases of iceberg rafting these phases could correlate with heinrich meltwater events h2 and h1 ca 25 and 17 ka bracketing the late wisconsinan glacial maximum ca 22 ka during holocene transgression some keel marks were reworked and reformed into oblique ridges where older sandier sediments crop out at the seafloor relict glacial features on the new jersey outer shelf provide a tie between the timing of laurentide glacial retreat and the evolution of shallow stratigraphy on this mid latitude shelf during the last global sea level cycle ca 120 ka to present

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

a native nickel heazlewoodite ferroan trevorite assemblage from mount clifford western australia the mount clifford nickel sulfide deposit consists of several stratabound lenses of disseminated sulfides averaging 1 5 to 2 0 percent ni in a serpentinized periodotite the sulfide assemblage is ni rich represented by varying proportions of godlevskite millerite heazle woodite and ni rich pentlandite in a single 50 cm section of core below the disseminated sulfide lenses native nickel has been found as a significant component of irregular oxide sulfide veins stringers and blebs in a serpentinized skeletal textured periodotite it is present as grains and granular aggregates ranging from 100 mu m to 3 mm in size and averaging about 500 mu m the chemical composition average wt from 24 electron microprobe analyses is ni 96 41 fe 1 41 co 0 45 and cu 1 51 total 99 78 euhedral to subhedral spinels dominate the veins approximate mode 75 and are typically zoned from cores of nickeloan magnetite to rims of ferroan trevorite the spinel both encloses and is included in an intergrowth of native nickel 15 and heazlewoodite 10 minor phases include chalcocite bornite and maucherite together with a rare unnamed mineral close to ni 4 s sn te sb in composition secondary sulfides have developed in response to weathering and or low temperature hydrothermal alteration processes pentlandite is replaced by violarite polydymite and millerite godlevskite is replaced by millerite heazlewoodite is replaced by godlevskite which in turn is replaced by millerite carbonate bearing solutions have caused alteration of coexisting native nickel and heazlewoodite to assemblages comprising native copper nickel iron carbonates nickel and nickel iron hydroxycarbonates and nickel rich silicates the native nickel heazlewoodite ferroan trevorite assemblage is believed to have crystallized in a hydrothermal environment characterized by high nickel and low sulfur activities and in which oxygen activity was sufficiently high to make nickel iron sulfides and nickel iron alloys unstable relative to nickel iron oxides nickel sulfides and native nickel

image analysis and composition of platinum group minerals in the j m reef stillwater complex detailed imaging bulk geochemical analyses and laser ablation icp ms analyses were conducted on mineralized samples from the platinum group element pge bearing j m reef stillwater complex montana the aims of the study were to determine which phases are the main hosts of the pge to examine the textural relationships of the pge bearing minerals and the associated base metal sulfides silicates and secondary magnetites the platinum group minerals pgm observed in the studied samples consist of pd or pt sulfides pt fe alloy isoferroplatinum pd or pt tellurides and pd cu alloy skaergaardite with minor native pd ru ir os s sub 2 laurite and au pd ag alloy palladian electrum these minerals account for all the pt half the pd ru and ir and a smaller proportion of the os in whole rocks the balance being found in the base metal sulfide most of these pgm are closely associated with base metal sulfides either included in the base metal sulfide or located at the contact between the base metal sulfide and the silicate or oxide minerals the textures of the pgm within and at the margins of the base metal sulfide suggest that they exsolved from the sulfides we suggest that the pge ni cu and te partitioned into a magmatic sulfide liquid that crystallized into base metal sulfide the pt fe alloys and some of the pd sulfides exsolved from the base metal sulfide during desulfurization at fairly high temperatures in contrast the skaergaardite and some pd sulfides are found in association with secondary magnetite and probably formed at relatively low temperatures approximately 250 degrees 465 degrees c during a second fluid migration event the pd pt ratio of the reef approximately 3 3 is higher than most mafic magmas and the pd pt ratio of the footwall is lower 0 3 0 7 it is possible that some of the pd in the reef was leached from the underlying rocks and deposited in the reef as pgm associated with secondary magnetite

geology geochemistry and origin of platinum group element chromitite occurrences in the heazlewood river complex tasmania three distinctive chromitite occurrences are recognized from the cambrian mafic ultramafic heazlewood river complex tasmania type i chromitite represents very thin schlieren comprising cr rich spinel and is hosted by primitive olivine rich cumulates type ii chromitite occurs as thicker schlieren containing more aluminous spinel and is hosted by an interlayered sequence of cumulate dunite and peridotite type iii chromitite is more massive occurring as irregular stringers pods and layers containing chromite having intermediate cr al ratios it is spatially associated with gabbroic dikes that intrude dunite harzburgite orthopyroxenite cyclic units and is interlayered with or enclosed by an unusual xenolith bearing gabbronorite both type i and ii chromitites are interpreted to have precipitated directly from a primitive magma and may reflect periods of magma replenishment type iii chromitite was formed in response to mixing between ultramafic magma and volatile rich intercumulus liquids producing hybrid magmas having chromite as the only liquidus phase all of the chromitites display enrichment in platinum group elements relative to the cumulate ultramafic sequences the maximum total platinum group element tenor in the chromitites is 6 ppm with average grades between 0 2 and 1 ppm the chromitites are richest in ruthenium platinum and rhodium reflecting the presence of discrete platinum group minerals laurite rus 2 is the most abundant platinum group mineral occurring as isolated inclusions in chromite a wide variety of pt and rh minerals are contained in the chromitites these include alloys arsenides and sulfarsenides the majority of which occur as altered anhedral grains in the silicate matrix the platinum group mineral assemblages recognized in this investigation are similar to bimodal platinum group mineral suites documented in chromitites from ophiolites legendre and auge 1986 and stratiform intrusions von gruenewaldt et al 1986 these include early crystallizing os ir and ru ir group elements phases and later formed rh pt and pd minerals pd group elements barnes et al 1985 the ir group elements display a preference for octahedrally coordinated sites in magmas similar to cr murck and campbell 1986 and like cr have a low solubility in primitive magmas the unusually low ir tenor of type i chromitites from the heazlewood river complex may reflect the prior removal of os ir alloys from the parental liquids which would account for the known occurrence of osmiridium in the complex localized magma mixing associated with the development of the type iii chromitites led to increasing a sub s 2 and a sub a s but not necessarily to s saturation in the resultant hybrid liquid under these conditions laurite was precipitated as an early formed cumulus phase and the more soluble pd group elements crystallized as intercumulus sulfarsenides and arsenides

comparison of archean dunites and komatiites associated with nickel mineralisation in western australia implications for dunite genesis the two important classes of archean fe ni cu sulphide deposits in western australia are those hosted by 1 coarse grained adcumulate dunites e g agnew mount keith and 2 komatiitic lavas e g kambalda close similarities in compositions of relict olivine pyroxene and chromite from dunitic and komatiitic host rocks and compositional and textural similarities between chilled margins of some dunites and some komatiites indicate origins from similar komatiitic liquids which contained 20 32 mgo as field relations suggest that the dunites are probably lateral stratigraphic equivalents of spinifextextured komatiites they are interpreted as the slowcooled end member of a continuum from spinifex textured komatiites former liquids through cumulate komatiites 40 90 olivine to adcumulate komatiitic dunite 90 olivine previous classification of these ni deposits as intrusive and volcanic related deposits now appears unwarranted and the terms komatiitic dunite hosted and komatiite hosted deposits are preferred

platinum group element distribution in komatiitic and tholeiitic volcanic rocks from munro township ontario platinum pd ir au mgo cr s ni co cu and zn were determined in volcanic rocks from pyke hill fred s flow and theo s flow in munro township ontario to establish the fractionation trends of the platinum group elements in these rocks and to evaluate the relevant processes controlling such fractionation distinctive and relatively consistent fractionation trends are characteristic of pt pd and ir platinum and pd are usually concentrated in the upper units of flows spinifex textured rocks relative to the lower olivine and pyroxene rich cumulates whereas the reverse is true for ir theo s flow does not show these patterns in that pt pd and ir behave in a similar fashion the platinum group element fractionation trends formed during the crystallization of these flows suggest that sulfide immiscibility has an insignificant role it is concluded that in general ir must be coprecipitated with olivine or pyroxene whereas pt and pd are rejected from the structures of these minerals the ir ferromagnesian association probably does not involve a substitution of ir in the mineral structure of olivine or pyroxene

holocene humidity changes in northern finnish lapland inferred from lake sediments and submerged scots pines dated by tree rings conclusive evidence for a rise in water levels has been found in connection with lake sediment studies undertaken partly in collaboration with the pine megafossil sampling and dendrochronological work in northern finnish lapland the change in lake level stands is shown by slow sedimentation rate in the early to mid holocene and an increase thereafter these data indicate a regional rise in water levels during the latter part of the holocene following a relatively dry period between 8000 and 4000 bp synchronous changes also indicating rising water levels have been observed in the diatom and cladoceran assemblages of the sediment cores subfossil scots pines pinus sylvestris l have been preserved in large quantities in small lakes in lapland because in many cases the rising water level has inundated the trunks after death the position of the subfossil trunks and stumps often indicates that the pines have been growing on dry land at the sides of the lakes in which they are now submerged traces of

tectonic stratigraphic and geochemical comparisons between ca 2500 2440ma mafic igneous events in the canadian and fennoscandian shields abstract palaeoproterozoic mafic igneous rocks 2500 2440 ma are exposed in the form of layered intrusions dykes and volcanic rocks in the southern canadian shield and the fennoscandian shield both igneous events were associated with episodes of continental rifting the tectonic and regional geological evolution of the two regions during the palaeoproterozoic are strikingly similar further comparisons of the mineralogical and stratigraphic characteristics of the layered intrusions the east bull lake suite in the canadian shield and the fennoscandian suite in the fennoscandian shield show additional similarities earlier investigations have identified possible parental magmas from mafic dykes for each of the layered intrusion suites these include the so called streich dykes associated with the east bull lake suite and the viianki dykes with the fennoscandian suite the viianki dykes are more primitive in composition mg 0 66 0 78 relative to the streich dykes mg 0 62 0 65 and probably represent primary mantle derived magmas geochemical modelling suggest that these two parental magmas cannot be related through fractional crystallization the differences in ree characteristics la yb cn 3 6 9 1 for viianki dykes 3 6 3 7 for streich dykes and selected incompatible element ratios can however be accounted for by either differential amounts of upper crustal contamination or different degrees of partial melting from a similar garnet bearing mantle source the data suggest that the viianki dyke parental magmas may have been produced by a degree of partial melting that was more than a factor of two lower than that which gave rise to the more primitive parent of the streich dyke magmas we conclude that these early palaeoproterozoic igneous events in the southern canadian shield and the fennoscandian shield could have been derived from a similar mantle source that was characterized by partial melt extraction of amorb and enrichment by subduction like lree enriched fluids during the archaean

the agnew nickel deposit western australia part i structure and stratigraphy the agnew formerly perseverance nickel deposit in the agnew wiluna greenstone belt of the yilgarn block has previously been assigned to the category of intrusive dunite associated deposits on the basis of detailed reinterpretation of the geology of the deposit we conclude that the nickel sulfide mineralization is hosted by komatiite flows which predate the associated dunite body which is itself extrusive nickel sulfide mineralization is hosted by a suite of metamorphosed olivine rich ultramafic rocks containing widely varying proportions of tremolite and chlorite chemical analyses of these rocks show them to be metamorphosed komatiites in which original asymmetric differentiation profiles can be recognized these provide facing directions which form the basis for structural and stratigraphic interpretation much of the massive sulfide has been structurally remobilized into fold noses and planar fractures but some retains primary igneous contacts with underlying flow tops primary contacts are characterized by a zone of skeletal chromite developed across the interface between massive sulfide and flow top the primary massive ore postdates a thick komatiite flow which hosts the matrix ore the dunite body shows a discordant lower contact which truncates stratigraphic marker units in the country rock at the northern limit of the mine area the base of the dunite is separated from the top of the main mineralized flow by more than 150 m of felsic volcanic rocks at the southern limit of mine development the dunite directly overlies the matrix ore zone of the mineralized flow this discordant contact is interpreted as the base of a thermal erosion channel formed by an extensive komatiite lava river the geometry of this contact and the competence contrast between the dunite body and the country rocks exerted a dominant influence on subsequent deformation and sulfide remobilization

ruthenium in komatiitic chromite abstract the distinction between ru in solid solution and ru bearing inclusions is essential for the predictive modeling of platinum group element pge geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites this study investigates the role of chromite in the fractionation of ru in ultramafic melts by analyzing chromite grains from sulfide undersaturated komatiites and a komatiitic basalt from the yilgarn craton in western australia in situ analysis using laser ablation icp ms yields uniform ru concentrations in chromites both within grain and on a sample scale with concentrations between 220 and 540 ppb all other platinum group elements are below the detection limit of the laser ablation icp ms analysis carius tube digestion isotope dilution icp ms analysis of chromite concentrates confirms the accuracy of the in situ method time resolved laser ablation icp ms analyses have identified the presence of sub micron ir bearing inclusions in some chromite grains from the komatiitic basalt however ru bearing inclusions have not been recognized in the analyzed chromites and this combined with the in situ data suggests that ru exists in solid solution in the crystal lattice of chromite these results show that chromite can control the fractionation and concentration of ru in ultramafic systems

layering and precious metals mineralization in the rincon del tigre complex eastern bolivia geologic investigations supporting recent technically successful platinum exploration of the rincon del tigre complex a ca 4 6 km thick layered sill have provided significant new understandings of the layering development and precious metals mineralization of one of south america s largest mafic ultramafic intrusions three macrocyclic units are recognized from the base up basal unit unit 1 and unit 2 each most likely the product of a major replenishment with mafic magma of one compositional type the ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite granular harzburgite olivine bronzitite each a few meters thick with occasional development of bronzitite at the top mafic rocks absent from unit 1 and present only locally in the basal unit form a well developed norite gabbro magnetite gabbro sequence in the upper half of unit 2 associated with the base of the magnetite gabbro is the precious metals zone a persistent zone of low grade subeconomic sulfide and precious metals mineralization 80 to 185 m thick the precious metals zone comprises an upper cu sulfide rich portion its base the sulfide phase boundary about 23 to 70 m above the magnetite phase boundary and a lower precious metals rich portion the lower part straddling the magnetite phase boundary the individual precious metals are concentrated in separate sulfide poor subzones each up to many tens of meters thick in the stratigraphic order rh pt pd au with peak offsets at the same scale modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic precious metals layering of the sulfides in the precious metals zone all at three different superimposed scales from about 3 m to several tens of meters a possible interpretive framework for the cryptic layering and perhaps for much of the other layering including the meter scale layering of the ultramafic rocks is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and in the magnetite gabbro by repeated convective overturns caused by the onset of magnetite precipitation the characteristics of the precious metals zone fit an orthomagmatic model the composition stratigraphic relations and modal correlation with magnetite of the sulfides all imply that s saturation was induced by loss of feo to magnetite precipitation this took place after prolonged closed system fractionation following the last major magma injection at the base of unit 2 during which crystallization of olivine pyroxene plagioclase and finally magnetite precipitation led to overall s cu and precious metals enrichment and ni and finally fe depletion of the magma the development of the mineralization and its complex layering was controlled by sulfide segregation rayleigh fractionation the different but generally very high partition coefficients dsilicate sulfide for each precious metal and repeated convective overturns at three different superimposed scales the decoupled metals distribution is analogous and similar in origin to that of the main sulfide zone of the great dyke zimbabwe and of the main sulfide layer offset portion of the munni munni complex western australia but the stratigraphic order of metal enrichment peaks is different pt pd au vs pd pt au this suggests a reversal in the relative magnitudes of the partition coefficients dpt and dpd that was possibly brought about by changes in the temperature and o s and fe activities of the magmas between the middle and late stages of crystallization constancy of d values during fractionation of mafic magma should not be assumed the precious metals zone is one of several skaergaard type strata bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions and the origin of the skaergaard mineralized zone itself including the potentially economic platinova reefs can be explained in terms of the precious metals zone model presented here the very low grade great thickness and wide separation of individual metal subzones in the precious metals zone are similar to those of the subeconomic axial facies of the main sulfide zone in the wider parts of the great dyke cf the economic more compact marginal facies on the basis of the lateral and vertical thermal model of the great dyke magma chamber the subeconomic status of the precious metals zone may be in large part a function of the specific fluid dynamics of the rincon del tigre magma chamber

subfossil bog pine chronologies from the puścizna wielka raised bog orawa basin southern poland abstract subfossil wood of the scots pine pinus sylvestris l appearing in the peat sediments in southern poland presents material for the construction of long tree ring chronologies that are of potential utility in palaeoclimatic reconstructions over 600 subfossil pine samples in form of discs were taken for dendrochronological analyses from the pu cizna wielka peat bog the only economically exploited bog in the carpathians based on these samples five floating chronologies covering ca 1700 years were constructed they were dated with the wiggle matching technique to the years 3485 3295 25 3205 2575 25 2445 2200 50 2210 1990 40 1100 610 10 cal bc these results indicate that the trees grew on the valley bog during the initial phase of its development the vast majority of the trees analysed grew on the peat bog between ca 3500 and ca 2000 cal bc with breaks in the record dated to ca 3290 3200 cal bc ca 2570 2450 cal bc pines invaded the peat bog in drier periods whereas a high groundwater level presumably related to increased precipitation was a factor determining mortality of the trees this study therefore fills a geographical gap in the coverage by subfossil pine records as there are no studies on subfossil pine trees from peat bogs in central eastern europe

late wisconsinan deglaciation of emerald basin scotian shelf emerald basin is a glacially overdeepened basin located on the central scotian shelf where quaternary sediments form a thin veneer over tertiary bedrock nineteen piston cores and 1200 line km of seismic data were obtained from the basin from which seven acoustic facies were recognized i cambro ordivician metasediments ii tertiary sediments iii scotian shelf drift till iv acoustically stratified emerald silt v acoustically unstratified emerald silt vi lahave clay vii diffused gas six depositional sequences were recognized in the emerald silt and lahave clay facics and were correlated with sediments recovered in piston cores depositional sequence 0 is a strongly stratified package which drapes over the scotian shelf drift it is correlated with stiff unbioturbated slightly gravelly sandy muds with no in situ molluscs and is characterised by an ice marginal foraminiferal assemblage depositional sequence 1 drapes over depositional sequence 0 and shows onlap on topographical highs it is correlated with slightly gravelly muds which are characterised by alternating bioturbated and unbioturbated hands and monospecific molluscan and foraminiferal assemblages depositional sequence 2 asymmetrically drapes over and onlaps onto depositional sequence 1 and is correlated with bioturbated slighty gravelly muds with monospecific molluscan and foraminiferal assemblages depositional sequence 3 infills the basin deeps erosionally truncating reflections of depositional sequence 2 it is correlated with bioturbated muds which are characterised by a monospecific foraminiferal assemblage and a slightly diverse molluscan assemblage depositional sequence 4 drapes over the erosional upper surface of depositional sequence 3 and is correlated with bioturbated muds which are characterised by a diverse molluscan assemblage depositional sequence 5 onlaps onto depositional sequence 4 infilling the basin deeps and is correlated with soft bioturbated sandy silts which are characterised by diverse molluscan and foraminiferal assemblages the present day seafloor in emerald basin is marked by small gas escape structures intepreted as pockmarks buried pockmards are observed in depositional sequences 4 and 5 buried iceberg scours are observed on the upper surface of depositional sequence 0 and within depositional sequences 1 and 2 small ridges interpreted as lift off moraines are formed on the upper surface of the scotian shelf drift and are oriented transverse to ice flow directions during glacial retreat wedge shaped acoustically unstratified features interpeted as till tongues are rooted in moraines on the basin flanks and were deposited simultaneously with depositional sequences 0 1 and 2 detailed interpretation of the seismic and core data suggest that glacial ice advanced across emerald basin reaching the shelf edge at approximately 24 ka melting of the ice sheet resulted in deglaciation of emerald basin at 17 5 ka by lift off of the grounded ice sheet to form a floating ice shelf of 10 100 yr duration sediment beneath the ice shelf was primarily supplied by subglacial meltwater streams prior to lift off till was squeezed into basal crevasses in the ice to form features known as lift off moraines the ice shelf calved at approximately 17 4 ka causing intense scouring of the seafloor by in situ icebergs and isolating ice rises on the banks surrounding emerald basin these ice rises disintegrated rapidly except for the rise on emerald bank which contributed sediment to emerald basin until 12 ka sediment was primarily supplied by subglacial meltwater and deposited out of suspension except near the ice margin where ice rafting and gravity flow mechanisms were important iceberg scouring occurred until about 14 ka the main ice margin remained on the northern flank of the basin until 15 4 ka at which time it retreated landward a widespread erosive event caused by storm driven currents occurred at 13 ka rain out from suspended sediment plumes was the dominant depositional mechanism until 12 ka lahave clay is primarily derived from inner shelf sediments reworked by rising sea level storm generated currents have created an erosional surface at the present day seafloor around the flanks of the basin

platinum group minerals in the middle group of chromitite layers at marikana western bushveld complex indications for collection mechanisms and postmagmatic modification the platinum group minerals in a drill core taken through the middle group of chromitite layers in the critical zone at marikana in the western bushveld complex were found to consist mainly of laurite as inclusions in chromite grains the platinum group minerals containing pt pd and rh are concentrated in the intercumulus silicates and frequently associated with base metal sulphides up to about 20 of all platinum group minerals in the investigated chromitite layers contain sub stantial amounts of as the base metal sulphides are strongly modified in the postmagmatic stage which led to a significant loss of fe and s in this way concentrating cu ni and the platinum group elements by factors of up to 10 interaction between chromite and base metal sulphides cannot account for all the fe lost in chromite poor samples and the importance of additional processes is indicated inclusions in chromite and orthopyroxene indicate the formation of discrete platinum group minerals and as rich phases before the

relict ice scour marks and late phases of lake agassiz in northernmost manitoba in northern manitoba intersecting grooves 300 1800 m long are ice scour marks created by the dragging of iceberg keels along rises in the bed of a glacial lake whose water plane was at about 305 m asl the lake was bounded by glacial ice on its northern and eastern margins the occurrence of scours on topographic divides indicates that a single extensive lake thought to be a northern extremity of lake agassiz occupied the area as far north as seal river at the time the ice scours were formed the lake extended as far west as sprott lake and eastwards into the hudson bay lowlands into an area later occupied by tyrrell sea the preservation of the scour marks suggests that the lake drained suddenly ice scour marks are easily recognized on air photographs and provide a means of identifying areas that have been inundated by glacial lakes scours in emerged marine sediment are generally obliterated by littoral processes

distribution and controls of platinum group element and base metal mineralization in the darwendale subchamber of the great dyke zimbabwe three major sulfide zones containing platinum group elements pge have been identified in the upper part of the ultramafic sequence of the great dyke the pge are at the bases of the sulfide zones the lower two have sparse sulfide mineralization over a thick interval the upper zone contains higher grade in a thinner stratigraphic layer none of the current models for the formation of stratabound sulfide deposits in layered intrusions adequately accounts for the mineralization the two lower sulfide zones are interpreted as resulting from progressive fractional segregation of sulfide from the silicate magma whereas the upper zone may have resulted from mixing of a relatively primitive magma at the base of the chamber with an evolved liquid derived from near the roof

iridium ruthenium and rhodium in komatiites evidence for iridium alloy saturation abstract a large body of new data over 400 samples on platinum group element pge concentrations in yilgarn craton komatiites is combined with all available high quality literature data to reveal trends in a spectrum of komatiite lithologies ranging from spinifex textured rocks to pure olivine and olivine chromite adcumulates the data set is restricted to carius tube isotope dilution analyses and high precision nickel sulfide fire assay icp ms analyses and we demonstrate good agreement between the two methods only s poor samples are considered and the data set is filtered to eliminate the effects of addition of cumulus chromite and addition or segregation of magmatic sulfide liquid the residual trends indicate that ir ru and rh are decoupled from one another and from pt and pd rhodium behaves more like pt and pd whereas ru is intermediate between ir and rh rhodium and ru both show indications of olivine control with partition coefficients less than unity the maximum ir whole rock content is a roughly linear positive function of mgo but the minimum is constant sub 1 ppb and independent of mgo ruthenium shows some evidence for limited control by chromite but anomalous ru enrichment is also found in some olivine cumulate samples with no evidence for accumulated chromite or sulfide neither rh nor ir shows evidence for any detectable control by chromite the distribution of ir and to a lesser degree ru and rh cannot be adequately explained by olivine control and a major influence of an ir rich phase is strongly indicated the pattern of ir variance in relation to mgo bears some important similarities to that of cr this similarity arises from the fact that both elements are controlled by a temperature dependent solubility limit the chromite saturation surface in the case of cr and saturation in ir or ir os alloy in the case of ir combining this effect with moderate compatibility in olivine allows us to successfully model the entire range of ir data the upper limit of ir concentration in olivine cumulates in an exact analogy to that for cr is interpreted as the cotectic ratio of olivine to alloy as determined by the average slope of the tangent to the alloy solubility curve sulfide undersaturated mantle melts of a wide range of compositions are likely to be ir alloy saturated experimental predictions of very low ir solubilities in magmas are borne out with the further implication that true thermodynamic sulfide silicate liquid partition coefficients at least for ir and by inference for the other pges are of the order of 107 or even higher

the volcanology of komatiites as deduced from field relationships in the norseman wiluna greenstone belt western australia abstract komatiites in the western part of the norseman wiluna greenstone belt in the yilgarn block of western australia display a wide variety of volcanic facies ranging from very thin differentiated munro township type flow units through to very thick olivine rich cumulate flow units containing high proportions of adcumulate dunite cumulate flow units have been mapped in detail in the agnew wiluna segment of the norseman wiluna greenstone belt in the yakabindie and mt keith areas thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet like sequences of finer grained olivine orthocumulates they show fine scale internal layering broad scale cryptic layering and upper fractionated sequences containing harrisites pyroxene bearing cumulates and in some cases gabbroic derivatives low grade disseminated sulphide mineralisation occurs within the dunite lenses in the stratigraphically equivalent kathleen east area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows a similar relationship is also seen further south where the perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows these field relationships indicate an extrusive origin for the adcumulate dunite bodies the walter williams formation in the west central part of the norseman wiluna belt is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent it consists of a central sheet like body of adcumulate dunite underlain and overlain by olivine orthocumulates the adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite at its northernmost extent the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros dolerites and pyroxene spinifex textured material interpreted as a periodically replenished lava lake sequence both the lenticular dunite bodies of the agnew wiluna belt and the sheet like dunite body of the walter williams formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates the adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates field evidence from perseverance and the general geometry of the dunite lenses suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low melting felsic volcanic substrates the textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion dunite sheets form close to the eruption sites and dunite lenses as a result of channellisation further away from the vent or close to the vent in the situation where the substrate is non refractory and thermal erosion can take place readily kambalda type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic and munro type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions or close to the vent of very small ones waning eruption rates leads to proximal facies being overridden by distal ones a common observation in komatiite sequences the size of thermal erosion channels requires very rapid eruption rates comparable to those in phanerozoic flood basalt terrains

sedimentary facies and depositional architecture of ice contact glaciomarine systems abstract the characteristics of ice contact glaciomarine systems are reviewed based on a range of weichselian and holocene examples from the fjordal basins in norway and svalbard these depositional systems occur as large morphological ridges end moraines including push moraines which are coarse grained and show large scale foreset bedding the paper addresses some of the major problems with regard to the recognition classification and stratigraphic analysis of these systems the ice proximal glaciomarine systems are classified here into three categories 1 ice contact submarine fans 2 ice contact deltas and 3 glaciofluvial deltas this classification integrates morphological and sedimentological criteria and renders these glaciomarine systems not only easier to recognize in the stratigraphic record but also possible to compare with common non glacial deltaic systems the emphasis is on the criteria for the recognition of the ice contact glaciomarine systems i e ice contact fans and ice contact deltas and their discrimination from common gilbert type deltas and fan deltas the review further focuses on ice contact glaciomarine systems as highly sensitive recorders of glacier dynamics and climatic fluctuations it is shown that both the periods of ice front stability and changes in the ice front position as well as the magnitude and rate of those changes are all recorded in the ice contact depositional system the sedimentary record further reflects glacio isostatic sea level changes and local palaeogeography a new conceptual model is suggested which places these ice contact glaciomarine systems in a dynamic stratigraphic context the model integrates sedimentary facies and their depositional architecture in an allostratigraphic framework and is meant to be a guide for the analysis of the sedimentary record of ice contact systems

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

the triple group and the platinova gold and palladium reefs in the skaergaard intrusion stratigraphic and petrographic relations this paper describes the setting structure and textural relations of the platinova reefs in the skaergaard intrusion the reefs comprise a strata bound occurrence of au pge estimated at 91 million tons of rock with 1 8 ppm au and similar amounts of pd the precious metals occur in two major zones a lower major pd reef which is concordant with the modal layering and followed by at least ten minor pd reefs also concordant and an au concentration which fringes the pd reefs outward and upward through the triple group units the major pd concentration and the au concentration are separated in a systematic way across the intrusion near the center of the intrusion the au concentration occurs some 60 m higher in the stratigraphy than the main pd reef but the separation decreases gradually outward being only a few meters near the margins the dominant precious metal minerals are small grains of cu fe au pd pt alloys which typically are included in interstitial sulfides bornite digenite and smaller amounts of chalcopyrite the textural relations suggest a close genetic association between the sulfides and au pge minerals an association which is confirmed by whole rock compositional data our observations demonstrate that the platinova reefs formed after significant concentration of the au pge and s in the residual magma during fractional crystallization before it became saturated with respect to a sulfide fe cu s o liquid this sulfide liquid acted as a collector of au pge and settled into the layered series cumulates we propose that the structure of the deposit evolved through fractional crystallization of the au pge bearing sulfide liquid as it was filter pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates the fractional crystallization resulted in an evolution of the au pge bearing phases with progressive fractionation from pd rich alloys at the base of the deposit to au rich alloys at the top we argue that the sulfides evolved to their present extreme cu rich fe poor composition during oxidation desulfurization reactions in the same event which oxidized the iron titanium oxides of the intrusion this oxidation desulfurization probably had no overall effect on the distribution of au and pge

tectonic styles of early proterozoic ore deposition in the fennoscandian shield the early proterozoic metallogenic events of the fennoscandian shield are connected with two major stages of crustal evolution the karelian continental development 2 5 2 1 ga and the svecofennian orogeny 2 0 1 75 ga the karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on archaean crust a tensional regime probably associated with a mantle plume and partial melting of the upper mantle culminated in the generation of an ene wsw trending rift zone which was intruded by tholeiitic magma 2 44 ga ago some 15 layered igneous bodies hosting cr pge ni cu and fe v ti deposits are known in the kemi suhanko and koillismaa belts these mafic ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km nw se and nnw sse trending dolerite dykes intruded the rifted and attenuated archaean crust 2 25 2 0 ga ago preceding ocean opening the coeval ne sw trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen the failed arm of a triple armed rift system oceanic crust is preserved in 1 96 ga old ophiolite complexes the cu co zn ni and cr contents of the outokumpu type deposits derive from hydrothermal leaching of oceanic crust and were discharged together with chemogenic cherts and carbonates from ascending hydrothermal fluids on the seafloor above fissure vents in the northern part of the shield rifting and probably ocean opening resulted in the formation of the layered tholeiitic sedimentary complex of the petsenga ni cu district intruded by mineralized mafic ultramafic magma the age of this event is poorly constrained the basin was subsequently closed and continental collision of the kola suture was accompanied by southward obduction of the lapland granulite belt on the archaean crust during the svecofennian orogeny the divergent continental margin was converted into an active margin early subduction of oceanic crust produced oceanic island arcs intruded by 1 93 1 9 ga old tonalites and trondhjemites subsequent subduction of oceanic crust generated 1 9 1 87 ga old continental island arcs associated with massive sulphide cu zn pb ag au deposits the island arcs were accreted to the archaean crust in a concave belt containing the ore districts of kiruna skellefte vihanti pyhasalmi aijala orijarvi and bergslagen bimodal volcanic suites with felsic calc alkaline and mafic tholeiitic volcanites are attributed to rifting of the overriding plate above subduction zones ni cu deposits in synorogenic 1 9 1 87 ga old mafic ultramafic intrusions occur in linear fault controlled belts at the margin of the archaean crust the generation of ni bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone magma differentiation and magma upwelling along transcurrent faults on the continental margin sulphur was probably assimilated from the supracrustal wall rocks 1 89 1 87 ga old early svecofennian granitoids are associated with porphyry type cu mo au deposits widespread late svecofennian s type 1 84 1 75 ga granitic magmatism generated minor mo u w sn and ree deposits emphasis is laid on the importance of geotectonic models as a basis for further exploration in the fennoscandian shield

sulfide poor platinum group mineralization in komatiitic systems boston creek flow layered basaltic komatiite abitibi belt ontario geochemical studies of platinum group elements pge au ag and other metals s se as sb cu ni cr co v ti and electron microprobe studies of them have provided new constraints on the character and origin of the recently discovered sulfide poor pge mineralization in the boston creek flow layered basaltic komatiite of the abitibi belt high pge levels characterize a 2 m thick interval overlapping the contact of the clinopyroxenite and gabbro layers 50 ppb pt 75 ppb pd 10 ppb au the basal part of the clinopyroxenite layer 20 ppb pt 33 ppb pd 49 ppb au and the basal part of the peridotite layer 3 ppb ir 15 ppb pd the pge rich clinopyroxenite and gabbro layers contain grains of pd pt and rh bismuthotellurides antimonides and arsenides and gold and silver minerals which occur in spatial association with sparsely disseminated interstitial chalcopyrite skeletal magnetite ilmenite and secondary amphibole alteration of chalcopyrite and magnetite ilmenite these rocks are spatially associated with zones of sheared amphibolite that are relatively poor in pge and rich in ni co s and se minor anorthosite patches dikes and discontinuous layers immediately up section of the mineralized gabbro are relatively poor in pge especially ir or 0 02 ppb and ni cu co and s the pge enriched peridotite is slightly richer in cr and ni but poorer in co ag s and se compared to the overlying peridotite the textural and geochemical characteristics indicate a sulfide control in the origin of the pge mineralization but they contradict an important role for settling of segregated sulfide cl rich late stage magmatic volatiles or hydrothermal fluid activity during metamorphism alternatively they suggest pge mineralization by in situ segregation of sulfide accompanying the crystallization of fe ti oxide from upwardly mobile volatile charged interstitial melt that evolved in the clinopyroxenite and basal gabbro and subsequently formed the anorthosite nickel co se s ag and cu were selectively mobilized relative to the pge in the mineralized rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism iridium and pd probably concentrated in the basal peridotite by coprecipitation of an alloy sulfide phase during the fractionation of olivine or chromite exploration criteria potentially useful in the search for economic concentrations of pge in geologically similar environments of the abitibi belt and elsewhere may include magnetite ilmenite rich clinopyroxenite and gabbro within thick layered mafic ultramafic flows and especially intrusions and cu v fe and ti fractionation trends and anomalies

geochemistry of the j m howland reef of the stillwater complex minneapolis adit area i sulfide chemistry and sulfide olivine equilibrium platinum mineralization in the stillwater complex occurs within a complex sequence of olivine bearing plagioclase rich cumulates about 400 m above the base of the banded zone the reef consists of a 2 m thick layer of disseminated sulfides extending over the entire strike length of the complex in the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine plagioclase cumulate mixed rock to a plagioclase olivine cumulate containing a high proportion of postcumulus bronzite with olivine preserved only as rare resorbed cores on bronzite grains platinum group elements within the reef correlate strongly with abundances of sulfides which show convincing textural evidence for having originated as immiscible sulfide liquid strong correlations are also observed between all six platinum group elements ni and cu despite the gradation in host rock mineralogy the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length containing 9 3 percent ni 6 9 percent cu and approximately 0 7 percent total platinum group elements principally pd these platinum group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt olivines from the reef show a range of compositions from fo 71 to fo 85 and from 2 400 to 4 500 ppm ni a positive correlation is observed between nio and feo in olivines from sulfide bearing samples indicating ni fe exchange between olivine and sulfides on a local scale such that ni contents of olivines were buffered by that of the sulfide phase a value of 9 70 or 1 49 is obtained for the ni fe distribution coefficient between sulfide and olivine in agreement with values obtained at magmatic temperatures by boctor 1982 these observations may be interpreted in the framework of the plume model of campbell et al 1983 a pulse of replenishing magma enters the stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling sulfide liquid attains high platinum group element concentrations during turbulent ascent and spreading of the plume crystallization of olivine from the hybrid layer gives rise to olivine sulfide boulders which settle to the floor of the chamber reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich subsequent reequilibration between olivine and sulfide grains gives rise to the observed ni fe distribution

the penikat intrusion finland geochemistry geochronology and origin of platinum palladium reefs the palaeoproterozoic penikat layered ultramafic mafic intrusion in northern finland is one of the most richly mineralized layered intrusions on earth containing at least six platinum group element pge enriched horizons exposed along 20 km of strike amongst them the sj reef which at 3 7 ppm pt pd over a width of 1 2 m is surpassed by few other pge reefs globally in terms of its endowment in pge important pge enrichments also occur in the pv reef average 2 6 ppm pd 4 ppm pt over 1 1 m and ap1 reef average 6 2 ppm pd 1 7 ppm pt over 0 7 m here we present new major and high precision trace element and nd isotope data from a traverse across the intrusion and a new u pb age of 2444 8 ma for the intrusion we show that the pge reefs formed by predominantly orthomagmatic processes as for example reflected by well defined positive correlations between pt pd and os ir ru contents late magmatic fluids played no significant role in concentrating pge there are at least six cyclic units in the intrusion displaying a progressive upward decrease in differentiation indices mg and cr v subdued stratigraphic variations in incompatible trace element ratios ce sm mostly 5 10 and nd isotope compositions end 3 to 1 indicate that mixing of magmas of distinct lineage or in situ contamination with country rocks was not required to form the pge reefs there is also no evidence for addition of external sulphur to the magma based on s se ratios at or below primitive mantle levels instead we propose that sulphide melt saturation at penikat was reached in response to fractionation of a siliceous high magnesium basalt and that the sulphides were concentrated through hydrodynamic phase sorting consistent with bonanza style pge grades in large potholes

chronology of late wisconsinan glaciation emerald basin scotian shelf eight new accelerator mass spectrometry ams radiocarbon dates from proglacial sediments in emerald basin indicate that the previously proposed chronology based on dates of total organic matter is much too old the new chronology indicates that grounded ice occupied emerald basin in the late wisconsinan

the reconstruction of boreal forest fire history from lake sediments a comparison of charcoal pollen sedimentological and geochemical indices abstract fossil charcoal fossil pollen sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change the rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically historical and dendrochronological records provide regional and local fire histories for a site in wood buffalo national park alberta canada the local and regional history of fires is compared with the microscopic charcoal content macroscopic charcoal content elemental carbon content fossil pollen content sedimentology and geochemistry of annually laminated sediments from a small lake there is no significant correlation between the abundance of microscopic charcoal macroscopic charcoal and total elemental carbon content of the sediments automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal none of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity however the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin variations in sedimentological measures and geochemistry do not correlate with local fire activity fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest what is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above ground biomass of the vegetation in an area at least as great as the drainage basin the inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent proximity intensity and impact of individual fires

stabilities of laurite rus2 and monosulfide liquid solution at magmatic temperature we have reversed the equilibria ru s2 rus2 and 2feo s2 2fes o2 under controlled temperature oxygen fugacity fo2 and sulfur fugacity fs2 at 1 atm to test if the platinum group mineral laurite rus2 has a stability field at magmatic temperature outside that of magmatic monosulfide liquid the fs2 in equilibrium with ru and rus2 is given by log fs2 6 9 12190 t 1173 to 1673 k whereas the fs2 at sulfide saturation of a basaltic melt may be approximated from log fs2 6 7 12800 t 2 log xfeo log fo2 1473 to 1673 k our experiments support the notion that the sulfide mineral laurite can crystallize directly out of a sulfur bearing sulfide undersaturated silicate melt as implied by laurite chromite intergrowth textures in natural chromite ore

lateral and vertical distribution of platinum group elements and petrogenetic controls on the sulfide mineralization in the p1 pyroxenite layer of the darwendale subchamber of the great dyke zimbabwe the great dyke of zimbabwe is a major resource of platinum group elements pge and represents the world9s second largest deposit after the bushveld complex the darwendale subchamber contains more than 75 percent of all the pge mineralization in the great dyke and is the subject of this investigation the mineralization is closely associated with sulfide bearing zones located in pyroxenites toward the top of the ultramafic sequence this environment contrasts with many other mineralized layered intrusions in that substantial plagioclase crystallization did not occur prior to the formation of the pge enriched sulfide layers several sulfide layers occur within the bronzitites of the topmost cyclic unit rather than at the base suggesting that a major mixing event of residual magma with a new influx of primitive liquid was not the primary control of the mineralization the lower sulfide zones are disseminated over 10 to 20 m and contrast with the uppermost sulfide zone s1 which is highly mineralized and generally less than 1 5 m in thickness in the s1 zone values for pt are as high as 8 ppm over a narrow interval with pt pd close to 2 the maximum enrichment of pge occurs at the first appearance of sulfide as a result of the high partitioning of these elements into sulfide liquid the order of enrichment in the sulfide is ir pd pt raquo au cu ni and as the abundance of sulfide increases this phase becomes highly depleted in these elements the enrichment pattern is consistent with fractional segregation of sulfide repetition of the mineralization results from periodic disturbance of the magma system which is related to relatively small pulses of new magma and overturns within the stratified magma column the stratified magma was sustained by the repeated influxes of new magma entering as fountains a series of peaks for pt pd and the base metals occurs in the s1 zone contrasting patterns in the vertical profile for pt and pd are attributed to the differences in the effective partition coefficients of these two elements in an environment of rapidly changing sulfide proportions marked lateral variations in the degree and style of mineralization are observed from axis to margin of the subchamber whole rock concentrations of pt are highest nearest to the margin in the very narrow and highly mineralized sulfide zone s1 but these fall to low values in the axis where the zone is more disseminated in any given vertical section the maximum concentration of pge in sulfide is similar in each of the sulfide zones the total amount of pt for the entire mineralized zone and the concentration of pt in sulfide are however significantly higher in the axial facies compared to the margin there are also major lateral variations in maximum values of cu ni and ni cu ni these lateral variations are attributed to a difference in the thermal regimes arising from the flared shape of the great dyke in cross section and its influence on the fluid dynamics of the magma chamber extensive borehole drilling has permitted detailed two dimensional contour patterns to be deduced for the distribution of pt and pd and the base metals in the s1 suboutcrop over a large area of the darwendale subchamber

partitioning of the platinum group elements and gold between silicate and sulphide magmas in the munni munni complex western australia geochimica et cosmochimica acta erratum to s j barnes 1993 57 1277 1290

fe ni exchange between olivine and sulphide liquid implications for oxygen barometry in sulphide saturated magmas abstract in order to better understand the behaviour of nickel in magmatic processes we have measured the apparent equilibrium constant k d for the exchange of fe and ni between coexisting olivine and sulphide liquid at controlled oxygen and sulphur fugacities f o 2 10 8 10 10 and f s 2 10 2 10 4 over the temperature range 1200 to 1400 c and with 5 to 50 wt nickel in the sulphide liquid measured values of k d are independent of temperature and sulphur fugacity but increase linearly with the nickel content of the sulphide liquid and follow a power law increase with oxygen fugacity behaviour that is consistent with previous measurements of k d under controlled conditions of f o 2 and f s 2 the variation of k d with melt nickel content and f o 2 is most likely the result of nonideal mixing in the sulphide liquid which results in a decrease in nis fes with melt metal sulphur ratio as a consequence of the systematic dependence of k d on f o 2 a new oxygen barometer is proposed for estimating oxygen fugacity in igneous rocks that were cosaturated in olivine and sulphide liquid application of the experimental results to natural samples shows that the relatively large variations that exist in k d values from different olivine sulphide saturated rock suites can be interpreted as arising from variations in f o 2 and or the nickel content of the sulphide liquid oxygen fugacities calculated for oceanic basalt samples using the proposed fe ni exchange oxybarometer are found to be relatively high 10 8 5 10 10 4 which is in accord with the range of values determined using glass ferric ferrous ratios moreover the very low f o 2 10 14 calculated for the mafic dike from disko island is consistent with the presence of native iron in these samples and is in quantitative agreement with indicators of f o 2 based on chromite and olivine melt partitioning of vanadium consideration of the f o 2 exhibited by olivine sulphide saturated intrusive suites reveals a range from relatively oxidized i e similar to oceanic basalts to values at or below that of disko island field petrographic and geochemical evidence suggests that the production of the low f o 2 suite of samples is probably the result of reduction accompanying the assimilation of carbonaceous country rock

experimental constraints on the partitioning of rhenium and some platinum group elements between olivine and silicate melt abstract we have performed partitioning experiments to assess the role of olivine in controlling the behavior of rhenium and the platinum group elements pges during basalt petrogenesis olivines were crystallized from an iron bearing basalt at 1 bar 105 pa and log fo2 of 2 6 4 9 and 7 4 fmq 4 3 2 and 0 5 respectively in situ analyses of olivine and glass by laser ablation inductively coupled plasma mass spectrometry la icp ms reveal a homogeneous distribution of ru rh pd re and pt but significant os heterogeneity at the m scale this latter behavior arises from the presence of undissolved os micronuggets suspended in the melt and included in olivine crystals olivine melt partition coefficients ds for re and the pges follow the order drh dru dpd dre dpt with decreasing fo2 rh and ru become more compatible with maximum partition coefficients of 2 6 and 2 respectively at log fo2 of 4 9 in contrast d values for pd become smaller with decreasing fo2 to a value of 0 006 at log fo2 of 7 4 olivine melt partitioning of rh ru pd re and pt derived from our experiments is confirmed by the behavior of these elements in lavas that have evolved by olivine fractionation an elastic strain model predicts the olivine melt partitioning of these elements excepting our measured value of dpt which is much lower the fo2 dependence on partitioning implies that at higher fo2 some portion of pges exist in higher valence states than predicted from their solubility

first known fire scar on a fossil tree trunk provides evidence of late triassic wildfire abstract fire scars are well known to fire ecologists and dendrochronologists worldwide and are used in dating fires and reconstructing the fire histories of modern forests evidence of fires in ancient forests such as fossil charcoal fusain is well known to paleontologists and has been reported in geologic formations dating back to the late devonian we describe what we conclude is a fire scar on a fossil tree trunk from the late triassic chinle formation of southeastern utah 200 225 ma the external features of the prehistoric scar match those of modern fire scars better than those of scars created by other kinds of wounding events the fossil specimen also exhibits a number of changes in wood anatomy similar to those reported in modern fire scarred trees including a band of very small tracheids that indicate growth suppression immediately associated with the scarring event an area with a tangential row of probable traumatic resin ducts and a significant increase in tracheid size following the scarring event that indicates a growth release no fire scar resembling those in modern trees has previously been described in petrified wood as far as we can determine the presence of a fire scar not only provides further evidence of ancient fires but also shows that at least some individual trees survived them indicating that fire could have been an ecological and evolutionary force in forests at least as early as the late triassic

quantitative mineral resource assessment of undiscovered pge resources in finland abstract most of the known platinum group element pge resources in finland are in contact type and reef type deposits in 2 45 ga mafic ultramafic layered intrusions these intrusions are also estimated to contain the majority of possibly existing yet undiscovered pge resources in finland the undiscovered pt pd au ni and cu resources in contact type and reef type deposits were estimated down to 1000 m depth using a three part quantitative assessment method this included the delineation of 19 contact type and 24 reef type permissive tracts the formulation of a grade tonnage model for the contact type deposits and a separate model for each reef type tract estimation of the number of undiscovered deposits for each permissive tract and monte carlo simulation to estimate the frequency distributions of metal tonnages in the undiscovered deposits the expected number of undiscovered contact type deposits is 29 and that of reef type deposits is 23 at the 50 probability level the total undiscovered pge resources of finland are estimated to be at least 5600 t pt 12 000 t pd 430 t au 4 2 mt ni and 5 7 mt cu at the 90 probability level the corresponding tonnages are 720 t pt 1700 t pd 59 t au 0 87 mt ni and 0 91 mt cu more than 80 of the pt pd in the undiscovered resources are estimated to be in reef type deposits the known pge resources in finland are 259 t pt pd in six contact type deposits plus 69 t pt pd in the atypical kevitsa deposit these figures indicate that the presently known deposits account for approximately 2 of the total assessed pge resources in finland they suggest that significant potential exists for the discovery of additional pge resources in the country

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

the orientation of buried iceberg scours and other linear phenomena abstract the average orientations of surficial iceberg scours can be obtained from sidescan sonograms using certain assumptions the average orientation of similar buried phenomena can be estimated from a pattern of intersecting seismic profiles two possible solutions are generated at each intersection of two seismic profiles each of which is oriented at an angle i from the profile which encounters the fewer scours where i tan 1 sin i r cos i and r is the ratio of the number of scours observed on the two seismic lines several pairs of solutions are required to determine which solution of each pair is significant iceberg scours observed on aerial photographs of the floor of glacial lake agassiz demonstrate that the estimates provided by this technique correspond with the average orientation of the population applying this method to buried iceberg scours in emerald basin scotian shelf gives orientations which are parallel to paleo bathymetric contours

komatiites of the wildara leonora belt yilgarn craton wa the missing link in the kalgoorlie terrane abstract the wildara leonora belt in the kalgoorlie terrane is located between the world class nickel provinces of the agnew wiluna belt to the north and the kambalda domain to the south it extends from the wildara station to the gold mining center of leonora and consists of a deformed package of mafic komatiitic and sedimentary rocks this package of rocks which hosts a number of small nickel sulfide occurrences has been explored extensively for komatiite hosted nickel deposits most recently within the sullivans unit in the south east of the wildara leonora belt lithostratigraphic similarities together with shared physical geochemical and textural properties support a common affiliation between the wildara leonora and the agnew wiluna belts on this evidence the wildara leonora belt represents the southern extension of the largest komatiite hosted nickel camp in the world the apparently lesser prospectivity for nickel sulfide deposits is interpreted as a consequence of contrasting volcanic facies assemblages komatiites in the wildara leonora belt present geochemical and volcanological characteristics consistent with turbulent lava flow emplacement however compared with the agnew wiluna belt the wildara leonora belt komatiites are not spatially associated with felsic volcanic units but rather overlie a thick sequence of pillowed basalts and sulfidic sediments the lower proportion of adcumulate greater degrees of fractionation and lower degrees of contamination are consistent with komatiites in the wildara leonora belt having been emplaced in a more distal position to the rift environment with lower magma flux and more tranquil emplacement than the agnew wiluna belt komatiites and consequently having lower potential for giant nickel sulfide deposits such as those at perseverance and mt keith

platinum group elements in kostomuksha komatiites and basalts implications for oceanic crust recycling and core mantle interaction we report precise pge os ir ru pt and pd concentrations for komatiite basalt lava sequences of the kostomuksha greenstone belt and for the komatiite standards kal 1 abitibi and wits 1 barber ton the flowtop breccia komatiites mgo 5 27 5 from the kostomuksha sequence which represent the composition of the primary komatiite liquid have os 5 1 5 6 0 1 ppb chondritic os ir n 5 1 06 6 0 05 and moderately fractionated pge patterns pd os n 5 6 5 6 0 4 based on the kal 1 standard the alexo primary liquid had os 5 1 8 6 0 1 ppb os ir n 5 1 05 6 0 05 and pd os n 5 8 5 6 1 0 pges in the kostomuksha komatiites exhibit incompatible behavior during magmatic differentiation with bulk d values of os ir 5 0 75 pt 5 0 52 and pd 5 0 pge abundances from kostomuksha komatiites and basalts were used to calculate the pt os and re os composition of a magnesian archean oceanic crust the pt re os isotopic evolution of this crust was modeled indicating that coupled 186 187 os enrichments observed in some modern plumes require a non crustal source a previously proposed model for core mantle interaction by physical admixture was evaluated and was shown to produce mantle sources with high pge abundances os 5 6 to 14 ppb pd 5 140 ppb for 1 outer core addition and os ir n 1 depending on the assumptions made these features were not observed in the kostomuksha komatiites g 187 os t 51 3 6 shown here to be derived from a reservoir with 25 lower absolute pge abundances than primitive mantle if the radiogenic initial 187 os 188 os in the kosto muksha komatiite is an outer core signature then core mantle interaction cannot have involved physical admixture of outer core material into mantle plume sources instead it must have occurred in the form of isotopic equilibration between liquid outer core and solid mantle at the core mantle boundary copyright 2000 elsevier science ltd

the fire history of danish heathland areas as reflected by pollen and charred particles in lake sediments sediments deposited during the last 5400 years in two danish closed oligotrophic lakes have been analysed for pollen dark coloured hyphae and microscopic charred particles chronologies are provided by numerous radiocarbon datings redundancy analysis is used to develop a model for the relationship between concentrations of sedimentary charred particles and pollen at lake solso the only pollen type which is significantly correlated with charred particles is calluna the model is used to predict charred particle concentrations in the sediments of lake skanso from the calluna concentrations and the predicted values are compared to the observed the results indicate that 1 the source area and the means of transportation for microscopic charred particles and pollen are approximately identical 2 charred particles and pollen may be focused differentially in a lake and 3 high fire intensities triggered heathland expansion and heaths were maintained by fires through millennia

the paleoproterozoic komatiite hosted pge mineralization at lomalampi central lapland greenstone belt northern finland several komatiite hosted ni cu pge deposits occur in archean and paleoproterozoic greenstone belts in finland some of these deposits are enriched in platinum group elements especially in pd and pt the lomalampi pge cu ni deposit is associated with a peridotitic cumulate body of the sattasvaara formation in the paleoproterozoic central lapland greenstone belt the sulfides in the deposit occur in disseminated form whole rock sulfur contents are 0 4 2 wt and ni contents are 0 5 wt and cu 0 4 wt while pge contents exceed 500 ppb the sulfides consist of magmatic pentlandite pyrrhotite and chalcopyrite and have not been substantially modified by metamorphic processes palladium minerals are associated with sulfides and silicates but the only pt bearing phase sperrylite occurs mainly within silicates the host rock of the deposit is a chromite undersaturated al undepleted high mg basalt or low mg komatiite in contrast to most other komatiite hosted ni cu pge deposits world wide that have pt pd around 0 5 the lomalampi deposit is enriched in pt over pd pt pd 2 only a weak contamination signal in the host cumulate is evident in ree data but a strong signal is evident in s isotope ratios 34s 10 to 15 which differ substantially from the mantle value 0 2 geochemical characteristics e g pge enrichment and r factor modeling indicate certain similarities between lomalampi and the raglan ni cu pge deposits of canada the combined data suggest that lomalampi formed through contamination of a pge rich magnesian magma with s rich country rocks this suggests that the extensive central lapland greenstone belt is favorable for komatiite hosted ni cu pge deposits which are substantially enriched in platinum and palladium

petrogenesis of the pt pd mineralized jinbaoshan ultramafic intrusion in the permian emeishan large igneous province sw china the jinbaoshan ultramafic intrusion is a sheet like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins pge are enriched in several disseminated sulfide zones in the intrusion olivine from the intrusion has low fo and depleted ni contents compared to olivine from coeval emeishan picrites whole rock major and trace element concentrations suggest that the jinbaoshan wehrlites originally contained 30 trapped liquid the total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid the jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma no clear evidence of crustal contamination is indicated by s nd and os isotopes we envision that sulfide saturation occurred at depth due to olivine and chromite crystallization immiscible sulfide droplets were transported to the jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high pge concentrations

partitioning of palladium iridium platinum and gold between sulfide liquid and basalt melt at 1200 c abstract iron nickel monosulfide and basalt glass containing trace amounts of pge equilibrated at 1200 c and fo 2 10 9 2 close to the wustite magnetite buffer and fs 2 10 0 9 have been analyzed for noble metals by radiochemical and instrumental neutron activation analysis the average contents of pge in coexisting fe ni sulfide and basalt glass respectively are pd 50 ppm and 0 5 ppb ir 50 ppm and 0 5 ppb pt 100 ppm and 10 ppb and au 0 7 ppm and 0 8 ppb the sulfide liquid silicate melt partition coefficients d values for the noble metals are 9 7 10 4 for pd 1 0 7 10 5 for ir 9 6 10 3 for pt and 1 0 9 10 3 for au the noble metals are strongly partitioned into sulfide liquid but the affinity of pd and ir for sulfide liquid is about 50 times greater than that of pt and about 500 times greater than that of au the d values indicate that equilibrium partitioning between immiscible sulfide liquid and basalt magma would result in fractionation of the noble metals which differs significantly from that generally observed in nature

platinum group element fractionation in a komatiitic basalt lava lake precise pge os ir ru pt pd abundance data for bulk rock samples and mineral separates from a deep differentiated komatiitic basalt lava lake in the vetreny belt baltic shield were obtained using an icp ms id technique the composition of the erupted liquid mgo 5 15 is characterized by a highly fractionated pge pattern pd os n 5 48 with subchondritic os ir n 5 0 43 analysis of separated olivine and chromite revealed that ru os and ir were slightly compatible to moderately incompatible in olivine d 5 1 7 0 8 and were compatible with chromite d 5 100 150 platinum and pd were highly incompatible with olivine d 5 0 08 0 03 and moderately compatible with chromite d 5 1 6 3 3 bulk solid liquid partition coefficients for pges ni cr and cu estimated from the whole rock regressions were os 7 9 ir 6 6 ru 4 5 pt 0 53 pd 0 09 ni 6 2 cr 4 4 and cu 0 01 the highly incompatible behavior of cu and pd indicates that there was no separation of fenicu immiscible sulfide liquid during the differentiation of the lava lake comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included 10 2561 mass fraction of pyrrhotite the data obtained provide new constraints on the effects of fractional crystallization on the pge compositions of mafic ultramafic magmas copyright 2001 elsevier science ltd