geology of unmineralized and gold bearing iron formation contwoyto lake point lake region northwest territories canada reply numerous gold occurrences including the lupin mine exist in archean iron formation within the contwoyto lake point lake region of the slave province early studies suggested that gold was a syn

debris jets in continental phreatomagmatic volcanoes a field study of their subterranean deposits in the coombs hills vent complex antarctica abstract the ferrar large igneous province of antarctica contains significant mafic volcaniclastic deposits some of which are interpreted to fill large vent complexes such a complex was re examined at coombs hills to map individual steep sided cross cutting bodies in detail and we found several contrasting types two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing non consolidated debris these transient conduits were opened because of the propagation of debris jets upward moving streams of volcaniclastic debris steam magmatic gases liquid water droplets following explosive magma aquifer interaction some debris jets probably remained wholly subterranean whereas others made it to the surface but the studied outcrops do not allow us to differentiate between these cases the pipes filled with country rock rich lapilli tuff or tuff breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex causing fragmentation of both magma and abundant country rock material in contrast some of the cross cutting zones filled with basalt rich tuff breccia or lapilli tuff could have been generated following explosions taking place within pre existing basalt bearing debris well away from the complex walls or floor we infer that once focused jets were formed they did not incorporate significant amounts of existing debris while travelling through them instead incorporation of fragments from the granular host took place near explosion sites other basalt rich tuff breccia zones accompanied by domains of in situ peperite and coherent basalt pods are inferred to have originated by less violent processes

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

syn and post eruptive volcanic processes in the yubileinaya kimberlite pipe yakutia russia and implications for the emplacement of south african style kimberlite pipes abstract the yubileinaya kimberlite pipe with a surface area of 59 ha is one of the largest pipes in the yakutian kimberlite province the devonian pipe was emplaced under structural control into lower paleozoic karstic limestone the pipe complex consists of several smaller precursor pipes which are cut by the large round main pipe while the precursor pipes show many features typical for root zones main pipe is younger cuts into the precursor pipes and exposes well bedded volcaniclastic sediments the maximum estimated erosion since emplacement is 250 m open pit mapping of a 180 m thick kimberlite sequence documents the waning phases of the volcanic activity in the kimberlite pipe and the onset of its crater infill by resedimentation three volcanic lithofacies types can be differentiated the deepest and oldest facies type is a massive volcaniclastic rock akb only accessible in drill core it is equivalent to tuffisitic kimberlite in south african pipes and thought to be related to the main volcanic phase which was characterized by violent explosions the overlying lithofacies type comprises primary and resedimented volcaniclastic sediments as well as rock avalanche deposits sourced from the exposed maar crater collar it represents the onset of sedimentation onto the crater floor during the waning phase of volcanic eruptions where primary pyroclastic deposition was contemporaneous with resedimentation from the tephra wall and the widening maar crater ongoing volcanic activity is also testified by the presence of a vertical feeder conduit marking the area of the last volcanic eruption clouds piercing through the diatreme this feeder conduit is overlain by the third and youngest lithofacies type which consists mainly of resedimented volcaniclastic material and lake beds during the sedimentation of this facies primary volcanic activity was only minor and finally absent and resedimentation processes dominated the crater infill the yubileinaya pipe complex exposes root zones contact breccias as well as diatreme and crater infill sediments it has all features typical of large south african style pipes and much can be learned from yubileinaya about the emplacement sequence and behaviour of these pipes emplacement of the pipe occurred over an extended time span with intermittent phases of volcanic quiescence and consolidation the akb reveals little direct evidence of what sort of emplacement process was dominant during the main period of volcanic activity there is neither textural evidence that violent degassing of a juvenile gas phase has caused pipe excavation nor that external water was present during the main phase of volcanic eruptions however there is clear evidence in rock textures that meteoric surface water was present during crater infill base surge deposits forming part of the bedded crater infill sequence indicate that water was present in the eruption clouds and hence the root zone of the pipe there is no reason to assume that groundwater did not also have access to the ascending magma during the main phase of volcanic activity that excavated the pipe and formed the akb

monogenetic v polygenetic kimberlite volcanism in depth examination of the tango extension super structure attawapiskat kimberlite field ontario canada

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

formation of tuffisitic kimberlites by phreatomagmatic processes the rock type tuffisitic kimberlite tk occurs in the deeper diatremes of some southern african canadian and siberian kimberlite pipes and has been considered a unique rock type related to a particular emplacement process the key features of this rock type are its fragmental nature its massive well mixed appearance and a specific matrix mineralogy characterized by the presence of serpentine and microlitic clinopyroxene and the absence or scarcity of carbonate historically all these features were thought to be related to a highly specific intrusive extrusive magmatic emplacement process in this process the expansion of magmatic volatiles drives the fluidization of a pre existing shallow crustal vertical magma reservoir the embryonic pipe and its consequent evolution into the final diatreme after breakthrough to the surface the specific matrix mineralization is explained by the dissociation of carbonate into co2 and cao while the expanding co2 drives the fluidization process cao reacts with sio2 released from xenolith and olivine alteration to form microlitic clinopyroxene with the phreatomagmatic process chain we offer an alternative model that can readily explain this specific rock type ongoing explosions in a downward penetrating root zone excavate a pipe consisting of a rather regular cone shaped diatreme underlain by the irregular root zone at this stage the pipe in its majority is infilled by warm to hot pyroclastic tephra during thermohydraulic explosions in the root zone the expansion of water vapor homogenizes the overlying diatreme tephra and mixes it with the adjacent country rock lithologies this result in the massive well mixed volcaniclastic rock type typical for tkbs post emplacement hydrothermal alteration and mineralization under epi to mesothermal conditions are considered responsible for the observed specific matrix mineralogy carbonate is frequently present in coherent root zone rocks but is rare or absent due to its dissolution in hot hydrothermal fluids ascending from the lower diatreme which in turn precipitate their dissolved carbonate higher up in the cooler bedded upper diatreme region clinopyroxene also grows under epi to mesothermal conditions for the crystallization of serpentine a x h2o x co2 ratio of 20 1 is required which suggests both a high availability of groundwater and a low abundance of co2 after and likely during emplacement isotope studies h c o sr of matrix minerals suggest participation of groundwater in the formation of these minerals

polygenetic quaternary volcanism at crater flat nevada abstract alkali basalts erupted during the quaternary at crater flat nevada record a complex history of polycyclic and polygenetic volcanism magmas from the two main centers black cone and red cone are petrographically and geochemically similar although field evidence suggests a number of separate eruptive events high incompatible element concentrations low nb la and high zr y indicate that the magmas were derived by small degrees of partial melting from the lithospheric mantle at red cone a significant range of sr la ce ba and th concentrations is observed with time e g sr range 1308 1848 ppm the youngest samples having the more elevated values however there is only limited variation in the compatible trace elements e g sc and ni the array of compositions at red cone could not have been produced by changes in the degree of partial melting or by fractional crystallization rather a model of magma mixing is proposed between relatively enriched and depleted end members the cluster of black cone data falls consistently at the least enriched end of the red cone sample arrays suggesting that the black cone magma represents one of the mixing end members the modeling indicates that the magmatic plumbing systems of the two centers were linked at least during the early stages of volcanism moreover volcanic activity may have occurred at a number of sites along the length of the magmatic feeder zone during a single eruptive phase this could have significant implications for volcanic hazard assessment in the region around yucca mountain and the proposed nuclear waste repository

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

late archean regional deformation and structural controls on gold quartz vein mineralization in the northwestern slave province n w t canada late archean crustal accretion in the northwestern slave province is suggested to have involved approximately west northwest east southeast directed horizontal compression that produced three episodes of deformation recognizable in the northwestern anialik river igneous complex aric and anialik river greenstone belt argb observations show that the aric was probably emplaced as a series of synvolcanic sills prior to the earliest deformational event regional shortening produced a pervasive foliation downdip lineations and folding in the argb and an early subsequently folded foliation in the northwestern aric postfold ductile and brittle ductile deformation produced regional and outcrop scale shear zones including the sheeted zone which defines the aric argb contact younger rocks of the northwestern argb appear to have been tectonically juxtaposed along the sheeted zone against the older rocks of the northwestern aric greater brittle response enhanced permeability and cyclical increases in f

tectonic and thermal history of the anialik river area northwestern slave province canada the anialik river area in the northwestern slave province comprises two geological domains of different age and origin that were tectonically juxtaposed at ca 2650 ma the older domain the kangguyak gneiss belt comprises ca 3300 2700 ma orthogneisses and paragneisses interpreted as the remnants of a mesoarchean continental margin the younger domain the anialik river greenstone belt consists of ca 2680 ma mafic to felsic volcanic rocks interpreted to have formed in an ensimatic island arc setting structural and geochronological evidence suggest collision of the two domains began around 2650 ma in a transpressive regime that involved oblique sinistral subduction of the greenstone belt beneath the kangguyak domain along the tokhokatak shear zone displacement continued until at least ca 2600 ma when late two mica granites intruded along and were deformed in the shear zone following ca 2600 ma rocks in both domains and along the fault cooled rapidly to about 350 c strongly overprinted musco

factors controlling the internal facies architecture of maar diatreme volcanoes most if not all kimberlite pipes show a multitude of facies types which imply that the pipes were emplaced under an episodic re occurrence of eruptive phases often with inter mittent phases of volcanic quiescence the majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur ing sedimentation of volcaniclastic deposits as well as their alteration and compaction after deposition an additional factor controlling pipe facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar diatreme volcano in a growing pipe the root zone moves downward and with that movement the overlying diatreme enlarges both in size and diameter however during the life span of the volcano the explosion chamber can also move upward backintothelower diatreme where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures nextto vertical shifts of explosion chambers the loci of explosions can also move laterally along the feeder dyke or dyke swarm this mobility of explosion chambers results in a highly com plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam ated crosscutting diatreme and maar crater with several lobes pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag ma recent mapping and logging results of canadian and african kimberlite pipes suggest that kimberlite magma frag mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter like production with scarce xenoliths on occasions spatter may even reconstitute and form a texturally coherent deposit on the crater floor in addition ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface respectively this highly variable emplacement behaviour is typical for basaltic maar diatreme volcanoes and since similar deposits can also be found in kimberlites it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes

tectonically controlled vent migration during maar diatreme formation an example from a miocene half graben basin in se korea abstract understanding the roles of external controls such as hydrology and tectonics is crucial for interpreting the evolution of a phreatomagmatic volcano and its subsurface structure called the diatreme the yangpori diatreme in a miocene terrestrial half graben basin of korea provides an opportunity to assess the roles of synvolcanic fault movement in forming a maar diatreme volcano the exposed part of the diatreme fill consists of two offset stacked sequences with contrasting lithofacies characteristics a lower sequence early diatreme fill of locally stratified or bedded basaltic lapilli tuff and tuff in the northeastern part and an upper sequence later diatreme fill of dacite clast rich and disorganized tuff breccia in the southwestern part the boundary between these sequences is abrupt suggesting eruption of the volcano in two distinct phases the first phase of eruption was driven by phreatomagmatic explosions mainly within the basin fill some of the diatreme fill was deposited in a local pond by debris flows and suspension settling of fines whereas the rest by fallout from eruption plumes and jets the second phase of eruption was driven by phreatomagmatic explosions at a fracture aquifer within the basement accompanying sudden lowering of the explosion locus the later diatreme fill therefore contains abundant basement derived dacite clasts its deposition was accomplished by en masse fallout of volcanic debris from dense collapsing columns a number of faults and shear fractures were generated within and in the vicinity of the diatreme together with penecontemporaneous basaltic intrusions structural and geochemical analyses show that many of these structures are genetically related with the formation of the diatreme the early diatreme fill was also tilted toward the southwest suggesting southwestward shift of the locus of explosion and diatreme excavation the explosion locus of the yangpori volcano is therefore interpreted to have migrated both vertically and laterally the abrupt lithofacies boundary between the early and later diatreme fills and the proximity of the diatreme to the basin margin suggests that the migration was caused by an abrupt tectonic activity near the basin margin most probably the feeder dike was dislocated by movement along the basin margin fault which created abundant fractures deep within the basement that redirected ascending magma and opened a new hydraulically active zone in the basement synvolcanic faulting is therefore interpreted to have played a crucial role in forming the offset stacked architecture of the yangpori diatreme sequences the faulting is also inferred to have influenced significantly the morphology and internal facies architecture of unpreserved ejecta rimbeds

positive correlation between li and mg isotope ratios in the river waters of the mackenzie basin challenges the interpretation of apparent isotopic fractionation during weathering during chemical weathering magnesium mg is released by the dissolution of both carbonate and silicate sources the degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs or cycling by a series of processes associated with weathering is poorly constrained in the river waters of the mackenzie basin canada the mg content is high and mg isotope ratios 26mg 24mg expressed as mg 26 show in excess of one per mil variability part of this variability is attributed to the 3 range in the carbonate and silicate rocks drained despite this inherent lithological control on river water mg 26 values there is also evidence for a fractionation control a linear positive covariation between lithium 7li 6li expressed as li 7 and mg isotope ratios in the river waters of the mackenzie basin is reported this covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with mg isotope ratios

distribution of mineral deposits in accreted terranes and cratonal rocks of western united states the western margin of the conterminous united states covering roughly 300 000 mi2 777 000 km2 is an agglomeration of tectonostratigraphic terranes accreted to the north american craton mainly during mesozoic time the terranes represent a number of fundamental crustal types oceanic crust island arc crust melange various combinations of the preceding three batholithic miogeoclinal and platform the distribution patterns of types of mineral deposits show that miogeoclinal terranes of the craton are characterized by replacement and vein type lead zinc silver skarn tungsten deposits molybdenum and tin whereas accreted terranes contain all the known volcanogenic massive sulfide deposits all chromite and chert associated manganese and all the large gold quartz vein deposits except goldfield nevada carlin type disseminated fine grained gold deposits occur mostly in windows of paleozoic miogeoclinal rocks in nevada but the only known fine grained gold deposit in california is in very youthful

stratigraphy of the intra crater volcaniclastic deposits of the victor northwest kimberlite northern ontario canada abstract the victor northwest vnw kimberlite is one of several steep sided pipes in the victor kimberlite complex in this paper detailed logging of 4 2 km of drill core and petrographic studies of hundreds of samples and thin sections are used to reconstruct the intra crater stratigraphy of the vnw kimberlite and to constrain the emplacement history of the pipe the results show that the vnw pipe comprises numerous contrasting small volume volcanic facies including dark and competent kimberlite sedimentary country rock breccias pyroclastic kimberlite and resedimented volcaniclastic kimberlite we interpret that the vnw pipe was formed by two separate eruption cycles during the first eruption cycle the main vnw crater is excavated and partly filled the second eruption cycle excavates a smaller nested crater within the existing lithified pipe fill both eruption cycles can be subdivided into three comparable stages the first stage in both cycles comprises a highly explosive eruption involving crater excavation and deposition of pyroclastic kimberlite the eruption products contain abundant broken olivines small angular country rock fragments and kimberlite ash all these features indicate high degrees of fragmentation resulting from high eruption intensities the high proportion of country rock fragments in these deposits suggests continuous pipe wall erosion the second stage in both cycles is represented by dark and competent deposits which have low country rock fragment abundances and mostly intact olivines based on contact relationships and textures within these units it is suggested that these rock types are formed by lower energy eruptions during which no major pipe wall erosion took place the eruption came to an end during the third and last stage in both cycles the uppermost deposits record resedimentation of kimberlite by water major pipe wall collapse results in the formation of voluminous sedimentary country rock breccia deposits that cap all previous deposits

revised conceptual model for maar diatremes subsurface processes energetics and eruptive products diatremes are debris filled structures beneath maars that result from many magma water phreatomagmatic explosions during a monogenetic volcano s lifetime a long standing model requires deepening explosions due to water table drawdown that eject progressively deeper seated country rock from the explosion sites while the overlying diatreme and its surface crater widen due to subsidence a revised model is proposed wherein explosions can take place at any level within a diatreme at a given time most effectively venting material from near surface explosions deep seated country rock lithics in tephra deposits record stepwise vertical mixing of material by upward directed debris jets and downward subsidence rather than direct ejection from deep explosions juvenile and lithic clasts erupted during a given explosion may have had a complex history within the diatreme and need not directly reflect fragmentation or brecciation during the explosion that ejects them

physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record we assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 1200 m and by a comparison with intraplate monogenetic basaltic volcanic fields eroded kimberlite fields are composed of pipes or diatremes and dikes and within any one kimberlite field regardless of erosion level kimberlite bodies vary in area at earth s surface over 2 3 orders of magnitude typically 60 70 of the bodies are

contrasting eruption styles of the 147 kimberlite fort a la corne saskatchewan canada abstract the cretaceous fort a la corne falc kimberlite field was active over a time span of 20 ma with contemporaneous terrestrial mannville group to marine lower colorado group background sedimentation steep sided pipes craters and positive landform volcanoes such as scoria or tuff cones are thought to have formed during that period the 147 kimberlite is located in the se section of the field s main cluster and is part of the large 377 5 ha orion north volcanic complex based on logging of 25 drill cores the morphology of the country rock kimberlite interface suggests excavation of a complex crater field down to the upper portion of the mannville group sedimentary deposits at least two types of volcaniclastic deposits are identified a main kimberlite unit that is typically characterized by crustal xenolith poor 1 2 normal graded beds possibly deposited as turbidites in a subaqueous environment originating from the nearby 148 tephra cone and infilling the adjacent 147 crater and a second unit located on the ne margin of the 147 kimberlite that represents a thick 60 m sequence of large up to 22 m sedimentary country rock blocks located at least 60 m above their original stratigraphic position we suggest the following time sequence of events crater excavation as a consequence of a shallow magma fragmentation level within the uppermost country rock sequences together with several closely spaced eruptive centres initially formed the complex intercalated crater field subsequently ongoing eruptions with a fragmentation level above the country rock produced the lithic fragment poor main infill of the 148 kimberlite resedimentation from the outer flanks of the 148 tephra cone resulted in the deposition of turbidites in the 147 area a consolidation phase solidified the lowermost portion of the main infill in 147 a subsequent explosion s occurred within the mannville group in the 147 area ejecting large blocks of sedimentary country rocks and fracturing the overlying volcaniclastic main infill finally blocks of the main infill tilted and possibly slumped into the subsidence structure developed above the emptied explosion chamber of 147 the different volcanic deposits reflect a change in eruption style and fragmentation level from highly explosive to spatter activity with little fragmentation potential cap rocks to build up the volatile overpressure necessary to blast the craters were not present at the time of emplacement no diatremes were observed in the study area assuming that the magma properties remained constant over time the change in eruption style has to be attributed to external factors such as water access to the rising magma the volcanic behaviour of the kimberlite magma appears to be comparable to that of other magmatic systems both in eruptive style and production rate no evidence was found for a high possibly plinian production rate or dispersion

deciphering the tectonic evolution of archaean greenstone belts the importance of contrasting histories to the distribution of mineralization in the yilgarn craton western australia abstract the distribution of archaean metallic mineral deposits reflects tectonic controls operating during the evolution of granitoid greenstone terranes tectonic analysis of these terranes not only allows recognition of parameters that may be important in interpreting the tectonic histories of greenstone belts but also the resultant tectonic control on mineral deposit distribution in the yilgarn craton of western australia the late archaean 2 7 ga norseman wiluna belt is most intensely mineralized with both komatiite associated ni cu deposits and epigenetic gold deposits this belt is dominated by 2 7 ga supracrustal successions and its tectonic history is that of an orogenic belt at an obliquely convergent craton or proto craton margin tectonostratigraphic associations preserved resemble younger volcanic arc and back arc basin crust and many represent the remains of a volcanic arc and back arc basin complex developed in response to westward dipping subduction these sequences were deformed metamorphosed and intruded by granitoids relatively soon 30 ma after the major period of volcanism with gold mineralization occurring late in the orogenic event possibly during uplift in contrast greenstone belts in the murchison and southern cross provinces are dominated by older supra crustal successions that contain 2 9 ga volcanosedimentary associations characterized by extensive oxide facies banded iron formations bifs bifs appear to have been deposited in shelf or platform environments in intra oceanic or passive craton margin settings komatiite associated ni cu mineralization and volcanogenic cu zn mineralization are locally hosted by favourable tectonostratigraphic associations the main period of deformation metamorphism and gold mineralization in these provinces was broadly synchronous with that in the norseman wiluna belt as was regional granitoid emplacement during this event strain was probably transmitted from the eastern margin of the craton via a yilgarn wide system of shear zones faults and fold belts gold mineralization is developed best in greenstone belts such as the meekathara mt magnet belt that contain large scale fault systems and internal granitoids similar tectonic controls on the distribution of mineral deposits can also be recognized in other archaean cratons and the heterogeneous distribution of archaean mineral deposits probably reflects a range of tectonic settings as well as different stages in the evolution of individual cratons

geology of the gahcho kue kimberlite pipes nwt canada root to diatreme magmatic transition zones abstract the cambrian gahcho kue kimberlite cluster includes four main pipes that have been emplaced into the archaean basement granitoids of the slave craton each of the steep sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite hk to diatreme facies tuffisitic kimberlite breccia tkb the tkb displays many diagnostic features including abundant unaltered country rock xenoliths pelletal lapilli serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate the hk contains common fresh olivine set in a groundmass composed of monticellite phlogopite perovskite serpentine and carbonate a number of separate phases of kimberlite display a magmatic textural gradation from tkb to hk which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity proportion of fresh olivine and the degree of xenolith digestion the pipe shapes and infills of the gahcho kue kimberlites are similar to those of the classic south african pipes particularly those of the kimberley area similar intrusive magmatic emplacement processes are proposed in which the diatreme zone results from the degassing after breakthrough of the intruding magma column the transition zones represent frozen degassing fronts the style of emplacement of the gahcho kue kimberlites is very different from that of many other pipes in canada such as at lac de gras fort a la corne or attawapiskat

the slave province a distinct archean craton the slave province is a relatively small late archean craton that exhibits distinctive rock associations and structures by comparison with the much larger superior province differences are evident in i the abundance of sedimentary versus volcanic rocks and of felsic versus mafic volcanic rocks ii the greater evidence for sialic basement iii the higher proportion of more evolved potassium rich granite iv the type setting and timing of gold and base metal mineralization and v a regional zonation of gold deposits that seems to be a unique feature of the slave province contrasts in structure are also significant the large scale linear belts and boundaries sutures that characterize the superior province have no obvious counterpart in the slave province despite some similarities with other archean cratons the distinct features of the slave province are important for they imply that no single archean craton should be used to develop a universal paradigm for the genesis and tectonic evol

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

the influence of complex intra and extra vent processes on facies characteristics of the koala kimberlite nwt canada volcanology sedimentology and intrusive processes the koala kimberlite northwest territories canada is a small pipe like body that was emplaced into the archean koala granodiorite batholith and the overlying cretaceous to tertiary sediments at 53 ma koala is predominantly in filled by a series of six distinct clastic deposits the lowermost of which has been intruded by a late stage coherent kimberlite body the clastic facies are easily distinguished from each other by variations in texture and in the abundance and distribution of the dominant components from facies analysis we infer that the pipe was initially partially filled by a massive poorly sorted matrix supported olivine rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe forming eruption this unit is overlain by a granodiorite cobble boulder breccia and a massive poorly sorted mud rich pebbly sandstone these deposits represent post eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim the crater then filled with water within which 20 m of non kimberlitic wood rich silty sand accumulated representing up to 47 000 years of quiescence the upper two units in the koala pipe are both olivine rich and show distinct grain size grading these units are interpreted to have been deposited sub aqueously from pyroclastic flows sourced from one or more other kimberlite volcanoes the uppermost units in the koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes

volcanic stratigraphy geochronology and gold deposits of the archean hope bay greenstone belt nunavut canada the archean hope bay greenstone belt is located in the bathurst block of the northeastern part of the slave structural province a predominantly archean granite greenstone metasedimentary terrane the greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks three main au deposits have been defined in the greenstone belt with a cumulative resource of over 10 million ounces moz of au as of april 2007 the hope bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by u pb geochronology on felsic suites the oldest felsic suite the flake lake suite ca 2700 ma is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity interpreted as a product of rift related volcanism overlying this suite is a series of well constrained calc alkaline mainly felsic volcanic rocks which have ages of ca 2690 ma square lake suite ca 2686 ma windy felsic suite ca 2677 ma koignuk suite and ca 2662 ma clover lake suite all of which are interpreted as products of arc volcanism the transition from rift to arc related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca 2690 ma the older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an fe rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle these fe rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to au mineralization acting as host strata to the main au deposits in the greenstone belt the younger cycle ca 2690 ma of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column detrital zircon work was carried out for all the main sedimentary successions recognized in the hope bay greenstone belt the detrital zircon results show that in each case the sediments were derived mainly from local sources as very few anomalously old zircons were recognized a conglomerate outcropping in the north part of the belt is dominated by locally derived material with detrital zircon work supporting this interpretation the three main au deposits at hope bay are all associated with the older cycle of volcanic rocks interpreted as products of rift related volcanism the structural setting of each deposit is different but each is related to d2 strain in structural and or stratigraphic settings which provided dilational environments to localize auriferous vein related mineralization or localized alteration and mineralization of favorable host lithologies the older cycle of tholeiitic felsic and fe rich mafic volcanic rocks are rift related volcanic products which are time equivalent to the kam group in the yellowknife supergroup overlying these rocks are calc alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks which represent the transition to calc alkaline arc related volcanism which correlates to the banting group of the yellowknife supergroup overlying the younger ca 2677 ma felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts representing collapse of the arc into a fluvial dominated environment this sedimentary succession is possibly equivalent to the regionally extensive burwash basin of the yellowknife supergroup

volcanology of tuzo pipe gahcho kue cluster root diatreme processes re interpreted abstract the middle cambrian 540 ma gahcho kue kimberlite field is situated about 275 km ene of yellowknife nwt canada the kimberlites were emplaced into 2 6 ga archean granitic rocks of the yellowknife supergroup four larger kimberlite bodies 5034 tesla tuzo and hearne as well as a number of smaller pipes and associated sheets occur in the field in plan view the tuzo pipe has a circular outline at the surface and it widens towards deeper levels the pipe infill consists of several types of coherent and fragmental kimberlite facies coherent or apparent coherent possibly welded kimberlite facies dominate at depth but also occur at shallow levels as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts the central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as tuffisitic kimberlites the definition geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant the fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi plastic state or even of welded material ragged clasts at low levels are inferred to form part of a complex peperite like system that intrudes the base of the root zone a variable often high abundance of local wall rock xenoliths between and within the kimberlite phases is observed varying in size from sub millimeter to several tens of meters wall rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt like geometry between 120 and 200 m depth in the pipe steeply outward dipping bedded deposits made up of wall rock fragments occur in deep levels of the pipe and are especially common under the downward widening roof segments the gradational contact relationships of these deposits with the surrounding kimberlite bearing rocks as well as their location suggest that they formed more or less in situ different breccia facies inside the pipe suggest an origin by slumping grain flows rock fall or pyroclastic deposition the shape and facies architecture of the tuzo pipe suggests that the studied section of the pipe lies at a root zone diatreme transitional structural level composite coherent kimberlite clasts imply that recycling processes were active over time while reworked wall rock rich deposits and ductily deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone the emplacement of the tuzo pipe did not occur in a single violent explosion but involved repetitive volcanic explosions alternating with periods of relative quiescence the observed features are typical of phreatomagmatic processes which may include phases of less explosive magmatic activity

the fekete hegy balaton highland hungary soft substrate and hard substrate maar volcanoes in an aligned volcanic complex implications for vent geometry subsurface stratigraphy and the palaeoenvironmental setting the fekete hegy volcanic complex is located in the centre of the bakony balaton highland volcanic field in the pannonian basin which formed from the late miocene to pliocene period the eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard substrate and soft substrate maars in one volcanic complex the analyses of bedding features determination of the proportion of accidental lithic clasts granulometry and age determination helped to identify and distinguish the two types of maar volcanoes ascending magma interacted with meteoric water in karst aquifers in mesozoic carbonates as well as in porous media aquifers in neogene unconsolidated wet siliciclastic sediments the divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks country rocks in hard substrate maars have a steep angle of repose leading to the formation of steep sided cone shaped diatremes enlargement and filling of these diatreme is mainly a result of shattering material by fci related shock waves and wall rock collapse during downward penetration of the explosion locus country rocks in soft substrate maars have much shallower angles of repose leading to the formation of broad bowl shaped structures or irregular depressions enlargement and filling of these diatremes is mainly the result of substrate collapse for example due to liquefaction of unconsolidated material by fci related shock waves and its emplacement by gravity flows the fekete hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

geology of the victor kimberlite attawapiskat northern ontario canada cross cutting and nested craters abstract the pipe shapes infill and emplacement processes of the attawapiskat kimberlites including victor contrast with most of the southern african kimberlite pipes the attawapiskat kimberlite pipes are formed by an overall two stage process of 1 pipe excavation without the development of a diatreme sensu stricto and 2 subsequent pipe infilling the victor kimberlite comprises two adjacent but separate pipes victor south and victor north the pipes are infilled with two contrasting textural types of kimberlite pyroclastic and hypabyssal like kimberlite victor south and much of victor north are composed of pyroclastic spinel carbonate kimberlites the main features of which are similar clast supported discrete macrocrystal and phenocrystal olivine grains pyroclastic juvenile lapilli mantle derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices these partly bedded juvenile lapilli bearing olivine tuffs appear to have been formed by subaerial fire fountaining airfall processes the victor south pipe has a simple bowl like shape that flares from just below the basal sandstone of the sediments that overlie the basement the sandstone is a known aquifer suggesting that the crater excavation process was possibly phreatomagmatic in contrast the pipe shape and internal geology of victor north are more complex the northwestern part of the pipe is dominated by dark competent rocks which resemble fresh hypabyssal kimberlite but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias volcaniclastic kimberlite current evidence suggests that the hypabyssal like kimberlite is in fact not intrusive and that the northwestern part of victor north represents an early formed crater infilled with contrasting extrusive kimberlites and associated breccias the remaining main part of victor north consists of two macroscopically similar but petrographically distinct pyroclastic kimberlites that have contrasting macrodiamond sample grades the juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically the nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different indicating that they derive from different magma pulses or phases of kimberlite and thus represent separate eruptions the initial excavation of a crater cross cutting the earlier northwestern crater was followed by emplacement of phase i a low grade olivine phenocryst rich pyroclastic kimberlite and the subsequent eruption of phase ii a high grade olivine phenocryst poor pyroclastic kimberlite as two separate vents nested within the original phase i crater the second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade thus the final pyroclastic pipe infill of the main part of the victor north pipe appears to consist of at least three geological macrodiamond grade zones in conclusion the victor kimberlite was formed by several eruptive events resulting in adjacent and cross cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal like kimberlite which is now interpreted to be of probable extrusive origin within the pyroclastic kimberlites of victor north there are two nested vents a feature seldom documented in kimberlites elsewhere this study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism

geochemistry of the archean yellowknife supergroup the archean yellowknife supergroup slave structural province canada is composed of a thick sequence of supracrustal rocks which differs from most archean greenstone belts in that it contains a large proportion 80 of sedimentary rocks felsic volcanics of the banting formation are characterized by hree depletion without eu anomalies indicating an origin by small degrees of partial melting of a mafic source with minor garnet in the residua granitic rocks include synkinematic granites hree depleted low 87sr86sr i post kinematic granites negative eu anomalies high 87sr86sr i and granitic gneisses with ree patterns similar to the post kinematic granites sedimentary rocks turbidites of the burwash and walsh formations have similar chemical compositions and were derived from 20 mafic intermediate volcanics 55 felsic volcanics and 25 granitic rocks jackson lake formation lithic wackes can be divided into two groups with group a derived from 50 mafic intermediate volcanics and 50 felsic volcanics and group b characterized by hree depletion derived almost exclusively from felsic volcanics ree patterns of yellowknife sedimentary rocks are similar to other archean sedimentary ree patterns although they have higher lanybn these patterns differ significantly from typical post archean sedimentary ree patterns supporting the idea that archean exposed crust had a different composition than the present day exposed crust

geology of the mwadui kimberlite shinyanga district tanzania abstract the mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in fill albeit without the tuff ring the geology of mwadui can be subdivided into five geological units viz the primary pyroclastic kimberlite pk re sedimented volcaniclastic kimberlite deposits rvk granite breccias subdivided into two units the turbidite deposits and the yellow shales listed in approximate order of formation the pk can be further subdivided into two units lithic rich ash and lapilli tuffs which dominate the succession and lithic poor juvenile rich ash and lapilli tuffs the lower crater is well bedded down to at least 684 m from present surface extent of current drill data the bedding is defined by the presence of juvenile rich lapilli tuffs vs lithic rich lapilli tuffs and the systematic variation in granite content and clast size within much of the lithic rich lapilli tuffs four distinct types of bedding have been identified in the pyroclastic deposits diffuse zones characterised by increased granite abundance and size and upward fining units represent the dominant types throughout the deposit lateral heterogeneity was observed in addition to the vertical changes suggesting that the eruption was quite heterogeneous or that more than one vent may have been present the continuous nature of the bedding in the pyroclastic material and the lack of ash partings suggest deposition from a high concentration ejecta sustained eruption column at times e g the massive very diffusely stratified deposits the paucity of tractional bed forms suggest near vertical particle trajectories i e a clear air fall component but the poorly sorted matrix supported nature of the deposits suggest that pyroclastic flow and or surge processes may also have been active during the eruption available diamond sampling data were examined and correlated with the geology data derive from the old 120 37 m 200 61 m 300 92 m and 1200 ft 366 m levels pits sunk during historical mining operations drill logs as well as more recent bench mapping correlating macro diamond sample data and geology shows a clear relationship between diamond grade and lithology localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post eruptive sedimentary in fill processes clear distinction can be drawn between upper re sedimented and lower pyroclastic crater deposits at mwadui both from a geological and diamond grade perspective finally an emplacement model for the mwadui kimberlite is proposed geological evidence suggests that little or no sedimentary cover existed at the time of emplacement the nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous but that the eruption column may have been heterogeneous both petrologically as well as geometrically volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re sedimented volcaniclastic material derived from the tuff ring the mwadui kimberlite exhibits marked similarities compared to the orapa kimberlite in botswana

an integrated model of kimberlite ascent and eruption diatremes are carrot shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock these unusual enigmatic and complex features are famous as the source of diamonds here we present a new model of kimberlite ascent and eruption emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes dyke initiation in a deep co2 rich source region in the mantle leads to rapid propagation of the dyke tip below which co2 fluid collects with a zone of magmatic foam beneath when the tip breaks the surface of the ground gas release causes a depressurization wave to travel into the magma this wave implodes the dyke walls fragments the magma and creates a ringing fluidization wave together these processes form the diatreme catastrophic magma chilling seals the dyke no precursor to the eruption is felt at the surface and the processes are complete in about an hour

the acheulean from atapuerca three steps forward one step back abstract in western europe acheulean cultural evidence is well attested by ca 0 5 ma however recent work has proven that it was present earlier at the end of the early pleistocene the timing and mode of the western european acheulean needs still to be investigated and this knowledge appears to be strongly dependent upon data from sites with sufficiently well preserved archeo stratigraphical sequences and or geophysical conditions that guarantee reliable dating the karstic deposits from the sierra de atapuerca burgos spain represent one such case they contain exceptionally long and continuous archeo paleontological deposits that have yielded hominins stone tools and faunal remains from a period covering the late early pleistocene up to the holocene previous work has allowed us to reconstruct the evolution of technology on a local scale from the late early to the middle pleistocene and to situate the appearance of the acheulean more or less simultaneously in the sites of galeria gran dolina td10 and the sima de los huesos at around 0 5 ma after a hiatus of archeological evidence of approximately 300 ka these acheulean assemblages appear in association with hominin fossils that have recently been identified as early members of the neandertal lineage delving further into the description of the acheulean from atapuerca we have recently focused on the technological features that we consider to be of particular evolutionary significance and attempted to identify them in other european key sites of comparable chronologies indeed we identified some internal evolutionary trends which led us to propose that there was a local transition at atapuerca from the acheulean to the early middle paleolithic techno complexes however the chronological setting resulting from the intensive dating programs carried out in the galeria and sima de los huesos sites and especially the correlation of these dates with the ones available from gran dolina td10 confronts us with some apparent paradoxes underlining the need to reconsider the technological interpretation of the lithic technical record from atapuerca while great efforts are being made in dating and analysis at many european middle pleistocene sites as well as in establishing correlations with which to build consistent regional sequences single long continuous and accurately excavated i e over a large surface area archeological successions remain crucial to properly interpret the appearance and evolution of the acheulean throughout europe we further stress the importance of taking into account conditioning factors such as raw material availability environmental constraints and site function issues

multiple dikes in lower kam group yellowknife greenstone belt evidence for archean sea floor spreading a mafic intrusive extrusive complex at the base of the kam group in the archean yellowknife supergroup at yellowknife northwest territories canada grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments although rock types and primary structures resemble those in upper parts of phanerozoic ophiolites the kam group lacks an ultramafic base and is much thicker than typical ophiolites because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins however the kam group is viewed as a remnant of archean oceanic crust rather than as a product of intracontinental rift volcanism the dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea floor spreading in an archean marginal basin setting

distinct kimberlite pipe classes with contrasting eruption processes abstract field and scott smith field m scott smith b h 1999 contrasting geology and near surface emplacement of kimberlite pipes in southern africa and canada proc 7th int kimb conf eds gurney et al 1 214 237 propose that kimberlite pipes can be grouped into three types or classes classical or class 1 pipes are the only class with characteristic low temperature diatreme facies kimberlite in addition to hypabyssal and crater facies kimberlite class 2 and 3 pipes are characterized only by hypabyssal and crater facies kimberlite in an increasing number of class 1 pipes a new kimberlite facies transitional facies kimberlite is being found in most cases this facies forms a zone several metres wide at the interface between the hypabyssal and diatreme facies the transitional facies exhibits textural and mineralogical features which are continuously gradational between the hypabyssal and the diatreme types the textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside both transitional and diatreme facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially co 2 rich volatiles from the volatile rich kimberlite magma the transitional facies is initiated by volatile exsolution at depths of about 3 km below the original surface with subsequent cracking through to the surface and resultant rapid decompression the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies thus diatreme facies kimberlite and class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism distinctly different petrographic features characterize crater facies kimberlite in each of the three pipe classes in crater facies kimberlites of class 1 pipes small pelletal magmaclasts and abundant microlitic diopside are characteristic these features appear to reflect the derivation of the crater facies material from the underlying diatreme zone most class 2 pipes have shallow craters and the crater facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli which are sometimes welded and have vesicles as well as glass possible kimberlite lava also occurs at two class 2 pipes in n angola the possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form class 2 pipes most class 3 kimberlites have very steep craters and crater facies rocks are predominantly resedimented volcaniclastic kimberlites in some cases characterized by the presence of abundant angular magmaclasts which are petrographically very similar to typical hypabyssal facies kimberlite found in class 1 pipes the differences in crater facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition specifically volatile composition kimberlite forming pipe classes 1 and 3 is thought to be relatively water rich and is emplaced by processes involving magmatic exsolution of volatiles the kimberlite magma forming class 2 pipes is co 2 rich can rise to shallow levels and can initiate phreatomagmatic emplacement processes

sedimentologic and stratigraphic constraints on emplacement of the star kimberlite east central saskatchewan abstract diamond bearing kimberlites in the fort a la corne region east central saskatchewan consist primarily of extra crater pyroclastic deposits which are interstratified with lower cretaceous albian and cenomanian marine marginal marine and continental sediments approximately 70 individual kimberlite occurrences have been documented the star kimberlite occurring at the southeastern end of the main fort a la corne trend has been identified as being of economic interest and is characterized by an excellent drill core database integration of multi disciplinary data sets has helped to refine and resolve models for emplacement of the star kimberlite detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex micropaleontologic and biostratigraphic analysis of selected sedimentary rocks and u pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at star the oldest kimberlite in the star body erupted during deposition of the predominantly continental strata of the lower mannville group cantuar formation kimberlites within the cantuar formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate successive eruptive events occurred contemporaneous with deposition of the marginal marine upper mannville group pense formation kimberlites within the pense formation consist primarily of terrestrial airfall deposits fine to medium grained cross stratified kimberlitic olivine dominated sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression the location of the source feeder vents of the cantuar and pense kimberlite deposits has not been identified the youngest and volumetrically most significant eruptive events associated with the star kimberlite occur within the predominantly marine lower colorado group joli fou and viking formations kimberlite beds which occur at several horizons within these units consist of subaerial and marine fall deposits the latter commonly exhibiting evidence of wave reworking black shale encased resedimented kimberlite beds likely deposited as subaqueous debris flows and turbidites are particularly common in the lower colorado group during its multi eruptive history the star kimberlite body is interpreted to have evolved from a feeder vent and overlying positive relief tephra ring into a tephra cone initial early joli fou volcanism resulted in formation of a feeder vent 200 m diameter and tephra ring subsequent eruptions dominated by subaerial deposits partly infilled the crater and constructed a tephra cone a late joli fou eruption formed a small 70 m diameter feeder pipe slightly offset to the nw of the early joli fou feeder vent deposits from this event further infilled the crater and were deposited on top of early joli fou kimberlite proximal to the vent and sediments of the joli fou formation distal to the vent the shape of the tephra cone was modified during multiple marine transgression and regression cycles coeval with deposition of the lower colorado group resulting in wave reworked kimberlite sand along the fringes of the cone and kimberlitic event deposits tempestites turbidites debris flows in more distal settings

three dimensional p and s wave velocity structure below unzen volcano abstract unzen volcano located in southwestern japan erupted on november 17 1990 after 198 years of dormancy and has been ejecting lava since may 20 1991 in this paper we present three dimensional p and s wave velocity variations below unzen volcano using 22 473 p and 14 349 s wave arrival times from 3457 local earthquakes recorded on a local network of twelve seismic stations the model was parameterized by 24 000 approximately 2 0 km cubic blocks targeting a volume of 120 100 20 km a prominent low velocity anomaly greater than 4 slowness perturbation starting from 2 5 to 5 0 km depth beneath the volcano is observed on both the p and s wave inversions below unzen volcano slightly lower amplitude low velocity anomalies are further observed to a depth of 12 5 km for p and 7 5 km for s waves shallow low velocity anomalies are observed below the chijiwa bay area northwest of unzen and deeper anomalies are seen below shimabara bay southwest of shimabara peninsula this nw se trending feature represents an elongated system of dykes which supply the shallow magma reservoirs

volcanic geomorphology an overview abstract the review examines the role of geomorphology in analyzing the volcanoes on earth five objectives are stressed first classifications of volcanic landforms should be improved to take care of the complexity in volcanic landform generation as magmatic systems style of eruption and the erupted material all influence the morphology second geomorphology should contribute to the science of volcanology through its capability in reconstructing growth stages in complex volcanoes and also in analyzing the structural factors which contribute to the catastrophic collapse of volcanoes third geomorphology can contribute to physical volcanology by assessing the effects of topography on transport erosion and deposition of volcanogenic flows and identifying the sources and climatic tectonic conditions which govern the emplacement of volcaniclastic deposits fourth volcanic geomorphology a identifies sedimentary facies associations b constructs facies models for dynamic volcano delivery systems and c analyzes the characteristics of sediment gravity flows in order to determine relevant parameters for modelling their behaviour fifth process oriented geomorphology is critical in developing accurate methods for measuring rates of geomorphic processes that shape ephemeral volcanic constructs and for evaluating and comparing geomorphic impacts on disturbed catchments and the related hydrologic response before during and after eruptions this should help to refine parameters for the exponential decay model finally volcanic geomorphology is essential for risk assessment through geomorphic hazard zonation and composite risk zonation such treatments are necessary in order to face the enhanced challenge posed by the combination of natural hazards and the increasing number of people who are at risk around volcanoes

phreatomagmatism and its relevance abstract phreatomagmatic explosive volcanism results from the interaction of magma with external water groundwater or surface water close to or at the earth s surface magma of any composition acid to ultrabasic may be involved in this type of explosive activity at present the relevance of phreatomagmatism is realized increasingly because it takes place in many diverse environments and affects not only small monogenetic volcanoes but also large volcanoes at some volcanoes as for example at lake taupo new zealand a volume of several tens of km 3 of magma may have been mixed with several km 3 of external water phreatomagmatism has thus become an important aspect in volcanic hazard assessments other fields involving the interaction of water with a melt are the study of nuclear and other industrial safety hazards of magma mixing of diamond carrying volcanism and of hydrothermal ore deposits as well as geothermal and groundwater reservoirs experiments on the dynamics of magma water interactions provide increasing insight into the principles and hazards of phreatomagmatic explosions this type of research will become a new field in experimental petrology

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

structural development of angular volcanic belts in the archean slave province narrow belts of metavolcanics lying between metasediments and granitoid rocks in the southwestern part of the archean slave province display a distinctive angular map pattern steep homoclinal segm

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

mineral deposits in the archean slave structural province lithological and tectonic setting abstract the slave structural province an archean granite greenstone terrane covering 190 000 km 2 in the northwest territories of canada has an extensive history that goes back to earth s oldest preserved rocks it has been subjected to archean and proterozoic orogenies the former built the craton the latter destroyed or reworked large areas reducing the slave to its present size a mounting body of data suggests that primitive sialic crust had a major influence on the evolution of most slave rock types supracrustals of the slave province comprise abundant greywacke mudstone 80 and subordinate volcanics 15 that form two distinct types of volcanic belt yellowknife type dominated by tholeiitic basalts and hackett river type dominated by felsic intermediate calc alkaline pyroclastics slave volcanogenic massive sulphides have high silver lead zinc contents rare element pegmatites are associated with highly evolved granitoids in four widespread parts of the province and gold deposit types define zones trending northeasterly across the craton conglomerates quartz sandstones and quartz arenites formed in a shelf environment define a northerly trending zone locally coincident with a long lived fault and a major lead isotope boundary which together demarcate a western part of the slave containing old basement tectonic models for the slave comprise two types those that attempt to explain its formation in plate tectonic terms and those that suggest formation by rifting of primitive sialic crust with volcanics and granitoids derived largely by melting of that crust many features of the slave seem more easily explained by a rift related model rather than by plate tectonics

lateral vent migration during phreatomagmatic and magmatic eruptions at tecuitlapa maar east central mexico abstract tecuitlapa maar located in the eastern central volcanic belt of mexico is a late quaternary 100 m deep 1 km diameter crater with an alignment of scoria cones building in height to the east within it analysis of stratigraphic sections around the maar indicates that the phreatomagmatic eruption began in the eastern part of the crater with interaction between the basaltic magma and liquefied tuffaceous sediments the explosion locus gradually moved westward as evidenced by bomb sag trajectories duneform axes and facies changes producing an elliptical crater the eruption then dried out and began to produce scoria spatter cones with nested craters that young in age and climb in height to the east along the same alignment exhibited by the phreatomagmatic eruption the lithic lapilli and ash in the deposits throughout the eruption are from the 10 40 m thick water saturated tuffaceous sediments partially exposed in the maar crater walls rather than the underlying lavas and limestones this implies that the vent migrated laterally rather than vertically during the eruption and this lack of deepening of the explosion loci may be related to the high lithologic contrast between the non consolidated sediments and the underlying fractured bedrock water movement in the bedrock would have been by fracture flow whereas movement within the sediments was through liquefaction failure and fluidization of the deposit low magma supply rates would allow collapse of the non lithified walls of the dike producing mingling and phreatomagmatic explosions whereas the same magma supply rate in fractured bedrock may seal off water access to the magma the alignment of the vents is parallel to regional structural trends so was probably set in the underlying bedrock and the distance that the vent migrated is likely related to the overall dike length the lack of fluidized sediment coating the scoria cone clasts both in the cones and in the deposits in the walls of the maar implies that the phreatomagmatic eruptions used up most of the liquefied sediment which ended the phreatomagmatic portion of the eruption the migration of the scoria cone vent loci can be explained by an actual migration of the vents or because the evidence on their locations is based on surficial crater overlap relations by a fissure eruption that closed up from the west to the east producing later activity at the taller eastern vents

volcanological features of a low viscosity melt the carbonatitic gross brukkaros volcanic field namibia in the upper cretaceous gross brukkaros volcanic field southern namibia a radial dyke system surrounds a dome structure and its 74 closely related carbonatite diatremes this paper focuses on volcanological features which seem to be typical for a low viscosity melt in various settings such as dykes sills and diatremes the total or near absence of vesicles in carbonatite ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes

origin and evolution of mid to late archean crust in the hanikahimajuk lake area slave province canada evidence from u pb geochronological geochemical and nd pb isotopic data abstract a geochronological major trace element geochemical and nd pb isotopic study of early to mid archean basement granitoids gneisses 2 68 ga greenstone belt and 2 58 2 62 ga late archean granitoids from the hanikahimajuk lake area of the northern slave province was undertaken to constrain the tectonomagmatic evolution of the north central area of slave province northwest territories canada the basement tonalites are precisely dated at 3378 1 ma by u pb zircon methods and are characterized by steeply fractionated ree patterns with no negative eu anomalies they were likely produced through partial melting of mafic crust at 0 8 gpa the endt of these rocks range from 0 3 to 2 1 implying involvement of even older crustal material in their genesis two different types of 2 68 ga mafic to intermediate volcanic rock are recognized in the hanikahimajuk lake area tholeiitic rocks with no ta nb depletions and calc alkaline rocks with slight negative ta nb depletions all of these rocks show clear evidence for contamination by the 3 4 ga basement tonalite thus precluding the possibility that some of these tholeiitic mafic rocks were produced in a mid oceanic ridge setting from the data presented here we conclude that the tholeiitic rocks were produced in a back arc setting likely similar to that of the japanese arc the calc alkaline rocks may either be a remnant of the volcanic arc or part of the same back arc system however the close proximity with the tholeiitic rocks in the field favours the latter possibility mafic to intermediate rocks with similar geochemical and isotopic characteristics are also found from the yellowknife area of the southwestern slave province and from the indin lake area of the westernmost slave province south of grenville lake area this observation may indicate that the mafic dominated volcanic belts of the western slave province may be a remnant of a single now collapsed and shortened back arc basin the existence of syn volcanic calc alkaline plutonic rocks with similar geochemical and isotopic features as the calc alkaline volcanic rocks supports this possibility the geochemistry of late archean granitoids suggests that they are produced mainly through partial melting of mafic igneous protoliths and clastic sedimentary rocks and possibly tonalitic rocks similarity in the end 2 6 values between these granitoids and the supracrustal rocks in the area is in accord with this model such protoliths are also consistent with the tectonic setting at which the 2 68 ga volcanic rocks were generated because mafic igneous rocks and clastic sedimentary rocks with lesser felsic igneous rocks are the major constituents of a back arc basin

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

geochemistry and origin of the regan intrusive suite and other granitoids in the northeastern slave province northwest canadian shield the regan intrusive suite of about 100 plutons of tonalite granodiorite and quartz diorite intruded the yellowknife supergroup and migmatite terrain in the northwest slave structural province 2 59 ga ago rare earth element ree trace element and major element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust producing a parental magma of tonalitic or granodioritic composition by analysing ree from different parts of a zoned pluton it was concluded that ree distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the ree were contained in early cumulate accessory apatite and monazite the residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks chondrite normalized ree patterns of single lithology plutons are similar to lithologies in zoned plutons and it is proposed they initially segregated during ascent

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

geology and evaluation strategy of the star and orion south kimberlites fort a la corne canada abstract diamond bearing kimberlites in the fort a la corne field east central saskatchewan consist primarily of volcaniclastic deposits which are interstratified with lower cretaceous continental marginal marine and marine sediments seventy kimberlite occurrences have been drill confirmed with the majority of those occurring in the fort a la corne main trend the star 352 ha and orion south 358 ha kimberlite complexes are located at the south eastern extremity of the main trend and contain some of the world s best examples of preserved primary volcaniclastic kimberlite deposits the advanced exploration programs on both the star and orion south complexes offer a rare opportunity to examine a wide variety of in situ volcaniclastic kimberlite deposits both kimberlite complexes are composed of multiple eruptive units that were deposited episodically over 4 to 6 million years separated by periods of volcanic quiescence marked by reworking of kimberlite and siliciclastic sedimentation detailed logging of core from 100 m spacing grid drilling has revealed well defined tuff ring tuff cone crater and extra crater deposits evaluation of these large tonnage lower grade coarse diamond size frequency kimberlite complexes requires a methodical and multi branched exploration strategy utilizing 1 grid pattern core drilling to gain an understanding of the three dimensional architecture of the kimberlite complexes 2 underground bulk sampling to provide appreciable carats for accurate estimates of diamond grade and price and 3 grid pattern mini bulk sampling via large diameter drilling to obtain an understanding of the diamond grade across the breadth of each kimberlite

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

the importance of the transport system in shaping the growth and form of kimberlite volcanoes transport of kimberlite magma from the mantle to the surface is a complex process that involves many often sudden and dramatic changes in transport style the pace and magnitude of changes in the transport system acclerate as the magma nears the surface culminating in eruptions during which multiple transport events take place simultaneously as well as single transport events that change significantly from their point of origin to final deposition understanding the range of transport styles recorded by kimberlite deposits is key to describing the type and style of eruptions the growth of cones and craters and the distribution geometry and structure of the deposits that make up kimberlite volcanoes in an economic context building a clear picture of the processes that shape deposits is essential for selecting exploration targets and evaluating the grade and value of diamond bearing kimberlites variations in diamond grade reflect differences in the diamond content of different magma parcels erupted during the lifetime of the kimberlite volcano sorting during transport of eruption products or re working of diamonds during crater growth cone collapse and erosion deep transport systems control delivery of magma and diamonds to eruption sites the initial stages of kimberlite transport involve rise of magma in narrow interconnected networks of fluidfilled fractures there is compelling evidence that kimberlite magmas are extracted from their source region and erupted without significant residence times in the crust many single kimberlite volcanoes have significant variations in diamond grade between different kimberlite phases suggesting eruption of distinct perhaps unrelated magma parcels at the same site

moderate overthickening of thinned sialic crust and the origin of granitic magmatism and regional metamorphism in low p high t terranes formation of low pressure p high temperature t metamorphic terranes by homogeneous shortening and thickening of tectonically thinned sialic crust and overlying sedimentary basins at geologically reasonable strain rates avoids the shortcomings of hypotheses linking the characteristically high geothermal gradients to doubling of normal thickness crust crustal extension alone intrusion of granitoids or extensive basic magmatism on the basis of geological and p t constraints from the archean slave province the p t histories of three points in a hypothetical block of crust are presented in p t p distance and p time diagrams the relations between the rock that ends up at the surface base of the basin base of the crust isotherms and depth determine the type of metamorphism and the degree of partial melting required to produce abundant granitoids this simple qualitative framework can account for the characteristic features of low p high t metamorphic terranes and reconcile those who believe granites are the cause of the metamorphism with those who favor the contrary

root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar diatreme volcanoes abstract the understanding of processes within the root zone of maar diatreme volcanoes is important for the interpretation of the geology volcanology and even hazard assessment of these volcanoes in the phreatomagmatic model of pipe formation the irregularly shaped root zone is the site of the phreatomagmatic explosions and thus functions as the engine for pipe formation in this model the root zone grows over a period of time in a series of many single thermohydraulic i e phreatomagmatic explosions the explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels the ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole like fashion into the root zone many phreatomagmatic eruptions consequently result in the formation of a cone shaped diatreme thus with ongoing eruptions the cone shaped diatreme has to grow systematically both in depth and diameter during its growth processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone at the surface the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter thermohydraulic explosions which fragment both magma and the surrounding country rocks mostly occur within the bottom part of the root zone violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme in larger pipes via expansion of the mixture of highly pressurized water vapor juvenile gas phases and explosively produced tephra the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt diatreme clearing events in large pipes are difficult or impossible to maintain since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes the combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types it is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar diatreme and related volcanoes

geology alteration and lithogeochemistry of the hood volcanogenic massive sulfide vms deposits nunavut canada the hood volcanogenic massive sulfide vms deposits are hosted by the 2 68 ga amooga booga volcanic belt abvb in the northwestern archaen slave craton and consist of three deposits hood 10 41 and 41a and three occurrences 46 461 and 462 the mineralized zones consist of massive to semi massive pyrrhotite pyrite chalcopyrite sphalerite and galena hosted predominantly by felsic volcanic flows within the predominantly mafic abvb the mineralized lenses occur at different stratigraphic levels and have textural alteration and stratigraphic features consistent with formation via subseafloor replacement the felsic volcanic rocks in the hood deposits can be subdivided into groups based on immobile trace element geochemistry the main felsic types a and b are petrographically indistinguishable type a has higher high field strength element hsfe and rare earth element ree contents than type b suggesting a higher temperature of formation type a rocks also have higher nb ta values indicative of a greater mantle input in their genesis compared to type b rocks mineralization is more closely associated with type a than type b rocks the two mafic volcanic rock types previously identified in the abvb type i and type ii both occur within the hood deposits the type ii mafic group is interpreted to be the result of variable crustal contamination of type i magma the volcanic rocks of the abvb are interpreted to have formed in a continental margin arc back arc setting the genesis of these magmatic suites involved magmatic underplating and emplacement through pre existing sialic basement that resulted in crustal melting mantle crust mixing and contamination leading to the aforementioned geochemical features in both mafic and felsic suites this type of extensional tectonic environment was likely associated with high heat flow and is similar to global vms environments proximal to extending continental margins e g sturgeon lake bathurst and finlayson lake the similarities of the abvb to other slave craton greenstone belts further highlights the overall potential for greenstone hosted vms mineralization in the slave craton chlorite sericite quartz is the dominant hydrothermal alteration assemblage in the hood deposits and is typical of vms style mineralization mass change calculations illustrate that elemental changes are typical of vms environments with gains in fe2o3 mgo and base metals associated with chlorite alteration near mineralized zones k2o gains associated with sericite alteration and losses of na2o in both alteration types

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

the nature of mesoarchaean seawater and continental weathering in 2 85 ga banded iron formation slave craton nw canada abstract banded iron formations bif have been extensively used as proxies to infer the chemical composition of ancient bulk seawater however their proximity to ancient crust suggests that they might also be used to reveal the composition of emergent continental landmass at the time of their deposition here we use the combination of geochemistry and sm nd isotopes on a layer by layer basis to interpret the relative contributions of hydrothermal hydrogenous and terrestrial input to one of the oldest documented superior type bif in the world the 2 85 ga central slave cover group bif is deposited within a rift basin related to a continental margin and is found associated with basement gneisses as well as shoreline and shallow shelf type facies such as fuchsitic quartzite and pebble to cobble conglomerate that confirm a near shore depositional setting for the bif the bif ranges from a pure chemical oxide magnetite silicate grunerite actinolite sediment with low al 2 o 3 2 o 3 10 wt and the occurrence of ferro hornblende biotite and garnet the silica bands have low trace metal content e g ni low ree average of 6 ppm and a shale normalized rare earth and yttrium rey pattern that is hree to lree enriched with pr yb sn values reaching sn of 0 5 during active rifting of the basement excess of eu 2 impacted the basin yielding seawater with eu anomalies eu eu sn as high as 3 85 average 2 75 larger than similarly aged bif high resolution geochemistry shows that there is more silica 19 4 wt sio 2 in the iron bands than iron 8 7 wt fe 2 o 3 in the silica bands implying that dissolved fe 2 came to the bif site in pulses and that silica likely represents background deposition consistently radiogenic e nd t values for the iron bands average 1 7 show that the dissolved rey in the source water during ferric iron precipitation was provided by submarine hydrothermal fluids with relatively uniform 143 nd 144 nd the silica bands by contrast reveal high variation in seawater 143 nd 144 nd as evident from the bimodal e nd t distribution with one segment exhibiting negative e nd t values averaging 1 1 and another with positive e nd t values averaging 2 5 this suggests input of dissolved rey into the upper seawater from weathering of isotopically different crustal components in the source region collectively we speculate that the low rey in the upper seawater and the overall low ni content implies a highly weathered crustal surface that was unable to contribute a significant dissolved load to the shelf environment

petrochemistry of the yellowknife volcanic suite at yellowknife n w t the petrochemistry of the yellowknife volcanic belt at yellowknife kam duck and banting formations shows the suite to be more chemically complex than previously thought the kam formation 65 basalt 27 basaltic andesite 17 andesite and 2 dactite has a strong tholeiitic trend involving pyroxene fractionation which shows iron enrichment to feo feo mgo 0 75 after altered samples are removed from consideration it can be shown that the kam is chemically analogous to modern ocean floor or ocean rise tholeiites the andesitic members of the kam were partially melted from a hornblendic region presumably former oceanic crust possibly with a slight sialic component the duck formation 11 basalt 21 basaltic andesite 59 andesite and 9 dacite shows considerable evidence of alkali mobility with varied carbonate content it is thought to have been largely derived as partial melts from former oceanic crust via hornblende fractionation the banting formation 14 basalt 7 basaltic andesi

emplacement and reworking of cretaceous diamond bearing crater facies kimberlite of central saskatchewan canada in central saskatchewan canada kimberlites were emplaced into cretaceous marine and nonmarine clastic sediments core recovered from one drill hole that intersects kimberlite smeaton fac uk core 169 8 was selected for an integrated study involving sedimentology volcanology mineralogy geochemistry palynology micropaleontology organic petrology and radiometric age determination only crater facies kimberlite has been observed there is no indication of the locations of feeder dikes four varieties of kimberlite occur all originating from subaerial volcanism 1 fluvial reworked kimberlite 2 diamondiferous kimberlite lapillistone air fall deposits 3 kimberlite olivine crystal tuff air fall deposits and 4 diamondiferous marine wave reworked kimberlite within the multiple primary eruptive phases of the kimberlite air fall deposits the volcanic style changed upward with time from violent strombolian to more explosive volcanism the bulk of the volcanism formed conformable air fall deposits on terrestrial sediments of the cantuar formation resulting in the development of positive relief tephra cones subsequent marine transgression associated with the westgate formation partially beveled the top of the cone the kimberlite air fall deposits contain microdiamonds 5 to 25 m in diameter the maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits although originally derived from magma at high temperatures did not thermally affect entrained surficially derived clasts or the country rock during emplacement the chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile at least two episodes of kimberlite volcanism occurred in the middle and late albian paleontologically assigned a u pb perovskite radiometric age of 101 1 2 2 ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post middle albian and pre late albian to late albian

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

geology and diamond distribution of the 140 141 kimberlite fort a la corne central saskatchewan canada abstract the cretaceous age fort a la corne falc kimberlite province comprises at least 70 bodies which were emplaced near the edge of the western canadian interior seaway during cycles of marine transgression and regression many of the bodies were formed during a marine regression by a two stage process firstly the excavation of shallow but wide craters and then subsequent infilling by xenolith poor crater facies subaerial primary pyroclastic kimberlite the bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin but high volume pyroclastic kimberlite deposits each body is composed of contrasting types of kimberlite reflecting different volcanic histories and therefore are considered separately the 140 141 kimberlite is the largest delineated body in the province estimated to have an areal extent below glacial quaternary sediments in excess of 200 ha the infilling of the 140 141 crater is complex resulting from multiple phases of kimberlite the central part of the infill is dominated by several contrasting phases of kimberlite one of these phases is a primary pyroclastic airfall mega graded bed up to 130 m in thickness the constituents grade in size from very fine to coarse macrocrystic kimberlite through to a basal breccia the mega graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe a second different phase of kimberlite is interpreted as representing a younger nested crater within the mega graded bed centrally located thicker intersections 450 m of this younger kimberlite may indicate a vent for the kimberlite crater the thickness of the mega graded bed increases with proximity to the younger kimberlite in the study area macrodiamond minibulk sample grades from the mega graded bed have been obtained from nine large diameter drill holes located within the northwest part of the body from an area of 20 ha which represents approximately 10 of the currently modeled kimberlite outline diamond grade increases with depth within the mega graded bed and also increases within the same unit towards the centrally positioned younger kimberlite macrodiamond sample grades vary from low at the top of the mega graded bed to considerably higher grades near the base total sample grade per drill hole varies from moderate near the vent feature to lower grades 200 300 m from the vent feature macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega graded bed there is a strong correlation between depth and increased stones per tonne and a similar correlation between stones per tonne and proximity to the vent feature the data supports the emplacement model of the mega graded bed and in turn this information is useful in understanding the macrodiamond distribution within this bed

rapid injection of particles and gas into non fluidized granular material and some volcanological implications in diatremes and other volcanic vents steep bodies of volcaniclastic material having differing properties particle size distribution proportion of lithic fragments etc from those of the surrounding vent filling volcaniclastic material are often found it has been proposed that cylindrical or cone shaped bodies result from the passage of debris jets generated after phreatomagmatic explosions or other discrete subterranean bursts to learn more about such phenomena we model experimentally the injection of gas particulate dispersions through other particles analogue materials glass beads or sand and a finite amount of compressed air are used in the laboratory the gas is made available by rapidly opening a valve therefore the injection of gas and coloured particles into a granular host is a brief 1 s discrete event comparable to what occurs in nature following subterranean explosions the injection assumes a bubble shape while expanding and propagating upwards in reaction the upper part of the clastic host moves upward and outward above the bubble forming a dome the doming effect is much more pronounced for shallow injection depths thin hosts with dome angles reaching more than 45 significant surface doming is also observed for some full scale subterranean blasts e g buried nuclear explosions so it is not an artefact of our setup what happens next in the experiments depends on the depth of injection and the nature of the host material with shallow injection into a permeable host glass beads the compressed air in the bubble is able to diffuse rapidly through the roof meanwhile the coloured beads sediment into the transient cavity which is also closing laterally because of inward directed granular flow of the host depending on the initial gas pressure in the reservoir the two phase flow can erupt or not non erupting injections produce cylindrical bodies of coloured beads whereas erupting runs produce flaring upward or conical deposits changing the particle size of the host glass beads does not have a large effect under the size range investigated 100 200 to 300 400 m doubling the host thickness injection depth requires a doubling of the initial gas pressure to produce similar phenomena such injections whether erupting or wholly subterranean provide a compelling explanation for the origin and characteristics of multiple cross cutting bodies that have been documented for diatreme and other vent deposits

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

subsidence of surtsey volcano 1967 1991 the surtsey marine volcano was built on the southern insular shelf of iceland along the seaward extension of the east volcanic zone during episodic explosive and effusive activity from 1963 to 1967 a 1600 m long east west line of 42 bench marks was established across the island shortly after volcanic activity stopped from 1967 to 1991 a series of leveling surveys measured the relative elevation of the original bench marks as well as additional bench marks installed in 1979 1982 and 1985 concurrent measurements were made of water levels in a pit dug on the north coast in a drill hole and along the coastline exposed to the open ocean these surveys indicate that the dominant vertical movement of surtsey is a general subsidence of about 1 1 0 3 m during the 24 year period of observations the rate of subsidence decreased from 15 20 cm year for 1967 1968 to 1 2 cm year in 1991 greatest subsidence is centered about the eastern vent area through 1970 subsidence was locally greatest where the lava plain is thinnest adjacent to the flanks of the eastern tephra cone from 1982 onward the region closest to the hydrothermal zone which is best developed in the vicinity of the eastern vent began showing less subsidence relative to the rest of the surveyed bench marks the general subsidence of the island probably results from compaction of the volcanic material comprising surtsey compaction of the sea floor sediments underlying the island and possibly downwarping of the lithosphere due to the laod of surtsey the more localized early downwarping near the eastern tephra cone is apparently due to greater compaction of tephra relative to lava the later diminished local subsidence near the hydrothermal zone is probably due to a minor volume increase caused by hydrous alteration of glassy tephra however this volume increase is concentrated at depth beneath the bottom of the 176 m deep cased drillhole

some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective and some suggestions five significant problems hinder advances in understanding of the volcanology of kimberlites 1 kimberlite geology is very model driven 2 a highly genetic terminology drives deposit or facies interpretation 3 the effects of alteration on preserved depositional textures have been grossly underestimated 4 the level of understanding of the physical process significance of preserved textures is limited and 5 some inferred processes and deposits are not based on actual modern volcanological processes these issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits the traditional steep sided southern african pipe model class i consists of a steep tapering pipe with a deep root zone a middle diatreme zone and an upper crater zone if preserved each zone is thought to be dominated by distinctive facies respectively hypabyssal kimberlite hk descriptively called here massive coherent porphyritic kimberlite tuffisitic kimberlite breccia tkb descriptively here called massive poorly sorted lapilli tuff and crater zone facies which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite rvk porphyritic coherent kimberlite may however also be emplaced at different levels in the pipe as later stage intrusions as well as dykes in the surrounding country rock the relationship between hk and tkb is not always clear sub terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis modern in vent volcanological processes should initially be considered to explain observed deposits crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes indicating that the pipe was largely empty at the end of the eruption and subsequently began to fill in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents classes ii and iii canadian kimberlite models have a more factual descriptive basis but are still inadequately documented given the recency of their discovery the diversity amongst kimberlite bodies suggests that a three model classification is an over simplification every kimberlite is altered to varying degrees which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in vent context few preserve original textures the effects of syn to post emplacement alteration on original textures have not been adequately considered to date and should be back stripped to identify original textural elements and configurations applying sedimentological textural configurations as a guide to emplacement processes would be useful the traditional terminology has many connotations about spatial position in pipe and of process perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies mining terminological scheme because it is so entrenched however for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics important variations in these and the process origins for example every deposit of tkb is different in componentry texture or depositional structure however because so many deposits in many different pipes are called tkb there is an implication that they are all similar and that similar processes were involved which is far from clear

integrated tectonic genetic model for volcanic hosted massive sulfide deposits volcanic hosted massive sulfide deposits exhibit a continuous range in metal composition from copper dominated cyprus type to lead rich zinc copper deposits kuroko type this spectrum is best viewed as a progressive enrichment in large ion lithophile elements lile in the ores e g pb ba and the magmatic systems e g k responsible for their generation thus lead rich volcanic hosted massive sulfide deposits are associated with calc alkalic magmatism in mature are systems e g buchans or in back arc sites where older continental crust can be implicated in magma genesis at the copper rich end of the spectrum ores are related to plagiogranites in supra subduction zone ophiolite complexes and lile enrichment is minimal the ancient back arc assemblages that now constitute greenstone belts host cu zn massive sulfide deposits associated with tholeiitic calc alkalic magmatic systems of intermediate lile enrichment i suggest that these variations in both the metal composition of such ores and the petrochemistry of coeval intrusions relate in a fundamental way to the siting of their emplacement in arc and back arc systems and the duration of the subduction process

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

on the volcanology of the gibeon kimberlite field namibia kimberlite volcanism in the upper cretaceous gibeon kimberlite field southern namibia consisting of at least 42 diatremes and a number of associated dykes is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the field the volcanology of the diatremes and dykes as well as their structural setting is reported here because of the paleohydrogeological setting and since juvenile kimberlite occurring in dykes intrusive plugs and spherical lapilli is devoid of vesicles a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes karoo dolerite basalt and sediment xenoliths in the diatremes provide evidence for the former extent of karoo strata at the time of eruption

the role of fluidisation in the formation of volcaniclastic kimberlite grain size observations and experimental investigation abstract deep steep sided kimberlite pipes in southern africa are normally partially filled with multiple units of a characteristically massive structureless volcaniclastic rock here termed massive volcaniclastic kimberlite mvk these units can be arranged in a pseudo concentric nested pipes within pipes structure several key observations on typical mvk from the venetia kimberlite pipes south africa indicate that fluidisation may have been an important process during its emplacement these include 1 the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and 2 a matrix composed of void filling minerals related to hydrothermal metamorphism mainly diopside and serpentine which indicates that the primary deposit had a high porosity 50 and 3 particle size distributions poor in fine ash 10 cm analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of mvk high pressure gas fed through the bed from a point source resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a gaussian distribution of vertical velocity in the cross flow direction a decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the pipes within pipes structures in kimberlite pipes the results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes we propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep wide conduit this bed is subject to quasi sustained fluidisation by the erupting gas particle flow which imparts on it the distinctive characteristics of a massive well mixed poorly sorted deposit

lithofacies characteristics of diatreme deposits examples from a basaltic volcanic field of sw sardinia italy abstract a deeply eroded diatreme field consisting in several decametric sized vertical mainly clastic volcanic bodies of basaltic composition is described for the first time in the variscan basement of sw sardinia the recognition and description of four different lithofacies in these diatremes allowed discussion of the role of the different processes which control magma eruption and conduit infilling and making general inferences about diatremes the studied diatremes have a cross sectional shape from elliptical to sub triangular and are slightly elongated nearly parallel to the main foliation of the intruded meta sedimentary rocks foliation of host rocks is locally reoriented or folded close to the contact with the diatremes suggesting that magma possibly rose to the surface through fissures oriented nearly parallel to host rock foliation textural features of the volcanic bodies show many analogies with kimberlitic diatremes despite the difference in petrography and composition juvenile lapilli are mainly made by ghosts of mafic phenocrysts olivine and clinopyroxene set in a groundmass formed by plagioclase microlites immersed in a cryptocrystalline chlorite rich matrix the four lithofacies were described mainly based on the shape and physical features of the clasts and textural anisotropy a globular juvenile rich lapilli tuff facies gjlt an angular juvenile rich lapilli tuff facies ajlt a lithic rich lapilli tuff facies lirlt and a coherent lava like facies coh all the clastic lithofacies are generally well sorted and typically lack a fine grained matrix juvenile fragments are lapilli sized and from equant to oblate in axial ratio and from rounded globular to very angular in shape conversely lithic clasts are largely variable in shape and size and are mainly represented by basement derived clasts the absence of bedding the scarcity of the coherent facies and the dominance of clast supported structureless volcaniclastic facies suggest that the outcropping portion of these volcanic bodies represents the lower diatreme zone the presence of diffuse welding and the globular shapes of some juvenile fragments together with their vesicularity suggest that magma fragmentation was mainly driven by magmatic gas exsolution occurring at a deeper level respect to classical basaltic explosive activity textural features facies association and facies architecture of the studied deposits are suggestive of an important affinity with kimberlitic and other ultramafic diatremes

comparative planetology and the origin of continental crust abstract this paper summarizes crustal evolution in silicate planets the moon mercury mars venus and meteorite parent bodies and outlines a general theory for the origin of continental crust based on comparative planetology and terrestrial evidence silicate planets and some meteorite parent bodies have undergone early global differentiation nonplate tectonic followed by a late basin forming bombardment and a long basaltic second differentiation evidence for analogous stages except a late bombardment can be found in terrestrial geology modified by conditions of hydrous magma generation and persistent tectonism continental crust is inferred to have largely been formed in the early archean by andesitic volcanism accompanying catastrophic early degassing this global crust was disrupted by basin forming impacts that initiated mantle upwelling and sea floor spreading basaltic overplating and underplating from mantle plumes continued on the continents producing greenstone belts mafic dike swarms and by partial melting of underplated basalt and sialic crust the granitoids of archean granite greenstone terrane this process produced most of the present continental crust by 2 5 ga ago when the global tectonic style evolved into the present one of tectonism marginal to and within continents subsequent crustal evolution has largely been deformation metamorphism anatexis and rearrangement of previously formed crustal segments not separation of new sialic crust from the mantle the crustal growth that has occurred since the archean has primarily been crustal thickening resulting directly or indirectly from generation of basalt in the subcontinental tectosphere viewed in the light of evidence from comparative planetology the continents are interpreted as the greatly altered remnants of an originally global primordial crust the result of an evolutionary sequence shared by all the silicate planets to some degree depending on their mass volatile content and location in the solar system this theory can be tested by deep drilling in archean terranes to locate remnants of the original crust by determination of the composition of the highland crust of mars and by investigations of the crust of venus

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the