thermal erosion by komatiites at kambalda western australia and the genesis of nickel ores huppert et al 1 calculated that komatiites flow turbulently transfer heat efficiently to their substrate and thus potentially produce thermal erosion channels in sequences onto which they were erupted their suggestion that komatiite hosted kambalda type2 nickel ores reside in such channels has led to resurgence of interest in volcanic models for ore emplacement3 5 and caused controversy6 7 such models have important implications not only to ore genetic studies but also to interpretation of the geochemistry of komatiites8 and their sm nd systematics9 10 here we demonstrate that interspinifex fe ni cu sulphide ores which underlie some hanging wall massive sulphide ores at kambalda the site of one of the world s major nickel deposits formed through thermal erosion of underlying komatiite flows and possibly also sediments providing proof of thermal erosion however we caution against the general application of thermal erosion models to the eruption of komatiites it seems that significant thermal erosion occurred rarely and only where basal concentrations of highly thermally conductive sulphide liquid were present at the base of localized lava channels isotope data11 12 and theoretical considerations13 suggest that the komatiites were contaminated by crustal rocks before eruption although there is limited evidence that localized contamination including sulphide assimilation4 may also have occurred below lava channels localized in pre existing embayments or troughs

geochemistry of komatiites from kambalda western australia i chalcophile element depletion a consequence of sulfide liquid separation from komatiitic magmas the type examples of archean volcanic peridotite associated ni sulfide deposits occur at or toward the base of the ultramafic lava pile at kambalda western australia spinilex textured komatiites considered to represent silicate liquids derived from the original magma are significantly depleted in chalcophile elements throughout the entire ultramafic sequence relative to unmineralized and poorly mineralized komatiites elsewhere and to a theoretically modeled sulfide undersaturated komatiite system this depletion is not attributed to postformational modification of the komatiites by sea floor alteration or metamorphism although very low sulfur contents in the same rocks probably result from such processes the depletion is best interpreted as the result of scavenging of metals by sulfide liquid in equilibrium with the silicate magma implying that all the silicate magma now represented by the ultramafic sequence equilibrated with sulfide liquids at the magmatic stage these results provide convincing evidence against formation of the sulfide ores by in situ sulfide separation from the thick komatiitic peridotitc host the magnitude of the depletion in the spinilex textured komatiite divisions of the peridotitc hosts is much less than that expected for such an origin but for the entire sequence it is greater than that expected from metal partitioning relations and mass balance calculations for the volume and composition of the ultramafic sequence and the volume of associated sulfides this suggests that a proportion of the sulfide liquid that formed from the ultramafic magma s was lost from the magmatic system prior to extrusion such loss is a natural consequence of models for ascent and eruption of sulfur rich komatiitic magmas that involve prior concentration of sulfides plus olivine in high level magma chambers or separation and segregation of sulfide liquid during ascent from the upper mantle

revised sm nd systematics of kambalda greenstones western australia sm nd isotope systematics in mafic ultramafic lavas from the norseman wiluna greenstone belt western australia have been reinvestigated a previous age based on mixing lavas and granites is not confirmed instead a sm nd age of 3 262 44 myr is indicated for the lavas 400 myr older than intruding granites and predating gneisses that have been proposed as basement rocks

the chemical and mineralogical variations in the nickel mineralization associated with the kambalda dome western australia the volcanic peridotite associated nickel deposits at kambalda western australia consist of a number of orebodies or shoots located on the flanks of a broad domal structure within an archean greenstone belt in the yilgarn block chemical and mineralogical analyses of carted ore samples demonstrate that there is a systematic variation in sulfide bulk composition and mineralogy between shoots this is shown by positive correlations between the ni tenor of the shoots and their co cu pyrite and spinel contents these variations permit division of the shoots into three broad groups occupying distinct geographic positions in relation to the kambalda dome the variation in ni tenor between shoots is suggested to be caused by oxidation occurring prior to or during extrusion resulting in disequilibrium in f sub o 2 between the sulfide and silicate melts as equilibrium is reapproached oxidation of the sulfide proceeds resulting in a change in the partitioning behavior of fe and consequently the ni content of the sulfide melt increases tapping of the silicate sulfide melts at different degrees of departure from equilibrium could account for the observed compositional variation between the shoots emplacement of the sulfides is interpreted to have taken place as a single eruptive event the coincidence of spinel rich zones with high pyrite and cr contents at most major sulfide rich sulfide poor interfaces e g between massive and disseminated sulfides within the ore zone suggests that post extrusive but premetamorphic oxidation of the sulfides occurred at these positions this oxidation is due to disequilibrium in f sub o 2 between the silicate and sulfide during cooling and crystallization of the magma

evidence of lateral thermomechanical erosion of basalt by fe ni cu sulfide melt at kambalda western australia the fe ni cu sulfide ores at kambalda western australia are interpreted to be the result of thermomechanical erosion of underlying rocks by the host komatiite lava flows however there is a long standing argument about the extent of the erosion process and the degree to which the linear embayments that host the ores were eroded by lava as opposed to formed by tectonic processes this controversy has fundamental implications for the origin of magmatic sulfide ore as well as for sinuous rilles on terrestrial planets the controversy at kambalda hinges on pinchout features where sulfide ore at the edges of embayments penetrates laterally into footwall rocks the most recently published studies of these features interpret them as forming by thrusting of basalts over sulfide komatiite contacts along the margins of tectonic embayments field evidence and x ray fluorescence element mapping on underground exposures in the moran deposit demonstrate that sulfide liquid melted its way both downward and laterally into basalt generating complex plumose melt layers melt emulsions and hybridized chromite decorated contacts these observations confirm an origin for the pinchouts by thermomechanical erosion driven by the high temperature high density and low viscosity of the sulfide melt they also provide some intriguing insights into the nature of interactions between sulfide melt and melting silicate rocks in magmatic ni cu platinum group element sulfide ore deposits in general

erosion by flowing lava field evidence erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on earth and other planets although there are many references to lava erosion on earth unambiguous evidence is rare geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion in order to assess possible erosion by flowing lava we carried out field studies of tube fed basalt flows sheet flows of the columbia river basalt province crb and precambrian komatiites unequivocal evidence for thermal erosion melted dacite substrate was found at the cave basalt lava tube mount st helens for which fluid dynamic analysis indicates laminar flow although erosion was enhanced in areas of locally steep slopes possibly as a result of localized turbulence other lava tubes in our study display strong but inconclusive evidence for erosion komatiite flows display good evidence for erosion of their substrate possibly in a turbulent regime but assessment of the extent of erosion is hampered by limited and disrupted exposures no evidence for thermal erosion was found in the crb our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high reynolds number flows high mass flux low viscosity lava steep slopes and substrates having a lower melting temperature than the lava or low mechanical strength e g regolith

roles of xenomelts xenoliths xenocrysts xenovolatiles residues and skarns in the genesis transport and localization of magmatic fe ni cu pge sulfides and chromite abstract most sulfide rich magmatic ni cu pge deposits form in dynamic magmatic systems by partial melting s bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components and generation of barren to weakly mineralized immiscible fe sulfide xenomelts into which ni cu co pge partition from the magma some exceptionally thick magmatic cr deposits may form by partial melting oxide bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components and generation of barren fe ti oxide xenocrysts into which cr mg v ti partition from the magma the products of these processes are variably preserved as skarns residues xenoliths xenocrysts xenomelts and xenovolatiles which play important to critical roles in ore genesis transport localization and or modification incorporation of barren xenoliths autoliths may induce small amounts of sulfide chromite to segregate but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors pge cu ni co and cr v ti respectively is required to make significant ore deposits silicate xenomelts are only rarely preserved but will be variably depleted in chalcophile and ferrous metals less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma but may aid downward sulfide transport by increasing the bulk density of the magma sulfide wets olivine so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets but other silicates and xenoliths may not be wetted by sulfides xenovolatiles may retard settling of or in some cases float dense sulfide droplets reactions of sulfide melts with felsic country rocks may generate fe rich skarns that may allow sulfide melts to fractionate to more extreme cu ni rich compositions xenoliths xenocrysts xenomelts and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas and are more likely to be preserved in less dynamic less turbulent systems domain phases than in more dynamic more turbulent systems domains phases massive to semi massive ni cu pge and cr mineralization and xenoliths are often localized within footwall embayments dilations jogs in dikes throats of magma conduits and the horizontal segments of dike chonolith and dike sill complexes which represent fluid dynamic traps for both ascending and descending sulfides oxides if skarns residues xenoliths xenocrysts xenomelts and or xenovolatiles are present they provide important constraints on ore genesis and they are valuable exploration indicators but they must be included in elemental and isotopic mass balance calculations

sulfide silicate textures in magmatic ni cu pge sulfide ore deposits massive semi massive and sulfide matrix breccia ores abstract much of the value of magmatic ni cu pge sulfide orebodies is contained within massive or semi massive ores that show a wide variety of textural relationships to included or adjacent silicate rocks we identify five mutually gradational textural types 1 pure inclusion free massive sulfide ores 2 sulfide matrix ore breccias of sharp wall soft wall or mixed character 3 emulsion textured ores formed by frozen mixtures of molten silicate and sulfide most commonly developed as melt films at thermal erosion contacts 4 vein hosted sulfides formed at late magmatic or high temperature post emplacement deformation stage close to the brittle ductile transition in the country rocks or host igneous bodies and 5 tectonic durchbewegung breccias formed by mechanical inter shearing of less ductile silicate inclusions and more ductile solid sulfides some deposits the moran shoot at kambalda being the type example record the invasion of country rock footwall by downward or sideways percolating superheated molten sulfide liquid generating vertical sequences of pure massive sulfide emulsion textured ores and finely spaced invasive sulfide veins these are referred to as sulfide melting infiltration fronts and may provide a clue to the mechanism of formation of sulfide rich magmatic ores as whole sulfide matrix ore breccias are particularly well developed in the voisey s bay and aguablanca deposits where they developed by flooding of percolating sulfide melt through the silicate matrix of magmatic intrusion breccias displacing silicate melt the lithology of the silicate or carbonate rock inclusions determines the nature of the inclusion matrix relationships non refractory inclusions typically disaggregate along original grain boundaries to leave coherent inclusions surrounded by clouds of inclusion derived or matrix derived crystals with the low melting silicate component preferentially displaced by sulfide liquid whereas refractory inclusions retain sharp boundaries zonation of inclusions and overgrowths preserves reaction between inclusion and silicate matrix that pre dates invasion of the intrusion breccia by sulfide liquid the process of percolation of dense low viscosity sulfide liquid into pore space and fractures within partially molten or melting silicate rock is a unifying theme that links sulfide matrix ore breccias and emulsion textured ores with distinctive textures in less sulfide rich rocks such as net texture matrix ore texture leopard texture poikilitic net texture and interspinifex ore vein hosted massive sulfides may be emplaced under magmatic conditions where the excess pressure of the sulfide liquid column drives or enhances fracturing of the country rock and injection of sulfide into the cracks such veins are commonly referred to as remobilised a term which may obscure process understanding and should be reserved for cases where tectonic solid state mobilisation of sulfide can be demonstrated on textural and structural grounds the tendency of sulfide liquids to invade country rocks and potentially to drive the propagation of their own magmatic containers may be a critical feedback loop in the development of magmatic sulfide mineral systems

textural development in sulfide matrix ore breccias in the voisey s bay ni cu co deposit labrador canada abstract magmatic ni cu sulfide ores at voisey s bay contain complex assemblages of extremely heterogeneous rocks these range from polymict breccias with rock fragments in sulfide rich and or sulfide poor matrices to heterogeneous vari textured gabbros with rapid short range variations in grain size and content of hydrous phases rock fragment populations in the breccias include endogenous olivine gabbros cumulate and non cumulate and cumulate peridotites along with extensively depleted plagioclase hercynite gneisses interpreted as restites from extensive partial melting of country rock quartzo feldspathic paragneisses using a combination of desk top microbeam xrf mapping at cm scale and 3d x ray tomography we show that both sulfide poor and sulfide rich breccias comprise heterolithic assemblages of clasts within a matrix of olivine gabbro this matrix is characterised by an interconnected 3d framework of plagioclase crystals highly variable in grain size at mm to cm scale with interstitial olivine and poikilitic clinopyroxene and is texturally indistinguishable from clast free olivine gabbro sulfide forms interconnected networks at cm to dm scale and possibly larger much of the plagioclase developed by outgrowth from the margins of paragneiss xenoliths when the porosity was occupied by silicate melt the observed range of textures is explained by a model of percolation of molten sulfide through variably crystalline inter clast matrix displacing the silicate melt to leave the refractory plagioclase olivine or in some cases plagioclase only component now entirely within a sulfide matrix the process is analogous to that believed to have formed interspinifex ore in komatiite hosted deposits biotite rims on plagioclase enclosed in sulfide are interpreted as the result of reaction between plagioclase olivine and a hydrous component derived from the sulfide melt itself with a possible component of migrating residual silicate melt wicking along sulfide silicate contacts this sulfide infiltration model offers an alternative to the current model for upward emplacement of a slurry of silicate melt sulfide melt and breccia fragments as a late stage injection into the dyke sill complex the preserved range of textures is interpreted as being due to gravity driven percolation of sulfide liquid through a pre existing partially molten intrusion breccia in this model the breccia serves as a physical trap site accumulating downward migrating sulfide liquid however the invariable close mutual association of sulfide and rock fragments at voisey s bay implies a common derivation

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

komatiites and nickel sulfide ores of the black swan area yilgarn craton western australia 2 geology and genesis of the orebodies the black swan ultramafic succession hosts a number of magmatic fe ni cu pge sulfide ore shoots ranging from high grade massive ore to low grade disseminated sulfides of these the most economically significant is the silver swan massive sulfide orebody associated with the basal contact of the succession the deposit varies in thickness between 5 and 20 m reaches a n s strike length of 75 m extends for at least 1 2 km of vertical plunge and is open at depth overlying matrix net textured ore is rare inclusions of dacite are abundant within the lower 5 m of the massive sulfide they range from angular fragments through smooth sinuous and plumose morphologies to fine lace like intergrowths with the sulfide matrix and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures some well above their liquidus temperature the composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30 komatiite the consistent thickness of the inclusion bearing basal layer within the massive sulphide is interpreted as the extent of 3 d physical connectivity between the inclusions and a partially molten underlying hybrid layer primary contacts between the silver swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse grained chevron textured chromites with skeletal textures compositions of these chromites match those from kambalda perseverance and other localities and are inconsistent with a metamorphic origin they are interpreted as markers of primary magmatic contacts the combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates taken together with observations on the internal fractionation of platinum group elements within the massive ores these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube the floor of the tube had been previously heated by passage of large volumes of lava such that it had reached its melting range the felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma this constitutes strong evidence for the operation of thermo mechanical erosion during ore emplacement the disseminated cygnet and black swan orebodies show a number of distinctive features cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip up transport and redeposition of sulfides from a pre existing massive sulfide orebody of which black duck may be a remnant the black swan orebody by contrast does not show xenolithic features but is characterised by an association of sulfide blebs with segregation vesicles and by unusually coarse grained olivine the black swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals

a komatiite analog to potential ultramafic materials on io ultramafic volcanism is one model which fits currently available data for some eruptions on jupiter s moon io assuming that such activity does occur it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to io we have used such analogs and models along with galileo data to investigate the nature of ultramafic materials that are assumed to occur at high temperature hotspots on io the unusual komatiites of the commondale greenstone belt south africa are consistent with available galileo data on the temperatures and composition of potential ionian ultramafic materials these komatiites had high sio2 50 wt and mgo 31 wt contents which are inferred to result in high liquidus temperatures 1610 c low dynamic viscosities 0 2 pa s and low densities 2680 kg m and in the crystallization of orthopyroxene phenocrysts not found in other komatiites we used the commondale composition and the model of williams et al 1998 to investigate potential ultramafic flow behavior on io compared to their terrestrial counterparts ionian ultramafic lava flows would have less potential for turbulent flow lower maximum emplacement distances and lower thermal erosion rates because of the lower ionian gravity and their more silicic composition shallow 10 m thermal erosion channels seem less likely on io unless 1 lavas are superheated 2 lavas are more ultramafic 31 mgo 3 eruption rates eruption durations or flow volumes are high or 4 substrates are unconsolidated partly consolidated or volatile rich and thus more erodable if volatiles are present in the lava as vesicles then increasing lava vesicularity results in decreasing maximum emplacement distances decreasing thermal erosion rates and decreasing erosion channel depths for given eruption durations

interspinifex ni sulfide ore from the coronet shoot kambalda characterization using microbeam x ray fluorescence mapping and 3 d x ray computed tomography interspinifex ores are developed where pools of sulfide liquid overlie thermally eroded komatiite flows such that sulfide occupies the space between spinifex olivine plates microbeam x ray fluorescence mapping and 3 d x ray computed tomography have been used to investigate microstructures and chemical zonation within interspinifex ores from coronet shoot kambalda sulfide compositions in the interspinifex space match the composition of the overlying sulfide pool aluminous silicate interpreted as displaced silicate melt forms a film at the silicate sulfide interface locally developing dome like plumes the film and plumes are characterized by fine skeletal chromite that diminishes in abundance over about a decimeter downward into the interspinifex zone these relationships are strong evidence for a primary magmatic origin for this ore type driven by the strong tendency of dense inviscid sulfide liquid to infiltrate and melt underlying rocks such infiltration melting interfaces may be a common feature at the base of massive sulfide ores taking different forms depending on the lithological and fracturing characteristics of the footwall rocks

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

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

recent depositional patterns of the nile deep sea fan from echo character mapping the nile deep sea fan was surveyed in 1998 using swath multibeam bathymetry backscatter imagery and 3 5 khz and seismic profiling on the basis of this new data set the fan has been divided into three main provinces western central and eastern recent sedimentary patterns as deduced from echo character mapping show that gravity induced sedimentary deposits are predominant and are expressed through either slides debris flows or turbidites turbidity processes are particularly active in the westernmost province through a well developed network of deep sea channels this network controls turbiditic flows that subsequently overflow levees mass wasting processes are extremely efficient all over the nile deep sea fan in the upper central province a broad allochthonous tongue of mass flow deposits generated by slope destabilizations may have led to channel avulsions and migrations in the eastern province a northwest southeast deformed belt resulting from combined deep seated and salt related tectonics contains large proportions of destabilized sediments i e originating from destabilization processes such as mass movements probably settled by recent tectonic activity this article highlights 1 the influence of both thin skinned and thick skinned tectonics on sedimentary distribution and 2 the predominance of gravity induced sediments turbidites slides and mass flows in such submarine fans the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential

highly siderophile element systematics of the 3 3 ga weltevreden komatiites south africa implications for early earth history highly siderophile element hse os ir ru pt pd and re abundances re os isotopic systematics and major minor and lithophile trace element abundances are determined for a suite of remarkably fresh komatiites from the weltevreden formation fm of the barberton greenstone belt south africa the komatiite lavas examined are calculated to have contained ca 31 mgo upon emplacement and are among the most mgo rich lavas ever erupted onto earth s surface based on the partitioning behavior of v indicating reduced conditions of magma formation as well as the low calculated h2o abundance of 0 02 in the mantle source anhydrous melting for the weltevreden fm komatiites is inferred the high calculated liquidus temperatures of the emplaced komatiite lavas of ca 1600 c require an unusually hot 1800 c mantle source 100 c hotter than the ambient contemporaneous mantle these observations are most consistent with formation of the weltevreden komatiites via deep 530 km melting in a mantle plume the re os isotopic data for twelve whole rock samples and an olivine separate define a precise re os isochron age of 3266 8 ma 2 mean and an initial 187os 188os of 0 10440 5 187os 0 13 0 05 the re os data coupled with the relatively high calculated hse abundances in the mantle source totaling 80 of those in modern primitive mantle pm estimates indicate that at least some portions of the deep early archean mantle evolved with essentially chondritic re os and absolute hse abundances that are typical of the sources of late archean komatiites if the hse budget of the terrestrial mantle was established via late accretion of several large planetesimals following the cessation of major chemical interaction between the core and the mantle our data indicate that by 3 3 ga these materials were well homogenized within the mantle domains sampled by the weltevreden komatiites and by inference within the mantle as a whole

structural setting and shape analysis of nickel sulfide shoots at the kambalda dome western australia implications for deformation and remobilization integrated three dimensional structural studies have led to new insights into the geologic controls on the distribution of magmatic nickel sulfide ore shoots associated with metakomatiites at the archean kambalda dome western australia exploration models emphasize volcanic channels in deep embayments trough structures modified by thermal erosion at the base of turbulent lava flows however analysis of three dimensional models of the kambalda exploration database reveals ore shoot scale controls consistent with the regional deformation sequence d1 to d4 ore shoots in d1 settings e g ken shoot are transposed into the north northwest d1 trend and have asymmetric trough structures with reentrant updip margins indicating a sense of movement opposite to that on the kambalda dome ore shoots in d2 settings e g lunnon shoot have asymmetric trough structures bound by low angle thrusts indicating a sense of movement typical of that on the dome with thrust movement of ore into the talc and carbonated hanging wall rocks lacking primary igneous features e g mcmahon shoot ore shoots in d3 settings are offset by normal faults which host gold mineralization at the south end of the kambalda dome e g hunt shoot or by major reverse faults at the north end of the dome e g otter juan shoots ore shoots in d4 settings fisher shoot are transposed from the north northwest trend into the north to north northeast d4 trend in contrast ore shoots associated with shallow poorly defined trough structures with or without serpentinized hanging wall showing relict igneous features e g durkin shoot indicate preserved volcanic control the spectrum of volcanic and structural controls indicates a continuum of kambalda ore shoots from a volcanic controlled end member 3 shoots in shallow trough structures with relict igneous features through structurally modified 7 shoots in deep trough structures without relict features to a structurally controlled end member 1 shoot tectonically emplaced in the hanging wall with deformation features shape analysis of the three dimensional models indicates that the structurally modified and controlled ore shoots have prolate shapes and two of the volcanic controlled ore shoots have oblate shapes comparison of the prolate shapes with the shapes of strain indicators reveals the importance of constrictional d1 deformation in elongation of the ore shoots particularly at the north and south ends of the kambalda dome the oblate volcanic controlled end members reflect low strain settings adjacent to major fault zones comparisons to previous experimental systems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow the importance of deformation controls on nickel sulfide distribution at the kambalda dome means that exploration for kambalda style mineralization should take into consideration the deformation history of terranes that host ore

emplacement and erosion by archean komatiite lava flows at kambalda revisited we have developed a mathematical model to evaluate the flow and erosional potential of submarine channelized komatiite lavas at kambalda western australia field data from kambalda were used to constrain the choice of important input parameters and model results were compared with data from field studies and geochemical analyses our results suggest that thermal erosion is strongly dependent upon the nature and behavior of the substrate if the substrate is treated as an unconsolidated hydrous sediment that can be fluidized by vaporized seawater then our model predicts that an initially 10 m thick basal kambalda komatiite lava could have produced very high thermo mechanical erosion rates 23 10 m day crustal thicknesses of 5 20 cm at distances of 5 35 km from the source and a high degree of lava contamination 3 12 in contrast if the substrate is treated as a more consolidated anhydrous sediment that could not be fluidized then our model predicts that a kambalda komatiite flow would have had much lower thermal erosion rates 1 2 0 4 m day and degrees of contamination 1 3 and would have had crustal thicknesses of 5 20 cm at longer flow distances of 30 165 km field constraints are generally consistent with our predictions for a non fluidized substrate the reentrant embayments at kambalda are thought to form from either erosion of deep channels in a flat basaltic seafioor huppert et al 1984 or erosion of a thin 5 m sediment with minor undercutting of basalt in pre existing topography lesher et al 1984 our modeling indicates that the former was possible for long eruption durations months whereas the latter was possible for short eruption durations 2 weeks as the latter hypothesis is more consistent with the existing field evidence for thermal erosion at kambalda we believe it is the preferred interpretation

a small s mif signal in martian regolith pyrite implications for the atmosphere abstract the past martian atmosphere is often compared to the archean earth s as both were dominated by co2 rich and o2 poor chemistries archean earth rocks preserve mass independently fractionated sulfur isotopes s mif non zero 33s and 36s originating from photochemistry in an anoxic atmosphere thus martian crustal rocks might also be expected to preserve a s mif signature providing insights into past atmospheric chemistry we have used secondary ion mass spectrometry sims to investigate in situ the sulfur isotope systematics of nwa 8171 paired to nwa 7034 a martian polymict breccia containing pyrite that formed through hydrothermal sulfur addition in a near surface regolith setting in this meteorite pyrite grains have a weighted mean of 33s of 0 14 0 08 and 36s 0 70 0 40 2 s e m so the s mif signature is subtle sulfur isotope data for four additional shergottites yield 33s values that are not resolvable from zero as in previous studies of shergottites at first glance the result for the polymict breccia might seem surprising but no martian meteorite yet has yielded a s mif signature akin to the large deviations seen on earth we suggest that s mif bearing aerosols h2so4 and s8 were produced when volcanic activity pushed a typically oxidising martian atmosphere into a reduced state after rain out of these aerosols s8 would tend to be oxidised by chlorate dampening the s mif signal which might be somewhat retained in the more abundant photolytic sulfate then in the regolith mixing of aqueous surface derived sulfate with igneous sulfide the latter with zero mif to form the abundant pyrite seen in nwa 8171 would further dampen the s mif signal nonetheless the small negative 33s anomalies seen in martian meteorites imply that volcanic activity was sufficient to produce a reducing atmosphere at times this volcanically driven atmospheric evolution would tend to produce high levels of carbonyl sulfide ocs given that ocs is a relatively long lived strong greenhouse gas the s mif signal implies that volcanism periodically generated warmer conditions perhaps offering an evidence based solution to the young wet mars paradox

petrology and crystallization history of multiphase sulfide droplets in a mafic dike from uruguay implications for the origin of cu ni pge sulfide deposits sulfide blebs in a mafic dike belonging to the uruguayan dike swarm replicate at a small scale many features observed in larger platinum group element enriched nickel copper sulfide deposits such as in sudbury and noril sk these blebs formed by the crystallization of droplets of immiscible sulfide liquid form 1 cm sized ovoid geopetal structures with pyrrhotite and pentlandite at the base chalcopyrite and cubanite at the top and titaniferous magnetite at the margins magnetite crystallized first followed by nickel and iron rich monosulfide solid solution which sank to the bottom of the droplets and from which pyrrhotite and pentlandite subsequently exsolved the remaining copper rich liquid in the upper part of the droplets crystallized to form an intermediate solid solution from which chalcopyrite and cubanite subsequently exsolved the final extremely evolved cu fe s rich pd sn pb mo ag bi te and sb bearing liquid crystallized as 10 to 20 sized veinlets filling fractures in the magnetite and adjacent silicate minerals the veinlets crossing the magnetite often terminate in 100 m sized subrounded globules at the contact of the magnetite and the surrounding silicate minerals indicating a direction of veinlet filling away from the center of the blebs textural evidence in the blebs illustrates that as crystallization proceeded and the immiscible sulfide liquid droplets within the dike sank a distance of less than 1 mm relative to the adjacent silicate onto already partially crystallized silicate minerals the sulfide liquid surrounded and resorbed the already solid magnetite grains at the top of the blebs magnetite grains were isolated above the sulfide liquid and partially surrounded by quartz plagioclase and amphibole an assemblage representing a late stage magma that had been drawn into the space created above the droplets as they sank and which is more evolved than that of the surrounding host dike the location of the blebs within the dike isolated from influences external to the dike indicates both that the fracture filling by evolved immiscible cu rich sulfide liquid is related to crystallization processes rather than later tectonic events and that the bismuth tellurium and antimony have a late magmatic origin associated with crystallization of the immiscible sulfide liquid

the geology of the kambalda nickel field western australia the kambalda nickel field contains the major concentration of volcanic peridotitc associated nickel ores in the archean of australia it occupies an area of 400 km 2 underlain by two sequences of ultramafic mafic and felsic volcanics and sedimentary rocks which can be broadly correlated with previously erected regional stratigraphic sections most ores occur in the lower kambalda sequence at the base of the kambalda ultramafic rocks the ultramafic rocks of the upper bluebush sequence contain only minor mineralization four phases of deformation occurred over an interval of more than 300 m y resulting in a dominant north northwest structural trend faulting was active during original volcanism and continued intermittently until after mafic felsic dike intrusion the ultramafic rocks have been serpentinized and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism the kambalda ultramafic rocks are of variable thickness and in places exceed 1 000 m the lower member of the ultramafic rocks occupying approximately one third of the sequence comprises thicker high magnesium flows i e generally 36 volatile free mgo commonly separated by sulfidic quartz albite sediments thinner lower magnesium flow units characterize much of the upper member the ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few grouped ribbonlike original orebodies eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt ultramafic contact these troughs are thought to be original volcanic depressions which focused later deformation the remainder is hanging wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow hanging wall orebodies occasionally grade into stratigraphically equivalent interflow sediments the ores contain varying proportions of massive matrix and disseminated sulfides the hypogene mineral assemblage is pyrrhotite pentlandite pyrite chalcopyrite but millerite is locally significant the massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position metamorphism has caused recrystallization of the ores and some redistribution of elements there is commonly marked variation of mineralogy and s ni ratios between contact and overlying hanging wall ores and within and between original ore surfaces within shoots but the relative contributions of primary composition and metamorphism to this variation are uncertain in ore environments the footwall basalt ultramafic contact is more deformed than in non ore environments the basal ultramafic flows are thicker and richer in magnesium the trend toward lower magnesium flows upward through the ultramafic formation is less well developed and there is better textural development and compositional differentiation within flows non ore environments usually contain interflow sediments whereas there is generally a sediment free prism of ultramafic rocks above ore environments most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor volcanic exhalations probably contributed to minor ore in sulfidic sediments

a reassessment of the emplacement and erosional potential of turbulent low viscosity lavas on the moon we have reevaluated the role of thermal erosion by low viscosity lunar lavas as a mechanism for the formation of the lunar sinuous rules we have adapted the model of williams et al 1998 and used the compositions of an apollo 12 basalt and a terrestrial komatiitic basalt to investigate the compositional and environmental effects on the flow behavior of low viscosity lavas on the moon and the earth our model predicts that lunar lavas could have erupted as turbulent flows that were capable of flowing hundreds of kilometers on a sufficiently flat unobstructed substrate these results are consistent with previous studies modeling of lavas over a substrate of the same composition shows that thermal erosion rates would have been low 10 cm d 1 as a result long duration eruptions approximately months to years would have been required to incise deep tens to hundreds of meters channels partial melting and mechanical removal of the substrate a mechanism suggested by hulme 1973 to enhance erosion only slightly increases thermal erosion rates other factors such as higher flow rates or lava superheating could have produced deep rules by thermal erosion during shorter duration eruptions a superheated lunar lava not only would have had a higher erosion rate 40 cm d 1 but also would have remained uncrusted for tens of kilometers which is consistent with the open channel morphology of most sinuous rules for lunar lavas with large volatile i e vesicle contents the presence of vesicles would have tended to increase viscosity at low strain rates resulting in shorter turbulent flow distances lower thermal erosion rates and thus shallower erosion channel depths for given eruption durations