composition and origin of intermediate solid solutions in the system thorite xenotime zircon coffinite abstract a comprehensive survey of the accessory mineral assemblages in variscan granites of the german erzgebirge and pan african granites from jordan revealed the occurrence of intermediate solid solutions of the tetragonal thorite xenotime zircon coffinite mineral group with partially novel compositions these solid solutions preferentially formed in evolved and metasomatically altered p poor leucogranites of either i or a type affinity thorite from the erzgebirge contained up to 18 8 wt y2o3 16 1 wt zro2 and 23 3 wt uo2 xenotime and zircon have incorporated th in abundances up to 36 3 wt and 41 8 wt tho2 respectively extended compositional gradation with only minor gaps is confined to hydrated members of this mineral group and is observed to exist between thorite and xenotime thorite and coffinite and y hree bearing thorite and zircon complex hydrous solid solutions containing elevated abundances of three or more of the endmembers are subordinate previously reported intermediate solid solutions between anhydrous zircon and xenotime and anhydrous zircon and thorite are not observed and are in conflict with experimental work demonstrating very limited miscibility between anhydrous species of endmember composition the majority of hydrous intermediate solid solutions in the th y zr u system are likely thermodynamically unstable instead they are probably metastable responses to unusual physico chemical conditions involving various parameters and conditions the relative importance of which is incompletely known leaching and dissolution of preexisting accessory phases during interaction with f bearing hydrous fluids enriched in th y hree zr and or u and common deposition of the various elements at disequilibrium supersaturation seems to play a key role but other processes may be of similar importance experimental work involving hydrous conditions and complex systems composed of more than two endmembers are needed to shed light into the stability relations of the chemically uncommon compositions treated in this study

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

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

changes in sulfides and platinum group minerals with the degree of alteration in the roby twilight and high grade zones of the lac des iles complex ontario canada the lac des iles complex hosts pd deposits with unusually high pd pt and pd ir ratios these high ratios have been attributed to the introduction of pd by late magmatic hydrothermal fluids the presence of hydrous silicates and low temperature sulfide mineral assemblages has been advanced as evidence for fluid overprint this idea has been investigated by documenting the change in silicate sulfide and platinum group mineralogy and texture in samples from the deposit ranging from fresh gabbronorite to extensively altered chlorite actinolite schist in addition whole rock analysis combined with laser ablation analysis of the sulfides was used to determine which phase controlled each of the pge in fresh gabbronorite the sulfide assemblage is igneous pyrrhotite pentlandite chalcopyrite and pd is present mainly in pentlandite and kotulskite pdte inclusions associated with the sulfide minerals all other pge except pt are present in solid solution in pyrrhotite and pentlandite platinum is present as sperrylite ptas2 and montcheite ptte2 inclusions associated with the sulfide minerals these observations suggest that the pge in the fresh gabbronorite were all collected by magmatic sulfide liquid in the chlorite actinolite schist the sulfide assemblage is a low temperature assemblage consisting of chalcopyrite pyrite millerite a minor amount of palladium is present in millerite but most is present as bismuth rich kotulskite and pd arsenides both of which are associated with hydrous silicate all other pge except pt are hosted by pyrite a minor amount of pt is present in pyrite but most is found as sperrylite and montcheite associated with hydrous silicate minerals the pyrite could have formed by fe loss from pyrrhotite resulting in the pyrite inheriting os ir ru and rh and fe loss from pentlandite to form millerite resulting in the millerite inheriting some pd the pge concentrations of the whole rock show there is no direct correlation between the sulfide assemblage the degree of alteration of the silicate minerals and ore grade thus although fluids have clearly altered the sulfide silicate and platinum group mineralogy and their textures it is not clear that these fluids have changed the pge content of the rocks

controls on the disseminated ni cu pge sulfide mineralization at the tubane section northern molopo farms complex botswana implications for the formation of conduit style magmatic sulfide ores abstract the molopo farms complex mfc is located near the western margin of the archean kaapvaal craton in southern botswana the complex hosts low grade disseminated ni cu and platinum group elements pge mineralization restricted to the lower ultramafic cumulates notably in the feldspathic orthopyroxenites the mineralized feldspathic orthopyroxenites contain up to 0 47 ppm pt pd au manifested in the form of platinum group mineral pgm assemblages dominated by pge bismuthotellurides asernides and minor au and ag alloys the pgm encountered are froodite pdbi2 moncheite ptte2 michenerite pdbite sperrylite ptas2 largely associated with altered silicates and sulfide minerals the association of pgm with sulfides suggests that they were originally concentrated by sulfide melt while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of pgm with altered silicates the cu pd ratios of most of the rocks fall within the primitive mantle range however the cu pd ratios of the feldspathic orthopyroxenites are more variable with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides this is supported by the cu zr ratios of these rocks which are also variable above and below unity trace element ratios on the th yb pm versus nb th pm plot indicate strong crustal contamination across all units including those not in proximity to the country rocks the 34s of the magmatic rocks are variable between 6 3 and 11 4 in contrast the 34s of dolomitic siltstones of the transvaal supergroup which constitute the immediate country rocks vary from 4 1 to 0 6 these data indicate that crustal contamination largely occurred prior to magma emplacement it is inferred that the most important contaminant and source of crustal s were the ramotswa and ditlhojana formations that include sulfidic shales at depth we consider that magnesian basaltic magmas of b1 bushveld magma composition underwent crustal contamination assimilation of crustal s and segregation of sulfide melt in the feeder conduit or staging chamber prior to final magma emplacement the feeder conduit or staging chamber was subsequently invaded by further b1 magma this magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final mfc chamber studied here after the staging chamber had already partially solidified it was invaded by yet another influx of magma possibly even more primitive than the b1 bushveld magma being a crystal mush this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final mfc chamber the presence of centimeter sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas therefore it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging chamber

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

ruthenium in chromite as indicator for magmatic sulfide liquid equilibration in mafic ultramafic systems abstract the platinum group element ruthenium ru is an important petrogenetic tracer of earth s accretion history core mantle interaction mantle evolution and the exploration for magmatic sulfide deposits however its geochemical behavior in mafic ultramafic systems is still not fully understood which limits its usefulness in the predictive modelling of geochemical systems to further develop the use of ru as a petrogenetic tracer we analyzed the ru contents of chromites from a global sample set of komatiites komatiitic basalts and ferropicrites by laser ablation icp ms and carius tube isotope dilution icp ms analysis the ru data are combined with full major and minor element microprobe analyses the data show that two groups of chromite can be distinguished on the basis of their ru contents this bimodal distribution occurs across komatiites komatiitic basalts and ferropicrites and corresponds to the sulfide saturation state of the magma during chromite crystallization chromites from sulfide undersaturated magmas contain between 150 and 600 ppb ru whereas chromites that crystallized from sulfide bearing magmas mostly contain less than 150 ppb ru the ru contents are independent of elements that typically document a modification of chromite suggesting that the determined ru concentrations reflect the primary magmatic chromite composition the ru contents are furthermore independent of magma type i e komatiites komatiitic basalts ferropicrites the magma source regions i e different cratons belt and localities the geochemical affinity i e munro type and karasjok type and age i e 2 7 ga and 2 0 ga and neither local fluctuations in fo2 nor emplacement styles i e intrusive vs extrusive can account for the bimodal ru distribution in chromite observed during this study as a consequence it is argued that the ru contents of chromites from mafic and ultramafic systems are indicative of the presence or absence of a sulfide liquid during chromite crystallization in sulfide saturated systems the chalcophile ru will dominantly partition into sulfides whereas in the absence of sulfides ru preferentially partitions into chromite over all other available phases because i high ru contents in chromites are exclusively associated with sulfide undersaturated systems and ii the ru contents of chromites can be measured via fast and cost effective laser ablation icp ms ru variability patterns in chromites allow the identification of magmas that have equilibrated with magmatic sulfide liquids prior to or during chromite crystallization and hence have potential to host metal sulfide orebodies

ruthenium variation in chromite from komatiites and komatiitic basalts a potential mineralogical indicator for nickel sulfide mineralization more than 390 chromite grains from komatiites and komatiitic basalts from the yilgarn craton of western australia and the finnish part of the fennoscandian shield were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry la icp ms to identify ruthenium ru signatures in chromite associated with nickel sulfide bearing rocks results indicate a potential method to discriminate mineralized and barren komatiite and komatiitic basalt units based on ru concentrations in chromite and indicate potential for chromite to be used as a resistate indicator mineral in exploration for komatiite associated nickel sulfide deposits chromites from barren komatiites and komatiitic basalts display ru concentrations mostly between 150 and 600 ppb chromites from mineralized units have distinctly lower ru contents 150 ppb these results can be interpreted in terms of the much higher partition coefficient for ru into sulfide liquid compared to that of ru into chromite resulting in much lower ru concentrations in chromite where both chromite and sulfide liquid are present and competing for ru as a result the ru content of chromite can be used to determine if a komatiite melt equilibrated with a sulfide liquid during crystallization and therefore if a system and or sequence is prospective for nickel sulfide mineralization the strength of this method compared to previous whole rock exploration techniques derives from combining 1 the geochemical properties of a chalcophile element that records an ore forming process while being strongly immobile during postmagmatic processes with 2 the in situ analysis of a mineral that is generally preserved even in highly altered and mildly weathered komatiites and that is a common constituent of detrital heavy mineral samples chromite ru content has potential as a prospectivity indicator applicable to a wide range of media including bedrock laterites and detrital resistates heavy mineral samples

platinum group minerals in the limoeiro ni cu pge sulfide deposit brazil the effect of magmatic and upper amphibolite to granulite metamorphic processes on pgm formation the limoeiro ni cu platinum group elements pge deposit is a recent discovery associated with an igneous tubular conduit system in northeastern brazil representative ores from the deposit have been used for platinum group minerals pgm identification and for pge in base metal sulfides bms quantification ninety eighty percent of the pgm in the massive sulfide ores is homogeneous pt ni bi bearing merenskyite pdte2 enclosed primarily by pyrrhotite suggesting that it is formed by exsolution from monosulfide solid solution mss merenskyite gradually but systematically becomes poorer in pt and ni with increasing fractionation which is interpreted to reflect a transition to a more evolved sulfide liquid that segregated in the eastern parts of the intrusion in massive sulfide ores merenskyite forms unusually large up to 5000 m2 euhedral grains commonly in contact with spherical silicate inclusions bms hosts 12 16 of the pd with the remainder hosted by pgm which is interpreted to indicate that merenskyite recrystallized from a pge bearing bismuthotelluride metamorphic melt formed during high grade metamorphism sperrylite ptas2 is the second most abundant pgm 18 of pgm in disseminated ore and in contrast to merenskyite occurs mainly as very small median of 25 m2 inclusions in high temperature silicates and oxides interpreted to have crystallized at high temperatures directly from sulfide blebs that formed and were transported within the limoeiro magma conduit

chromite in komatiites 3d morphologies with implications for crystallization mechanisms high resolution x ray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies chromite contents and degrees of alteration and metamorphism to reveal the wide range of sizes shapes and degrees of clustering that chromite grains display as a function of cooling history dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops at slower cooling rates in the interiors of thick flows chromite forms predominantly euhedral grains large clusters up to a dozen of grains are characteristic of liquidus chromite whereas fine dustings of mostly individual 20 m grains form by in situ crystallization from trapped intercumulus liquid chromite in coarse grained olivine cumulates from komatiitic dunite bodies occurs in two forms as clusters or chains of euhedral crystals developing into chicken wire texture where chromite is present in supra cotectic proportions and as strongly dendritic semi poikilitic grains these dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite in some cases this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs this texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures our 3d textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocks

microchemical and sulfur isotope constraints on the magmatic and hydrothermal evolution of the black swan succession western australia the black swan succession consists of an archean bimodal dacite komatiite association largely dominated by ultramafic cumulates hosting a number of massive and disseminated magmatic sulfide orebodies although it was affected by multiple alteration events the low degree of penetrative deformation allowed the preservation of magmatic textures and stratigraphic relationships in this context we apply a combined multiple sulfur isotope and microchemical approach to unravel the potential of sulfides to simultaneously retain information about their magmatic and hydrothermal history negative 33s signatures in both massive 0 56 and disseminated 0 66 sulfide orebodies reflect the assimilation of photochemically derived sulfur into the komatiite magma which attained sulfide liquid supersaturation through the assimilation of crustal material analogous to the intercalated breccia dacite hosting colloform sulfides 33s 0 91 the slightly different 33s signatures between disseminated and massive orebodies reflect the assimilation of different proportions of crustally derived sulfur and imply that the respective sulfide liquids segregated from distinct komatiite magma pulses subsequent alteration episodes imparted to the black swan succession a concentric zonation consisting of a serpentinite core and talc carbonate margins the black swan disseminated orebody extends through both alteration styles without major variations in the dominant millerite pyrite sulfide assemblage pyrite in both serpentinite and talc carbonate samples exhibits a consistent trace element signature suggesting a relative immobile behavior of ni and co during the percolation of the talc carbonate fluids conversely variable 34s signatures between the two alteration styles reflect mobilization of sulfur during talc carbonate alteration whereas the 2 range of 34s values in serpentinites can be attributed to moderate equilibrium fractionation processes the 10 range of 34s values in talc carbonate rocks suggests that highly oxidizing fluids promoted the mobilization of sulfur as so42 which decreased the 34s of the disseminated sulfides furthermore sulfur isotope and microchemical signatures of euhedral pyrite from intercalated dacitic breccia indicate that this pyrite generation was deposited from a late hydrothermal event which is characterized by variable and least negative 33s signatures between 0 20 and 0 48 relative to all sulfide orebodies these features suggest that the late fluids developed in a heterogeneous hydrothermal system with a minor component of sulfur recycled from the sulfide orebodies

redistribution of ore elements during serpentinization and talc carbonate alteration of some archean dunites western australia serpentinization of barren and weakly mineralized archean dunites in the yilgarn block western australia occurred without any apparent loss of major components including the important ore elements ni and fe sulfur addition during initial serpentinization resulted in about 30 percent of original silicate ni being redistributed to newly formed sulfide by reactions similar to olivine water lizardite brucite and lizardite brucite s lizardite brucite magnetite heazlewoodite water h 2 serpentinization at a higher temperature or metamorphism of the lizardite bearing assemblage formed antigorite serpentinite in which up to 60 percent of bulk rock ni is incorporated in sulfide form lizardite brucite magnetite heazlewoodite co 2 s antigorite magnesite magnetite ni sulfide water h 2 complete co 2 metasomatism resulted in talc carbonate rocks with still more ni redistributed to sulfides and the formation of some fe sulfide antigorite magnesite magnetite ni sulfide co 2 s talc magnesite pentlanditc fe sulfide or magnetite water o 2 in areas where low s activity precluded sulfide formation ni was strongly enriched in magnetite in serpentinites or talc in talc carbonate rocks preexisting disseminated ni sulfides of magmatic origin were significantly upgraded by serpentinization up to 30 percent of sulfide ni in ores averaging about 0 7 percent ni is of metamorphic origin and this proportion may be higher in talc carbonate ores fe sulfides of metamorphic origin form up to 20 percent of the total sulfide fraction in talc carbonate altered disseminated ni ores so that about half the sulfide content of these ores may be metamorphically derived by redistribution of elements originally held in olivine

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

the honeymoon well nickel sulfide deposits western australia the honeymoon well ni fe sulfide deposits occur in a thick assemblage of altered intrusive dunires and extrusive komatiites in an archean greenstone belt near wiluna in the eastern yilgarn block western australia the sulfides host rocks and surrounding intermediate to basic volcanics have undergone greenschist facies metamorphism in a low strain environment ultramafic rocks have been totally serpentinized and or carbonated but igneous textures are commonly well preserved the coarse grained dunitic rocks are interpreted as slowly cooled igneous cumulates on the basis of textural features and mineralogical and cryptic layering spinilex textures within the ultramafic assemblage and rare interbedded black shale suggest an extrusive origin for some of the rocks and the dunite bodies are possibly consanguineous subvolcanic sills disseminated ni fe sulfide mineralization occurs in two zones up to 100 m wide near the western and eastern contacts of the ultramarie assemblage pentlandite is the dominant sulfide in lizardite serpentinites and occurs in an intercumulus position moulded around former olivine grains in talc carbonate rocks locally developed adjacent to the ultramarie contacts pyrrhotite is also common exploration to date has outlined two major zones of ni fe sulfide mineralization with mean grades ranging from 0 5 to 1 percent ni honeymoon well sulfide mineralogy is ni rich in comparison with most other intrusive dunite associated deposits in the region and regular differences in s ni fe ni and ni cu ratios appear to correlate with the metamorphic setting and host rock mineralogy

pt and pd mobility in hydrothermal fluids evidence from komatiites and from thermodynamic modelling abstract komatiite host rocks and ores provide a natural laboratory for the study of pt and pd during hydration and carbonation of ultramafic rocks the degree of correlation between pt and pd and of those elements with lithophile elements such as ti in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock pt and pd are typically very strongly correlated within s poor komatiites and in disseminated ores containing 5 volume or less magmatic sulfides in contrast they are almost completely decoupled in sulfide rich ore samples thermodynamic modelling based on the existing body of published experimental data on pt and pd solubilities suggests that pd and pt can be readily transported as bisulfide complexes in acidic neutral solutions under reduced and moderate oxidation condition at 300 c and chloride complexes are only important under extremely acidic oxidised conditions pd is much more soluble than pt in the case of fluids in sulfide rich environments in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites solubilities of both elements are much lower in sulfide poor environments especially where ph values are constrained to neutral values by the near ubiquitous presence of carbonate minerals in the alteration assemblage this accounts for the evident near immobility of pt and pd in even the most altered silicate host rocks as compared with their wide dispersion in sulfide rich ores

a reappraisal of mount st helens ash clusters depositional model from experimental observation abstract ash clusters depositing the distal ash of the may 18 1980 eruption of mount st helens are supposed to be generated due to electrostatic forces within the eruption cloud therefore an experimental attempt was made to test electrostatic ash clustering which produced fallout with artificial ash clusters in the laboratory its grain size characteristics were similar to the mount st helens ash proximal fallout was coarse grained but showed a fine tail on the distribution curve medial ash was bimodal with the distribution minimum at 45 63 m distal ash was fine grained 63 m and cluster modifying ash 63 m clustering and subsequent deposition of two different typs of ash aggregates explain the grain size characteristics of the mount st helens and the experimental fallout ash proximal fallout was deposited as single grain clusters composed of a large particle 63 m with adhering fines settling at the grain specific rates of the largest particle distal fallout settled as fine ash clusters made up of ash 90 wt of which were

platinum group element and nickel sulphide ore tenors of the mount keith nickel deposit yilgarn craton australia a set of platinum group element pge analyses of about 120 samples from a 250 m continuous drill core through the mount keith komatiite hosted nickel orebody combined with ni cu co s and major elements reveals a complex trend of covariance between the original cumulus components of a thick sequence of nearly pure olivine sulphide liquid adcumulates the intersection is divided into informal chemostratigraphic zones defined primarily by combinations of fine scale cyclicity in original olivine composition defined by mg and sulphide composition defined by pt s and ni s contents of ni and pge in 100 sulphides tenors were determined from linear regressions of the ni s and pge s covariance for each zone inferred olivine compositions range from about fo92 to fo94 6 and show a broad decrease from bottom to top of the sequence complicated by numerous reversals revealing crystallisation in an open conduit system ni and pge tenors of mount keith sulphide ores have typical values similar to the type i deposits of the kambalda dome mobility of s at least on the scale of 2 m sample composites is evidently relatively minor tenors for the various zones range 12 22 ni 370 1540 ppb pt 970 3670 ppb pd 100 460 ppb ir 170 460 ppb rh and 710 1260 ppb ru pt pd and rh tenors are very strongly correlated but the iridium group of platinum group elements ipges ir and ru less so tenor variations are predominantly controlled by variations in magma sulphide ratio r 100 350 with a minor component of variance from equilibrium crystallisation trends in the parent magma pge depletion in the silicate melt due to sulphide liquid extraction is limited by entrainment of sulphide liquid droplets and continuous equilibration with the transporting silicate magma ratios of the pges to one another are similar to those in the host komatiite magma with the exception of pt which is systematically depleted in ores relative to rh and pd and relative to host magma by a consistent factor of about 2 to 2 5 this anomalous pt depletion relative to pge element ratios in unmineralized komatiitic rocks matches that observed in bulk compositions of many komatiite hosted orebodies the highly consistent nature of this depletion and particularly the very strong correlation between pt pd and rh in the mount keith deposit argue that this depletion is a primary magmatic signal and not an artefact of alteration differential diffusion rates between pt and the other pges giving rise to a low effective partition coefficient for pt into sulphide liquid is advanced as a possible but not definitive explanation

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

vesicles in thick komatiite lava flows kambalda western australia traditionally vesicles have been considered to be rare in komatiites however field studies at kambalda western australia and critical literature examination suggest that vesicle presence and content in komatiites are grossly underestimated at kambalda vesicles are observed in significant amounts up to 30 in internal horizons within komatiite lavas the vesicular horizons are inferred to represent bubbles frozen in place below the upper crust during multiple episodes of flow through of bubble rich lava we conclude that vesicle and crystal distribution are compatible with many komatiites being emplaced under laminar flow conditions analogous to models for basalt lava emplacement

relationship between microstructures and grain scale trace element distribution in komatiite hosted magmatic sulphide ores komatiite hosted nickel sulphides from the yilgarn craton australia consist of two main sulphide phases pyrrhotite fe7s8 and pentlandite fe ni 9s8 two minor sulphide phases chalcopyrite cufes2 and pyrite fes2 and trace arsenides samples of massive sulphides from three deposits with diverse deformation and metamorphic histories the silver swan perseverance and flying fox deposits have been studied by electron backscatter diffraction and laser ablation inductively coupled plasma mass spectrometry and nano scale secondary ion mass spectrometry these ore bodies were selected to investigate the relationship between microstructures and mineral trace element chemistry in three dominant sulphide species in each deposit in all three samples pyrrhotite preserves a strong evidence of crystal plasticity relative to both pentlandite and pyrite the trace element composition of pyrrhotite shows significant variation in specific elements pb bi and ag this variation correlates spatially with intragrain pyrrhotite microstructures such as low angle and twin boundaries minor signatures of crystal plasticity in pyrite and pentlandite occur in the form of rare low angle boundaries pentlandite and mild lattice misorientation pyrite trace element compositions of pentlandite and pyrite show no correlation with microstructures variations in pyrrhotite are interpreted as a result of intragrain diffusion during the syn and post deformation history of the deposit intragrain diffusion can occur either due to bulk diffusion dislocation impurity pair diffusion or by pipe diffusion i e along fast diffusion pathways at high and low angle and twin boundaries this contribution examines three different diffusion models and suggests that dislocation impurity pair diffusion and pipe diffusion are the most likely processes behind increased trace element concentration along the microstructures in pyrrhotite the same phenomenon is observed in samples from three different deposits that experienced widely different metamorphic conditions implying that the final disposition of these elements reflects a post peak metamorphic stage of the geological history of all three deposits

komatiites and nickel sulphide orebodies of the black swan area yilgarn craton western australia 1 petrology and volcanology of host rocks the black swan succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km nne of kalgoorlie within the 2 7 ga eastern goldfields greenstone province of the yilgarn craton the komatiite stratigraphy comprises a steep dipping east facing package about 700 m in maximum thickness and about 2 5 km in strike length fig 1 which hosts a number of economically exploitable ni sulphide orebodies including the silver swan massive ore shoot approximately half a million tonnes at about 10 5 ni the sequence can be subdivided into a lower felsic unit comprising coherent and autobrecciated facies of multiple dacite lava flows an upper eastern and lower western ultramafic unit each showing marked lateral facies variation and an upper felsic unit coeval with the eastern ultramafic unit the komatiite sequence has been metamorphosed at sub greenschist facies in the presence of high proportions of co2 rich fluid giving rise to pervasive talc carbonate and talc carbonate quartz assemblages with extensive preservation of pseudomorphed igneous textures cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession the degree of penetrative deformation is generally very low and original stratigraphic relationships are largely intact in much of the sequence the eastern ultramafic unit and western ultramafic unit are interpreted as components of a single large komatiite flow field representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows the western ultramafic unit which hosts the bulk of the high grade massive and disseminated ores is a sequence dominated by coarse grained olivine cumulates 2 km wide and up to 500 m thick with major magma pathways represented by thick homogenous olivine mesocumulate piles at its northern and southern ends respectively 400 and 200 m thick the sequence between the two major pathways consists of olivine orthocumulates ooc with minor spinifex textured intervals the unit is capped by a persistent spinifex textured crust less than 1 m thick and is locally vesicular the eastern ultramafic unit contains the black swan cumulate zone a 500 m thick sequence of very coarse grained hopper textured locally vesicular ooc containing disseminated sulphides in its lower 200 m the zone is flanked to the north and south by complexly interdigitated sequence of highly irregular spinifex capped olivine cumulate rich flow lobes between 1 and 100 m thick and dacitic lavas and tuffs the complexity of the 3 d spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite combined with thermal or thermomechanical erosion the eastern and western units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava which developed localised thermo mechanical erosion channels in the dacitic substrate komatiite and dacite eruption was synchronous giving rise to complex interdigitation and extensive contamination and hybridisation

structural mineralogical and geochemical constraints on the atypical komatiite hosted turret deposit in the agnew mt white district western australia in the norseman wiluna belt yilgarn craton the agnew mt white district is the host of many gold deposits located in the hinge of the regional lawlers anticline the turret gold deposit is structurally controlled by the table hill shear zone that transects the agnew ultramafic unit geochemistry coupled with petrographic data allowed the delineation of the paragenetic sequence associated with gold mineralisation and include 1 a pervasive talc carbonate alteration assemblage 2 a pre mineralisation stage associated with pervasive arsenopyrite chalcopyrite pyrrhotite pyrite alteration followed by 3 a late deformation event along a dilatational segment of the main table hill shear zone leading to the formation of a breccia hosting a cu bi mo au ag zn te w metal assemblage the presence of au ag cu alloys native bismuth chalcopyrite and other bi te s phases in the mineralisation stage suggest that gold may have been scavenged from the hydrothermal fluids by composite bi te cu au ag s liquids or melts using this mineral paragenetic sequence together with mineralogical re equilibration textures observed we show that the gold deposition at turret occurred over a temperature range approximately between c 350 and 270 c this temperature range together with the structural control and typical mesothermal alteration pattern including carbonate chlorite alteration shows that the turret deposit shares common characteristics with the orogenic gold deposit class however the metal association of cu au bi and mo the quartz poor and high copper sulphide content up to 15 are characteristics that depart from the typical orogenic gold deposit mineralogy through comparison with similar deposits in the yilgarn craton and worldwide we propose that the turret deposit represents an example of a porphyry derived au cu bi mo deposit

droplets and bubbles solidification of sulphide rich vapour saturated orthocumulates in the norilsk talnakh ni cu pge ore bearing intrusions

textural relationship of sulfide ores pge and sr nd os isotope compositions of the triassic piaohechuan ni cu sulfide deposit in ne china the 216 ma piaohechuan no 4 intrusion in northeastern china situated in the easternmost part of the central asian orogenic belt is composed of hornblende olivine gabbro overlain by hornblende gabbro the intrusion is about 630 m long 40 to 250 m wide and 50 to 150 m thick nickel sulfide mineralization is restricted to the lower part of the intrusion and contains 10 000 metric tons t of ni with average pge concentrations of 40 ppb os 30 ppb ir 36 ppb ru 26 ppb rh 255 ppb pt and 161 ppb pd in 100 sulfide

fall out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of st vincent abstract the 1979 explosive activity of soufriere volcano st vincent commenced on april 13th and continued for two weeks producing eleven eruption columns the activity produced air fall ash with abundant accretionary lapilli plus minor pyroclastic flows and base surge deposits the dispersal of the columns was observed by an sms 1 geostationary satellite all but two of the plumes were dispersed entirely to the east the last event on april 26th formed a minor lobe to the east and a major lobe to the south southeast the april 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour the magma discharge rate was greater than 3000 m3 s the plume expanded at a rate of 4000 km2 hr and was dispersed at a mean velocity of 15 m s the ash deposits show bimodal grain size distribution with 95 of the ash collected being sub 1 mm diameter modelling of the ash fall out has allowed interpretation of grainsize parameters in a quantitative manner the fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density the coarse population of the april 26th deposit displayed a systematic change in md and away from source the fine mode displays uniform md and everywhere and is interpreted as having been deposited in the form of accretionary lapilli

primary stratigraphic controls on ore mineral assemblages in the wannaway komatiite hosted nickel sulfide deposit kambalda camp western australia abstract the 18 m long uwa 04 02 drillcore from the fe ni cu pge wannaway deposit in the widiemooltha dome district eastern goldfields western australia intersects the whole sequence of a komatiite hosted layer of metal rich sulfide magma in spite of regional deformation and amphibolite facies metamorphism the sequence in the drillcore still preserves some of the original magmatic textures and assemblages and these were examined in a great detail the magmatic orebody typically consists of basal massive sulfides grading to net textured matrix and disseminated sulfide mineralization upward into the komatiite host the ore zone is underlined by sulfide rich black shale passing to basalts country rock xenoliths are locally enclosed in the massive sulfides portions of the drillcore untouched by penetrative deformation and with minimal imprint by late stage serpentinization allow the construction of a fairly complex framework where mineral assemblages and mineral chemistry of sulfides spinels and silicates vary systematically with stratigraphy and may reflect original conditions of ore deposition the general ore assemblage is dominated by fe sulfide and pentlandite with minor sphalerite and chalcopyrite spinels zn rich chromite ti magnetite alabandite mns accessory pge rich sulfarsenides and tellurides and rare molybdenite monoclinic and high s hexagonal pyrrhotite and fresh zn mn rich chromite characterize the basal massive facies whereas the matrix ore facies is marked by magnetite sphalerite and a gradually s depleted and reduced assemblage now represented by troilite exsolving low s hexagonal pyrrhotite and alabandite compositional modifications of the fe sulfides across the whole orebody and occurrence of alabandite testify to progressive sulfur loss concomitant with the establishment of low fo2 conditions over several meters upsequence in the matrix ore facies pge rich sulfarsenides disseminated across the whole mineralized sequence display igneous textures and pge fractionation trends the composition of olivine intergrown with matrix sulfides and in the serpentinized hangingwall komatiite deviates from the typical unmetamorphosed komatiite related highly forsteritic type however the fe mn and zn contents of olivine crystals decrease systematically and gradually with distance from mineralization towards the hangingwall komatiite contamination may be an alternative to metamorphic recrystallization of olivine as the cause of these trends the role of contamination is also shown by the trends of whole rock data from the mineralized sequence across the entire drillcore textures and mineral chemistry of minerals from the different rock facies in the drillcore are evaluated in terms of metamorphic effects although the remarkable relationship observed between stratigraphy and several major and accessory phases over metric distances is suggestive of alternative options including primary processes involving the komatiitic lava flow in its interaction both with the black shale substrate and with the sulfide melt ponding at its base

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

laacher see tephra a widespread isochronous late quaternary tephra layer in central and northern europe a late quaternary tephra layer widespread in central and northern europe resulted from explosive plinian and phreatomagmatic eruptions of the laacher see volcano 11 000 yr b p the tephra is distinguished from other late quaternary andesitic rhyolitic airfall tuff layers in northern europe and from basaltic or trachytic tuff deposits in southern europe by its phonolitic composition and abundance of sanidine plagioclase clinopyroxene amphibole and sphene the proximal tephia sequence at laacher see is divided into three main deposits the predominantly plinian deposits of lower and middle laacher see tephra llst and mlst and phreatomagmatic deposits of the upper laacher see tephra ulst the mlst member is further subdivided into beds a b and c1 c2 and c3 the chemical composition of the magma is highly differentiated phonolite in the llst to mlst b sections but mafic phonolite in the mlst c1 to ulst sections all deposits are considered to be isochronous the frequency maximum of 16 radiocarbon datings indicating an eruption about 11 000 50 yr b p distal ash was deposited in three main fans directed to the northeast lst traced up to 1 100 km distance south lst traced up to 600 km and southwest lst traced up to 100 km tephrostratigraphic correlation of the distal ash deposits is based on a the major element composition of glass shards b lithology and c heavy mineral analyses the northeastern fan consists of deposits from llst mlst b and mlst c1 eruptive phases the southern fan comprises mlst a mlst c2 and ulst deposits and the southwestern fan consists exclusively of ash from the ulst eruptive phase northeastern transport of ash during eruptive phases with high plinian eruption columns but southern and southwestern transport of ash along phases of relatively low eruption columns are interpreted in terms of prevailing southwesterly paleowinds at high altitudes tropopause level but northerly winds dominating in the lower atmosphere the laacher see eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions which also explains the near vent shifting of llst mlst b and mlst c1 iso pach axes from east southeast to northeast within the first 20 km of transport

sulphide mineral evolution and metal mobility during alteration of the oceanic crust insights from odp hole 1256d fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere supporting microbial life an

three dimensional morphology of magmatic sulfides sheds light on ore formation and sulfide melt migration the morphology of magmatic sulfides in igneous cumulates is controlled by the wetting properties of sulfide liquids against silicates the formation of nickel sulfide ores the behavior of sulfide liquids during mantle melting and potentially the segregation of the earth9s core are all controlled by the ability of sulfide liquids to migrate through the pore space of partially molten silicates three dimensional x ray tomographic images of sulfide aggregates in komatiitic olivine cumulates indicate that sulfide liquids have a limited tendency to wet olivine crystals forming interconnected networks only in the absence of silicate melt consequently the ability of sulfide liquids to migrate through the pore space of olivine cumulates is limited we conclude that disseminated sulfide ores in komatiites formed by accumulation of transported sulfide blebs a few millimeters in size and not by settling of sulfide olivine aggregates and that sulfides accumulated in the proportions in which they are now found rather than by percolation through cumulate pore space it is unlikely that sulfide droplets can be entrained and carried from the mantle at low degrees of partial melting our results also support the hypothesis that segregation of the earth9s core took place from a magma ocean rather than by percolation of sulfidic melt through partially molten mantle

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

microscale data to macroscale processes a review of microcharacterization applied to mineral systems microanalysis can provide rapid quantitative characterization of mineral systems that complements the field and core scale observations traditionally made in ore deposits we review recent innovations in microanalytical procedures and their application to studies of ore deposits case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems synchrotron x ray fluorescence shows centimetre scale chemical variations associated with proximity to mineralization in samples from sunrise dam gold mine western australia pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid driven brecciation with very little shearing both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization a second case study of sulphide mineralogy at the mt keith nickel sulphide deposit western australia demonstrates how x ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems large scale interaction between silicate and sulphide melts shown by homogenous palladium enrichment in pentlandite leads to a large proportion of globular ores with a high nickel content increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro and macroscale datasets

cotectic precipitation of olivine and sulfide liquid from komatiite magma and the origin of komatiite hosted disseminated nickel sulfide mineralization at mount keith and yakabindie western australia an empirical equation for the relationship of sulfur solubility to silicate magma composition and temperature has been combined with numerical simulations of the fractional crystallization of olivine from komatiite magma to predict the relative proportions of olivine and sulfide liquid which should segregate at equilibrium from a fractionating komatiite the predicted cotectic ratio of olivine to sulfide is higher and the s content of the cumulate assemblage lower by a factor of about three compared with the peak observed in type ii mount keith type dunite hosted disseminated ni sulfide deposits for the observed range of olivine compositions in the dunites this implies that a purely cotectic precipitation model for this deposit type is incorrect and that most of the sulfide liquid found in type ii deposits is transported and mechanically deposited

interfacial tension between magmatic sulfide and silicate liquids constraints on kinetics of sulfide liquation and sulfide migration through silicate rocks we have measured the interfacial tension sf between sulfide and silicate melts over the temperature range from 1250 to 1325 c by in situ measurement of the shapes of sessile drops of sulfide immersed in silicate melt sitting upon level alumina substrates the silicate melt was an alkali free synthetic basaltic liquid the sulfide melts were synthetic mixtures of fe cu ni s or fe s o the interfacial tension ranges from about 500 mj m 2 for sulfide of fes composition to approximately 600 mj m 2 for sulfide containing 16 wt cu or 6 5 wt ni and up to 650 mj m 2 for fe s o liquid containing 4 wt o contact angles vary from 150 to 180 showing that sulfide liquid does not wet oxide minerals in silicate magmas the density of sulfide melt does not depend strongly on cu or ni content remaining between 4 0 and 4 5 g cm 3 over the range of cu and ni contents expected in sulfide magmas but increases substantially to about 5 5 g cm 3 in fe s o melts containing 4 wt o our results can be combined with conventional concepts of nucleation theory to indicate that when silicate magmas exist at small degrees of supersaturation sulfide drops may nucleate rarely and at widely spaced intervals leading to kinetic control of the compositions of the resulting droplets in light of our results we use simple scaling arguments to argue that sulfide magmas are not capable of migrating through partially molten silicate rocks by capillary forces alone but may be forced through narrow grain boundaries or grain edges only by the flow of the enclosing silicate melt during melt extraction by compaction of partially molten mantle peridotite it is highly unlikely that silicate flow rates could be sufficiently rapid to permit the entrainment of droplets of sulfide liquid at temperatures below the solidus of enclosing silicate rocks sulfide melt will be free to travel along fractures or grain boundaries the mobility of sulfide melts through completely solidified silicate rocks may account for the widespread observation that late stage highly fractionated sulfide liquids have escaped from cooling bodies of massive sulfide to form veins and disseminations of cu rich sulfide minerals in their silicate host rocks

introduction to nickel sulfide orebodies and komatiites of the black swan area yilgarn craton western australia the black swan district 70 km north east of kalgoorlie in the archaean yilgarn craton of western australia hosts massive and disseminated nickel sulfide orebodies associated with komatiites the host rocks and ores preserve some remarkable primary igneous features which provide important clues as to the origin of komatiite hosted nickel ores the series of papers that follow report an extremely detailed study of the petrology volcanology and geochemistry of these unusually well preserved orebodies and their host rocks