sulfur isotopic studies of the dunka road cu ni deposit duluth complex minnesota the dunka road deposit is one of a series of cu ni occurrences located within troctolitic rocks of the southern portion of the duluth complex minnesota sulfide minerals principally pyrrhotite chalcopyrite cubanite and minor pentlandite occur interstitially with silicate minerals and average about 2 volume percent in ore grade horizons areas of sulfide concentrations in excess of 1 to 2 percent occur erratically throughout the troctolitic sequence but are in general concentrated near the basal contact with rocks of the metasedimentary virginia formation the delta 34 s values of sulfides in the troctolites range from 0 2 to 15 8 per mil with an average value near 7 5 per mil pyrrhotite in the underlying virginia formation hornfels and pyrite from unmetamorphosed virginia formation are characterized by a similar range in delta 34 s values the high positive delta 34 s values and similarity to values found in the virginia formation strongly suggest that most of the sulfur in the dunka road deposit was derived from the breakdown of pyrite in the virginia formation the sulfur isotopic compositions of minerals in the troctolites show no consistent lateral or vertical trends and are in general characterized by a high degree of variability such fluctuations in delta 34 s values indicate that sulfur isotopic homogenization was not attained throughout the troctolitic sequence several mechanisms may have contributed to the variations in delta 34 s values found within the troctolites including mixing of sedimentary and igneous sulfur assimilation of sedimentary sulfur characterized by variable delta 34 s values fractionation caused by fluctuations in oxidation states during sulfur volatilization and assimilation and periodic introduction of isotopically distinct magma batches in light of the erratic sulfide mineral and delta 34 s distributions the predominant mechanism is thought to be the introduction of both isotopically and spatially variable country rock sulfur into crystallizing troctolitic magma

physical controls associated with the distribution of sulfides in the voisey s bay ni cu co deposit labrador the voisey s bay ni cu co sulfide deposits occur within troctolites and olivine gabbros of the 1 34 ga voisey s bay intrusion formerly known as the reid brook intrusive complex the voisey s bay intrusion is the oldest known member of the predominantly anorthositic nain plutonic suite which straddles the ca 1 85 ga suture between archean gneisses of the nain province to the east and paleoproterozoic rocks of the churchill province to the west the reid brook discovery hill mini ovoid and ovoid zones are mineralized domains within a subvertical conduit dike system that appears to span between two large troctolite intrusions the eastern deeps chamber and the lower western deeps chamber the conduit trends east west and extends from near the top of the western deeps chamber at a depth of 700 m toward the north margin of the eastern deeps chamber close to surface all the mineralized zones in the voisey s bay deposits occur as magmatic textured sulfides associated with fragment bearing troctolites and olivine gabbros related to the conduit dikes rather than simply as basal accumulations at the floor of the chambers sulfides within the system are preferentially concentrated in traps where physical irregularities and changes in conduit morphology favor the precipitation capture and preservation of sulfides as a result of changes in the velocity and viscosity of a magma there is a consistent relationship between different sulfide textures and the physical environment existing in the conduits where each of the mineralized zones occurs this suggests that sulfides were deposited in their present sites by consistent magmatic processes within the conduits but were locally influenced by the physical parameters existing at these sites later remobilization or mass movement of sulfides would not generate the consistent textural relationships observed therefore sulfide deposition is ultimately related to a complex interplay of dike geometry i e changes in dike orientation and thickness and the fluid dynamics in the fragment bearing magma within a conduit system i e the ovoid mineralization appears to fill a bulge in the conduit and the reid brook zone is located close to the axis of inflection along its strike the eastern deeps zone although it does occur at the base of the eastern deeps chamber is more specifically associated with the line of entry for the feeder conduit close to a structural low at the base of the eastern deeps chamber the voisey s bay deposit does not conform to the traditional model for magmatic sulfide generation such as sudbury in which sulfides accumulate largely by gravitational settling within a magmatic chamber following attainment of sulfur saturation as a result of local assimilation in fact current evidence suggests in the eastern deeps mineralized zone that little gravitational settling took place because a large proportion of sulfides occur as high ni tenor minerals trapped interstitially in sulfides in coarse phases of the variable textured troctolite furthermore sulfide saturation and generation of magmas with varying sulfide contents clearly appear to have occurred in chambers at a depth of several kilometers rather than at the levels currently exposed in contrast to gravitational settling sulfide distribution within the voisey s bay deposit is controlled by magma emplacement through multiple braids of a dynamic channel like conduit dike system

the nature and genesis of marginal cu pge au sulphide mineralisation in paleogene macrodykes of the kangerlussuaq region east greenland the kangerlussuaq region of east greenland hosts a variety of early tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral iceland plume these intrusive bodies include a number of gabbroic macrodykes two of which the miki fjord macrodyke and the newly discovered togeda macrodyke contain cu pge au sulphide mineralisation along their margins sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some fe ti oxides and platinum group minerals comprising largely pd bismuthides and tellurides the globules are interpreted to have formed from fractionation of trapped droplets of an immiscible cu and pd rich sulphide melt and show geopetal indicators sulphur isotopes imply a local crustal source of s in these from pyritic sediments of the kangerlussuaq basin thus generation of these sulphide occurrences was controlled by local country rock type low ni cu and pt pd ratios also present in the platinova reefs in the skaergaard intrusion indicate that early fractionation of olivine may have depleted the magma of ni and suggest the likely presence of a large magma chamber at depth xenoliths of ni rich olivine cumulates in the miki fjord macrodyke may have been sourced from such a body the location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies

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

geologic and geochemical relationships between the contact sublayer inclusions and the main mass of the sudbury igneous complex a case study of the whistle mine embayment more than half of the giant ni cu platinum group element pge sulfide ores of the sudbury igneous complex are associated with a discontinuous unit at the base of the main mass known as the sublayer the sublayer is comprised of two fragment rich members 1 a metamorphic textured footwall breccia and 2 an igneous textured contact sublayer the contact sublayer occurs as a thick unit in depressions at the base of the sudbury igneous complex termed embayments it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase melanorite olivine melanorite metamorphosed melanorite wehrlite and dunite we present data for a case study of the whistle mine embayment and show that the igneous textured sublayer matrix is geochemically unlike the main mass norites quartz gabbros and granophyres for example the igneous textured sublayer matrix in the whistle mine has la sm 4 0 la nb 5 1 and th zr 0 02 and the main mass norites quartz gabbros and granophyres have ratios of 5 5 to 7 2 8 to 4 2 and 0 04 to 0 05 respectively this matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase the igneous textured sublayer matrix at the whistle mine can be modeled with small amounts of assimilation of local country rock granitoids approximately 10 large degrees of assimilation of diabase that are not derived from the immediate country rocks approximately 70 and small contributions from the main mass magma type approximately 20 mafic norite the amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma even with a superheated magma 70 percent assimilation is too large a model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments these fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer there are significant differences in the composition of the igneous textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments the melanorite inclusions in the sublayer at the whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous textured sublayer matrix but they have similar high incompatible element concentrations e g olivine melanorites have 20 65 ppm ce in rocks with 15 21 wt mgo 1 to 10 percent interstitial sulfide up to 0 5 percent apatite 1 to 15 percent biotite and 1 85 ga age zircon and baddeleyite the mafic ultramafic inclusions are interpreted to be the broken up remnants of an earlier cumulate formed at depth from a main mass magma this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase olivine compositional data for the mafic inclusions fo sub 71 79 500 3 800 ppm ni in the absence of reequilibration indicate that olivine crystallization both predated and postdated sulfur saturation of the magma cr rich spinels from these rocks confirm that the parental magma was especially cr rich and based on the olivine compositional data had a tholeiitic mg fe ratio

selective crustal contamination and decoupling of lithophile and chalcophile element isotopes in sulfide bearing mafic intrusions an example from the jingbulake intrusion xinjiang nw china abstract the jingbulake mafic ultramafic intrusion in the south tianshan orogenic belt xinjiang nw china is a zoned intrusive body composed of gabbro diorite olivine gabbro and wehrlite locally intruded by pyroxenite within a major w e structure both olivine gabbro and wehrlite contain disseminated ni cu sulfides whereas pyroxenite hosts an ore body containing massive net textured and disseminated sulfide ores both silicate rocks and sulfide ores from the jingbulake intrusion have low pd ir ratios 10 63 the silicate rocks contain ni poor olivine generally 187 os 188 os i ratios 0 277 to 0 916 of the silicate rocks are consistent with a major contribution of os from crustal materials on the other hand the net textured sulfide ores from no 8 mineralization zone have high os contents 15 1 to 32 6 ppb low re os 2 0 to 2 3 and low 187 os 188 os i ratios 0 125 to 0 141 indicating a mantle derived origin modeling indicates that 30 crustal contamination is required to explain the radiogenic os isotopic composition if average upper crust is adopted as the crustal contaminant or 5 25 if sulfide rich crustal rock is used as the crustal contaminant however only 5 10 crustal contamination is required to explain the nd and sr isotopes we propose that the decoupling of os from sr nd isotopes in the silicate rocks was due to selective crustal contamination addition of external sulfur from a crustal source may be the key factor triggering sulfide saturation the silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a crustal os isotope signature continued reaction of sulfide melts with new pulses of mantle derived magma increased the os content and decreased the 187 os 188 os i ratios effectively masking the crustal os contribution in the sulfides

geochemical and fluid dynamic modeling of compositional variations in archean komatiite hosted nickel sulfide ores in western australia stratigraphic volcanological petrological and fluid dynamic studies suggest that archean komatiite associated nickel sulfide deposits in western australia formed by thermal erosion of unconsolidated sulfidic sediments beneath channelized lava flows geochemical and nd isotope data for the type area at kambalda suggest that mineralized lava channels are less contaminated than barren flanking segments of the flow owing to high discharge rates and continuous replenishment within the channel this interpretation has several implications for the mode of emplacement of sulfides their degree of equilibration with the host komatiite and the compositions of sulfide ores 1 the solubility of sulfur in komatiite is relatively low so most of the sulfide in eroded sediments will be melted and remain as a dense layer at the base of the flow or be incorporated into the turbulently flowing komatiite as immiscible sulfide droplets 2 sulfide ores will not necessarily be in equilibrium with overlying komatiites but will record equilibration with variably contaminated lavas during initial stages of emplacement and crystallization and 3 ore compositions will vary from channel to channel shoot to shoot depending on i the composition of the hybridized komatiite ii the fluid dynamics of the system and the effective magma sulfide ratio r factor and iii the sulfide silicate partition coefficients under the prevailing conditions of temperature and f sub o 2 f sub s 2 the size of immiscible droplets and the interface between two immiscible layers are affected by inertial forces during turbulent flow turbulence causes coalescence of small droplets and breakup of large droplets leading to an equilibrium droplet size if the average size of the droplets is small and or the flowage rate is high they may be carried in suspension in the flowing komatiite droplets have a large surface area to volume ratio and will be effective in scavenging chalcophile elements since they are carried by the turbulent motion of the flowing komatiite if the average size of the droplets is large or if the flowage rate is low they will settle and collect at the base of the komatiite komatiite and sulfide liquids may continue to flow but the sulfide layer will have a low surface area to volume ratio and will be ineffective in extracting chalcophile elements from the overlying komatiite layer thus the initial stages of emplacement when sulfides are initially melted and are most likely to be in suspension if at all control the effective r factor and therefore the tenor of the sulfide ores numerical modeling of the observed variations in ore compositions constrained by known komatiite compositions and experimentally or empirically determined distribution constants indicates that the observed range of ore compositions in western australian komatiite associated nickel sulfide deposits can be accounted for primarily by variations in the effective sulfide silicate ratio 500 r 100 although required variations in the partition coefficient for ni 200 d ni 100 suggest that f sub o 2 f sub s 2 may also have varied systematically assimilation of h 2 o fes rich sediments should increase f sub o 2 f sub s 2 because of the relative abundances of the two components in the magma and unconsolidated sediments if low tenor ores represent equilibration at lower r and higher f sub o 2 f sub s 2 and high tenor ores represent equilibration at higher r and lower f sub o 2 f sub s 2 as suggested by the modeling then d ni appears to decrease with increasing f sub o 2 f sub s 2 as suggested by doyle and naldrett 1987 the range of calculated r values is relatively low suggesting that low tenor sulfides were not entrained into the komatiite lava but remained as a segregated layer at the base of the flow and that high tenor sulfides were not transported long distances in suspension but settled rapidly to the base of the flow magma compositions vary only slightly from an inferred parental komatiite low tenor ores appear to have equilibrated with only slightly more evolved contaminated and fractionated lavas

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

role of degassing of the noril sk nickel deposits in the permian triassic mass extinction event the largest mass extinction event in earth9s history marks the boundary between the permian and triassic periods at circa 252 ma and has been linked with the eruption of the basaltic siberian traps large igneous province slip one of the kill mechanisms that has been suggested is a biogenic methane burst triggered by the release of vast amounts of nickel into the atmosphere a proposed ni source lies within the huge noril sk nickel ore deposits which formed in magmatic conduits widely believed to have fed the eruption of the slip basalts however nickel is a nonvolatile element assumed to be largely sequestered at depth in dense sulfide liquids that formed the orebodies preventing its release into the atmosphere and oceans flotation of sulfide liquid droplets by surface attachment to gas bubbles has been suggested as a mechanism to overcome this problem and allow introduction of ni into the atmosphere during eruption of the slip lavas here we use 2d and 3d x ray imagery on noril sk nickel sulfide combined with simple thermodynamic models to show that the noril sk ores were degassing while they were forming consequent bubble riding by sulfide droplets followed by degassing of the shallow sulfide saturated and exceptionally volatile and cl rich slip lavas permitted a massive release of nickel rich volcanic gas and subsequent global dispersal of nickel released from this gas as aerosol particles

sulfur isotope exchange and metal enrichment in the formation of magmatic cu ni pge deposits sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic cu ni pge deposits although low sulfur pge rich deposits e g j m reef of the stillwater complex merensky reef of the bushveld complex show little or no isotopic evidence for the involvement of crustally derived sulfur several high sulfur deposits e g noril sk duluth complex show strong evidence for contamination of mafic magmas by sulfur of crustal origin processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas for this reason 34s values are primarily dependent on sulfur source characteristics and variations can often be treated using a two component mixing approach however deposits such as those at noril sk and voisey s bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures these deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle derived melts that traversed the accumulated sulfide on their way to the surface our calculations illustrate the efficiency of reactions that involve the exchange of fe for cu ni or pge in producing increases in the metal tenor of the accumulated sulfide sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur sulfur isotope exchange is distinct from the addition of sulfur in the form of immiscible sulfide droplets from a sulfide saturated magma to the accumulated sulfide although both additive mixing and exchange can lead to similar isotopic effects stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems

os nd o and s isotope constraints on country rock contamination in the conduit related eagle cu ni pge deposit midcontinent rift system upper michigan abstract the eagle cu ni pge deposit is hosted by mafic to ultramafic intrusive rocks associated with the marquette baraga dike swarm in northern michigan sulfide mineralization formed in a conduit system during early stages in the development of the 1 1 ga midcontinent rift system the conduit environment represents a prime location for melt rock interaction in order to better assess the extent of country rock contamination in the eagle system a combined trace element nd os o and s isotope study of country rocks sulfide bearing igneous rocks and massive sulfide was undertaken both the eagle and the weakly mineralized east eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development the trace element os nd and o isotopic values of the igneous rocks are consistent with os 1100 and e nd 1100 values for the magmas are 34 and 2 18 o values of pyroxene in feldspathic pyroxenite range from 6 5 to 6 6 and provide the only indication that bulk contamination may locally have exceeded 20 sulfur isotopic values of disseminated and massive sulfide in the eagle intrusion range from 0 3 to 4 6 the 34 s values are much lower than those that characterize most of the country rocks but could still be indicative of a contribution of s from country rocks of up to 50 33 s values of the disseminated and massive sulfides range from 0 10 to 0 09 indicating a source in paleoproterozoic country rocks semi massive sulfide in the eagle deposit has 34 s values between 2 2 and 5 3 and 33 s values show a broad range between 0 86 and 0 86 indicating a major contribution from an archean source isotopic data from the eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the ni cu pge mineralization and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the eagle deposit our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end member values for mixing calculations sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios in addition sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and 34 s values near those of mantle sulfur

description of a new enantiornithine bird from the early cretaceous of hebei northern china this paper describes a new enantiornithine fossil bird vescornis hebeiensis nov sp from the early cretaceous of china we refer vescornis to the crown clade euenantiornithes based on several characteristics observed in the thoracic girdle and wing vescornis also exhibits characteristics that separate it from other enantiornithine birds such as the short alular phalanx the vestigial manual claws and the well developed and long foot claws these features suggest an adaptation towards an improved flight capability while the ability of vescornis to climb is reduced compared with many other enantiornithine birds

dynamic fluid connectivity during steady state multiphase flow in a sandstone abstract the current conceptual picture of steady state multiphase darcy flow in porous media is that the fluid phases organize into separate flow pathways with stable interfaces here we demonstrate a previously unobserved type of steady state flow behavior which we term dynamic connectivity using fast pore scale x ray imaging we image the flow of n2 and brine through a permeable sandstone at subsurface reservoir conditions and low capillary numbers and at constant fluid saturation at any instant the network of pores filled with the nonwetting phase is not necessarily connected flow occurs along pathways that periodically reconnect like cars controlled by traffic lights this behavior is consistent with an energy balance where some of the energy of the injected fluids is sporadically converted to create new interfaces

paragneiss assimilation in the genesis of magmatic ni cu co sulfide mineralization at voisey s bay labrador δ34s δ13c and se s evidence the voisey s bay ni cu co deposit is located in ca 1 33 ga troctolitic rocks of the nain plutonic suite northern labrador it is widely regarded as a prime example of sulfide mineralization associated with a magmatic conduit system previous geological and geochemical studies have indicated that the voisey s bay intrusion was contaminated by interaction with both sulfide poor archean gneisses and with paleoproterozoic sulfide bearing tasiuyak metasedimentary gneiss limited sulfur isotope data previously indicated that only one of the four ore zones at voisey s bay is characterized by 34s values outside the range considered normal for uncontaminated mantle derived mafic magmas 0 2 new s isotope data presented here extends the previously reported range in 34s values for the tasiuyak gneiss from 17 0 to 18 3 per mil with an average of 1 4 per mil assimilation of this sulfur followed by isotope homogenization in the magmatic system is consistent with the isotope data from the ore zones and in line with the wealth of evidence that suggests that interaction with tasiuyak gneiss was an essential ingredient for ore formation in all the zones se s values of sulfide minerals in the ore zones at voisey s bay are also in the same range as those of whole rock samples from the tasiuyak gneiss 13c values of both mineralized and nonmineralized parts of the voisey s bay intrusion are anomalous 18 3 to 24 8 and similar to values of graphite in the tasiuyak gneiss both s and c contamination of the voisey s bay intrusion via diffusive transfer in a fluid phase derived from inclusions and wall rocks are considered unlikely because the area had been subjected to granulite grade metamorphism prior to emplacement of the nain plutonic suite either fractional melting or total fusion of xenoliths is considered a more likely mechanism for s addition to the mafic magma via pyrrhotite melting and for c addition in the form of co2 and ch4 produced by reaction of graphite with o and h bearing melt species

evidence for sulfide and fe ti p rich liquid immiscibility in the duluth complex minnesota apatite and oxide ilmenite magnetite rich dunite pyroxenite peridotite melatroctolite and gabbro occur as transgressive bodies in the boulder lake area of the southern duluth complex nelsonite apatite ilmenite is locally found as vein or dikelike bodies whose geometric form resembles veinlike massive cu ni sulfide mineralization in the babbitt deposit massive sulfide mineralization at babbitt has been attributed to the separation of an immiscible sulfide magma from a tholeiitic parent nelsonite contains assessory minerals such as zn rich hercynitic spinel biotite zircon baddeleyite and sulfide minerals massive sulfide mineralization at babbitt is characterized by the presence of minor 1 10 vol amounts of euhedral apatite as well as fe ti oxides and the same assemblages of accessory minerals that is found in the nelsonite textures developed in the nelsonite together with the unusual composition are supportive of an origin involving fe ti p and s liquid immiscibility apatite from the nelsonites and related rocks contain very low ree and cl contents relative to that in the massive sulfide these data are thought to reflect the evolution of a cl and ree bearing fluid from the magma that gave rise to the nelsonite and a later separation of an fe ti p rich immiscible liquid in comparison to the early separation of an immiscible sulfide rich liquid from a basaltic magma in the babbitt area sulfur oxygen and hydrogen isotope data are not supportive of an origin for the apatite oxide rich rocks that is linked to assimilation of metasedimentary country rock however elevated delta 18 o values up to 13 2 in plagioclase in many of the apatite oxide rich rocks together with anomalously high an content of plagioclase suggest that subsolidus interaction with a ca rich fluid derived from dehydration reactions in the pelitic rocks of the virginia formation has been important although an origin involving immiscibility of an fe ti p rich liquid is strongly indicated for the nelsonites the origin of spatially associated apatite oxide rich rock types is problematic the sio 2 content of most of the crosscutting bodies is less than 30 wt percent and derivation by extreme closed system fractional crystallization of a tholeiitic parent is considered unlikely due to predicted early saturation in fe ti oxides the oxideapatite rich rocks may themselves be products of liquid immiscibility reflecting a continuum in compositions with nelsonite as an end member

district to camp controls on the genesis of komatiite hosted nickel sulfide deposits agnew wiluna greenstone belt western australia insights from the multiple sulfur isotopes the agnew wiluna greenstone belt of western australia is the largest komatiite hosted nickel sulfide belt in the world and contains two world class ni cu pge deposits and a host of smaller deposits this study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite hosted ni cu pge deposits with specific focus on camp to district controls we apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems but above all that the sulfur required to generate world class deposits is most likely derived from barren volcanic massive sulfide lenses which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows multiple sulfur isotope data can be utilized in exploration at the deposit to district scales at the deposit scale the spatial pattern of mass independent s isotope values 33 s provides crucial insight into the identification of proximal high grade and high tenor ores in mineralized systems in fact sulfur data reflect the assimilation process that occurred upon komatiite emplacement whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero 33 s values whereas less turbulent flows interacted with distal sulfidic shales having 33 s values above 0 per mil accordingly the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high grade ores of poorly known systems at the district scale rather than ascertaining what controls the distribution of komatiite hosted ni cu pge deposits the appropriate question to ask is what controls the distribution of country rock sulfides considering that exhalative sulfides may be crucial to ore genesis in komatiite systems we propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data this work provides the first step in identifying district scale control on komatiite hosted ni cu pge deposits this is the scale that has high impact on exploration for new komatiite hosted nickel sulfide belts globally

the nature of sulfidic metasediments at kambalda and their broad relationships with associated ultramafic rocks and nickel ores sedimentary horizons at kambalda occur at the base of and at interflow boundaries within the same lower one third of the ultramafic sequence as do the major ni cu orebodies although sediments occur on all the same stratigraphic horizons as the ores they are almost entirely absent from within the volume containing the ores being separated by zones free of both sediments and ni cu sulfides generally they are broadly lenticular are not very extensive in area and correlate poorly laterally mineralogically and chemically the sediments are very variable but much of this is attributable simply to a variable iron sulfide content geographic and stratigraphic location and lithology are other significant causes of these variations proximity to the ni cu ores has at best an exceedingly subtle control on chemistry or mineralogy the kambalda sediments are not compositionally very different from other archean sediments in broadly similar environments however their sulfur and chalcophile element levels are high and au is especially enriched sulfur isotopes have a narrow spread near zero range 1 5 5 and the s se is around 15 000 to 20 000 indicating a little modified magmatic source for the sulfur apart from transfer of sulfur due to pyrite breakdown at peak metamorphism to the ultramafics there was negligible chemical exchange across sediment ultramafic interfaces virtually no ni was extracted from the ultramafic silicates during this sulfurization and migration of ni into the sediments was negligible nickel sulfide ore formation by sulfurization or sedimentary replacement is rejected ores intimately involved with sediments make up only 3 to 5 percent of the total ni reserve there is a strong inverse relationship between the quality of ore environments and the amount of their ore which is in contact with sediments the four largest ore locations accounting for about 70 of the reserve each has considerably less than 1 percent of ore with sediments extensive ore sediment contact occurs locally and can result from either 1 physical injection of preexisting ni cu ore into previously barren sediments 2 a synsedimentary intermixing of ores and sediments or 3 a passive and largely coincidental relationship the volcanic exhalative model of lusk 1976 for the ni ores is rejected the sediments are interpreted to be a mixture of local mafic ultramafic and distant felsic granitic detritus s pb zn fe and cu from ultramafic exhalations s co ni and cu from local sea floor leaching of ultramafics a biological carbonaceous component and probably minor chemical silica sea floor alteration affecting na k and mn diagenesis and metasomatic k rb ba sr and co 2 introductions are evident

geology genesis and metamorphic history of the namew lake ni cu deposit manitoba the namew lake fe ni cu sulfide deposit 60 km south of flin flon manitoba is hosted by an undifferentiated metapyroxenite sill enclosed within early proterozoic orthogneisses and covered by paleozoic sedimentary rocks the namew lake pyroxenite contains a relict igneous mineral assemblage of olivine orthopyroxene clinopyroxene or spinel or sulfides suggesting derivation from a low mg komatiitic or high mg basaltic magma sulfide ore in the namew lake pyroxenite occurs as 3 to 10 mm blebs of disseminated fe ni cu sulfides that accumulated during crystallization of olivine and pyroxene a 0 5 to 5 m thick layer of massive sulfide at the hanging wall contact may represent a gravitationally segregated basal layer of a sill and suggests that the mine stratigraphy is inverted although the massive sulfides were compositionally modified during metamorphism and supergene alteration the disseminated sulfides exhibit coherent geochemical trends and may preserve the average composition of the sulfide melt recalculated to 100 sulfide ca 36 s 42 fe 16 ni 6 1 cu 0 23 zn 148 ppm pb 5 300 ppb pd and 88 ppb ir assuming a silicate sulfide mass ratio of ca 560 calculated from the platinum group element pge contents the ni content of a parental magma that would be in equilibrium with sulfides of that composition is similar to that of typical komatiites but the cu zn and pb concentrations are much higher thus a komatiitic magma with ordinary ni and pge contents may have partially assimilated volcanogenic cu zn pb sulfides resulting in a high mg basaltic magma from which the silicates and sulfides subsequently crystallized and equilibrated the namew lake pyroxenite and associated sulfides were affected by five phases of deformation and three phases of metamorphism the first deformational event in the area d 1 involved migmatization and isoclinal folding at upper amphibolite facies conditions in addition sheeted tonalites and diorites intruded prior to or during d 1 d 2 folded the gneisses into domes synforms and antiforms d 3 involved minor boudinage of the pyroxenite intrusion of dikes and displacement of the sulfides updip d 4 shearing locally increased the thickness of the pyroxenite d 5 and subsequent deformations produced brittle faulting with minor displacement open folds of the ore horizon and large scale domain bounding faults the namew lake pyroxenite was metamorphosed at upper amphibolite facies m 1 at lower amphibolite facies m 3 and at greenschist facies m 5 m 1 metamorphism of the pyroxenite produced partial replacement of the host pyroxenite by pargasitic amphibole m 3 produced successive partial replacement by talc dolomite tremolite and biotite with greatest alteration at the edges of the pyroxenite and near crosscutting dikes m 5 produced minor serpentine and cummingtonite most or all elements including rare earth elements ree were mobile to varying degrees during m 1 m 3 and m 5 resulting in the observed compositional variability of the namew lake pyroxenite

platinum group elements and gold in the komatiite hosted fe ni cu sulfide deposits at kambalda western australia the concentrations of all platinum group elements and gold in monthly composited samples of mined ores have been determined by neutron activation analysis average values from a typical medium ni tenor ore shoot 10 ni in 100 sulfides are os 480 ir 230 ru 980 rh 320 pt 1 650 pd 2 050 au 500 all values are in ppb and are recalculated to 100 sulfides these values are similar to those reported from other komatiite hosted ores os ir ru rh and pd correlate strongly with ni r 0 63 to 0 92 whereas pt r 0 50 correlation with ni is only moderate au does not correlate with ni r 0 28 geochemical profiles of platinum group element values recalculated to 100 percent sulfides through typical ore zones show that pt pd and au are lower in massive ore relative to overlying matrix and disseminated ores whereas other platinum group elements have a more uniform distribution through these ore types pt pd and au are strongly concentrated at the footwall metabasalt massive ore contact and in small sulfide filled fractures within the footwall metabasalt other platinum group elements are concentrated to a lesser extent at this position some magnetite rich selvedges immediately above massive ores are depleted in os ir and ru pt pd and au largely occur in discrete mineral phases hudson and donaldson 1984 and os ir ru and rh are interpreted to occur in solid solution in pentlandite the present concentration of pt pd and au at the base of massive ores in late fractures is attributed to metamorphic remobilization although this does not preclude an original magmatic concentration of platinum group elements at the base of the massive ore the variation in the platinum group element content of bulk ore samples is similar to that of ni cu and co and is controlled by a variation in fe content at constant sulfur levels

evolution of the sublayer of the sudbury igneous complex geochemical sm nd isotopic and petrologic evidence abstract the mineralized sublayer at the base of the sudbury igneous complex sic consists of two variants the noritic contact sublayer and radial and concentric quartz dioritic offset dykes both are characterized by the presence of significant quantities of ni cu pge sulphides and by a prominent population of recrystallized diabasic textured and melanocratic to ultramafic fragments the two variants of the sublayer contain compositionally distinct inclusion populations and inclusion bearing matrices contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments the compositional variation in sic rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt s the basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase and is geochemically and isotopically identified with huronian basalts which comprise the south range footwall with ce yb n ratios of around 2 5 and e nd 1850 between 2 to 5 the melanocratic inclusions in the sublayer are typically coarse grained and undeformed with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet which has ce yb n ratios of around 10 and e nd 1850 around 9 calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet it is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact this basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling offset dykes were emplaced and the magmas effectively removed from the system subsequently continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer no extracrustal e g mantle component is envisioned in this model to explain the silicate compositional distributions in the sic and mafic crustal rocks in the target zone are implicated as the metal source for the sic deposits

analytical numerical modeling of komatiite lava emplacement and thermal erosion at perseverance western australia abstract we have applied a thermal fluid dynamic geochemical model to investigate the emplacement and erosional potential of archean komatiite flows at perseverance western australia perseverance has been proposed as a site of large scale thermal erosion by large volume komatiite eruption s resulting in a 100 150 m deep lava channel containing one of the world s largest komatiite hosted fe ni cu pge sulfide deposits using constraints based on field theoretical and geochemical data we have modeled the emplacement of a range of flow thicknesses over felsic tuffaceous substrates with various degrees of consolidation and water contents thermo mechanical erosion becomes more effective for substrates that are increasingly unconsolidated and water rich for thermo mechanical erosion to be responsible for the formation of the 100 m deep concave perseverance embayment and the highly contaminated 10 20 perseverance komatiites the most likely scenarios require emplacement of thick e g 10 30 m turbulent channelized liquidus or superheated komatiite lavas over a welded or unconsolidated submarine tuff flow distances must have been long tens to hundreds of kilometers and flow volumes must have been very high hundreds to thousands of km 3 lava channels and tubes 110 km long have not been observed on earth but are consistent with those formed by low viscosity lavas on venus the moon mars and io flow volumes are consistent with those in continental flood basalt eruptions and oceanic plateau large igneous provinces and may represent the initial outpourings of komatiite lavas from archean mantle plume activity

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

the role of paragneiss assimilation in the origin of the voisey s bay ni cu sulfide deposit labrador multiple s and fe isotope evidence isotopic and geochemical studies conducted on the voisey s bay deposit labrador canada suggest crustal contamination of the primary magma as a trigger for sulfur saturation and formation of the deposit the use of multiple s isotopes has allowed for the identification of a bacterial sulfate reduction biosignature in the tasiuyak gneiss in the footwall to the voisey s bay deposit this putative biosignature is preserved in the deposit even at high silicate magma sulfide melt ratios r factor and links the s present in the voisey s bay deposit to the tasiuyak gneiss iron isotopes in the voisey s bay deposit have been reset to magmatic values at r factors 100 but s isotope data can be used to model higher r factors a contamination model results in calculated r factors of 433 177 the multiple s isotope data are a new proxy to directly link s from the deposit to crustal s sources even in deposits with high r factors where the equilibration with large amounts of silicate magma can make interpreting a link between the deposit and the sulfur source difficult

siderophile and chalcophile metals as tracers of the evolution of the siberian trap in the noril sk region russia abstract in this study cu ni and platinum group elements pge were determined in a sequence of basaltic and picritic lavas from the siberian trap in the noril sk area of russia to constrain genetic relationships between the basalts and the petrogenesis of ni cu pge sulfide deposits associated with the talnakh and noril sk intrusions in the most primitive basalts 8 19 wt mgo of the tuklonsky tk suite pt and pd concentrations range from 4 13 ppb increasing with decreasing mgo content whereas ir contents decrease with mgo from 0 8 0 05 ppb the contrasting behavior of these elements which all have very high sulfide silicate partition coefficients as well as the primitive mantle like ratios of cu y and pd y suggests that these magmas were not sulfide saturated the high pge abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle due to sulfide segregation the overlying basalts of the nadezhdinsky nd series are low in cu and ni 52 and 38 ppm respectively highly depleted in all pge and have very low cu y pd y and pd cu ratios however in stratigraphically higher levels cu ni and pge concentrations increase systematically through the morongovsky mr suite to reach a concentration plateau in the uppermost mokulaevsky mk suite pt 8 ppb pd 9 ppb ir 0 12 ppb rh 0 4 ppb at the same time ratios such as cu y increase and approach primitive mantle values however ratios involving pge such as pd y remain low suggesting the removal of small amounts of sulfide 0 01 0 03 the compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open or closed system magma chamber the latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma with crustal components contaminated overlies less contaminated denser magma crustal contamination caused sulfide saturation and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber in this model the sulfide liquid is not in equilibrium with the whole magma mass and sulfide segregation is compared with the zone refining process of metallurgy the sulfides become more enriched as they move through the magma and although the magma left behind is depleted in pge cu and ni their concentrations also increase with depth eventually the magma chamber is emptied from the top to the bottom producing the flood basalt sequence and the associated intrusions and ore deposits in the open system model sulfide saturation was initially caused by assimilation of crustal material by the tuklonsky magma continuous and simultaneous replenishment assimilation and crystallization processes formed the lower nd lavas the concurrent removal of 0 5 1 sulfide strongly depleted these magmas in chalcophile and siderophile metals due to the continuous replenishment of the magma chamber with uncontaminated pge rich magma succeeding lavas mr mk show diminishing signs of crustal contamination and become less sulfide saturated as indicated by the increasing ni cu and pge abundances during the evolution of the chamber the magma remained sulfur saturated and sulfides accumulated at the base the composition of the sulfide ores could be regarded as a mixture consisting of low ni cu and pge sulfides derived with a low silicate sulfide ratio 100 from the tk nd magma and high ni cu and pge sulfides formed with a high ratio 10 000 from the mr mk magma

melting and dissolution kinetics application to partial melting and dissolution of xenoliths kinetics of melting and dissolution rates and textures in magmatic systems is reviewed and applied to partial melting and dissolution of xenoliths special attention is paid to melting dissolution of solid solution crystals in which characteristic cloudy textures are formed and diffusion of elements in the crystals plays an important role due to compositional change of the crystals based on the above discussion model calculations are made for xenolith digestion using the haplogabbro system diopside plagioclase xenolith digestion can proceed by partial melting of the xenolith itself as well as dissolution in this case xenolith size is not important the digestion rates suggest that many xenoliths could survive only if they were incorporated into a host basaltic magma just prior to eruption natural partial melting found in a spinel lherzolite xenolith from kilbourne hole new mexico is also examined with additional heating experiments it is concluded that spongy clinopyroxene in the spinel lherzolite was formed by partial dissolution reaction between clinopyroxene and partial melt during partial melting at depth for a short run duration a month or less both the model calculations and the heating experiments show that even mafic to ultramafic xenoliths can easily undergo partial melting or react with basaltic melts and many xenoliths found in basaltic rocks could be incorporated into the basalts just prior to eruptions

sulfide paragenesis and sulfur mobility in fe ni cu sulfide ores at lunnon and juan main shoots kambalda textural and sulfur isotopic evidence sulfur isotope data from a range of ore types at lunnon and juan main shoots kambalda have been determined in an attempt to constrain models for the crystallization of the sulfide ores the place of pyrite in the sequence of ore formation and modification is evaluated from data representing the widest possible range of pyrite types showing variable textural relations to coexisting pyrrhotite the sulfur isotopic distribution 68 mineral analyses yields a mean value of 2 4 per mil range 0 5 to 3 8 standard deviation or 0 8 the distribution is characterized by a small spread in values and isotopic disequilibria between coexisting minerals there is no statistical difference between pyrrhotites in massive ores and matrix ores at lunnon shoot and no systematic variation in isotopic composition of pyrrhotite and sparse pyrite in a single ore section from the same deposit these data are consistent with previous suggestions that massive and matrix ores crystallized independently at the magmatic stage variable but overall less positive delta 34 s values for sulfides from disseminated ores from lunnon shoot are consistent with control of isotopic compositions by fe related redox mechanisms during alteration of the ultramafic hosts the delta 34 s values of pyrite and pyrrhotite from massive ores at lunnon and juan main shoots are indistinguishable and the origin of coarse grained pyrite remains equivocal despite the isotopic data the data are however consistent with derivation of fine grained pyrite by late stage relatively low temperature sulfurization processes as suggested by textural studies the greater permeability of the massive ore relative to more disseminated ores and barren ultramafics due to thermal contraction following magmatic and or metamorphic cooling is considered the controlling factor the secondary nature of a significant proportion of the pyrite in massive ores severely limits attempts to use present bulk compositions to predict magmatic crystallization sequences within ore and the use of sulfur metal ratios to estimate parameters such as f sub o 2 at the magmatic stage

anatomy of the primitive bird sapeornis chaoyangensis from the early cretaceous of liaoning china two new nearly completely articulated skeletons of sapeornis chaoyangensis provide much new information about the anatomy of this basal avian particularly in the skull pectoral girdle forelimb and hind limb this new material shows that the hand of sapeornis with a phalangeal formula of 2 3 2 was more derived than previously reconstructed the skeleton of sapeornis has several unique features such as a distinctively elongated fenestra on the proximal end of the humerus a robust furcula with a distinctive hypocleidum and an elongated forelimb sapeornis exhibits a combination of derived and primitive features including a short robust non strut like coracoid and a fibula reaching the distal end of the tarsal joint as in archaeopteryx a pygostyle reduced manual digits and a well fused carpometacarpus as in more advanced birds these features further indicate the mosaic pattern in the early evolution of birds and confirm the basal position of sapeornis near archaeopteryx and jeholornic in

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa the platreef the putative local analogue of the merensky reef forms the floor to the mafic succession in the northern limb of the bushveld complex we define the platreef as the lithologically variable unit dominated by pyroxenite which is irregularly mineralised with pge cu and ni between the transvaal metasedimentary footwall or archaean basement and the overlying main zone gabbronorite we define the mineralisation around calcsilicate xenoliths within the main zone in the far north of the limb as a platreef style mineralisation the platreef ss has a strike extent of 30 km whereas platreef style mineralisation occurs over a strike length of 110 km the platreef varies from 400 m thick in the s to 30 modal in some intersections these are dominated by pyrrhotite with lesser pentlandite and chalcopyrite minor pyrite and traces of a wide compositional range of sulphides in the southern sector mineralised zones have cu grades of 0 1 0 25 and ni 0 15 0 36 massive sulphides are localised commonly but not exclusively towards the contact with footwall metasedimentary rocks magmatic sulphides are disseminated or net textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite much of the sulphide is associated with intergranular plagioclase or quartz feldspar symplectites along the margins of rounded cumulus orthopyroxenes the pges in the southern sector occur as tellurides bismuthides arsenides antimonides bismuthoantimonides and complex bismuthotellurides pgm are rarely included in the sulphides but occur as micron sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones the pt pd ratio 1 and pge grade may be decoupled from s and base metal abundance

a textural mineralogical and statistical study of the footwall breccia within the strathcona embayment of the sudbury structure the footwall breccia is one of the main ni cu pge ore hosting units of the sudbury camp this polymict matrix supported contact metamorphosed breccia occurs between the sudbury igneous complex and the footwall rocks along the west north and east margins of the sudbury structure in areas where there are depressions or embayments in the footwall the footwall breccia and basal units of the sudbury igneous complex are thickened and the footwall breccia is variably mineralized mineralogical and textural variations of footwall breccia samples from the strathcona embayment in proximity to the fraser mine suggest that the mineralogy and texture of this unit vary as a function of proximity to the sudbury igneous complex proximity to ore and location within the embayment the most mafic footwall breccia mineralogy is present proximal to the base of the sudbury igneous complex within a transition zone the presence of relatively mafic hydrous and cl rich silicate minerals e g biotite and amphibole characterize the footwall breccia matrix around ore zones and form an alteration halo of up to 5 m thick textural variations include a coarsening of quartz and feldspar grains with increasing proximity to the sudbury igneous complex contact textural coarsening also occurs from the margins of the strathcona embayment to the center together these mineralogical and textural variations influence the color of the footwall breccia matrix and can explain some of the gray colors that are typical of mineralized footwall breccia these variations also constrain the timing of formation of this unit and suggest that at least three phases of footwall breccia exist

wall rock structural controls on the genesis of the voisey s bay intrusion and its ni cu co magmatic sulfide mineralization labrador canada the voisey s bay intrusion of the mesoproterozoic nain plutonic suite hosts a world class magmatic ni cu co sulfide deposit here we document the brittle and ductile structure of gneisses surrounding the intrusion using field data borehole intersections and subsurface structural data obtained from acoustic and optical televiewer surveys we elucidate the role that wall rock structure played on controlling the geometry of the intrusion and the formation of structural traps that host mineralization the voisey s bay intrusion consists at first order of two km scale magma chambers interpreted to represent an upper and lower chamber of the intrusion that are connected by a system of dikes three types of gneiss associated with the paleoproterozoic torngat orogen form the wall rocks of the intrusion archean orthogneiss paleoproterozoic tasiuyak paragneiss and paleoproterozoic enderbitic orthogneiss three phases of ductile deformation d1 d3 predating intrusion emplacement are recorded in the wall rock gneisses d1 produced the dominant s1 fabric subparallel to original bedding s in the tasiuyak gneiss d2 shortening caused shallowly plunging upright to moderately inclined f2 folds in the enderbitic gneiss the prototype of which was likely emplaced toward the end of d1 d3 ductile deformation resulted in steeply dipping shear zones and flat lying to shallowly dipping c3 shear planes at least two later phases of brittle deformation d4 and d5 are present d4 is a preemplacement event characterized by fractures with orientations similar to d3 shear zones and shear planes d5 faults and joints formed during syn to postemplacement brittle deformation likely associated with e w brittle sinistral transtension associated with the mesoproterozoic gardar voisey s bay fault zone the geometry of the voisey s bay intrusion and its mineralization was controlled by pre to synemplacement brittle structures and preemplacement ductile structures which acted as passive wall rock anisotropies mineralization hosted within tasiuyak gneiss is controlled by the intersection of a dike with s1 s d3 shear zones and d4 fractures where the intrusion occurs within enderbitic gneiss mineralization locally coincides with shallow fabrics associated with f2 fold closures the margins of the voisey s bay dikes are controlled by steeply and shallowly dipping d4 fractures changes in dike thickness which control the locations of mineralization can be explained by thermomechanical erosion of the walls of the dike which occurs by thermal expansion and contraction of wall rock due to repeated magma pulses and the mechanical spalling of this wall rock along anisotropies the strong brittle structural control on emplacement along with previous geothermobarometry is consistent with a mid crustal 9 11 km depth of emplacement lithosphere scale e w trending d5 faults may have acted as conduits for the crustal scale transport of primitive voisey s bay magmas from their source rather than an older orogenic suture zone

thermomechanical erosion of ore hosting embayments beneath komatiite lava channels textural evidence from kambalda western australia abstract archean komatiite hosted nickel deposits at kambalda western australia are located in embayments within channelized lava flows the origin of these embayments is controversial the currently favoured model invokes pre existing topographic features filled in by the komatiite and sulfide melt and subsequently modified by thermomechanical erosion new evidence from sulfide deposits on the eastern limb of the kambalda dome includes well preserved magmatic textural relationships of the sulfides with the surrounding rocks suggesting the entire embayment was formed by thermomechanical erosion these textures are particularly well preserved within the moran deposit of the long victor complex which lacks the younger intense structural deformation that other kambalda deposits have undergone the vestiges of an older broad concave embayment are represented by sediment sulfide textures present on the flanks up to 150 m away from the younger steep sided embayment sediment sulfide contacts are marked here by a globular silicate sulfide intergrowth interpreted as emulsion and cumulate like mono sulfide solid solution mss grains surrounded by interstitial silicate melt textures preserved from the formation of a steep sided embayment are frozen into the basalt sulfide contact and include an undulating contact with sulfide filled microfractures and a ferrichromite layer basalt sulfide breccia and interpreted crystallized silicate sulfide liquid emulsions and basalt plumes rising into the sulfides textures from the last step of embayment formation are found in the pinchouts i e where the massive sulfide ore is bounded top and bottom by basalt they include the same textures that were formed during the excavation of the steep sided embayment but they also feature interpreted basalt sulfide emulsions on the upper contact cm scale silicate sulfide ferrichromite layering on the upper contact and a vesicular basalt scum layer floating on the sulfides beneath the solid older basalt all these textures represent frozen examples of different stages during the excavation and formation of the embayment and reflect physical processes responsible for thermomechanical erosion the initial disposition and orientation of ore hosting channels was controlled by paleo topography probably defined by syn volcanic faults but the textures we describe show that within the channel the geometry of the entire embayment was the product of erosion the sulfide melt itself played a major role in the process owing to its high density high heat content and very low viscosity but the hydrated nature of the underlying altered basalt was also critical fluids were released from the basalt due to the high temperature in the 1 2 cm immediately beneath the sulfide melt with instant flash boiling creating a network of microfractures pre existing fracture networks and breccia in the basalt allowed the sulfide melt to locally infiltrate the substrate to a greater depth increasing the hydraulic head at the tip of the fracture thus accelerating the erosion process once a critical sulfide melt thickness is reached partly aided by a growing pile of matrix net textured sulfides the pressure difference on the embayment floor compared to the basalt at the bottom edges of the embayment becomes high enough to allow the liquid sulfides to melt sideways to form a pinchout

the physical volcanology of archaean komatiite sequences from forrestania southern cross province western australia abstract the archaean supracrustal sequence at forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km lithologies range from olivine cumulates which crystallised at the interface between substrate and flowing lava to spinifex textured rocks which underwent rapid crystallisation within ponded lava lobes characteristic associations of different rock types are interpreted in terms of variations in eruption rate which presumably reflect proximity to the vent or to a major lava river thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment where lava flow was continuous and eruptive velocities were high sheeted bodies of olivine cumulate formed from voluminous turbulent flow of lava over relatively refractory substrates such as basalt and oxide facies banded iron formation ribbon shaped olivine cumulate bodies formed in an equally dynamic environment within major lava channels where the substrate was less refractory and consisted of quartz rich sedimentary rocks which were susceptible to thermal erosion distributary lava channels in more distal settings are represented by thinner ribbon like accumulations of olivine mesocumulate to orthocumulate unusual poikilitic pyroxene bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks pyroxene and plagioclase bearing cumulates are present at the top of other channel sequences and reflect late stage ponding and in situ differentiation of komatiitic lava within the channel the channel facies rocks are flanked by sequences comprising multiple thin spinifex textured flow units which formed from the episodic overflow of lava from the distributary channels similar rocks also occur stratigraphically above many of the thick olivine cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle

the role of black shales as a source of sulfur and semimetals in magmatic nickel copper deposits example from the partridge river intrusion duluth complex minnesota usa the basal unit of the duluth complex minnesota usa contains ni cu sulfide deposits the s in these is thought to be derived from a sulfide rich black shale unit known as the bedded pyrrhotite unit a stratigraphic unit within the virginia formation host rocks however the mechanism of s transfer has not been clearly established in order to understand how this transfer occurs we have undertaken petrography and whole rock geochemistry of the rocks from the contact aureole and the basal unit in the contact aureole the bedded pyrrhotite unit consists of a very fine grained graphitic shales with thin beds of sulfides consisting of pyrrhotite with minor chalcopyrite 1 the basal unit contains numerous bedded pyrrhotite unit xenoliths surrounded by norites the bedded pyrrhotite unit xenoliths are partially melted and the sulfide beds are disrupted leucosomes are present and these contain blebs of sulfides consisting of pyrrhotite pentlandite cubanite and chalcopyrite in the mafic rocks surrounding the xenoliths small patches of sulfide bearing leucosome are found in addition to being rich in s the bedded pyrrhotite unit is rich in as 38 ppm sb 4 1 ppm and bi 0 6 ppm and te 0 4 ppm and has high 34s values the 34s as s bi s and sb s decrease with distance from the xenoliths similarly the ni s cu s se s and platinum group elements s ratios are higher in the mafic rocks and increase with distance from the xenoliths our model proposes that droplets of sulfide melt derived from the bedded pyrrhotite unit xenoliths were entrained in the anatectic silicate melt of the xenoliths and transferred to the mafic magma the sulfide droplets equilibrated with the mafic magma those close to the xenoliths did not have the opportunity to react with a large quantity of magma and hence their composition is similar to the sulfides of the bedded pyrrhotite unit i e rich in semimetals and poor in ni cu and pge farther away from the xenoliths the sulfide droplets could have reacted with more magma and the composition of these sulfides approach that of sulfides derived mainly from mafic magma

sulfur and oxygen isotopic evidence of country rock contamination in the voisey s bay ni cu co deposit labrador canada abstract the emplacement of basaltic magma into sulfide bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation induced changes in magma composition pelitic country rocks of the proterozoic tasiuyak gneiss in the area of the voisey s bay ni cu co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the voisey s bay intrusion sulfur isotopic compositions of the tasiuyak gneiss range from 0 9 to 17 0 values typical for sulfides produced via bacterial sulfate reduction in an open marine environment archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally derived sulfur sulfur isotopic compositions of the sulfide minerals from the voisey s bay deposit show consistent variations both spatially and with rock types disseminated and massive sulfides show a decrease in 34 s to the west with values typically between 0 and 2 in the eastern deeps ovoid and discovery hill zone and between 2 and 4 in the reid brook zone 34 s values of the mushua intrusion to the north and the normal troctolite in the eastern deeps are more positive ranging between 0 5 and 1 8 this range is taken to represent the isotopic composition of primary mantle derived sulfur in the area because the mushua intrusion and normal troctolite show the least geochemical evidence for contamination by country rocks sulfur isotopic data from the reid brook zone are consistent with up to a 50 sulfur contribution from the tasiuyak gneiss correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low sulfide enderbitic and quartzofeldspathic gneisses oxygen isotopic values of gneiss fragments in the basal breccia sequence and feeder breccia suggest that the assimilation process involved a greater proportion of high 18 o contaminant to the west 18 o values of the tasiuyak gneiss 5 9 to 14 0 enderbitic gneiss 6 4 to 8 7 and archean quartzofeldspathic gneiss 9 5 to 9 7 are consistent with an increased proportion of tasiuyak gneiss contaminant to the west isotopic data strongly indicate that sulfur from the tasiuyak gneiss has been involved in ore deposition at the voisey s bay deposit however sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization and that assimilation of both proterozoic and archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic physical mechanisms

geochemistry and mineralogy of platinum group element mineralization in the river valley intrusion ontario canada a model for early stage sulfur saturation and multistage emplacement and the implications for contact type ni cu pge sulfide mineralization the river valley intrusion within the 2 48 ga east bull lake intrusive suite in ontario canada is an example of a mafic igneous intrusion with contact type ni cu pge sulfide mineralization along its base whereas many contact type deposits are thought to form from in situ contamination of the magma by the addition of crustal s during emplacement there are some intrusions including the river valley intrusion which appear to have a much more complex history where the timing of s saturation and thus the critical ore genesis processes may have occurred much earlier prior to emplacement the river valley intrusion is made up of a basal 100 m of unlayered inclusion bearing units overlain by layered cumulates the basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites all within a gabbroic matrix platinum group element rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides the matrix and inclusions can be separated into two distinct textural types hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages the platinum group mineral pgm assemblages vary only between textural types and not between inclusions and matrix being dominated tellurides in all rock types the hydrothermally altered rocks however have fewer tellurides and an increased amount of sb and as bearing pgm indicative of minor fluid interaction although the pgm have not been mobilized significantly away from the base metal sulfides precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements pge indicating the presence of a single well homogenized pge rich sulfide liquid however au and cu appear to be decoupled from the pge at low concentrations although correlate well with each other which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins the overlying layered units above the mineralized units are not pge depleted trace element data including th yb pm and nb th pm ratios demonstrate that all river valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber although some samples have s se ratios indicative of crustal s most have s se ratios lower than the mantle range indicative of s loss we propose a multistage model for the formation of the mineralization in the river valley intrusion with a major contamination event at depth with the addition of s from local crustal rocks inducing sulfide saturation sulfide droplets were then enriched in pge within a conduit system with possible further upgrading of sulfide metal tenors and reduction of s se ratios via partial dissolution of sulfide the pge enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid eroded blocks of precrystallized and mineralized gabbro and footwall rocks and emplaced an inclusion bearing package as the lower 100 m or so of the river valley intrusion later emplacement of main river valley magma was from an s undersaturated pge fertile magma the river valley intrusion is thus an example whereby contact type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization rather than contamination triggering sulfide saturation in situ in such cases processes at depth determine the generation and subsequent tenor of the mineralization in particular dissolution of the sulfide can upgrade metal tenor but subsequently will reduce s se ratios masking the signature of crustal contamination in addition a multistage emplacement such as this will not necessarily preserve the characteristic increase in cu pd ratios in the overlying cumulates that is often used in exploration for pge deposits in mafic intrusions thus a full understanding of all the field geochemical and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of s se and cu pd ratios as an indicator of crustal contamination and the presence of pge mineralization

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

analog experimental insights into the formation of magmatic sulfide deposits abstract magmatic sulfide deposits result from transport and settling of dense sulfide liquid droplets in basic magma to model these processes we carried out analog experiments in which dense silicone oil droplets were transported in a viscous aqueous polymer solution the results of the experiments indicate that sulfide droplets can be carried in suspension by magma flowing at typical velocities due to the large surface tension of sulfide droplets coalescence does not occur during transport transport ceases when upward magma velocity decreases because of conduit enlargement or change from vertical to horizontal flow coalescence of droplets is controlled by magma viscosity and the presence of crystals

controls on the formation of contact type platinum group element mineralization in the east bull lake intrusion the east bull lake intrusion is the type example of a series of 2 48 to 2 49 ga layered mafic intrusions that developed within the rifted southern margin of the superior craton central ontario the east bull lake intrusion contains contact type pge mineralization e g strata bound pge rich disseminated sulfide mineralization occurring within 200 to 300 m of the contact with footwall and sidewall units although the mineralization locally occurs in massive plagioclase cumulates anorthosite zone most of the sulfides are concentrated within an underlying heterolithic inclusion rich unit inclusion bearing zone that grades into an intrusive breccia disseminated sulfides also occur in fine grained gabbroic veins within brecciated footwall rocks border zone geochemical characteristics of the contact type sulfide mineralization and its host rocks include 1 high average pd abundances throughout the east bull lake intrusion including an average of ca 200 ppb in the lower series 2 low average s contents typically contact type pge mineralization in the east bull lake intrusion reflects flow enhanced gravitational settling of a cu rich sulfide liquid followed by local fractionation of ni rich monosulfide solid solution to produce minor amounts of cu and pt rich residual sulfide liquid most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid sulfide liquid mass ratios field and geochemical observations suggest that the parental magmas to the mineralized parts of the east bull lake intrusion were s saturated and pge rich at the time of their emplacement these fertile parent magmas most likely originated as buoyant feldspathic residual magmas that escaped from a deeper more primitive subchamber although first described from the east bull lake intrusion contact type pge mineralization is now recognized in all of the major east bull lake suite intrusions similar styles of magmatic sulfide mineralization are developed in the contact environments of the bushveld igneous complex platreef deposits and the duluth complex minnesota e g dunka road deposit these contact type deposits are part of the family of basal accumulation magmatic sulfide deposits the economics of low sulfide tenor contact type pge cu ni mineralization such as that developed in the east bull lake intrusion will be most favorable in systems in which 1 the parent magmas possessed high initial pge tenors and reached s saturation prior to emplacement 2 the parent magmas experienced vigorous convection in the early stages of chamber development and 3 segregated sulfide was not diluted by coprecipitating silicate minerals a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high temperature liquidus mineral

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

plant mobility in the mesozoic disseminule dispersal strategies of chinese and australian middle jurassic to early cretaceous plants abstract four upper middle jurassic to lower cretaceous lacustrine lagerstatten in china and australia the daohugou talbragar jehol and koonwarra biotas offer glimpses into the representation of plant disseminule strategies during that phase of earth history in which flowering plants birds mammals and modern insect faunas began to diversify no seed or foliage species is shared between the northern and southern hemisphere fossil sites and only a few species are shared between the jurassic and cretaceous assemblages in the respective regions free sporing plants including a broad range of bryophytes are major components of the studied assemblages and attest to similar moist growth habitats adjacent to all four preservational sites both simple unadorned seeds and winged seeds constitute significant proportions of the disseminule diversity in each assemblage anemochory evidenced by the development of seed wings or a pappus remained a key seed dispersal strategy through the studied interval despite the rise of feathered birds and fur covered mammals evidence for epizoochory is minimal in the studied assemblages those early cretaceous seeds or detached reproductive structures bearing spines were probably adapted for anchoring to aquatic debris or to soft lacustrine substrates several relatively featureless seeds in all assemblages were potentially adapted to barochory or to endozoochory the latter evidenced especially by the presence of smooth seeds in vertebrate gut contents and regurgitant or coprolitic masses hydrochory is inferred for several aquatic plants that notably bear small featureless seeds particularly aggregated into detachable pods

petrologic evolution of gneissic xenoliths in the voisey s bay intrusion labrador canada mineralogy reactions partial melting and mechanisms of mass transfer nickel copper cobalt sulfide ores in the voisey s bay intrusion are closely associated with troctolitic to gabbroic rocks that contain abundant country rock xenoliths potential sources of the xenoliths include pelitic paragneiss enderbitic orthogneiss and mafic to quartzofeldspathic gneisses that form immediate country rocks to different parts of the intrusion regardless of location all xenoliths have a similar refractory mineral assemblage composed of hercynite magnetite ca rich plagioclase and corundum the refractory mineral assemblage formed via partial melting immediately after xenoliths were engulfed by magma rapid thermal equilibration allowed the xenoliths to survive prolonged interaction with magma corundum was formed by the incongruent melting of na rich plagioclase in pelitic and quartzofeldspathic gneisses corundum and ca rich plagioclase assemblages are aluminous their origin involved either multistage melting of protoliths where the production of a granitic minimum melt was followed by the liberation of a more silica rich and al poor melt or one stage disequilibrium melting density differences between the xenoliths restite assemblages and enclosing mafic magma facilitated the separation of partial melt from restite no evidence for the melts in the form of channels or interstitial glass is observed in the restite flow of mafic magma in the conduit system is thought to have dispersed the si and alkali rich melts derived from the xenoliths the only record of the xenolith derived melts is in the form of concentric rims of plagioclase and biotite which crystallized from a hybrid melt in boundary layers around most xenoliths hercynite in the restitic assemblage was produced either by partial melting involving fe and mg bearing minerals such as garnet and pyroxene or by replacement of corundum feo and mgo that were excluded from the boundary layer diffused inward and reacted with corundum to form hercynite the thickness of the hercynite bands suggests formation times between 3 000 and 23 000 years where the insulating rims of plagioclase and biotite were not present diffusion of feo and mgo from the surrounding crystal mush continued resulting in near complete conversion of corundum to hercynite the highly refractory mineral assemblages which characterize xenoliths present in the voisey s bay intrusion provide evidence for a complex history of magma country rock interaction the transfer of xenomelts and sulfur to flowing magma may have been essential for the formation of the magmatic sulfide ores

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

petrogenesis of pelitic xenoliths at the babbitt cuni deposit duluth complex minnesota u s a abstract the babbitt deposit consists of disseminated cu fe ni sulfides found within mafic rocks of the duluth complex generally near contacts with underlying metasedimentary rock types host rocks for the deposit include troctolites olivine gabbros gabbronorites norites and occasionally country rock hornfels xenoliths of country rocks are abundant in the deposit and suggest a relationship between sulfide mineralization and country rock contamination country rocks in the babbitt area include those of the middle precambrian biwabik iron formation and both calcareous and non calcareous pelites of the virginia formation xenoliths contain the assemblage cordierite plagioclase biotite orthopyroxene and are thought to have been derived from virginia formation protoliths comparison of protoliths and xenoliths using composition volume element ratio and mass balance techniques suggests that xenoliths have been strongly depleted in volatiles alkalis and si footwall rocks show only a depletion in volatiles neither fluid phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred extensive partial melting of xenoliths with residual enrichment of feo mgo and al2o3 is the most viable transfer process the lack of sio2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible and prior to final emplacement sulfide saturation may have been initiated due to si assimilation with an auxiliary magma chamber however the composition of ores in the babbitt deposit is consistent with saturation being achieved by addition of sediment derived volatile sulfur independent of major element assimilation

sulfide saturation in mafic magmas is external sulfur required for magmatic ni cu pge ore genesis the importance of externally derived sulfur in the genesis of sulfide rich magmatic ni cu platinum group element pge deposits remains a key yet unresolved issue calculations utilizing a variety of mafic magma types indicate that in general sulfide saturation by fractional crystallization occurs after ni has been depleted due to olivine crystallization cu and pge rich layers may form during relatively later stages of closed system crystallization but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient the mass of sulfide is too small to produce an economic deposit we show that there are numerous processes that may lead to early sulfide saturation in mafic ultramafic magmas contamination of mantle derived magmas by siliceous country rocks or their partial melts will lower the sulfur content needed to induce sulfide liquid saturation typically by amounts ranging from 200 to 700 ppm the mixing of magmas particularly if the result is to lower the liquidus temperature of the mixed magma may also lower the sulfur content needed to attain sulfide saturation by similar amounts an increase in magma fo2 related to the addition of volatiles such as h2o and co2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation contamination processes that lead to an increase in the activity of sio2 in the melt and hence may promote orthopyroxene rather than olivine crystallization aid in generating relative ni enrichment in remaining liquid as a result of the lower dni mineral melt value of orthopyroxene relative to olivine although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur ni rich sulfide deposits can form in such cases only from large volume open systems where the efficiency of sulfide collection is high no matter what the liquidus minerals may be without the addition of country rock derived sulfur the mass of sulfide necessary to generate economic ni cu pge concentrations requires efficient sulfide collection from large but not necessarily unrealistic volumes of magma small deposits 4 30 mt of sulfide may form from the collection of cotectic proportions of sulfide from less than 50 km3 of magma larger deposits such as those at noril sk could involve more than 200 km3 of magma this volume of magma is not unreasonable particularly in rift plume related settings despite such possibilities sulfur isotope data clearly indicate that externally derived sulfur has been involved in the formation of many large deposits and that collection of mantle derived sulfide in sufficient quantities to produce orebodies is a rare process we propose that magmatic ni cu pge sulfide ore formation normally requires significant sulfide supersaturation and that the addition of sulfur derived from xenoliths is the most viable mechanism for producing sulfide well above the cotectic proportion in mafic ultramafic magmas

characteristics of ni cu pge mineralization and genesis of the jinchuan deposit northwest china the jinchuan ni cu sulfide deposit is hosted by a northwest striking ultramafic body located at the southwest margin of the sino korean platform china sulfide mineralization occurs in the lower part of the intrusion and comprises three main orebodies 24 and 1 in the west and 2 in the east of the intrusion three types of sulfide ores are identified net textured disseminated and a small amount of massive ore which occurs only in orebody 2 the principal ore minerals are pyrrhotite pentlandite violarite chalcopyrite and cubanite a systematic study has been made of the s ni cu co platinum group elements pge and au of the ore samples a lateral zonation is defined by these elements in the two western orebodies the concentrations of ni cu and pge increase from the margins toward the center of the orebodies this trend is also present when the metal elements are normalized to the s content of the samples a vertical variation of elements is present in the east orebody where the amounts of ni cu and pge as well as s decrease upward however when normalized to the s content pge increase upward but ni and cu remain more or less constant a plot of pge versus s clearly shows three sample sets 1 samples from orebody 24 have lower s contents mostly less than 5 wt percent and their pge contents roughly increase with that of sulfur 2 orebody 1 has higher pge contents but the pge plots are more scattered suggesting multiple processes during pge mineralization and 3 orebody 2 has very low pge concentrations ore samples studied show an average ni cu ratio of 2 and pt pd ru ir os ratios of 5 to 30 with most values falling in the range of 10 to 15 these are typical features of sulfide ores that relate to mafic magma there is little difference in the pt pd os ir ru ratios and the chondrite normalized pge distribution patterns between the pge rich and pge poor ores suggesting that they originated from magmas with similar major element compositions the close association of pge enrichment with sulfide mineralization and the strong correlations exhibited in all ore types between s and ni co and pge indicate that the ores are the consequence of the segregation of sulfide from silicate magmas the high pge contents of ore bodies 24 and 1 and the low pge contents of orebody 2 suggest that two batches of sulfide liquid were involved the pge rich sulfide liquid equilibrated with a relatively primitive silicate magma whereas the pge poor sulfide liquid equilibrated with a silicate magma from which pge had been depleted by prior probably fractional sulfide segregation the low values of ni cu ratios about 2 and high values of pt pd ru ir os ratios 10 15 of the samples together with the chemical composition and rare earth element data of the intrusive rocks provide convincing evidence that the initial magma of the intrusion was gabbroic rather than ultramafic

the eagle and east eagle sulfide ore bearing mafic ultramafic intrusions in the midcontinent rift system upper michigan geochronology and petrologic evolution 1 the eagle and east eagle intrusions are small subvertical dike like mafic ultramafic bodies that cut proterozoic sedimentary strata in the baraga basin in northern michigan the eagle intrusion hosts a newly discovered magmatic ni cu pge deposit the nearby east eagle intrusion also contains sulfide mineralization but the extent of this mineralization has yet to be determined by further drilling both intrusions contain olivine bearing rocks such as feldspathic peridotite melatroctolite and olivine melagabbro sulfide accumulations range from disseminated at both eagle and east eagle to semimassive and massive at eagle u pb baddeleyite dating gives a crystallization age of 1107 2 5 7 ma for the eagle intrusion coeval with eruption of picritic basalts at the base of the volcanic succession in the midcontinent rift system mrs the fo contents of olivine cores in the eagle and east eagle intrusions vary between 75 and 85 mol higher than those of olivine in larger layered intrusions in the mrs such as the duluth complex the feo mgo ratios and al2o3 contents of the parental magmas for the eagle and east eagle intrusions inferred from olivine and spinel compositions are similar to those of picritic basalts in the base of the mrs volcanic succession these petrochemical data suggest that the eagle and east eagle intrusions are the intrusive equivalents of high mgo basalts that erupted in the early stages of continental magmatism associated with the development of the rift variations in mineral compositions and incompatible trace element ratios suggest that at least three major pulses of magmas were involved in the formation of low sulfide rocks in the eagle intrusion lower fo contents of olivine associated with semimassive sulfides as compared to that of olivine in low sulfide rocks suggest that the magma associated with the semimassive sulfide was more fractionated than the parental magmas of the low sulfide rocks in the eagle intrusion accumulation of suspended olivine crystals and sulfide droplets from ascending magmas as they passed through wide parts of the conduits at eagle and east eagle played a critical role in the genesis of olivine rich rocks and sulfide ores in the intrusions the eagle ni cu pge deposit typifies the conduit style of magmatic sulfide deposition that is associated with continental basaltic magmatism

melting reactions of gneissic inclusions with enclosing magma at voisey s bay labrador canada implications with respect to ore genesis the voisey s bay ni cu co sulfide deposit is associated with a 1 333 ga troctolitic gabbroic intrusion which transects the 1 85 ga collisional boundary between tasiuyak paragneiss of the proterozoic aged churchill province to the west and orthogneiss of the archean aged nain province to the east the intrusion comprises an upper chamber to the east the eastern deeps subchamber a lower chamber to the west the reid brook subchamber and a conduit connecting them sulfide mineralization is closely associated with a breccia which occurs close to the basal contact of the eastern deeps subchamber and within the conduit the breccia is composed of abundant tasiuyak paragneiss inclusions with a troctolitic gabbroic matrix the gneiss inclusions have reacted extensively with the enclosing magma in general the degree of reaction increases from the reid brook subchamber through the conduit to the eastern deeps subchamber in the course of this reaction garnet has been oxidized to form hercynite and magnetite with the release of sio2 to the magma cordierite has been dehydrated to form hercynite with the release of sio2 hypersthene and k feldspar have reacted together to produce hercynite with the release of sio2 and k2o plagioclase has broken down to produce corundum losing sio2 and na2o to the magma and itself becoming enriched in anorthite and the corundum has subsequently reacted with feo and mgo from magma to form hercynite the color of the hercynite changes progressively from beige in the reid brook subchamber through green within the conduit to black in the eastern deeps subchamber this color change reflects an increase in feal2o4 at the expense of the mgal2o4 in the hercynite the inclusions in the eastern deeps subchamber characteristically show a series of reaction rims ranging from labradorite through biotite to hypersthene it is seen that reaction with the gneiss inclusions results in the addition of sio2 k2o and na2o to the magma this felsification of magma has resulted in a decrease in its ability to dissolve sulfide and therefore has promoted sulfide immiscibility

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

physical constraints on the migration of immiscible fluids through partially molten silicates with special reference to magmatic sulfide ores the migration of dense fe rich immiscible liquids through partially molten silicates is of critical importance to several issues including the formation of magmatic sulfide ore deposits and the formation of metallic cores in terrestrial planets the migration of immiscible dense liquids can be considered to occur in three distinct regimes dictated by the relative sizes of the dense liquid droplets and the crystals making up the silicate crystal mush or partially melted silicate rock if dense microdroplets are smaller than the diameter of the narrowest portions of pore throats between silicate crystals then downward motion is possible the maximum rate of downward settling in loosely packed crystal mushes occurs when the droplet size is approximately one tenth the crystal size as long as the silicate melt between crystals remains microdroplets are capable of migrating distances of hundreds to thousands of meters vertically through crystal mushes contained in cooling dikes with typical grain sizes stranding of larger droplets similar in size to the crystals results from capillary forces preventing droplet deformation as they attempt to pass into pore throats narrower than themselves only in very coarse grained mushes with grain sizes greater than about 2 cm can droplets the size of intergranular pores migrate downwards on the other hand coalescence of many microdroplets into pre existing stranded droplets may generate connected net textured domains of the dense liquid that are many times larger than the grain size of the mush when the vertical height of the connected domain is great enough the pressure gradient inside the dense phase exceeds the capillary force impeding downward motion through narrow pore throats and the immiscible phase is able to move down along vertically oriented networks these networks should not suffer stranding until they reach a solid layer or the base of the intrusion we suggest that in most cases sulfide droplets formed by fractional segregation in large magma bodies should come to rest at the top of the crystal pile whereas sulfides introduced to magmas by assimilation of fine grained sulfidic sediments should be capable of passing downward through crystal mushes to form massive or net textured accumulations at the bottom of the partially molten zone

reevaluation of the role of external sulfur addition in the genesis of ni cu pge deposits evidence from the nebo babel ni cu pge deposit west musgrave western australia the nebo babel ni cu pge sulfide deposit located in the west musgrave block western australia is hosted within a tube like concentrically zoned gabbronorite magma conduit emplaced at 1068 ma the nebo babel deposit is associated with the ca 1078 ma giles complex which is part of the warakurna large igneous province previous geologic and geochemical studies have shown that the nebo babel consists of chemically related but temporally distinct magma pulses the initial magma pulses which were the most primitive and the most contaminated formed the marginal units of the conduit the later pulses were emplaced into the core of the conduit the chilled margin rocks contain sulfide globules mineralization is confined to the early magma pulses and consists of massive sulfide breccias and stringers and disseminated gabbronorite hosted sulfides the ni tenor of the massive sulfides which have undergone sulfide liquid fractionation and the disseminated sulfides are almost the same indicating that both formed at similar r factors the whole rock massive and disseminated sulfides contain 650 ppb and 60 to 500 ppb total pge respectively all of the nebo babel rock units are slightly pge depleted with cu pd ratios higher than primitive mantle indicating early saturation and segregation of a sulfide and or pgm rocks with trace cumulus sulfides have the lowest cu zr and highest cu pd ratios and the most fractionated metal patterns indicating that these units have undergone the highest degrees of ni cu and pge depletion the average 34s value of all nebo babel disseminated and massive sulfides is 0 4 0 2 per mil which is consistent with the s originating in the mantle sulfur saturation did not occur in situ and is inferred to have been triggered by assimilation of felsic orthogneiss perhaps in the parental magma chamber this initial magma pulse with entrained sulfides formed marginal units of the nebo babel intrusion some of the entrained sulfides accumulated within physical traps on the way into the conduit and or at the exit from the chamber these sulfides were later emplaced along the footwall contact as massive and or marginal fragment laden sulfide liquid distribution of the disseminated sulfides was influenced by the conduit geometry and magma flow dynamics the final and volumetrically dominant barren magma pulse underwent crystal fractionation and oxide precipitation in the core of the nebo babel intrusion which in turn caused sulfide saturation and formed trace but pge depleted sulfides in these units the nebo babel deposit has a number of features in common with other ni cu pge deposits hosted in dynamic magma conduits e g voisey s bay such as multiple magma pulses and sulfide entrainment from depth rather than in situ sulfide segregation however si addition was the primary cause of sulfide saturation and external s addition did not play a role in the genesis of the nebo babel deposit

multiple sulfur and iron isotope composition of magmatic ni cu pge sulfide mineralization from eastern botswana we report here high precision multiple sulfur and iron isotope compositions for a series of mineralized samples from ni cu pge sulfide deposits in the archean tati greenstone belt and the phikwe complex of eastern botswana mineralized samples from the phoenix and selkirk ni cu pge deposits in the tati greenstone belt display slightly positive 34s isotope values ranging from 0 2 to 0 8 v cdt 33s values of sulfides at phoenix and selkirk are 0 01 to 0 08 v cdt suggesting either a dominantly mantle sulfur source or effective eradication of a crustal 33s anomaly through equilibration with large amounts of silicate melt in the selebi phikwe belt a granite gneiss terrane with abundant amphibolite lenses of either volcanic and or intrusive nature mineralized lower grade samples from the phikwe phokoje and dikoloti ni cu pge deposits have more variable 34s values ranging from 3 1 to 0 3 and display significant mass independent anomalies 33s values ranging from 0 89 to 0 27 suggesting that barren sulfides associated with distal or low temperature sea floor hydrothermal activity contributed sulfur to these deposits iron isotopes of sulfides from these deposits show a relatively small range of negative 56fe values 0 29 to 0 04 consistent with high temperature fractionations in magmatic systems with the exception of one sample from the dikoloti ni cu pge deposit of the selebi phikwe greenstone belt which shows a more negative 56fe value of 0 61 consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high temperature magmatic systems data from this study highlight the complexity and variability that characterize ore forming processes in magmatic systems we suggest that the presence of sulfur bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides geologic settings without any or little sulfur in the stratigraphy which have been traditionally neglected in terms of their prospectivity should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits

architecture and emplacement of the nebo babel gabbronorite hosted magmatic ni cu pge sulphide deposit west musgrave western australia the nebo babel ni cu platinum group element pge sulphide deposit in the west musgrave block western australia is the largest nickel sulphide discovery in the last 10 years the deposit is hosted within a concentrically zoned olivine free tube like chonolithic gabbronorite intrusion associated with the approximately 1 078 ma giles complex layered intrusions in the warakurna large igneous province emplaced into sulphide free amphibolite facies orthogneiss the fault offset nebo babel chonolith extends for 5 km and has a cross section of 1 0 5 km igneous mineralogy fabrics and textures are well preserved the lithostratigraphy includes variably textured leucogabbronorites vlgn that form an outer shell around mineralised gabbronorite mgn with barren gabbronorite bgn and oxide apatite gabbronorite oagn in the middle and lower parts of the chonolith mineral and whole rock geochemistry indicate that the units become progressively evolved in the order vlgn mgn bgn and oagn and that incompatible trace element concentrations increase downwards within the mgn and bgn the mineralisation which is confined to the early more primitive units vlgn and mgn occurs as massive sulphide breccias and stringers and as disseminated gabbronorite hosted sulphides the massive sulphides were emplaced late in the intrusive sequence have different pge chemistry and cu tenor to the disseminated sulphides and have undergone sulphide fractionation the distribution of disseminated sulphides which are primary magmatic in origin is related to chonolith geometry and magma flow regimes rather than to gravitational settling sulfur bearing country rocks are absent in the nebo babel deposit area and thus local crustal s addition was unlikely to have been the major mechanism in achieving sulphide immiscibility the nebo babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses the parental magma was medium to low k tholeiite with 8 9 wt mgo the initial magma pulse vlgn the most primitive and sulphide saturated was probably emplaced along a linear weakness in the country rock after crystallisation of vlgn marginally more fractionated sulphide saturated magma was injected through the thermally insulated core of the conduit forming the mgn successive pulse s of more fractionated magma bgn were emplaced in the core of the intrusion after magma flow ceased closed system crystal fractionation produced consistent mineral and chemical fractionation trends within bgn and oagn after crystallisation the intrusion was overturned and then offset by the jameson fault resulting in the apparent reverse chemical and mineral trends in nebo babel

up down or sideways emplacement of magmatic fe ni cu pge sulfide melts in large igneous provinces the preferential localization of fe ni cu pge sulfides within the horizontal components of dike sill lava flow complexes in large igneous provinces lips indicates that they were fluid dynamic tra

heterogeneity of s isotope compositions recorded in the sudbury igneous complex canada significance to formation of ni cu sulfide ores and the host rocks sulfur isotope ratios of sulfide ores and their host rocks and the overlying main mass sequence of norites quartz gabbros and granophyres from the sudbury igneous complex have been analyzed to establish the extent of isotopic heterogeneity of the complex and the impact melt sheet from which it was produced noritic rocks located above the sublayer in the lower portion of the main mass of the sudbury igneous complex are characterized by uniform sulfur isotope ratios with 34s values of 2 59 0 48 rocks from the granophyric portion of the main mass are characterized by elevated and variable 34s values from 4 4 to 14 1 the more 34s enriched s in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled footwall and contact style mineralization from the sublayer show variable 34s values from 0 2 to 4 3 south range mineralization is characterized by low ratios from 0 2 to 2 5 north and northeast range mineralization exhibits locally elevated but variable 34s values for example mineralization from whistle mine varies from 3 1 to 4 3 whereas mineralization from victor varies between 2 4 and 2 9 there is typically only a small difference in s isotope ratio of contact and proximal footwall mineralization country rocks in the area of the sudbury igneous complex are also characterized by a wide range in 34s values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources however the variability present between deposits at the lower contact of the complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet there is no evidence that deposit size correlates with 34s value variations between deposits may involve local s isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall this study illustrates the application of s isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet

chalcophile element ni cu pge and au variations in the tamarack magmatic sulfide deposit in the midcontinent rift system implications for dynamic ore forming processes the tamarack magmatic sulfide deposit is hosted by the tamarack intrusive complex 1105 6 1 2 ma in the midcontinent rift system the most important sulfide mineralization in the complex occurs in the northern part which consists of two separate intrusive units an early funnel shaped layered peridotite body containing relatively fine grained olivine referred to as the fgo intrusion at the top and a late gabbro troctolite peridotite dike like body containing relatively coarse grained olivine referred to as the cgo intrusion at the bottom disseminated net textured and massive sulfides occur in the base of the fgo intrusion as well as in the upper part of the cgo intrusion the widest part of the cgo intrusion also hosts a large semi massive net textured sulfide ore body locally surrounded by disseminated sulfide mineralization small massive sulfide veins occur in the footwall of the fgo intrusion and in the wall rocks of the cgo dike the sulfide mineralization is predominantly composed of pyrrhotite pentlandite and chalcopyrite plus minor magnetite pyrrhotite containing the highest ni and co contents occurs in the fgo disseminated sulfides and in the cgo semi massive sulfide ores respectively the most important platinum group minerals associated with the base metal sulfides are sperrylite ptas2 sudburyite pdsb and michenerite pdbite nickel shows a strong positive correlation with s in all types of sulfide mineralization and cu shows a strong positive correlation with s in the disseminated sulfide mineralization at a given s content the concentrations of pt pd and au in the cgo disseminated sulfides are significantly higher than those in the fgo disseminated sulfides the semi massive sulfide ores are characterized by significantly higher ipge ir os ru and rh concentrations than most of the massive sulfide ores with few exceptions all of the various textural types of sulfide mineralization collectively show a good positive correlation between pt and pd and between individual ipge at a given pt or pd content however the semi massive sulfide ores have higher ipge contents than the disseminated sulfide samples modeling results show that the variations in pge tenors metals in recalculated 100 sulfide in the tamarack magmatic sulfide deposit are mainly controlled by variable r factors magma sulfide liquid mass ratios during sulfide liquid segregation and subsequent monosulfide solid solution mss fractionation during cooling the initial contents of ir pt and pd in the parental magma estimated from the metal tenors of the disseminated sulfides are 0 2 2 and 1 8 ppb respectively which are 1 5 of the values for the pge undepleted primitive basalts of the midcontinent rift system the variations of pge tenors in the semi massive and massive sulfide ores can be explained by mss fractional crystallization from sulfide liquids extreme variations in the pge contents of the massive sulfides may also in part reflect metal mobility during post crystallization hydrothermal processes the higher pge tenors for the disseminated sulfides in the cgo dike relative to those in the fgo intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the fgo magma were subsequently upgraded by the subsequent surge of the cgo magma the relatively low pge tenors for the semi massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit

transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles emissions of sulphur 1 2 and transition metals 3 4 from magmas in the shallow crust have global impacts on human society by affecting climate after volcanic eruptions 5 6 and by forming giant copper and gold ore deposits which are massive anomalies of sulphur as well as of the ore metals themselves 3 4 7 8 these emissions vastly exceed the amounts that could have been derived from the associated magmas themselves 2 intrusion of denser hotter vapour and sulphide saturated magma beneath the cooler and more buoyant bodies that are finally erupted is the most probable source of the excess sulphur and metals in eruptive or hydrothermal systems 1 4 9 10 however the mechanism for the selective upward transfer of sulphur and metals remains poorly understood because their main carrier phase sulphide melt is dense and is generally assumed to settle to the bottoms of magma bodies here we show by experimental observation and by gas speciation and mass balance models that such transfers may be achieved by the flotation of compound drops 11 comprising droplets of sulphide melt attached to vapour bubbles by proposing and demonstrating the feasibility of a novel physical mechanism for upward mobility of sulphide liquids our work provides a mechanism for massive atmospheric releases of sulphur and contradicts widely held assumptions that dense sulphide liquids rich in sulphur copper and gold will remain sequestered in the crust

the trophic habits of early birds abstract although direct associations that reveal diet are extremely rare in the fossil record the rich lower cretaceous jehol lagerstatte has produced dozens of specimens preserving ingested items which together reveal important information regarding the early evolution of the avian alimentary canal direct evidence indicates jeholornis and sapeornis ate seeds and like living granivores utilized a gastric mill although only the more derived sapeornis possessed a crop for food storage despite their smaller numbers in the early cretaceous most direct evidence pertains to the ornithuromorpha indicating a structurally and functionally modern alimentary canal was present in even the earliest members of this clade similar evidence is altogether lacking in the enantiornithes suggesting this clade was characterized by a primitive alimentary canal potentially factoring into the ultimate extinction of this successful cretaceous lineage the role of the gizzard in the loss of teeth in aves is more complex than in non avian theropod lineages as in neornithines the presence of gastroliths in basal ornithuromorphs may not be entirely indicative of herbivory although this diet does appear to correlate with complete tooth loss in this clade positive selection for specialized tooth morphologies persists throughout the evolution of aves and in the ornithuromorpha appears to be linked to piscivory although represented by the greatest numbers no direct indicator of diet preference is preserved in any confuciusornithiform or enantiornithine suggesting these clades utilized different nutritional strategies the absence of teeth and gastroliths in all confuciusornithiforms suggest this lineage may have secondarily switched to a non herbivorous soft diet similarly the absence of gizzard stones in enantiornithines also suggests a soft diet while their small body size suggests early cretaceous species would have fed on invertebrates the diversity of recognized dental patterns including enamel specializations observed in jehol enantiornithines suggests effective resource partitioning of available invertebrate faunas and that teeth had an active participation in enantiornithine feeding strategies

extension of laramide magmatism in southwestern north america into trans pecos texas the red hills intrusion hosts the easternmost porphyry copper molybdenum system in southwestern north america and consists of quartz sulfide stockwork veins in sericitized porphyritic quartz monzonite zircon u pb and molybdenite re os analyses yield ages of 64 2 0 2 ma and 60 2 0 3 ma respectively indicating that the red hills intrusion and mineralization are distinctly older than all other tertiary magmatism 48 17 ma in the trans pecos region of texas including the nearby 32 ma chinati mountains caldera the red hills intrusive system is contemporaneous with and genetically related to other laramide magmatic systems 75 54 ma that host porphyry copper deposits in arizona southwestern new mexico and northern mexico these results significantly extend the laramide magmatic province eastward and suggest that laramide subduction related magmatism and deformation are coextensive over a broad area of southwestern north america

s saturation history of nain plutonic suite mafic intrusions origin of the voisey s bay ni cu co sulfide deposit labrador canada the voisey s bay deposit is hosted in a 1 34 ga intrusion composed of troctolite olivine gabbro and ferrogabbro the sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites and also occurs as veins along structures in adjacent paragneiss a dyke is connected to the base of the north wall of the eastern deeps intrusion and the entry point of this dyke into the chamber is the locus of the eastern deeps nickel sulfide deposit a detailed exploration in the area between the eastern deeps and the ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke the eastern deeps deposit is surrounded by a halo of moderately to weakly mineralized variable textured troctolite vtt that reaches a maximum thickness above the axis of the eastern deeps deposit along the northern wall of the eastern deeps intrusion the massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the vtt and there is a marked break in ni tenor of sulfide between the two sulfides hosted in the dyke tend to have low metal tenors ni 100 2 5 3 5 sulfides in eastern deeps massive and breccia ores have intermediate ni tenors ni 100 3 5 4 and disseminated sulfides in overlying rocks have high ni tenors ni 100 4 8 four principal processes control the compositions of the voisey s bay sulfides coarse grained loop textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the pd ir ratio which is due to mineralogical variations where pentlandite is enriched in pd and ir is dispersed throughout the mineral assemblage a decrease in ir and rh from the margin of the ovoid toward cubanite rich parts at the central part of the ovoid is consistent with fractionation of the sulfide from the margins toward the center of the ovoid the ovoid ores have higher ni and pd tenor than the eastern deeps massive sulfides this is consistent with both a higher r factor and greater degree of silicate parental magma evolution in the ovoid than the eastern deeps the disseminated sulfides surrounding the eastern deeps deposit have some of the highest ni and pd tenors at voisey s bay which are indicative of not only more primitive magmas but also higher r factors than the ovoid or the eastern deeps vtt and normal textured troctolite of the eastern deeps that contain trace sulfide have 0 1 3 ppb pt and 0 1 3 ppb pd whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of pt and pd troctolites and olivine gabbros from other parts of the voisey s bay intrusion and other nain plutonic suite intrusions including the kiglapait newark bay barth island mushua and nain bay south intrusion also have low platinum group element abundances although it is possible that this is a signature of a widespread sulfide saturation event that pre dated ore formation at voisey s bay it is more likely that platinum group element pge depletion is a product of the source melting process where low degrees of melting resulted in the retention of pge in the mantle source if so this indicates that pge depletion should be used with caution as an exploration tool in the nain plutonic suite

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

petrology and geochemistry of mafic ultramafic fragments from the aguablanca ni cu ore breccia southwest spain aguablanca southwest spain is the first economic ni cu pge deposit found in southern europe two features make it an unusual example of magmatic sulfide ore it is related to the development of an andean type continental magmatic arc and it is hosted by a subvertical magmatic breccia the structural style and the geodynamic context of the deposit contrast with most plutonic ni cu pge deposits elsewhere which occur at specific levels of layered mafic intrusions in rift environments the ni cu deposit is hosted by the aguablanca intrusion a mafic body composed of gabbronorite and minor quartz diorite gabbro and norite sulfides are concentrated in a gabbronorite matrix along a subvertical dip of 70 80 n funnel like magmatic breccia that contains barren or slightly mineralized ultramafic mafic cumulate fragments modal compositions of the fragments reflect a wide variety of rock types including peridotite hornblende rich werhlite dunite and hornblende rich harzburgite pyroxenite ortho and clinopyroxenite gabbro gabbro gabbronorite and hornblende gabbro and anorthosite the primary silicate assemblage includes olivine fo91 fo79 orthopyroxene mg no 0 85 0 73 clinopyroxene mg no 0 93 0 62 plagioclase an99 an38 amphibole mg no 0 87 0 68 and phlogopite mg no 0 89 0 64 the wide range of rock types and the fe enrichment trends in the primary ferromagnesian silicates suggest magmatic differentiation processes from the parent melts with the fragments representing different stages of cumulate formation the ore bearing breccia contains both semimassive and disseminated sulfides in the gabbronorite matrix textures vary between meso and orthocumulate and the rock forming magmatic silicates are orthopyroxene mg no 0 83 0 74 clinopyroxene mg no 0 89 0 78 plagioclase an50 an77 and intercumulus amphibole mg no 0 86 0 70 phlogopite 0 84 0 69 and minor quartz the gabbronorite in the matrix of the breccia is petrographically and chemically very similar to that of the unmineralized parts of the main aguablanca intrusion and exhibits a similar differentiation trend suggesting that the matrix of the ore bearing breccia and the unmineralized rocks belong to a same magmatic suite the local presence of mafic ultramafic fragments in the barren aguablanca intrusion supports this suggestion the presence of highly ni depleted olivine whole rock cu zr ratios below 1 and the local occurrence of disseminations of magmatic sulfides in the peridotite fragments point to sulfide segregation before and or during the formation of the peridotite cumulates mantle normalized incompatible trace element patterns of the fragments along with published sulfur isotope data are consistent with crustal contamination suggesting that addition of crustal sulfur from pyrite bearing black slates led to sulfide saturation these results support a model in which sulfides segregated and settled during the differentiation of an unexposed mafic ultramafic complex now sampled as fragments in the breccia whereas the overlying silicate magma most probably fed by successive fresh magma injections underwent fractional crystallization giving rise to this cumulate sequence the emplacement of the ore breccia took place at temperatures above the monosulfide solid solution mss solidus but below the olivine and pyroxene solidus likely owing to the explosive injection of a new pulse of magma into the chamber which mingled with the sulfide liquid and disrupted the overlying cumulate sequence as a consequence fragments reached their current position in the breccia injected along with the sulfide and the silicate melts which subsequently formed the sulfide rich gabbronorite

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

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

the association between ni cu pge sulfide and ni co lateritic ores and volcanic facies within the komatiites of the 2 7 ga east yilgarn craton large igneous province western australia abstract a critical factor in the generation of komatiite hosted ni cu pge sulfide deposits the addition of external sulfur via assimilation of s bearing substrate beneath channelised lava flows is a function of the komatiite flow field architecture this architecture is also critical in the development of magnesium rich bedrock lithologies that are prerequisites for the formation of komatiite hosted ni co laterite deposits to better understand the controls on the spatial distribution of both these deposit types a gis referenced map of komatiite flow regimes within the komatiitic component of the c 2 7 ga east yilgarn large igneous province and from parts of adjacent older greenstone belts in the yilgarn craton has been constructed using data from the field and the literature the regimes essentially divide komatiite sequences into those composed of thin 0 5 100 m laterally restricted 50 m 3 km cooling units and those composed dominantly of thick 100 m 1 km and laterally extensive 3 100 km cooling units the regimes are subdivided into six facies and five sub facies based on both stratigraphic and lateral variations in lithologies the facies define regional scale variations reflecting markedly different spatial and temporal crystallisation conditions within komatiite flow fields there is a marked spatial association between known magmatic nickel sulfides and particular komatiite facies facies favourable for hosting magmatic sulfides are those that contain early formed very magnesian cumulate sequences requiring high effusion rates of very hot turbulently flowing lava to form and thus have the potential to be contaminated by s bearing substrate rocks sulfide mineralised komatiites make up about 9 of the area of the current outcrop of the 2705 ma komatiite lip facies that have a low potential for nickel sulfide mineralisation are sheet like bodies in which basal rocks were emplaced under relatively quiescent conditions and where hot turbulently flowing lava and crystallisation of olivine rich cumulates developed only after the build up of a pile of high porosity olivine cumulates allowing no interaction with the substrate nickel cobalt laterite deposits are associated with thick originally laterally extensive high magnesian sequences some such sequences are also favourable for magmatic sulfide mineralisation but others occur in sheet like sequences

the langmuir volcanic peridotite associated nickel deposits canadian equivalents of the western australian occurrences the 2 7 b y old abitibi greenstone belt is a crudely s shaped zone of metamorphosed volcanic subvolcanic and sedimentary rocks and associated intrusions through it are scattered a dozen small nickel deposits in volcanic peridotites their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous mgo with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows at least one deposit dumont is hosted by a dunitic intrusion others e g dundonald langmuir 2 are partially hosted in sulfidic iron formations mining of the langmuir 2 deposit permitted extensive underground mapping core logging and sample collecting the other deposits are known mainly from poor surface exposures and drill core the langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in western australia a magmatic origin is supported by the localization of the main ore zone in a paleotrough by the presence of a possible volcanic feeder beneath the ore and by the regular sequence through the basal ultramafic flow of massive ore overlain by net textured ore then by barren peridotite and lastly by a spinilex textured cap to the flow an unusual spinilex textured and sulfide bearing rock and skeletal chromite also support a magmatic origin as do the base and precious metal contents of the ores the high s se ratios averge 23 000 and slightly lower sulfur isotopic ratios average 0 5 per mil delta 34 s compared to most magmatic nickel sulfide deposits are similar to the ratios for windarra western australia and may be explained by assimilation of sulfide rich cherty iron formation other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore and the high sulfur content of some ores these features result from deformation and greenschist facies metamorphism but surviving features of an earlier magmatic history argue against the ores being formed metamorphically evidence of contemporaneous fumarolic activity was found at the south end of the mine but its importance as an ore forming process was not supported by detailed petrological and geochemical studies limited data from the other deposits in the abitibi belt suggest a common origin for the langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced the volume of contained nickel sulfides the presence of sulfidic iron formations and the severity of alteration and metamorphism the deposits in western australia probably were formed by similar processes in particular the langmuir deposits display strikingly similar sulfide chemistries to the windarra deposits which are probably dae to assimilation of sulfide rich cherty iron formation