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

impact of horizontal spreading on light propagation in melt pond covered seasonal sea ice in the canadian arctic 1 melt pond covered sea ice is a ubiquitous feature of the summertime arctic ocean when meltwater collects in lower lying areas of ice surfaces horizontal transects were conducted during june 2008 above and below landfast sea ice with melt ponds to characterize surface and bottom topography together with variations in transmitted spectral irradiance we captured a rapid progression from a highly flooded sea ice surface with lateral drainage toward flaws and seal breathing holes to the formation of distinct melt ponds with steep edges as the mass of the ice cover decreased due to meltwater drainage and rose upward with respect to the seawater level the high scattering properties of ice above the water level i e white ice were continuously regenerated while pond waters remained transparent compared to underlying ice the relatively stable albedos observed throughout the study even as ice thickness decreased were directly related to these surface processes transmission through the ice cover of incident irradiance in the 400 700 nm wave band ranged from 38 to 67 and from 5 to 16 beneath ponded and white ice respectively our results show that this transmission varied not only as a function of surface type melt ponds or white ice areal coverage but also in relation to ice thickness and proximity to other surface types through the influence of horizontal spreading of light thus in contrast to albedo this implies that regional transmittance estimates need to consider melt pond size and shape distributions and variations in optical properties and thickness of the ice cover

flow differentiation phenocryst alignment and compositional trends within a dolerite dike at rockport massachusetts systematic variations in the volume percent size and orientation of plagioclase phenocrysts up to 12 0 cm long occur across a 5 6 m thick porphyritic alkaline dolerite dike in rockport massachusetts field measurements indicate that phenocryst concentrations increase from nearly zero at the dike margin to 46 0 vol at its center average phenocryst size increases inward from 4 1 2 2 mm at the dike margin to 19 2 7 9 mm at the center the increase in size and abundance of phenocrysts toward the center of the dike is interpreted as resulting from flow differentiation the magma flow direction is assumed to have been upward and parallel to the dike margins n7 w stiike 88 e dip the strikes and dips of all elongate phenocrysts viewed in cross section within one traverse across the dike were measured and compared to the dike attitude to determine the degree of flow alignment across the dike average phenocryst strike deviations from dike strike increase inward 21 8 from the dike margin to its midpoint phenocryst dip angle deviations from dike dip increase inward by 18 8 this more pronounced clow alignment of phenocrysts nearer the dike margins is interpreted as being a function of the more extreme velocity gradients and resulting shear due to flow within the marginal zones of the magma than within its interior at least the outer few centimetres of phenocryst free chilled dike margins may form primarily by rapid quenching of phenocryst free magma rather than by flow differentiation whole rock major and minor element trends across this dike may be produced in part by flow differentiation a1 2 o 3 cao p 2 o 5 and na 2 o increase inward with plagioclase phenocryst concentrations whereas sio 2 k 2 o feo tio 2 mgo and mno decrease this inward decrease in certain oxides together with an inward within dike decrease in the anorthite content of the cores of plagioclase phenocrysts cannot be attributed to flow differentiation the inward decreasing chemical trends are believed to have resulted from one or a combination of several of the following processes the concentration of silica and incompatible elements in glass now devitrified within chilled dike margins compared to interiors plagioclase fractionation continuous tapping of partial melts increasingly depleted of silica and incompatible elements tapping of a zoned magma chamber and greater crustal contamination of marginal liquids superposed on the chemical trends the inward decrease in plagioclase anorthite content resulted from fractional crystallization

cu isotope variations between conduit and sheet style ni cu pge sulfide mineralization in the midcontinent rift system north america abstract disseminated ni cu pge sulfide mineralization in sheet like intrusions of the duluth complex are characterized by elevated bulk rock cu ni ratios typically in the range of 3 to 8 in order to evaluate whether the addition of crustal cu to parental magmas may have contributed to the high cu ni ratios cu isotope ratios were measured from sulfide minerals in the disseminated mineralization found within the sheet style partridge river and south kawishiwi intrusions of the duluth complex as well as sulfide mineralization in geometrically distinct conduit style deposits in the eagle intrusion and tamarack igneous complex these intrusions were emplaced during early stages of rift evolution 1110 1105 ma from picritic parental magmas with associated mineralization characterized by lower cu ni ratios in addition cu isotope ratios were determined from sulfidic and carbonaceous sedimentary country rocks to the intrusions 65cu values of disseminated sulfide mineralization in intrusions of the duluth complex range from 0 85 to 0 45 whereas values from the eagle intrusion and the tamarack igneous complex range from 0 69 to 1 84 65cu values of the sedimentary rocks range from 0 33 to 1 18 with two samples showing higher values of 2 65 and 3 12 the 65cu values of the duluth complex mineralization is consistent with the assimilation of significant amounts of sedimentary cu only if the 65cu value of the parental mantle derived magma was lower than 0 25 a value less than that proposed for the mantle based on analyses of terrestrial mafic igneous rocks the distinct difference in 65cu values between the sheet style and conduit style mineralization suggests that either mantle sources of the parental magmas in the rift differed or that cu isotope fractionation accompanied magmatic processes i e variable degrees of melting and sulfide retention sulfide liquid fractionation that were responsible for the relative enrichment of cu in the sheet style intrusions of the duluth complex

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

chironomid inferred holocene temperature changes in the south carpathians romania we present a holocene summer air temperature reconstruction based on fossil chironomids from lake brazi 1740 m a s l a shallow mountain lake in the south carpathians summer air temperature rec

world class ni cu pge deposits key factors in their genesis magmatic ni cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt sulfide saturation of a magma is not enough in itself to produce an ore deposit the appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals and then is concentrated in a restricted locality so that the resulting concentration is of ore grade the deposits of the noril sk region have developed within flat elongate bodies 15 2 0 2 km that intrude argillites evaporites and coal measures adjacent to a major trans crustal fault and immediately below the centre of a 3 5 km thick volcanic basin studies of the overlying basalts have shown that lavas forming a 500 m thick sequence within these have lost 75 of their cu and ni and more than 90 of their pge overlying basalts show a gradual recovery in their chalcophile element concentrations to reach normal values 500 m above the top of the highly depleted zone the ore bearing noril sk type intrusions correlate with those basalts above the depleted zone that contain normal levels of chalcophile elements the high proportion of sulfide 2 10 wt associated with the noril sk type intrusions the high pge content of the ores the extensive metamorphic aureole 100 400 m around the bodies and the heavy sulfur isotopic composition of the ores 8 12 34s are explicable if the ore bearing bodies are exit conduits from high level intrusions along which magma has flowed en route to extrude at surface the first magma to enter these intrusions reacted with much evaporitic sulfur at a low r value and thus gave rise to sulfides with low metal tenors successive flow of magma through the system progressively enriched the sulfides in the conduits losing progressively less of their chalcophile metals and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows the voisey s bay deposit lies partly within a 30 100 m thick sheet of troctolite interpreted as a feeder for the 1 334 ga voisey s bay intrusion and partly at the base of this intrusion where the feeder adjoins it studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was ni depleted but that subsequent pulses were much less depleted trace element re os and s and o isotope data and mineralogical studies indicate that the magma pulses interacted with country gneiss probably principally in a deeper level intrusion extracting sio2 na2o k2o and possibly sulfur form the gneiss which accounts for the magma becoming sulfide saturated the jinchuan deposit of north central china occurs within a 6 km long dyke like body of peridotite the compositions of olivine within the dyke the igneous rocks themselves and the ore are all inconsistent with derivation of the body from ultramafic magma as originally supposed and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion the west australian komatiite related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma the sediments are sulfide rich and may have contributed substantially to the sulfide of the ores the mineralisation in the duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite and sulfide bearing sediments no magma flow channels have been identified so far and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade when most major ni cu sulfide deposits are compared they prove to have a number of features in common olivine rich magma proximity to a major crustal fault sulfide bearing country rocks chalcophile element depletion in related intrusive or extrusive rocks field and or geochemical evidence of interaction between the magma and the country rocks and the presence of or proximity to a magma conduit the features are thought to explain the three key requirements sulfide immiscibilty adequate mixing between sulfides and magma and localisation of the sulfides discussed and have important implications with respect to exploration

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

multiple sulfur isotopes reveal a magmatic origin for the platreef platinum group element deposit bushveld complex south africa the platreef ore horizon of the bushveld complex south africa is the third largest platinum group element ore deposit in the world but the origin of its ore remains enigmatic a complex contact relationship between the igneous and footwall rocks of the bushveld complex coupled with evidence for widespread late stage hydrothermal processing obscures the original mineralization history of the platreef we constrain the parental magmatic origin of the platreef by exploiting multiple sulfur isotope contrasts across bushveld complex contact zones in order to see through the effects of postmineralization hydrothermal activity we report s isotope measurements made on samples collected along two profi les through the platreef into underlying metapelitic and metacarbonate footwall rocks in both profi les igneous rocks far from the contact have low 33 s values average 33 s 0 15 whereas metasedimentary rocks far from the contact have high 33 s values 33 s to 5 04 with a smoothly varying profi le between the two end members the midpoint in both isotope profi les is displaced into the footwall defi ning a classic advective dispersive tracer geometry this geometry is not present in the associated 34 s values the displacement of the 33 s front suggests fl uid transport and advection of s into the country rocks this was accompanied by back diffusion of the s isotope tracer into the platreef the platreef magma was apparently s saturated prior to emplacement and counterintuitively lost s during the formation of the present platreef ore horizon

micro scale s isotope studies of the kharaelakh intrusion noril sk region siberia constraints on the genesis of coexisting anhydrite and sulfide minerals abstract the coexistence of magmatic anhydrite and sulfide minerals in non arc related mafic magmas has only rarely been documented likewise the s isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured in the kharaelakh intrusion associated with the world famous noril sk ore district in siberia coexisting magmatic anhydrite and sulfide minerals have been identified sulfur isotope compositions of the anhydrite sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite sulfide pairs magmatic anhydrite and chalcopyrite are characterized by 34 s values between 18 8 and 22 8 and 9 3 and 13 2 respectfully coexisting anhydrite and chalcopyrite show values that fall between 8 5 and 11 9 anhydrite in the kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks partial reduction to sulfide led to mixing between sulfate derived sulfide and sulfide of mantle origin the variable anhydrite and sulfide 34 s values are a function of differing degrees of sulfate reduction variable mixing of sulfate derived and mantle sulfide incomplete isotopic homogenization of the magma and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling the 34 s values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide sulfate ratio variations in both sulfide and anhydrite 34 s values indicate that isotopically distinct domains existed on a centimeter scale late stage hydrothermal anhydrite and pyrite also occur associated with ca rich hydrous alteration assemblages e g thomsonite prehnite pectolite epidote xonotlite 34 s values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals anhydrite pyrite values are in the 9 1 10 1 range and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages the small values are suggestive of either sulfate sulfide isotopic disequilibrium or closure of the system to further exchange between 550 and 600 c our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated 34 s values in the kharaelakh intrusion but highlight the sulfur isotopic variability that may occur in magmatic systems in addition our results confirm the need for more precise experimental determination of sulfate sulfide sulfur isotope fractionation factors in high t systems

wetting of mantle olivine by sulfide melt implications for re os ratios in mantle peridotite and late stage core formation this study investigates the effects of variations in the relative fugacities of oxygen and sulfur on the wetting of mantle olivine by molten sulfide experiments were performed on mixtures of san carlos olivine and synthetic fes at 1 bar and 1350 c crucibles were fabricated from san carlos olivine and the fugacities of oxygen and sulfur were controlled by mixing co2 co and so2 gases experimental conditions ranged from logfo2 7 9 to 10 3 and from logfs2 1 5 to 2 5 our experimental results demonstrate that at a given temperature and pressure the olivine sulfide melt dihedral angle is controlled by the concentration of o dissolved in an anion rich melt trace amounts of o dissolve in sulfide melt at fo2 conditions near the iron wustite oxygen buffer and the dihedral angle is 90 at fo2 conditions near the fayalite magnetite quartz oxygen buffer the concentration of dissolved o is near 9 wt and the dihedral angle is 52 allowing small amounts of sulfide melt to form an interconnected network in olivine rich rocks and to migrate via porous flow these results indicate that sulfide melt is likely to be mobile at current upper mantle fo2 and fs2 conditions in mantle peridotite the addition or removal of sulfide melt by porous flow will variably fractionate re os u pb and th pb ratios because os and pb are more chalcophile than re u and th the re os ratio of the peridotite is especially sensitive to this process the mobility of sulfide melt at oxidizing conditions implies that the addition of oxidized chondritic material during the later stages of the accretion of the earth may have facilitated the segregation of core forming material by porous flow if temperatures were in excess of the sulfide solidus

analysis of sulfides in whole rock matrices by elemental analyzer continuous flow isotope ratio mass spectrometry abstract potential matrix effects in the sulfur isotopic analysis of whole rocks by elemental analyzer continuous flow isotope ratio mass spectrometry ea cfirms have been investigated by measuring mixtures of a well calibrated laboratory sulfide reference mineral and common rock forming minerals mixtures of sulfide and quartz orthoclase plagioclase kaolinite olivine magnetite ilmenite garnet halite fluorite calcite and graphite show consistent s yields and 34s values generally within 0 2 of the measured pure sulfide value however erratic results are obtained for mixtures of reference sulfide and pyroxene amphibole or biotite these minerals may suppress sample peaks cause extreme peak tailing and prohibit isotope ratio measurements pure sulfide minerals introduced after the mixtures also showed suppressed sample peaks suggesting so2 adsorption on mineral and residual ash surfaces removal of the residual ash resulted in normal measurements for reference sulfide minerals whole rocks with s concentrations in excess of 100 ppm can be successfully analyzed using ea cfirms provided that measurements are carefully monitored for evidence of so2 retention during and after combustion

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

the penokean orogeny in the lake superior region abstract the penokean orogeny began at about 1880 ma when an oceanic arc now the pembine wausau terrane collided with the southern margin of the archean superior craton marking the end of a period of south directed subduction the docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back arc extension both in the initial arc and adjacent craton margin to the north a belt of volcanogenic massive sulfide deposits formed in the extending back arc rift within the arc synchronous extension and subsidence of the superior craton resulted in a broad shallow sea characterized by volcanic grabens menominee group in northern michigan the classic lake superior banded iron formations including those in the marquette gogebic mesabi and gunflint iron ranges formed in that sea the newly established subduction zone caused continued arc volcanism until about 1850 ma when a fragment of archean crust now the basement of the marshfield terrane arrived at the subduction zone the convergence of archean blocks of the superior and marshfield cratons resulted in the major contractional phase of the penokean orogeny rocks of the pembine wausau arc were thrust northward onto the superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 ma in the south baraga group rocks and 1835 ma in the north rove and virginia formations a thick succession of arc derived turbidites constitutes most of the foreland basin fill along with lesser volcanic rocks in the southern fold and thrust belt tectonic thickening resulted in high grade metamorphism of the sediments by 1830 ma at this same time a suite of post tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the penokean orogeny the penokean orogen was strongly overprinted by younger tectonic and thermal events some of which were previously ascribed to the penokean principal among these was a period of vertical faulting in the archean basement and overlying paleoproterozoic strata this deformation is now known to have post dated the terminal penokean plutons by at least several tens of millions of years evidence of the penokean orogen is now largely confined to the lake superior region comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive penokean orogen have been removed or overprinted by younger tectonic events

geodynamics of magmatic cu ni pge sulfide deposits new insights from the re os isotope system in this study we reassess crustal contamination and sulfide ore forming processes in some of the largest magmatic ore deposits using published re os isotope data and a modeling methodology that incorporates the r factor defined as the effective mass of silicate magma with which a given mass of sulfide magma has equilibrated in an re os isotope mixing equation we show that there is less disparity between conclusions based on re os isotope data compared to other isotopic systems if the r factor is considered komatiite associated ni sulfide ore systems typically have high os concentrations low re os ratios and near chondritic initial os isotope compositions for magmatic sulfide ores that are interpreted to have experienced relatively low r factors 10 000 for noril sk talnakh and the j m reef stillwater complex in these very dynamic magmatic ore systems crustal contamination processes are more difficult to assess using re os isotopes as the effects of contamination are masked by the r factor process in which sulfide magma equilibrates with extensive amounts of asthenospheric mantle derived magma sulfide protore for these systems may then have been very radiogenic and of crustal origin prior to r factor processes that occurred during transport in feeder conduits and in upper crustal magma chambers this study therefore highlights the need for caution when interpreting the re os isotope geochemistry of sulfide ores from dynamic magmatic systems the results of our reinvestigation of these giant ore deposits suggest that geodynamic processes associated with large magmatic systems including major lithospheric pathways to the surface changes in flow regime coupled magma flow through and magma mixing providing enhanced r factors may be critical to our understanding of the emplacement localization and quality of magmatic sulfide deposits thus the timing and exact mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localization of economic concentrations of magmatic sulfides

sulfur isotope variations within the platreef ni cu pge deposit genetic implications for the origin of sulfide mineralization the platreef the world s third largest platinum group element pge deposit is a 10 400 m thick pyroxenitic unit at the base of the northern limb of the mafic bushveld complex hosting pge mineralization in association with base metal sulfides the sulfide mineralization is thought to have been either largely magmatic in origin with contamination by assimilation of local floor rocks considered an ore modifying process or a product of assimilation of country rock s and silicic contamination which caused s saturation in the magma with the contamination being fundamental to the ore forming process we have performed an extensive and detailed sulfur isotope study of the platreef which indicates that magmatic signatures 34s 0 to 2 are preserved in early formed sulfide droplets within the platreef pyroxenites in the area from sandsloot to witrivier these values are comparable to sulfide inclusions in diamonds in the nearby klipspringer kimberlite and are considered to have a primary mantle origin there is no indication of any significant external s in any of the primary sulfides in the sandsloot witrivier area however later sulfides found in calc silicate floor rocks at sandsloot and zwartfontein and in xenoliths of calc silicate throughout the section indicate an input of country rock s anhydrite bearing horizons in the malmani dolomites may have exchanged s with magmatic platreef sulfide during platreef intrusion imparting a heavier s isotope signature in the fluid affected rocks than those in the early sulfides the archean basement although containing minor amounts of sulfide with a negative 34s value is not a significant contributor to the s budget of the platreef previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides rather than sulfates such as at turfspruit the platreef sulfides are extensively contaminated with country rock s assimilation of pyrite bearing shales in the turfspruit area has locally upgraded the s content of the platreef and given the basal platreef sulfides a heavier isotopic signature sulfur saturation in the platreef magma took place before contamination probably in a staging chamber prior to intrusion a major pulse of magma entrained the preformed pge rich sulfides and was injected to form the platreef where assimilation of country rock sulfides upgraded the s content on a strictly local scale and hydrothermal leaching introduced s from country rock sulfates into later stage sulfides again on a local scale south of zwartfontein it is proposed that the platreef was intruded into sediments of the transvaal supergroup which formed the floor and the roof of the platreef north of zwartfontein the platreef intruded the boundary between the malmani supergroup sediments and the archean basement this could have potentially mineralized the roof calc silicates north of zwartfontein in a manner similar to the floor in the sandsloot area the later intrusion of the main zone magma formed a magmatic unconformity on top of the cold platreef and entrained large rafts of mineralized calc silicate for tens of kilometers north of the last footwall outcrop of calc silicate

sulfur isotopic studies of continental flood basalts in the noril sk region implications for the association between lavas and ore bearing intrusions previous studies of both ore and non ore bearing intrusives in the permo triassic flood basalts of the siberian platform in the noril sk area have shown that high grade ni cu platinum group elements pge mineralization is associated with anomalously high 34s values of 8 to 12 in addition several researchers have proposed that observed depletions in the cu ni and pge content of basaltic lavas of the nadezhdinsky nd formation are related to diffusional exchange with and upgrading in metal tenor of sulfides in the volcanic conduit system sulfur isotopic studies of the lavas at noril sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident and if the nd lavas in particular were characterized by an anomalous isotopic signature 34s values of the lavas range from 4 5 to 8 7 vienna canon diablo troilite vcdt with s concentrations from 40 to 1373 ppm the majority of 34s values range from 0 to 4 and are similar to those from s poor intrusions in the noril sk area although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas recrystallization may have erased expected mineralogical and textural evidence mineralogical data indicate that hydrothermal alteration of the lavas has occurred but s redistribution has been restricted to localized areas and 34s values have not been affected the relatively low s concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of 900 c at qfm leads to only minor 34s depletion of sulfur remaining in the melt and decreases in 34s values of less than 2 at 90 degassing for this reason all lavas with 34s values in excess of 2 require a contribution of 34s enriched country rock sulfur because of the high s content and 34s value 16 20 of evaporites in the country rocks at noril sk contamination of less than 0 5 is required to explain the most 34s enriched lavas the nd lavas have an average 34s of 2 9 but show no difference in s isotopic composition relative to the other lavas suggesting that metal depletion involved only limited s transfer or that exchange between mantle derived s and s of crustal origin buffered 34s values to less than 5 anomalously positive 34s values similar to those of the ore bearing intrusives in the noril sk region are not consistently found in low s rocks either lavas or intrusives although the mechanism for the derivation of sulfide in the ore bearing intrusions remain speculative it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin

field relationships and geochemical constraints on the emplacement of the jinchuan intrusion and its ni cu pge sulfide deposit gansu china field mapping and petrological geochemical investigation of the jinchuan intrusion in north central china clarifies how the intrusion was emplaced and provides a new model that explains how its large and rich ni cuplatinoid deposits may have formed the intrusion was emplaced into high grade gneisses and marbles along a disconformity at the base of an overlying cover sequence indicating that it was emplaced as a sill not a nearvertical dike as previously proposed after emplacement the intrusion was rotated to its present orientation and deformed and metamorphosed under greenschist facies conditions relative enrichment of incompatible trace elements coupled with negative u th and nb ta anomalies in all samples from the intrusion provide evidence that the parental magma assimilated granitoid rocks in the lower crust the presence of abundant marble xenoliths now decarbonatized to diopside rich skarns and chemical indices such as high cao sio2 indicate that the magma assimilated carbonate on reaching its present site this contamination may be linked to the formation of the ni cu platinoid ores we propose that the assimilation of carbonate rich fluids increased the oxygen fugacity of the magma and led to the segregation of metal rich sulfides

age of volcanic rocks and syndepositional iron formations marquette range supergroup implications for the tectonic setting of paleoproterozoic iron formations of the lake superior region a rhyolite in the hemlock formation a mostly bimodal submarine volcanic deposit that is laterally correlative with the negaunee iron formation yields a sensitive high resolution ion microprobe shrimp u pb zircon age of 1874 9 ma but also contains inherited archean zircons as old as 3 8 ga this precise age determination for the classic paleoproterozoic stratigraphic sequence of northern michigan the marquette range supergroup mrs necessitates modification of previous depositional and tectonic models our new data indicate that the menominee group previously ascribed to continental rifting during early pre collision phases of the penokean orogenic cycle is coeval with arc related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit we interpret these to be second order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin along with a recently reported age for the gunflint formation in ontario of 1878 2 ma our data suggest that an extensive foredeep in the western lake superior region was the locus of iron formation deposition during arc accretion from the south further we interpret the lower mrs chocolay group a glaciogenic and shallow marine succession that lies atop archean basement to be equivalent to the upper part of the huronian supergroup of ontario and to represent the original continental rifting and passive margin phase of the penokean cycle the upper mrs baraga group represents deeper marine basins dominated by turbidites and lesser volcanic rocks resulting from increased subsidence and continued collision a stitching pluton which cuts correlatives of the hemlock formation in a thrust sheet yielded a u pb zircon age of 1833 6 ma consistent with other post tectonic plutons in wisconsin and northern michigan indicating that penokean convergence lasted no longer than 40 million years

mineralogic and stable isotope studies of hydrothermal alteration at the jinchuan ni cu deposit china the jinchuan intrusion hosts one of the largest magmatic ni cu sulfide deposits in the world net textured and disseminated pyrrhotite pentlandite and chalcopyrite are found in olivine rich rocks 60 80 vol that occur as a northwest southeast striking dike olivine in the intrusion has been variably converted to mixtures of serpentine and fine grained magnetite interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite amphibole epidote clinozoisite na rich feldspar and lesser amounts of calcite although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved up to 30 vol percent of sulfide minerals are replaced by magnetite serpentine and chlorite an external origin for mg and si needed for the isovolumetric replacement of sulfides cannot be discounted but a local origin related to olivine and pyroxene alteration is favored serpentine and chlorite may be enriched in cu and ni but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides some of the metals and sulfur that were removed from the net textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine chlorite and amphibole sulfur isotope values of the pyrrhotite pentlandite chalcopyrite assemblage at jinchuan fall in the range of 2 to 8 per mil with over 80 percent of the values between 2 and 2 per mil immediate country rocks in the jinchuan area are precambrian granitoids schists and marbles with sulfur concentrations less than 100 ppm although the 34 s values of the main sulfide assemblage at jinchuan are in the range that is characteristic of mantle derived sulfur assimilation of sulfur from unexposed country rocks cannot yet be discounted secondary pyrite occurs in veins and stringers and rarely as a replacement of pyrrhotite and is characterized by 34 s values between 7 and 27 per mil the strongly negative 34 s values of secondary pyrite are thought to be a result of oxidation of primary magmatic sulfide assemblages and partial reduction of aqueous sulfate oxygen and hydrogen isotope values of silicate minerals suggest that at least two fluids have been involved in the hydrothermal alteration at jinchuan the 18 o values of plagioclase amphibole and serpentine all suggest the involvement of a fluid with a 18 o value between 2 and 5 per mil at 300 to 400 c the d values of amphibole and one population of serpentine suggest a d value for the fluid in the range of 45 to 63 per mil these values are consistent with the involvement of evolved seawater or a metamorphic fluid in the early stages of alteration the possible involvement of evolved seawater in hydrothermal alteration is consistent with a rift setting for the emplacement of the jinchuan intrusion however the wide range in serpentine d values computed d of the fluid 45 to 103 suggest either that serpentinization involved a mixture of evolved seawater and or metamorphic water and meteoric water or more likely that serpentine continued to exchange hydrogen isotopes with late stage meteoric waters after it formed

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

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

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

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

spatial distribution of sulfur isotope ratios associated with the eagle ni cu sulfide deposit in upper peninsula michigan implications on country rock contamination in a dynamic magma conduit abstract the eagle ni cu magmatic sulfide deposit in the upper peninsula of michigan is hosted in mafic ultramafic rocks associated with the 1 1 ga midcontinent rift event the deposit consists of massive semi massive and disseminated sulfide textural zones located in two closely spaced funnel shaped intrusive bodies called the eagle and eagle east intrusions the intrusions are surrounded by metamorphosed supracrustal paleoproterozoic rocks of the michigamme formation and at depth by a neoarchean migmatitic banded gneiss 34s values were obtained from sulfide minerals separated by micro drilling from different parts of the intrusions and the surrounding rocks the 34s values from the intrusions cluster within a small range between 1 and 3 relative to v cdt regardless of the mode of occurrence textural type mineral type and location of the sample however disseminated sulfide minerals in the metasedimentary country rocks of the michigamme formation show a substantially greater range in 34s values between 4 and 35 with a median of 7 7 while disseminated sulfides in the basement granite gneiss show values ranging from 9 to 13 with a median of 2 5 the narrow range of 34s values within the intrusions despite wide variabilities in the surrounding country rocks can be explained by the incorporation of crustal sulfur by a mantle derived magma followed by a homogeneous mixing of the immiscible sulfide liquid with a silicate liquid to sulfide liquid ratio r factor of 200 or more the assimilation of crustal rocks and the mixing of the resultant immiscible sulfide liquid must have been facilitated by the rapid movement of magma along a narrow conduit system

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

compositional variations of olivine and sulfur isotopes in the noril sk and talnakh intrusions siberia implications for ore forming processes in dynamic magma conduits olivine from the noril sk i main and northwestern talnakh ore bearing intrusions in the noril sk region occurs as inclusions discrete grains and poikilitic crystals the original compositions of olivine inclusions have been well preserved by their host oikocrysts from reaction with interstitial silicate and sulfide liquids in contrast the compositions of discrete and poikilitic olivine crystals have been variably modified by reequilibration with trapped silicate and sulfide liquids there are abrupt changes in the contents of nickel in olivine inclusions 500 1 500 ppm between different rock units in all three intrusions which indicates that the different rock units in these intrusions are not related to each other via in situ differentiation of a common parental magma but rather are consistent with separate pulses of magma with variable degrees of differentiation possibly involving olivine crystallization and assimilation of siliceous crustal material in a staging chamber the 34s values of the samples with high sulfur contents 800 ppm sulfur from all three ore bearing intrusions vary between 8 and 14 per mil two sulfur poor samples 600 ppm sulfur from the northwestern talnakh intrusion yield 34s values of 1 5 and 3 6 per mil these values are similar to the values of the sulfur poor rocks from the ore barren intrusions in the region and illustrate the fact that high sulfur contents couple with increasing 34s values such a relationship is consistent with the addition of crustally derived 34s enriched sulfur to the magma of the ore bearing intrusions in contrast to other models of sulfide segregation in closed systems we propose that sulfide segregation occurred in an open system and the magma involved was undersaturated in sulfur and undepleted in chalcophile elements we envision that the 34s enriched sulfur was introduced via circulating fluids that transported sulfur from evaporite in the underlying sedimentary sequence to the magma in a dynamic magma conduit the immiscible sulfide liquids that segregated during sulfur contamination settled down in the wider parts of the conduit as the magma continued to travel successive pulses of new magma entered the conduit reacted with the early sulfide and displaced much of the early magma in the conduit the ore bearing intrusions are thought to have crystallized within the wider parts of the conduits and their voluminous sulfide poor peripheral sills represent some if not all of the magma that flowed out of the conduits

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 midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting