variations in os isotope ratios of pyrrhotite as a result of water rock and magma rock interaction constraints from virginia formation duluth complex contact zones abstract os isotope ratios in pyrrhotite bearing pelitic rocks of the 1 85 ga virginia formation are variable with perturbations linked to the emplacement of the 1 1 ga duluth complex pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1 1 ga involving a low 187 os 188 os fluid however because rocks with perturbed pyrrhotite os isotope ratios occur 1 km or more from the duluth complex the fluid is unlikely to have been of magmatic origin fluid inclusions in layer parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between 300 and 400 c analyses of fluid inclusions via la icp ms show that the fluids contain up to 1 7 wt na 1 1 wt k 4330 ppm fe 2275 ppm zn and 415 ppm mg the veins also contain pyrite or pyrrhotite plus minor amounts of chalcopyrite bornite pentlandite and sphalerite the re os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187 os 188 os fluids 18 o values of vein quartz range from 7 7 to 9 5 consistent with an origin involving fluid with a relatively low 18 o value between 2 and 5 meteoric water with such a low 18 o value could have interacted with the igneous rocks of the complex and would have acquired os with a low 187 os 188 os ratio strongly serpentinized olivine rich rocks of the complex are commonly characterized by such low 18 o values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the os isotopic system recorded by pyrrhotite in the virginia formation two important observations are that only pyrrhotite bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing os exchange are spatially restricted and not uniformly distributed os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite and where time integrated water rock ratios in the aureole were high enough to provide a supply of os troctolitic and gabbroic rocks of the partridge river intrusion duluth complex are characterized by os isotope ratios that are indicative of variable degrees of crustal contamination os values of 0 543 xenoliths of carbonaceous and sulfidic pelitic rocks of the virginia formation found in the igneous rocks provide evidence that os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle derived magma however residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187 os 188 os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole the os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of re os liberation during devolatilization kerogen maturation and pyrite to pyrrhotite conversion processes that contribute to magma contamination followed by os uptake by pyrrhotite during back reaction involving magma and or fluid characterized by a relatively low 187 os 188 os ratio the extreme os uptake recorded by pyrrhotite in the xenoliths as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole is in line with the high os diffusivity in pyrrhotite experimentally determined by brenan et al 2000 our data confirm that os isotope ratios in pyrrhotite bearing rocks may be readily perturbed for this reason caution should be exercised in the interpretation of os isotope ratios in rocks where pyrrhotite may be the primary host of os

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

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

sulfur isotope and trace element data from ore sulfides in the noranda district abitibi canada implications for volcanogenic massive sulfide deposit genesis we examine models for volcanogenic massive sulfide vms mineralization in the 2 7 ga noranda camp abitibi subprovince superior province canada using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals the noranda camp is a well preserved vms deposit rich area that is thought to represent a collapsed volcanic caldera due to its economic value the camp has been studied extensively providing a robust geological framework within which to assess the new data presented in this study we explore previously proposed controls on mineralization within the noranda camp and in particular the exceptional au rich horne and quemont deposits we present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different vms deposits and showings within the noranda camp multiple sulfur isotope data for this study have 34sv cdt values of between 1 9 and 2 5 and 33sv cdt values of between 0 59 and 0 03 we interpret the negative 33s values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the noranda camp vms deposits the contribution of seawater sulfate increased with the collapse and subsequent evolution of the noranda caldera an inference supported by select trace and major element analyses in particular higher concentrations of se occur in samples with 33s values closer to 0 as well as lower fe zn ratios in sphalerite suggesting lower pressures and temperatures of formation we also report a relationship between average au grade and 33s values within au rich vms deposits of the noranda camp whereby higher gold grades are associated with near zero 33s values from this we infer a dominance of igneous sulfur in the gold rich deposits either leached from the volcanic pile and or directly degassed from an associated intrusion

oxygen isotopic studies of the interaction between xenoliths and mafic magma voisey s bay intrusion labrador canada abstract sulfide mineralization in the voisey s bay intrusion labrador canada is closely associated with country rock xenoliths that have extensively reacted with basaltic magma in order to better understand the processes that control the assimilation of country rocks by mafic magma a detailed study of oxygen isotope systematics related to magma country rock interaction in the voisey s bay area was undertaken protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths 2 10 cm in diameter composed of ca rich plagioclase corundum hercynite and minor magnetite overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix the 18o values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are plagioclase 8 7 12 3 orthoclase 9 5 9 8 biotite 5 2 8 7 garnet 8 3 10 8 pyroxene 8 0 10 1 and quartz 9 6 14 0 the 18o values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths hercynite plagioclase and corundum separates from the xenoliths have 18o values that vary from 2 9 to 10 5 5 6 to 10 9 and 2 0 to 6 8 respectively although a siliceous 18o enriched melt has been lost from the xenoliths corundum and feldspar 18o values are significantly lower than expected through melt loss alone the relatively low 18o values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a 18o value of 5 5 to 6 0 the high 18o melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths 18o values from 6 2 to 10 7 and in hercynite that replaced corundum however mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system 18o and d values of biotite surrounding the plagioclase overgrowth range from 5 0 to 6 2 and 58 to 80 respectively these data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma the current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high 18o partial melt from the xenoliths the feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma the minerals within the xenoliths are not in oxygen isotopic equilibrium with each other due in part to rapid thermal equilibration partial melting and partial exchange with flow through magma

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

in situ measurements of moisture and salt movement in freezing soils the paper presents the results of field studies on the movement of moisture and salts during freezing of prairie soils it is shown that large fluxes of water can migrate to the freezing front and move upward into the frozen soil above the fluxes are largest in light textured soils e g silt loam having a water table at shallow depth however substantial amounts of soil moisture may also move in silty clay silty clay loam and clay soils under dryland farming provided there is sufficient water present to support capillary flow the dynamics of soil moisture transfer under natural conditions as a result of freezing involves movement of water in both vapor and liquid phases in the shallow surface layer of soil to a depth of 300 400 mm vapor flow predominates in the depth below water usually moves primarily as a liquid it is demonstrated that the accumulation of ice with time increases because of the downward movement of the freezing front and the upward movement of water into the frozen soil above in a silt loam with large fluxes the ice content of the frozen zone rapidly reaches a level 80 85 pore saturation where measurable migration ceases conversely in a silty clay the movement of moisture into the frozen soil is observed to continue throughout most of the freezing period and the ice content reaches 93 pore saturation the greater movement in the finer grained soil is attributed to a higher freezing point depression a larger number of capillary pores and a higher concentration of soluble salts in the liquid films a close association is observed between changes in the ice content and electrical conductivity of a silt loam after freezing in a silty clay the agreement is less clear probably the result of the exchange of ions between the migrating liquid water and the clay particles maximum amounts of exchangeable ions moving into a 1 m depth of soil by the freezing action are estimated to be 11 9 t ha in a silt loam and 15 7 t ha in a silty clay loam data showing the redistribution of water and salts during thawing are also presented and discussed

geochemical and nd pb isotopic evidence for the provenance of the early proterozoic virginia formation minnesota implications for the tectonic setting of the animikie basin the early proterozoic animikie basin is located at a major crustal boundary between archean crust of the superior craton and penecontemporaneous early proterozoic crust of the wisconsin magmatic terranes the provenance of sedimentary rocks within the animikie basin constrains the tectonic evolution of the early proterozoic margin of the superior craton published geological petrographic and nd pb isotopic data indicate that the pokegama quartzite lower passive margin unit of the animikie group has a late archean provenance in contrast the virginia formation upper deep water turbidite shale unit of the animikie group was derived from dominantly early proterozoic sources with isotopic compositions similar to igneous rocks of the wisconsin magmatic terranes shales of the virginia formation have nd depleted mantle model ages 2 14 to 2 35 ga and mu 1 of 7 81 to 8 09 ashes within the virginia are more evolved geochemically than shales and have younger model ages of 1 86 and 1 99 ga and simila

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

magma mixing and constitution zone refining in the lac des iles complex ontario genesis of platinum group element mineralization the lac des iles complex is a 2 74 ga old composite intrusion emplaced into a granite and or granite gneiss terrane of the wabigoon subprovince northwestern ontario the complex consists of a gabbroic and ultramafic part the latter has been subdivided into northern and southern intrusive phases both the gabbroic and ultramafic parts contain platinum group element bearing ni cu sulfide mineralization although only the roby zone in the gabbro is of economic importance the southern ultramafic complex consists of a wehrlite to olivine clinopyroxenite core which is surrounded by websterite and gabbronorite cumulates petrographic and geochemical studies suggest that the northern ultramafic complex consists of at least eight cyclic units which ideally are composed of dunite and wehrlite to olivine clinopyroxenite cumulates at the base followed by clinopyroxenite and websterite locally grading into gabbronorite this indicates similar crystallization sequences in both ultramafic centers olivine olivine clinopyroxene clinopyroxene clinopyroxene orthopyroxene or plagioclase in the ultramafic complex platinum group element bearing sulfide mineralization occurs at the top of cyclic units in websterite and orthopyroxenite cumulates sulfide bearing samples up to 1 wt sulfide are enriched in au pd pt and cu relative to ni ir os and ru with pd ir ratios on the order of 250 sulfide immiscibility probably was caused by mixing of fractionated residual melt with more primitive liquid which periodically intruded the magma chamber evidence of mixing is seen in gradational chemical variations across the interface of cyclic units and in the occurrence of inclusions of olivine crystals and crystal aggregates in websterite and gabbronorite cumulates these inclusions are not in equilibrium with their present host rock and presumably formed before mixing in the more primitive magma a mineralized orthopyroxenite cumulate at the top of cyclic unit 2 could not have been produced by the normal crystallization sequence consideration of the phase relationships in the ternary system olivine clinopyroxene plagioclase quartz suggests that these cumulates are the crystallization product of a hybrid magma formed during magma mixing at the interface of cyclic units the gabbroic complex consists mainly of largely altered gabbro and gabbronorite cumulates and is intruded by mafic ultramafic dikes the center of the gabbroic complex contains the economically important roby zone with its platinum group element bearing cu ni sulfide mineralization this is a lithologically and texturally very complex zone in which high grade platinum group element mineralization is associated with pegmatite and mafic ultramaflc dikes the roby zone is characterized by strong enrichments of au pd and pt relative to ir and os and by variable but extremely high pd ir ratios of greater than 10 000 high au pt and pd pt ratios indicate a fractionation of pt from au and pd it is suggested that the high volatile content of the residual magma of the gabbroic complex triggered partial remelting of surrounding gabbro cumulates a process called constitutional zone refining by mcbirney 1987 during the partial melting event compatible elements such as ir and os are buffered by the residual silicate and oxide mineralogy whereas incompatible elements such as au pd and pt as well as volatile components such as h 2 o and h 2 s become highly concentrated in the partial melt phase eventually this newly formed magma may become sulfide saturated either by cooling and fractional crystallization or due to the continuous dissolution of sulfides when the zone of melting proceeds through the gabbro cumulates ratios such as pd ir and pd pt would strongly depend on the degree of remelting and fractional crystallization until the time of sulfide saturation the matrix of the varitextured zone is interpreted as the residuum of this melting process whereas pegmatoids and pegmatites segregationlike gabbro phases and quartz bearing plagioclase and incompatible element rich dikes are regarded as the final crystallization products of the partial melting

sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the babbitt and serpentine cu ni deposits duluth complex minnesota sulfur and oxygen isotope compositions of two pelitic virginia formation xenoliths and surrounding igneous rocks have been measured in drill core intersects at the babbitt and serpentine cu ni deposits duluth complex the xenolith at the babbitt deposit xenolith 1 shows compositional and textural evidence for partial melting whereas the xenolith at the serpentine deposit xenolith 1 may still be in part connected to the footwall and primary sedimentary mineral assemblages are present in its center the delta 34 s values of the xenoliths range from 6 to 27 per mil a well developed vertical profile of upward increasing delta 34 s values in xenolith 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary environment where the rate of sulfate reduction was rapid relative to sulfate supply the extreme variation in delta 34 s values over a vertical interval of less than 20 m illustrates the potential variability of delta 34 s values in the virginia formation the delta 34 s values of magmatic sulfides that formed in response to s assimilation may be equally variable depending upon local sites of contamination and convective homogenization in the magma in situ conservation of sulfur is suggested for xenolith 2 where the fe mg ratio of biotite decreases with increasing bulk rock sulfur content sulfidation reactions involving iron bearing silicates or oxide minerals limited the amount of sulfide liberated by the xenolith assimilation of country rock sulfide occurred either at depth within crustal staging chambers or during magma ascent an oxygen isotope gradient is well developed at the upper contact between gabbronorite norite and xenolith 2 oxygen isotope exchange fronts are poorly developed at the lower contact of xenolith 2 and at both contacts of xenolith 1 at the babbitt deposit the igneous rock at the lower contact is a compositionally distinct troctolite that shows little evidence for country rock contamination intrusion of distinct thin magmatic pulses that cool rapidly after previous devolatilization and possible partial melting of the xenoliths can cause the obliteration of well developed isotopic profiles the lack of concentration gradients that coincide with the oxygen isotope gradient together with results of modeling of mass transfer by diffusive processes suggests that the observed delta 18 o profile was produced during subsolidus cooling the observed profile may be produced by oxygen isotope exchange via fluid assisted grain boundary diffusion within 500 000 yr a duration consistent with the 0 5 to 1 m y time span estimated for emplacement of the major intrusions of the duluth complex

glaciations in the united kingdom the basic objective of project 24 was correlation of glaciations this inevitably involves consideration also of interglacials and interstadials as well as periods of cold climate shown by biological indicators or by oxygen isotope ratios in deep sea cores but not cold enough to generate glaciers in this country before giving conclusions about correlation which have been reached during the last 8 9 years it is important to summarise what progress has been made in absolute dating which if reliable is the best tool for correlation

re os sm nd and pb isotopic constraints on mantle and crustal contributions to magmatic sulfide mineralization in the duluth complex abstract previous petrologic and stable isotopic studies of sulfide mineralization in the duluth complex have led to the premise that sulfide genesis is strongly linked to the interaction between mantle derived magmas and sedimentary country rocks in a rift zone environment in order to more fully evaluate the nature of this interaction and to gain an insight into the possible importance of externally derived metals in the ore forming process pb sm nd and re os isotopic studies of the babbitt cu ni deposit were initiated rock types examined include low sulfide troctolite that show very little petrologic evidence for contamination disseminated sulfide bearing troctolitic to gabbroic rocks that occur close to the basal contact with country rocks and contain metasedimentary xenoliths and massive sulfide lead isotopic values of whole rocks plagioclase mineral separates and massive sulfides show only subtle differences and are compatible with 3 to 5 contamination of a mantle derived melt with a proterozoic crustal contaminant e nd 1 1 ga values of the troctolite and gabbro samples are chondritic and only the massive sulfides show strong evidence for contamination based on sm nd isotopic values massive sulfides tend to be more sensitive indicators of contamination in the sm nd system because of late stage incorporation of a light rare earth element rich fluid into a ca po 4 apatite component of the immiscible sulfide liquid os 1 1 ga values are also strongly anomalous and range from 500 to 1200 in disseminated sulfide bearing troctolites and massive sulfides these values are also consistent with from 1 to 3 contamination by c rich and strongly radiogenic proterozoic sedimentary rocks elemental and isotopic mass balance calculations suggest that up to 50 of the pb and at least 35 of the os in the sulfide mineralization have been derived from external sources a corollary is that other metals may also be in part derived from external sources which would aid in explaining the compositional diversity of the sulfide mineralization selective assimilation of country rocks by mantle derived magmas may occur in crustal staging chambers during ascent to shallower levels or in situ as a result of devolatilization reactions and partial melting

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

chemical and mineralogical heterogeneity in the basal zone of the partridge river intrusion implications for the origin of cu ni sulfide mineralization in the duluth complex midcontinent rift system copper nickel sulfide mineralization in the partridge river intrusion of the 1 1 ga duluth complex is restricted primarily to a 100 m thick zone near the base of the intrusion which is heterogeneous at meter scales in terms of both sulfide contents and rock types which include dunite melatroctolite troctolite leucotroctolite gabbro olivine gabbro gabbronorite and rare norite olivine rich troctolites and melatroctolites appear to have required mineral accumulation on a substrate whereas augite troctolite and gabbros are thought to have formed via in situ crystallization of magmas ranging in composition from high al olivine tholeiite to high ti tholeiite 18o values of orthopyroxene poor rocks in the partridge river intrusion range from 5 2 to 6 7 18o values of 6 7 are consistent with less than 20 contamination by high 18o metasedimentary country rock either via devolatilization or local partial melting rocks with greater than 15 orthopyroxene gabbronorites and norites are characterized by 18o values in excess of 6 9 and required the assimilation of larger amounts of siliceous country rocks sulfur isotopic values in leucotroctolitic rocks that contain less than 400 ppm s and that overlie the basal zone range between 1 5 and 2 values that are consistent with those of mantle derived sulfur in contrast 34s values in the basal zone range from 1 4 to 10 5 where the 34s enriched samples require an input of sulfur from metasedimentary country rocks 34s values of the rocks in the basal zone correlate with variations in olivine fo content but not with s abundance the wide range in 34s values of rocks in the basal zone strongly suggests that magmas interacted with layers in the sedimentary country rocks that were themselves characterized by variable sulfide contents and 34s values the s isotopic data suggest that the heterogeneity observed in the basal zone results from the emplacement of relatively thin sheets of compositionally distinct magma all rock types present in the basal zone can be produced as a result of variable degrees of fractionation of a parental high al olivine tholeiite followed by varying degrees of contamination of derivative liquids by country rocks the s contamination process was essential for the development of cu ni mineralization and was restricted to the earliest stages in the development of the duluth complex at a time when volatile species such as s and h2o and low t partial melts of country rocks were available to magmas

discovery of distal ejecta from the 1850 ma sudbury impact event a 25 70 cm thick laterally correlative layer near the contact between the paleoproterozoic sedimentary gunflint iron formation and overlying rove formation and between the biwabik iron formation and overlying virginia formation western lake superior region contains shocked quartz and feldspar grains found within accretionary lapilli accreted grain clusters and spherule masses demonstrating that the layer contains hypervelocity impact ejecta zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca 1878 ma and 1836 ma the sudbury impact event which occurred 650 875 km to the east at 1850 1 ma is therefore the likely ejecta source making these the oldest ejecta linked to a specific impact shock features particularly planar deformation features are remarkably well preserved in localized zones within the ejecta whereas in other zones mineral replacement primarily carbonate has significantly altered or destroyed ejecta features

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

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

re os isotopic composition and pge contents of proterozoic carbonaceous argillites virginia formation northeastern minnesota abstract carbonaceous sulfidic argillites from the proterozoic virginia formation in minnesota have been investigated in terms of platinum group elements pge distribution and re os isotopic signatures kerogen from protoliths and slightly metamorphosed equivalents is characterized by re and os enrichment factors relative to whole rock concentrations that range from 4 to 272 re os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1 85 ga reference isochron this age is consistent with that estimated for the virginia formation and suggests that dating of organic rich precambrian sedimentary rocks and the retrieval of data pertinent to the os isotopic evolution of seawater may be possible using the re os isotopic system