petrology mineral chemistry and geochemistry of late triassic ni cu ore bearing mafic ultramafic intrusions hongqiling northeastern china petrogenesis and tectonic implications the hongqiling magmatic ni cu sulfide deposit situated on the southern margin of the eastern central asian orogenic belt caob is composed of over 30 mafic ultramafic intrusions these ore beari

spinels and mg ilmenites from the noril sk 1 and talnakh intrusions and other mafic rocks of the siberian flood basalt province spinels from the mineralized noril sk 1 and talnakh intrusions show a broad and distinctive compositional spectrum from normal chromite to ti enriched aluminous magnetite in the talnakh intrusion in particular the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase spinels enclosed in olivines are less ti enriched than those outside olivine grains compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid the advanced nature of the process is attributed to the open nature of the intrusions continuous prolonged flow through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness the barren chibacheka river intrusion shows some of the same features but differs from talnakh in that spinel olivine pairs record equilibration temperatures higher than those at talnakh temperatures recorded by spinel olivine pairs at talnakh are anomalously low for such a small intrusion again indicating prolonged slow cooling consistent with a flow through model for the sill spinel compositions support a hypothesized comagmatic relationship between the chibacheka river intrusion and the ti cr rich gudchichinsky picritic basalts spinels in the noril sk and talnakh intrusions imply a parent magma with a lower cr al ratio than the gudchichinsky picritic basalts spinels from the siberian intrusions show a similar range in compositions to the feeder intrusions of the karoo flood basalt province jinchuan spinels share the features of high ti contents and anomalously low equilibration temperatures with the noril sk talnakh spinels mg ilmenites are a feature of the siberian and jinchuan intrusions they are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding mg rich cumulus phases their existence implies moderately but not unusually ti rich parent magmas consistent with compositions of the siberian traps lavas

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

controls on the composition of ni cu sulfide deposits as illustrated by those at noril sk siberia the ni cu pge ore deposits of the noril sk region have developed in feeder channels for permo triassic continental flood basalt volcanism particularly within 100 to 300 m thick elongate stratified intrusions that merge with an extensive system of peripheral sills through which the magma is thought to have continued to surface the mineralization is centered within two ore junctions the noril sk and talnakh ore junctions principal ore types include massive ore which occurs typically below or at the lowest margin of the intrusions disseminated ore in picritic and taxitic gabbrodolerite which constitute the lower part of the intrusions and copper ore which either forms an impregnation in hornfelsic country rock adjacent to massive ore or occupies the matrix of breccia zones that have developed at the front and upper contact of the intrusions although showing a close spatial relationship to the intrusions the massive ore particularly that underlying the northwest talnakh intrusion appears to have been injected separately over 500 samples have been analyzed for ni cu s pt pd rh ru ir os and au by nis fire assay acid dissolution and neutron activation the massive ore of the kharayelakh orebody beneath the northwest talnakh intrusion has fractionated as injection proceeded from east to west with the fractionated liquid moving progressively farther west this liquid has also soaked into the surrounding hornfels to produce the cu rich mantle and above the western part of the kharayelakh orebody where the cu enrichment is most pronounced has also affected the lower part of disseminated ore zones within the intrusion study of the disseminated ore has revealed that this has not fractionated to a great degree and that adequate sampling gives an average composition close to that of the primary sulfide liquid disseminated ore from different parts of the talnakh ore junction are characterized by very different metal tenors with rh ranging from 4 000 to 10 000 ppb the composition of the disseminated ore is modeled as the result of initial low tenor sulfides becoming trapped in the ore bearing intrusions and then interacting with successive surges of new magma passing along the feeder channels represented by their host intrusions on the way to the surface the different tenors of the different ore junctions and parts of ore junctions are the consequence of the effective number of times the magma passing through a particular channel exceeds the amount of sulfide within the channel higher ratios leading to higher tenors

compositional variations of olivine from the jinchuan ni cu sulfide deposit western china implications for ore genesis the jinchuan ni cu sulfide deposit is hosted by an elongated olivine rich ultramafic body that is divided by subvertical strike slip faults into three segments central eastern and western the central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides pyrrhotite pentlandite chalcopyrite intergrowth whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts in all segments sulfides are concentrated at the expense of intercumulus silicates olivine re crystallization is found to be associated with actinolite alteration in some samples the compositional variations of primary olivine from the sulfide poor samples can be explained by a small degree of olivine crystallization 50 modal and sulfide averaging 5 wt in the rocks are consistent with the interpretation that the jinchuan deposit was formed by olivine and sulfide laden magma successively ascending through a conduit to a higher now eroded level sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have in part resulted from flow differentiation and gravitational settling during magma ascent respectively

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

8 precambrian geodynamics and ore formation the fennoscandian shield abstract compared with present day global plate tectonics archaean and palaeoproterozoic plate tectonics may have involved faster moving hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle this scenario also fits with the complex geodynamic evolution of the fennoscandian shield from 2 06 to 1 78 ga when rapid accretion of island arcs and several microcontinent continent collisions in a complex array of orogens was manifested in short lived but intense orogenies involving voluminous magmatism with a few exceptions all major ore deposits formed in specific tectonic settings between 2 06 and 1 78 ga and thus a strong geodynamic control on ore deposit formation is suggested all orogenic gold deposits formed syn to post peak metamorphism and their timing reflects the orogenic younging of the shield towards the sw and west most orogenic gold deposits formed during periods of crustal shortening with peaks at 2 72 to 2 67 1 90 to 1 86 and 1 85 to 1 79 ga the ca 2 5 to 2 4 ga ni cu pge deposits formed both as part of layered igneous complexes and associated with mafic volcanism in basins formed during rifting of the archaean craton at ca 2 5 to 2 4 ga svecokarelian ca 1 89 to 1 88 ga ni cu deposits are related to mafic ultramafic rocks intruded along linear belts at the accretionary margins of microcratons all major vms deposits in the fennoscandian shield formed between 1 97 and 1 88 ga in extensional settings prior to basin inversion and accretion the oldest cyprus type deposits were obducted onto the archaean continent during the onset of convergence the pyhasalmi vms deposits formed at 1 93 to 1 91 ga in primitive bimodal arc complexes during extension of the arc in contrast the skellefte vms deposits are 20 to 30 million years younger and formed in a strongly extensional intra arc region that developed on continental or mature arc crust deposits in the bergslagen uusimaa belt are similar in age to the skellefte deposits and formed in a microcraton that collided with the karelian craton at ca 1 88 to 1 87 ga the bergslagen uusimaa belt is interpreted as an intra continental or continental margin back arc extensional region developed on older continental crust iron oxide copper gold iocg deposits are diverse in style at least the oldest mineralizing stages at ca 1 88 ga are coeval with calc alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone younger mineralization of similar style took place when s type magmatism occurred at ca 1 80 to 1 77 ga during cratonization distal to the active n s trending subduction zone in the west possibly interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation finally the large volumes of anorthositic magmas that characterize the sveconorwegian orogeny formed a major concentration of ti in the sw part of the sveconorwegian orogenic belt under granulite facies conditions about 40 million years after the last regional deformation of the sveconorwegian orogeny between ca 930 and 920 ma

ni cu pge magmatic sulfide deposits in the yangliuping area permian emeishan igneous province sw china the ni cu pge sulfide deposits in the yangliuping area sw china are hosted in mafic ultramafic sills the four mineralized sills are located in the yangliuping tectonic dome and intrude devonian carbonaceous marble graphitic schist the sills are 200 300 m thick and 1 000 2 000 m in strike length and now consist chiefly of serpentinite talc schist tremolite schist and meta gabbro disseminated ni cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite although the sulfide ores have been modified by hydrothermal activity there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores the yangliuping intrusions and the dashibao formation have similar primitive mantle normalized trace element and platinum group element pge patterns indicating that they are derived from a common parental magma type the positive correlation between cu concentrations and cu zr ratios of the dashibao formation basalts indicates that the chalcophile elements were removed before eruption we propose that fractional crystallization of the yangliuping magma accompanied by the introduction of s and co2 from the wall rocks caused the magma to become s saturated leading to the segregation of magmatic sulfides that became enriched in ni cu pge the sills acted as conduits for the overlying dashibao formation basalts with the sulfide liquid along with early crystallizing olivine and pyroxene segregating from the magma as it passed through the conduits prior to eruption

reducing subjectivity in multi commodity mineral prospectivity analyses modelling the west kimberley australia predicting realistic targets in underexplored regions proves a challenge for mineral explorers knowledge driven prospectivity techniques assist in target prediction and can significantly reduce the geographic search space to a few locations the mineral prospectivity of the underexplored west kimberley region was investigated following interpretation of regional gravity and magnetic data emphasis was placed on identifying geological structures that may have importance for the mineral prospectivity of the region subsurface structure was constrained through combined gravity and magnetic modelling along three transects crustal scale structures were interpreted and investigated to determine their depth extent these interpretations and models were linked to tectonic events and mineralization episodes in order to map the distribution of minerally prospective regions using a knowledge driven mineral systems approach a suite of evidence layers was created to represent geological components that led to mineralization and then applied to each mineral system where appropriate this approach was taken to provide a more objective basis for prospectivity modelling the mineral systems considered were 1 magmatic ni sulphide 2 carbonate hosted base metals 3 orogenic au 4 stratiform hosted base metals and 5 intrusion related base metals including sn w fe oxide cu au and cu au porphyry deposits these analyses suggest that a geologically complex belt in the kimberley basin at the boundary to the king leopold orogen is prospective for magmatic related hydrothermal mineral systems including ni au and cu the lennard shelf is prospective for carbonate hosted base metals around a feature known as the 67 mile high and parts of the king leopold orogen are prospective for stratiform hosted base metals these results show that knowledge driven mineral system modelling is effective in identifying prospectivity in regional scale studies of underexplored areas as well as drastically reducing the search space for explorers working in the west kimberley

a review of the cu ni sulphide deposits in the chinese tianshan and altay orogens xinjiang autonomous region nw china principal characteristics and ore forming processes abstract several cu ni sulphide deposits and occurrences have recently been discovered along parallel deep faults in the chinese tianshan and altay orogenic belts xinjiang province nw china the kalatongke and several other cu ni mineralized intrusions are located along the irtysh fault which separates the altay orogenic belt from the junggar basin the huangshan huangshandong xiangshan tudun erhongwa tula ergen and hongling deposits occur along the kanggurtag suture which separates the jueluotage orogenic belt from the turpan hami tuha basin the baishiquan tianyu and tianxiang deposits are located along the arqikekuduke fault which separates the jueluotage orogenic belt from the central tianshan precambrian terrane the poyi poshi and luodong deposits are located along the baidiwa fault which separates the central tianshan precambrian terrane from the beishan paleozoic rift re os dating of cu ni sulphide ores reveals that these cu ni ore belts formed in a narrow age range of 298 282 ma this age range is about the same as those of associated intrusions and dykes dated by the shrimp zircon u pb method tectonic and geochronological constraints suggest that the amalgamation of continental blocks mainly occurred during the late carboniferous in the central asian orogenic belt large scale hydrothermal and magmatic metallogenesis in the region occurred during post collisional stages of latest carboniferous to early permian age the cu ni sulphide deposits are part of this metallogenic event the cu ni orthomagmatic sulphide deposits in northern xinjiang are represented by 1 net textured type deposits by segregation of a cu ni metal sulphide melt and 2 magma conduit type some mafic ultramafic suites exhibit lithological zoning caused by strong differentiation stratiform orebodies are hosted by ultramafic rocks at the base of the magma chamber good examples are the kalatongke huangshandong and poshi deposits others such as the xiangshan baishiquan tianyu and tula ergen deposits are hosted by magma conduits consisting of peridotite troctolite and pyroxenite these ultramafic rocks either occur within faults or are surrounded by gabbroic and dioritic intrusions these two types of orthomagmatic cu ni sulphide deposits are also distributed along the same ore belts for instance the differentiated sill related huangshandong deposit co exists with the magma conduit type tula ergen deposit in the jueluotage orogenic belt orthomagmatic cu ni sulphide deposits in northern xinjiang formed during post collisional extension and are possibly related to a late carboniferous early permian mantle plume event the mafic ultramafic suites and associated cu ni deposits are commonly accompanied by dyke swarms and are characterized by elongated outcrops occurring along parallel e w trending regional faults these mafic ultramafic suites and accompanying dyke swarms are generally fractionated implying that they were feeders of presently eroded flood basalts

the formation of low volume high tenor magmatic pge au sulfide mineralization in closed systems evidence from precious and base metal geochemistry of the platinova reef skaergaard intrusion east greenland the majority of magmatic platinum group element pge deposits in layered mafic ultramafic intrusions are characterized by visible cu ni sulfide mineralization often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination the platinova reef hosted by the 55 ma skaergaard intrusion east greenland is an example of a rarer type of pge deposit typified by a pd au cu dominant ni pt poor sulfide assemblage present in the upper parts of the host intrusion such deposits are generally considered to form through prolonged fractional crystallization of the magma with magnetite crystallization playing a role in late stage s saturation in the case of the skargaard intrusion the crystallization of the cumulate rocks and the generation of the mineralization hosted within them occurred within a closed system from a single homogenized batch of magma the platinova reef is characterized by offsets in the stratigraphic position of individual metal peaks an anomalously low sulfide content 1 ppm the intermediate zone pd 0 1 1 ppm the au zone au 1 ppm and the upper cu zone cu 200 ppm with precious metals 3 to 10 4 ppm low cu pd ratios and a remarkable upward decrease in pd au ratio from 40 to 1 whereas the sulfides in the cu zone are lower tenor 10 3 ppm pd higher volume and have high cu pd ratios and pd au to explain these features we present a multistage model of preconcentration dissolution upgrading and further metal enrichment this involves initial s saturation of the skaergaard magma at a late stage in the crystallization history the sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter fe rich and s undersaturated magma at the bottom of the chamber producing a highly metal rich magma layer at the bottom of the chamber this layer then became s saturated as a result of continued magnetite crystallization with the tiny sufide droplets that segregated into this precious metal enriched magma layer becoming highly enriched in pge themselves these were then trapped in situ by the crystallizing cumulate pile forming the subzone pd zone and intermediate zone sulfides formed in the rest of the chamber which was pge depleted at this stage would have been of low tenor these sulfide droplets grew and settled concentrating any remaining au when they encounter the floor to form the au zone and formed the low tenor sulfides of the cu zone above this we propose that the platinova reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through sulfide dissolution upgrading in the latter stage of crystallization of a closed system the offsets in individual metal peaks mineralization demonstrate the relative partitioning of pd pt au cu into a sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low volume magmatic sulfide melts this feature can lead to the characteristic stratigraphic offsets in peak metal abundances the role of magnetite crystallization appears to be an important control on s saturation in such systems and thus may be an important indicator of the location of mineralized horizons in addition in such deposits indicators of crustal contamination by external s and multiple injections of magma common features of more ni rich pge reefs are less likely to be present

mineralogical and isotopic constraints on the genesis of the jingchong co cu polymetallic ore deposit in northeastern hunan province south china abstract the jingchong co cu polymetallic deposit and a few other co polymetallic ore deposits are located in northeastern hunan province in the central segment of the jiangnan orogen south china these deposits occur along the ne trending changping changsha pingjiang deep fault zone and show a spatial association with the late jurassic early cretaceous lianyunshan granitoids which intruded the archean to paleoproterozoic lianyunshan group comprising metaclastic sedimentary rocks interlayered with mafic intermediate igneous rocks the jingchong deposit is hosted within an altered fracture zone at the footwall of the changping fault zone and comprises an upper pb zn ore zone and a lower co cu ore zone wall rock alteration is dominated by silicification and chloritic alteration three types of ores were identified i e quartz sulfide veins altered structural breccias and altered fractured rocks integrated field work and microscopy observations reveal a three stage hydrothermal process related to mineralization 1 quartz chlorite chli pyrite 2 quartz chlorite chlii pyrite chalcopyrite cobaltite and 3 quartz carbonate galena sphalerite pyrite chalcopyrite la icpms analyses reveal that the co contents are 1315 6473 ppm 76 3866 ppm and 5 435 ppm for pyrite from the first the second and the third stages respectively epma analyses of chlorite show that chli and chlii have fe fe mg ratios of 0 65 0 80 and 0 76 0 92 and aliv contents of 1 30 1 65 apfu atom per formula unit and 1 18 1 48 apfu respectively empirical geothermometry of chlorite yielded temperatures of 256 318 c av 281 c and 239 290 c av 256 c for first and second stage chlorite respectively the 34svcdt values of sulfides 4 9 to 0 2 av 1 6 suggest a magmatic source for the sulfur and the 3he 4he 0 017 0 30 ra and 40ar 36ar 285 306 ratios of fluid inclusions in pyrite suggest that the ore forming fluids were mainly crust derived magmatic hydrothermal fluids with possible meteoric water inputs the 206pb 204pb 18 042 18 746 207pb 204pb 15 592 15 741 and 208pb 204pb 38 237 39 150 ratios of ore sulfides are similar to those of the lianyunshan group suggesting that the metals may have been derived from the latter integrating the geological mineralogical and geochemical data documented in this study we propose that the jingchong co cu polymetallic deposit formed from a hydrothermal system related to the lianyunshan pluton which was derived from crustal anataxis of the lianyunshan group in a back arc extensional setting in the late mesozoic

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

origin of the inconsistent apparent re os ages of the jinchuan ni cu sulfide ore deposit china post segregation diffusion of os abstract apparent re os ages of some magmatic sulfide ore deposits are older than the zircon and baddeleyite u pb ages which are interpreted as the formation age of the host intrusions the jinchuan ni cu pge deposit of china the world s third largest is such a case we report apparent re os isochron ages of 1117 67 ma 1074 120 ma and 867 75 ma with initial 187os 188os ratios of 0 120 0 012 0 162 0 017 and 0 235 0 027 for disseminated ores sulfides from the disseminated ores and massive ores from jinchuan respectively using these data and re os ages from the literature we find that the oldest apparent re os age and lowest initial os isotope ratio are from disseminated ores which contain small amounts of sulfide minerals the highest initial os isotope ratios and youngest apparent re os ages consistent with the zircon and baddeleyite u pb ages are from massive ores containing 90 100 modal sulfide and net textured ores with about 25 modal sulfides yield apparent re os ages and initial os ratios intermediate between those of the disseminated and massive ores because os diffusion between sulfides is inhibited by the intervening silicates even at high temperatures re equilibration did not occur in the disseminated ore and the samples retained the os ratios of the contaminated magma leading to geologically meaningless ages that are older than the formation age of the rocks while os bearing sulfide minerals and magnetite show low closure temperatures of os diffusion and the sulfide minerals in the massive ore are closely connected with each other facilitating fast diffusion of os re equilibration of os was achieved during cooling of the ore from about 850 c after the segregation to about 400 c thus an age corresponding to the formation time and an elevated initial os ratio were yielded by the massive ore os isotopes in the net textured ore behave in the way intermediate between the disseminated and massive ores pb isotope data support the os results disseminated ores have heterogeneous pb isotope ratios whereas pb in the massive ores is more uniform consistent with pb isotopic equilibration in the massive ores but not in the disseminated ores

geochemistry of the siberian trap of the noril sk area ussr with implications for the relative contributions of crust and mantle to flood basalt magmatism the sequence investigated of the siberian trap at noril sk ussr consists of at least 45 flows that have been divided into six lava suites the lower three suites consist of alkalic to subalkalic basalts the ivakinsky suite overlain by nonporphyritic basalts the syverminsky suite and porphyritic and picritic basalts the gudchikhinsky suite the upper three suites are tholeiitic the uppermost 750 m of dominantly non porphyritic basalt belong to the mokulaevsky suite and are characterized by a nearly constant mg number 0 54 0 56 sio2 48 2 49 1 wt ce 12 18 ppm and ce yb 5 8 the underlying 1100 m of dominantly porphyritic basalt belong to the morongovsky and nadezhdinsky suites there is a continuous increase in sio2 48 1 55 2 wt ce 12 41 ppm and ce yb 5 18 from the top of the mokulaevsky to the base of the nadezhdinsky with little change in the mg number 0 53 0 59 mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid ocean ridge basalts the change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by mokulaevsky magma the nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the noril sk talnakh mining camp the mokulaevsky and morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the noril sk region these magmas may have risen up out of a different conduit system

u pb zircon geochronology and geochemistry of paleoproterozoic magmatic suite from east sarmatian orogen tectonic implications on columbia supercontinent abstract extensive paleoproterozoic magmatism occurred in the east sarmatian orogen eso located at the junction of the sarmatian and volga ural segments of the east european craton generating numerous felsic to mafic ultramafic complexes here we report geochemical and zircon u pb age data from a suite of ultramafic mafic and diorite granodiorite intrusive rocks from this orogen shrimp u pb dating of zircons yielded the following concordant ages for the magmatic suites 1 2085 17 to 2093 20 ma sub volcanic high mg intrusions of shmb like melanorite melaquartz diorite melagranodiorites and quartz diorite tonalite granodiorites 2 2072 4 8 2073 8 3 and 2068 13 ma ultramafic mafic diorite layered intrusions 3 2053 14 to 2058 22 ma magnesian ferroan quartz diorite tonalite granodiorite intrusions and 4 2035 18 ma ferroan norite diorite intrusions the trace and rare earth element geochemistry indicate arc related features the enrichment in lree and lile in the mafic ultramafic and diorite granodiorite suites is correlated with melting of metasomatised lithosphere mantle along with involvement of crustal components in their petrogenesis the third magmatic event conforms to the age of the post collision s and a granitoids of the region our data indicate two major magmatic events in the post collisional setting the former immediately after metamorphism with the formation of schistosity in the host rocks and the latter two during the culmination of the orogenic cycle in the voronezh crystalline massif the duration of the paleoproterozoic orogeny during the collision of the sarmatian and volga ural segments of the east european craton is estimated to be approximately 100 million years from 2140 to 2035 ma we propose that the layered orogenic plutons were derived from relatively deeper mantle source whereas the sub volcanic rock series are derivatives of the slightly shallow mantle source the paleoproterozoic collision in the region broadly coincides with timing of amalgamation of the global supercontinent columbia

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

high precision u pb zircon baddeleyite dating of the j m reef platinum group element deposit in the stillwater complex montana usa the stillwater complex montana usa is one of the world s largest layered intrusions and hosts the johns manville j m reef a world class platinum group element pge deposit in this study zircon and baddeleyite were successfully extracted from three mineralized samples of the j m reef the samples which were collected over 15 km of strike length of the deposit provide high precision crystallization ages for the reef and the stillwater complex all samples are coarse grained to pegmatitic and range in composition from olivine bearing gabbronorite to anorthosite and troctolite irregular anhedral morphology and prominent sector zoning characterize interstitial zircon u pb crystallization zircon ages determined by chemical abrasion thermal ionization mass spectrometry ca tims for the three samples are indistinguishable within uncertainty frog pond adit 2709 11 0 56 ma east boulder mine 2709 28 0 32 ma and west fork area 2709 00 0 45 ma a u pb baddeleyite date from the frog pond adit sample 2708 85 0 46 ma is identical within uncertainty with the zircon dates collectively the results illustrate that the j m reef is an intrusion wide time marker that formed at ca 2709 ma during crystallization of the lower banded series of the stillwater complex the recognition that zircon can be successfully extracted from mafic ultramafic rocks associated with magmatic ore deposits provides new opportunities for testing the timing of mineralization and duration of mineralization processes in layered intrusions worldwide

zoned orthopyroxenes in the ni co sulfide ore bearing xiarihamu mafic ultramafic intrusion in northern tibetan plateau china implications for multiple magma replenishments abstract the xiarihamu mafic ultramafic intrusion is situated in the east kunlun orogenic belt e kob northern tibetan plateau china with ni co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite whole rock and mineral compositions show cyclic variations in drill hole intersections through the xiarihamu mafic ultramafic intrusion microbeam x ray fluorescence xrf mapping shows reverse zoning cr poor core cr rich rim and oscillatory zoning cr poor core cr rich mantle and cr poor rim in orthopyroxene of olivine orthopyroxenite and orthopyroxenite zoned orthopyroxenes are euhedral or subhedral and there are relatively sharp interfaces between zones with distinct cr contents crystal size distribution csd curves of orthopyroxenes in olivine orthopyroxenite show concave upward curvature at size larger than 3 0 mm and larger orthopyroxenes commonly display zoning implying a secondary recrystallization and grain growth of larger orthopyroxenes quantitative analyses show that cr and enstatite contents of cores are relatively lower but incompatible element e g ti zr ga and sr contents of cores are higher than rims in reverse zoned orthopyroxenes and chemical profiles of unzoned orthopyroxenes resemble those of rims but different from those of cores in reverse zoned orthopyroxenes chemical profiles of orthopyroxenes indicate that cr zoning is not due to the change in cr partitioning behavior between orthopyroxene and silicate melt which shows significant temperature and redox dependence textural feature and crystal size distribution of orthopyroxenes suggest the formation of cr rich zones attests to the multi stage growth in the process of magma replenishment rather than a reactive liquid flow model or in situ trapped liquid overgrowth orthopyroxene zoning together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the xiarihamu mafic ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition

magma generation and sulfide saturation of permian mafic ultramafic intrusions from the western part of the northern tianshan in nw china implications for ni cu mineralization compared to permian magmatic ni cu sulfide deposits in the eastern part of the northern tianshan such as huangshan and huangshandong those in the western part e g baixintan and lubei are poorly documented zircon id tims u pb age data 279 7 0 2 ma of lherzolite and la icpms u pb age data 277 8 3 4 ma of a hornblende gabbro suggest the lubei intrusion was emplaced in the early permian the baixintan and lubei mafic ultramafic intrusive rocks are characterized by enrichments of large ion lithophile elements and th and u relative to the high field strength elements as well as depletion of nb and ta relative to mid ocean ridge basalt the 87sr 86sr t 0 70354 0 70713 end t 2 80 to 7 05 and ehf t 2 50 18 4 values for the intrusions indicate that they have assimilated tianshan basement rocks whereas the se s ratios 181 10 6 418 10 6 and 34scdt values 0 13 to 0 90 of the sulfides suggest that crustal sulfur has not been incorporated into the magma the decrease in pd zr with increasing th nb pm indicates that sulfide saturation accompanied crustal contamination the olivine compositions and whole rock chalcophile elements of the baixintan and lubei intrusions imply that the parental magma of the intrusive rocks in the west contains higher chalcophile elements than those in the east which suggests that the mafic ultramafic intrusions in the western part of the northern tianshan should be regarded as favorable ni cu sulfide exploration targets

the early permian mafic ultramafic complexes in the beishan terrane nw china alaskan type intrusives or rift cumulates abstract the petrogenesis and geodynamic setting of the early permian mafic ultramafic complexes in the beishan terrane nw china have important bearing on the prospects of ni cu pge sulfide and ti fe oxide deposits as well as in understanding the history of evolution of the southern central asian orogenic belt caob here we present results from a detailed study on a representative suit of a mafic ultramafic rocks from the bijiashan complex in the beishan terrane the complex is composed of dunite troctolite olivine gabbro and gabbro without clinopyroxenite or hornblendite in addition to olivine and clinopyroxene orthopyroxene and plagioclase are also present in all these rock types in the absence of chromite or primary hornblende the minerals display marked compositional variations with the fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite the whole rock geochemistry shows good correlations among the major elements and the trace elements are characterized by flat ree patterns with eu positive anomalies and hfse depletion relative to lile these features are comparable to other mafic ultramafic complexes in the beishan and eastern tianshan terranes but obviously deviate from the typical features of alaskan type intrusives the present study suggests that the beishan complexes were formed in a post orogenic extensional environment with potential for ni cu mineralization rather than in arc related setting with alaskan type pge mineralization our study confirms that the beishan area does not conform to an arc related setting but was located within a rift setting in permian probably related to permian mantle plume event suggesting that the subduction of the paleo asian ocean had culminated by this time and the southern caob witnessed a post orogenic extensional regime in late paleozoic

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

formation of the jinchuan ultramafic intrusion and the world s third largest ni cu sulfide deposit associated with the 825 ma south china mantle plume we report here shrimp u pb geochronological geochemical and nd isotopic data for the jinchuan ultramafic intrusion gansu province china which hosts the world s third largest magmatic ni cu sulfide deposits u pb baddeleyite analyses yield an age of 812 26 ma for the ultramafic intrusion this age is indistinguishable within analytical uncertainties from the u pb zircon ages of 827 8 ma and 828 3 ma for the sulfide bearing ultramafic rocks and the dolerite dykes that cut the ultramafic intrusion respectively these u pb dating results show beyond doubt that the jinchuan ultramafic intrusion and associated ni cu sulfide deposit were formed at 825 ma rather than 1500 ma as has been widely believed the ultramafic rocks exhibit large negative nd t values 8 9 to 12 0 that decrease with increasing la sm suggesting that their parental magmas were derived from a long term enriched lithospheric mantle and experienced crustal contamination mineralogical petrological and geochemical data all indicate that the jinchuan intrusion was generated by melting of the enriched lithospheric mantle heated by an anomalously hot plume the u pb ages of 825 ma for igneous baddeleyites and zircons and 900 880 ma for inherited zircons in the jinchuan mafic ultramafic rocks are comparable with those in the qaidam block and qilian belt the western extension of the qinling belt that was likely derived from northern yangtze craton the jinchuan ni cu sulfide bearing intrusion along with coeval regional plume related mafic dykes and tholeiites and mafic ultramafic complexes with associated v ti and ni cu pge mineralization is interpreted to be genetically related to the 825 ma south china mantle plume

in situ strontium and sulfur isotope investigation of the ni cu pge sulfide ore bearing kevitsa intrusion northern finland the 2 06 ga kevitsa mafic ultramafic intrusion in northern finland hosts a large disseminated ni cu pge deposit the deposit occurs in the ultramafic olivine pyroxene cumulates and shows a range in ni tenors varying from 4 7 wt regular ore to 10 wt ni pge ore there are also a metal poor sulfide mineralization false ore and contact mineralization that are uneconomic ni tenor 0 7045 in comparison to the estimated depleted mantle sr isotope ratio of 0 702 at 2 06 ga the sulfur 34s values are generally higher than 2 which together with the sr isotope data imply involvement of crustal material in the genesis of the kevitsa intrusion and its ores the 87sr 86sr i values obtained from the ore bearing domain of the intrusion show stratigraphic variation and exceed 0 7050 with the maximum value reaching up to 0 7109 in contrast in rocks around the ore domain the initial sr isotope compositions remain more or less constant 0 7047 0 7060 throughout the intrusive stratigraphy the isotope data suggest that the ore bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma whereas the surrounding part of the intrusion experienced a less vigorous emplacement history no correlation is observed between the strontium and sulfur isotope compositions this is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the kevitsa magma chamber the low level of metals in false ore and the ni depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement cannibalization of early formed sulfides by later magma injections may have been important in the formation of the economic ore deposit

erratum to a new style of ni cu mineralization related to magmatic breccia pipes in a transpressional magmatic arc aguablanca spain the aguablanca deposit represents a new style of magmatic ni cu mineralization in discordant sulfide rich pyroxenitic breccia pipes the orebody is hosted by variscan calc alkaline diorites and gabbros which intruded during an oblique subduction collision event transpressional transtensional left lateral structures facilitated the intrusion of primitive magmas to shallow depths in the crust a two stage genetic model is proposed in the first stage a transitional deep magma chamber formed the primitive magma interacted at depth with wall rocks resulting in extensive crustal contamination concomitant sulfide magma immiscibility and settling of orthopyroxene clinopyroxene and sulfide rich cumulates to form a layered magmatic complex geochemical and mineralogical evidence including the virtual disappearance of olivine heavy 34s values 7 4 distinctive nd sr and pb signatures high au contents and the presence of spinel and graphite indicate a major interaction with the upper crust probably with pyrite rich carbonaceous slates of late proterozoic age the second stage was related with the emplacement of residual calc alkaline gabbroic to noritic melts and the development of an intrusive breccia containing fragments of the consolidated layered complex rocks and associated disseminated to massive sulfides

volcanic associated nickel deposits from around the widgiemooltha dome western australia volcanic peridotitc associated nickel sulfide deposits occur in archean komatiitic flow sequences around the widgiemooltha dome in the eastern goldfields region of western australia the deposits are typically small shoots 40 volatile free al 2 o 3 2 content of metamorphic fluids and the timing of fluid access to the rocks along the eastern side of the widgiemooltha dome host rocks have undergone serpentinization and variable but generally extensive prograde talc carbonate alteration e g redross dordie rocks north and widgiemooltha 3 deposits in the north munda deposit they have experienced serpentinization with minor carbonate alteration and in addition prograde metamorphic reconstitution to forsteritc talc rich assemblages dunitic units in the west wannaway and wannaway north deposits escaped prograde serpentinization talc carbonate alteration and significant metamorphic reconstitution but have been retrogressively serpentinized sulfides from the different alteration environments show systematic mineralogical and compositional differences at all deposits peak metamorphic temperatures 550 degrees 600 degrees c resulted in reversion of sulfides to a dominant fe ni co cu bearing monosulfide solid solution accompanied by pentlanditc or fe cu intermediate solid solution in some cases most of the present ore mineralogies and textures relate to exsolution of low temperature sulfides from metamorphic monosulfide solid solution with reequilibration and recrystallization during slow postmetamorphic cooling deformation produced a tectonite fabric in the ores characterized by preferred orientation of sulfide grains and mineral layering breccia ores were formed in response to brittle deformation in host rocks and continued ductile behavior in the sulfides deformation also affected the overall configuration of the ore shoots and led to significant sulfide mobilization and ore shoot distension after allowing for the metamorphic imprint a number of primary features of the ore environment can be isolated and there is strong evidence for the prior existence of magmatic sulfides the association of ores with early formed largely dunitic units similar bulk ni cu and ni co ratios for ores from different deposits precious metal data low s se ratios and distinctive magmatic spinels in the sulfides all point to a magmatic control on the ores strong positive correlation between ni and mgo in host units and similar ni mgo ratios in both mineralized and unmineralized units suggest that sulfide liquids formed and equilibrated with the parent magma at depth before being extruded as part of olivine rich crystal mushes

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

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

geology and geochemistry of intrusions and flood basalts of the noril sk region ussr with implications for the origin of the ni cu ores geochemical studies indicate that five principal magma types characterize the 3 8 km vertical thickness of flood basalt of the noril9sk region from base to top these are 1 a fractionated type with alkalic affiliations ivakinsky and syverminsky 2 an ni rich suite including picritic basalt gudchichinsky 3 a primitive but ni depleted suite with fiat rare earth element ree profiles and also including picritic basalts tuklonsky 4 a light ree enriched crustally contaminated suite lower nadezhdinsky nd 1 above 4 there is a transitional suite upper nadezhdinsky nd 2 and morongovsky mr 1 and mr 2 which is overlain by the fifth type a second primitive suite without picrites mokulaevsky similar but not identical in its chemistry to the tuklonsky type this is followed by tuffs and basalts of the kharayelakhsky kumginsky and samoyedsky suites which are largely composed of similar uncontaminated tholeiitic magma the latter three suites have not been studied by us intrusions of the region fall into five main groups 1 alkalic intrusions 2 ti rich dolerite dikes 3 dolerite sills and dikes 4 differentiated bodies not related to ore junctions and 5 differentiated bodies related to ore junctions the ni cu ores are associated with ni and cr rich type 5 olivine rich intrusions with flat ree profiles that are geochemicaly similar to the mokulaevsky magma light ree enriched ni and cr poor olivine rich type 5 intrusions with uneconomic mineralization occur closely associated with the strongly mineralized variants and resemble geochemically the basalts transitional between the lower nadezhdinsky and upper morongovsky they are cut by the strongly mineralized bodies indicating that the latter are of late morongovsky to mokulaevsky age the mineralized intrusions are associated with about 1 5 to 1 10 of their mass in sulfides this is a factor of 25 to 50 more than could have been carried in solution and deposited by in situ liquation from the magma of these intrusions the platinum group element pge concentrations in the ores require that the sulfides giving rise to them equilibrated with 15 to 200 times more magma than is represented by the intrusions much of the ore is very massive and crosscuts the host intrusions the sequence of igneous activity and mineralization in the immediate vicinity of noril9sk and talnakh from the base of the nadezhdinsky upward is thus interpreted to be 1 extrusion of highly crustally contaminated ni and cu poor basalt 2 extrusion of a series of basalt flows each progressively less contaminated and less ni and cu depleted than the preceding one accompanied by intrusion of contaminated ni depleted magma with weak mineralization 3 extrusion of uncontaminated ni and cu rich basalt accompanied by intrusion of uncontaminated ni rich magma with strong mineralization followed very closely by introduction of associated massive ore and 4 continued extrusion of ni and cu rich basalt it is proposed that crustal contamination of the original ni and cu rich magma occurred at the top of a vertically extensive fault controlled magma chamber and caused segregation of immiscible sulfides with a low r factor these depleted this magma in chalcophile elements and settled deeper in the chamber as they settled they reacted with less contaminated magma scavenging additional chalcophile elements they finally came to rest near the base of the chamber to form a zone of sulfide and thus chalcophile element enriched magma eruption and in some instances intrusion of progressively stratigraphically lower less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions the answer to the strong enrichment in ni cu and pge associated with the ore bearing intrusions is that these metals came from the magma that is now represented by the depleted basalts

geochemistry and petrogenesis of 270 ma ni cu pge sulfide bearing mafic intrusions in the huangshan district eastern xinjiang northwest china implications for the tectonic evolution of the central asian orogenic belt abstract the central asian orogenic belt caob is composed of remnants of microcontinents island arcs and ophiolites the presence of voluminous phanerozoic a type granites suggest that continental growth occurred through addition of juvenile crust and the presence of mafic ultramafic intrusions provides direct evidence for a mantle contribution the origin of the mafic ultramafic intrusions in the huangshan area eastern xinjiang has been debated because of the lack of precise age data or trace element and isotopic analyses we have studied the geology petrology geochemistry and geochronology of two of the intrusions the huangshanxi and huangshandong intrusions both are typical layered mafic ultramafic intrusions composed of diorite troctolite norite gabbronorite gabbro wehrlite websterite and lherzolite with systematic geochemical variations that are consistent with the following crystallization sequence ol chr ol pyx chr cpx pl and hbl bt igneous zircons separated from a diorite in the huangshanxi intrusion yield a 269 2 ma shrimp u pb age the rocks are enriched in rb ba u k sr relative to th and lree depleted in nb ta relative to th and exhibit positive end values 6 7 to 9 3 ni cu pge sulfide mineralization occurs in the lower parts of the intrusions primarily strata bound within wehrlite or lherzolite and locally stratiform along the margins of the intrusions the parental magmas appear to have been derived from a mantle source that was previously contaminated by subduction of oceanic crust and to have been variably contaminated by upper continental crustal components the intrusions appear to have formed by plume related activity within a continental setting likely related to post orogenic extension following carboniferous amalgamation of arc related terranes in the caob this plume activity was most likely also responsible for providing heat sources to generate large amounts of a type granites across the caob

multi element variations in olivine as geochemical signatures of ni cu sulfide mineralization in mafic magma systems examples from voisey s bay and pants lake intrusions labrador canada olivine from the olivine gabbro to troctolite intrusions at voisey s bay and at pants lake labrador was analyzed for multiple elements ca sc mg si ti v cr mn fe co ni cu zn sr y and zr with secondary ion mass spectrometry sims both intrusions have similar lithologies and petrographic characteristics and are approximately coeval 1 34 and 1 32 ga respectively members of the mesoproterozoic nain plutonic suite the voisey s bay intrusion hosts a producing economic ni cu sulfide deposit whereas the pants lake intrusion displays evidence of ni cu sulfide mineralization but to date a viable ore deposit has not been discovered olivine from both barren and mineralized lithologies was analyzed to assess the potential of the olivine composition for providing a record of silicate melt evolution and sulfide saturation related to formation of a massive sulfide deposit two detailed transects were sampled using five diamond drill holes that laterally approach the basal massive sulfide in the eastern deeps portion of the voisey s bay intrusion from the barren central part olivine displays distinct trace element distributions that vary coherently with host lithology and proximity to sulfide mineralization in particular olivine shows an increase in fe fo80 to fo60 mn 2500 to 5000 ppm and zn 280 to 700 ppm generally coupled with a decrease in ni 1600 to 900 ppm co 270 to 190 ppm and cr 110 to 45 ppm from barren troctolite normal troctolite nt and weakly mineralized troctolite variable textured troctolite vtt towards the heavily mineralized brecciated basal succession basal breccia sequence bbs the enrichment in fe mn zn is most pronounced in samples that laterally approach but do not directly intersect the massive sulfide deposit at the base of the intrusion particularly in samples from the lower variable textured troctolite and the basal breccia olivine from gabbro lithologies within the basal gabbro bg and upper gabbro ug of the pants lake intrusion is fairly homogeneous fo60 4300 ppm mn 460 ppm zn 340 ppm ni 50 ppm cr with a more evolved composition 2100 ppm ca 20 ppm sc 300 ppm ti 4 ppm y than the average olivine of the voisey s bay intrusion 570 ppm ca 6 ppm sc 65 ppm ti 0 3 ppm y pants lake olivine also commonly lacks the mutually mn zn rich signature of olivine from the basal breccia at voisey s bay that is characteristic of contamination of the parent magma by country rock gneiss and there reflects a close proximity to massive sulfides in conjunction with petrographic observation and the stratigraphic context a multiple element v cr mn fe co ni zn regression can be applied to calculate a lateral proximity of olivine to massive sulfide mineralization in the eastern deeps part of the voisey s bay intrusion over a distance exceeding 150 m the compositional variations in olivine from the economic voisey s bay intrusion bimodal and primitive and the mainly barren pants lake intrusion homogeneous and evolved provide potential as a regional scale mineralogical fertility indicator for mafic intrusions with comparable types of sulfide mineralization

unconventional pge occurrences and pge mineralization in the great dyke metallogenic and economic aspects platinum group elements pge are strategic materials because 96 99 of the world production is derived from just five mining districts and because they cannot be replaced as catalysts in many chemical processes in order to lessen the strategic character of pge both conventional deposits and unconventional pge mineralizations were investigated in an attempt to locate viable deposits which would diversify the supplier base in the great dyke conventional pge mineralization occurs in the form of pristine sulfide ores mined underground and oxidic surface ores new observations such as bimodal distributions of the pge in the main sulfide zone msz elevated pt pd ratios in the oxidized msz compared to the sulfidic part and distinct differences between the platinum group mineral pgm assemblages of the msz and stream sediments of adjacent rivers emphasize the fact that even though the great dyke seems to be the second or third largest pge occurrence in the world the complicated pge distributions and supergene redistributions should be kept in mind during planning and mine operation investigations of unconventional pge occurrences in ophiolites alaskan type intrusions porphyry copper deposits and in the kupferschiefer show that economically exploitable pge concentrations can be expected in a broader variety of host rocks than considered favourable in the past in the albanian mirdita ophiolite average contents of 860 ppb pt and 60 ppb pd were detected flotation concentrates of porphyry copper deposits for example from mamut malaysia santo tomas philippines elacite bulgaria and ok tedi papua new guinea contain between 827 and 1860 ppb pd pt in selected profiles of the polish kupferschiefer average contents of 255 ppb pt 94 ppb pd 2 4 ppm au and 13 0 ppm ag were analysed the distribution of the pge resources in the world and the annual production rates however underline the fact that the chances for a significant change in the supplier base are relatively low the bushveld complex will remain the largest producer followed by noril sk talnakh sudbury and stillwater if the operations in the great dyke reach their planned capacities the great dyke will rank in third place among the pge producing deposits in the world

origin of extremely pge rich mafic magma system an example from the jinbaoshan ultramafic sill emeishan large igneous province sw china abstract the 260 ma jinbaoshan sill is part of the permian emeishan large igneous province in sw china and is mainly composed of wehrlite and hosts a pge deposit containing 45 tonnes of pt and pd with ore grades ranging from 1 to 5 ppm pt pd the sill is composed of several pge rich horizons at different stratigraphic heights a 5 m thick pge rich horizon at the base of the sill which is underlain by a chromite rich pge poor layer accounts for 44 of the total ore reserve the jinbaoshan sill is rich in pges with ir ranging from 1 7 to 138 ppb ru from 0 9 to 39 ppb rh from 3 1 to 133 ppb and a total pt and pd from 76 to 5371 ppb all the rocks from the sill display primitive mantle normalized chacophile element patterns enriched in pges relative to ni and cu the low cu pd ratios 20 to 1938 of the rocks relative to the primitive mantle cu pd 7000 imply that there was no sulfide fractionation of the magmas the pge enrichment of the sill indicates that it formed from extremely pge rich magmas modeling results show that such pge rich magmas likely resulted from a multistage dissolution upgrading process in an open magma conduit system large amounts of chromite crystallization triggered sulfide saturation when pge rich magmas emplaced into the upper magma chamber of the system settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest pge rich horizon at the base of the sill the rest minor sulfide liquid accumulated to form pge rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber

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

re os pseudo isochron of disseminated ore from the kalatongke cu ni sulfide deposit xinjiang northwest china implications for re os dating of magmatic cu ni sulfide deposits abstract re os dating of disseminated ore from the kalatongke cu ni sulfide mineral deposit xinjiang northwest nw china yields an apparent isochron age of 433 31 ma with an apparent initial 187os 188os 433 ma ratio of 0 197 0 027 this apparent age is older than not only the zircon u pb age of the host intrusion 287 5 ma han et al 2004 but also the stratigraphic age of the intruded country rock thus the regression line is a pseudo isochron however previous re os dating of massive ores of the same deposit yielded an age that is consistent within analytical uncertainty with the zircon u pb age zhang et al 2008 this relationship is similar to that observed in the jinchuan deposit nw china therefore we suggested that the same mechanism post segregation diffusion of os yang et al 2008 is applicable to the kalatongke deposit re os isotopic studies of kalatongke jinchuan and representative magmatic cu ni sulfide deposits suggest that the massive ores of mafic ultramafic rock associated cu ni sulfide deposits would yield geologically meaningful re os age whereas a pseudo isochron would be obtained for the disseminated ores therefore to obtain a geologically meaningful re os age the type of the deposit the type of the ore and the ore forming process should be taken into account

the role of discontinuous magma inputs in felsic magma and ore generation for a long time granites have been considered as passive bodies ascending under intrinsic negative density and viscosity contrasts with their host rocks chemical variations within a granitic body resulted from in situ differentiation and crystal fractionation since the mid 1980s this global view has been significantly modified by i shifting melting from water saturated conditions to fluid absent reactions ii increasing the role played by the mantle during granite generation iii reassessing the rheology of partially molten rocks iv demonstrating stepwise segregation and ascent of magmas by analogue and numerical models v combining structural geophysical and geochemical studies to reveal the internal structures in granitic plutons it results that a granitic body is built up by a discontinuous accumulation of successive magma intrusions the discontinuous nature of magma emplacement has also significant consequences for its ability to generate ore the processes that lead to ore deposits are examined with a brief review of the magmatic and fluid phases that concentrate ore forming elements examples are taken from crustal derived granites and porphyry type deposits those are considered as the two end members of magmatic and hydrothermal ore deposits the source characteristics of the magma the emplacement mechanisms and magma mixing processes are the frame that controls the potential to carry base metals with the magma the distribution of elements is controlled by diffusion partition between minerals and melt solubility and redox conditions variations of those parameters are examined by considering their activation energy which controls the exponential dependence with temperature a characteristic length depending on the activation energy temperature variation and time is estimated for a characteristic time lag of 30 ka the intrusion of a magma into a magma chamber of similar composition hence temperature has few effects on diffusion partition coefficient and redox conditions because of a too low temperature contrast the intrusion of a mafic magma into a felsic one induces a variation of 300 c in both magmas the characteristic length of diffusion may vary by up to two orders of magnitude whereas the variation of partition coefficients is only one order of magnitude the redox conditions are about 2 5 log unit in the mafic magma but they can vary by 7 log units in the felsic magma hence a strong decrease in d values is observed in porphyry type deposits the effect is a removal of the elements with higher activation energy w sn zr from the mafic to the felsic magma deformation during the late stages of emplacement also controls ore formation

postmagmatic variability in ore composition and mineralogy in the t4 and t5 ore shoots at the high grade flying fox ni cu pge deposit yilgarn craton western australia the high grade flying fox komatiite hosted ni sulfide deposit located in the forrestania greenstone belt of the archean yilgarn craton western australia is hosted in a deformed and metamorphosed volcano metasedimentary succession postmineralization events have sheared and modified the texture and composition of the original massive sulfide ore creating up to 11 distinct ore shoots including massive stringer vein and breccia sulfides composed of pyrrhotite pentlandite chalcopyrite and variable abundances of pyrite ranging up to 40 vol nickel and platinum group elements pge tenor variations were investigated in two ore shoots t4 and t5 all mineralization styles show considerable variability in ni tenor pges show strong linear correlations between ir os ru and rh but poor correlation between pt pd and cu the normalized molar proportions of fe ni and s projected into the fe ni s ternary system show a distinct linear trend of pyrite addition to a typical primary magmatic composition and no correlation with mineralization style the high pyrite content present throughout the flying fox ore is also associated with elevated cu and as contents and is interpreted to be primarily due to the addition of pyrite from circulating fe s cu and as enriched fluids creating pyrite pentlandite intergrowths localized mechanical segregation of pyrite sulfidation of pyrrhotite to pyrite and oxidation of pyrrhotite to pyrite magnetite has also contributed to these increased pyrite contents although to a lesser extent the addition and segregation of pyrite has diluted the ni tenor with no evidence to suggest chemical mobilization of ni

ground melting and ocellar komatiites a lead isotopic study at kambalda western australia stratigraphically and geographically restricted ocellar komatiite flows at kambalda western australia appear to represent the products of ground melting of sulphidic sediments by komatiites in lava channels that localized the fe ni cu sulphide ores an immiscible sulphide liquid formed and gravitationally separated from the melted sediment xenomelt the resultant buoyant silicate liquid being partly or wholly assimilated by the turbulently convecting komatiite magma rarely the xenomelt gravitationally migrated to the top of flows and overflowed into the less turbulent lava levees where it collected to form a separate layer overlying a komatiitic layer within a single flow there was selective preservation of the hybrid felsic layer as an upper ocellar unit within an ocellar komatiite flow in lava levees flanking lava channels the ocellar unit is enriched in elements previously concentrated in the sediments and shows u th pb isotopic systematics akin to the underlying sediments moreover the partitioning relationships of u and pb between the immiscible xenomelt and sulphide liquid enhances the range of u pb ratios for components of the ocellar unit thus allowing sufficient spread of modern uranogenic pb isotopic ratios to form isochrons albeit imprecise ones the range and similarity of model th u data from these flows 2 8 3 9 and adjacent sulphidic sediments 2 3 4 4 mostly 2 8 3 9 contrasts with the generally invariable th u within kambalda ultrabasic basic flows 3 6 3 9 and further supports the ground melting hypothesis

re os isotopic variations in carbonaceous pelites hosting the duluth complex implications for metamorphic and metasomatic processes associated with mafic magma chambers abstract previous mineralogic and isotopic studies of sulfide mineralization in the 1 1 ga duluth complex have suggested that sulfur and possibly metals have been derived from proterozoic sedimentary country rocks the re os isotopic system offers great potential in the evaluation of crustally derived metals in magmatic ore forming processes but an understanding of the mechanisms that could cause isotopic variations in country rock contaminants is a prerequisite a suite of sulfidic and carbonaceous pelitic rocks of the virginia formation country rock to the duluth complex ranging from unmetamorphosed through strongly recrystallized and partially melted xenoliths have been analyzed for their os isotopic composition sulfide mineral separates mainly pyrrhotite kerogen graphite isolates and whole rocks were analyzed whole rocks kerogen and graphite from metamorphosed samples plot between 1 85 ga and 1 1 ga chondritic reference isochrons sulfide separates from all metamorphosed samples plot nearer the chondritic 1 1 ga reference isochron and require exchange with a fluid or melt characterized by a chondritic os isotopic composition potentially derived from the duluth complex devolatilization and partial melting may perturb isotopic systematics when accompanied by the loss of re and os bearing fluids these processes could in part explain the scatter in isotopic values found in kerogen and metamorphosed whole rocks however metasomatic addition of os via interaction with a fluid or melt is required to explain the sulfide isotopic ratios and is consistent with sulfide minerals being more susceptible to os exchange with a fluid than kerogen the relative ease with which pyrrhotite may be reset during fluid interaction as well as the variable degrees of exchange shown by kerogen graphite suggest that a knowledge of the petrologic and isotopic evaluation of country rocks is necessary before a meaningful assessment of the isotopic variations found in igneous rocks or associated ore deposits can be undertaken

the role of crustal contamination in the formation of ni cu sulfide deposits in eastern tianshan xinjiang northwest china evidence from trace element geochemistry re os sr nd zircon hf o and sulfur isotopes tulaergen xiangshan huangshan tianyu and baishiquan magmatic ni cu sulfide deposits which are all located in the eastern part of the central asian orogenic belt and associated with small mafic ultramafic intrusions with high grade cu and ni orebodies have similar ages of formation their magma sources are morb like and oib like depleted mantle modeling calculations of sr nd and zircon hf isotopic data show that about 10 15 of oceanic slab components are added into the source of baishiquan and tianyu intrusions but only less than 5 are in the three other intrusions the higher degree of subduction metasomatism in the baishiquan and tianyu magma sources is likely the result of the paleozoic multiple stage subduction moreover the relative enrichment of lile depletion of hfse and lower ce pb ratios indicate that these intrusions experienced crustal contaminations the effects of crustal contamination are also identified in the magmatic sulfide deposits by the higher zircon d 18 o higher sulfide cos and d 34 s values observed we propose a two stage crustal contamination model of the tulaergen xiangshan tianyu and baishiquan intrusions however the huangshan intrusion which has the highest degree of crustal contamination likely experienced only single stage upper crustal contamination 14 2 the addition of crustal s appears to be directly related to triggering s saturation in these deposits according to a simple s isotopic modeling calculation 11 2 9 1 2 and 2 1 of the crustal sulfur addition into the parental magma would be required to form the tianyu baishiquan xiangshan and tulaergen deposits respectively

factors affecting carbon 14 activity of unsaturated zone co2 and implications for groundwater dating summary unsaturated zone processes may influence the carbon 14 14 c activity of infiltrating groundwater and thus introduce error in derived groundwater residence times however unsaturated zone 14 c activities are rarely measured and there is little understanding of how they may vary spatially in a groundwater basin in this study we measured 14 c activity in unsaturated zone gas at five sites with different watertable depths 8 2 31 5 m in the arid ti tree basin central australia we observed a relatively uniform decrease in 14 c activity of unsaturated zone gas with depth at most sites with variation in unsaturated zone depths leading to variation in 14 c activities directly above the watertable at each site ranging from 54 to 106 percent modern carbon pmc through modelling we show that the profiles are influenced by co 2 production at different depths from sources with different isotopic ratios including production of modern co 2 in the root zone and production of old co 2 above the watertable scenario modelling showed that these processes are independent of recharge when recharge is low 0 10 mm y 1 but that higher recharge rates 100 mm y 1 result in more advective transport of atmospheric co 2 to the watertable the variation in 14 c above the watertable was more sensitive to watertable depth and shallow and deep co 2 production rates these findings offer insight into how unsaturated zone 14 c activities may vary spatially and provide guidance as to when 14 c depletion in unsaturated zone co 2 may become important for groundwater dating particularly in arid settings

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

classification and depositional environments of quaternary pedogenic gypsum crusts gypcrete from east of the fayum depression egypt abstract the eastern boundary of the fayum depression 45 m below sea level is bounded by a 99 m high hill inselberg of girza that slopes gently eastward towards the nile valley the inselberg and the surrounding plain are encrusted with 50 130 cm thick gypsum crusts that exhibit well defined pedogenic horizons also east of the nile valley the qattamia plateau is encrusted with gypsum crusts that form 50 m wide pavements that are scattered at different levels the gypsum crusts range in thickness from 20 to 50 cm and do not form characteristic vertical horizons the individual gypcrete pavements exhibit massive mottled or powdery structure using structural fabric and textural criteria three main types of crusts are distinguished 1 eluvial horizon massive to spongy like powdery gypsum crusts found at the land surface and consisting of gravels and sands cemented with gypsum 2 illuvial horizon a massive indurated gypsum crusts that consist dominantly of white gypsum with horizontal and sub vertical black gypsum veinlets 3 illuvial horizon b massive mottled gypsum crusts that consist of carbonate or shale fragments floating in gypsum cement petrographically the gypsum crusts in the girza and qattamia areas comprise a varying abundance of microcrystalline fine lenticular coarse lenticular porphyroblastic prismatic or poikilotopic gypsum lenticular and microcrystalline gypsum crystals are usually observed filling rhizotubules in all crusts the origin of the gypsum crusts is related to pedogenic processes which involved the leaching of gypsum from the underlying or adjoining gypsiferous shale or marl layers and the subsequent displacive crystallization of gypsum in the upper soil horizon cycles of wetting rain fall and drying are believed to account for the dissolution and translocation of gypsum from the upper eluvial horizon and its recrystallization as microcrystalline and fine lenticular gypsum in the illuvial horizon a the poikilotopic gypsum crystals in the illuvial horizon b reflect the abundance of gypsum enriched groundwater in the phreatic zone the dominant processes of gypsum growth are by displacive and or inclusive crystallization reflecting formation in pedogenic setting the existence of gypsum crusts capping the inselberg and the surrounding plain of girza area resist eastward deepening of the fayum depression on the other hand the gypsum crusts on the surface of the qattamia plateau favor the development of relief inversion due to the removal of the surrounding soft materials therefore the gypsum crusts provide useful paleoclimatic and geomorphic indicators to the study areas