precious metals in magmas of east greenland factors important to the mineralization in the skaergaard intrusion a survey of precious metals ni and cu levels and their mutual relationships in several magma suites of the east greenland volcanic passive margin shows that the magma type which was parental to the skaergaard intrusion was markedly enriched in pt pd and especially au and relatively depleted in cu compared to the preceding voluminous plateau basalt volcanism it is likely that elevated levels of platinum group minerals pgm and au were critical to the generation of ore grade precious metals in the intrusion the discovery of a major pcm and au deposit within the intrusion combined with these results suggests a set of criteria which might be critical to the formation of skaergaard type pcm deposits elsewhere in the province and worldwide the skaergaard mineralization appears to be a hitherto undescribed class of pgm and au mineralizations

petrochemistry of the okiep copper district basic intrusive bodies northwestern cape province south africa copper mineralization is associated with noritic and pyroxenitic rocks in the granulite facies metamorphic terrane in the okiep district of namaqualand in the northwestern cape province of south africa these bodies have been interpreted to be postmetamorphic and the mineralization has been inferred to be magmatic however the sulfide mineralogy of many of the orebodies is dominated by bornitc whereas immiscible magmatic sulfide ores contain pyrrhotite and chalcopyrite hence the okiep ores cannot be primary magmatic sulfides geochemical evidence is presented to demonstrate that the ores have undergone an extensive oxidative event in which pyrrhotite and chalcopyrite were converted to magnetite and bornite up to 90 percent of the sulfur has been lost in the case of some samples from the carolusberg mine the high fe 2 o 3 tio 2 and cu s ratios the low s se ratio of whole rock samples and their variation in different orebodies provide quantitative evidence for this process it is possible to recalculate the original sulfide from these data and a cu content of approximately 10 percent is indicated however these ores only contain on the order of hundreds of parts per million of ni and so the cu ni ratio is extremely high compared to that of most sulfide ores derived from a differentiated basic magma the textural and mineralogieal evidence for this oxidation is best seen in samples from the carolusberg mine in many of the other mines a later low temperature hydrothermal alteration has obliterated the evidence for these reactions

investigations of the stillwater complex part v apatites as indicators of evolving fluid composition variations in the f cl and oh contents of apatite are not constrained by crystal chemical factors in contrast to micas and amphiboles and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks microprobe analysis of intercumulus apatites from the stillwater complex reveal that cl rich apatites typically containing 6 0 wt cl occur throughout the lower 1 3 of the complex excluding the basal series a change from cl rich to more f rich apatite occurs within olivine bearing zone i ob i of the banded series the host zone of the platiniferous j m reef although apatite compositions are somewhat variable above the j m reef more f rich apatites predominante and typically contain 1 2 wt f and 3 0 wt cl the most f rich apatites occur in the uppermost exposed cumulates pristine apatites from coeval sills and dikes from below the complex and from the basal series are similarly f rich in all apatites the cl and f contents are lower in rocks affected by later metamorphic fluids rare earth element ree concentrations in chlorapatites show a marked peak in the olivine rich rocks of the j m reef and contain up to 2 wt ce2o3 la2o3 the trend of first increasing then decreasing cl f ratios with stratigraphic height is modeled by a vapor driven zone refining process occurring within the cumulate pile causing cl enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of cl from the magma as crystallization proceeded the abrupt change from cl rich to more f rich apatites within ob i is interpreted as the result of a breakdown of the cl rich zone refining front and mixing with cl poor supernatant melt any high temperature fluids that exsolved and circulated through the lower 1 3 of the complex must have been enriched in cl and could have transported ree and trace metals

a gold and platinum mineralized layer in gabbros of the kap edvard holm complex field petrologic and geochemical relations the lower layered series a cumulates of the kap edvard holm complex contain a stratiform layer 1 to 20 m thick and 6 km long that is mineralized with gold and platinum group elements pges the lower layered series a cumulates 650 m thick are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2 to 30 m thick layers that are most abundant above the mineralized layer minor volumes of poikilitic wehrlite form a few 1 to 2 m thick layers throughout the section field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the lower layered series a formed in an open system magma chamber from repeated magma injections within the mineralized layer average concentrations over 3 m thickness are 250 ppb pt 40 ppb pd and 50 ppb au with individual samples containing up to 5 ppm pt and 6 ppm au the concentrations of ir os ru and rh are uniformly low 20 ppb stratigraphic zoning of pt pd and au is consistent along strike with peak au concentrations overlying pt and pd peaks platinum minerals are primary pt fe alloy sperrylite and moncheite with secondary pt sulfide overgrowths gold occurs as alloys with ag and cu these minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite chalcopyrite digenite deposition of the mineralized layer is associated in space and time with injections of plagioclase saturated primitive magma that mixed with more evolved resident magma these events caused efficient partitioning of au and pge into sulfides derived from a large volume of magma deposition of these sulfides to form the mineralized layer and subsequent deposition of the overlying anorthosite layers the wehrlite layers however were formed by postcumulus reactions between gabbro cumulates and hydrous intercumulus melts probably derived from the compacting cumulus pile subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro oxidation and desulfurization of cu fe sulfides and local recrystallization of pge minerals

oxygen isotope exchange and mineral alteration in gabbros of the lower layered series kap edvard holm complex east greenland multiple intrusions of gabbros mafic dikes and syenites in the kap edvard holm complex gave rise to prolonged circulation of meteoric hydrothermal solutions and extreme isotope exchange and mineral alteration in the 3600 m thick lower layered series gabbros in the lower layered series 18o of plagioclase varies from 0 3 to 5 8 and it decreases with an increase in the volume of secondary talc chlorite and actinolite in the same gabbros pyroxenes have a more restricted range in 18o from 5 0 to 3 8 and values of 18opyroxeneare independent of the abundance of secondary minerals which ranges from 14 to 30 these relations indicate that large amounts of water continued to flow through the rocks at temperatures of 500 600 c altering the gabbros to assemblages of talc chlorite actinolite epidote albite and causing significant oxygen isotope exchange in plagioclase but not in pyroxene the extensive low temperature secondary mineralization and 18o depletion of plagioclase in the lower layered series are associated with the later emplacement of dikes and gabbros and syenites which created new fracture systems and provided heat sources for hydrothermal fluid circulation this produced subsolidus mineral alteration and isotope exchange in the lower layered series that are distinct from those in the skaergaard and cuillin gabbros of the north atlantic tertiary province but are similar to those observed in some oceanic gabbros

weathering of pge sulfides and pt fe alloys in the freetown layered complex sierra leone fresh and weathered rocks and saprolite from horizon b of the freetown layered complex contain platinum group minerals pgm the pgm in the fresh rocks are 1 7 m across including cooperite pts isoferroplatinum pt3fe minor tetraferroplatinum ptfe tulameenite pt2fecu os bearing laurite rus2 and other base metal sulfide bms bearing pgm the weathered rocks contain fewer of those pgm but a high proportion of disordered cu pd bearing pt fe alloys the saprolite hosts scarce smaller 1 3 m ordered ptfe and disordered ptfe3 the pt fe alloys became increasingly fe rich as weathering proceeded pt fe oxides appeared during weathering copper sulfides associated with the primary pgm and cooperite with 3 pd were destroyed to provide the minor cu and pd found in some of the disordered pt fe alloys platinum and pd bearing saprolites have retained the original rock fabric and to a depth of about 2 m surround residual rocks that show progressive weathering corestones ground water passing through the saprolite has transported pt and pd and probably au in solution down slope into saprolite over unmineralized rocks transport is marked by changes in the pt pd ratio indicating that the metals have moved independently palladium is present in marginally higher concentrations in the deeper saprolite than in the corestones suggesting some retention of pd in the deeper saprolite platinum and pd are less concentrated in the upper saprolite than the deeper saprolite indicating surface leaching alteration occurred over a long period in an organic and microbial rich environment that may have contributed to the leaching and transport of pge

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

horizontal and vertical variations in noble metal distribution in the great dyke of zimbabwe a model for the origin of the pge mineralization by fractional segregation of sulfide abstract a study based largely on diamond drilling undertaken specifically for research purposes has shown that in the darwendale subchamber of the great dyke of zimbabwe there are three principal sulfide zones that are located in subunits 1a and 1b of cyclic unit 1 sulfide zone 1 the main sulfide zone msz is subdivided into three subzones the base of the msz occurs at the top of the main bronzitite layer of cyclic unit 1 extending into the overlying websterite the noble metals are concentrated at the bases of each sulfide zone and subzone and decrease in concentration rapidly upwards whereas variations in the concentrations of base metals such as cu correlate well with variations in the amounts of sulfide across each zone when calculated in terms of their concentration in 100 sulfides it is found that pd decreases upwards most rapidly followed by pt and then au with the cu concentration remaining essentially constant the decrease of the noble metals is interpreted in terms of simple rayleigh fractionation as the consequence of the removal of sulfide the observed correlation between the tops and bottoms of sulfide zones with small variations in the mg number of bronzite is taken to indicate that in zones 2 and 3 sulfide segregation started as a result of crystallization increasing the s content of the resident magma in the chamber until it reached that of sulfide saturation coupled with fresh inputs of sulfide unsaturated more primitive magma causing the resident magma to become unsaturated again this recharge by primitive magma also raised the pge content of the sulfides segregating from the trapped intercumulus liquid indicating that the resident magma was becoming recharged in pge in this way subunits i and ii of sulfide zone 1 are thought to contain too much sulfide to be explicable in this way and it is suggested that mixing with a plagioclase saturated magma possibly generated around the flanks of the intrusion was responsible for these subunits the sulfides of each of the sulfide zones are more concentrated at the margin than at the axis of the great dyke but have a lower tenor of noble metals it is proposed that the axially located sulfides were the consequence of magma sulfide equilibration at a higher r factor magma sulfide ratio than those at the margin the magma sulfide partition coefficients required to explain the percentage difference in the tenors of the different noble metals between axis and margin in this way are the same as those required to explain the vertical variations in noble metal tenor in terms of the fractional segregation model

an ocean ridge type magma chamber at a passive volcanic continental margin the kap edvard holm layered gabbro complex east greenland the gabbros of the tertiary kap edvard holm layered serieshave a stratigraphic thickness of more than 5000 m earlier work has shown that the range in cumulus mineral compositions is restricted plagioclase an 81 an 51 olivine fo 85 fo 66 pyroxenes ca 43 mg 46 fe 11 to ca 43 mg 37 fe 20 field evidence of magma injections is common which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma a detailed study of a 600 m section 900 1500 m in the lower layered series reveals a period of crystallization when the magma chamber behaved as a closed system 900 1300 m the rocks formed during this periodare well laminated olivine gabbros 900 110 m which evolved to well laminated oxide gabbros 1100 1300 m compositional trends in the cumulusminerals are towards more evolved compositions plagioclase an 64 an 58 pyroxene mg from 80 to 76 with stratigraphic height from 1300 m to 1500 m granular olivine gabbros dominate with moreprimitive mineral compositions plagioclase an 67 an 76 pyroxene mg from 78 to 82 the transition olivine gabbro to oxide gabbro at 1100m is a consequence of fractional crystallization and it is shown how changes in activities of feo and fe 2 0 3 in the magma are reflected in the total iron content of plagioclases the transition from oxide gabbro to olivine gabbro at 1300 m results from replenishment by less evolved basaltic magma on the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the kap edvard holm layered series crystallized from a tholeiitic magma similar to morb melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill like bodies 0 2 2 0 m thick the melanogabbroic units consist of cr rich augite olivine plagioclase heteradcumulates and contain deformed mica crystals of pre emplacement origin these units crystallized from a wet mgo rich magma which was injected into the layered host gabbros after the formation of the cumulus pile but before the magma was completely solidified the kap edvard holm layered series has several parallels with the plutonic part of ophiolite sequences these include cumulus mineral assemblage compositions of the minerals and the restricted range in compositions with stratigraphic height field evidence of repeated replenishment of basaltic magma dyke swarms overlying the roof zone of the magma chamber and the existence of a late intrusive suite of wet mgo rich magma these parallels suggest that the processes involved in the formation of the kap edvard holm layered series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present day spreading ridges the kap edvard holm layered series is therefore believed to represent a shallow level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the north atlantic ocean

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

effects of h2o on phase relations during crystallization of gabbros in the kap edvard holm complex east greenland in the kap edvard holm complex of east greenland layered monly results in the formation of discordant igneous bodies within crystallizing cumulus sequences in some olivine gabbros host numerous small discordant bodies of ultramafic cases partially molten cumulates appear to have reacted rock and magnetite gabbro that formed by late magmatic metasomatic with liquids or fluids that originated within the cumulus replacement of partially solidified gabbroic cumulates similar repile resulting in resorption of cumulus minerals crysplacive rocks are also present as large semi conformable sheet like tallization of intrusive or replacive bodies and rebodies within the layered gabbro sequence replacive bodies of all distribution of mass within the intrusion e g robins sizes are spatially associated with xenoliths of metabasaltic lava 1982 butcher 1985 butcher et al 1985 helz 1987 the small bodies cluster around xenoliths and the large bodies host irvine 1987 boudreau 1988 mcbirney sonnenthal swarms of xenoliths the replacive bodies are inferred to have formed 1990 larsen brooks 1994 scoon mitchell 1994 by reaction of gabbroic cumulates with pore liquids that were mathez 1995 meurer et al 1997 boudreau 1999 in enriched in h2o by degassing or dehydration melting of the other cases partially molten cumulates are inferred to metabasalts as these liquids migrated through the cumulus pile have reacted with externally derived magmas that inthey resorbed plagioclase and produced ultramafic bodies where the truded the cumulus pile resulting in the development liquids came to rest they reacted with gabbroic cumulates to produce of discordant replacive bodies via the reaction of the bodies of magnetite rich gabbro the most evolved pore liquids cumulates with intruding magmas e g morse et al 1987 segregated to form hornblende rich pegmatitic dikes of roughly dacitic bedard et al 1988 tegner wilson 1993 tegner composition suggesting that hydration and possibly oxidation of the robins 1996 because cumulus rocks preserve records magma shifted the liquid line of descent from a tholeiitic trend toward of magmatic fractionation and replenishment in large a calc alkaline trend these inferences imply that contamination of mafic intrusions recognizing and understanding postmagmas by dehydration of hydrothermally altered crustal rocks may cumulus processes such as these can provide insights significantly influence the evolution of mafic magma systems into the evolution of mafic magma chambers and the differentiation of basaltic magmas within the crust near the western margin of the kap edvard holm

dehydration partial melting and assimilation of metabasaltic xenoliths in gabbros of the kap edvard holm complex east greenland

tertiary plutons monitor climate change in east greenland the d values of amphibole and biotite in tertiary plutons in central eastern greenland decreased by 30 between 55 and 50 ma and then increased by 35 between 50 and 43 ma the d values of amphibole and biotite in in the plutons range from 160 to 123 these low values reflect isotopic exchange between the igneous rocks and hydrothermal fluids that were dominated by meteoric water the changes in mineral d values between 55 and 43 ma are explained most plausibly by shifts in the isotopic composition of meteoric waters that fed hydrothermal recharge areas at 50 ma uplift related to the iceland mantle plume elevated hydrothermal recharge areas by 1 2 km high altitude low d meteoric waters collected by the uplifted recharge areas caused the 30 decrease in mineral d values between 55 and 50 ma the subsequent increase in mineral d values from 50 to 43 ma corresponds to widening of the north atlantic ocean and a change from continental to maritime climate thus tertiary plutons in central eastern greenland represent a network of paleoclimate stations that monitored the d of meteoric water during opening of the north atlantic ocean basin

the mediterranean outflow splitting a comparison between theoretical models and canigo data abstract the mediterranean outflow in the atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima hydrological data collected west of the strait of gibraltar and off the south and south west coasts of portugal in september 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the mediterranean outflow these models are i a steady one dimensional streamtube model and ii a local time dependent model that includes cross stream variations the streamtube model applied at the northern respectively the southern part of the outflow appears capable of reproducing the observed density difference between the two cores the vertical separation however was underestimated the results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

40ar39ar geochronology of tertiary mafic intrusions along the east greenland rifted margin relation to flood basalts and the iceland hotspot track abstract the east greeland tertiary igneous province includes the largest exposed continental flood basalt sequence within the north atlantic borderlands more than ten layered gabbro complexes including the 55 ma skaergaard intrusion and a large dolerite sill complex are the plutonic equivalents of flood basalts both lavas and intrusions have been regarded as synchronous with continental breakup at 57 54 ma we report ten new ages of the mafic intrusions determined by40ar 39ar incremental heating experiments demonstrating that the mafic intrusions formed in two distinct time windows only intrusion ii of the imilik gabbro complex the skaergaard intrusion and the sorgenfri glestcher sill complex formed at 57 55 ma coeval with the eruption of regional flood basalts and continental breakup other layered gabbro intrusions at imilik intrusion iii kruuse fjord igtutarajik nordre aputite q kap edvard holm and lilloise are distinctly younger and formed between 50 and 47 ma plate kinematic models indicate the axis of the ancestral iceland mantle plume was located under central greenland at 60 ma and subsequently crossed the east greenland rifted continental margin we propose that tholeiitic magmatism along the east greenland rifted margin largely occurred in three distinct pulses at 62 59 ma lavas and dykes 57 54 ma lavas dykes sills and some gabbros and 50 47 ma gabbros dykes and rare lavas related to discrete mantle melting episodes triggered by plume impact continental breakup and passage of the plume axis respectively this model implies northwestward continental drift of greenland relative to the plume axis by 3 9 5 0 cm yr between 60 and 49 ma consistent with estimates from seismic studies of submerged flood basalts

the triple group and the platinova gold and palladium reefs in the skaergaard intrusion stratigraphic and petrographic relations this paper describes the setting structure and textural relations of the platinova reefs in the skaergaard intrusion the reefs comprise a strata bound occurrence of au pge estimated at 91 million tons of rock with 1 8 ppm au and similar amounts of pd the precious metals occur in two major zones a lower major pd reef which is concordant with the modal layering and followed by at least ten minor pd reefs also concordant and an au concentration which fringes the pd reefs outward and upward through the triple group units the major pd concentration and the au concentration are separated in a systematic way across the intrusion near the center of the intrusion the au concentration occurs some 60 m higher in the stratigraphy than the main pd reef but the separation decreases gradually outward being only a few meters near the margins the dominant precious metal minerals are small grains of cu fe au pd pt alloys which typically are included in interstitial sulfides bornite digenite and smaller amounts of chalcopyrite the textural relations suggest a close genetic association between the sulfides and au pge minerals an association which is confirmed by whole rock compositional data our observations demonstrate that the platinova reefs formed after significant concentration of the au pge and s in the residual magma during fractional crystallization before it became saturated with respect to a sulfide fe cu s o liquid this sulfide liquid acted as a collector of au pge and settled into the layered series cumulates we propose that the structure of the deposit evolved through fractional crystallization of the au pge bearing sulfide liquid as it was filter pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates the fractional crystallization resulted in an evolution of the au pge bearing phases with progressive fractionation from pd rich alloys at the base of the deposit to au rich alloys at the top we argue that the sulfides evolved to their present extreme cu rich fe poor composition during oxidation desulfurization reactions in the same event which oxidized the iron titanium oxides of the intrusion this oxidation desulfurization probably had no overall effect on the distribution of au and pge

speciation and solubility of palladium in aqueous magmatic hydrothermal solutions combining thermodynamic data at 25c and 1 bar with correlation algorithms enables prediction of equilibrium dissociation constants for aqueous platinum group element pge complexes at high pressures and temperatures we used these data 1 to calculate the speciation of pd ii in nacl solutions at temperatures to 750c pressures to 2 kbar and values from 3 to 8 and 2 to predict the solubility of palladium at 2 kbar as a function of temperature oxygen fugacity f sub o2 ph and total chloride concentration m sub t cl sup minus calculations indicate that as ph increases there is a sharp transition from chloride to hydroxide complex dominance at all chloride concentrations this shift occurs at or below neutral ph in dilute solution but at m sub t cl sup minus ge 1 0 pd ii chloride complexes account for the majority of dissolved pd ii well into alkaline conditions solubility of palladium increases with increasing temperature m sub t cl sup minus and f sub o2 at 700c 2 kbar and the f sub o2 set by the fmq buffer palladium solubility in a 3 0 m nacl solution is approximately 0 3 ppm at a ph of 4 and one unit ph variation can change palladium solubility by two orders of magnitude these results suggest that pge can be transported as chloridemore complexes in cl rich magmatic hydrothermal solutions like those inferred from petrologic studies of the stillwater and bushveld complexes less

variation in the composition of apatite through the merensky cyclic unit in the western bushveld complex microprobe analysis of interstitial apatite from a 100 m section through the merensky reef of the bushveld complex reveals that this stratigraphic section marks a transition from cl rich to more f rich compositions in the critical and main zones respectively apatite compositions across the section vary by more than 80 mole percent in the chlorapatite component the trend of decreasing cl concentration with stratigraphic height is not regular and cl contents in apatites from some samples may vary as much as 50 mole percent la 2 o 3 and ce 2 o 3 tend to be enriched in the more melanocratic orthocumulate parts of the section sro concentrations of apatite show an increase with stratigraphic position a trend which is opposite that observed in coexisting cumulus plagioclase the data are consistent with mineralogic and textural evidence for an influx of cl rich vapor into the cumulus section which has affected the cl f variations through the lower portion of the bastard unit this influx occurred above the crystallization temperature of apatite 1 000 degrees 1 050 degrees c it is suggested that much of the modal variation seen in the section may be the result of shifts in liquidus field boundaries caused by gain or loss of volatile components from the melt

partitioning of the platinum group elements and gold between silicate and sulphide magmas in the munni munni complex western australia geochimica et cosmochimica acta erratum to s j barnes 1993 57 1277 1290

variation in the composition of apatite in the munni munni complex and associated intrusions of the west pilbara block western australia abstract apatite is a ubiquitous intercumulus mineral in the archean munni munni radio hill and mount sholl layered igneous complexes of the west pilbara block in western australia apatite compositions from the three intrusions are relatively rich in the fluorapatite endmember and become more so in cumulates derived from compositionally evolved magmas the most extensive stratigraphic coverage is from the munni munni complex where the mole fraction of fluorapatite increases from approximately 60 70 in cumulates of the ultramafic zone to over 90 near the top of the gabbroic zone the chlorapatite component decreases from a high of 20 30 mol in the ultramafic zone to less than 10 in the uppermost part of the gabbroic zone the ree content of apatite parallels stratigraphic changes in whole rock incompatible element zr rb concentrations and is consistent with equilibration with the bulk assemblage the stratigraphic decrease in the cl f and oh f ratio of apatite can be modeled by assuming that fluid saturation of intercumulus liquids is a function of pressure and occurs prior to apatite saturation imposed on the stratigraphic trend are the effects of the combined processes of continuing fluid loss and fractional crystallization of apatite and locally biotite during solidification of the intercumulus liquid these processes produced relative compositional variations in endmember components on the order of 50 mol or more in individual thin sections because apatite crystallizes late in the solidification of intercumulus liquids it is estimated that the cl f ratio of the original trapped parent liquid prior to degassing could have been more than 2 orders of magnitude higher than is now recorded by intercumulus apatite compositions a comparison of literature apatite compositions from other intrusions shows that with the exception of the unusually cl rich assemblages of the stillwater and bushveld complexes processes affecting halogen geochemistry of layered intrusions do not appear to have changed significantly over the past 3 ga

petrology and geochemistry of the kruuse fjord gabbro complex east greenland the kruuse fjord gabbro complex is a composite intrusion of layered gabbro and troctolite with subordinate ultramafic rocks and minor trondhjemitic bodies it was emplaced into archaean continental crust of east greenland during early tertiary rifting of greenland from eurasia the work to date has identified an outer gabbro series and an inner troctolite series and these are separated by a narrow zone of trondhjemitic intrusions in the southeast the partially crystallized cumulates of the gabbro series were intruded by a lenticular ultramafic pluton 800 m in thickness volumetrically minor syenite trachyandesite net veined dykes and later diabase dykes cross cut the plutonic rocks structural and topographic features suggest that the layered rocks were affected by synmagmatic subsidence and deformation but not by monoclinal coastal flexure the gabbro series is composed of a marginal gabbro unit about 20 m wide bordering more than a 2 km thickness of layered olivine and magnetite gabbro cumulates the marginal gabbro is interpreted to be chilled magma the layered cumulates are the product of repeated injections of magma that fractionated in an open system magma chamber anorthositic and troctolitic layers in the lower part of the sequence may represent inputs of magma and suggest that the order of cumulus mineral crystallization was 1 plagioclase an 39 85 2 olivine fo 46 82 3 augite wo 28 47 en 39 58 fs 8 18 and 4 magnetite the disappearance of cumulus magnetite and a reversal in mineral compositions at 1 5 km from the base of the succession suggests a major input of magma occurred at this height in the troctolite series the composition of cumulus minerals mineral crystallization sequence and style of emplacement are similar to those in the gabbro series the ultramafic pluton is composed of coarse grained wehrlite olivine melagabbro and troctolite that were formed by at least three injections of magma the typical mineral crystallization sequence was 1 cumulus chromite and olivine fo 84 88 2 poikilitic chrome diopside wo 29 51 en 43 63 fs 3 13 and 3 intercumulus plagioclase an 75 90 phlogopite apatite and localized disseminated sulphides containing au and platinum group elements comparison of crystallization sequences and the major and trace element compositions of clinopyroxene suggests that the gabbroic and troctolitic rocks formed from a magma represented by the chilled marginal gabbro a tholeiitic basalt magma similar to e morb whereas the ultramafic rocks formed from a magma that was relatively enriched in incompatible trace elements and volatiles the association of these two magma types is an example of bimodal mafic ultramafic magmatism in a rifting environment

halogen geochemistry of the great dyke zimbabwe apatite from the great dyke of zimbabwe is relatively rich in the hydroxy fluorapatite end members the mole fraction of fluorapatite increases from approximately 40 in cumulates of the ultramafic sequence to over 60 in a sample near the top of the exposed mafic sequence the chlorapatite component decreases from a typical high of 10 20 mole in the ultramafic sequence to about 1 in the uppermost part of the mafic sequence however within sample variation may be as great as the entire stratigraphic variation halogen contents of marginal samples generally are similar to axial samples but tend not to have as high cl concentration and tend to oh enrichment biotite compositions approach hydroxyl end member compositions and apatite biotite oh f exchange geothermometers give an average closure temperature of 564 c apatite from the umvimeela dyke an unlayered dike that parallels the great dyke over much of its length contains less cl than is seen in the ultramafic sequence cumulates of the great dyke while the overall stratigraphic trend is characterized by a decrease in the cl f ratio with stratigraphic height within the p1 unit at the top of the ultramafic sequence there is a positive correlation between cl and other incompatible elements such as na and ce the apparent contradiction between the general stratigraphic trend of decreasing cl f ratio with fractionation and the apparent increase in cl and other incompatible elements seen in the p1 unit can be explained by assuming that the great dyke magma chamber was degassing near its top where confining pressure was lowest and cl was preferentially lost to a separating volatile rich fluid as cumulates formed on the floor they entrapped liquid that was increasingly depleted in cl at the higher stratigraphic levels however at any given stratigraphic interval either local fluid enrichment or the eventual crystallization of halogen bearing minerals that incorporate the smaller f ion in preference to the larger cl ion led to a local increase in the cl f ratio

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

inclusions in mount st helens dacite erupted from 1980 through 1983 abstract inclusions of plutonic metavolcanic and volcanic rocks are abundant in dacite pumice and lava from the 1980 1986 eruption sequence at mount st helens point counts of inclusions exposed in talus blocks from the dome from 1980 through 1983 show that inclusions form approximately 3 5 vol of the lava eighty five percent of the inclusions are medium grained gabbros with an average diameter of 6 cm additional rock types include quartz diorite hornfelsic basalt dacite andesite and vein quartz disaggregated inclusions are common and define shear planes within the dome these fragmented inclusions may significantly contaminate analyses of the dacite the gabbroic inclusions are of four distinct types all with mineral assemblages consistent with crystallization pressures of less than 9 kb textures and major element compositions indicate that most of the gabbroic inclusions are cumulates the most abundant inclusion type is laminated gabbronorite which contains up to 9 interstitial glass derived from partial melting the presence of quartz veins and hornblende bearing veins within sheared zones in the laminated gabbronorite indicates that the source of these inclusions was holocrystalline rock that had been penetrated by water rich fluids the gabbronorite contained sufficient water to be susceptible to partial melting when the magma that fed the 1980 1986 eruption sequence was emplaced nearby various types of gabbroic inclusions including the laminated gabbronorite are common in mount st helens lavas of approximately the last 3000 years this coincides with the interval in which mount st helens first erupted basalt and basaltic andesite lavas these observations together with the fact that the gabbroic inclusions are compositionally unlike any of the tertiary intrusive rocks in the mount st helens area strongly suggest that the inclusions are related to the introduction of basalt to the mount st helens magmatic system the source of the gabbros could be a layered mafic pluton formed through crystal accumulation from multiple batches of basaltic magma emplaced at mid to upper crustal depths beneath the volcano the prevalence of explosive eruptions at mount st helens may play a part in bringing the inclusions to the surface the eruptive products of the cataclysmic eruption of may 18 1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently this suggests that the may 18 eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series