Smith 2009 - Sources and evolution of mineralising fluids in IOCG systems

The sources and evolution of mineralising fluids in iron oxide–copper–gold systems,Norrbotten,Sweden:Constraints from Br/Cl ratios and stable Cl isotopes of fluid inclusion
leachates
S.A.Gleeson a,*,M.P.Smith b
a
Department of Earth &Atmospheric Sciences,University of Alberta,Edmonton,Alta.,Canada T6G 2E3b
School of the Environment,University of Brighton,Cockcroft Building,Lewes Road,Brighton BN24GJ,UK
Received 25February 2009;accepted in revised form 11June 2009;available online 16June 2009
Abstract
We have analysed the halogen concentrations and chlorine stable isotope composition of fluid inclusion leachates from three spatially associated Fe-oxide ±Cu ±Au mineralising systems in Norrbotten,Sweden.Fluid inclusions in late-stage veins in Fe-oxide–apatite deposits contain saline brines and have a wide range of Br/Cl molar ratios,from 0.2to 1.1Â10À3and d 37Cl values from À3.1&to À1.0&.Leachates from saline fluid inclusions from the Greenstone and Porphyry hosted Cu–Au prospects have Br/Cl ratios that range from 0.2to 0.5Â10À3and d 37Cl values from À5.6&to À1.3&.Finally,the Cu–Au deposits hosted by the Nautanen Deformation Zone (NDZ)have Br/Cl molar ratios from 0.4to 1.1Â10À3and d 37Cl values that range from À2.4&to +0.5&,although the bulk of the data fall within 0&±0.5&.
The Br/Cl ratios of leachates are consistent with the derivation of salinity from magmatic sources or from the dissolution of halite.Most of the isotopic data from the Fe-oxide–apatite and Greenstone deposits are consistent with a mantle derived source of the chlorine,with the exception of the four samples with the most negative values.The origin of the low d 37Cl values in these samples is unknown but we suggest that there may have been some modification of the Cl-isotope signature due to fractionation between the mineralising fluids and Cl-rich silicate assemblages found in the alteration haloes around the depos-its.If such a process has occurred then a modified crustal source of the chlorine for all the samples cannot be ruled out although the amount of fractionation necessary to generate the low d 37Cl values would be significantly larger.
The source of Cl in the NDZ deposits has a crustal signature,which suggests the Cl in this system may be derived from (meta-)evaporites or from input from crustal melts such as granitic pegmatites of the Lina Suite.Ó2009Elsevier Ltd.All rights reserved.
1.INTRODUCTION
The iron oxide–copper–gold (IOCG)deposit class has attracted much attention in recent years,both in terms of academic research and exploration activity.The deposits are characterised by Cu-sulphides ±Au hydrothermal ores with abundant magnetite or hematite and occur in rocks ranging from Late Archaen to the Cenozoic in age
(Williams et al.,2005).IOCG deposits do not have a clear spatial association with igneous rocks,have disputed tec-tonic settings and variable geological characteristics.Also,the sources of the major components in many of the depos-its are unknown.However,all the deposit types are com-monly found in sequences which have undergone large scale sodic alteration and contain Cl-rich silicate minerals such as scapolite,biotite and amphiboles.
The relationship between Fe-oxide–apatite (e.g.Kiruna-type)and the IOCG deposits has also been a contentious point in the literature.The common spatial relationship
0016-7037/$-see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.gca.2009.06.005
*
Corresponding author.
E-mail address:sgleeson@ualberta.ca (S.A.Gleeson).
/locate/gca
Available online at
Geochimica et Cosmochimica Acta 73(2009)
5658–5672
of these deposit types(and in some cases their direct super-position)has lead to suggestions that they are part of the same deposit class or that there are genetic links between the two(Hitzman et al.,1992).This is supported by early stage magnetite alteration in many IOCG deposits(e.g. Smith et al.,2007),the late-stage occurrence of pyrite,chal-copyrite and gold in and near massive magnetite deposits, and the common features in alteration associated with both deposit types(Sillitoe,2003).However,recent work has highlighted a significant difference in timing of the two de-posit types in some areas(Hitzman,2000)indicating that a direct genetic link between the two may not exist.
One of main difficulties in constructing a genetic model for these deposits is determining thefluid sources at differ-ent stages of mineralisation,in part due to the modification offluid stable isotope characteristics by water–rock interac-tion in the deposit class as a whole(Haynes,2000).A num-ber of different genetic models have been suggested for the deposit class including(1)a magmatic source for the miner-alisingfluids(e.g.Pollard,2000,2006);(2)a magmatic source that has been modified by large scalefluid circula-tion forming the regional sodic alteration and adding met-als to the mineralisingfluid(e.g.Oliver et al.,2004);(3)a non-magmatic,evaporite or near-surface continental brine derived origin for thefluids(Barton and Johnson,1996, 2000;Xavier et al.,2008);(4)a metamorphic source for thefluids(Fisher and Kendrick,2008);(5)a mixed mag-matic–basinal brine origin(Chiaradia et al.,2006;Baker et al.,2008;Kendrick et al.,2008).Metamorphism of evap-orites has also been invoked to explain the regional distri-bution of sodic alteration in the Kiruna district, Norrbotten,Sweden although no Fennoscandian evaporitic sediments are preserved(Frietsch et al.,1997).However,it has been suggested that some occurrences of apparently stratigraphically restricted scapolite in the Greenstone group have are related to former evaporite beds(Martins-son,1997).It is now recognised that the class represents a diverse group with the potential for a wide range of poten-tialfluid sources(Williams et al.,2005).
In Norrbotten both deposits hosted by the Greenstone and Porphyry Group metavolcanic rocks,and deposits hosted regionally significant deformation zones such as the Nautanen Deformation Zone(NDZ),have been pro-posed to belong to the IOCG class.They are linked by the ubiquitous presence of magnetite as an alteration phase, and the common occurrence of scapolite in alteration assemblages,despite other variations.It is,therefore,perti-nent to examine the deposits together in order to investigate the range of brine sources operating over time in the area, and in particular to investigate if common sources periodi-cally suppliedfluid accounting for the geochemical similar-ities in the deposit types.In this study,we examine the Cl and Br concentrations and,using an on-line mass spectro-metric technique,the stable chlorine isotopic composition of dilute leachates(>20ppm total chloride)derived from microthermometrically well characterised vein quartz sam-ples.We compare the source of Cl in Kiruna-type Fe-oxide–apatite and IOGC deposits of the Norrbotten dis-trict,Sweden and provide new constraints on thefluid source and water–rock interaction history in deposits and prospects in this important metallogenic province.Bulk leachates are currently the only way to examine the chlorine stable isotope chemistry of inclusionfluids.
2.BACKGROUND GEOLOGY
The major iron ore province of northern Sweden is lo-cated in Norrbotten County and is mainly hosted by Palae-oproterozoic rocks(see reviews by Carlon(2000)and Bergman et al.(2001)).These deposits are mainly Karelian (2.5–2.0Ga)and Svecofennian(1.9–1.88Ga)in age(Fig.1) and are preserved in deformed and metamorphosed belts, intruded by a range of granitoid plutons.Metamorphic conditions peaked at upper greenschist or lower amphibo-lite facies during the Svecofennian Orogeny from1.9to 1.8Ga(Skio¨ld,1987).A detailed lithostratigraphy of these rocks has been proposed by Martinsson(1997).The Green-stone Group(>1.9Ga),consisting of mainly tholeiitic(Ek-dahl,1993)to komatiitic(Martinsson,1997)volcanic rocks overlies Archaean basement.These are overlainfirst by the Middle Sediment Group(Witschard,1984),and then by the Porphyry Group,which consists of volcanic and sub-volca-nic rocks,subdivided in the Kiruna area into the domi-nantly andesitic Porphyrite Group,and the syenitic and quartz-syenitic Kiruna Porphyries which host the Kiruna-vaara magnetite–apatite deposit.In view of their proximity to the Kirunaavaara deposits the Kiruna porphyries may have acquired their syenitic character via alkali metamor-phism,and it is likely that the original volcanic rocks were calc-alkaline in character.The Haparanda and Perthite–monzonite calc-alkaline and alkali-calcic granite suites in-truded these rocks between1.9and1.8Ga(Skio¨ld,1987) followed by the Lina Suite granitoids at around1.79Ga (Skio¨ld,1987;Bergman et al.,2001).The youngest plutonic rocks in the area are TIB2granitoids,at around1.71Ga in age,exposed at the Swedish–Norwegian border(Romer et al.,1994).The area is cross-cut by a series of large scale shear systems including the NDZ,which is notable for its association with mineralisation.It is a NNW trending tec-tonic structure where strongly schistose or mylonitic rocks occur in several high strain branches in a zone up to3km wide(Martinsson and Wanhainen,2004).
The Palaeoproterozoic rocks of Norrbotten area are af-fected by scapolite and albite alteration at both the regional and deposit scale,where they are associated with both iron oxide and Cu–(Au)mineralisation(Frietsch et al.,1997). Samples for this study were taken from three groups of mineralising systems;late-stage quartz veins cutting Fe-oxide–apatite deposits and associated alteration;minerali-sation-related quartz veins from Cu–(Au)prospects hosted by the Greenstone and Porphyry Groups,and mineralisa-tion-related quartz veins from the heavily deformed meta-volcanic rocks of the NDZ.
Data on the timing of the scapolization of the area are limited,but Smith et al.(in press)report a U–Pb titanite age of1903±8Ma for titanite in a scapolite altered diorite at Nunasvaara.The Fe-oxide–apatite deposit at Kiirunava-ara has been dated as forming between1884±6and 1875±9Ma(Romer et al.,1994),whilst Storey et al. (2007)showed that titanite from Luossavaara had distinct
Chlorine isotopes in iron oxide–copper–gold deposits Sweden5659
cores with ages of ca.2050Ma,and rims with ages of 1870±24Ma,indicating that the Fe deposits are truly epi-genetic.The timing of Cu-deposition has been addressed by Billstro ¨m and Martinsson (2000),who identified two main periods of IOCG mineralisation of 1850–1880Ma,and from 1750to 1800Ma.The early period is supported by re-cent work on the Rakkurija ¨rvi prospect (Smith et al.,2007),where Re-Os analyses of molybdenite give ages of 1853±6Ma and 1862±6Ma,whilst the later period is associated with the major deformation zones at Nautanen and elsewhere.The later period also corresponds to a peri-od of metamorphism of pre-existing deposits identified at Malmberget (initial mineralisation ages of 1920±23Ma with lead loss and re-equilibration of titanite down to 1708±20Ma,Storey et al.,2007),in titanite from Cu-prospects (Smith et al.,in press),and in secondary modifi-cation and sulphide mineralisation at Kiirunavaara (Cliffand Rickard,1992;Romer et al.,1994).The data available at present are consistent with an overlap in time between re-gion Na-alteration and Fe-oxide–apatite mineralisation,with the initial period of Cu mineralisation overlapping with,or slightly postdating the late stages of Fe-oxide min-eralisation.They indicate periods of both Fe-oxide and in-ital Cu mineralisation overlap with the intrusion period of the Haparanda and Perthite monazite suite granitoids and
with the initial period of Svecofennian metamorphism,whilst the later group of Cu deposits overlap with the intru-sion of the Lina Suite granitoids and a later period of meta-morphism (Bergman et al.,2001;Martinsson,2004).2.1.Fe-oxide–apatite deposits
The Fe-oxide–apatite bodies are typified by the Kiruna-vaara–Luossavaara magnetite dominated bodies,and the Per Geiger ores which contain both hematite and magnetite (Geijer,1910,1931;see Martinsson,2004for a recent re-view).The deposits are accompanied by sodic and potassic alteration (albite–K–feldspar–biotite),which in some in-stances includes scapolite.Samples from the Kirunava-ara–Luossavaara deposit are either from quartz–carbonate–(magnetite–titanite)veins which cut the ore body,or one sample from the summit of Kirunavaara which consists of a quartz–calcite–actinolite–hematite–titanite vein which forms a late-stage fracture fill in the ma-trix of the magnetite-cemented,Na–K altered metavolcanic breccia exposed in the hanging wall of the ore body.A quartz–albite–tourmaline vein was sampled cutting the main magnetite–apatite body at Mertainen.Samples from the magnetite–hematite–apatite bodies of the Per Geiger ores are taken from the Nuktus and Henry bodies,and
in-
Fig.1.Location map of the study area with regional geology after Bergman et al.(2001).Samples were collected from Fe-oxide–apatite deposits,Greenstone and Porphyry-hosted Cu–Au deposits and Cu–Au deposits associated with the Nautanen Deformation Zone.
5660S.A.Gleeson,M.P.Smith /Geochimica et Cosmochimica Acta 73(2009)5658–5672
clude quartz–chalcopyrite or quartz–hematite veins cutting the oxide ore bodies or the surrounding rock.
2.2.Porphyry and Greenstone hosted Cu–Au deposits
Quartz vein samples related to Cu–Au mineralisation were taken from the Pahtohavare(Lindblom et al.,1996; Martinsson,2004),Gruvberget(Martinsson and Virkkunen, 2004),Kiskamavaara(Martinsson and Wanhainen,2000) and Kallosalmi(Wa¨gman and Ohlsson,2000)deposits and prospects.Both Pahtohavare and Kallosalmi are Green-stone group hosted mineralisation,and Pahtohavare was mined from1990to1997.The mineralisation in both cases is developed in basaltic metavolcanics and metasedimentary schists affected by sodic alteration including both scapolite and albite,potassic alteration and carbonate metasomatism (Martinsson,2004).At Kiskamavaara a quartz vein was sampled from hematite cemented K-altered metavolcanic breccia.At Gruvberget Cu mineralisation directly over-prints a magnetite–hematite–apatite body associated with intense scapolite–albite and later potassic alteration.The Cu mineralisation is associated with the potassic(K–feld-spar)alteration(Martinsson and Virkkunen,2004).
2.3.Nautanen Deformation Zone
Quartz veins associated with Cu mineralisation where also taken from sites associated with the NDZ.The miner-alisation is associated with scapolite,tourmaline,sericite and K–feldspar–epidote alteration,alongside the develop-ment of metasomatic garnet,and is hosted within a major shear zone(Martinsson and Wanhainen,2004).The sam-ples come from both the main Nautanen prospect,and from the Ferrum vein hosted Cu–Au prospect on the mar-gins of the main deformation zone.
3.ANALYTICAL TECHNIQUES
Petrographic and microthermometric studies were car-ried out on doubly-polishedfluid inclusion wafers for all sample veins.Microthermometric data was collected using a Linkam THMSG600heating and freezing stage which was calibrated atÀ56.6,0,and10°C using syntheticfluid inclusion standards and distilled water,and at high temper-atures using a range of pure solids.Reported temperature measurements have a precision of±0.2°C on cooling runs, and for heating runs within±1°C.
Quartz veins were isolated from the host rock using a small rock saw,coarsely crushed and hand-picked for pur-ity,and then crushed and leached using the technique de-scribed in Gleeson and Turner(2007).Leachate samples were analysed for anions(including ClÀand BrÀ)using a Dionex DX600ion chromatographfitted with an AS-14 analytical column.The detection limit for the anions was 0.008ppm.Analyses of standard solutions and replicate analyses of leachates were reproducible within5%.
The samefluid inclusion leachate analysed by ion chro-matography was used for Cl-isotope analysis.Chloride in the leachate was isolated by precipitation of silver chloride salt(AgCl)as described by Long et al.(1993)and Wassenaar and Koehler(2004).Filters were placed inside foil wrapped10ml glass serum vials to prevent photo-disso-ciation(Long et al.,1993).Quantitative conversion of AgCl to CH3Cl gas was carried out at the Stable Isotope Hydrol-ogy and Ecology Laboratory of Environment Canada in Saskatoon,Saskatchewan using the Iodomethane(CH3I) reaction as described by Wassenaar and Koehler(2004). The d37Cl values were obtained using a multicollector GV Instruments Isoprime TM IRMS.Multiple injections of 100%CH3Cl were repeatable within(±SD)0.06&for d37Cl analyses(Wassenaar and Koehler,2004).To correct the CH3Cl d37Cl values relative to the Standard Mean Ocean Chloride(SMOC)reference(Kaufmann et al., 1984),standards were prepared using200l L of Ocean Sci-entific Internal Stock Atlantic Seawater and G-10953sea-water(internal east coast seawater standard).
4.RESULTS
4.1.Fluid inclusion petrography and microthermometry
The Palaeoproterozoic rocks of the northern Norrbotten district have typically reached lower greenschist facies meta-morphic conditions syn-to post-mineralisation(Bergman et al.,2001),and vein quartz commonly shows some evi-dence for deformation and recrystallisation along grain mar-gins.As a result careful characterisation was necessary in order to ensure that inclusions are related to the mineralising event.All thefluid inclusions analysed were classified according to their room temperature phase assemblage (see Table1and Figs.2and3)and the results of thefluid inclusion microthermometric study are summarised in Table 1.The full details of all heating–freezing experiments will be the subject of a future paper.Many of the observations made here are in agreement with previous work by Broman and Martinsson(2000)and Lindblom et al.(1996).Weight% NaCl equivalent salinities were calculated from halite disso-lution temperatures using the equation of Sterner et al. (1988)for halite bearing inclusions,from the ice melting temperature using the equation of Bodnar(1993)for aque-ous inclusions,and from clatherate melting in the Q2assem-blage using the equation of Diamond(1992).
The late-stage quartz veins from the Fe-oxide–apatite deposits contain a large number offluid inclusions,and the paragenetic setting of inclusions is sometimes difficult to determine.However,along with inclusions trapped on annealed trails,inclusions of consistent microthermometric properties sit in unequivocally primary settings,typically trapped in arrays that parallel growth zones or isolated in grain cores(Fig.2A and B).Hence,we interpret the mic-rothermometric properties of these inclusions to represent thefluid present during and immediately post-quartz vein formation.The inclusions are dominantly halite saturated (Fig.3A)with a salinity of30–40wt.%NaCl eq.and homogenisation temperatures(T h)in the range of100–150°C.The exception to this is a single sample from the Mertainen magnetite body which had salinities in the range 40–60wt.%NaCl eq.The sample from the Henry Fe-oxide–apatite body is also notable in that halite-bearing inclusions showed a double meniscus and a small amount of CO2
Chlorine isotopes in iron oxide–copper–gold deposits Sweden5661
within the inclusion,and was the only occurrence of a min-or aqueous–carbonic inclusion population in these veins.
In Pahtohavare and other relatively undeformed IOCG deposits(Fig.2C and D),the inclusions are likewise trapped in growth zones with some secondary trails,which are dominated by CO2-rich inclusions(Lindblom et al., 1996).In both deposit types previous workers have also identified inclusions as primary or pseudosecondary in ori-gin(Lindblom et al.,1996;Broman and Martinsson,2000; Harlov et al.,2002).Thefluids associated with quartz veins from Greenstone and Porphyry hosted Cu–(Au)deposits and prospects,ranged from much higher salinities(around 50–60wt.%NaCl eq.)at Pahtohavare and Kallosalmi and one example from Gruvberget,to those comparable with the late-stage veins associated with the Fe-oxide bodies at Kiskamavaara and an additional sample from Gruvberget (Fig.3B–D).In a number of cases a secondary carbonic inclusionfluid inclusion population occurred composed of virtually pure CO2(all inclusions had solid CO2melting atÀ56.6±0.2°C).
At Nautanen and other NDZ related deposits all the inclusions analysed are trapped in annealed fractures.High salinity inclusions are similar to those noted as primary in the Aitik deposit by Wanhainen et al.(2003),who also noted that such inclusions are associated with solid inclu-sions of chalcopyrite,and inclusions with daughter or trapped chalcopyrite grains.
The inclusion assemblage from deposits associated with the NDZ includes coexisting inclusions containing aqueous liquid,a halite daughter and a vapour phase(Lw+Sh+V; Fig.3E)and some contained a liquid carbonic phase also (Lw+Lc+V and Lc+V:Fig.3F).In some instances
Table1
Summary of the microthermometric data.
N Inclusion types Para.XCO2Salinity Th
Mean Min Max Mean Min Max Mean Min Max Magnetite–(Hematite)–apatite
KR228Lw+Sh+V P/PS/S ND33.231.436.610294117 L4.128Lw+Sh+V P/PS/S ND36.533.441.2122102156 03HENRY0416Lw+Sh(+Lc)+V P/PS/S0.020.010.0334.733.337.911896165 03HENRY0410Lw+Lc+V PS/S0.040.030.0621.421.421.5212189242 N128Lw+Sh+V P/PS/S ND33.331.535.3150106202 N2.224Lw+Sh+V P/PS/S ND34.932.939.410381124 N2.521Lw+Sh+V P/PS/S ND35.331.538.511087131 MER225Lw+nS+V PS/S ND52.040.259.5145105164 Greenstone-and Porphyry-hosted Cu
KALL9100259.53m15Lw+nS+V P/PS ND49.132.761.1139111154 11Lw+Sh+V PS/S ND32.231.632.910190107
6Lw+V PS/S ND24.223.924.8120110138 PAH8821721.20m16Lw+nS+V P/PS/S ND53.646.357.4119102132 PAH85117152.14m19Lw+nS+V P/PS/S ND49.041.457.0161122193
Lc+V PS/S Lc+V inclusions(S)
P1123Lw+nS+V P/PS/S ND42.034.149.1137124164 K227Lw+Sh+V P/PS/S ND33.531.137.2118103150 G622Lw+nS+V P/PS ND42.838.148.811997159 8Lw+Sh+V PS/S ND41.140.541.6128120134 G5.116Lw+nS+V P/PS ND34.533.237.1130120138
Lc+V PS/S Lc+V inclusions(S)
Nautanen Deformation Zone(NDZ)-hosted Cu
FERR6920572.75m3Lw+Sh+V PS/S ND31.328.133.9141135147 18Lw+V PS/S ND21.718.624.0152130214 NAU77006210.4m5Lw+Lc+V PS/S0.180.170.29.1 3.712.4263243300 17Lw+Sh+V PS/S ND34.831.541.021*******
22Lw+V PS/S ND19.412.028.3144106203
Lc+V PS/S Lc+V inclusions(S)
NAU77006281.7m18Lw+Sh+V PS/S ND33.231.335.0133112183 11Lw+Sh+V PS/S ND28.927.829.9140117143
Lc+V PS/S Lc+V inclusions(S)
03NAU0220Lw+Sh+V PS/S ND33.230.235.4146132183
Lc+V PS/S Lc+V inclusions(S)
NAU83009110.90m30Lw+Sh+V PS/S ND29.326.232.2158113206 3Lw+Lc+V PS/S0.230.190.2910.3 6.712.9
Lc+V PS/S Lc+V inclusions(S)269250281 N,number of inclusions analysed;Lw,liquid water;Sh,halite;V,vapour;Lc,carbonic liquid;nS,multiple solids;P,primary;PS, pseudosecondary;S,secondary;XCO2,Mole fraction CO2calculated from estimated phase volumes;T h,Liquid–vapour homogenisation temperature;ND,not determined.
5662S.A.Gleeson,M.P.Smith/Geochimica et Cosmochimica Acta73(2009)5658–5672
these are preserved along the same secondary fluid inclusion trails.Lw +Sh +V inclusions from some veins at Nauta-nen had similar salinities and homogenisation temperatures to those from the Fe-oxide and Cu–Au deposits already dis-cussed.At Nautanen,complex inclusion assemblages were hosted on secondary trails,suggesting phase separation oc-curred in the system.Salinities varied from $12–18wt.%NaCl eq.for Lw +Lc +V inclusions to $29wt.%NaCl eq.for halite bearing inclusions.Total homogenisation tem-peratures for Lw +Lc +V inclusions were typically around 250–300°C,whilst L ÀV homogenisation for halite bear-ing inclusions was around 100–120°C.At both Nautanen and Ferrum a further assemblage of brines was observed.These are reported with NaCl-equivalent salinities in Table 1,but their freezing behaviour was consistent with a Ca-rich brine,with Ca constituting up to 50%of the cations present (Oakes et al.,1990).
4.1.1.Suitability of the samples for bulk extraction
All the samples from Fe-oxide–apatite bodies contained halite-bearing fluid inclusions at room temperature (Table 1),with the exception of a chalcopyrite-bearing quartz vein from the Henry deposit,which also contained inclusions of an aqueous–carbonic fluid.The samples from the Green-stone and Porphyry-hosted Cu mineralisation were also dominated by single populations of inclusions with multiple daughter solids (at Pahtohavare identified as including syl-vite,calcite,hematite,graphite and others see Lindblom et al.,1996),including halite in all cases.Lower salinity sec-ondary inclusion populations occurred in samples from Kallosalmi,and secondary CO 2-rich inclusions were a fea-ture in some samples,however,these will not affect the overall Cl budget in the sample.These samples are,there-fore,ideally suited to crush–leach analytical techniques in-tended to determine the composition of the high salinity inclusions.
The samples from the NDZ-related deposits typically showed more complex inclusion populations.Samples 03NAU02and FERR6920572.75m were both dominated by a single fluid inclusion population –halite-bearing inclu-sions at Nautanen and aqueous-liquid plus vapour at Fer-rum,and so are directly suitable for crush–leach analysis.Other samples from Nautanen showed complex,multistage inclusion assemblages,in most case with evidence for
co-
Fig.2.Evidence for primary origin of inclusions.(A)Quartz grain from sample L4.1.Halite-bearing aqueous inclusions occur in growth zone parallel bands and isolated settings (I and II)and in secondary trails (III).(B)Quartz grain from sample N2.The grain is nucleated on magnetite wall rock,and shows lath shaped magnetite growth along a crystal face.Primary bands of halite-bearing fluid inclusions (II–IV)and solid magnetite inclusions are parallel to the growth face.The crystal is also cut by secondary trails of halite bearing inclusions (I).(c)Quartz grain from sample PAH85117152.14m.Vein margin parallel bands of fluid inclusions are overgrown by relatively inclusion free quartz with actinolite inclusions.(D)Quartz grain from sample KAL9100259.53m.The core of the grain shows a high inclusion density with occasional large inclusions (I)and is overgrown by an inclusion free rim.In all cases the quartz is cut by trails of secondary inclusions.The scale bar in photomicrographs of fluid inclusions is 10mm unless otherwise shown.
Chlorine isotopes in iron oxide–copper–gold deposits Sweden 5663
existing halite plus liquid plus vapour (Lw +Sh +V)and aqueous–carbonic inclusions (Lw +Lc +V)indicative of aqueous–carbonic phase separation.In these cases the leachate will be a mixture of material from all inclusion gen-erations,but the halogen signature will be dominated by the most saline populations –typically the Lw +Sh +V inclu-sions which are part of the unmixed aqueous–carbonic assemblage.
4.2.Halogens and d 37Cl values
Halite bearing fluid inclusions in the Fe-oxide–apatite deposits have a range in Br/Cl molar ratios,from 0.2to 1.1Â10À3,although the bulk of the data span the range 0.2–0.3Â10À3(Table 2;Fig.4).These samples have d 37Cl values from À3.1&to À1.0&.Leachates with salin-ities dominated by ultra saline fluid inclusions from the Greenstone and Porphyry hosted Cu–Au prospects have Br/Cl ratios that range from 0.2to 0.5Â10À3and d 37Cl values from À5.6&to À1.3&(Table 2;Fig.4).The latter value is the lowest d 37Cl value obtained from fluid inclusion leachates to our knowledge.Finally,the NDZ hosted Cu–Au deposits have Br/Cl molar ratios that range from 0.4to 1.1Â10À3and have significantly different d 37Cl values that range from +0.5&to À2.4&,although the bulk of the data fall within ±0.5&of 0&(Table 2).
5.DISCUSSION
The aim of this study was to compare and contrast the source of Cl in the three spatially related mineralizing sys-tems.The Br/Cl ratios of sedimentary brines have been successfully used to distinguish between Br-rich fluids gen-erated by evaporative concentration of seawater and Cl-rich fluids derived from the dissolution of halite (e.g.Kes-ler et al.,1995).The Br/Cl ratios of metamorphic fluids can vary widely but seem to be strongly controlled by the original source of the salinity rather than metamorphic grade (Yardley and Graham,2002).The Br/Cl composi-tion of magmatic fluids is less well constrained,mostly due a paucity of data on the behaviour of Br during the exsolution of a hydrothermal fluid from a crystallizing melt and subsequent processes such as phase separation (see discussion in Nahnybida et al.(in press)).However,previous studies on porphyry deposits suggest that many magmatic systems are associated with hydrothermal fluids with molar Br/Cl ratios in the range 0.5to 2.0Â10À3(Ir-win and Roedder,1995;Bo ¨hlke and Irwin 1992;Banks et al.,2000a,b;Kendrick et al.,2001;Nahnybida et al.,in press ).
There are two main reservoirs of Cl on the planet.The crustal chlorine reservoir is dominated by the oceans,with isotopic values around 0&in the Phanerozoic (Kaufmann et al.,1984;Godon et al.,2004a ).Analyses of Phanerozoic evaporites yield d 37Cl data with a total range of 0&±0.9&(Eastoe et al.,2007).Likewise shield brines and most formation waters also have compositions within this range (e.g.Eastoe et al.,1999,2001;Shouakar-Stash et al.,2007),although there is one study of formation waters in the North Sea that yield data down to –4.7&(Ziegler et al.,2001).Sharp et al.(2007)report d 37Cl val-ues from Precambrian cherts in the range of À3.16&to +1.04&with the bulk of the data falling within 1&of 0.The Cl in these samples is presumed to be hosted in sal-ine fluid inclusions of unknown age.If these samples truly are representative of Precambrian seawater,this may sug-gest that the Cl isotopic composition of Precambrian oceans were not dissimilar to those of the
Phanerozoic.
Fig.3.Representative examples of fluid inclusion types in vein quartz samples.(A)Lw +Sh +V inclusion,Kiirunavaara.(B)Lw +Sh +V inclusion,Nuktus.(C)Lw +nS +V inclusion,Gruvberget.Daughter solids include halite and a carbonate phase.(D)Lw +nS +V inclusions,Pahtohavare.Daughter solids include halite and a carbonate phase.(E)Lw +Sh +V inclusion,Nautanen.(F)Lw +Lc +V (carbonic phase dominated)inclusions,Nautanen.
5664S.A.Gleeson,M.P.Smith /Geochimica et Cosmochimica Acta 73(2009)5658–5672。