Fluid inclusions of the massive sulfide deposits in the Skellefte district, Sweden

Chemtcal Geology, 61 (1987) 161-168 Elsewer Scmnce Pubhshers B V, Amsterdam - - Printed m The Netherlands

161

FLUID INCLUSIONS OF THE MASSIVE SULFIDE DEPOSITS IN THE SKELLEFTE DISTRICT, SWEDEN CURT BROMAN Ore Research Group, Department of Geology, Stockholm Unwers~ty, S-106 91 Stockholm (Sweden) (Accepted for pubhcatmn July 11, 1986)

Abstract Broman, C , 1987 Fired mclusmns of the masswe sulfide deposits m the Skellefte district, Sweden In E E Horn and H -J Behr (Guest-Editors), Current Research on Flmd Inclusmns, ECRFI, Gottmgen, April 10-12. 1985 Chem Geol, 61 161-168 The masswe sulfide deposits of the Skellefte district are located m northern Sweden The ores are hosted by Precambrmn submarine volcanlcs and sedimentary rocks Both rocks and ores have been deformed and metamorphosed, mainly to greenschlst facms Aqueous two-phase (hqmd-vapor) primary inclusions m sphalente, quartz and calcite from six mines (Ravhden, Kmstmeberg, Ostra Hogkulla, Bjurtrask, Renstrom and L~ngdal) mdmate that the sulfide ores were formed under non-boiling condltmns during two major stages of hydrothermal actlwty at temperatures of 210 ° and 295 ° C, respectively The ore-forming flmds were Ca-Na-Cl-bearmg hydrothermal solutmns with a salt content of 4_+ 2 eq wt % NaC1 Later recrystalhzatmn of the ores occurred at lower temperatures mvolwng solutmns with higher sahmtms

1. Introduction

2. Geology

The Skellefte district is a metallogenlc province of Precambnan massive sulfide ores situated along the northern margin of the Norrland geosynchne m northern Sweden (Fig 1) The ores have been referred to as having a volcanogemc-exhalahve origin, being formed on the sea floor (Rlckard and Zweffel, 1975) during a permd of rifting within a volcamc arc (Vwallo, 1985 ) This paper reports a fired mclusmn study of some of these sulfide deposits The purpose of the mvestlgatmn was to corroborate the mltml data of Broman and Lmdblom (1984) and test to the model of ore formatmn proposed by Rlckard and Zweffel (1975)

The geology of the Skellefte district has been previously described by Gavehn and Grip (1946), Helfrlch (1971), Rmkard and Zwelfel (1975) and Lundberg (1980) The district is dominated by Precambnan volcamcs and sedimentary rocks ofsubmarme ongm (Fig 1 ) that have been folded and intruded by two mam generations of gramtolds and metamorphosed in relation to the Svecokarehan orogney The degree of metamorphism is low greenschlst facms, except m zones near the gramte contacts ( Rlckard and Zwelfel, 1975 ) The geologmal setting is characterized by the mteractmn of volcamc actlwty and sedimentation (Helfnch, 1971) The general stratl-

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Fig 1 Generahzedgeologicalmap of the Skellefte dmtnct (after Grip, 1973) showinglocation of mines dmcussed m text graphical successton as shown m Fig 2, is divided into a volcamc and a phylhte serms (Gavehn and Grip, 1946) Recent prehmmary reports on the petrology and geochemistry of the volcamc rocks m the area have been presented by Claesson (1982) and Vlvallo (1985) Briefly the lower parts of the volcamc series consist of acid to intermediate volcamc rocks wtth a rhyohtm and dacltm composltmn whereas the upper parts have a more basxc composltmn with andesltes and basalts (Fig 2) The volcamc pde grades into an overlying sedimentary sequence (Helfnch, 1971), the phylhte serms, that consmts of bedded phylhtes and greywacke phylhtes w~th mtercalatmns of sandstones and conglomerates ( Lundberg, 1980) Most of the sulfide deposits are emplaced close to the contact between the volcamcs and the overlying phylhtes and often associated with calcareous horizons (Fsg 2) Some deposits,

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Fig 2 Slmphfied stratigraphy of the Skellefte dmtmct, modlfmd from Gavehn and Grip (1946), Helfnch (1971), Rlckard and Zwelfel (1975), and Lundberg (1980) Letters show position of ore deposits described m text M (mare ore position) = Ravhden, Ostra Hogkulla, B]urtrask, Renstrom and L~ngdal, K = Knstmeberg

163 such as Knstlneberg, are located lower down m the stratigraphy The deposits form elongated lens-shaped stratabound massive F e - Z n - C u - P b - s u l f i d e bodies which are underlain by crosscutting stringer zones w~th d~ssemmated Fe-Cu-sulfides and alteratmn zones where the volcamcs have been transformed into senclte or chlorite schists (Rlckard and Zwelfel, 1975 ) 3. F l u i d i n c l u s i o n s m e t h o d All the samples selected for study are Zn-rlch specimens of the massive ore and collected from six mines Ravhden, Krlstlneberg, Ostra Hogkulla, Bjurtrask, Renstrom and Lfingdal (Fig 1 ) Sphalerlte together with pyrite are the predominant ore minerals Pyrrhot~te, chalcopyr~te, galena and arsenopynte appear irregularly m varying amounts The major gangue minerals, quartz and calcite, occur as individual grains between the sulfides Typical ore samples are more or less banded with alternating sphalerlte and pyrite concentrations, but in the K n s t m e b e r g samples sphalente occurs with chlorite as a fine-grained groundmass to the pyrite Flmd inclusions were investigated using doubly pohshed 100-/~m-thlck u n m o u n t e d thin sections of sphalente, quartz and calcite With the exceptmn of Krlstlneberg the sphalente was dark red which sometimes made it very difficult to find statable flmd inclusions In contrast the sphalerlte from Knstlneberg was more transparent ( light yellow ) The mmrothermometrlc analyses were made with the md of a heating-freezing stage of Chmxmeca ® constructmn ( P o t y et al, 1976) that was cahbrated against known melting points of pure substances 4. R e s u l t s

4 1 Fluid mclusmn morphology The mveshgatlon was performed principally on primary fluid mclusmns ( Roedder, 1981 ) but

a small number of secondary inclusions were also measured The inclusions were all of the aqueous twophase ( h q m d - v a p o r ) type No CO2 was observed and no evidence for boihng was found The size of the inclusions was generally between 5 and 10/~m, but occasional secondary inclusions were larger (up to ~ 20/~m ) The following varieties of flmd inclusions were observed in the Skellefte ores (1) Primary inclusions m sphalente and quartz from Ravllden, Ostra Hogkulla, Bjurtrask and L~ngdal that were rounded in shape and contained ~ 2-5 vol % vapor (2) Primary lnclusmns m calote from Krlstmeberg, Bjurtrask and Renstrom that were typically flat and rhombohedral-shaped with up to 20 vol % vapor (3) Secondary mclusmns m sphalerlte from Langdal that occurred along twin planes The twins are presumed to be deformation twins The inclusions were irregularly shaped with ~ 2 vol % vapor (4) Secondary lnclusmns m sphalente from Krlstmeberg Often rounded but sometimes tetrahedral-shaped mclusmns were found The vapor phase occupmd ~ 1 vol % ( 5 ) Secondary mclusions that occurred along rehealed mlcrofractures in quartz from Krlstlneberg These were irregular and often very angular m shape The vapor bubble occupmd ~ 5 vol %

4 2 Homogemzatmn temperatures The measured homogemzatlon temperatures for fired inclusions from the different mines are shown m Fig 3 All inclusions, both primary and secondary, homogenized into the llqmd phase Primary inclusions m samples from Ravhden, Ostra Hogkulla, Bjurtrask and L~ngdal indicate two temperature groups one between 130 ° and 220°C and the other from 220 ° to 340°C No differences between sphalerlte, quartz and calcite could be recogmzed Primary inclusions m calcite from K n s t m e b e r g

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and Renstrom fall within the upper temperature group The secondary inclusions in quartz from Kristineberg gave temperatures (150-240 °C) that coincide with the lower-temperature group Significantly lower temperatures ( ~ 90 ° C) were recorded for secondary inclusions m sphalente from K n s t m e b e r g and Lfingdal

4 3 Melting temperatures The first melting temperatures were consistently very low and close to the eutectlc points

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for the systems CaClz-NaC1-H20 ( - 5 2 °C) and CaC12-MgClz-NaC1-HzO ( - 5 7 ° C ) (see Crawford et al, 1979) Overall, first melting was m the order of - 50 ° C, mchcatlng that the fluids were Ca-Na-C1 solutions However, the sphalente from Krlstlneberg demonstrated the lowest first melting temperatures, mean value - 5 6 ° C, suggesting an additional Mg component The final ice melting temperatures are presented m Fag 4 The main characteristic features of these results are (1) The ummodal distribution of data around - 2 ° C for primary inclusions from Ravhden,

165

Ostra Hogkulla and Bjurtrask Inclusmns m quartz from Lfingdal display a similar pattern, however with a larger scatter and with most values close to - 1 ° C (2) The somewhat lower temperatures ( mean value - 6 ° C) of primary inclusions m calcite from K n s t m e b e r g One calcite sample ymlded a value as low as - 1 7 ° C (3) The clearly lower melting temperatures obtained for secondary mclusmns than those of primary fired mclusmns

4 4 Sahn~ty determmatmns Sahmtms are estimated from the final meltmg temperatures of ice, approximated to the NaC1-H20 system and expressed as equivalents of weight percent NaC1 (Potter et al, 1978) Errors from the presence of other salts (e g, CaC12) when estimating the sahmty are small ( Crawford, 1981 ) The final ice melting temperatures for the primary fluid inclusions, except for calcite from Knstmeberg, correspond to sahmtms of ~ 4 + 2 eq wt % NaC1 Inclusmns in the K n s t m e b e r g calcite showed higher concentrations with salmitres of ~ 10 eq wt % NaC1 Secondary mclusmns in sphalente from Lfingdal demonstrated sahmtms of ~ 1 9 eq wt % NaCl whilst mclusmns m quartz from K n s t m e b e r g showed a w~de range of sahmtms from 12 to 21 eq wt % NaC1 In sphalente from the same locality the distinctly low melting points indicated a fluid with high concentratmns of dissolved salts Daughter minerals were not observed m any of the mclusmns examined

5. D i s c u s s i o n and i n t e r p r e t a t i o n Although the Skellefte district ores have been subjected to regmnal low-grade metamorphism (Rlckard and Zweffel, 1975) the deposits are relatively well preserved Etching techmques have revealed many rehct primary textures, such as growth zoning and colloform textures, which are commonly found m younger volcan-

ogemc massive sulfide deposits (Rlckard and Zwelfel, 1975) Thus data for the primary inclusions are interpreted as mdmatlve of the original deposltmnal cond~tmns Fig 5 presents a summary of homogemzatmn temperatures and sahmty data for the Skellefte deposits Homogemzatmn temperatures show two m a m phases of mmerahzatmn, one at temperatures o f ~ 2 7 0 ° C and another near 180°C ( H and L m Fig 5, respectively) Perhaps their small size (5-10 tim) made it possible to survive the metamorphm heatmg without decrepitation In this respect Rlpley and Ohmoto (1977) reported small ( ~<5/lm) fluid inclusions m quartz from the p y n t e - c h a l c o p y n t e ores at the Raul mine, Peru, that were presumed to have resisted metamorphlc temperatures of 100-150 ° C m excess of their filhng temperatures Rlckard and Zwelfel (1975) and Lundberg (1980) interpreted the Skellefte district to be a Precambrmn island arc equivalent The deposits show most of the charactenstms of volcanogemc ores that have formed m a marine environment (Rlckard and Zweffel, 1975) The ore-forming fluids are suggested to be Ca-Na-C1 solutmns w~th the composltmn resulting from heated seawater modffmd by water-rock rateractions in the volcamc pile This is in agreement with the observation that the volcamcs underlying the ores are depleted in Ca and Na (Vlvallo, 1985) The sahmty of the primary mclusmns representing the two main phases of mmerahzatmn (4 _+2 eq wt % NaC1 ) is considered to reflect the concentration of the fluids in which the ores were hydrothermally deposited on the sea floor The shghtly higher sahmty (10 eq wt % NaC1) of the primary mclusmns m calcite from K n s t m e b e r g may be explained by mixing of the modified seawater with a local more sahne component, probably a magmatm fluid, related to the older gramtold m the wclnlty of the deposit (see Figs 1 and 2) Many slmllantms have been found between the Skellefte district ores and the Tertmry kuroko sulfide deposits of J a p a n (Rmkard and Zwelfel, 1975, Vlvallo, 1985) The kuroko

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Fig 5 Relatmn between homogemzatmntemperatures (uncorrectedfor pressure) and sahmty data for fluid mclusmnsm sphalente, quartz and calcite from the Skellefte district sulfide deposits Sohd symbols mdmate primary mclusmns interpreted to represent omgmalore-formingflmds Note the two distract groups labeledH and L Primary mclusmnsm calcite from Krlstmebergare mdmatedseparately (squares w~thdots), dependingon thmr differentsalt concentratmn {seediscusstun m text) Opensymbols refer to later generationsof mclusmns Isopycnlchnes are from Haas (1976) deposits are considered to have a submarine exhalatlve volcanic origin, related to blmodal volcamsm in an Island-arc environment (Tanlmura et a l , 1983) Interesting slmllamtms can be seen when a compamson is made with fluid mclusion data for some of the kuroko deposits Plsutha-Arnond and Ohmoto (1983) suggested that the kuroko ore-forming process revolve four periods of mmerahzation In period I, a fine-grained ore type was precipitated at 200-!-_ 50°C when the discharging ore-bearing hydrothermal solution was rapidly cooled by

seawater In periods II-IV, replacement of the first type occurred when later hydrothermal fluids migrated through the ore Period I might be represented by the lower-temperature phase of mlnerahzatlon m the Skellefte district deposits The higher-temperature phase m the present study is similar to pemod II, the major Z n - P b mmerahzatlon at 290 + 50 ° C, and period IV, the minor sphalemte mmerahzatlon at 2 8 0 + 2 0 ° C The only period m the kuroko model t h a t ts not comparable w~th the present mmrothermometrm data for the Skellefte dis-

167 trict is period III, major c h a l c o p y n t e + q u a r t z mineralization at 330_+ 30°C. Thas difference could be accounted for by the fact that samples from the Skellefte distract were taken only from the massive ore type The salmatles remained constant at 3 5-6 eq wt % NaC1 throughout all periods m the kuroko ores (Pasutha-Arnond and Ohmoto, 1983 ) However, subsequent metamorphic processes, with later generations of fluids revolved, have affected the Skellefte district ores Secondary mclusmns outlining rehealed mlcrofractures m quartz from Krlstmeberg represent a later fluid This fired healed the fractures around 200°C and was a moderately sahne aqueous solution with a salt concentratmn of ~ 15 eq wt % NaC1 The results from secondary mclusmns m sphalerite from Krlstaneberg are not shown in F~g 5 but they indicate that recrystalhzataon of sphalente took place at temperatures of 100°C m a highly saline, presumably C a - M g - N a - C 1 brine The sphalerlte from K n s t m e b e r g as accompamed by talc and Mg-rach chlorate (Du Rmtz, 1953) and seems to have become completely recrystalhzed an assoclatmn with the formatmn of these Mg-rlch minerals A separate type of fired mclusmns was found Inclusions m sphalente from the L~ngdal deposit are assumed to have formed when parts of the ore were recrystalhzed during deformation after the two mare phases of mmerahzahon These mclusmns mdmate trappmg condatmns for a solution with a salinity of 19 eq wt % NaC1 at low temperatures, in the anterval 70-120°C This could mdmate conditions during a later metamorphac event Further studms may give more detailed constraants on the relatmnsh:p between primary ore-deposition condatmns and the metamorphm overprint No pressure correctmns have been added to the homogemzatmn temperatures so the values presented here for the two mare phases of mineralization are only the mmamum temperatures of formatmn Since no evidence for boll-

mg was found the depth of seawater must have been at least 1 km as est:mated from pressure-temperature-density data for an aqueous solutmn with a sahmty of 4 eq wt % NaC1 (Haas, 1976) Based on foramamferal ewdence the kuroko manerahzatmn as eshmated to have occurred at a depth of 3500 m ( Guber and Merill, 1983) So if a samdar formatmn depth as assumed for the Skellefte distract ores a pressure correction for the homogenization temperatures should be 30-25 ° C (Potter, 1977) gnvlng formatmn temperatures of ~ 210 ° and ~ 295 ° C, respect:vely

6. Conclusions Finding statable fluid inclusions m metamorphosed ore deposxts is difficult and their xnterpretataon :s open to much d~scussaon as to whether the data truly represent the cond:tmns of ore formataon or not In spate ofth:s inherent amblgu:ty the data obtaaned in thas study are remarkably cons:stent and tend to support the hypothesis of a kuroko-type ore-forming environment for the deposlhon of the Skellefte distract sulfide ores Fired anclusmns an sphalerate, quartz and calcite mdmate that the ores were depos:ted from Ca-Na-C1 solutmns during two mare phases of mmeralazatlon around 210 ° and 295°C The salanitms of the ore-forming fluads remained generally constant (4 + 2 eq wt % NaC1) Secondary mclusmns demonstrated that the ores have subsequently been affected by more salane solutions at lower temperatures

Acknowledgements The project was lmtlated by David Rlckard and supervised by Sten Lmdblom I thank Waldo Vlvallo for stimulating discussions and constructive comments I am grateful to the Bohden Mineral A B for giving access to material and to the:r geologists for assistance with the fieldwork I am also grateful to the State M m m g Property Commlsion (N S G ) and the

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Swedish Geological Company (S G A B ) m Malh that allowed me to use drill-core samples from the Bjurtrask deposit The figures were drawn by Inger Arnstrom and Solvelg Jevall Doubly pohshed thin sectmns were prepared by K3ell Helge and Fernando Miranda Fmancml suppolt was provided by the Swedish Board for Technical Development (S T U ) and Stockholm Umverslty Finally I would hke to thank Tom Shepherd and John Rose-Hansen ior their revmws of the manuscript

References Broman, (' and Lmdblom, S , 1984 Studms of fired inclusions in the Ostra Hokulla deposit. Skellefte dlstrmt, Sweden S T U (Styr Tek Utv ), Stockholm, Rep '84, 16 pp {m Swedish) Clae~on, L .~, 1982 Stratigraphy and petrochemlstry of the ore-bearing Skelleffe Group volcamtes Geol Foren Sto~khohnForh, 104 378 379 Crawford, M L , 1981 Phase eqmhbrla m aqueous fired mclu~mns In L S ttolhster and M L Crawford, (Editors), Short Course m Flmd Inclusmns, Vol 6 Apph¢at~on to Petrology Mineral Assoc Can, Calgary, Alta, pp 75 100 Crawtc~fd, M I , Kraus, D W and Holhster, L S, 1979 Petrologac and fired mclusmn study of calc-slhcate rocks, Prince Rupert, British Colombia Am J S o , 279 1135-11,59 Du Rmtz, T . 1953 Geology and ores of the Knstmeberg deposit. Vesterbotten, Sweden Sver Geol Unders, Ser (', 524 1 90 Gavehn, S and Grip, E , 1946 Skellette- och Arvlds]aurfalten Geol Foren Stockholm Forh, 68 152-170 Grip, E , 1973 SkelletteFaltets sulfldmalmer In E Grip and R Frmtsch (Editors), Malta 1 Svenge, Vol 2 Almq~lst & Wlksell, Stockholm, pp 194-273 Guber, A L and Merrill III, S , 1983 Paleobathymetmc slgmtlcance of the foraminifera from the Hokuroko district Econ Geol Monogr,5 55 70 Haas, I t , J L , 1976 Thermodyn~rmc propertm~ of the coexisting phases and thermochemlcal properties of the

NaCI component m bmhng NaC1 solutions (prehmlnary steam tables for NaC1 solutions) U S Geol Surv, Bull 1421-B, 71 pp Helfrlch, H K , 1971 Stratlgraphm, Tektonlk, Petrochemm and montangeologische Zuge am Nordrand der zentralen "Norrlandgeosynclmale" Sver Geol Unders, Set C, 654 1-195 Lundberg, B , 1980 Aspects of the geology of the Skellefte field, northern Sweden Geol Foren Stockholm Forh, 102 155-166 P~sutha-Arnond, V and Ohmoto, H , 1983 Thermal history, and chemmal and isotopic compositions of the oreforming flmds responsible for the kuroko massive sulfide deposits m the Hokuroko district of Japan Econ Geol, Monogr, 5 523-558 Potter II, R W , 1977 Pressure corrections for fired inclusion homogemzatlon temperatures based on the volymemc properties of the system NaCI-H,O J Res U S Geol Surv, 5(5) 603-607 Potter II, R W , Clynne, M A and Brown, D L , 1978 Freezing point depression of aqueous sodmm chloride solutions Econ Geol, 73 284-285 Poty, B , Leroy, J and Jachlmow~cz, L , 1976 Un nouvel apparell pour la mesure des tempdratures sous le microscope L'mstallatlon de mmrotermomdtne Chalxmeca Bull Soc Fr Mmdral Cmstallogr,99 182-186 Rmkard, D and Zweffel, H , 1975 Genesis of Precambrlan sulfide ores, Skellefte dlstrmt, Sweden Econ Geol, 70 255-274 Rlpley, E M and Ohmoto, H , 1977 Mmeralogic, sulfur isotope, and fired inclusion studms of the stratabound copper deposits at the Raul Mme, Peru Econ Geol, 72 1017-1041 Roedder, E , 1981 Origin of fired mclusmn and changes that occur after trapping In L S Holhster and M L Crawford (Editors), Short Course m Fluid Inclusmns Vol 6 Apphcatmn to Petrology Mineral Assoc Can, Calgary, Alta, pp 101-137 Tammura, S , Date, J , Takahashl, T and Ohmoto, H , 1983 Geological setting of the kuroko deposits, Japan, Part II Stratigraphy and structure of the Hokuroko district Econ, Geol Monogr, 5 24-39 Vlvallo, W , 1985 Early Proterozmc blmodal volcamsm, hydrothermal activity and massive sulfide deposltmn in the Bohden-L~ngdal area, Skellefte district, Sweden S T U (Styr Tek Ut~ ), Stockholm, Rep '85, 50 pp