MASTER'S THESIS. Bastutjärn Ni-Cu-Co mineralization. Anders Zettergren Master of Science in Engineering Technology Natural Resources Engineering

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1 MASTER'S THESIS Bastutjärn Ni-Cu-Co mineralization Anders Zettergren 2013 Master of Science in Engineering Technology Natural Resources Engineering Luleå University of Technology Department of Civil, Environmental and Natural resources engineering

2 Bastutjärn Ni-Cu-Co mineralization Anders Zettergren Master of Science Natural Resources, Exploration Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering

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4 Sammanfattning Bastutjärn Ni-Cu-Co mineralisering är belägen i en gabbroid intrusion ca 10 km nord-öst om Norsjö i de syd-centrala delarna av Skelleftefältet. Mineraliseringen uppträder i kontakten mellan den gabbroida bergarten, klassificerad som gabbronoriter, samt en grafit- och sulfidrik metasedimentär, delvis skiffrig, bergart. Det förekommer tre olika typer av mineralisering; disseminerad låghaltig mineralisering som dock upptar stora volymer, semimassiv sulfidmineralisering samt en semimassiv mineralisering rik på grafit. Malmmineralen som förekommer är magnetkis, pentlandit samt kopparkis. Bildningen av Bastutjärns Ni-Cu-Co mineralisering innefattar en inblandning av svavel från de omgivande krustala bergarterna för att uppnå svavelmättnad i magman. De avblandade svaveldropparna bildar sedan lager med massivare mineralisering. Kontaktzonen mellan gabbroiden och metasedimentet består av en kraftig uppmixning av de två bergarterna. Dels genom xenoliter av sedimentärt ursprung i gabbron samt genom en hybridisering av gabbron. I mineraliseringen förekommer grafit rikligt vilket pekar på en kraftig inblandning av sedimentbergarter i bildningsprocessen. Abstract Bastutjärn Ni-Cu-Co mineralization is located within a gabbroic intrusion approx. 10 km north-east of Norsjö village in the south-central parts of the world class massive sulphide Skellefte District. The mineralization is situated at the contact zone between a gabbroic unit, classified as gabbronorite, and a graphite- and sulphide-rich metasedimentary unit. Three different types of mineralization occurs, a disseminated low grade, extensive volume with disseminated pyrrhotite, a semi-massive pyrrhotite mineralization and a semi-massive pyrrhotite mineralization-rich in graphite and sedimentary fragments. Ore bearing minerals are solely pyrrhotite, pentlandite and chalcopyrite. Formation of the Bastutjärn Ni-Cu-Co deposit includes an involvement of sulphur from the surrounding crustal rocks to achieve sulphide immiscibility. The separated droplets of sulphides creates layers of more massive mineralization. The contact zone between the two rock units is consisting of an area where a major mixing process has occurred, which can be seen by xenoliths and hybridization of the gabbroic rock. Large amount of graphite is found within the mineralization, suggesting that an intense mixing with the surrounding sedimentary unit took place during formation. 3

5 Table of content 1. Introduction Studied area Origin of Ni-Cu ores Geological setting Regional geology Geology of the Bastutjärn intrusion Method Mapping Core logging Rock chip logging Thin sections Geochemistry Recalculation of whole rock geochemistry data Recalculation of metal content in 100% sulphides Rock types Gabbroic rocks Relations between the gabbroic rocks Contact gabbro-country rock Metasedimentary rocks Dikes Alteration Rock geochemistry Major element Classification diagrams Chemical layering Trace element CIPW normalization of whole rock geochemistry Mineralization Profiles Discussion Parent magma

6 7.2 Gabbro-sediment interaction Ore forming processes Sulphur source Pathway Comparison with other Cu-Ni mineralization Älgliden Näsberg Lainijaur Deposits within the Nickel Belt, Västerbotten Bastutjärn compared to similar deposits Ore potential Conclusion Future work Acknowledgements References Appendices

7 Illustrations Fig. 1. Simplified geological map over the Skellefte District and its deposits. Bastutjärn mineralization outlined in the center. Modified after Skyttä et al Fig. 2. Simplified geological map over Bastutjärn project area. Modified from SGU Bedrock map 23J Norsjö SV and SO (Bergström et al Sveriges geologiska undersökning Fig. 3. Stages in the conception, delivery, and development of a theoretical magmatic Ni-Cu sulphide deposit. A model where the formation of the mineralization at Bastutjärn could fit in to, as discussed in later chapters. Naldrett (2010) Fig. 4. Geological provinces of the Baltic Shield. (Weihed et al 1992) Fig. 5. Location and number of channel samples taken from outcrops within the project area. Note that outcrops are only found in north and east of the gabbroic intrusion. Profile 1 and Profile 2 refers to Fig Fig. 6. Plot with all gabbroic samples, with respect to their modal composition determined from thin section estimation. (Streckeisen, 1976) Fig. 7. Gabbroic samples from Bastutjärn. TAS Na 2 O + K 2 O vs SiO 2 plutonic after Cox et al., Fig. 8. Photomicrographs (plane light) of gabbronorite samples from Bastutjärn A: Secondary biotite foliation within plagioclase and pyroxene grains, 10X plane polarized light sample B: Chlorite alteration of pyroxene, 10X plane polarized light sample C: Biotite and phlogopite intergrowth within pyroxene grain, 20X plane polarized light sample Fig. 9. AFM-diagram (Irvine & Baragar 1971). Samples from gabbroic rocks (Black squares), hybride rocks (Violet stars) and granodiorite (blue triangles) from Bastutjärn Fig. 10. K 2 O- versus SiO 2 -diagram (Middlemost 1974). Samples from rocks at Bastutjärn. Legend see Fig Fig. 11. A + B. Plots of downhole data of major oxides from drill hole 4 at Bastutjärn Fig. 12. Plots of Mg/(Fe+Mg) ratio versus major element oxides for the Bastutjärn intrusion. A: SiO 2 B: Al 2 O 3 C: CaO D: Na 2 O E: K 2 O F: Cr 2 O 3 G: TiO 2 H: P 2 O Fig. 13. Plots of Mg/(Fe+Mg) ratio versus trace elements of the Bastutjärn intrusion Fig. 14. Photomicrographs (reflected light) of mineralized samples from Bastutjärn. CcP = Chalcopyrite, Grp = Graphite, Mag = Magnetite, Pent = Pentlandite, Po = Pyrrhotite. A: Pyrrhotite with pentlandite and chalcopyrite inclusions. Chalcopyrite as fracture filling and pentlandite exsolution texture, 20X sample B: Magnetite in center. Lower part of the picture is rich in graphite. 20X sample C: Pentlandite exsolution in pyrrhotite grain, 50X sample D: Chalcopyrite vein within semi-massive pyrrhotite, 5X sample E: Large pentlandite grain within pyrrhotite, 20X sample F: Part of the mineralization rich in graphite, 10X Fig. 15. Relationships between elements in the sulphide mineralization at Bastutjärn. Fig. a,b,c n=108 from ore sample analyzes Fig. d,e n=148 from rock chip samples Fig. 16. Profile 1 Section 940 W. For location see Fig

8 Fig. 17. Profile 2 Section 1000 W. For location see Fig Fig. 18. Plot of Ni grade (%) vs resource of Ni ore (Mton) in world class deposits divided into different deposit types. After Jaireth et al. (2005) Tables Table 1. Elements analyzed in whole rock geochemistry package CCP-PKG01 at ALS Minerals Table 2. Rock names and definitions by IUGS Table 3. Average composition of major elements in % for gabbronorite samples Table 4. Average composition in % of 19 metasedimentary rock chip samples southwest of the mineralization. Trace metals in ppm Table 5. CIPW norm vs modal composition TS Table 6. Data on selected Swedish nickel deposits. Note that all data except Blackstone data are historical and not NI compliant. Ni S is the content of Nickel in 100 % sulphides.. 36 Table 7. Elements in 100 % sulphides, n = 108 ore samples. % Me S = percentage of metal in 100 % sulphide phase Table 8. Historical estimation of the mineralization at Bastutjärn. Ore zone includes the upper Lens 1 and lower Lens 2 plus marginal ore between these lenses. Internal Report Boliden Mineral Appendix Appendix 1 Graphic drill core logs Appendix 2 Raw data whole rock geochemistry Appendix 3 Recalculated whole rock geochemistry Appendix 4 CIPW normalization 7

9 1. Introduction The Bastutjärn gabbro intrusion is located in the Västerbotten county, 10 km NE of Norsjö and 70 km W of Skellefteå. The intrusion, which hosts a Ni-Cu-Co mineralization, belongs to the central Palaeoproterozoic Skellefte District (Årebäck et al. 2005). Within this district several deposits, mostly of VMS-type, have been mined and five deposits are being exploited today, all of them operated by Boliden Mineral AB. Gold deposits of quartz vein type are also found in the area, one of them is currently mined by Björkdalsgruvan AB. Besides from volcanogenic massive sulphides this mining district hosts economic important deposits of mafic intrusion nickel and porphyry copper type (Weihed et al. 1992). Historically polymetalic VMS-type deposits have been of greatest interest for exploration and mining companies within the district since the first discoveries of the famous gold-rich Boliden deposit in the 1920 s, but other metals have also been mined. At the Lainijaur Ni-Cu deposit, associated with a gabbroic intrusion, a total of ton, averaging 2.20 % Ni, 0.93 % Cu and 0.1 % Co have been mined during the years (Grip 1961 & Nilsson 1985). In the central part of Västerbotten, south of the Skellefte District, a nickel belt containing several deposits with Ni-Cu occur within an area from Bureå at the coastline to Örträsk in the west. Most exploration in this area was conducted in the 1970 s and today more than 40 prospects are known, where Lappvattnet and Rörmyrberget are the largest deposits (Fig 1). All deposits in this area are associated with ultramafic bodies, except for the Rörmyrberget deposit where gabbroic rocks are found. Besides Lainijaur no other deposit with Ni as primary metal has been mined within the Skellefte District or northern Sweden, despite several promising occurrences. One of these is the Bastutjärn Ni-Cu-Co mineralization, hosted within the Bastutjärn gabbro intrusion. The area has been known since 1940s when several nickel mineralized boulders where found at Bastutjärn and Kvavisträsk, which were located downstream in the ice direction (Grip 1961). In 1971, promising boulders was found in the area and an exploration program lead by Boliden Mineral resulted in the discovery of mineralized outcrops (Årsrapport 1973). A drilling program with 11 holes was undertaken during 1974 and Ni-Cu mineralization where found in core (Årsrapport 1974). The area has been a target for PGE-Au exploration by SGAB during a 8

10 field campaign 1987, but all analyzed samples showed PGE-values under detection limit and no clear magmatic layering was found (Filén 1988). Fig. 1. Simplified geological map over the Skellefte District and its deposits. Bastutjärn mineralization outlined in the center. Modified after Skyttä et al The aim in this study is to get a better understanding of the geology in the project area. This will be done by: -Relogging and reinterpretation of the geological model. -Description of rock types. -Description of mineralization and ore minerals. -Geochemical characteristics. -Discussion of the link between gabbro and surrounding sedimentary units. 9

11 1.1 Studied area The Bastutjärn project area is located 45 km west of Boliden and 10 km NE of Norsjö, Västerbotten county (Fig.1 and 5). The intrusion is located south of the river Malån and north of the village Bastutjärn. Dimensions of the gabbro complex is 5x3 km and the shape is oval based on geophysical gravity maps. Earlier mapping was undertaken by SGU, that resulted in a geological map of semi-local scale (1:50 000). More intense mapping and also trenching have been done by Boliden Mineral, which has given a better detail of the surface geology. The area is affected by glacial activities and a majority of the bedrock is covered by glacial till, like the rest of northern Sweden. Compared to other sites in this part of Sweden this area has a quite high percentage of outcrops, mainly in the northern and eastern parts. In the south-west, where the mineralization is situated, the thickness of till reaches its maximum 30 meters. This fact makes it difficult to map, from an economic point of view, the most interesting areas in outcrop. Fig. 2. Simplified geological map over Bastutjärn project area. Modified from SGU Bedrock map 23J Norsjö SV and SO (Bergström et al Sveriges geologiska undersökning. 10

12 1.2 Origin of Ni-Cu ores Magmatic Ni-Cu mineralizations are formed by a primitive or differentiated magma of maficultramafic nature. The sulphur saturation stage is an important key in the ore forming process, which must be achieved during cooling in order to create a sulphide liquid. Chalcophile elements like Cu and Ni elements accumulate into the sulphide melt which will deposit, either in the top of the cumulate or in embayments within the magma conduit (Naldrett 2010; Maier 2011). Ni-Cu ores are associated with different types of ultramaficmafic rocks, including komatiites, dunites, harzburgite, pyroxenite, olivine gabbro, gabbronorite and troctolite (Naldrett 2004). The general host bodies for the mineralization are intrusions, often small, as dykes, sills, magma channels or other irregularly shaped intrusions. (Naldrett 2010). Magma of mantle origin needs to have a high degree of melting to be able to host critical amounts of nickel, at a ore forming point of view. This is due to the fact that nickel is primarily situated in olivine. The content of olivine in the melted mantle is therefore controlling the nickel content in the magma (Naldrett 2010). To produce a Cu-rich magma partial melting is needed, as Cu is situated in the sulphides in the mantle. In basaltic magmas sulphur has a relatively low solubility, so there a partial melting of approx. 20 % is required (Naldret 2004). When magma is produced it needs pathways to rise upwards. Magma convection at a regional scale generates extensional areas, which acts as pathways for the magma (Naldrett 2010). Due to pressure and solubility conditions for sulphur, primitive magmas are generally S- undersaturated. Saturation is critical in the ore forming process as it generates the sulphide liquid. Through differentiation it is possible to enrich the magma in S to achieve saturation, because S is not present in silicate minerals (Maier 2011). Other processes that could be important to achieve S-saturation is magma mixing and pressure changes. For many of the most important Ni-Cu deposits in the world isotopic studies show that external sulphur must have been added into the system during magma emplacement. External sulphur is extracted from crustal rocks such as black shales, banded iron formations, evaporites and other rocks which host a high level of sulphur (Maier 2011). In some cases isotopic studies show little or no influence of crustal addition of sulphur. This could prove that the magma in some cases derives enough sulphur from the mantle source to generate a liquid phase. A good example 11

13 where isotopic studies show no external contribution of sulphur is the deposit Nebo-Babel, western Australia (Seat et al. 2007). Fig. 3. Stages in the conception, delivery, and development of a theoretical magmatic Ni-Cu sulphide deposit. A model where the formation of the mineralization at Bastutjärn could fit in to, as discussed in later chapters. Naldrett (2010). 2 Geological setting 2.1 Regional geology The project area is located in the south central part of the Palaeoprotorezoic Skellefte District, which is part of the larger Fennoscandian shield. The Fennoscandian shield (Fig. 4) is a segment in the east European craton that also comprises areas more to the south which consist mainly of Precambrian rocks covered by platform sediments (Lahtinen et al. 2005). Formation of the Fennoscandian shield is divided into four different accretion phases according to their age. Oldest is the basement formed at the Saamian orogeny ( Ga), which mainly comprises tonalites and trondhjemites. The period was followed by the Lopian orogeny ( ) that was a period with formation of high grade gneisses, granites as well as greenstones. The Archean rocks was followed by a period of intracratonic sedimentation and volcanism in early Proterozoic age. Activation of a passive margin started the Svecofennian orogeny ( Ga) (Gaal & Gorbatschev 1987), which formed the Skellefte 12

14 District from a destructive margin at 1.89 Ga. This island arc environment hosts volcanogenic massive sulphides and also porphyry type deposits. According to the classification of massive sulphide deposits, Skellefte District fits best into the Kuroko type of deposits (Weihed 1992). Rifting south of the area generated an extensive greywacke sequence, called the Bothnian basin (~1.95 Ga), which was intruded by mafic to ultramafic rocks that host Ni deposits in the so called Nickel belt. These deposits are believed to be in same age or slightly younger than the sedimentary unit (Weihed 1992). Geochemical data gives the mafic rock a MORB or Island arc type of signature, reflecting an extensional environment (Weihed 1992).This does not exclude a back arc setting which Gaal & Gorbatschev (1987) have considered the mafic rocks to belong to an oceanic island arc at Ga. Closing of the sedimentary basin, that is spanning 500 km south to the Bergslagen area, halted at 1.87 Ga (Nironen 1997). Subduction in the Skellefte District resulted in many granitoid intrusions, classified according to age of emplacement by Gaal & Gorbatschev (1987). Early orogenic ( Ga), late orogenic which comprises the Skellefte-Härnö granites ( Ga) and the post orogenic Revsundgranite ( Ga). The Revsundgranite is of same age as the extensive Transscandinavian igneous belt, that covers a large area from southeastern Sweden to the north (Gaal & Gorbatschev 1987). 13

15 Fig. 4. Geological provinces of the Baltic Shield. (Weihed et al 1992). 2.2 Geology of the Bastutjärn intrusion The project area (Fig. 2) consists of an oval shaped gabbroic intrusion, approximately 3 x 5 km, surrounded by graphite- and pyrite-rich metasedimentary units (shales), granites of Revsund type (according to SGU bedrock map sheet 23J) and volcanic rocks belonging to the Skellefte group. Based on available drill holes, the thickness of the gabbroic intrusion seems to be around 200 meters. Within the area a sulphide mineralization is present and hosts significant amounts of nickel, copper and cobalt. This mineralization is hosted by the gabbro unit near and at the contact to the sedimentary unit in the south of the area. The sedimentary unit comprises mainly graphite-rich shales with decent amounts of pyrrhotite. Shales rich in sulphides are a common feature in the Skellefte District (Martinsson 1996). 14

16 3 Method 3.1 Mapping During the summer year 2011 fieldwork was undertaken at the Bastutjärn area. Reconnaissance work was done to briefly map outcrops within the explored area, were outcrops where chosen for whole rock geochemistry and thin section samples. Totally 9 channel samples were taken with a hand held rotation diamond saw. Localities for these samples are indicated in Figure 5. Due to the glaciated till conditions, most of the outcrops are located in the northern part of the intrusion. Profile 2 Profile 1 Fig. 5. Location and number of channel samples taken from outcrops within the project area. Note that outcrops are only found in north and east of the gabbroic intrusion. Profile 1 and Profile 2 refers to Fig Core logging From earlier exploration work 11 drill holes have been done and the cores are since that time stored at Boliden Minerals drill core facilities in Boliden. Six of these cores were selected for further detailed studies, based on their spatial relationship to the mineralization. These cores have earlier been logged, sampled and analyzed. Except for 3 drill cores, which were relogged in 2010, they are only analyzed for nickel, copper and 15

17 sulphur. For this thesis the cores have been reanalyzed, both for ore analyzes and for certain sections lithogeochemical analyze. Ore analyzes where performed at Labtium Oy, Rovaniemi, lithogeochemistry samples was sent to ALS Minerals, first prepared in Piteå and then analyzed in Vancouver. 3.3 Rock chip logging Together with drill cores and outcrop mapping, rock chip samples have been logged and analyzed to achieve further detailed information of the geology. Rock chip sampling was performed on a number of localities within the project area by percussion drilling. Sampling was done from the bedrock surface and approximately 1 meter down in the rock. Coarsegrained material (>4 mm and >2 mm) was then separated from the fine fraction and the resulting two products where used for further studies. To determine the rock type, mineralogy and in some cases structures, optical microscope was used. 3.4 Thin sections Thin sections were taken from both drill core and outcrop samples. A total of 12 samples were sent to Vancover Petrographics for preparation, of which 7 were covered thin sections (30 microns, 26x46 mm), 3 polished thin sections (30 microns, 26x46 mm) and 2 polished thick sections (300 microns, 25 mm circular). A Nikon stereo microscope was used, both for opaque and transparent mineralogy. Determination of plagioclase composition was done by the Michel-Levy method. In this method a single grain of plagioclase is selected, where the two extinction angles are measured for each grain. This was repeated several times on every thin section which resulted in a mean value. Inserting this value in a diagram gives the plagioclase composition of the sample. 3.5 Geochemistry A total number of 36 (excluding the rock chips) samples were sent to ALS Minerals in Vancouver for whole rock analysis, code CCP-PKG01. A complete list of analyzed elements is shown in Table 1. Gold was analyzed in rock chip and outcrop samples. 16

18 Table 1. Elements analyzed in whole rock geochemistry package CCP-PKG01 at ALS Minerals Major Elements: Si, Al, Fe, Ca, Mg, Na, K, Ti, Mn, P, C, S, LOI Base Metals: Ag, Cu, Cd, Mo, Ni, Pb, Sc, Zn Trace elements and REE s: Ba, Ce, Cr, Cs, Dy, Er, Eu, Ga, Gd, Hf, Ho, La, Lu, Nb, Nd, Pr, Rb, Sm, Sn, Sr, Ta, Tb, Th, Tl, Tm, U, V, W, Y, Yb, Zr Volatiles: As, Bi, Hg, Sb, Se, Te Recalculation of whole rock geochemistry data Whole rock analysis data reflects the average composition of the rock. When sulphides are present they are included in the analysis so that the data is the composition of both host rock and sulphides. In that case the data must be modified to get the correct composition of the host rock without sulphides. Elements hosted in the sulphides are iron (pentlandite, chalcopyrite and pyrrhotite), copper (chalcopyrite) and nickel (pentlandite). The method of recalculation includes the following steps (modified after Li et al & De Waal et al. 2004). Remove all Cu by forming chalcopyrite (CuFeS 2 ), as this is the only major mineral hosting Cu. Next step is to remove Ni, which is hosted in pentlandite or olivine if present. Petrographic studies and geochemical modeling show that these rocks contain no olivine. With this background all Ni is calculated for creation of pentlandite. Since the mineralization is rich in Co, which is considered to be situated within the pentlandite. The formula Fe 4 Ni 4 CoS 8 were used when calculating pentlandite. Remaining sulphur is consumed by creation of pyrrhotite, the dominating iron sulphur mineral. Pyrite is only present in some samples and mostly within the footwall, so the amount of the total sulphur situated in the pyrite is negligible Recalculation of metal content in 100% sulphides To compare the different mineralization types the metal content from whole rock geochemistry have been recalculated into 100 % sulphides. The formula by Barnes and Lightfoot (2005), was applied: 17

19 C (100% sul) = C wr * 100/(2.527 * S * Cu * Ni) Where C (100% sul) = concentration of an element in 100 % sulphides; C wr = concentration of the element in the whole rock geochemistry; S, Cu and Ni =concentration of the elements in whole rock geochemistry. 4 Rock types By macroscopic and microscopic studies several different rock types where distinguished. 4.1 Gabbroic rocks The intrusion consists of a gabbronorite which varies in chemical composition and grain size. In the Table 3 the average composition of this rock is shown. By definition a gabbronorite consists of plagioclase and equal amounts of orthopyroxene and clinopyroxene (IUGS, Table 2). If orthopyroxene is dominating the rock name should be norite and a clinopyroxene dominating rock should be gabbro (s.s.). Table 2. Rock names and definitions by IUGS Gabbro (s.s.) Plagioclase + Clinopyroxene Norite (s.s.) Plagioclase + Orthopyroxene Gabbronorite Plagioclase + equal amount Ortho and Clinopyroxene Orthopyroxene-Gabbro Plagioclase + Clino > Orthopyroxene Clinopyroxene-Norite Plagioclase + Ortho > Clinopyroxene Modal mineral composition were approximated in thin section samples. From the gabbroic samples the main mineral composition consists of plagioclase, clino- and ortho- pyroxene and also biotite. Based on the nomenclature set up by IUGS the rocks would range from orthopyroxene-gabbro to clinopyroxene-norite, but the general rock name would be gabbronorite. All samples plot within the main gabbronorite zone (Fig. 6). Two of the samples plot on the line that shows equal amounts of the two pyroxenes. In a strict sense the four samples on the left hand side of the line should be named clinopyroxene-norite and the two on the right hand side should be named orthopyroxene-gabbro. 18

20 Fig. 6. Plot with all gabbroic samples, with respect to their modal composition determined from thin section estimation. (Streckeisen, 1976) The TAS alkali-silica diagram by Cox et al. (1979) in Figure 7, shows that all samples plot within the gabbroic field. Fig. 7. Gabbroic samples from Bastutjärn. TAS Na 2 O + K 2 O vs SiO 2 plutonic after Cox et al.,

21 Average composition in percent for eight gabbronorite samples are displayed in table 3. Table 3. Average composition of major elements in % for gabbronorite samples. SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO Na 2 O K 2 O Cr 2 O 3 TiO 2 MnO P 2 O 5 SrO BaO The gabbronorite rock is equigranular and holocristalline. The rock is in general weakly altered. Some of the samples are rich in biotite, from 5 % (sample , ) up to 25 vol% (sample ). The biotite occurs as primary anhedral flakes up to 7 mm, often associated with magnetite and with inclusions of mainly plagioclase and minor apatite. Biotite also occurs as a secondary recrystallised mineral. Figure 8 from sample (Appendix 1) is showing orientated biotite, indicating a weak tectonic foliation Relations between the gabbroic rocks From studies of drill core it is clear that the intrusion consists of different generations of gabbroic rocks. Younger magma injections have intruded older rocks, based on sharp contacts, with an overall similar composition. There are also gabbros that are crosscutting mineralized gabbros showing that there must be later magmas that were sulphur undersaturated. Even if there are clear contacts between the gabbroic rocks, both geochemical and modal compositions seem to be similar Contact gabbro-country rock Most studied drill cores show that the contact zone between the intrusion and the metasedimentary country rock is complex. The zone consists of a mix of gabbroic mingled by metasediments. The gabbroic rock exhibit more xenoliths closer to the contact zone, where in some cases the contact zone consists of a gabbro-sediment hybrid-rock which are more fine-grained than the typical gabbronorite. Due to the fact that most of the drilled cores only are intruding the metasedimentary unit a few tens of meters, it is not possible to exclude that the gabbro is continuing at depth. Isolated outcrops with metasediments inside the gabbroic unit have been found at the north western part of the area and one drillhole (BAB9) intersected metasediment at the start of the hole. 20

22 4.2 Metasedimentary rocks Country rock below the intrusion in the southern contact is a metasedimentary unit that exhibits a black to dark gray color. The rock is fine-grained and a weak foliation can be seen. As listed in Table 4 these rocks are generally rich in sulphur, mainly as the mineral pyrrhotite, and carbon hosted as graphite. The texture in rock chip samples and in drill core varies from homogeneous samples without any structures to more foliated, shale like rock. These varieties are best seen in drill core, where the sediments occur also as xenoliths within the gabbroic intrusion. Table 4. Average composition in % of 19 metasedimentary rock chip samples southwest of the mineralization. Trace metals in ppm. SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Cr2O3 TiO2 MnO P2O5 SrO BaO C S Co Cu Mo Ni Pb Zn Dikes Several dikes cross-cut the intrusion. One type is mafic dikes, which are fine-grained and black to grey in color. The dikes show a sharp contact to the gabbroic rocks, and are partly infiltrated with carbonate minerals. The more felsic dikes are often granitic in composition, fine-grained and with a slightly red tint. These felsic dykes are typically crushed and brecciated. A few quartz-carbonate veins intrude the gabbroic rock as well, typically they are pink and fine-grained. 4.4 Alteration The gabbroic rocks at Bastutjärn show only a weak alteration of the primary minerals. The main alteration mineral is chlorite, which is found as a product of alteration of the ferromagnesian pyroxenes (Fig. 8B). Secondary biotite is as well a common alteration product which commonly coexists with phlogopite (Fig. 8C). 21

23 Fig. 8. Photomicrographs (plane light) of gabbronorite samples from Bastutjärn A: Secondary biotite foliation within plagioclase and pyroxene grains, 10X plane polarized light sample B: Chlorite alteration of pyroxene, 10X plane polarized light sample C: Biotite and phlogopite intergrowth within pyroxene grain, 20X plane polarized light sample Rock geochemistry The rock types within the Bastutjärn intrusion show a small variation in the silica content (mainly between 52 to 54 % SiO, Fig. 12). The plots of major and trace elements are against the MgO/(MgO + FeO + Fe 2 O 3 ) ratio (Mg-number), as plots against silica do not show the differentiation trends within the different rock types in the intrusion. 5.1 Major element Raw data from the geochemical analyses are presented in Appendix 1. The results have been recalculated to a sulphur free and volatile free basis (Appendix 2). Recalculated samples with large amount of sulphides should be taken with care. The effect of later alteration on the rocks seems to have a minor effect on the chemical composition as discussed later. 22

24 Plots including the MgO/(FeO+MgO) ratio versus major element oxides shows in most cases a clear systematical trend (Fig. 12). Systematics that can bli explained by fractionated crystallization when the magma evolves. The MgO/(FeO+MgO) ratio shows a positive correlation with SiO 2, Al 2 O 3 and Cr 2 O 3, for Na 2, TiO 2 and P 2 O 5 the correlation is negative. K 2 O and CaO show an unclear trend versus the ratio. Four samples have been classified to not belong to the typical Gabbronorites at Bastutjärn, the hybridized gabbro/metasediments and the fine-grained gabbro Classification diagrams From the assay data plotting was done to classify the gabbroic rocks according to magma character. Plotting in AFM (Fig. 9) diagram gives a tholeiitic trend for the magma. Gabbroic samples plotted in the discrimination diagram SiO 2 vs K 2 O (Fig. 10) results in a subalkalic rock. Fig. 9. AFM-diagram (Irvine & Baragar 1971). Samples from gabbroic rocks (Black squares), hybride rocks (Violet stars) and granodiorite (blue triangles) from Bastutjärn. 23

25 Fig. 10. K 2 O- versus SiO 2 -diagram (Middlemost 1974). Samples from rocks at Bastutjärn. Legend see Fig Chemical layering Drill hole BAB 4 was sampled systematically in order to define an eventual cryptic layering (Fig. 11 A + B). Samples below 100 m are gabbroic rocks surrounded by metasediments (Appendix 1). A major peak in MgO and a weak depletion of CaO can be seen in the lower part of the intrusion (Fig. 11 A). Also Cr 2 O 3 increases down hole, showing a more mafic character in the lower samples (Fig. 11 B). 24

26 A B Fig. 11. A + B. Plots of downhole data of major oxides from drill hole 4 at Bastutjärn. 25

27 Fig. 12. Plots of Mg/(Fe+Mg) ratio versus major element oxides for the Bastutjärn intrusion. A: SiO 2 B: Al 2 O 3 C: CaO D: Na 2 O E: K 2 O F: Cr 2 O 3 G: TiO 2 H: P 2 O 5. 26

28 5.2 Trace element Fig. 13. Plots of Mg/(Fe+Mg) ratio versus trace elements of the Bastutjärn intrusion. Results from trace element analysis of gabbroic samples are listed in Appendix 2 and plotted against the MgO/(FeO+MgO) ratio (Fig. 11). Europium, Niobium, Sulphur and Zirconium show a clear correlation with the ratio. Vanadium also shows a correlation, but in this case it is more unclear. This could be a product of the hybridization process in the area, where the input of Vanadium is from the surrounding sediments. 27

29 5.3 CIPW normalization of whole rock geochemistry CIPW normalization was undertaken on a number of samples with whole rock geochemistry (Appendix 4). Modal composition was estimated on most of the samples, apart from TS which was fine-grained and individual grains where impossible to define. CIPW normalization gives a rough overview about which mineral assemblage that could be expected from the crystallized melt. In Table 5 samples with CIPW normative composition is compared against the estimated mineral volumes from thin section, which gives a slightly different result. The gabbronorites mineralogy is mainly consisting of plagioclase and pyroxenes, but from CIPW there should be some quartz, orthoclase and diopside. Although the CIPW gives a pure theoretical composition, it follows the main mineralogical variations estimated from thin section. Table 5. CIPW norm vs modal composition TS Thin section TS CIPW % % Quartz 3.23 Plagioclase Orthoclase 6.93 Diopside 2.48 Hypersthene Augite 30 Olivine 0.00 Ilmenite 0.50 Magnetite 0.59 Apatite 0.19 Chromite 0.04 Biotite 10 Total

30 6 Mineralization The Bastutjärn mineralization occurs as disseminated sulphide droplets and as semi-massive to massive lenses. The disseminated zone consists of dropletlike pyrrhotite grains with pentlandite inclusions and minor chalcopyrite. This zone is rather subeconomic and the amount of nickel and copper are low. On the other hand this zone is widespread and the area between highgrade zones is mostly of disseminated style. Zones with highest grades are solely found in semi-massive to massive zones of pyrrhotite lenses, rarely wider than 40 cm. These lenses are concentrated and are therefore forming wider zones with interesting grades. Chalcopyrite occurs frequently and is mainly forming veins within the semi-massive pyrrhotite. In general the PGE grades within the mineralization and the intrusion is very low or below detection limit. Grades of Gold are in general very low, but follow the pattern for the main mineralization and the grades are highest where the richest mineralization occur. One element that is elevated and may be of further interest is molybdenum. From this study three different types of sulphide mineralization are found. First, and most widely spread, the disseminated pyrrhotite mineralization which is characterized by droplet textured evenly distributed sulphides in the gabbroic matrix. Chalcopyrite and pentlandite are common as inclusions within pyrrhotite grains. Optical observation shows that the ratio between pentlandite and pyrrhotite is higher in the disseminated mineralization than in the more massive type of mineralization. This observation is also done for chalcopyrite. The second type is the semi-massive to massive sulphide (more than 15 % S), with pyrrhotite as the dominating sulphide. Chalcopyrite is a common mineral and is together with pentlandite found as inclusions in the pyrrhotite. Chalcopyrite is also found as later veins that intrudes the mineralization. The third and the least abundant type is the graphite-rich semi-massive sulphides, with pyrrhotite as main sulphide, occurring in fine-grained matrix. This type of mineralization is unevenly distributed and occurs only in drill hole 4 and 11. The width of this zone is rarely more than 50 cm and in drill hole 11 it shows sharp contacts to the surrounding gabbro. Sample LK (Fig. 14 A and B) is from a section with graphite-rich (5.67 % C) and 29

31 semi-massive pyrrhotite (11.9 % S). Based on a few sample points, the content of Ni and Cu seems to be rather low (0.229 % vs %) while carbon content is high. Sulphides in this type of mineralization are more anhedral and fine-grained compared to the second type of mineralization. Graphite is very common in the sample where it occurs frequently as minor inclusions in the pyrrhotite (Fig. 14 C-F). Chalcopyrite is less abundant in the sample and occurs mainly as inclusions in pyrrhotite, at the pyrrhotite grain rims and as fracture fillings. Compared to the graphite-rich samples, no chalcopyrite have been seen to create vein textures, and local areas with higher concentrations seem to be missing. Pentlandite occurs as inclusions at pyrrhotite grain boundaries and as exsolution textures. The former type of pentlandite exhibits the largest crystals. Magnetite is also abundant in the thin sections. Parts of the mineralization are richer in molybdenum (sample ), a metal which should not be present in any greater amount in a magmatic nickel sulphide deposit. Higher values of the other economical important metals are not always followed by molybdenum. A relationship can be seen between Mo and C (Fig. 15 D). A relationship to the S values can be seen as well but highest Mo values seem to coincide with higher carbon grades (Fig. 15 E). 30

32 Po Pent Ccp Mag Grp Po Ccp Pent Po 50 µm Pent Po Grp Pent Po Fig. 14. Photomicrographs (reflected light) of mineralized samples from Bastutjärn. CcP = Chalcopyrite, Grp = Graphite, Mag = Magnetite, Pent = Pentlandite, Po = Pyrrhotite. A: Pyrrhotite with pentlandite and chalcopyrite inclusions. Chalcopyrite as fracture filling and pentlandite exsolution texture, 20X sample B: Magnetite in center. Lower part of the picture is rich in graphite. 20X sample C: Pentlandite exsolution in pyrrhotite grain, 50X sample D: Chalcopyrite vein within semi-massive pyrrhotite, 5X sample E: Large pentlandite grain within pyrrhotite, 20X sample F: Part of the mineralization rich in graphite, 10X Grp 31

33 Fig. 15. Relationships between elements in the sulphide mineralization at Bastutjärn. Fig. a,b,c n=108 from ore sample analyzes Fig. d,e n=148 from rock chip samples. 32

34 6.1 Profiles Fig. 16. Profile 1 Section 940 W. For location see Fig. 5. Fig. 17. Profile 2 Section 1000 W. For location see Fig

35 7 Discussion 7.1 Parent magma Tools to determine the origin and character of a magmatic rock is to plot geochemical data in discrimination diagrams. The problem in this case is that the rocks are differentiated from the magma, chilled margins should then be used as a key to the parental magma. However, in the investigated drill cores in this study, chilled margins were lacking so no good data was achieved for this kind of plotting. Despite this fact, the gabbroic rocks at Bastutjärn were plotted in this type of diagrams in order to give a rough estimation of the geotectonic environment. The Skellefte District have by many authors (e.g. Gaal & Gorbatschev 1987) been interpreted as an island arc environment. From the AFM diagram (Fig. 9, Irvine & Baragar 1971) the samples plot in the tholeiitic trend. The K 2 O vs SiO 2 - diagram (Fig. 10, Middlemost 1985) gives a sub-alkalic character for the rocks. From the sampling no clear cryptic or modal layering of the gabbroic intrusion can be distinguished, although a weak tendency was noticed (Fig. 9). More sampling would be needed to give a complete image of the instrusion. 7.2 Gabbro-sediment interaction Most of the major Ni-Cu deposits worldwide are believed to be formed from a process where the magma has interacted with sulphur-rich country rock (Maier, 2011). The reason why this is the case in many formation processes is found in the initial magma composition, which generally contains to little sulphur in order to create a sulphide immiscibility. The possible source of the sulphur at Bastutjärn is deeper discussed in the next chapter. At Bastutjärn the interaction between the gabbroic intrusion and the underlying sulphidebearing sediments has not only affected the initial magma composition. There has also been a huge effect on the rocks in the contact zone, more than 100 meters from the sediments. This can be seen by large xenoliths of sediments inside the gabbroic body and also as hybridized rocks where the sediment assimilation changes the texture and composition of the gabbro. 34

36 7.3 Ore forming processes Sulphur source In magmatic sulphur ores there are two possible sources for the sulphur, which is required to achieve saturation. A first source is internal sulphur from the magma and the second is external crustal sulphur, extracted from country rocks. A way to distinguish between these sources is to commence an isotopic analysis for sulphur. Crustal sulphur would then give negative δ 34 S values compared to the internal source which would result in mantle like near zero values for δ 34 S. This study does not include isotopic analyses of sulphur, so it is not possible to suggest a source from isotopic evidence. However, it is possible to suggest that the sulphur source in this scenario is external and crustal. This is based on features seen in drill core, outcrops and thin sections. In drill core and also outcrop there is a large amount of sulphur-rich fragments of sedimentary origin, most likely from the unit that is in contact with the gabbroic intrusion. The intruding gabbro shows a variety of compositions which may be a hybridization of the rock due to assimilation of the surrounding sediment. From thin sections of ore zones large amounts of graphite are noted which may be a result of contamination from the graphite-rich sediments. Whole rock geochemistry of the surrounding sedimentary unit shows that these rocks are rich in sulphur and also partly very rich in carbon, in form of graphite. All these features show that the country rock is a possible source for the sulphur. To further evaluate and maybe confirm this theory an isotopic study is needed. Sulphur isotopic studies done on black schists at Mörttjärn, close to the Lainijaur deposit, shows near zero values which also is reported from pyrrhotite from Ni-Cu mineralization (Martinsson 1996). Near zero sulphur isotopic values are also reported from a majority of the massive sulphide ores within the Skellefte District (Rickard et al. 1979). This means that if sulphur is extracted from the surrounding schists to achieve sulphur saturation, this could not always be shown by isotopic studies if this results in near zero values Pathway One important key to generate a magmatic sulphide deposit is the pathway of magma (Fig.4 Naldrett 2010). This pathway is found where the crust is weak, in crustal lineaments or faults. Bastutjärn is situated on a regional structural lineament that is suggested to crosscut the Skellefte District and continue to the Älgliden mafic dyke. From an ore forming point of view this structure is possibly the pathway for the magma that created the Bastutjärn 35

37 mineralization. From this structure the magma intruded the metasedimentary unit, creating an irregularly shaped intrusion. 7.4 Comparison with other Cu-Ni mineralization Within the Skellefte District a number of mafic to ultramafic intrusions occur, some of them hosting Ni-Cu mineralization. Most notable are Älgliden, Storbodsund, Näsberg and Lainijaur. These mineralizations have through the history been studied in a much greater extent than the Bastutjärn occurrence and can be a tool to give an explanation of the processes included in the formation. For this thesis a number of deposits have been studied (Table 6), but only a few were selected for deeper comparison. A brief description for the most comparable deposits follows. Table 6. Data on selected Swedish nickel deposits. Note that all data except Blackstone data are historical and not NI compliant. Ni S is the content of Nickel in 100 % sulphides. Deposit name Type Metric tonnes Grade Reference Ni Cu Kton % Ni % Co % Cu % S % Ni S Kukasjärvi Ultramafic sill Boliden mineral Gabbroid border zone of Fiskelträsk granodiorite pluton Hansson 1985 Layered, funnel shaped, Notträsk intrusion 0.5/ / / /30 5.1/1.2 Arvantidis 1982 Storbodsund Gabbroid Grip 1961 Blackstone report, may Lainijaur Mafic dyke /Grip 1961 Älgliden Differentiated mafic dyke Boliden Mineral Bastutjärn Gabbroic intrusion Boliden Mineral Lappvattnet Ultramafic lenses and fragmental ore in paragneiss Ultramafic lenses and Blackstone report, may 2009/Grip 1961 Mjövattnet fragmental ore in paragneiss Nilsson 1985 Blackstone report, may Rörmyrberget Differentiated multiple sill /Grip Älgliden Älgliden is a mafic dyke crosscutting the G1-phase of the Jörn granitoid complex, SW of Jörn in the eastern part of the Skellefte District. The length of this dike is 2.7 km and the width is m and it is oriented subvertical in a NE trending direction. Rock composition in the dike is mainly diorite, gabbro and ultramafic rocks. Mineralization occurs as disseminated and massive including pyrrhotite, chalcopyrite, minor pentlandite and sporadically pyrite (Wilson et al., 1987). 36

38 7.4.2 Näsberg The Näsberg mafic instrusion is located within the GI phase of the Jörn granitoid complex, comprising two different generations of gabbroic rocks (Filen, 2001). Later studies have resulted in discoveries of three zones in the intrusion divided into a lower, a main and an upper zone. The lower and upper zone is poorly exposed and hard to define, but is generally consisting of gabbronorite, hornblende gabbro and quartz-bearing gabbro. The main zone is consisting of cumulates and well developed layering in a olivine gabbronorite. Pyrrhotite and magnetite dikes are found within the younger fault controlled gabbro. Iron was earlier mined here in a very small scale (Årebäck, 2006) Lainijaur Lainijaur is situated in the NW area of the Skellefte District and 50 km NW of the Bastutjärn project. The deposit is the only one mined in this area, with an accumulated production of tonnes of ore averaging 2.20% Ni, 0.93% Cu and 0.01% Co during the years (Grip, 1961). Host rock is consisting of gabbroic and diorite rocks, emplaced by several injection pulses of magma. The mineralization consists of two sulphide layers of mainly pyrrhotite, occurring at the base of the gabbro and close to the contact to the metasedimentary country rock. At the deposit also a nickel-arsenic mineralization occurs as veins and fissure fillings (Martinsson, 1996) Deposits within the Nickel Belt, Västerbotten The Nickel belt is a larger area with several known occurrences located south of the Skellefte District in the central of Västerbotten. All of them are associated with ultramafic rocks. Major rock types are metamorphosed dunites, peridotites and pyroxenites. Ultramafic intrusions located within the Nickel belt are in general small and the largest known deposit is at the Rörmyrberget intrusion with a size of 1.7 km x 320 m. Host to that intrusions are gneisses and migmatites, often rich in graphite and pyrrhotite. Origin of the graphite is interpreted to be sapropelic, because if its high values of vanadium and molybdenum. Values at the graphite-rich gneisses at Lappvattnet range from ppm V and ppm Mo, which can be considered as elevated. Graphite is also found within the ultramafic rocks as isolated inclusions and aggregates, giving indication of possible contamination of the gneisses to the magma (Nilsson, 1985). 37

39 Table 7. Elements in 100 % sulphides, n = 108 ore samples. % Me S = percentage of metal in 100 % sulphide phase. % Ni S % Cu S % Co S Bastutjärn Bastutjärn compared to similar deposits Most obvious similarities are found to the Lainijaur deposit not only because its spatial presence, where multiple gabbroic injections intrude a metasedimentary unit. The behavior of the mineralization shows similarities, it consists of two layers with a larger area of disseminated type of mineralization and it is emplaced in vicinity to the contry rock. Also here fragments and xenoliths of sedimentary origin are found in the host rock. A feature at Lainijaur that is not shared with Bastutjärn is the nickel-arsenic mineralization. Genetic model for the magmatic Lainijaur deposit is mantle-derived sulphur, based on isotopic data by Martinsson (1996). However there is a possibility that pyrrhotite-bearing schists have contaminated the magma, schist with close to zero sulphur isotopic composition. In this case the two deposits would be similar even in the forming aspect. Grades at Lainijaur is much higher compared to Bastutjärn, both in total Ni content and Ni in 100 % sulphides (Table 6 and 7). Bastutjärn on the other hand shows a much higher Ni:Co ratio (approx. 0.3 compared to approx 0.07), in fact highest among all the major occurrences in this area. Other deposits that are possible to compare to Bastutjärn are the deposits along the Västerbotten Nickel belt, e.g. Lappvattnet and Rörmyrberget deposits. Those deposits are of ultramafic intrusive type compared to the gabbroic associated mineralization at Bastutjärn, but there may be similarities in the formation process. Deposits along this belt are situated within intrusions intruding gneisses and migmatites, often rich in graphite and pyrrhotite, similar to Bastutjärn. At Bastutjärn these sedimentary units have given an enrichment of molybdenum, which is found in elevated grades as well in the country rocks for these deposits. Since the gneisses and migmatites are rich in pyrrhotite, this is a possible sulphur source for the formation of an economic interesting deposit. The Älgliden deposit shows only a few similarities, mainly in rock and ore mineral composition. Although the grades for nickel is at the same magnitude, even if the copper grades are higher at Älgliden. Mineral assemblage is similar in the mineralized zones. 38

40 The mineralization at Bastutjärn is not easy to completely fit into any other deposit of same type. Many differences are found when comparison is undertaken. A deposit that shows most similarities is the Lainijaur deposit, which in a way can be used as a tool to better understand the Bastutjärn mineralization. It must be noted that the extent of data from Bastutjärn is much lesser in many of the other deposits, so more information may be needed to do a complete comparison. 7.5 Ore potential Since the number of drill holes intersecting the mineralization is only six, it has not been possible to calculate an accurate volume of ore. Historically it has been done a rough estimation of the potential in the area (Table 8). In this case a two layered mineralization dipping slightly to the north was used as the model. In a global scale the grades and tonnage are far away from a major deposit (Fig. 18) and would be plotted in the lower left corner in the Figure 18. Noted from the figure is that stratabound Ni-Cu deposits often are of low grade character. Favorable conditions for the Bastutjärn mineralization is the high grades of cobalt and from a mining point of view the near surface localization. Table 8. Historical estimation of the mineralization at Bastutjärn. Ore zone includes the upper Lens 1 and lower Lens 2 plus marginal ore between these lenses. Internal Report Boliden Mineral Mton Ni % Cu % S % Lens Lens Ore zone It must be noted that there is a big potential to discover more mineralization at Bastutjärn. The mineralization is not limited in any direction and further drilling is needed to define the borders. On a more regional scale there is potential to find more Ni-Cu mineralization hosted in other parts of the Bastutjärn gabbroic intrusion. The targeted area must be to investigate the near contact zone of gabbro and metasediments, since they are interpreted to be a crucial key in the ore formation and also the area where mineralization is deposited at Bastutjärn. 39

41 Fig. 18. Plot of Ni grade (%) vs resource of Ni ore (Mton) in world class deposits divided into different deposit types. After Jaireth et al. (2005). 8 Conclusion The Bastutjärn gabbroic intrusion consists of rocks ranging from gabbroic to dioritic composition, with only minor amounts of ultramafic rocks. Based on nomenclature set up by IUGS a majority of the gabbroic rocks are gabbronorites, containing almost equal amounts of clino- and orthopyroxene. Optical and geochemical studies suggest that the rocks here are more or less affected by assimilation of the country rock, consisting of graphitic- and pyrrhotitic schist. A process that have resulted in abundant xenoliths and fragment of sedimentary units within the gabbroic body. The mineralization at Bastutjärn is consisting of multiple layers of semi-massive sulphides, mainly pyrrhotite with minor chalcopyrite and pentlandite. Layers are interpreted to form two zones where the mineralization is concentrated. Between the high grade zones the mineralization is disseminated with droplets of pyrrhotite with pentlandite and chalcopyrite 40