MASTER'S THESIS
Mineral Chemistry and Texture
Paragenesis of Alteration Minerals in the
Pahtohavare Cu-Au Deposit, Sweden
Valentin Alain
2014
Master of Science (120 credits)
Exploration and Environmental Geosciences
Luleå University of Technology
Mineral chemistry and texture paragenesis of
alteration minerals in the Pahtohavare Cu-Au
deposit, Sweden
Valentin Alain
Master of Science
Exploration and Environmental Geosciences
Luleå University of Technology
Abstract
The Proterozoic Pahtohavare Cu-Au deposit in the northernmost part of Sweden within the Fennoscandian shield consists of a syngenetic stratiform sulphide-magnetite mineralisation (East Ore) which is uneconomic and three stratabound to discordant epigenetic Cu-Au mineralisations (Central, South-East and South Ores) hosted by the Viscaria formation. These epigenetic deposits are hosted by fine-grained albite felsite formed by alteration of graphitic schist while the East Ore is hosted by tuffite. The black graphitic schist have acted as a chemical trap for the mineralising fluids explaining the decomposition of the graphite within the schist proximal to mineralised zones and altering it into albite felsite. The past tectonic events made the Kiruna area having a favorable permeability for epigenetic solutions like saline hydrothermal fluids. This favourable permeability is one of the main important characteristic which explains the formation of Pahtohavare ores.
A scapolite-biotite alteration is enveloping the albite-altered mineralised zone and occurs in all stratigraphic units. One albite alteration of the tuffite is related to the intrusion of the footwall mafic sill and the other one is an additional ore-related mineralised albitization which is distinguishable by the lack of spatial relationship with the mafic sill and the occurrence of disseminated Ferro-dolomite. Chlorite has been formed by replacement of biotite and amphibole. A negative correlation between Mg and Cl contents of amphiboles is distinguishable which indicates that Mg-Cl avoidance mechanisms can control the incorporation of halogen in the amphibole structure. Scapolite from scapolite-biotite alteration surrounding the ore-bearing albite felsites and ore veins have a dominantly marialitic composition which indicates that the alteration must have been due to highly saline fluids. The occurrence of dipyre in Pahtohavare can be explained by the fact that the formation of the deposit happened in a Na-Cl rich environment. The main ore minerals are chalcopyrite and pyrite occurring disseminated, as veinlet, or filling breccias, and they are often associated with quartz and carbonate. Pyrrhotite is locally significant. Accessory minerals such as sphalerite, galena, millerite, native gold, tellurobismuthite, altaite, molybdenite, tellurides, and native gold in epigenetic ores occur as inclusions in sulphides and quartz.
Alteration zones surrounding the Pahtohavare ores have chemical and mineralogical
zonations similar to Rakkurijärvi.The Mg-content of biotites decreases toward the ores of the
South, East and South East zone. Addition of potassium and depletion of calcium and manganese are characteristic of the biotite-scapolite alteration zone. The ore-bearing albite carbonate alteration zone shows a depletion of K2O and an addition of Na2O, CaO and MnO. The relation between the depletion of Na and scapolitisation-albitisation is close as it is likely due to Na-Ca exchange reactions. Co-variation diagrams are good geochemical discriminants which can be used as exploration tool. The alteration characteristics of the Pahothavare deposit share similar features with other iron oxide and sulphide deposits such as e.g. Bidjovagge, Norway. The sodic alteration is the most important one because of its association to the mineralisation.
There are two generations of ore-forming fluids at Pahtohavare. The main physicochemical parameters that controlled hydrothermal alteration and gold mineralisation are pH, ƒO2 and temperature. The decrease of ƒO2 triggered the replacement of pyrite by pyrrhotite and its occurrence is spatially related to the mineralisation. The magnetite-pyrite and hematite-pyrite assemblages may have buffered the pH increase and ƒO2 decrease of the ore fluids. The chloride complexes of copper and gold are the most important one concerning the transport. The destabilization of gold chloride complexes is the main mechanism of gold deposition. This destabilization is due to an increase of pH from CO2 loss, cooling and dilution of the solution. The high salinity of the fluid can be explained by the metasomatic hydration of biotite and amphibole formation. Salinity is an important factor which determines the precipitation of metals from chloride complexes. Pahtohavare is considered as a copper deposit because of the low concentrations of gold due to the low initial concentrations of gases in the ore fluid and the fluids have not reach the native gold solubility
103 ppb limit for Cl complexes but has crossed the copper 100 ppm limit for Cl complexes.
The copper content of the hypersaline brines at Pahtohavare have a range of 100-500 ppm which is comparable to saline magmatic fluids of the Cloncurry district in Australia.
Table of Contents
Introduction ... 1
Geological settings ... 2
2.1. Regional geology ... 2
2.2. Local geology ... 5
2.3. Geology of the Pahtohavare deposit ... 9
Methodology ... 12 Results ... 13 4.1 Geochemistry ... 13 4.2 Microscopic study ... 17 4.3 Mineral chemistry ... 23 4.3.1 Silicates ... 23 4.3.2 Iron oxides ... 30 4.3.3 Sulphides ... 31 4.3.4 Carbonates ... 33 Discussion ... 36 Conclusion ... 42 Acknowledgments ... 42 References ... 43 Appendix... 46
1
Introduction
The main purpose of this thesis is to study the mineral chemistry variations, the texture and paragenesis of alteration minerals in rocks within and surrounding the ores of the Pahtohavare deposit to be able to determine if any correlations between the gold-copper occurrence and those variations exist.
The first aim of this thesis is to identify the different mineral chemistry variations occurring in the different ores to be able to deduce the mechanisms and sources of those variations. The second objective is to make simplified paragenetic sequences and detailed descriptions of specific paragenesis of alteration minerals of each ore in order to identify any differences between the ores. When these objectives are completed, it would be possible to determine if any zonation of alteration can be used as exploration tools pointing toward the mineralization The Proterozoic Pahtohavare deposit is located 8km southwest of Kiruna and about 10 km south of the Viscaria deposit, in a volcanoclastic unit of the Kiruna greenstones. The discovery of this deposit is the result of intensive gold exploration in Norrbotten by the Swedish Government since 1982.
In 1984, NSG initiated an exploration program at Pahtohavare with the intention to find ore of Viscaria type using geological, geophysical, geochemical work and diamond drilling. As very little number of outcrops was found, geophysics, geochemistry and drilling have been mainly used (Carlson, 1988). The past exploration activities four different ores was identified: South East Pahtohavare, East Pahtohavare, South Pahtohavare, and Central Pahtohavare. At the end of the 1987 exploration program, 108 diamond drill holes and 22000 metres of drilling had been completed.
Pahtohavare has been mined minor contributions from 1990-1997. The majority of the production came from the main Southern orebody with the South-East ore. The Central ore have not been mined mainly because it consists of secondary Cu-minerals which is a problem in a metallurgical point of view. Due to the results of a recent EM survey, the Central ore is now the main subject of interests. As a large untested conductor located in a down-dip position of the Central oxide ore body has been identified. This thesis has been sponsored by Kiruna Iron AB.
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Geological settings
2.1. Regional geology
The Pahtohavare Cu-Au deposit is located in the northern part of the Norrbotten County which is an important mining region within the Fennoscandian shield. The apatite irons ores (Kiruna and Malmberget) are economically the most important deposits for this province however it also hosts the Aitik deposit which is the Europe’s largest Cu mine. Gold is a minor element in some Cu-deposits like Pahtohavare (Fig.1.).
Fig.1. Simplified bedrock map of the northern art of Norrbotten County with epigenetic deposits occurrences
(Bergman, Kübler, & Martinsson, 2001).
The bedrock in the northern Sweden is a part of the Baltic Shield which has been formed during three consecutive major orogenic events: Lopian orogeny (2.9-2.6 Ga), Svecofennian
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orogeny (2.0-1.7 Ga) and Gothian orogeny (1.75-1.50 Ga). As showed in Fig.2, the study area is located in a transition zone between the Archean domain of the Baltic Shield in the northeast and the Svecofennian terrains on the southwest. The Archean domain has been affected by a rift event during the Lopian orogeny which has produced rift related greenstones. At the same time, a westward subduction of ocean crust has generated calc-alkaline volcanic rocks. It ended with a collision envent with the Belomorian belt which is outlining the suture (Gaál & Gorbatschev, 1987; Martinsson, 1997).
Fig.2. Major geological units in the Fennoscandian Shield and surrounding areas. (Bergman, Kübler, &
Martinsson, 2001). KADZ= Karesuando-Arjeplog deformation zone, PSZ= Pajala shear zone.
The basement of the Norrbotten Region is an Archean granitoid-gneiss basement consisting of c. 2.8 Ga tonalite-granodiorite intrusions with some mafic-intermediate volcanic rocks, clastic sediments and undeformed red c. 2.7 Ga granites (Bergman et al., 2001). Several generations of 2.1 Ga mafic dyke swarms have intruded these rocks and are related to the continental breakup (Martinsson, 1997). The deformed-metamorphosed Archean basement is unconformably overlain by Paleoproterozoic greenstones, porphyries and sediments (Martinsson O. , 2004). In the Kiruna area, the Kovo group (c. 2.5-2.3 Ga) consists of clastic sediments and basaltic-andesitic volcanic rocks formed during a rifting event. During a second rifting event at c. 2.1 Ga, the Kiruna Greenstones were formed. This rifting ended with
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the opening of a SE-NW-directed ocean. MORB-type pillow lavas in the upper part of this sequence represents a deeper marine facies within a NNE-directed failed rift arm while the lower part of the greenstones have been deposited in a shallow water environment. The Haparanda and Perthite suites (1.89-1.87 Ga) are synorogenic intrusions and have a gabbroic to granitic range of composition (Martinsson, 2004). The extensive subduction (1.9 Ga) which had generated island arc magmatism and a basin closure has created a crustal accretion in Svecofennian time. The collisional orogeny and Svecofennian porphyries (1.83-1.77 Ga) have followed (Martinsson, 1997).
Fig.3. Simplified diagram with schematic illustration of main rock types and units of the northern Norrbotten
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2.2. Local geology
The Pahtohavare deposit is situated in the Kiruna area which consists of Paleoproterozoic rocks as greenstones, porphyries and clastic sediments lying unconformably on a deformed Archean basement.
Fig.4. A. Simplified geological map of the Baltic Shield outlining the major tectonic units. B. Generalized
geology of the Kiruna area with location of the Viscaria deposit (Martinsson, 1997).
The stratigraphic succession of the Kiruna area can be described according to the following subdivisions. The Kovo Group (2.4-2.3 Ga) which consists of clastic metasedimentary and mafic-intermediate metavolcanic rocks unconformably overly the Archean basement (Bergman et al., 2001). This stratigraphically lowest unit of the Karelian rocks consists of basal conglomerate, tholeiitic lava, volcanoclastic sediments and calcalkaline mafic to intermediate volcanic rocks (Martinsson, 1997). The following unit is called the Kiruna Greenstone Group which is deposited during a rift-related event based on a combination of lithological and geochemical criteria divided into six formations (Martinsson O. , 1997). The Såkevaratjah Formation consists of amygdaloidal basalt with intercalations of conglomerate in the lower part. It is followed by the Ädnamvaara Formation which is an ultramafic unit with a komatiitic composition. Next is the Pikse Formation, another unit of tholeiitic basalt flows with some intercalations of chemical sediment. On top of it follows the Viscaria Formation which is a tuffitic unit with minor black schists, magnetite-sulfide ores, carbonate rocks hosting the Viscaria and Pahtohavare deposits. An upper basaltic pillow lava
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unit with some intercalations of mafic tuff, tuffite and iron-rich sediments is called the Peuravaaara Formation. The top of the Kiruna Greenstone consists of volcanoclastic rocks of pyroclastic origin, the Linkaluoppal Formation. In the central Kiruna area, it has been lost by erosion. The Såkevaratjah, Pikse and Viscaria formation have been intruded by mafic sills, probably related to the same magmatic event forming the Peuravaaara Formation (Martinsson, 1997). Then, the Kurravara Conglomerate overlay unconformably the Peuravaara Formation, followed by the Kirunavara Group and Hauki Quartzite (Martinsson, 2004). Both Kurravara Conglomerate and Kirunavara Group have an approximate age of 1.89-1.91 Ga. This volcano-sedimentary sequence deposited on the Archean basement has a total thickness of approximately 8-10 km.
Fig.5. Stratigraphy and chronology of Paleoproterozoic greenstones in the northern part of the Baltic
Shield (Martinsson, 1997).
The Kiruna area has been affected by several types of alterations, some related to ore formation and others have a more regional character. Scapolitisation for example is rather widespread and particularly intense in the north of the Norrbotten County. Intense scapolitisation is believed to be mainly linked to shear zones, contact of intrusive rocks or mineral deposits. The strong scapolite alteration of the lowest part of the Greenstone group is possibly due to the metamorphism of evaporate beds (Martinsson, 1997). According to Lindblom et al (1995) and Bergman et al (2001), regional scapolite replaces plagioclase in gabbroic rocks and in mafic sills which has also been observed from core logs study. A second important type of alteration in Kiruna area is a locally intense albitization of
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greenstones producing Na-rich felsic rocks along the contact with mafic sills in the Viscaria Formation and associated to epigenetic copper-gold mineralisations. The albitization of tuffite has an extension 1-15m from the contact of the mafic sills and has been initiated from the reaction between hot basaltic intrusions and unconsolidated volcanoclastic rocks with saline interstitial water (Martinsson, 1997).
Metamorphism & Deformation
The Pahtohavare deposit is located within the Kiruna area where the metamorphosed rocks may contain zoisite and chlorite. It is characterised as a low-grade metamorphic area. In this area hornblende, epidote, plagioclase, chlorite, actinolite and albite are the minerals present in the mafic metavolcanic rocks which indicates those rocks have undergone an upper greenschist to lower amphibolite metamorphic facies.
As a part of the Norrbotten County, the Kiruna area went through several ductile and brittle deformation events. This E domain has a strong North-South structural orientation (Bergman et al., 2001). Four different deformation events has been identified in the Kiruna area: an early westward thrusting linked to the formation of mineral lineation, folding, a formation of thrust ramps and last, the formation of open folds and shear zones (Bergman et al., 2001). The major ductile shear zones affecting the north of Sweden as KADZ, KNDZ, NDZ and PSZ (Fig.7) have been active at ca. 1.8 Ga. It has been noticed that the crustal-scale shear zones have a close relationship with metamorphic grade changes which indicate that the shear zones have been active after the peak of regional metamorphism. Many epigenetic gold and copper-gold deposits have a close spatial relationship with crustal-scale shear zones, however it appears that their local control depend of second-fourth-order faults and shear-zones (Billström et al., 2010).
Today, it is possible to say that the Kiruna area has been affected by one major compressional deformation episode and correspond to eastern limb of an anticline by studying the bedding-cleavage relationships and fold symmetries. The Kiruna Greenstone Group dipping toward southeast is part of this anticline steeply. The asymmetric folds dipping toward south are Z form (Vollmer, Wright, & Hudleston, 1984). In the Pahtohavare area, it is possible to identify a local anticline. This anticlinal structure has been partly overturned and truncated by a major WNW shear zone and then, secondary shear zones have been formed mainly at the fold hinge and limbs (Martinsson O. , 1997).
8 Fig.6. Metamorphic map of the northern part of Norrbotten County. Examples of localities where boundaries
between medium-and high-grade metamorphism are controlled by deformation zones are marked by A. KADZ= Karesuando-Arjeplog deformation zone. KNDZ= Kiruna-Naimakka deformation zone, NDZ= Nautanen deformation zone, PSZ= Pajala shear zone (Bergman et al , 2001).
9 Fig.7. Map with generalised structural trends from magnetic connexions and form lines of tectonic foliations.
KADZ= Karesuando-Arjeplog deformation zone. KNDZ= Kiruna-Naimakka deformation zone, NDZ= Nautanen deformation zone, PSZ= Pajala shear zone (Bergman et al, 2001).
2.3. Geology of the Pahtohavare deposit
In 1984, the NSG (The State Mining Property Commission) start to explore for Viscaria-type deposits in the Pahtohavare area where resulting it the discovery of a syngenetic stratiform sulphide-magnetite mineralisation genetically similar to the Viscaria deposit, the East Pahtohavare deposit. The mineralisation consists of thin intercalations of magnetite, chalcopyrite, pyrrhotite, pyrite situated in the middle part of the Viscaria Formation between two black schist formations. However, it is considered has an uneconomic deposit due to its small size and low grade. After few drilling operations 1985-1988 the South, South-East and Central deposits was discovered. Those three stratabound to discordant epigenetic Cu-Au mineralisations are hosted by the Viscaria formation in an anticlinal structure dipping to the south-east with an over-turned south limb which makes the axial plane dipping approximately sixty degrees to the north-east. The epigenetic deposits are hosted by fine grained albite felsite which has been formed by alteration of the graphitic schists while the Eastern deposit is hosted by tuffite (Martinsson, 1997). The black graphitic schists have probably acted as
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chemical trap for the mineralising fluids explaining the decomposition of the graphite within the schists proximal to mineralised zones and altering it into albite felsite.
Fig.8. Geological map of the Pahtohavare area. All rocks belong to the Greenstone group except the
metaconglomerate (Svecofennian metasediment) (Bergman et al, 2001).
The host rocks are highly altered by regional and local alteration occurring within the Pahtohavare area. The scapolite-biotite alteration is enveloping the albite altered mineralised zone and occurs in all stratigraphic units. There are two types of albite alteration occurring in Pahtohavare, one is related to the intrusion of the footwall mafic sill as mentioned previously and one is an additional ore-related mineralised albitisation which is distinguishable by the lack of spatial relationship with the mafic sill and the occurrence of disseminated Ferro-dolomite.
The main ore minerals are chalcopyrite and pyrite occurring disseminated, as veinlet or filling breccias and often associated with quartz and carbonate. Pyrrhotite is locally significant. Accessory minerals as sphalerite, galena, millerite, tellurobismuthite, altaite, molybdenite, tellurides and native gold in epigenetic ores occur as inclusions in sulphides and quartz (Lindblom et al., 1995). Copper and gold have probably precipitated at the same time according to their similar distribution patterns (Martinsson et al., 1997). The gangue minerals are Ferro-dolomite, quartz and minor marialitic scapolite, biotite and albite. Moreover, the host rock age and timing of the mineralisation is c. 2.1 Ga. In addition to Au and Cu, the components enriched are Co, U, S and Te (Martinsson et al., 1997).
Only the South and South-East deposits have been mined starting as open pits respectively from 1990 to 1992. Both ores have been developed for underground mining in
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1993 until their closure in 1997. Totally 1.68 Mt of ore with 1.89% Cu, 0.88ppm Au has been produced (Martinsson et al., 1997). The Central deposit has not been mined mainly because of its small size and the extensive supergene oxidation which has replaced the primary sulphides
by oxides as malachite, chrysocolla, limonite-goethite and Cu-oxides.
Pahtohavare deposit have an analogue at Bidjovagge (Norway) which also is a copper-gold deposit occuring in albitized rocks comprising graphitic schist and tuffite in the Proterozoic Kautokeino Greestone Belt. They occur similar to Pahtohavare in antiforms close to shear zones. In the Bidjovagge case, the poor correlation between gold and copper indicate that the precipitation of the metals was controlled by different factors while Pahothavare has a coeval precipitation of Cu and Au shown by their distribution patterns and as gold inclusions occur in chalcopyrite. Albitisation is the most common alteration at Bidjovagge and is often associated with carbonatization. The alteration may have been due to seawater circulation or a sodium-rich brine of seawater. Scapolite is formed at the expense of albite. Biotite is present in restricted zones with albite, amphibole and carbonate (Bjørlykke et al., 1997).
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Methodology
There are four main methods which have been used to get enough information to be able to complete the objectives of this thesis.
Core logging
Thirteen drill cores from the SGU drill core archives in Malå have been selected by Olof Martinsson. These cores (Appendix 1) have been previously logged by Leif Carlson and some by Heikki Markkula, Stina Danielson and Bose Gustafsson. The author has logged these cores during spring 2013. There are two from the East, three from the Central, five from the South and three from the South-East ore. They have been drilled from 1984 to 1988.
Geochemistry
Nineteen samples from the three drill cores of the South-East ore have been sent to ALS laboratory in Piteå for wholerock analysis using the XRF method (Appendix 2). The samples have been preferentially selected from less altered units which are representative precursors of the altered ones.
Microscopy
Fifty-three samples have been taken from the thirteen logged drill cores and sent to Vancouver Petrographics, Canada. The thin sections were analysed by optical microscope in order to define and identify the texture paragenesis of alteration minerals. Alterations zone, mineralisations and veins have been preferentially selected. The microscopic observation of thin sections is necessary to be able to collect enough information about the mineralogy and in order to select the minerals for microprobe analysis.
Microprobe
The microprobe analyses have been preferentially done on silicates, carbonates, sulphides and oxides in order to determine any changes in composition of those minerals. The JEOL JXA-733 electron microprobe analyser of the Oulu University (Finland) is believed to be the best one according to the quality of the past results. A total of 22 analyses were performed on biotite, 22 were acquired on chlorite, 12 on titanium minerals, 7 on scapolite, 5 on feldspars, 7 on iron oxides and 6 on amphiboles. An additional 22 analyses were made on carbonates and phosphates and 54 on sulphides.
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Results
4.1 Geochemistry
Classification of magma
The use of the cation plot from Jensen (1976) makes it possible to get the general character of the volcanic rocks as the main elements reflects the mineralogy of the rocks. From the Jensen plot, it is possible to identify three major rock types: tholeiites, komatiites and calcalkalines rocks. The analysed volcanic rock samples have a Fe-tholeiitic character. The only sample near to the calc-alkaline area is believed to be a highly altered albite felsite.
Fig 10. Cation plot for volcanic rock from the Kiruna Greenstone Group at South-East Pahtohavare (Jensen,
1976).
Alterations
As the sampled rocks have been more or less affected by several types of alterations as albitization, scapolitisation, chloritisation and biotite alteration in accordance to the logging description, it is necessary to plot the data in order to find the least altered sample (Fig.11)
14 Fig 11. Igneous spectrum plot for the Kiruna greenstone group rocks of South-East Pahtohavare (Hughes, 1973).
The Alteration Index (AI=100.K2O+MgO/K2O+MgO+CaO+MgO) have also been calculated
for each mafic volcanic samples (Appendix 2). The least altered sample have been selected
because it has an AI<50, K2O< 2%, Na2O< 3% and plot within the igneous spectrum.
Understand the geochemical discrimination between altered and least altered volcanic rocks is very important in order to identify the alterations. From Fig.12, it is possible to say that the volcanic rocks that have been exposed to biotite-scapolite alteration exhibit addition of potassium and the depletion of calcium and for some sample manganese as well. Barium has also been added, this addition can also be due to the biotite-scapolite alteration or/and by the presence of some small barite veins. L.O.I. is mostly increased compared to the least altered sample which could be explained by the presence of dolomite and calcite in altered rocks. According to Martinsson (1997), in the South-eastern ore the surrounding biotite-scapolite alteration can be identified by the enrichment of K, Ba and a depletion of Mn. Plotting geochemical compositions of samples from different ores (Fig.13) has been decided in order to identify the relationship between alterations and several ores of Pahtohavare. The results from these graphs confirm the previous observations that the closer we get to the ore-bearing
albite carbonate alteration zone, K2O got depleted and Na2O, CaO and MnO are enriched
which reflects the destruction of biotite-scapolite and amphibole during albitization (Martinsson, 1997). The samples showing a %Cu >0.5% are mostly at the left side of the graphs. It is also possible to notice from these graphs that the samples from the surrounding biotite-scapolite alteration which caused an enrichment of K and a depletion of Mn are on the right side. Moreover, it is possible to say that the Cu occurrence is related to albitization even if some samples from the albite alteration zone do not show a Cu enrichment.
15 Fig. 12. Chemical changes in mafic volcanic wall rocks at the South-eastern ore. Normalized to the least altered
rock. -0.5
0 0.5 1
1.5 Log WR88057E/ Least altered
-0.5 0 0.5 1
1.5 Log WR85115A/ Least altered
-0.5 0 0.5 1
1.5 Log WR88034C/ Least altered
-0.5 0 0.5 1
1.5 Log WR88057I/ Least altered
-0.5 0 0.5 1
16 Fig. 13. Figures showing the correlation between copper and depleted/enriched elements in ore and wall rocks.
Blue: South-eastern ore; Red: Central ore; Green: Eastern ore. 0 1 2 3 4 5 6 7 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 %Cu %K2O 88057 85115 88034 87119 87104 85108 84004
Albite alteration Biotite-scapolite alteration
0 1 2 3 4 5 6 7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 %Cu K2O/CaO+Na2O+K2O % 88057 85115 88034 87119 87104 85108 84004
Albite alteration Biotite-scapolite alteration
0 1 2 3 4 5 6 7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 %Cu K20/K2O+Na2O % 88057 85115 88034 87119 87104 85108 84004 Albite alteration
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4.2 Microscopic study
Biotite-Scapolite alteration
Two types of biotite-scapolite alterations have been identified during the core logging study, one has regional character and one is ore-related. In a petrographic view, the scapolitisation close to the South-East ore (Fig.14 A) consist of 0.5-1.0 mm subhedral porphyroblasts of scapolite in a matrix of 20-50 µm euhedral biotites. This is the result of the change of mafic sill into a fine grained biotite rock with large scapolite due to the replacement of amphibole by biotite and scapolite replacing plagioclase (Martinsson, 1997). It is also possible to observe some few 0.75-0.50 mm euhedral pyrites mainly in the matrix but some rare and small pyrite grains can be present in scapolites. A biotite-scapolite ore-related alteration from the Central ore (Fig.14 B) shows 250-500 µm anhedral porphyroblasts of scapolite in a matrix of 20-50 µm subhedral biotites. Minor quartz and plagioclase grains are also present. It is also possible to see that amount and the size of the scapolite grains are different between the two samples. Some scapolite grains are sericitized along cleavages in sample B while none have been observed in sample A. The A and B biotite-scapolite altered rocks are believed to be the result of the replacement of amphibole by biotite which is also believed to come from the alteration of plagioclase and pyroxene. Pyrite is the only sulphide which has been observed into the biotite-scapolite alteration zone. Subhedral magnetites are mostly present and carbonates are often very rare.
Albitization
There is two different albite alterations one is regional formed during the basaltic intrusion in volcanoclastic rocks containing saline interstitial water and another one which is ore-related. The ore-related one is distinguishable by the fact that it is associated with Ferro-dolomite. Albite is formed from replacement of biotite and scapolite. Figure 14 E shows the mineral assemblage which consists of biotite-scapolite alteration with albite, containing disseminated magnetite, pyrite associated with albite veining and mm scale carbonate grains. Figure 14 F also shows the minerals assemblage from the transition zone between biotite-scapolite and albite alteration. The hand specimen shows a “zebra texture”. Sericitisation of scapolite, albite and other feldspar is quiet common but weak compared to carbonatisation (Fig.14 B, D). Albite grains usually occur as patches with carbonate and biotite grains while albite veins have ~100µm elongate grains rather orientated rather approximately parallel to the vein orientation if it had not been deformed by folding and shearing.
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Chloritisation
It is possible to differentiate two types of chloritisation, one concerning amphibole grains by the size and shape of the primary amphibole (Fig.20E), the second one
Fig 14. Microscopic pictures; A: scapolite alteration of mafic sill South-East Pahtohavare, B:
Biotite-scapolite alteration of gabbroic wallrock Central Pahtohavare, C: Chloritisation Central , D: Albite vein in a gabbroic wallrock South Pahtohavare, E Scapolite-biotite/albitization transition zone South Pahtohavare, F
Scapolite-biotite/albitization transition zone South-East Pahtohavare.
A
B
C
D
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related to the replacement of biotite which form small scattered grains particularly in potassic alteration zones. Two main events of chloritisation have been deduced from microscopy study: before and after the formation of sulphides. The chloritisation zone is widespread and is most important in mafic gabbroic units. Chlorite is mainly related to amphibole alteration in mineralised samples.
Paragenesis sequences
East
The microscopic study of the Eastern ore has been done on a thin section showing the mineralisation in veinlets and wallrock which consists of disseminated magnetite, chalcopyrite and pyrite but only pyrite and chalcopyrite in the veinlet. The amount of pyrite is much more important than chalcopyrite while pyrrhotite is minor compared to chalcopyrite. Chalcopyrite is anheudral, bordering the pyrite and showing open-space filling texture. Vermicular chalcopyrite in quartz has also been observed. As it is possible to see from Figure 20 C, quartz grains are rather anheudral and contain sulphides and a few chlorite grains. Rounded chlorite aggregates have a size of 350 to 50 µm in the veinlet while in the wallrock chlorites show a complete different texture. Quarts grains are much smaller and feldspars have not been seen. Carbonatisation occurs as veins of mm to µm scale grains. It is possible to
Minerogenetic periods and stages
Time
Early
Late
SILICATES Pyroxene Amphibole Albite Quartz Chlorite Biotite Scapolite CARBONATES Dolomite Calcite OXIDES Magnetite Illmenite SULPHIDES Pyrite Chalcopyrite Pyrrhotite Sphalerite ? ?20
notice a halo of reaction between chlorite and carbonates which give information for the paragenesis sequence Figure 20 E. Hematite is rather less abundant than magnetite and is associated with pyrite while magnetite is disseminated. Quartz occurs as interstitial µm scale grains or as veinlets. Few 100-300 µm rounded garnets have been identified in carbonate veining network in a volcano sedimentary unit.
South
Samples from the Southern ore gave some additional information in and completed the rest of the paragenesis sequence. The studied thin section shows a porphyroblastic texture composed of subhedral 100µm-1mm dolomite porphyroblasts in a matrice of pyrite, chalcopyrite and pyrrhotite (increasing proportion order). Pyrite and pyrrhotite are sometimes vermicular through carbonate grains but rather euheudral to subheudral. Chalcopyrite is disseminated, anheudral and filling cracks carbonates and shows an interstitial, free space filling texture. Carbonates have been observed in cracks of albite phenocrysts. Weak sericite alteration is also identifiable. Quartz mostly occurs to be related to veining events which post-date the carbonate one. It is possible to distinguish the original porphyroblastic texture from a biotite-scapolite altered sample from the albtisation zone. Scapolite and biotite have been
Minerogenetic periods and stages
Time
Early
Late
SILICATES Pyroxene Amphibole Albite Quartz Chlorite Sericite Biotite Scapolite CARBONATES Dolomite Calcite OXIDES Hematite Magnetite Rutile SULPHIDES Pyrite Chalcopyrite Pyrrhotite21
replaced to albite. The matrice is composed of small elongated 100-200 µm grains which show a very low extinction angle from the Carlsbad twins.
Central Minerogenetic periods and stages
Time
Early Late
SILICATES Pyroxene Amphibole Albite Quartz Chlorite Sericite Biotite Scapolite CARBONATES Dolomite Calcite OXIDES Hematite Magnetite Illmenite Rutile SULPHIDES Pyrite Chalcopyrite PyrrhotiteThe amount of magnetite in the Central ore thin section is much more abundant than sulphides. Pyrite is more abundant than chalcopyrite. Chalcocite has been observed associated with chalcopyrite which may be due to supergene oxidation. Chalcopyrite and pyrite can be within magnetite grains but in the matrix as well. Chalcopyrite has not been seen in pyrite. Pyrite shows an open space filling and jointing texture. Quartz occurs mainly between the oxides and sulphides grains but few times inside magnetite. Hematite is also occurring mainly at the contact with pyrite and along magnetite. Pyrite and pyrrhotite occur rather sporadically which means that these phases are subordinate. Some cuprite, tenorite, malachite, hematite and azurite have been identified in previous microscopic studies of the ore (Hålenius, 1988). 10µm grains of gold occur with oxides and carbonates but mainly in the quartz-rich parts of the mineralisation.
22 South-East Minerogenetic periods and stages
Time
Early Late
SILICATES Pyroxene Amphibole Albite Quartz Chlorite Sericite Biotite Scapolite CARBONATES Dolomite Calcite OXIDES Hematite Magnetite Illmenite Rutiles SULPHIDES Pyrite Chalcopyrite Pyrrhotite PHOSPHATES ApatiteThe thin section from South-East ore studied shows 100-300 µm carbonate grains close to the intense carbonatisation while grain size decrease when carbonates proportion decrease. Magnetite grains have a size from 0,1 to 2mm and are rather euheudral. Magnetite and pyrite 10-100 µm grains are subheudral to anheudral in the fine grained part. Chalcopyrite is often on the border of pyrite and magnetite grains. It is also possible to see pyrite and chalcopyrite inclusions in magnetite while magnetite inclusions in pyrite are rare. Albite and other feldspars have not been identified which is probably due to the intense carbonatisation. The abundance of magnetite is much more important than pyrite which occurs in a higher proportion than chalcopyrite. Two generations of chlorite have been identified, one occurring as fine grained matrix (biotite replacement) or phenocrysts (previously amphibole) and one weaker consisting of chlorites in late quartz veins crosscutting albite veins.
23
4.3 Mineral chemistry
4.3.1 Silicates Biotites
In order to identify any chemical changes and zonation of the biotite alteration, analysed biotites come from the biotite-scapolite alteration of both gabbroic and volcanosedimentary units and from mineralised albite felsite. The #Mg (Mg/Mg+Fe) has been calculated for each biotite. Most of biotites are phlogopites but annites have been identified from the east mineralisation in graphitic banded volcanosedimentary unit (Appendix 3, No1, 50) and also from a biotite-
Table 1. Representative electron microprobe analyses of biotites.
scapolite altered gabbroic unit situated between two significant mineralisation (Appendix 3, No36, 41) at the South zone. Concerning the East zone, the analysed biotite of a sample following the mineralisation has a higher #Mg (Appendix 3, No 8) than the ones of the mineralisation. The Central zone biotite analyses show a different pattern. The biotites from the mineralised albite felsite have a high #Mg (Appendix 3, No 19 24) while biotites from gabbroic unit surrounding the albite felsite have a lower #Mg. The South and South East zone
Zone South East Central South East
Sample 88219C CHL 88218D2 BIOT 84004Fbiot 85108F BIOT 87104J1 BIOT 87127E BIOT 85115J BIOT 88057A BIOT
No. 57 101 8 50 36 67 74 120 Na2O 0,171 0,113 0,163 0,217 0,163 0,118 0,134 0,178 FeO 14,184 19,816 19,706 23,996 26,499 18,686 21,462 18,008 Cr2O3 0 0 0 0,021 0,03 0,087 0,069 0,016 Cl 0,523 0,681 0,467 0,603 0,764 0,819 0,458 0,645 MgO 17,783 12,834 11,136 8,134 9,731 13,363 12,128 13,506 MnO 0,057 0,129 0,576 0,202 0,149 0,069 0,167 0,055 K2O 7,506 8,883 9,8 8,965 6,3 6,714 5,834 10,072 Al2O3 15,03 15,04 16,639 16,337 15,967 14,87 17,716 14,865 NiO 0,067 0,007 0 0,059 0,087 0,035 0,012 0,013 CaO 0,05 0,058 0,021 0 0,066 0,141 0,194 0,003 SiO2 35,44 36,079 36,07 34,7 32,379 34,789 32,796 37,282 TiO2 0,357 1,589 1,304 1,636 1,454 1,479 1,101 1,788 Total 91,05 95,075 95,777 94,734 93,417 90,985 91,968 96,285
Number of cations on the basis of 11O
Si 2,757 2,779 2,765 2,745 2,614 2,766 2,598 2,821 Al 1,378 1,366 1,503 1,523 1,519 1,393 1,654 1,325 Ti 0,021 0,092 0,075 0,097 0,088 0,088 0,066 0,102 Cr 0,000 0,000 0,000 0,001 0,002 0,005 0,004 0,001 Mg 2,062 1,474 1,272 0,959 1,171 1,584 1,432 1,523 Fe2+ 0,923 1,277 1,263 1,587 1,789 1,243 1,422 1,139 Mn 0,004 0,008 0,037 0,014 0,010 0,005 0,011 0,004 Ni 0,004 0,000 0,000 0,004 0,006 0,002 0,001 0,001 Na 0,026 0,017 0,024 0,033 0,026 0,018 0,021 0,026 K 0,745 0,873 0,958 0,905 0,649 0,681 0,589 0,972 Ca 0,004 0,005 0,002 0,000 0,006 0,012 0,016 0,000 Cl 0,069 0,089 0,061 0,081 0,105 0,110 0,061 0,083 OH 1,931 1,911 1,939 1,919 1,895 1,890 1,939 1,917 #Mg 0,69 0,536 0,50 0,38 0,40 0,56 0,502 0,572
24
biotites show a similar pattern as the ones from the East zone. The biotites of the samples taken in a gabbroic units and albite felsite prior to the mineralisation show a #Mg>0,59 while biotites from samples close and within the mineralisation have a #Mg≤ 0,57 (Appendix 3 No106, 110, 74, 88, 95, 120).
Chlorites
The only chlorite which show a low #Mg have been found from the same samples where annites (low #Mg) have been previously identified (samples 85108F and 87104J). However, Mg rich chlorites are the most common ones and are also present with Fe-rich chlorites and biotites. Using the classification of Hey (1954) and Melka (1965), chlorites plot as delessite, pennine and corundophilite. It is not possible to distinghish the chlorites using optical microscope. However, chlorites replacing amphibole all plot in the corundophilite field.
25 Fig 15. Classification diagram of chlorites (Melka, 1965)
26
Amphiboles
Concerning the chemical analysis of a hornblende from a carbonate altered mafic gabbroic unit in the South zone (Fig 17A, 88218D1), pyrite grains have been observed into it. An undefined amphibole has been studied from a quartz-chlorite vein with chalcopyrite, pyrite and Cr rich magnetite in a volcanosedimentary unit at the Central zone. The amphibole has been named as a ferro-tschermakite and occurs as an early mineral in the paragenesis sequence of the vein. The majority of amphiboles occur to have actinolite compositions. From Fig. 17 B, we can see a similar relationship between pyrite and the amphibole. We can now have an idea that the generation of amphibole and pyrite are close in the time scale and pyrite first generation rather post-date the amphibole generation. Actinolite may have been formed by alteration of albitic felsite by the ore-forming solution.
Table 3. Representative electron microprobe analyses of amphiboles.
Zone South East Central
Sample 88218D1 HBL 88219B FELDS 84004Mamph 87104J1 HBL 87119C ACT Name Actinolitic Hbl Actinolitic HblFerro-Hbl Actinolitic HblActinolite
SiO2 52,342 52,928 42,789 50,271 55,192 TiO2 0,140 0,143 0,145 0,128 0,037 Cr2O3 0,000 3,026 0,047 0,031 0,000 Al2O3 3,736 3,026 11,131 4,470 1,932 FeO 12,420 9,824 20,026 16,446 7,982 MnO 0,301 0,129 0,494 0,338 0,153 NiO 0,000 0,000 0,000 0,000 0,027 MgO 15,778 17,423 6,538 12,331 19,105 CaO 11,826 12,502 11,672 12,139 12,633 Na2O 0,477 0,526 1,198 0,569 0,438 K2O 0,043 0,083 0,634 0,244 0,014 H2O 0,000 0,000 0,000 0,000 0,000 F 0,000 0,000 0,000 0,000 0,000 Cl 0,055 0,000 0,943 0,273 0,045 Total 97,063 99,610 94,674 96,967 97,513 Si 7,477 7,376 6,705 7,420 7,732 Ti 0,015 0,015 0,017 0,014 0,004 Cr 0,000 0,333 0,006 0,004 0,000 Al 0,629 0,497 2,056 0,778 0,319 Fe 1,484 1,145 2,624 2,030 0,935 Mn 0,036 0,015 0,066 0,042 0,018 Ni 0,000 0,000 0,000 0,000 0,003 Mg 3,359 3,619 1,527 2,713 3,989 Ca 1,810 1,866 1,959 1,919 1,896 Na 0,132 0,142 0,364 0,163 0,119 K 0,008 0,015 0,127 0,046 0,003 H 0,000 0,000 0,000 0,000 0,000 F 0,000 0,000 0,000 0,000 0,000 Cl 0,013 0,000 0,250 0,068 0,011 #Mg 0,694 0,760 0,368 0,572 0,810
27 Fig. 17 A: 88218 Hornblende with its pyrite from South albite-carbonate mineralised sample; B: 87119, Central
28 Fig 18. Calcic amphiboles with diagram parameters: CaB≥1.5; (Na+K)A≤0.5; CaA≤0.5, according to (Leake,
2013)
Due to a very low Si value, an amphibole from Central zone could be named as Mg-Gedrite or Cummingtonite because it is magnesium rich and calcium, sodium and potassium concentrations are low. Another possibility explaining the low Si value is that the amphibole has been replaced by chlorite partially conserving the initial shape and cleavage as the sample has been taken within chloritisation alteration zone.
Feldspars and scapolites
Scapolite have been analysed in order to determine their nature which is mainly marialitic in composition. From Fig.14F, it has been possible to see scapolites occurring in sample from an albitization zone. The analyse of scapolite in sample 85115N1 show a marialitic scapolite closer to the albite field on the ternary classification diagram compared with other scapolites from samples outside the albite alteration zone. The two anorthites have been found from both gabbroic and volcanosedimentary units few meters above and below the mineralisation respectively. Plagioclase has also been found mainly in potassic alteration zone but also in albite felsite.
29 Fig 19. Plot of compositions of feldspars and scapolites in Ab-An-Or ternary classification diagram.
Table 4. Representative electron microprobe analyses of feldspars and scapolites.( Ab=Na/Na+K+Ca; An=Ca/Na+K+Ca; Or=K/Na+K+Ca)
Zone South Central South East
Sample 88218C FELDS 88093D FELDS 88221A SCP 87104J1 SCP 87104J2 SCP 87104J2 FELDS 87127E SCP 85115N1 SCP 88057A FELDS Anorthoclase Anorthoclase Anorthoclase Scapolite Scapoilte Albite Scapolite Albite Albite SiO2 55,672 56,646 55,552 55,100 55,598 69,585 55,599 67,200 67,690 TiO2 0,000 0,003 0,000 0,000 0,000 0,012 0,000 0,000 0,000 Al2O3 23,070 22,409 22,586 24,167 24,084 20,345 23,286 19,340 19,904 Cr2O3 0,000 0,004 0,000 0,000 0,023 0,000 0,004 0,000 0,000 FeO 0,173 0,089 0,049 0,015 0,065 0,179 0,000 0,566 0,099 MnO 0,044 0,000 0,000 0,065 0,024 0,053 0,000 0,024 0,010 MgO 0,000 0,000 0,000 0,002 0,000 0,000 0,000 0,147 0,000 CaO 7,947 6,319 7,300 9,506 7,696 0,404 7,614 0,476 0,295 Na2O 9,815 10,297 9,655 8,497 8,571 11,363 10,007 10,278 11,759 K2O 0,325 0,675 0,668 0,387 0,395 0,070 0,376 2,133 0,034 NiO 0,000 0,037 0,000 0,000 0,000 0,018 0,000 0,000 0,005 Cl 3,020 3,397 3,169 2,563 2,737 0,409 3,196 0,059 0,000 Total 97,046 96,479 95,810 97,739 96,456 102,029 96,886 100,164 99,796
Number of ions on the basis of 32 O
Si 10,446 10,652 10,539 10,271 10,424 11,918 10,437 11,872 11,876 Al 5,102 4,966 5,050 5,309 5,322 4,107 5,152 4,027 4,116 Ti 0,000 0,000 0,000 0,000 0,000 0,002 0,000 0,000 0,000 Cr 0,000 0,001 0,000 0,000 0,003 0,000 0,001 0,000 0,000 Mg 0,000 0,000 0,000 0,001 0,000 0,000 0,000 0,039 0,000 Fe2+ 0,027 0,014 0,008 0,002 0,010 0,026 0,000 0,084 0,015 Mn 0,007 0,000 0,000 0,010 0,004 0,008 0,000 0,004 0,001 Ni 0,000 0,006 0,000 0,000 0,000 0,002 0,000 0,000 0,001 Na 3,571 3,754 3,551 3,071 3,116 3,773 3,642 3,520 4,000 K 0,078 0,162 0,162 0,092 0,094 0,015 0,090 0,481 0,008 Ca 1,598 1,273 1,484 1,899 1,546 0,074 1,531 0,090 0,055 Alb 0,68 0,72 0,68 0,61 0,66 0,98 0,69 0,86 0,98 An 0,30 0,25 0,29 0,38 0,33 0,02 0,29 0,02 0,01 K 0,01 0,03 0,03 0,02 0,02 0,00 0,02 0,12 0,00
30
Titanium minerals
Rutile occurs with two different shapes. No. 113 is euheudral associated with quartz and carbonate grains in a sample from silicified part of a volcanosedimentary unit at the South zone close to the ore. The second rutile shows an elongated shape. Rutiles are present in a none negligible amount in a of graphitic volcano-sedimentarty unit situated close to the ore at the South East zone. Ilmenite grains are rather small (<50µm) subheudral to anheudral, filling disseminated cracks. The only ilmenite with a size over than 50µm has been observed in a sample of gabbroic unit from the Central zone.
Table 5. Representative electron microprobe analyses of Ti-minerals.
4.3.2 Iron oxides
The magnetite having a size less than 50µm usually occur as disseminated subheudral grains in gabbroic, mafic volcanic and volcanosedimentary wallrock. It also occurs as euheudral 200-500 µm grains in wallrock (sample 88219B) or in quartz vein as (sample 87104DMg). As mentioned before, the magnetite of this quartz vein which also contains pyrite and
Zone South East Central SouthEast
Sample 88221B MG 84004MMg 85108G MG 87104J1 PO SIL 85115J MG 85115N2 ILL 88057Nb mg
No. 113 4 54 35 72 83 93
Rutile Ilmenite Ilmenite Ilmenite Rutile Ilmenite Ilmenite
Na2O 0,023 0,045 0,031 0,046 0,135 0,025 0,02 FeO 0,048 41,75 41,588 45,099 4,023 41,774 40,34 Cr2O3 0,474 0,01 0,009 0,009 0,034 0,005 0,021 Cl 0 0,012 0,021 0,014 0,013 0 0 MgO 0,066 0,688 0,01 0,061 0,042 0,014 0,009 MnO 0,014 4,13 5,285 2,485 0,683 3,068 7,578 K2O 0 0,02 0 0,011 0,179 0,023 0 Al2O3 0,014 1,002 0 0,039 0,233 0,578 0,021 NiO 0 0,068 0 0,013 0,037 0,04 0 CaO 0,039 0,025 1,459 0 0,604 2,844 0 SiO2 0,108 1,996 0,012 0 3,102 3,174 0,012 TiO2 102,227 51,34 53,087 54,236 93,211 51,333 55,238 Total 103,013 101,083 101,497 102,01 102,293 102,878 103,239
Number of cations on the basis of
2O 3O 3O 3O 2O 3O 3O Si 0,001 0,049 0,000 0,000 0,041 0,076 0,000 Al 0,000 0,029 0,000 0,001 0,004 0,016 0,001 Ti 0,993 0,946 0,992 1,006 0,924 0,925 1,010 Cr 0,005 0,000 0,000 0,000 0,000 0,000 0,000 Mg 0,001 0,025 0,000 0,002 0,001 0,000 0,000 Fe2+ 0,001 0,855 0,864 0,930 0,044 0,837 0,820 Mn 0,000 0,086 0,111 0,052 0,008 0,062 0,156 Ni 0,000 0,001 0,000 0,000 0,000 0,001 0,000 Na 0,001 0,002 0,001 0,002 0,003 0,001 0,001 K 0,000 0,001 0,000 0,000 0,003 0,001 0,000 Ca 0,001 0,001 0,039 0,000 0,009 0,073 0,000 Cl 0,000 0,000 0,001 0,001 0,000 0,000 0,000
31
chalcopyrite is enriched in chromium. In contrary, the analyses of magnetite present within the mineralisation show a rather pure magnetite composition (sample 87104H).
Table 6. Representative electron microprobe analyses of magnetite.
4.3.3 Sulphides Pyrite
Table 7. Representative results of electron microprobe analyses of pyrites.
Zone South Central
Sample 88209F MG 88219B MG 87104D MG 87104D MG2 87104H MG1 87104H MG2 87119B2 Mg SiO2 0,394 0 0 0,003 0 0 0,19 TiO2 0,117 0,017 0,035 0,001 0,009 0 0,305 Al2O3 0,205 0,028 0,1 0,053 0,035 0,038 0,122 Cr2O3 0 0,141 1,254 0,314 0,007 0 0,556 FeO 90,682 90,61 91,461 89,722 90,962 92,857 91,298 MnO 0 0,016 0,084 0 0,042 0,063 0,068 MgO 0,284 0,01 0,015 0 0,01 0,008 0 CaO 0 0 0 0,003 0 0 0 Na2O 0,01 0,083 0,035 0,073 0,028 0,059 0,017 K2O 0,13 0 0,009 0 0,001 0 0,008 Total 91,822 90,911 93,043 90,171 91,129 93,025 92,564 Si 0,015 0,000 0,000 0,000 0,000 0,000 0,007 Ti 0,003 0,001 0,001 0,000 0,000 0,000 0,009 Cr 0,000 0,004 0,038 0,010 0,000 0,000 0,017 Al 0,009 0,001 0,005 0,002 0,002 0,002 0,006 Fe3+ 1,960 2,000 1,958 1,993 2,000 2,003 1,947 Fe2+ 0,988 0,987 0,990 0,989 0,993 0,989 1,011 Mn 0,000 0,001 0,003 0,000 0,001 0,002 0,002 Ni 0,000 0,000 0,002 0,000 0,001 0,000 0,000 Mg 0,016 0,001 0,001 0,000 0,001 0,000 0,000 Ca 0,000 0,000 0,000 0,000 0,000 0,000 0,000 Na 0,001 0,006 0,003 0,006 0,002 0,004 0,001 K 0,006 0,000 0,000 0,000 0,000 0,000 0,000
Number of cations on the basis of 4 O
Zone South Central East South East
Sample 88209I PY 88219G PY 87104D PY 87119BPY 84004FPy 84004QPy 85115O PY1 88057A PY
Name py py py py py py py py No. 23 32 13 10 7 8 36 52 Fe 46,957 45,982 46,559 44,95 45,505 46,643 45,826 42,609 Mn 0 0,052 0,019 0,027 0,021 0,003 0 0 Au 0,009 0 0 0,014 0,025 0,046 0,024 0 S 52,301 53,693 53,399 52,692 52,721 53,172 53,164 52,288 Cu 0,026 0 0,045 0,036 0 0,033 0,046 0 Ag 0 0,014 0,025 0,018 0,013 0 0 0 Zn 0 0,092 0 0,062 0,09 0,043 0,026 0,013 Ti 0 0 0,019 0,019 0 0 0,014 0 Total 99,293 99,833 100,066 97,818 98,375 99,94 99,1 94,91
32
The samples have been taken prior to, within and following the mineralisation for each zone. For example, 88209I and 88219G samples have been taken within the mineralisation. Microscopically, 88219G pyrite is a unique grain whiles the other pyrite as a pore-filling texture. Cu, Au, Ag and Zn values are very low and below the detection limit (0,1% wt.)
(Tab.6). The hornblende has been formed before pyrite grains which probably come from a
first pyrite generation. The other pyrite grains out of the actinolitic hornblende shows similar composition values as the first generation pyrites (see Annexe 3 analyses 43-44, Fig. 17 A).
Pyrrhotite
Table 8. Representative results of electron microprobe analyses of pyrrhotite.
Pyrrhotite has been found in vein far from the mineralisation with significant amount of chalcopyrite. From thin section observation of sample 88034A, pyrrhotite is replaced by pyrite (South-East paragenesis sequence). Few samples show low values of Cu and Au below the detection limit (0,1% wt.) (Tab.8).
Chalcopyrite
Table 9. Representative results of electron microprobe analyses of chalcopyrite.
Zone South East South East
Sample 88209I PO 84004FPo 85108F PY 85108G PO 88034A PO
Name po po po po po No. 22 6 25 27 48 Fe 59,078 59,451 57,959 60,463 57,93 Mn 0 0 0 0 0,021 Au 0 0 0,063 0,009 0,037 S 39,359 38,526 38,94 39,009 39,063 Cu 0,038 0,047 0,051 0 0 Ag 0,012 0 0 0 0 Zn 0 0 0,006 0 0 Ti 0,006 0,004 0 0 0,036 Total 98,493 98,028 97,019 99,481 97,087
Zone South Central East South-East
Sample 88209I CPY 88218c cpy 87104D CPY GOLD 87104H CPY 84004MCPy 85108G CPY 85115O CPY2 88034A CPY
Name cpy cpy cpy cpy cpy cpy cpy cpy
No. 21 41 15 17 3 28 35 47 Fe 30,167 30,669 29,831 29,691 30,419 30,572 30,436 30,052 Mn 0 0 0 0,012 0,006 0,02 0,008 0 Au 0 0 0 0 0,044 0 0,004 0 S 34,317 34,741 34,214 34,729 35,024 34,783 34,609 34,784 Cu 34,019 33,987 34,119 34,628 34,104 33,726 33,833 33,798 Ag 0 0,013 0,002 0 0 0,013 0 0,016 Zn 0,105 0,127 0,044 0,112 0,159 0,095 0,121 0,109 Ti 0,021 0,008 0,007 0 0,003 0 0,007 0 Total 98,629 99,545 98,217 99,172 99,759 99,209 99,018 98,759
33
Zinc values are below the detection limit (0,1% wt.). The average iron content in chalcopyrite is 30,11wt. % and the average copper content is 34,43wt. %. Few samples show low values of Ag and Au over the detection limit (0,1% wt.) (Tab.9).
Others
Table 10. Representative results of electron microprobe analyses of sphalerites.
Sphalerites have only been observed in the East zone (Fig.19 C). They all are associated with pyrite or/and chalcopyrite. It consists of sub to anheudral 200-500 µm grains. Sphalerites contain low values of Mn and an iron content between 5-6 wt. % and one has a significant Cu value of 1.654 wt.% (Tab.10). All samples have been taken from a volcanosedimentary unit which host the mineralisation and these samples are spatially close to the mineralisation. 4.3.4 Carbonates
Table 11. Representative results of electron microprobe analyses of calcites and Ferro-dolomites.
Zone East
Sample 84004MHem sulphid 84004FHem sul 85108F HEM as sulf 85108G MG SULPH
Name Sph Sph Sph Sph No. 1 4 26 29 Fe 6,629 5,351 5,356 6,668 Mn 0,093 0,031 0 0,03 Au 0 0 0,004 0 S 32,991 33,305 33,131 33,709 Cu 0,023 0,086 0,239 1,654 Ag 0 0,001 0 0 Zn 60,309 62,808 59,336 60,494 Ti 0,019 0 0,034 0 Total 100,064 101,582 98,1 102,555
Zone SOUTH EAST SOUTH EAST CENTRAL
Sample 88209I CARB 88219C CARB 84004Mcarb 85108G CARB 85115O CARB 88057A CARB 87119B2 Carb 87119B1 Carb
Name Fe-Dol Calc Calc Calc Calc Fe-Dol Calc Calc
FeO 7,741 0,209 1,171 1,77 0,971 8,104 0,383 0,864 MnO 0,422 0,309 1,111 1,644 1,206 0,597 0,314 0,364 NiO 0 0 0 0 0 0 0 0 MgO 16,028 0,076 0,505 0,666 0,932 15,213 0,446 0,947 CaO 28,136 52,052 51,224 49,266 49,564 27,844 50,465 51,038 CO2 47,673 47,354 45,988 46,655 47,327 48,242 48,392 46,788 Total 100 100 99,999 100,001 100 100 100 100,001 Fe2+ 0,106 0,003 0,017 0,026 0,014 0,113 0,006 0,013 Mn 0,006 0,005 0,016 0,025 0,018 0,008 0,005 0,005 Ni 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 Mg 0,393 0,002 0,013 0,018 0,025 0,379 0,012 0,025 Ca 0,495 0,990 0,954 0,932 0,943 0,499 0,977 0,957 Na 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 #Mg 0,787 0,393 0,435 0,401 0,631 0,770 0,675 0,661 CaCO3 0,495 0,990 0,954 0,932 0,943 0,499 0,977 0,957 FeCO3 0,106 0,003 0,017 0,026 0,014 0,113 0,006 0,013 MnCO3 0,006 0,005 0,016 0,025 0,018 0,008 0,005 0,005 MgCO3 0,393 0,002 0,013 0,018 0,025 0,379 0,012 0,025
34
The amount of Fe ~8 % in dolomite indicate that it consist of dolomite. The both Ferro-dolomites have been taken from albite felsite part of the mineralisation from the south and south east zone (Tab.11).
Minor amount of 200-300µm subheudral apatite grains have only been observed and analysed in quartz veins from mafic volcanic to volcano-sedimetary unit only at the south east zone (Tab.12).
Table 12. Representative results of electron microprobe analyses of apatites.
Zone South-East
Sample 85115N2 APATITE 88057O AP1 88057O AP2
No. 82 87 91 Na2O 0,037 0 0 FeO 0,079 0,138 0,1 Cr2O3 0,016 0,035 0 Cl 1,271 1,253 1,689 MgO 0 0,015 0,014 MnO 0,095 0,22 0,138 K2O 0,016 0,013 0 Al2O3 0,016 0,06 0 NiO 0,015 0 0,021 CaO 53,327 53,542 54,09 SiO2 0 0 0 TiO2 0 0 0 Total 54,585 54,993 55,671
35 Fig. 20 A: Ilmenite grains from Central sample of gabbroic unit; B: Ilmenites in a biotite-scapolite altered
volcano-sedimentary unit; C: Mineralised sample from volcano-sedimentary unit of East zones; D: Graphitic volcanosediment from South-East zone with albite veins; E: Amphibole replaced by chlorite which has been
carbonate altered from East zone; F: Carbonate-albite hosted South mineralisation.
A
B
C
D
36
Discussion
Alteration zones surrounding the Pahtohavare ore bodies show chemical and mineralogical zonation (Martinsson et al, 1997). The alteration characteristics of the Pahothavare deposit share similar features with other iron oxide and sulphide deposits of the Fennoscandian shield such as e.g. Bidjovagge (Bjørlykke et al, 1987; Bjørlykke et al, 1993) and Rakkurijärvi (Smith et al, 2007).
Similar to Pahtohavare, the alteration associated to the Bidjovagge copper-gold mineralisation includes carbonatisation, scapolitisation, sodic and potassic alteration. The sodic alteration is in both cases the most important one because of its association to mineralisation. The Ernest Henry deposit in Australia shares identical chemical and temporal progression from sodic- to potassic-dominant alteration. This transition may have occurred during the interaction between rock and fluid which evolved via alkali exchange (Mark et al, 2006). The occurrence of sphalerite as anhedral inclusions in chalcopyrite (Figs. 20A and B), subhedral ilmenite grains from mineralised volcano-sedimentary unit surrounding the albite felsite (Fig. 20C), and disseminated rutile in graphitic felsite (Fig. 20D) enhances the similarity between the two deposits (Bjørlykke et al, 1987).
Biotite from Pahtohavare show similar depletion of Al and K as biotite from Rakkurijärvi, a nearby IOCG deposit. These depletions comforts the deduction that chloritisation of biotite is widespread (Smith et al, 2007). Enrichment in Ti has also been noticed from Rakkurijärvi biotite. Few chlorites of the East and Central zone samples from mineralisation within volcanosedimentary and gabbroic units of the Kiruna Greenstone Group show Fe enrichment compared to chlorite from the surrounding alteration. A similar feature has been observed at Rakkurijärvi and Smith et al. (2007) have deduced that the composition of secondary chlorite of the surrounding alteration indicates temperatures from ~300° to 250°C, and chlorites replacing biotite suggest temperatures from ~175° to 100°C. The presence of Fe-rich chlorite support the hypothesis that chlorite have been formed by replacement of biotite and amphibole.
Amphibole showing the richest Cl content (Table 3) is also from the sample which has Fe-rich chlorite and biotite (Table 1-2, Appendix 3). The Cl content of amphibole and biotite is at a similar low range (<0.6 wt %) as the ones from Tjårrojåkka (Edfelt et al, 2005). Another explication of this content could be that the analyses have been done in the Fe-rich rims of actinolite from the Kurravaara conglomerate (Smith et al, 2007). An increase of Cl
content in amphibole requires an increase of Fe2+, K and Al content according to Oberti et al.
(1993) which is relevant with the tendency of the amphiboles from Pahtohavare (Table 3). If
we do not take in consideration the 87104D amphibole due to the zero values, a negative correlation between Mg and Cl contents is distinguishable which probably indicates that Mg-Cl avoidance mechanisms can control the incorporation of halogen in the amphibole structure (Monteiro, et al., 2008). Amphibole comes from samples of least altered units or from the ore
37
in the case of 87119C. It is possible that the generation of amphibole and pyrite are
temporally close and that first generation pyrite rather post-date the amphibole generation. The fact that the amount of amphibole and plagioclase decrease towards the alteration zones makes it possible to conclude that the biotite-scapolite alteration has been formed by the decomposition of plagioclase and amphibole in accordance with Martinsson (1997).
Scapolite from scapolite-biotite alteration surrounding the ore-bearing albite felsites and ore veins have a dominant marialitic composition which indicates that the alteration must have been due to highly saline fluids (Pan, 1998). The analyse of scapolite in sample 85115N1 show a marialitic scapolite closer to the albite field on the ternary classification diagram compared with other scapolite grains from samples outside the albite alteration zone. It supports the previous microscopic observations that albite is formed by replacement of scapolite and biotite. The fact that it has been possible to identify albites in a mafic volcanic unit far below the mineralisation indicates that the albite alteration zone is wide spread but strong albitisation is mainly localised at the mineralisation and around vein network. No clear spatial relationship between albite/plagioclase occurrence and mineralisation has been identified. This might be due to the fact that not enough feldspar have been analysed. The An (=Me) content of scapolite is intermediate (0,29-0,42) which is in accordance with what have been agreed in the past (Frietsch et al, 1997). Dipyre seems to be a more accurate name to chemistry of scapolite. The occurrence of dipyre in Pahtohavare can be explained by the fact that the formation of the deposit happened in a Na-Cl rich environment (Frietsch et al, 1997). Moreover, a reddened and silicified part of the metavolcanic unit has been described in core log (Appendix 1, page 72, sample 88218D). There are similar observations from drill cores of Rakkurijärvi. Apatite also occurs as an accessory mineral in the Rakkurijärvi deposit (Smith et al, 2007).
Fluid evolution
The past tectonic events made the Kiruna area having a favorable permeability for epigenetic solutions like saline hydrothermal fluids. This favourable permeability is one of the main important characteristic which explains the formation of Pahtohavare ores. There are two generations of ore-forming fluids at Pahtohavare. The first is a high temperature supersaline fluid which has also deposited iron oxides and most of the carbonate. The second one is a CO2-bearing saline solution. This solution was very reactive with the volcanic rocks of the Pahtohavare area mainly because these rocks were already albitised (Lindblom et al, 1995).
The main physicochemical parameters that controlled hydrothermal alteration and gold mineralisation at Bidjovagge were pH, ƒO2 and temperature (Ettner et al, 1993; Frietsch et al, 1997). The main stage of co-deposition of chalcopyrite-gold at Pahtohavare is characterized by the second generation of ore-forming fluids, aqueous fluids with a salinity up to 30 eq. wt.% NaCl, a temperature below 350°C and a pressure of 1-2 kbar (Lindblom et al, 1995). The high salinity of the fluid can be explained by the metasomatic hydration of biotite
