Characterization of the Hydrothermal Alteration around the Björkdal Au Deposit,
Skellefte District, Sweden
Madeleine Erneholm
Natural Resources Engineering, masters 2017
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
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Table of Contents
1. Abstract ... 3
Key Words ... 3
2. Introduction ... 4
3. Geological Setting ... 4
3.1. Regional Geology ... 4
3.2. Gold Deposits in the Skellefte District and Gold Line ... 7
3.3. Local Geology ... 8
3.3.1. The Björkdal Gold Deposit ... 8
3.3.2. Structural Setting ... 9
4. Method ... 10
4.1. Sampling Procedure and Logging ... 10
4.2. Lithogeochemistry ... 11
4.3. Petrography ... 11
5. Results ... 12
5.1. Mineralogy... 12
5.1.1. DDP2015─012 ... 12
5.1.2. DDT2015─008 ... 18
5.1.3. DDU2015─007 ... 23
5.1.4. DDP2015─046 ... 28
5.1.5. DDE2015─007 ... 32
5.1.6. DDE2015─010 ... 36
5.1.7. DDER2014─003 ... 40
5.1.8. DDE2015─001 ... 44
5.2. Geochemistry ... 48
6. Summary of the Mineralogy and Geochemistry ... 53
6.1. Petrology and Geochemistry ... 53
6.1.1. Mineralogy ... 53
6.1.2. Geochemistry ... 54
6.1.3. Alteration Zonation ... 56
6.2. Outlier Samples ... 59
6.2.1. SO-000056 from DDE2015─007: Green Banded Unit ... 59
6.2.2. SO-000068 from DDU20115─007: Sheared Unit ... 60
6.2.3. SO-000063 from DDER2014─003: Amphibolite unit ... 60
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6.2.4. DDE2015─001: Mafic Unit ... 60
7. Discussion ... 62
7.1. Interpretation of Mineral Paragenesis and Hydrothermal Alteration ... 62
7.2. Origin of the Björkdal Gold Mineralization ... 63
8. Conclusions ... 68
8.1. Lithologies and Alteration Features ... 68
8.2. Alteration Zonation ... 68
8.3. Paragenesis ... 69
9. Acknowledgements ... 69
10. References ... 70
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1. Abstract
Sixteen samples from 8 locations within and surrounding the Björkdal mine area in northern Sweden were chosen in order to reevaluate and characterize previous interpretations of the lithology and hydrothermal alteration. Geochemical analysis by ICP-MS was made in order to chemically classify the lithology of the area and petrological studies were made by study of thin sections and core logging.
Three different sets of major alteration types with similar protolith were noted where two dominated:
a) a silicified, sericitic, deformed unit; b) a felspathic altered unit with various intensity of epidote; c) amphibole and a so called green banded unit that has undergone Ca-Mg-metasomatism. The main host rock lithology could be identified as a coarse grained, equigranular, plagioclase and (Na-K)- feldspar dominated rock with slightly elongated, stubby grains that have an interlocked, igneous texture. Apatite is a reoccurring accessory mineral in all samples and remains relatively undamaged.
Fragmental quartz occur in the samples and is primary to later forming alteration minerals. Amphibole is also considered to be primary, but can also been formed during regional metamorphism. Two lithologies could be identified in addition to the main protolith. Two samples were located above the marble horizon and was classified as basaltic unit with Ca-plagioclase, biotite and amphibole. One sample mainly consisted of aligned amphibole and biotite and is considered to be an amphibolite xenolith or a dyke intruding the main protolith rock.
An increase in the alteration minerals albite, amphibole, epidote and allanite, with peak intensity found north-east of the mine site. This is confirmed by the geochemistry of major elements Fe
2O
3, MgO, CaO and the ratio between Na
2O and K
2O. The zonation is explained by a suggested increase in temperature condition during metamorphosis. This could be explained by either an underlying intrusion or by structurally controlled variation in metamorphism by e.g. major faults running through the area.
Comparisons with previous work in Björkdal favors an intrusion-related origin for the gold deposit.
Key Words
Gold Ore, Mine, Skellefte District, Sweden, Quartz Lode, Hydrothermal Alteration, Petrography,
Lithogeochemistry, Intrusion-related Gold.
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2. Introduction
The Björkdal gold deposit is located ca. 20 km northwest of the town Skellefteå in the Paleoproterozoic Skellefte District in Northern Sweden. The lode gold mineralization was first discovered by Terra Mining AB between 1983 and 1985 and mining started in 1988. The deposit has been a subject to several ownership changes throughout the years and is currently owned by Mandalay Resources. The geology of the deposit has previously been described as steeply dipping auriferous quartz veins occurring in a dome-shaped felsic intrusion overlaid by a marble unit with surrounding volcanics and sediments.
Recent, renewed viewings on the geology made in 2014 and 2015 by Mandalay Resources has resulted in a renewed interest of reclassify previous interpretations made of the petrology in the Björkdal area.
The purpose of the study is to interpret and characterize the lithology and investigate a possible alteration zonation within and surrounding the mine site by the use of geochemistry and observation of cores and thin sections. By improving the understanding of the Björkdal geology, there is a higher chance of identifying possible exploration targets in extension of the mining area as well as identify high grade areas within the mine site. Eight cores with two rock samples (a total of 16) from different locations that has been drilled at the mine site in the past is chosen to represent the area.
3. Geological Setting
3.1. Regional Geology
The Skellefte District is a 1.9 Ga geological district in northern Sweden. The district hosts numerous ore deposits of different types and has been explored since the early 18
thcentury. The mining in the area has been active since the discovery of the world-class Boliden massive sulfide deposit and today there are seven mines active in the district, making it one of the most important mining districts in Europe (Weihed et al., 1992; Weihed and Billström 1996). The orogeny of the Skellefte District has been thoroughly discussed in the past. Today, the district is considered to have been formed on a mature arc or a continental crust in an extensional intra-arc region covering a subduction zone which was dipping to the north (Allen et al., 1996; Carranza and Sadeghi, 2014; Weihed et al., 2005). There are also hypotheses considering the geological district to have been formed in a rift zone of a >1.9 Ga basement, where the original rocks of the basement where emplaced in a back-arc marginal basin (Allen et al., 1996; Carranza and Sadeghi, 2014). The Björkdal gold deposit is located in the eastern part of the district and its location is indicated in Figure 1.
The Skellefte District mainly consist of sediments and bimodal marine and subaerial metavolcanics, all which have been exposed to regional metamorphism of greenschist facies in the center of the district to lower amphibolite facies in the south, east, and west (Allen et al., 1996). The highest metamorphic facies can be observed in the south-western parts of the district. Peak metamorphism (including partial melting) in the southern and western parts of the Skellefte District is associated with a tectonic deformation event, aged 1.89─1.87 Ga (Bauer et al., 2014), and the area has been a subject to sporadic peak metamorphic conditions and ductile deformation from 1.87 Ga to 1.80 Ga (Weihed et al., 1992;
Billström and Weihed, 1996; Rutland et al., 2001; Weihed et al., 2002; Bark and Weihed, 2012).
The district is regarded as a transitional zone between the Bothnian Basin (Bothnian Supergroup) in
the south and volcanic arc assemblages (Skellefte, Vargfors and Arvidsjaur Groups) in the north. The
Bothnian Supergroup (2.0─1.86 Ga) comprise marine epiclastic sediments and have been proposed to
form the basement for the Skellefte Group volcanic rocks (Kathol and Weihed, 2005).
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Fig. 1: Geology of the Skellefte District (modified from Weihed, 2001). The Björkdal deposit is indicated in red.
The Skellefte Group volcanic and sedimentary rocks are a 1.89─1.88 Ga old and can be further subdivided into three formations (Weihed et al., 1992). The lowest stratigraphical formation is mainly a felsic, homogenous volcanic formation with pyroclastic rocks and lavas and minor mafic volcanics interbedded. This formation is overlain by a second volcanic formation that includes both mafic and felsic rocks, volcanic sediments, porphyry domes, lavas and tuffites (Weihed et al., 1992). The third, and youngest, formation is mainly consisting of greywackes with a grading of grain size going from fine- grained to coarse-grained conglomerates (Weihed et al., 1992).
The base of the Skellefte Group is not known, but the unit has a thickness measured of three kilometers
in the northern parts of the district (Allen, Weihed and Svenson, 1996). The Skellefte Group is overlain
by the Vargfors Group (1.88─1.87 Ga), which mainly comprise epiclastic sediments deposited in rivers
or near-shores and also locally interbedded volcanic rocks of basalt-andesite composition (Weihed et
al., 1992; Weihed and Billström, 1996; Allen et al., 1996).
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At least three generations of granitoid suites intrude the Skellefte District. The earliest suite (calc- alkaline I-type Jörn granitoids) are
supposedly coeval with the volcanic rocks (1.89─1.87 Ga), followed by the Gallejaur monzonite-gabbro intrusions, aged 1.88─1.86 Ga (Weihed et al., 1992; Weihed et al., 2003). The late-orogenic S-type Skellefte- Härnö granitic suite is dated at 1.82─1.80 Ga. The post-orogenic A- to I-type (1.80─1.78 Ga) Revsund and Sorsele granites are considered to be the latest generation of intrusions occurring in the Skellefte District (Weihed et al., 1992; Claesson and Lundqvist 1995; Billström and Weihed 1996;
Weihed et al., 2003). The complete evolution of the stratigraphy in the Skellefte District is illustrated in Figure 2.
The structural geology in the Skellefte District is characterized by several isoclinal and open folds which are cut by brittle structures and larger shear zones (Allen et al., 1996; Bergman-Weihed, 2001).
Faulting in the area follows a
pattern of faults that strike from WNW-ESE and NNE-SSW (Bergman-Weihed, 2001; Bauer, 2014) and the primary structures; folds, shear zones, cleavage and lineation, trends parallel to the belt (major fault patterns is seen in Figure 1). The faults with a northerly trend are steep, dip-slip faults which are cross-cut by the abundant WNW-ESE faults (Allen et al., 1996; Bergman-Weihed, 2001). The folds are upright and the axis and lineation plunge moderately to the west in the western parts of the Skellefte District and more steeply to the NE and SE in the eastern parts of the district (Allen et al., 1996, Bergman-Weihed, 2001).
In the central parts of the district, the folds plunge more shallowly along a strike from the NW to the SE. These folds shift to northern striking faults in the north. An S
1foliation occurs all over the Skellefte District, often parallel to the bedding, and with various expressions in different lithologies (Allen et al., 1996; Bergman-Weihed, 2001).
Fig. 2: Stratigraphic evolution of the Skellefte District (Weihed et al., 1992).
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3.2. Gold Deposits in the Skellefte District and Gold Line
The Skellefte District is known for gold-rich massive sulfide deposits. Several known lode gold deposits occur in the Skellefte District. Deposits includes the Åkerberg and Björkdal deposits. Gold is also found in the massive sulfide ores that occurs in the Skellefte District, although these share a different fluid source. Whereas gold in the massive sulfide ores are precipitated simultaneously as the base metals or during a later event, the gold in the lode gold deposits in the Skellefte District occur in epigenetic, auriferous quartz veins and are often associated with intrusive rocks (Billström et al., 2012). The Svartliden and Fäboliden deposits are a part of the so called Gold Line located in the Lycksele-Storuman area, where the Fäboliden is the largest with respect to tonnage. The four deposits mentioned above (Åkerberg, Björkdal, Svartliden and Fäboliden) have gold as the only economic commodity and share, other than being hosted in quartz veins, the characteristics of being a low sulfide, low base metal, and structurally controlled gold deposit (Bark and Weihed, 2012). The Fäboliden deposit is hosted in arsenopyrite-rich, auriferous quartz veins that occurs in metagreywackes. The mineralization is mainly hosted by the metagreywackes, but also occurs in interbedded metavolcanics. The mineralized unit is intruded by (1.81─1.77 Ga) Revsund granitoids (Bark and Weihed, 2012). The timing of mineralization is determined to be 1.8 Ga and the deposit is interpreted to be an orogenic gold deposit (Bark and Weihed, 2012).
The Åkerberg deposit is located a few km NNE of Björkdal, and the location of the deposit can be seen in Figure 1. The Åkerberg deposit has been classified and compared to the Björkdal deposit by Billström et al. (2012) as an intrusion-related gold deposit in a back-arc basin. The gold at Åkerbers occur in auriferous sheeted, thin, parallel quartz vein which is hosted by a gabbro which is intruded by a granodiorite , and dated at around 1.88 Ga (Billström et al., 2012). However, several characteristics (structural settings, mineral association, and limited hydrothermal alteration) at Åkerberg also fit the orogenic gold model. The mineralization is hosted by a gabbro, within thin, but densely disposed auriferous quartz veins. A granodiorite pluton (1.88─1.87 Ga) of the Jörn suite and pegmatite bodies intrudes the gabbro (Billström et al., 2012). Mafic dykes cross-cut both the granodiorite and the gabbro. Scheelite is found in both quartz and feldspar veins as well as in the gabbro, and are often gold-associated. There is a lack of sulfides as well as significant alteration, although no detailed studies have been performed on deformation processes or metamorphism in the area (Billström et al., 2012).
The gabbro is silicified and feldspar are usually locally altered to albite, preferably in association with
the quartz veins. Pyrite, minor arsenopyrite and pyrrhotite occur both disseminated in the host rock
and occasionally as thin sulfide veins (Billström et al., 2012). The orogin of the deposit is suggested by
Billström et al. (2012) to be of an intrusion-related gold deposit origin.
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3.3. Local Geology
3.3.1. The Björkdal Gold Deposit
The Björkdal gold deposit is located ca. 20 km northwest of the town Skellefteå in northern Sweden.
The geology of the Björkdal deposit has previously been described as a dome-shaped, felsic, medium- grained, quartz-monzonite to tonalite intrusion of the Jörn suite (Weihed et al., 1992). The margins of the intrusion is covered by a marble unit and surrounded by mud-, sand- and siltstone and felsic volcanic rocks (Fig. 3).
Fig. 3: Modified map of the geology of the Björkdal area. The mine is indicated in the north-western part of the intrusion (SGU, 2016).
The intrusion at Björkdal includes the main minerals plagioclase, quartz, biotite, amphibole and accessory minerals calcite, epidote, titanite, apatite and zircon (Åberg and Weihed, 1999). The sedimentary rocks are located in the north, south and east and have a bedding that dips outward from the intrusion (Weihed et al., 2003). The volcanic rocks are in contact with the intrusion in the north and west.
Recent interpretation of the structures and geology made by the geologists at Björkdalsgruvan and by
MacCormack (2015), suggests that the intrusion might be seated further down. What was previously
interpreted as the 1.9 Ga intrusion might rather be volcanics of the Skellefte Group than a pre-orogenic
Jörn-granitoid. Based on previous geochemical samples made, the gold rather seems to be hosted in a
basalt-andesitic volcanic rocks and mafic-intermediate intrusive rocks (Gniel, 2016). A shallow dipping
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foliation structure that strikes to the NNW overprints all rocks and is assumed to be the result from the main deformation event (MacCormack, 2015).
The Björkdal deposit is located at the north-western parts of the intrusion and the gold is hosted in (0.1─1 m wide) auriferous semi-vertical quartz-veins associated with tourmaline, scheelite and tsumoite (Weihed et al., 2003; Roberts et al., 2006). Scheelite crystals in the quartz veins are often cross-cut by thin quartz-calcite-sulfide-gold-veinlets (Roberts et al., 2006). Pyrite, chalcopyrite, pyrrhotite and galena occur as accessory minerals associated with the quartz veins. Weak alteration halos occur surrounding several quartz veins with the alteration minerals chlorite, sericite, biotite and sulfide minerals (Broman et al., 1994). The primary quartz and scheelite crystallized from a CO
2-rich, gold-bearing fluid at 375°C (Broman et al., 1994). This fluid reacted with the host-rock and tellurides and sulfide crystallized from an alkaline, reducing CH
4--rich fluid. Fluid inclusions in microfractures suggests a later metamorphic brine at 145°C-220°C which caused gold to remobilize (Broman et al., 1994)
3.3.2. Structural Setting
The Björkdal intrusion has been dated by U-Pb zircon analysis to be 1.90 Ga (Billström et al., 2009), which pre-dates the earlier mentioned Jörn suite intrusions and would be formed at the same times as the supposedly surrounding Skellefte Group volcanics (aged 1.89-1.88 Ga). Dating of titanite in shear zones provided ages of 1.80 Ga, placing peak temperature and deformation at this stage (Weihed et al., 2003). Whether the pluton intruded into surrounding rocks or the supracrustal rocks have been deposited on top of a eroded intrusion is unknown, since the age of surrounding volcanics and sediments are unknown (Weihed et al., 2003).
The Björkdal pluton is surrounded by (at least two) large faults striking north-northeast on the east and west side of the pluton. Weihed et al. (2003) suggests that these faults forms large crustal shear zones, which caused the intrusion with its surrounding rocks to be uplifted and emplaced amongst stratigraphically younger rocks. The rocks at Björkdal share a gently sloping, north-dipping S
1-foliation, suggested to be associated with peak metamorphism (MacCormack, 2015). High-strained and ductile shear zones overprints the S
1-foliation and share a weak to moderately dip angle towards N and NE.
The steeply dipping gold-bearing veins are interpreted to be tension veins, locally associated with
steeply north-dipping brittle-ductile faults (MacCormack, 2015).
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4. Method
4.1. Sampling Procedure and Logging
Eight drill cores were chosen from different locations at the mine site and its surrounding areas (Fig.
4). From these, a total of sixteen rock samples were chosen to undergo whole-rock lithogeochemistry analysis and petrography. The locations are visible in Figure 4. From the mine site, two drill cores were chosen from the underground mine (DDU2015─007 and DDT2015─008), and one was picked from the open-pit (DDP2015─012). The remaining cores (DDE2015─001, DDP2015─046, DDE2015─007, DDE2015─010 and DDER2014─003) were collected from previous exploration programs and are found in the vicinity of the mine site. The cores were selected based on the attribute of how well they represent each geographical area.
Fig. 4: Location of drill cores in the Björkdal mine and surroundings (Mandalay Resources, 2015). The red lined area illustrates the open-pit operation and the grey section is the underground mine. The map is modified to mine grid and each square represents 500x500 m.
The drill cores were visually logged, focusing mainly on host rock lithology and alteration features with the purpose to identify changes in alteration style outside of the mine site. Important attributes such as grain size, sulfide content, mineral composition, structures and alteration features were recorded.
Alteration features associated with quartz veins, such as fractures, shear zones etc. were noted but
will not be addressed in detail in this study. Two rock samples from each location were chosen to
represent the main lithology and alteration of the area. All samples are presented in Table 1.
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Table 1: Presented below are the samples for every location (a total of 16 samples), with the sample location and length of core pointed out.
Hole ID From (m) To (m) Length (m) Rock Sample
DDT2015─008 36.35 36.95 0.6 SO-000054
DDT2015─008 91.73 92.17 0.44 SO-000055
DDE2015─007 116.84 117.4 0.56 SO-000056
DDE2015─007 59.52 59.95 0.43 SO-000057
DDP2015─046 42.3 43 0.7 SO-000058
DDP2015─046 35.61 35.97 0.36 SO-000059
DDP2015─012 104.1 104.62 0.52 SO-000060
DDP2015─012 145.7 146.2 0.5 SO-000061
DDER2014─003 144.17 144.7 0.53 SO-000062
DDER2014─003 128.6 129.3 0.7 SO-000063
DDE2015─010 172.72 173.34 0.62 SO-000064
DDE2015─010 34.67 35.15 0.48 SO-000065
DDE2015─001 64.5 64.85 0.35 SO-000066
DDE2015─001 18.85 19.45 0.6 SO-000067
DDU2015─007 6.6 7.1 0.5 SO-000068
DDU2015─007 54.69 55.22 0.53 SO-000069
To present the documented features in a simple and illustrative way, the information was recorded as hand-made stratigraphic sequences. These were later processed with the vector graphics software Adobe Illustrator CC to create the stratigraphic logs presented in this paper.
4.2. Lithogeochemistry
The samples chosen for the whole-rock geochemistry analysis were sent to CRS Minlab Oy in Finland.
A whole rock analysis and trace element analysis was performed by ICP-OES and ICP-MS (CRS, 2016).
The geochemistry sample names are corresponding with the SO-0000xx number given in Table 1. The handling and recalculation of data was done using Microsoft Excel 2013 and the GCD 4.1 toolkit (Janoušek et al. 2006).
4.3. Petrography
Rock samples were send to a lab in the Czech Republic for preparation of thin sections. The thin
sections are named after their sample ID in Table 1 and share name with its corresponding
geochemistry sample. The thin sections were visually investigated in detail with a polarizing
petrographic microscope at the Luleå University of Technology. By using the optical properties of
minerals (color, relief, shape etc.) the mineral abundancy (a subjective evaluation of the amount of
minerals), rock types, alteration features and the paragenesis may be identified.
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5. Results
In section 5.1. Mineralogy, the results based on the visual logging will be presented with a stratigraphic description of petrography, structures and alteration features. The stratigraphic log presents the alteration features both as intensity of each separate mineral and intensity of the combined alteration intensity. Following the stratigraphic log is a mineralogical description based on the thin section microscopy. The order of which the samples are presented is based both on the location (starting at the mine site and moving outwards to the east) and lithology resemblance (where DDE2015─001 will be presented last due to its geological and geochemical dissimilarities to the other cores). The geochemistry of all samples will be addressed in chapter 5.2.
5.1. Mineralogy 5.1.1. DDP2015─012
The DDP2015─012 core was drilled in 2015 in the eastern pit at the mine site (Mine Grid: X: 370.621, Y: 1929.821, Z: -93.385). The location can be seen in Figure 4 in chapter 4.1. The stratigraphy and variations in alteration features is presented in Figure 5. The core was logged as an intermediate, dark grey, fine- to medium-grained, weakly foliated, volcanoclastic rock. The clasts are 2─4 cm in size and consist of finer grained black minerals, which was considered to be dominantly biotite.
Fig. 5: The stratigraphy of DDP2015─012. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description of the alteration minerals occurring in the drill core is shown on the right side of the columns, where bio=biotite, alb=albite, ser=sericite/muscovite, chl=chlorite, kfs=k-feldspar, car=carbonate/calcite and epi=epidote.
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Short sections of very fine grained, foliated, biotite-rich units interrupted the otherwise homogenous rock type, which can be seen at 38, 50, 95 and 140 m (Fig. 5). Biotite alteration and albitization reoccurred throughout the stratigraphy and a weak potassic alteration and epidote substitutes the albitization at the end of the hole. Carbonates usually occurred as thin veinlets or patches, but also as thin alteration envelopes in association to quartz veins. The occasional minor increase of grain size throughout the core was considered to be the result of albitization and seems generally restricted to surround fractures. A strained shear zone cuts the core at 163.08 m and contains a large irregular boudinaged quartz vein along with intensified carbonate-biotite-chlorite alteration. Remaining quartz veins occur sporadically throughout the stratigraphy and share no significant alteration halos associated with them other than the minor carbonate occurrences. Occasional fragments of blue quartz follows beneath the shear zone. Minor pyrite and traces of chalcopyrite and pyrrhotite occurs throughout the drill core and their presence is occasionally enhanced in association with quartz veins and the shear zone.
The first sample (SO-000060) was collected at 104.1 m, and represented the finer grained, least albitized type (Fig. 6A). SO-000060 is silicified with a weak foliation present. The second sample (SO- 000061) was collected at 145.7 m. This rock had a slight increase in grain size compared to SO-000060 and distinct grain boundaries (Fig. 6B).
5.1.1.1. Petrographic Description SO-000060
Most of the original plagioclase grains have been destructed and altered into sericite. The sericite occurs as elongated needles with two preferred alignments (Fig. 7) and occasionally grows into larger flakes of muscovite. Along with quartz, sericite constitutes the most significant alteration minerals of this sample. The quartz is present both as a cryptocrystalline mass, as well as larger, more angular grains with minor grain boundary recrystallization (Fig. 7). The muscovite is occasionally overprinting the larger quartz grains (Fig. 7). Biotite grains occur both as tabular and irregular subhedral grains, in semi-aligned segregated assemblages, aligning with one of the preferred alignments of the sericite.
They share low interference colors and is overprinted by quartz. Minor allanite (Ce-epidote) and zircon are associated with these biotite assemblages and form irregular brown radioactive halos in the surrounding biotite (Fig. 9A-B and Fig. 12A-B). A few well developed diamond-shaped grains of titanite can be found in the main matrix. Traces of euhedral and subhedral grains of pyrite are found disseminated in the main grain matrix. Calcite is primarily occurring restricted within a thin fracture crossing the thin section, overprinting biotite, muscovite and quartz. The calcite is also present disseminated, overprinting biotite, early quartz and resulting in one of the latest minerals to be formed
Fig. 6: A) Sample SO-000060, B) Sample SO-000061
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(Figure 7 and Figure 8A and 8B). Traces of microcline alteration can also be seen with a typical tartan twinning replacing primary plagioclase grains (Fig. 8A and 8B).
Fig. 8: A) Hydrothermal biotite (Bt( occur as brown-green tabular and irregular assemblages to the right. Within, there is a messy allanite (Al) grain with high relief. The microcline to the left is colorless and will not be noticed under the plane polarized light. A simple twinning can be spotted in the calcite (Cal) grain above the microcline (K). TS (Plane Polarized Light) 10x. 8B) In this picture the microcline alteration of a plagioclase grain is apparent with an uneven distribution of tartan twinning. Calcite can be seen above and within the microcline grain. Thin Section (Crossed Polarized Light) 10x.
Fig. 7: Sericite/muscovite (Ms) is overprinting most of a plagioclase grains in the bottom of the photograph. These elongated grains have two preferred alignments, which are pointed out in the picture. The quartz (Qz) assemblage in the middle is overprinted by I) muscovite, which crosses over the boundaries of the lowest parts of the quarts grain, and II) calcite at the top of the picture. The calcite veinlet (Cal) cuts through the muscovite fabric in the bottom of the picture. Thin Section (Crossed Polarized Light) 10x.
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A primary elongated larger (1─2 mm) grain of Na-K-feldspar with Carlsbad twinning can be found in the sample (a similar one displayed in the descriptions of SO-000061, in Figure 11A and 11B), less altered than its surrounding matrix, but still overprinted by both sericite and calcite. Stubby hexagonal prisms of apatite occur as an accessory mineral, with minor inclusions and widely spaced cross fractures (Fig. 10A and 10B). These grains are not overprinted and have a relatively clean look compared to its surroundings.
SO-000061
SO-000061 displays some similarities with its twin sample SO-000060, however with less deformation.
The sericite alteration is less pervasive, which leads to a simplified interpretation of the rock, as the primary structure is more detectable. The minerals alignment also seem to occur less defined. The
Fig. 10: A) Two apatite (Ap) grains occur in the middle of the picture within weakly muscovite altered plagioclase grain. The apatite grains differ from the surrounding colorless minerals by the high relief. TS (PPL) 10x. B) Displays the same grain as 10A under cross-polarized light. The apatite occurs as hexagonal prisms throughout the entire sample and shares a widened cross fracture (indicated by red arrows) that can also be noticed in the picture 10A on the left. Thin Section (Crossed Polarized Light) 10x.
Fig. 9: A) A “messy” assemblage of allanite (Al) grains with high relief. A vague zonation and fractures can be spotted within the colorless zircon (Zr) in the middle of the picture. TS (Plane Polarized Light) 20x. B) Same mineral displayed under cross-polarized light. Thin Section (Crossed Polarized Light) 20x.
16
plagioclase grains are considerably larger than the surrounding minerals and share a typical multiple, parallel twinning and occur as interlocked minerals, suggesting an igneous origin of the rocks. A minor k-feldspar alteration can also be seen in this sample, similar to the one in SO-000060, with the tartan twinning of microcline overprinting plagioclase grains. A minor plagioclase albitization with parallel twinning is also present. The cryptocrystalline quartz is less abundant, but larger, angular fragments of the mineral that displays minor grain boundary migration, as well as rounded secondary quartz grains with undefined recrystallized grain boundaries occur. The biotite shows no preferred alignment or segregation and can be seen in clusters, suggesting that the segregation visible in SO-000060 is the result of deformation. The biotite has a dark, discolored and shady color, and is overprinted by the quartz. The mineral occur both as euhedral grains as well as subhedral, bended, irregular grains. Na-K- feldspar occur as larger (1─2 mm) euhedral angular grains with the typical Carlsbad twinning visible, similar to the one seen in SO-000060 and can be seen in Figure 11A and 11B. Traces of zircons are found with a characteristic zoning and a radioactive halo stains and fractures the surrounding biotite, caused from the interaction between the radioactive elements in the zircon and the biotite (Fig. 13A and 13B). The other radioactive mineral allanite is also abundant in the thin section, occurring as messy assemblages with a compact core, causing the biotite to obtain a decay discoloring (Fig. 12A and 12B).
A minor amount of calcite occur disseminated in the sample, with a variation in grain size and shapes, overprinting plagioclase and biotite. Small, rounded epidote crystals constitute a minor component of the rock sample composition, and only occur adjacent of plagioclase grain boundaries. Apatite grains are also present in this rock sample, similar to the crystals presented in Figure 10A and 10B.
Fig. 11: A) The elongated feldspar (Na-K) crystal can be distinguished from the surrounding alteration minerals. TS (PPL) 5x. B) The distinct Carlsbad twinning reveals its identity. Sericite and minor epidote alteration (indicated by red arrows) can be spotted at the grain boundaries. Thin Section (Crossed Polarized Light) 5x.
17
Fig. 12: A) This picture shows a biotite (Bt) assemblage of irregular, bended brown-green biotite. The brown high relief mineral in the middle is allanite (Al). TS (PPL) 10x. B) Same picture under cross-polarized light. Thin Section (Crossed Polarized Light) 10x.
Fig. 13: A) A colorless zircon mineral (indicated by arrow) occur as an inclusion within a biotite (Bt) grain. Cracks and discoloring of the biotite are the result of interacting between the biotite and the radioactive elements released from the zircon. Two elongated allanite grains is seen in the bottom left corner. TS (PPL) 20x. B) Same picture under cross-polarized light. The weak zonation is seen in the zircon. Thin Section (Crossed Polarized Light) 20x.
18 5.1.2. DDT2015─008
DDT2015─008 was drilled in 2015 at the mine site, in the south-eastern parts of the underground mine (Mine Grid: X: 1961.974, Y: 997.17, Z: -327.1458). The stratigraphy and variations in alteration features is presented in Figure 14 and the location can be seen in Figure 4. At the beginning of the hole, the rock consists of an intermediate volcanoclastic rock that is carbonatized, silicified and foliated with weak to moderate sericite alteration. The clasts are 2─4 cm in size and consist of finer grained black minerals, which was considered to be dominantly biotite and can only be seen the first 10 meters.
Downwards in the stratigraphy the grain size is coarser, interpreted to be the result of increased albite content and lower strain due to the foliation becoming weaker. This coarser-grained unit was previously logged as a granodiorite. A “green-banding” alteration is visible at around 50 m, a carbonized unit with alteration bands of amphibole, sericite and chlorite. Amphibole veinlets are found in association to the green bands, and have a characteristic bleached alteration envelopes, consisting of silica and albite. The decrease of grain size at 25 m is caused by a brecciated section that has been intensively silicified and albitized.
Fig. 14: The stratigraphy of DDT2015─008. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description these is shown on the right side where bio=biotite, alb=albite, ser=sericite/muscovite, chl=chlorite, car=carbonate/calcite, epi=epidote and amph=amphibole.
Pyrite is found as small disseminated grains throughout the stratigraphy. Although, an increasing
amount of pyrite along with other sulfides, for instance pyrrhotite and chalcopyrite, can be found in
association with quartz veins and the previously mentioned amphibole veinlets. Small fragments of
blue quartz is noted at the end of hole. The first sample (SO-000054) is collected from the finer grained
part of the stratigraphy (at 36.35 m). This sample has a fine grained, foliated structure with sericite
and biotite visible (Fig. 16A). The second sample (SO-000055) is picked from the coarser grained,
deeper located, albitized parts of the core (at 91.75 m), where the color is slightly darker and no
foliation is present (Fig. 16B).
19 5.1.2.1. Petrographic Description
SO-000054
SO-000054 shares a lot of similarities with SO-000060. The rock constitutes a fine grained intensely sericite altered matrix with quartz and calcite replacing aligned biotite assemblages (Fig. 16). The sericite occur as tabular needles and share two preferred alignments, although one seems more dominant and matches the alignment of segregated biotite assemblages. The level of deformation and sericitization is higher than both previous samples described in DDP2015─012 which can also be noted in the grabsample in Figure 15A. The higher strain and intensive destruction of primary plagioclase grains by sericite, causes the initial rock composition and textures to be undistinguishable.
Cryptocrystalline recrystallized quartz constitutes a big part of the rock composition, but angular grains with only minor bulging recrystallization are also common (Fig. 16). The larger quartz grains are in places overprinted by sericite and calcite. The biotite occur mostly as subhedral irregular grains with cloudy brown color with one cleavage occasionally visible. The calcite occur both cryptocrystalline and as larger grains in the vicinity of biotite (Fig. 17). Euhedral epidote grains are concentrated within the biotite assemblages, but can be found scattered randomly in the thin section (Fig. 16). Allanite is commonly altering the euhedral epidote grains (Fig. 16 and Fig. 18), giving them a soiled look in the center of the grain or entirely replacing the primary grain. A weak microcline alteration similar to the one in SO-000060 is occurring in the sample. Minor apatite and traces of zircons occur as accessory minerals. The paragenesis is similar as in the previous samples with plagioclase is intensively broken down by sericite, biotite is deformed and overprinted by quartz and calcite (Fig. 17A and 17B), and in places there is a weak albite or microcline alteration present. The quartz is in its turn occasionally overgrown by sericite and calcite.
Fig. 15: A) Sample S0-000054. B) Sample SO-000055.
20
Fig. 16: The picture shows an alteration of muscovite/sericite (Ms) of plagioclase in cross-polarized light. The tabular minerals shows two preferred alignments, the first following the biotite (Bt)-calcite (Cal)-quartz assemblage alignment (SW-NE) and is the more dominant of the two. The second alignment has the direction N-S. The preferred alignments are shown in the bottom left corner. Larger grains of quartz (Qz) can be seen in top of the photograph. Thin Section (Crossed Polarized Light) 5x.
Fig. 17: A) This picture displays calcite (Cal) and quartz (Qz) overprinting the brown biotite (Bt) assemblage. TS (Plane Polarized Light) 10x. B) Same picture in crossed-polarized light. Thin Section (Crossed Polarized Light) 10x.
21
Fig. 18: A) In the middle of the picture, there is a brown allanite (Al) core with a colorless epidote rim surrounding (indicated by red arrow). The rim is slightly cracked due to radiation from the allanite. TS (Plane Polarized Light) 10x. B) Same picture in crossed-polarized light. The epidote has distinctive high colors compared to the allanite. Thin Section (Crossed Polarized Light) 10x.
SO-000055
SO-000055 shares similarities with SO-000061. The matrix composition consists of large, about 1 mm,
stubby, interlocked plagioclase grains with occasional well developed polysynthetic twinning (Fig. 19)
and abundant assemblages of biotite and amphibole (Fig. 19 and 20). There are no preferred
alignments visible in this sample. The amphibole is primary to the biotite and overprinted by quartz
and calcite (Fig. 20). The plagioclase is overprinted by the calcite and quartz and hosts finer grains of
minor epidote and sericite. The epidote can occasionally also be found related to the biotite
assemblages with similar allanite alteration as earlier mentioned in SO-000054 (Fig. 18). The allanite
also occurs as the irregular, messy assemblages visible in Figure 12A and 12B in SO-000061. The quartz
occurs as larger angular grains interlocked with the plagioclase (Fig. 19), with occasional bulging
recrystallization. A minor amount of microcrystalline quartz is found in the overprinted biotite-
amphibole assemblages. A weak tartan twinning and cloudy multiple parallel twinning can be spotted
in a few of the primary plagioclase grains, caused by microcline and albite alteration. A few grains of
disseminated subhedral pyrite occurs disseminated in the sample. Apatite and zircons occur as
accessory minerals, where the zircons displays a radioactive halo in the surrounding biotite, similar to
the one seen in Figure 9A and 9B from SO-000060. Calcite is mainly concentrated to biotite-amphibole
assemblages but can occasionally be seen overprinting the plagioclase.
22 c
Fig. 19: The typical texture of sample SO-000055 is stubby, 1 mm, interlocked plagioclase (Pl) with polysynthetic twinning and quartz grains (Qz) with amphibole (Amph) and biotite (Bt) assemblages. The amphibole is completely replaced with biotite and later overprinted by calcite (Cal). Thin Section (Crossed Polarized Light) 5x.
Fig. 20: A) This picture displays the irregular amphibole (Amph) assemblages with a later biotite (Bt) alteration. A large angular quartz (Qz) grain can be seen in the top left corner. TS (Plane Polarized Light) 5x. B) Same view in crossed-polarized light. Thin Section (Crossed Polarized Light) 5x.
23 5.1.3. DDU2015─007
DDT2015─008 was drilled in 2015 at the mine site, in the north-western parts of the underground mine (Mine Grid: X: 1432.916, Y: 1536.553, Z: -382.0827). The stratigraphy and variations in alteration characteristics are presented in Figure 21 and the location can be seen in Figure 4. The first 172 m were logged as a fine to medium grained, foliated, intermediate metavolcanic rock. This unit is deformed with a smeared, sericitized version of green banding (carbonate, chlorite and amphibole) at the start of hole. Fractured and silicified sections occur in the unit (at 126 m) and decreases the grain size to very-fine grained. The green banding reoccurs towards the transition zone at the end of the unit.
Fig. 21: The stratigraphy of DDU2015─007. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description of these is shown on the right side where bio=biotite, ser=sericite/muscovite, chl=chlorite, car=carbonate/calcite and amph=amphibole.
24
The transition zone between the volcanics and next unit is an intensely carbonized shear zone. The next unit consists of a foliated metasedimentary rocks from 172 to 214 m. The main alteration feature in this unit is the sericite alteration with minor amphibole and carbonate close to the upper contact zone. Another brecciated shear zone with a fractured quartz vein highlights the transition zone between the sediments and the next unit. From 214 to about 236 m there is a porphyritic mafic metavolcanic unit with larger grains of amphibole in a finer grained matrix. This rock is slightly foliated and thin veinlets of carbonate interrupts the unit. Between this unit and the next one is a 7 m carbonized shear zone. From 236 m to 266 m, there is another foliated sedimentary unit with sericite and carbonate alteration. At 266 m there is a sharp contact between the sediments and a mafic metavolcanic rock with amydoils of a white unidentified mineral. This unit is 42 m wide (to 278 m).
From 278 m and onwards, there is a metasedimentary unit, similar to the previous mentioned ones.
At the very end, there is a 2 m wide transition zone to an amygdoil unit, similar to the previous mentioned unit.
Traces of pyrite occur disseminated in host rock the first 220 m. Increased amounts of pyrite occur in association with the contacts of a 0.5 m wide gold bearing quartz vein. Quartz veins occur in all units and have often sharp contacts with none to minor alteration features of carbonate and/or biotite. The first sample (SO-000068) is collected from the finer grained volcanic part of the stratigraphy (at 6.6 m).
This sample has a fine grained, foliated texture with sericite and biotite visible (Figure 22A). The second sample (SO-000069) is picked from the coarser grained, deeper located, albitized parts of the volcanics (at 54.69 m), where the color is slightly darker and no foliation is present (Figure 22B).
Fig. 22: A) Sample SO-000068, B) Sample SO-000069.
5.1.3.1. Petrographic Description SO-000068
Sample SO-000068 displays a highly strained rock (Figure 23). The rock composition is mainly consisting of sericite with a strong preferred alignment which also can be seen in the grab sample in Figure 22A.
Crenulation cleavage in the mica can be seen in Figure 23 and is an indication that two separate deformation events have occurred. Larger fragments of quartz occurs in the muscovite matrix with cryptocrystalline irregular recrystallized quartz surrounding, so called pressure shadows (Figure 23).
Traces of cryptocrystalline microcline and albite can occasionally be found in association with the cryptocrystalline quartz. Occasional destructed remnants of amphibole and biotite can be seen.
Apatite occurs as subhedral hexagonal accessory minerals in the muscovite matrix as well as within
the biotite and amphibole. Allanite (Figure 24A, 24B and Figure 25) occurs as both anhedral and
euhedral, zoned crystals with high relief. They often also occur as irregular rims in association with
larger pyrite grains (Figure 25) and as occasional fine dark rounded relics. The pyrite often occurs as
25
euhedral large crystals disseminated in the fine grained matrix. Traces of anhedral microcrystalline calcite can occasionally be found in association with the micas.
Fig. 23: This image displays the rock under cross-polarized light. The high strained rock is intensely altered by sericite (Ms) and a distinct crenulation cleavage can be found in the center of the image (indicated by red arrow). Cryptocrystalline quartz (Qz) and larger fragments of quartz occurs disseminated in the matrix. These have so called pressure shadows of recrystallized quartz at the grain boundaries. Thin Section (Crossed Polarized Light) 5x.
Fig. 24: A) A idioblastic allanite (Al) grain with high relief occurs with surrounding biotite (Bt) and minor pyrite (indicated by red arrow). A zonation can be spotted. TS (Plane Polarized Light) 10x. B) Same picture under crossed-polarized light. The zonation in the allanite grain is easier located and a variation of colors can be detected. Thin Section (Crossed Polarized Light) 10x.
26
Fig. 25: This image displays two large euhedral pyrite (Py) grains with a zoned allanite (Al) rim attached. TS (Plane Polarized Light) 10x.
SO-000069
The main alteration features in thin section SO-000069 is sericitization by destruction of plagioclase
and a later silicification. The sericite occurs as elongated needles and sometimes developed into flaky
grains. They share two preferred alignments (Figure 26) similar to earlier deformed samples. The strain
is less intense than in sample SO-000068. Biotite occurs as anhedral thinned segregated alignments
overprinted by quartz and occasional calcite (in association with a calcite fracture). The direction of
the fracture is similar to the biotite and assumed to be of later origin than the deformation event
causing the alignments. Calcite can also be found disseminated in the sample, replacing the biotite and
overprinting the plagioclase. Quartz occur as larger fragments with migrating boundaries and as
cryptocrystalline secondary quartz (Figure 27). Traces of minor destructed anhedral amphibole grains,
similar to the ones seen in sample SO-000068, also occurs in this sample. Apatite occurs in the biotite
assemblages, as well as in the main matrix as subhedral hexagonal crystals with minor inclusions and
cross fractures. Allanite occurs as prismatic grains with a high relief, as well as the messy irregular
samples, similar to SO-000068.
27
Fig. 26: This image illustrates the two orientations of sericite (Ms) present in the thin sections. The alignments are indicated by the two red arrows. Abundant amount of recrystallized quartz (Qz) occurs disseminated in the matrix. Thin Section (Crossed Polarized Light) 10x.
Fig. 27: A) A quartz fragment (indicated by red arrow) with minor surrounding mineral recrystallization and close by anhedral biotite (Bt) grains aligned to a curved structure. TS (Plane Polarized Light) 5x. B) Same image displayed in crossed-polarized light. Thin Section (Crossed Polarized Light) 5x.
28 5.1.4. DDP2015─046
DDP2015─046 was drilled in 2015 from the surface in the Nylund area, east of the mine (Mine Grid: X:
2510.5, Y: 2.1, Z: -83.2). The core was logged mainly as a medium- to coarse-grained intermediate volcanoclastic rock, intruded by mafic dykes at top and middle of the stratigraphy. The stratigraphy and variations in alteration features is presented in Figure 28 and the location can be seen in Figure 4.
Closer to the surface the rocks are dominantly altered by k-feldspar, causing the rocks to shift to a pink color. Further down in the stratigraphy, the rocks have more albite-dominant alteration. Amphibolite veinlets and quartz veins occur with alteration envelopes that matches the dominant host rock alteration, meaning that in the section where the potassic alteration is dominant, the halo surrounding these veinlets would be red, whereas in the albite-dominant section, they are bleached pale green. A weak epidote alteration can be seen throughout the core and has a slight increase towards the end of the hole. The finer grained units in the stratigraphy corresponds to foliated units of higher deformation and increased sericitization. The dykes share a texture of elongated, euhedral, medium grained plagioclase grains within a finer grained matrix of dark grey minerals. These rocks are unaltered and have intruded after alteration of the host rock.
Fig. 28: The stratigraphy of DDP2015─046. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description of these is shown on the right side where bio=biotite, alb=albite, ser=sericite/muscovite, car=carbonate/calcite, epi=epidote, kfs=k-feldspar and amph=amphibole.
Traces of disseminated pyrite occurs throughout the core and locally increased amounts of pyrite and
chalcopyrite is found in association with quartz veins and actinolite veinlets. Occasional fragments of
blue quartz is seen at 70 and 116 m. The first sample (SO-000058) is collected from the albitized parts
29
of the stratigraphy (at 42.3 m). This sample has a coarse grained phaneritic texture (Figure 29A). The second sample (SO-000059) is picked from the upper parts of the core, where potassic alteration presumably dominates (at 35.6 m). This sample share the same texture as SO-000058 but has a pink color tone (Figure 29B).
Fig. 29: A) Sample SO-000058, B) Sample SO-000059.
5.1.4.1. Petrographic Description SO-000058
The rock composition consists mainly of anhedral, interlocked and overprinted plagioclase grains with irregular assemblages of amphibole. The boundaries between the plagioclase grains are diffuse which makes it impossible to detect initial grain size. A very weak alignment can be seen in the assemblages.
The plagioclase is overprinted mainly by small grains of sub- to euhedral epidote and traces of
elongated needles of sericite (Figure 30A and 30B). The sericite seems to hold two preferred alignment
types in the plagioclase which can be seen in Figure 30A and 30B. The amphibole are primary, hollow
and moderately to strongly altered into epidote and biotite. The biotite share the same appearance as
earlier samples, with deformed, subhedral shape and a dusty brown color. Within the amphibole
assemblages the epidote grains are significantly larger and appears more prismatic than the grains
occurring in the plagioclase matrix. Inclusions and fractures are a common sight in the larger epidote
crystals. Comparing the epidote found in this sample to the ones at the mine site (in DDP2015─012,
DDT2015─008 and DDU2015─007), there is a distinct increase in amount, grain size and shape, and the
epidote appears more well-developed in SO-000058. Well-developed allanite grains are found in
association with the amphibole and biotite and causes a brown radiation halo in the surrounding
mineral matrix (Figure 31A and 31B). Allanite can also be seen as the altered assemblages described in
earlier thin sections (Figure 32A and 32B). Minor titanite is also occasionally found in association with
the amphibole and forms subhedral diamond-shaped crystals. Apatite occurs as accessory minerals,
similar to the ones found in previous samples (Figure 10A and 10B). Albite is a common alteration
mineral (Figure 30A and 30B) and occur as replacement of primary plagioclase grains and is recognized
with the parallel twinning. The albitization is often associated with quartz and calcite. The quartz has
been recrystallized and overprints the earlier plagioclase and amphibole. It occurs both as larger grains
with grain boundary migration as well as cryptocrystalline masses. Traces of k-feldspar alteration can
occasionally be found instead of the albite, with a typical tartan twinning of microcline that can be
seen in Figure 8A and 8B, SO-000060. Calcite is found in small amount in the sample. It appears in
different sizes and anhedral grains, overprinting the plagioclase. It can occasionally be found
overprinting earlier amphibole grains. A few disseminated subhedral grains of pyrite occurs in
association with the amphibole.
30
Fig. 30: A) This image displays an altered and overprinted plagioclase (Pl) grain. The high relief mineral in the top is a well- shaped epidote (Epi). Smaller epidote grains can also be distinguished by the higher relief. TS (Plane Polarized Light) 10x. B) The epidote displays high interference colors. Minor sericite with two alignments can be noted in the plagioclase. A weak albitization is seen in the middle and identified by the cloudy parallel twinning. Thin Section (Crossed Polarized Light) 10x
Fig. 31: A) Allanite grain (Al) in association with the green amphibole (Amph). TS (Plane Polarized Light) 5x. B) Same minerals under cross-polarized light. Primary twinning in the plagioclase (Pl) can be seen to the right. Minor amount of calcite can be noted in the middle of the picture (indicated by red arrow). Thin Section (Crossed Polarized Light) 5x.
Fig. 32: A) The allanite (Al) is the brown mineral in the bottom left corner in association with biotite (Bt) and grains of epidote surrounding (indicated by red arrows). The allanite holds minor grains of unidentified opaque minerals. TS (Plane Polarized Light) 10x. B) Primary twinning of plagioclase can be noted to the right along with minor calcite overprinting the biotite. Thin Section (Crossed Polarized Light) 10x.
31 SO-000059
The initial rock composition in this sample is the same as in SO-000058 and similar to earlier mentioned samples (SO-000055 and SO-000061). The plagioclase still dominates the ground mass but is extensively overprinted by euhedral and commonly fractured epidote grains and traces of elongated sericite needles. The amphibole assemblages has almost entirely been altered into biotite and large epidote crystals (Figure 33A and 33B) and only remains of amphibole grains is seen. The epidote dominates this sample and appears more developed as larger crystals than SO-000058 (Figure 33A and 33B) and also appears as finer grained in the ground mass. The biotite is anhedral and bent with a dusty brown color. Abundant microcline occurs as patchy, irregular alterations with tartan twinning that overprints the primary plagioclase grains. It often occurs together with quartz, occasional albite and calcite, which are also overprinting the plagioclase. The microcline is more dominant than the previous samples and is probably the cause of change in color that can be seen in the grab samples in Figures 29A and 29B. The biotite and amphibole has been chloritized (Figure 33A and 33B). A few larger grains of euhedral pyrite occurs disseminated in the sample. Minor accessory apatite occurs. Allanite occurs in slightly less amount than in SO-000058, but are still found as well developed grains related to the amphibole-biotite assemblages and causing discoloring in surround biotite due to radioactive elements. The silicification is more intense in this sample compared to SO-000058 and the quartz occurs as fine grains recrystallized assemblages with occasional larger grains. These can be noted overprinting the amphibole and plagioclase.
Fig. 33: A) This picture displays intense chloritization (Chl) of amphibole and biotite (Bt) assemblages. Fractured epidote grains with various size can be identified by the high relief and interference colors (in figures indicated by red arrows). TS (Plane Polarized Light) 5x. B) Displays same minerals under crossed polarized light. Thin Section (Crossed Polarized Light) 5x.
32 5.1.5. DDE2015─007
DDE2015─007 was drilled in 2015 from the surface east of the open pit (Mine Grid: X: 3651.316, Y: - 1091.633, Z: -70.274). The core was logged mainly as a fine- to medium-grained intermediate volcanoclastic rock, with fragments of biotite rich rocks present. The stratigraphy and variations in alteration features of the core is presented in Figure 34 and the location can be seen in Figure 4. The volcanic rock is interrupted by several fine grained, foliated units, presumably dominated by biotite and/or amphibole. Weak potassic alteration envelopes occur at the start of the hole, surrounding the quartz veins. At about 110 m, the rocks are foliated and carbonized, creating a transition zone of green banding alteration surrounding an irregular and unpure limestone unit at 121 m. The limestone has a deformed appearance with intense amphibole alteration and volcanic rock mixed up with the carbonate. Below the limestone the alteration features are intense, but mainly occur as envelopes surrounding the irregular quartz veins.
Fig. 34: The stratigraphy of DDE2015─007. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description of these is shown on the right side where bio=biotite, alb=albite, ser=sericite/muscovite, car=carbonate/calcite, epi=epidote, kfs=kfelspar and amph=amphibole.
Pyrite occurs disseminated and in low amounts (subjective evaluation of concentration: <0.1 %) throughout the core. Below the limestone, the pyrite is associated with the alteration envelopes of amphibole and albite and can be found in large amounts intergrown with the amphibole.
The first sample (SO-000056) was collected at 116.8 m, and represented the green banded unit (Fig.
35A). The second sample (SO-000057) was collected at 59.5 m and is a weakly albitized, dark grey-
green, medium grained rock with minor epidote (Fig. 35B).
33
Fig. 35: A) Sample SO-000056, B) Sample SO-000057.
5.1.5.1. Petrographic Description SO-000056
Sample SO-000056 mainly consists of aligned banded assemblages of amphibole and recrystallized quartz. The amphibole are associated with traces of biotite, small grains of epidote and allanite. The epidote and allanite occurs as sub- to euhedral grain assemblages, often within and at the borders of the amphibole assemblages. The biotite occurs as anhedral grains with a dusty brown color. Two types of amphibole occurs. One elongated secondary actinolite and one primary amphibole, presumably hornblende (Fig. 37) than can be found in the other samples. The quartz can be seen overprinting all amphibole. Between these amphibole assemblages the sample has been intensely silicified. The cryptocrystalline quartz matrix between the amphibole assemblages contains cryptocrystalline, anhedral albite, abundant calcite and minor microcline (Fig. 38). Epidotes can also be seen in between the amphibole assemblages as smaller, broken up grains scattered across the quartz-albite matrix (Fig.
36). Occasionally the quartz, calcite and feldspars have grown larger and can be seen overprinting the amphibole, where mainly the calcite dominates. Traces of fine grained, slightly broken, large diamond shaped grains within amphibole, suggestively titanite (Fig. 37). The allanite occur in minor amount within the amphibole as the diffuse assemblages mentioned in previous samples. Apatite occurs as accessory mineral similar to previous mentioned samples.
Fig. 36: A) Abundant amount of epidote grains in a silicified matrix (epidote grains are indicated by red arrows and Qz=quartz).
Minor amount of primary amphibole can be seen at the top of the picture as the dark green mineral (Amph). TS (Plane Polarized Light) 5x. B) The epidotes are distinguished from the matrix by its high interference colors. Thin Section (Crossed Polarized Light) 5x
34
Fig. 37: A) Two types of amphibole (suggestively hornblende and actinolite) occurs together where the actinolite (Act) is the lighter elongated grains and the hornblende (Hbl) has a darker green color and an indistinct crystal structure. The actinolite seem to replace the more primary hornblende. Diamond shaped titanite (Ti) is located in the amphibole in the middle of the picture. TS (Plane Polarized Light) 5x. B) The actinolite and hornblende can in this picture be recognized by the different interference colors, where the actinolite shares an intense blue and red color, while the hornblende is darker green. Thin Section (Crossed Polarized Light) 5x.
Fig. 38: The calcite (Cal), microcline (K), albite (Ab) and quartz (Qz) dominates the matrix between the amphibole bands (Amph) that can be seen in the grab sample (Fig. 35A). Thin Section (Crossed Polarized Light) 5x.
SO-000057
The SO-000057 holds similarities with SO-000055 and mainly consists of weakly sericite altered
plagioclase grains and clusters of amphibole and secondary biotite. There is no apparent alignment
visible and the grains are larger than in the previous sample. The biotite have in places proper cleavage
surfaces but usually occur as irregular, anhedral masses (Figure 39A and 39B). The plagioclase occur as
35
altered, interlocked, anhedral grains, with parallel twinning overprinted by quartz, patches of albite, minor microcline, small grains of epidote and irregular calcite. A initial shape of the plagioclase can locally be spotted as a elongated, euhedral grains (Figure 40A and 40B). Quartz and calcite is also found as infill minerals in fractured plagioclase (Figure 40A and 40B). The amphibole-biotite assemblages contains epidote grains and allanite with radioactive staining in the surrounding biotite. The amphibole and biotite are overprinted by calcite, albite and quartz, (causing the amphibole to look hollow).
Fig. 39: A) Amphibole (Amph) altered to irregular biotite (Bt). The biotite have been stained by small radioactive minerals of allanite (indicated by red arrows). TS (Plane Polarized Light) 5x. B) Minor albitization (Ab) and silicification (Qz) is visible in the top right and bottom left corner. Thin Section (Crossed Polarized Light) 5x.
Fig. 40: A) The majority of this figure displays a large fractured elongated plagioclase grain (Pl) with minor amphibole (Amph) and biotite (Bt) in the bottom left corner. TS (Plane Polarized Light) 5x. B) The plagioclase have a primary polysynthetic twinning and the cryptocrystalline quartz and calcite (indicated by red arrows) can be seen intruding in fractures perpendicular to the length of the plagioclase mineral. Thin Section (Crossed Polarized Light) 5x.
36 5.1.6. DDE2015─010
DDE2015─010 was drilled in 2015 from the surface east of the mine (Mine Grid: X: 4303.701, Y: - 2028.675, Z: -63.443). The core was logged mainly as a fine- to medium-grained mafic volcanoclastic rock, with fragments of biotite rich rocks present. The stratigraphy and variations in alteration features of the core is presented in Figure 41 and the location can be seen in Figure 4. Potassic alteration is the main noticeable feature in the drill core, ranging from weak to intense grade which gives the rock a deep red color. The intensity is associated with the intensity of epidote and occasionally amphibole.
Epidote occur disseminated in various amounts throughout the core, and can also be found in monomineralic veinlets consisting of only epidote.
Fig. 41: The stratigraphy of DDE2015─010. In the left column the variation in grain size is presented. The middle column displays the variation in intensity of the combined alteration features. A more detailed description these is shown on the right side where bio=biotite, car=carbonate/calcite, epi=epidote, kfs=k-feldspar and amph=amphibole.
