Petrography, alteration and structure of the Bronäs Zn-Pb-Ag deposits, Bergslagen,

Petrography, alteration and structure of the Bronäs Zn-Pb-Ag deposits, Bergslagen,

Sweden

Luleå University of Technology

Tom Turner

Natural Resources Engineering, master's level (120 credits) 2020

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering

i Table of Contents

Abstract iii

1. Introduction and Aim 1

2. Background information 2

2.1 Regional geology 2

2.2 Local Bergslagen geology 3

2.3 Mineral deposits in the Bergslagen region 4

2.4 Local geology of the Sala area 5

2.5 The Sala mine 6

2.6 Local geology of the Sala deposit 6

2.7 The Bronäs deposit 8

2.8 Local geology of the Bronäs deposit 8

3. Methods 11

3.1 Logging 11

3.2 Sampling 12

3.3 Laboratory analysis 12

3.4 Sulphur isotope analysis 13

3.5 SEM 14

4. Results 14

4.1 Lithology descriptions 14

4.1.1 Marble 15

4.1.2 Rhyolitic ash siltstone (RAS) 18

4.1.3 Granite 19

4.1.4 Aplite 20

4.1.5 Porphyritic intrusion 21

4.2 Lithogeochemistry 23

4.2.1 Rhyolitic ash siltstone (RAS) 23

4.2.2 Granite and Aplite 23

4.2.3 Porphyritic intrusion 25

4.3 Cross section 26

4.4 Marble lithogeochemistry 30

4.4.1 Dolomite marble 30

4.4.2 Spatial distribution of major oxides in relation to mineralization in Bronäs 31 4.4.3 Spatial distribution of metals in relation to mineralization in Bronäs 32

4.4.4 Metal concentration relationships in Bronäs 33

4.4.5 The anomalous point at the beginning of 55Enr11 hole 35

4.5 Sulphur isotope 36

4.6 SEM analysis 37

4.7 Characterization of alteration types 37

4.7.1 Marble alteration 37

4.7.2 Skarn 40

4.7.3 Alteration of igneous rocks 43

ii

4.8 Mineralization 43

5. Discussion and interpretation of results 47

5.1 Major element distribution 47

5.2 Metal distribution 49

5.3 Element relationships 49

5.4 Au Pb hotspot 50

5.5 Discussion of cross section 50

5.5.1 Overview 50

5.5.2 Possible models for formation 50

5.5.3 Other potential modes 52

5.6 Alteration 52

5.6.1 Overview 52

5.6.2 Marble alteration 52

5.6.3 Skarn 52

5.7 Mineralization 53

5.8 Deposit type / ore genetic model – CRD vs SVALS vs distal Zn-Pb skarn 53

5.8.1 Conclusion 54

6. Recommendations for future exploration 54

7. Acknowledgements 55

8. Appendix I – Geological core logs 56

9. Appendix II – Thin section descriptions 70

10. Appendix III – SEM results 97

11. References 370

iii Abstract

This report aims to characterize the alteration styles and mineralization of the Zn-Pb-Ag Bronäs deposit, in the Sala area of the Bergslagen mining district, Sweden. It presents data collected through 237m of lithological logging and subsequent lithogeochemical data, which, coupled with thin section and SEM analysis has led to the first geological descriptions of the rocks in the deposit.

The results are presented in the report through logs and a cross-section interpreted from these logs.

The Bronäs deposit, mined between 1945 and 1962, is a satellite deposit to the well-known Sala mine, and similarly to the Sala deposit, it is hosted within an extensive marble unit interbedded with layers of felsic volcaniclastic material. The rocks in the area have first been dolomitized and then undergone regional metamorphism to greenschist facies at 1.87 Ga. There is a large granitic batholith to the south and east of the deposit, and a thin porphyritic intrusion with a complex geometry closely spatially associated with the mineralization.

The marble unit is the host unit for the mineralization and is the most commonly occurring rock type in the study area. It is commonly quite impure and variably skarn altered throughout, with common gangue minerals including serpentine, chlorite, tremolite, diopside and phlogopite. The mineralized sections of the marble are spatially related to the skarn-rich areas of marble, and common sulphides include pyrite, galena and sphalerite.

The interbedded volcaniclastic beds have a rhyolitic composition, and also contain Mg-silicates such as chlorite, tremolite and phlogopite, giving them a green colour too. The contacts between volcaniclastic and marble beds is often marked by contact skarns.

The porphyritic intrusion has a granodioritic composition. It exhibits chilled margins, and the contact between it and the marble is often marked by increased skarn alteration in the marble. The feldspar crystals in the unit are zoned, with a white outer rim and a green to yellow sericitic core.

The cross section shows that all the units are steeply dipping to the south-west, with steeply dipping stratabound lenses of mineralization in the marble, sub-parallel to the volcaniclastic beds in the north east. The south-western part of the section is thick uninterrupted marble. Lithogeochemical results have been divided into two groups – a distal group within this thick marble, and a proximal group close to the mineralization and volcaniclastic beds.

The proximal zone is enriched in SiO

2

and Al

2

O

3

and depleted in FeO, MnO and MgO relative to the distal zone. The Mn and Mg values are enriched when compared to regional levels however. The proximal zone also contains more Pb, Zn, Ag, As and Sb than the distal zone, but contains lower amounts of Au and Cu than the distal zone. There is one hotspot in the distal zone which contains high concentrations of Pb, Zn, Ag, Au, Cu and Sb.

The chemical signature is similar to the mined ore at Sala. The common occurrence of Fe, Mg and Mn-bearing skarn minerals near the mineralization suggests these elements should be higher in the mineralized zone, but skarn minerals are common throughout the study area, and the high levels of Si and Al in the mineralization zone could be diluting the other oxides.

The lenses of mineralization are closely spatially related to the porphyritic intrusion however the intrusion is relatively minor in terms of size and is younger and unaltered. It could have followed an existing fault which was related to the mineralization though. Various examples of sulphide

replacement and retrograde alteration can be seen, such as galena replacing amphiboles and barite.

The similarity in mineralogy between regional metamorphic calc-silicates and ore-related skarn

minerals makes it hard to differentiate between the two, however the presence of high temperature

clinopyroxene indicates the deposit is likely a prograde metasomatic skarn deposit. This is backed

up by a similar calc-silicate assemblage, mineralization style and replacement textures to those

associated with skarn deposits. It is also spatially related to magmatism with mineralization

occurring along lithological contacts.

1 1. Introduction and Aim

This thesis aims to characterize the mineralization and alteration of the Bronäs deposit and explore the alteration history of the Paleoproterozoic marble in the Sala area of the Bergslagen mining district, Sweden. The main research objective is to characterize the alteration styles around the sulphide ore bodies, and to investigate whether exploration relevant zonation patterns and vectors can be determined for Zn-Pb-Ag sulphide deposits.

This type of deposit occurs in the area around the Sala mine, a historically important silver mine in Sweden. The Sala area also hosts industrial dolomite and calcite deposits. Both the sulphide and carbonate deposit types are linked, spatially, to the marble units in the area. This thesis is part of the larger SIP Strim research project VectOre, which is jointly funded by Vinnova, Boliden Mineral and Björka Mineral. It aims to increase understanding of potential links between industrial carbonate deposits and nearby sulphide deposits. Alteration haloes around marble-hosted sulphide deposits in the Bergslagen area locally host high quality industrial carbonate deposits, so the VectOre project aims to investigate the chemical, mineralogical and isotopic zonation of these sulphide deposit alteration haloes.

It is anticipated that improved alteration zonation models for marble hosted mineral deposits such as the Bronäs deposit will lead to new discoveries of both sulphide and industrial carbonate deposits. A greater level of understanding of these systems will lead to a greater level of accuracy, and lower exploration costs for new deposits. As the two types of deposit are commonly spatially related, more knowledge of the association could lead to currently active mines lengthening their life of mine as their ‘waste’ rocks could now be viewed as a by-product with significant value.

The Bronäs deposit was the second largest sulphide deposit in the Sala area and until recently there was very little available data on the Bronäs deposit, despite its relatively recent mining history – the Bronäs mine closed in 1962. With the second largest deposit in the area being a data blind spot, it makes it hard to get an overview of the alteration system in the Sala area, which is a main aim of the VectOre project.

Recently however, old drill core from the mine was discovered at the SGU core storage in Malå.

Historical metadata was uncovered for these drill cores, meaning that by using drill core logging, optic microscopy and whole rock lithogeochemistry, the first modern geological descriptions of these rocks can now be given, and a geological cross-section can be produced.

As the Bronäs deposit is regarded as the same deposit type as the Sala deposit, albeit with some

differences, this study aims to compliment earlier studies of the Sala deposit and to add more

information on the sulphide-related alteration systems, as well as allowing a comparison with the

Sala deposit.

2 2. Background information

2.1 Regional geology

The Fennoscandian Shield (figure 1) covers the majority of Sweden, Finland, Norway and parts of NW Russia, making up the north-westernmost part of the East European craton (Weihed et al., 2005). Rocks as old as 3.5 Ga have been found in the shield (Weihed et al., 2005). During the Paleoproterozoic, from 2.5 to 1.9 Ga, episodes of continental rifting caused several periods of sedimentation and magmatism (Weihed et al., 2005) leading to the formation of supracrustal volcano-sedimentary sequences overlying the Archaean granitic and gneissic basement rocks (Stephens and Weihed, 2013; Weihed et al., 2005).

Within Sweden the shield is made up of three orogens; the Svecokarelian orogen, the Blekinge- Bornholm orogen and the Sveconorwegian orogen (Stephens and Weihed, 2013). The largest of the three, the Svecokarelian orogen, is situated in the eastern part of Sweden, whilst the

Sveconorwegian orogen is in the south-western part and the Blekinge-Bornholm orogen is in the south-eastern part of Sweden (Stephens and Weihed, 2013).

The Svecokarelian orogen is dominated by syn-orogenic rocks forming between 2.0 – 1.8 Ga

(Stephens and Weihed, 2013). The Bergslagen region is located largely within the southwestern part of this orogen (Stephens et al., 2009) and as such the rocks are predominantly Paleoproterozoic.

They have been deformed and metamorphosed to varying degrees due to this Svecokarelian orogeny, and in western areas of the Bergslagen region there has been some overprinting by the Sveconorwegian orogeny between 1.0 and 0.9 Ga (Stephens et al., 2009). There are also Proterozoic rocks that formed post-orogeny (in relation to the Svecokarelian orogeny) which are variably

affected by Sveconorwegian deformation (Stephens et al., 2009).

Figure 1: Simplified map showing the major geological units in the Fennoscandian shield, with the Bergslagen region highlighted. (From Stephens et al., 2009)

3 Some models regard the Bergslagen region as the western part of the Bergslagen-Uusimaa belt which stretches from Sweden to Finland and which is regarded as an intra-arc basin of a mature continental arc (Weihed et al., 2005).

2.2 Local Bergslagen geology

The oldest rocks in the syn-orogenic supracrustal sequence in the Bergslagen region are dominated by turbiditic metagreywackes (Stephens et al., 2009), which shallow upwards into quartzite and then to felsic metavolcanic rocks (Stephens et al., 2009; Allen et al., 2013). The metavolcanics are mostly rhyolitic to dacitic composition and are often interbedded with metamorphosed well-sorted clastic rocks (Stephens et al., 2009). The stratigraphically upper part of the metavolcanic succession

contains interbedded carbonates (calcite- or dolomite-rich) and calc-silicates (skarn). This upper part of the metavolcanic succession is also where the majority of the metal sulphides are situated

(Stephens et al., 2009).

Overlying the volcanic rocks is another, younger succession of clastic metasedimentary rocks hosting metamorphosed turbidite and conglomerate, and quartzite (Stephens et al., 2009; Allen et al., 2013).

This youngest sequence of metamorphosed, siliciclastic sedimentary rocks was deposited at the same time as mafic dykes and sills intruded, whilst the whole supracrustal succession has been intruded by felsic rocks (Stephens et al., 2009). The entire succession has been extensively

metamorphosed by the Svecokarelian orogeny to upper greenschist to amphibolite facies (Allen et al., 2013; Weihed et al., 2005), and in some areas in the southern part of Bergslagen to granulite facies (Stephens et al., 2009).

After the Svecokarelian orogeny the evolution of the Bergslagen region continued with the

emplacement of a suite of granite-diorite intrusions of roughly 1.7 Ga, with associated volcanic and sedimentary rocks (Stephens et al., 2009). The region also contains Mesoproterozoic sandstone, dolerite, granite and quartz syenite, and locally, Neoproterozoic sandstone and lower Palaeozoic sediments (Stephens et al., 2009). The vast majority of post-Svecokarelian igneous events are far- field tectonic responses and can be related to more widespread tectonic activity outside of the Bergslagen region (Stephens et al., 2009).

According to Allen et al. (1996), the supracrustal succession represents low energy, deep-water sedimentation followed by a volcanic episode and thermal doming at 1.9 Ga, with shallow water or subaerial, intense, explosive volcanism. In this model the upper part of the volcanic succession represents the waning volcanic stage, which is accompanied by regional subsidence and the deposition of the carbonate units, followed by more deep-water sedimentation. Allen et al. (1996) suggests the succession formed on an extensional back-arc basin on continental crust. Stephens et al. (2009) backs this theory up, saying that the “deposition and intrusion of the rocks in a continental back-arc setting along a convergent, active continental margin is inferred”. Both Stephens et al.

(2009) and Allen et al. (2013) infer that the basement in the Bergslagen region is probably a

continental basement of around 2.1 – 1.9 Ga.

4

Figure 2: A simplified map showing the bedrock geology of the Bergslagen region, with Sala labelled. (From Stephens et al., 2009)

2.3 Mineral deposits in the Bergslagen region

The Bergslagen ore district is an arc-shaped region in the north-west of the Bergslagen region,

containing a vast range of different ore types, including banded iron formation, apatite iron ore,

5 carbonate and skarn hosted iron ore, W skarn, REE deposits and polymetallic base metal sulphide ores, as well as various industrial mineral deposits (Allen et al., 2013). The majority of these are associated with skarn, which is very common in Bergslagen – so much so that the word skarn originates from this region (Törnebohm, 1875). Focusing on the base metal sulphide deposits in Bergslagen, these are generally split into two distinct types.

The first of these two types is known as “stratiform ash-siltstone-hosted Zn-Pb-Ag sulphide deposits”

or “SAS-type” (Allen et al., 1996) as this type is typically hosted by metavolcanic rocks after originally fine-grained, commonly banded-laminated, reworked volcanic ash. They are stratiform, sheet-like and Zn-Pb-Ag rich (Allen et al., 1996; Stephens et al., 2009; Jansson et al., 2017). These deposits also have crystalline carbonate and calc-silicate (skarn) rocks present, and the most important example of this type is the c. 65 Mt Zinkgruvan deposit (Jansson et al., 2017; Stephens et al., 2009).

The second type in known as “stratabound, volcanic-associated, limestone-skarn Zn-Pb-Ag-(Cu-Au) sulphide deposits” or “SVALS-type” (Allen et al., 1996) and are usually hosted by interbedded metavolcanic rocks and crystalline carbonate rocks. They are stratabound, irregular, multi-lens and podiform, massive and disseminated Zn-Pb-Ag-Cu rich deposits (Allen et al., 1996; Stephens et al., 2009). Examples of this type would be the currently operating Garpenberg deposit (Stephens et al., 2009), or the Falun deposit (Kampmann et al., 2017). SVALS-type deposits are closely associated with magnesium-rich calc-silicate rock, otherwise known as skarn (Stephens et al., 2009).

2.4 Local geology of the Sala area

The Sala area is located in the central area of Bergslagen (see figure 2) and consists of thick

dolomitized and hydrothermally altered carbonate units, predominantly dolomitic marble (Jansson, 2013). Within the marble units in the Sala area are microbial laminations which are attributed to stromatolites (Allen et al., 2003). These laminations are on a sub-mm scale, but nonetheless are still present in areas that have undergone high degrees of alteration and deformation (Allen et al., 2003).

These units probably formed in waters around 100-200 meters deep (Allen et al., 2003).

The stromatolitic marbles are interbedded with fine grained, thinner beds of felsic ash-

siltstone/sandstone, interpreted as re-worked felsic pyroclastic rocks (Allen et al., 2003). Across the Sala area the marbles have zones containing large amounts of calc-silicate minerals associated with reaction skarns adjacent to these volcaniclastic interbeds (Allen et al., 2003). The rocks in the area were dolomitized and then metamorphosed to greenschist facies regionally (Jansson, 2013).

The Sala area is dominated by gently plunging F

1

folds which are generally tight to isoclinal (though some are open), WNW trending structures with steep axial surfaces which have a foliation parallel to the axial surface and subparallel to bedding (Allen et al., 2003; Jansson, 2013). These are

overprinted by open, NE-trending F

2

folds associated with a steep stretching lineation (Jansson, 2013). The second period of deformation is thought to have occurred during early regional metamorphism, and peak metamorphism outlasted the main deformation (Allen et al., 2003). The area has locally been highly deformed by later shear zones and faults which adds to the structural complexity of the area (Jansson, 2013).

The western part of the Sala area is interpreted as an F

1

synclinorium which is truncated by a WSW- dipping, roughly north-south trending shear zone with inferred dextral reverse displacement, with felsic metavolcanic rocks to the west (Jansson, 2013).

To the east, the marble is cut by a large calc-alkaline granite-granodiorite batholith (Jansson, 2013).

This batholith has been dated at c. 1.89 Ga by Ripa et al. (2002), overlapping within error with the

6 age of the supracrustal rocks in the area (Stephens et al., 2009; Allen et al., 2013), including

interbedded metavolcanic units within the marble (Jansson, 2013).

2.5 The Sala mine

The Sala mine is situated 60km west of the city of Uppsala (Allen et al., 2013) and is considered one of the most important historical mines in Sweden. The ore-deposits of the Sala mine were originally discovered prior to 1510, and initially, rich outcropping ore-bodies were mined easily, which meant that the average annual production of the Sala mine was approximately 4,000kg of silver (Sjögren, 1910). However, by 1571 this was down to roughly 200kg, and as little as 10kg by 1609 due to numerous collapses in the mine prior to the introduction of mining with shafts (Sjögren, 1910).

Production varied over the centuries from an average of 1,000kg per year between 1650 - 1700, to 200kg per year between 1720 – 1750, but it is thought that the initial period up to 1600 is the only time the Sala mine made a profit (Sjögren, 1910). The decrease in the price of silver in the 1880’s and 90’s eventually forced the mine to exploit the zinc ore in the deposit. Although the mine eventually ceased production in 1908 (Sjögren, 1910), minor zinc mining took place in the middle of the 20th century (Jansson, 2013, 2016).

The majority of the ore produced at Sala mine was argentiferous galena, and at the end of the 1800’s also large amounts of sphalerite (Jansson, 2016; Sjögren, 1910). The galena is particularly rich in silver in the Sala mine, which earnt the mine the nickname “the treasury of Sweden” (Jansson, 2016) and the mine is also commonly known as Sala Silvermine. In addition to Ag, Pb and Zn, several accessory minerals occur at the mine, including minerals of copper, iron, arsenic, and native varieties of mercury, silver and antimony (Jansson, 2016).

It is estimated over the course of the mines life it produced approximately 400 (Sjögren, 1910) to 450 tons (Jansson, 2016) of silver and roughly 35,000 tons of lead (Jansson, 2016). The total quantity of zinc that was mined is unknown (Jansson, 2016).

2.6 Local geology of the Sala deposit

The Sala deposit is a SVALS-type deposit, hosted by an extensive marble unit over 300m thick (Jansson, 2016). This was originally deposited in a shallow marine environment, and was originally made up of calcitic, stromatolitic limestone interbedded with layers of reworked, felsic volcaniclastic material (Allen et al., 2013; Jansson, 2013, 2016).

It has been suggested that the marble unit is underlain by sub-alkaline metavolcanic rocks of

rhyolitic to dacitic composition west of the marble (Ripa et al., 2002). Jansson et al. (2019) provide a recent re-interpretation of stratigraphy. In this model, stratigraphically the underlying metavolcanic rocks occur SW of the marble, consisting of a lower succession of stratified, rhyolitic-dacitic siltstone- sandstone with local volcaniclastic breccia intervals, and an approximately 300m thick upper unit of rhyolitic pumice breccia. Stratigraphically overlying rocks occur NW of the marble unit, including metamorphosed volcanic-limestone conglomerates of rhyolitic volcanic breccia interbedded with rhyolitic silt-sandstone (Jansson et al., 2019).

The rocks have been dolomitized semi-regionally (Ripa et al., 2002; Jansson, 2013) before being

hydrothermally altered and regional metamorphosed to greenschist facies (Jansson, 2016). The

dolomitization is not linked spatially to mineralization (Jansson, 2013) but the altered marble is

clearly genetically connected with the orebodies (Sjögren, 1910). The mineralization is almost

exclusively hosted by the carbonate rocks, and these sulphide bearing rocks always contain calc-

silicates (Sjögren, 1910). Sjögren (1910) also noted that the higher the ore concentration, the more

7 abundant the silicates appear to be. The most common silicates are pyroxene, actinolite, tremolite, biotite, chlorite, serpentine, phlogopite and diopside, and the ore minerals are Ag-rich galena and sphalerite (Sjögren, 1910; Jansson, 2013, 2016).

The Sala deposit is located in an upright NNW trending open-close F

1

syncline (figure 3) known as the

‘Sala syncline’, which has a fold axis plunge of 35

^o

NNW, and is parallel to the plunge of the mined deposit (Jansson, 2013, 2016). This F

1

fold has, like most of the folds in the east of the Sala area, been refolded in a sigmoidal fashion around NE trending, steep F

2

folds (Allen et al., 2003; Jansson, 2013, 2016).

This syncline has been cut by steep SW dipping shear zones such as the Storgruveskölen shear zone (SSZ) which the mineralization follows, discordant to stratigraphy (Jansson, 2013). The N-NW trending SSZ is the largest of a series of parallel shear zones in Sala mine which represents D

3

shearing (Jansson, 2016), though it is argued that this structure was reactivated during D

3

and that an earlier proto-SSZ already existed at the time of mineralization (Jansson, 2016). There is a final period of deformation (D

4

) characterized by sub-horizontal, dextral strike-slip faults (Jansson, 2016).

The exact timing of mineralization is unknown, but an epigenetic origin has been proposed (Sjögren, 1910). The similar plunge of the mineralization and the F

1

fold axis and the presence of galena in S

1

veins parallel to the axial surface of F

1

folds points to a pre- to syn-D

1

origin (Jansson, 2013).

8

Figure 3: A map showing the simplified bedrock geology and structures of the Sala area. The location of the Sala mine is shown, as well as the location of the Bronäs deposit in relation to the Sala mine. (Jansson et al., 2019)

2.7 The Bronäs deposit

There is limited literature available on the Bronäs deposit, and much of the background provided in this section and section 2.8 is translated and summarized from a mine map description by Heuberg (1952).

The Bronäs deposit is a satellite deposit to the Sala mine, with the main access shaft being situated approximately 600m east of the Sala mine (see figure 3). After the Sala mine was abandoned in the early 20

^th

century, exploration activities increased in the adjacent areas, eventually leading to the discovery of the Bronäs deposit. Exploration drilling began in 1943, targeting a magnetic anomaly.

This led to significant discoveries of galena, hosted in a skarny dolomite marble. This discovery led to a mining concession being granted in 1945, with the mining rights being owned by Avesta Jernverks AB at Sala Silvermine. The deposit was actively mined from 1945 to 1962, with the first drifts at depths of 40 and 55 meters. Following a systematic drilling program at the 55m level, the mineralization was defined in 1950 as a roughly 40m by 100m zone of silver-rich polymetallic sulphide mineralization. This led to further drifts and an additional underground access shaft being sunk at this level. In total, 171,000 tonnes of ore were mined at 350 g/t Ag, 2% Zn and 4.2% Pb between the years 1951 and 1962 (Grip, 1974).

2.8 Local geology of the Bronäs deposit

Figure 4 shows a digitised version of the 55m level mine map, whilst figure 5 shows the 100m level.

There are also levels at 80- and 155-meters depth, though the mined area does not extend below the 100m level.

The deposit is located to the west of the large ‘granitic’ pluton in figure 3 and as such there are many rocks classified as ‘granitic’ material in the mine maps. This material is found as larger coherent bodies at the 55m level around the shaft (figure 4), as well as in numerous exploration drill holes.

The mine map also shows that some drill holes and drifts contain several small, dyke-like granites.

The host rock is shown on the mine map as skarn-altered dolomite of varying levels of alteration with minor felsic metavolcanic rocks interbedded, referred to as ‘hälleflinta’ (fine grained) and

‘leptite’ (coarse grained). The contact between the rocks are mostly sharp contacts.

On the 55m level mine map (figure 4) close to the shaft, at the contact between the dolomite and

granite, there is a roughly three-meter zone of biotite mica schist (shown in black in figure 4). On

the 55m level there is also a roughly north-south trending dolerite dyke which is approximately 30

meters thick. It is located in the drift between the mine workings and the access shaft, and has

9 steep, sub vertical contacts with the marble. This dyke is also visible on the bedrock geology map (figure 3), and could be part of the youngest suite of dolerites in the Bergslagen region, which trend in a similar NNW-SSE direction (Stephens et al., 2009). This suite of dolerite dykes has been dated at 0.98–0.95 Ga and as such are likely related to the Sveconorwegian orogen (Stephens et al., 2009).

Figure 4: Mine map of the 55m level of the Bronäs deposit, with section lines shown in red. The line marked 6 is the section that will be analysed in this report in greater detail as a number of exploration holes were drilled from the drift that follows this section. (Digitized from Heuberg, 1952)

On the 100m level mine map (figure 5) the dolerite dyke is seen again, in the drift between the mine workings and the shaft. The smaller granite dykes are seen more commonly at the 100m level, and they appear to have an irregular shape and strike, suggesting a complex intrusion geometry. At this level there are also more metavolcanic beds which appear to be associated with the granite

55m level mine map – Bronäs deposit

10 intrusion spatially. There also seems to be a spatial relationship between them and the

mineralization too. The silver-rich lead and zinc mineralization occurs as stratabound lenses and disseminations, almost exclusively within carbonate beds where the marble contains abundant metavolcanic interbeds, indicating that this is probably a SVALS-type deposit.

Figure 5: Mine map of the 100m level of the Bronäs deposit, with section lines shown in red. (Digitized from Heuberg, 1952)

100m level mine map – Bronäs deposit

11 3. Methods

3.1 Logging

A systematic exploration drilling program up until 1952 produced approximately 30 drill holes which are displayed on the mine map (Heuberg, 1952) which is available online from the Mining

Inspectorate of Sweden’s database. The mine map contains an index of all these drill holes, which allowed metadata to be collected for these drill holes, allowing them to be drawn in a 3D modelling software. There were no deviation measurements made on these holes, so it is unknown if the logs represent the true trajectory.

Logging of the core has been carried out on boreholes drilled from the 55m level. The boreholes that have been logged were selected as they lie in a 2D cross-section and geology can be correlated between them, as shown by figure 6 below. The drift in figure 6 from which the holes have been drilled is the drift that lies along cross section 6 in figure 4, and is orientated SW to NE. The logging took place at SGU’s logging shed in Malå, where the cores have been stored.

Figure 6: A simplified profile of the exploration holes drilled from the 55m level at the Bronäs deposit. The outline of mine workings around the mineralized zone is also shown. 0m is ground level, and the drift has a strike of approximately 50 degrees to the north-east. (Digitized from Heuberg, 1952)

12 The first drill hole to be logged was drill hole 55Cnr1, which was logged between December 12

^th

and December 14

^th

, 2018. Between February 4

^th

and 8

^th

2019, boreholes 55Enr11, 55Enr9, 55Dnr2, and 55Cnr5 were subsequently logged, in that order, meaning a total of 237m has been logged for this study. Hole 55Enr10 was not logged.

The cores were logged systematically from the collar down, using logging paper, pencil and a folding log rule. A scribe with a magnet was also used to test the hardness and magnetic properties of the core, whilst diluted hydrochloric acid (HCl) was used to determine the presence of carbonates.

Initially the lithology and contacts were logged, followed by the alteration and mineralization. When logging the alteration and mineralization, the type of mineral, character, and estimated percentage was recorded, using an achromatic 20x hand lens. The structures such as foliation and veins were then logged, using a protractor to record the orientation in relation to core axis. Finally, the cores were photographed for future reference. The cores were not logged for recovery percentage or rock quality.

3.2 Sampling

In the first logging session, whole rock lithogeochemistry samples were taken in each of the four different lithologies. As the core diameter was only 22mm, roughly one meter of each lithology was needed to be split in half using a diamond saw to provide a sufficient sample weight. Thin section samples were also taken from various depths in the core and were also sawed in half so that half the core remained for future analysis. In the mineralized sections, the core had already been split in half previously, using force rather than a diamond saw, such that the half core that remained was very broken up, and not a smooth slab. This meant taking samples in these sections was difficult, as it was hard to find pieces that were large enough to make a thin section from.

In the second logging session, eight further whole rock lithogeochemistry samples were taken. They were taken in any distinctly new lithologies, and also within the marble units at points that would give good coverage spatially across the section, so potential alteration patterns could be analyzed later. Thin sections were also taken at interesting points across the cores. Hand samples were taken within the sulphide mineralized sections for future reference, and so that sulphur isotope analyses could be done. In total, 12 lithogeochemical samples, 12 corresponding reference hand specimens, 26 thin sections, and 16 hand samples were taken across the five cores.

The thin section samples were marked out and labelled, and the order for 26x46mm polished thin sections was sent to Precision Petrographics’ lab in British Colombia, Canada. The whole rock lithogeochemistry samples were bagged and labelled and sent to ALS for preparation, who in turn sent them to Bureau Veritas for analysis, along with two blank samples so that reference samples could also be analyzed for quality control.

3.3 Laboratory analysis

The lithogeochemical data was analyzed by Bureau Veritas Commodities Canada Ltd. using total whole rock characterization package LF202, which is a combination of LF100-EXT and LF302-EXT analyses.

LF302-EXT analyzes whole rock major and minor elements by ICP-ES, using lithium borate fusion to digest the mineral phases. The method provides results for a standard suite of major oxides as well as Ce, Co, Cu and Zn.

LF100-EXT combines LF100 with AQ200 to measure a total of 45 trace elements. LF100 uses ICP-MS,

again using lithium borate fusion to digest the mineral phases, to analyze refractory and rare earth

13 elements. AQ200 uses a modified aqua regia digestion of (1:1:1 HNO

3

:HCl:H

2

O) to leach sample splits of 0.5g and then ICP-ES/MS to analyze 36 elements.

These methods combined provide a determination of the element content of each sample for a large range of elements. Full details of which elements and their detection limits can be found in the Bureau Veritas schedule of services and fees (Bureau Veritas Minerals, 2017). The data was then analyzed using ioGas-64 software.

3.4 Sulphur isotope analysis

Four hand specimen samples were taken from borehole 55Enr11 at a depth of 44m, in the mineralization zone. These samples were BRO55E.11-44b, BRO55E.11-44c, BRO55E.11-44d and BRO55E.11-44f. Samples 44b and 44c consist of massive galena with some sphalerite, 44d is predominantly barite with some minor pyrite, and 44f is barite with pyrite, galena and sphalerite.

44b, 44c and 44d were crushed by hand, using a hammer and steel plate, as the samples were too small to be put into a rod mill. Sample 44f was kept as a reference, and also as a spare if required.

Each sample was then separated into 5 size fractions by sieving. Four sieves of mesh sizes 212 microns, 149 microns, 106 microns and 75 microns were used. First each sieve was cleaned in an ultrasound bath before being dried in a kiln and cleaned again with compressed air. Each sample was then sieved for ten minutes in a sieving shaker machine (Haver and Boecker Ro-Tap RX-29).

Each size fraction was then collected separately and analyzed under a binocular microscope.

Under the microscope, individual grains of galena, sphalerite, pyrite and barite were separated and collected by hand using tweezers. Galena and sphalerite were collected from sample 44b, whilst barite and pyrite were collected from 44d. The galena and barite crystals are quite coarse, and a large enough sample for each could be collected from the >212 micron and the 212 to 150 micron size fractions of their respective samples. The sphalerite was only found fully liberated from the galena in the size fractions under 106 microns, which lead to a much smaller sample being collected.

In total four samples were produced; one each for galena, sphalerite, barite and pyrite. These samples were 44b-GN, 44b-SPH, 44d-BA and 44d-PY respectively, and each sample was weighed using a milligram scale before being sent to the laboratory for analysis. Approximately 178mg of galena and 99mg of barite were collected, whilst 26mg of pyrite was collected and only 6mg of sphalerite was collected due to it being so fine grained.

The samples were analyzed to determine the δ

³⁴

S at the Institute of Applied Geosciences at Karlsruher Institut fur Technologie (KIT) in Karlsruhe, Germany. The analysis was done using a EuroVector Elemental Analyzer and isotope-ratio mass spectrometry (IRMS) Isoprime GV instruments on continuous-flow mode.

The powdered sample is weighed into small tin capsules, measuring 4x6mm, with Vanadium pentoxide additive, and the sample is pressed to remove oxygen. The minimum sample size is 130 µg S

tot

so the amount of sample is dependent on the S

tot

content – typically a few mg are used. Three individual subsamples are measured from each sample, then the mean and standard deviation is calculated.

The automated sampler drops the tin capsule containing the prepared sample into a preheated

reactor which is 1030

^o

C. The reactor material is Tungsten oxide, which allows catalytic oxidation of

bound inorganic sulphur to SO

2

gas, and reduced copper chips which reduces SO

3

to SO

2

. In the

reactor the sample is burned in an oxygen atmosphere at a temperature of 1800

^o

C. The evolving

gases are transported with the Helium carrier gas (99.999% purity) at a flow rate of 100 ml/min.

14 They pass the reactor and the magnesium perchlorate water trap and eventually reach the GC- column. The GC-column is 0.8m long and is 45

^o

C, and in it the different gas components are separated. The element content (S

tot

) is then determined by an integrated thermal conductivity conductor (TCD). Afterwards the gas passes open split to reach the ion source of the IRMS.

The results are calibrated using the following certified standard materials;

IAEA S1 (silver sulphide): δ

³⁴

S = -0.3‰

IAEA S3 (silver sulphide): δ

³⁴

S = -32.3‰ +/- 0.2 NBS 127 (barium sulphate): δ

³⁴

S = 21.17‰ +/- 0.12

During measurement the stability of the signals are checked regularly using an in-house CdS standard: δ

³⁴

S = 11‰ +/- 0.2. All the results are given relative to the V-Canyon Diabolo Troilite (VCDT) standard.

3.5 SEM

Seven thin sections were analysed in a scanning electron microscope (SEM). These thin sections were BRO55E.11-44a, BRO55E.11-44e, BRO55C.1-13, BRO55C.5-33, BRO55E.9-28, BRO55C.5-17 and BRO55D.2-23. In preparation for the analysis, these seven slides were cleaned with ethanol to remove any organic matter on the surface, and then the target spots were marked with a permanent marker. They were then carbon coated with a 20nm thick layer of carbon.

The slides were then analysed in a high-resolution Zeiss Merlin FEG-SEM microscope at LTU, at a working distance of 8.566mm, with a 20.00kV electron high tension (EHT) and 1.0nA probe, and the results processed using Aztec software from Oxford Instruments.

4. Results

4.1 Lithology descriptions

Drill core logging suggests that there are 5 main lithologies present in the study area: dolomitic marble, rhyolitic-ash-siltstone, aplite, granite, and a felsic porphyritic intrusion

¹

. The latter two were both initially logged as granitoid, as the differences were not apparent in hand sample. The dolerite dyke in the mine maps (figures 4 and 5) was not intersected. The following descriptions are based on the logs, lithogeochemistry analysis, and microscopy and SEM analysis of thin sections. The full, detailed logs, full thin section descriptions and SEM results can be found in Appendixes I, II and III.

Figure 7 shows simplified digitized logs on the cross-section, to give an overview of the distribution of the different lithologies, and figure 8 shows the location of each sample that was taken during logging. The logs in Appendix I shows clearly which lithology each sample was taken from.

1Note that even though pre-metamorphic rock names are used for the igneous rocks, all igneous rocks in the area have been metamorphosed and are thus strictly speaking metavolcanic rocks, eg. Metagranite, except for the dolerite dykes which post-date metamorphism. Similarly, any reference to limestone is a reference to the protolith of the marble.

15

Figure 7: The five logs overlain onto the cross section, giving an overview of the distribution of the major lithologies.

4.1.1 Marble

As suggested by figure 3, dolomitic marble is by far the most common rock type in the study area, dominating every core that was logged and in places in uninterrupted intervals for over 50 m. It is often interbedded with thinner beds of fine-grained siliceous material termed rhyolitic ash siltstone by Allen et al. (1996), which are occasionally over one meter thick but generally around 0.5 m thick, and quite widely spaced.

The marble has a grain size of around 2 mm and is light grey in colour, although it is commonly quite

impure which gives it a patchy appearance and often a green tint in hand specimen, as shown in

figure 9A-D. Common silicate components in the marble include serpentine, chlorite, tremolite,

diopside and phlogopite (figure 9D), which are responsible for the green colour. The marble does

react with acid although this too is patchy, suggesting a varying content of dolomite and calcite

throughout – SEM analysis has confirmed the presence of both dolomite and calcite in different

samples across the section.

16

Location of all samples taken during logging

Figure 8: A diagram locating every sample that was taken during logging. The logs in Appendix I show which lithology each sample was taken from.

17 In hand specimen, coarse marble with patchy skarn-alteration is the dominating texture, and in places there is foliation visible, as shown in figure 10A-C. The areas with foliation tend to be defined by higher amounts of soft, fine, yellow serpentine interfingering with black serpentine (see figure 10A-C). Figure 10C also shows a thin brown sphalerite vein parallel to foliation. The varying skarn alteration will be discussed in more detail in a later section. Figure 10C-D also shows that the marble is occasionally cut by calcite veins up to 1cm thick. In figure 10C a calcite vein cuts a foliation and likely also cuts the sphalerite that is parallel to the foliation, and therefore the calcite vein is a later

occurrence. The calcite vein also contains dark minerals within which could be fine grained

sulphides – this is potential textural evidence for multiple phases of mineralization or for ore mineral remobilization. There are also occasional thin veins of magnetite around 4mm thick, and even rarer pyrite veins, the thickest of which is c. 1mm thick in 55D nr2. The marble also contains varying magnetite dotted throughout, up to 0.5 mm, and rarer, fine pyrite porphyroblasts around 0.2 mm, often found in association with magnetite.

The dolomitic marble is the host for the sulphide mineralization within the area, and the marble nearest the mineralization in the area is skarn-rich. In hand specimen the main visible sulphides are pyrite, galena and sphalerite, which range in style from massive to disseminated, though pyrite is commonly found as small euhedral porphyroblasts. The mineralization will be discussed in further detail in a later section.

A B

C D

Figure 9: A = relatively silicate-poor, pale grey dolomitic marble at 29.2m in borehole 55E nr11. B = relatively silicate- poor, pale grey dolomitic marble at 40.8m in borehole 55E nr11, with patchy black serpentine and magnetite. C = pale green and black colour of marble with some silicate minerals at 22m in borehole 55C nr1. D = pale to emerald green silica-rich marble, and a 10cm thick dark brown phlogopite section at 22.7m in borehole 55C nr1.

PHL

18 4.1.2 Rhyolitic ash siltstone (RAS)

Allen et al. (1996) gave the name rhyolitic ash siltstone to silt-sized, laminated metavolcanic rocks in the area which have been interpreted as being composed of fine-grained rhyolitic ash.

Metamorphosed rhyolitic ash-siltstone is found interbedded with the dolomitic marble, with beds ranging from 2 cm to 1 m. It is most common in core 55C nr1, but it is found within the marble in all cores except 55D nr2. It contains Mg-silicates and more aluminous minerals such as chlorite, tremolite, sericite and phlogopite which overall gives is a faint green or yellow tint. The contacts with the marble are usually sharp contacts, and often marked by thin bands of chlorite and tremolite, or occasionally phlogopite, as shown in figure 11C-D.

In hand specimen the siltstone is a fine grained, dark grey rock with a flint-like texture. It is very hard, and is commonly laminated (see figure 11A,C,D). There is a variable calc-silicate distribution which can define or accentuate certain laminations. Alteration and metamorphic processes involved in forming the observed features will be discussed in a later section. The rhyolitic ash-siltstone beds contain rare pyritic porphyroblasts which are semi rounded and up to 1.5 mm, but other than pyrite, sulphide mineralization is lacking.

A thin section analysis from 55C nr5 (BRO55C.5-17) shows that the siltstone is largely clast- supported but in places matrix supported, containing quartz, K-feldspar, phlogopite, and chlorite.

The opaque mineral content is surprisingly high compared to expectations from the hand specimen.

SEM analysis shows that these opaque minerals are pyrrhotite, pyrite and arsenopyrite, found with rutile, and minor monazite. These minerals are fine-grained and relatively rare compared to the sulphide content of the marble.

C D

B A

Figure 10: A = soft yellow serpentine interfingering with black serpentine in the marble at 9.34m in borehole 55C nr1.

B = Skarn texture and alteration plus foliation in the marble at 38.27m in borehole 55E nr11. C = calcite vein crosscutting pale yellow/green marble with some foliation and a thin sphalerite vein, at 35.6m depth in borehole 55C nr1. D = calcite vein crosscutting dark emerald green altered marble at 21.8m in borehole 55C nr1.

SPH

19 4.1.3 Granite

The original mine maps (figures 4 and 5) did not distinguish between different granitic units, rather mapping it all as coherent granite bodies and smaller dyke-like granitic intrusions (Heuberg, 1952).

Looking at the surface geology (figure 3) as well as the logs and cross-section, it becomes apparent that these are in-fact two different types of intrusion (Ripa et al., 2002). The larger bodies of granite are part of the large batholith to the south and east, known as the Sala-Vänge batholith (Jansson, 2013, 2016). This calc-alkaline granite-granodiorite batholith is approximately 1.89 Ga (Jansson, 2013 – from Ripa et al., 2002) which coupled with the surface map and Ripa’s interpretation, indicates that it truncates the older marble and metavolcanic beds at depth.

As borehole 55E nr10 was not logged, the only occurrence of this granite batholith in the section is in borehole 55E nr9 where it is over 20 meters thick, though the lower contact is not seen before the end of the core. The granite is intruded by multiple intrusions of finer-grained aplite, which range in thickness from a few centimeters up to one meter. The aplite is described in more detail later.

In hand specimen, the granite is a very hard, coarse-grained rock, though the uppermost c. 50cm of the lithology is fine grained with no individual crystals visible, shown in figure 12. This chilled margin indicates that the overlying dolomitic marble was cool when the granite intruded it, and the contact is marked by intense skarn alteration in the marble (see figure 12). The granite consists of coarse, 4- 5mm white plagioclase crystals. The rounded plagioclase crystals are zoned, with a white outer rim and a green sericite-altered core. There are also white, angular, 2-3mm microcline crystals, and a siliceous groundmass of grey quartz. There is a very low mafic content, with some minor ragged biotite visible. Overall the rock is a pink colour, with varying shades, and occasionally appears green too due to the sericite alteration. With depth, the green colour fades and pink becomes prominent.

A B

C D

Figure 11: A = grey to green RAS bed starting at 15.73m depth in borehole 55C nr1. Some weak laminations can be observed. B = a tremolite vein within the RAS bed shown in A at approximately 15.8m in borehole 55C nr1. C = A RAS bed at 33m depth in borehole 55C nr1, with tremolite occurring in laminations, and both tremolite and phlogopite occurring in reaction skarns at the contact between RAS and marble. D = The lower contact between a grey RAS bed and green marble at 3.83m depth in borehole 55C nr1. The contact is marked by a thin tremolite and phlogopite occurrence. Phlogopite is commonly found within the RAS due to the aluminium content of RAS, whereas tremolite occurs in the marble too.

Marble

RAS tremolite

20

Figure 12: The contact between the granite and the marble at 65m depth is marked by skarn alteration (A), and the uppermost 50cm of granite is a fine grained chilled margin (B). With depth the grain size increases (C).

Figure 13: A large plagioclase crystal with epidote +/- sericite alteration in its core in PPL (left) and XPL (right) in thin section BRO55E.9-71.

Thin section analysis (figure 13) shows that that the plagioclase cores do contain epidote alteration (saussuritization), as too do the rare biotite and microcline. The degree of alteration varies but compared to thin sections from the other granitic units, it is not as altered. The grain size of all of the crystals is significantly larger in this granite than in other the other granitic units too, though there are rare patches of fine-grained biotite and quartz. This granite also contains very rare, extremely fine-grained pyrite porphyroblasts.

4.1.4 Aplite

The aplite unit is only found in core 55E nr9 where is occurs as intrusions within the granite, varying in thickness from 2 cm thick to one meter, but typically around 0.5 m thick. The space between the different aplite intrusions also varies from a couple of decimeters to over seven meters. It is compositionally very similar, but it is much finer grained, and with a lower mafic content – biotite is significantly less abundant in the aplite than the granite. The contact to the granite is always sharp,

A B

C

contact

21 and the aplite always displays a chilled margin within it, as shown in figure 14. This indicates that it intruded into the granite not the other way round, and that the granite had already cooled

somewhat when it was intruded.

In hand specimen the aplite has a fine grained sugary texture. It also has the same green to pink alteration to the granite indicating that the minor sericite alteration occurred after the felsic intrusions. At the upper contact with the granite there are coarse pink pegmatite minerals which can also be found in the aplite. This is more evidence for the aplite intruding into the granite.

The thin section shows abundant, fine-grained granoblastic quartz crystals, with irregular-shaped plagioclase distributed evenly throughout. As with the granite it has intruded into, these plagioclase crystals do exhibit some epidote +/- sericite alteration, but are not as obviously zoned. There are also rare and fine-grained microcline and biotite crystals, but as figure 10 above shows, the aplite is dominated by quartz, containing over 75% SiO

2

. There are rare pyrite porphyroblasts up to 1 mm which can be seen in hand specimen, and thin section analysis shows that there is also rare, angular, fine grained magnetite present.

4.1.5 Porphyritic intrusion

The porphyritic intrusion is intersected by three different boreholes – 55C nr1, 55C nr5 and 55E nr11. It is always found at depth, towards the bottom of the cores, intruding into the dolomitic marble. Cores 55E nr11 and 55C nr5 end shortly after intersecting with the porphyritic unit, but 55C nr1 drills through the granite to discover the marble interbedded with siltstone continues beneath.

In this core the apparent thickness of the intrusion is 4.5 m, but in the other two cores the thickness is unknown. In core 55C nr1 there is also a second, thinner intrusion towards the end of the hole which is only ca. 25cm thick.

Figure 14: A and B = Sharp contacts between the coarse granite and the fine aplite at 72.2m and 81.2m in borehole 55E nr9. A = a thin chilled margin at the contact at 72.2m.

C = coarse pink pegmatite minerals at 72.2m contact.

A

B

C

22

Figure 15: A = soft crumbly gouge at the porphyritic intrusions upper contact with marble at 33.79m in borehole 55C nr5. B = the sharp lower contact between the porphyritic intrusion and dark grey RAS at 30.67m in borehole 55C nr1.

The intrusion is fine grained and dark at the contacts with marble, but it gets coarser and lighter with depth. This chilled margin indicates the marble was significantly cooler when the intrusion occured.

The intrusion has marble above it, but in 55C nr1 it has metavolcanic rocks directly beneath it, as shown in figure 18. The contact is often marked by increased skarn alteration within the marble but can also be marked by the occurrence of a soft, crumbly gouge which reacts strongly to acid. This gouge can be found at the upper contact at 33.79m in borehole 55Cnr5 (see figure 15).

The intrusion has a porphyritic texture with approximately 15-30% white feldspar crystals up to 5 mm in size, as shown in figure 16. These feldspar crystals are zoned, with a white outer rim and pale yellow/green epidote/sericite core. The rock also consists of microcline, minor quartz, and varying amounts of ragged biotite, ranging from approximately 5 to 25%, and there are often large

variations in biotite abundance over a short distance. There is a general weak sericitic alteration throughout, as the feldspars and groundmass are often tinted yellow. Biotite content decreases in the more sericite rich haloes around thin sericitic veins. A sericitic vein in 55C nr1 is accompanied by traces of pyrrhotite.

In hand specimen the intrusion rarely contains any sulphides, with only a trace of fine pyrite visible towards the upper contact. Thin section analysis shows the occurrence of minor magnetite and pyrrhotite too. The pyrrhotite is often highly fractured, and forms alongside pyrite. The thin section shows that the cores of the plagioclase crystals are heavily altered to epidote and sericite (see figure 16), and that biotite and microcline also show some less severe sericitization.

A B

gouge

RAS

intrusion

23

Figure 16: A = the porphyritic intrusion at 27.5m depth in borehole 55C nr1 - the porphyritic texture is a distinguishing feature of the intrusion. The epidote +/- sericitic alteration can also make the intrusion appear pale green. B = small quartz grains surrounding a large plagioclase crystal in XPL with epidote alteration in its core seen in thin section BRO55C.1-27. C = quartz and biotite in PPL in thin section BRO55C.1-27.

4.2 Lithogeochemistry

4.2.1 Rhyolite ash siltstone (RAS)

There was only one homogenous siltstone bed intersected that was thick enough to take a lithogeochemical sample from – sample LK20180600. Compositionally, using major elements the metavolcanic rock is classified as a rhyolite, close to the boundary of a dacite (figure 17) and using trace elements it is classified as an evolved rhyolite dacite (Volcanic Rocks Modified, Pearce 1996), or as dacite-H (Volcanic rocks classification, Stanley 2017).

4.2.2 Granite and Aplite

On top of the visible differences between the granitic rocks, lithogeochemical analysis shows the compositional differences of the lithologies. Figure 18 shows that the granite batholith (and the aplite intrusions) are classified as a granite by the Middlemost 1994 classification diagram, whereas the porphyritic intrusion contains less silica and is classified as a granodiorite. The aplite is

compositionally very similar to the granite and is still classified as a granite (see figure 18).

B C

A

Fsp

Qtz

Qtz

Bt

Fsp

24

Figure 17: TAS classification diagram from Le Maitre et al, 1989, showing that the metavolcanic interbeds are rhyolite. The sample is shown as the green circle on the diagram.

Figure 18: The granite batholith and aplite intrusions are classified as granites, whereas the porphyritic intrusion is classified as a granodiorite by Middlemost, 1994.

25 4.2.3 Porphyritic intrusion

The whole rock geochemistry analysis shows that this intrusion has a granodiorite composition (see figure 18), although figure 19 classifies it as a granite, albeit very close to being a quartz diorite. This illustrates that the main compositional difference between the porphyritic intrusion and the granite batholith is that the intrusion has a significantly higher mafic content, as well as being slightly finer grained.

The porphyritic intrusion contains less K than the granite, despite containing more biotite. This could indicate that the feldspars in the granite are orthoclase whereas in the intrusion they are plagioclase. The higher Ca content in the porphyritic intrusion could also indicate this.

Figure 19: Classification diagram adapted by Wilson 1989 from Cox et al. 1979, showing that the granite batholith has a granitic composition, whereas the intrusion contains more mafic minerals, and is closer to a granodiorite or quartz diorite.

On this diagram, the aplite plots to the right of granite, off the scale, and would in this diagram be classified as a quartzolite.

26 4.3 Cross section

To produce the 2D cross section interpretation of the study area’s geology and mineralization, the logs and lithogeochemical sampling have primarily been used. To try and produce a more accurate cross section, two historical cross sections have been studied and used, and acted as an underlay for the section. Figure 20 shows these two legacy cross sections. In these sections red represents granitoid, yellow is metavolcanic rock, and white/colourless is dolomite, whilst the mineralization is shown by purple (galena) and brown (sphalerite) dashed lines.

Figure 20: Two historical cross sections from old mine maps. The first section (left) shows the basic geology in boreholes 55Cnr1 and 55Cnr5, as well as 55Bnr1 which was not logged in this study, and also shows the mineralization and geology found in the mine workings. The second section (right) shows the basic geology in boreholes 55Enr9, 55Enr11 and 55Dnr2, as well as 55Enr10 and 55Snr190 which were not logged in this study. (Source: Heuberg, 1952)

When the logs in section (as seen in figure 7) were projected over these historical sections, it was discovered that the geology seen in these logs did not correlate with what was expected in the boreholes 55Enr9 and particularly in 55Enr11, for example the mineralized section of this log appears to be outside the area marked on the historical mine workings as containing the

mineralization in figure 20. This is because these two drillholes were drilled slightly off section, and to counter this the logs have been projected by 6.5 meters to the left, so that the mineralization in the log occurs where we know the mineralization was in the past. This projection is illustrated in figure 21, where the geology of the log 55Enr11 lines up nicely with the geology and mineralization that was recorded in the historical mine map. In the top diagram in figure 21, the mineralization seen in the log is represented by the thicker, white patch, which lies outside the known mine area.

The section marked as mineralized on the historical map actually correlates to a section of the log

which contains dolomitic marble with no sulphides. Also the bottom right corner of the historical

mine workings shows a steeply dipping metavolcanic bed. In the lower diagram in figure 21, the two

logs have been projected 6.5m to the left, and the mineralization lines up with the mineralization in

the old map, and the metavolcanic bed in the old map also lines up with a metavolcanic bed seen in

the log.

27

Figure 21: Projecting boreholes 55Enr11 and 55Enr9 6.5m to the left to correlate the known geology with the logs

In the final cross section (figure 22), the right hand side (NE) is characterized by dolomitic marble interbedded with steeply dipping rhyolitic to dacitic metavolcanic beds. Two of these interbeds appear to be truncated by the porphyritic intrusion which has a similar steep dip. This truncation indicates that the intrusion is discordant and must be younger than the metavolcanic beds, and the

Original

Offset

28 intrusion could have followed an older fault. The intrusion thickness is known to be approximately 4.5m in core 55Cnr1, and it is a similar thickness in core 55Bnr1 on the historical section, therefore this thickness has been inferred as continuous to depth, though the actual intrusion geometry appears to be quite complex in the mine maps (see figures 4 and 5). To the right of this intrusion the marble and metavolcanic interbeds continue, with another smaller intrusion on the far right. As only one drill hole that was logged actually continued through the intrusion, it is hard to interpret how these beds behave at depth. To the left of the succession of steeply dipping metavolcanic beds, the centre of the section is made up of thick, uninterrupted dolomitic marble, which is variably skarn altered but on too small a scale to include in the section, and in general less skarn altered than the interbedded marble and metavolcanic beds.

The left hand side (SW) of the section shows the large granitic pluton which is seen at the end of

core 55Enr9, and has been mapped on the historical maps at the SW end of the drift, and at the end

of core 55Enr10, and has been inferred between these points. The granite batholith is known as the

Sala-Vänge batholith and is approximately 1.89 Ga (Jansson, 2013, 2016), and as such it truncates

the older marble and metavolcanic beds which formed on the seafloor. The marble in the Bronäs

deposit has not been dated but marble from the nearby Sala mine has been dated at 1894±2 Ma

(Stephens et al., 2009). The age of the porphyritic intrusion is not known, so it is unknown whether

it is truncated by the granite batholith or not, however a feldspar-quartz porphyritic intrusion to the

north-west of the Sala mine has been dated at 1892 +5/-4 Ma by Stephens et al. (2009) which is a

similar age to the marble. The age of this intrusion is also very similar to the age of the granite

batholith, which suggests that they could be related.

29

SW NE 2D Cr os s sec ti on in te rpr et at ion of the s tu dy ar ea

Figure 22: A 2D cross section of the study area showing the lithologies and mineralization. The outline of the mine drift and mine workings are shown, as are the simplified logs. The scale on the right shows meters below the surface. A dashed line with question marks indicates a very low degree of certainty as these rocks have only been seen in one location (the end of hole 55Cnr1) and have been extrapolated to depth based on this observation and one historical observation of the rocks occurring at the end of a horizontal drill hole from the NE end of the drift

? ? ? ? ? ? ?

? ?

? ?

30 4.4 Marble lithogeochemistry

4.4.1 Dolomite marble

During logging different sections of marble reacted with different ferocities to hydrochloric acid, suggesting varying calcite/dolomite concentrations. Microscopy work was not particularly effective at determining the difference either. Some (seven) of the thin sections were analysed using SEM, including BRO55D.2-23 and BRO55E.9-28, which showed that some of the grains were calcite, and some were magnesium bearing dolomite. In some instances (E.9-28) calcite crystals were seen containing dolomite inclusions which in turn also contain calcite inclusions.

Whole rock geochemistry can be used to define whether it is a dolomitic marble or a calcitic marble.

Even if the crystallography/composition has changed during metamorphism, the whole rock geochemistry will remain the same assuming it is a closed system, and therefore the original pre- alteration chemistry can be determined. Figure 23 shows that when the MgO to CaO ratio if plotted, the Bronäs samples plot in the same area as the Sala dolomite samples and the Tistbrottet dolomite sample, and the Sala calcite sample has significantly less MgO. Therefore the Bronäs samples can be classified as dolomitic marble.

Figure 23: MgO:CaO ratios show that the Bronäs marble samples are dolomite marble.

The seven dolomite marble sample points have been divided into two groups for analysis; the three

points on the right hand side of the study area, relatively proximal to the mineralization, make up

what is henceforth referred to as the mineralized zone. These samples are lithogeochemistry

samples 3, 4 and 12 in figure 8. The four more distal samples make up the second group, and these

are lithogeochemistry samples 5, 6, 7 and 8 in figure 8.

31 4.4.2 Spatial distribution of major oxides in relation to mineralization in Bronäs

As figure 24 shows the mineralized zone is low in MgO, MnO and FeO, and high in SiO

2

and Al

2

O

3

. Mg, Mn and Fe content all increase with distance from the mineralized zone, whereas Al

2

O

3

and SiO

2

decrease with distance from the mineralized zone. These patterns remain the same even when the ratio of these elements to CaO is compared. Whilst the mineralized zone is low in Mn (and Fe) relative to the more distal samples, they still contain 0.18% and 0.22% MnO, whereas an unaltered regional sample contains 0.04%, and a white dolomite sample from the Tistbrottet mine contains 0.06% MnO, so the mineralized zone in Bronäs is still high in MnO relative to these.

Figure 24: Graphs showing the spatial distribution of major oxides in the study area. MgO, MnO and FeO are lower in the mineralized zone and higher in the more distal samples, whereas SiO2 and Al2O3 are higher in the mineralized zone and lower in the distal samples.