Three-Dimensional Modelling of the Källfallsgruvan Iron Oxide Deposit

Three-dimensional modelling of the Källfallsgruvan iron oxide deposit, Riddarhyttan ore field, Bergslagen, Sweden: Integrating existing and new data to aid understanding of structural controls and mineral exploration

Edna Spahic

Natural Resources Engineering, master's 2021

Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Acknowledgements The Geological Survey of Sweden (SGU) has provided the financial support to this study (FoU project 36-1341/2019). I therefore extend my greatest gratitude towards SGU for making this project possible.

I would like to give a special thanks to my supervisor Tobias Kampmann for the continued assistance, patience, and motivation during this thesis. I could not have asked for a better supervisor.

I am also thankful for my external supervisor Stefan Luth (SGU) who has been of great help both during the field work and in the understanding of structural geology. Your input throughout this project has been of great value. I would also like to thank Alexander Lewerentz (SGU) for his aid during the field work as well but also for providing me with valuable information to improve this thesis. In addition, I would like to thank Peter Hedin (SGU) for providing me with a detailed magnetic map over the Riddarhyttan ore field.

I would like to thank Jerry Hedström and the whole SGU team at the national drill core archive in Malå for making it possible to conduct the drill core logging and for the support during sampling of drill core sections.

I would like to extend my gratitude to Wilfried Tsoblefack working in the support department for helping me with any questions regarding SKUA-GOCAD 19, you always made time to help and supported me from start to finish.

I would like to express my sincere gratitude to my family, friends, and boyfriend. They have provided constant encouragement and pushed me all the way during this journey and always believed that I can achieve anything I want if I just believe in myself. I would like to highlight my brother Edin, he has always been my role model and been of great support during my time at LTU. Ti si najbolji brat što se moze poželjeti!

Luleå, June 2021 Edna Spahic

Abstract The Bergslagen ore province, located in the Fennoscandian shield in the south-central part of Sweden hosts several metallic mineral deposits, one of them being the Källfallsgruvan iron- oxide deposit in the Riddarhyttan ore field, situated in a high-strain shear belt denoted as the West Bergslagen Boundary Zone (WBBZ).

An accurate 3D geological model of the Källfallsgruvan iron-oxide deposit has been generated to aid understanding of the ore body geometries and calculating volume and tonnage. The methodology of this study consisted of assessing existing data in the form of legacy mine maps and to integrate new data from field work, drill core logging and 3D geological modelling. The work has resulted in structural interpretations that are put in the context of the regional structural framework in Bergslagen consisting of three deformation events (D 1, D2 and D3) and two metamorphic events (M1 and M2). The field work resulted in six rock units being defined used to construct a geological map, in addition the structural measurements resulted in a hypothetical semi-regional fold and evidence of ductile strike-slip/dip-slip shearing. Logging of drill cores resulted in three rock units being defined, two of them related to the mineralization and one characterizing the host rock (± local variations), correlating to the observed host rock from the field work. The rock units discovered from both the field work and drill core logging are all interpreted to be the metamorphic products of volcanic rocks subjected to alteration of varying degree.

Based upon geological field observations with subsurface data and 3D geological modelling it is concluded that, 1) The deposit comprises multiple ore bodies that jointly resemble an S- shaped synform that is steeply inclined-upright, moderately-steeply plunging towards the southwest with an axial plane striking northeast-southwest. The deposit is interpreted to be geometrically controlled by an F2 fold, possibly displaying an interference pattern of type 1, favouring progressive shearing and deformation solely related to D2, 2) Evidence of at least one generation of transpressional tectonic regime exists, interpreted to be D2, 3) The estimated tonnage of 4 938 610 tons of the massive magnetite and semi-massive mineralization revealed that a deposit of such tonnage is presently not economically viable. However, if the Källfallsgruvan iron-oxide deposit or similar is determined to have a significant REE content, such deposit may then be of economic interest and, 4) Possible mineral exploration indicators around Källfalls-like deposits are intensely altered rocks related to magnesium alteration, consisting dominantly of quartz, biotite (increasing towards mineralization), muscovite, chlorite, anthophyllite and cordierite, affected by parasitic folding. Sammanfattning Bergslagen malmprovins, belägen i den Fennoskandiska skölden i sydcentrala delen av Sverige innehåller flera metall mineralfyndigheter, varav en av dem är Källfallsgruvan järnoxidfyndighet i Riddarhyttan malmfält, belägen i en skjuvzon kallat för ”West Bergslagen Boundary Zone (WBBZ)”.

En representativ 3D geologisk model av Källfallsgruvan järnoxidfyndighet har genererats för att öka förståelsen av malmkroppens geometrier samt beräkna volym och tonnage. Metodiken bestod av att bedöma existerande data så som gruvkartor och integrera nya data genom fältarbete, borrkärnekartering och 3D geologisk modellering. Arbetet resulterade i strukturella tolkningar som sattes i kontexten av den regionala strukturella bilden i Bergslagen bestående av tre deformations event (D1, D2 och D3) och två metamorfa event (M1 och M2). Fältarbetet resulterade i att sex bergartsenheter definierades som användes för att konstruera en geologisk karta samt så har de strukturella mätningarna resulterat i ett hypotetiskt semi-regionalt veck och bevis för duktil strike-slip-/normal-skjuvning. Borrkärnekarteringen resulterade i att tre bergartsenheter definierades, varav två var relaterade till mineraliseringen och en kännetecknade värdbergarten (± lokala variationer) som korrelerar med den observerade värdbergarten från fältarbetet. Bergartsenheterna som upptäcktes från både fältarbetet och borrkärnekarteringen har tolkats vara metamorfa produkter av vulkaniska bergarter som omvandlats i varierande grad.

Baserat på de geologiska fältobservationerna med underjordsdata och 3D geologisk modellering dras följande slutsatser, 1) Fyndigheten består av multipla malmkroppar som gemensamt liknar en S-formad synform som är brant lutande-upprätt, måttligt-brant stupande mot sydväst med ett axialplan som stryker nordost-sydväst. Fyndigheten tolkas vara geometriskt kontrollerat av ett F2 veck, som möjligtvis påvisar ett interferensmönster av typ 1 bildat genom progressiv skjuvning och deformation endast relaterat till D2, 2) Bevis för minst en generation av transpressionstektonisk miljö existerar vilket tolkats vara D2, 3) Det estimerade tonnaget på 4 938 610 ton för den massiva magnetit och semi-massiva mineraliseringen visade att en fyndighet av sådant tonnage är för nuvarande inte ekonomiskt hållbart. Om Källfallsgruvan järnoxidfyndighet eller liknande fyndighet bestäms att ha ett betydande REE innehåll så skulle en sådan fyndighet vara av ekonomiskt intresse och, 4) Möjliga prospekteringsindikatorer runt Källfalls-liknande fyndigheter är intensivt omvandlade bergarter relaterad till magnesiumomvandling främst bestående av kvarts, biotit (ökar mot mineraliseringen), muskovit, klorit, antofyllit och kordierit, påverkad av parasitisk veckning. Table of contents 1. Introduction ...... 1 2. Geological background ...... 3 2.1 Regional geology of the Bergslagen ore province ...... 3 2.2 Semi-regional geology of the Riddarhyttan ore field ...... 7 2.3 Källfallsgruvan iron-oxide deposit ...... 8 3. Method...... 10 3.1 Existing data...... 10 3.2 New data...... 12 3.2.1 Field Work ...... 12 3.2.2 Drill core logging ...... 12 3.2.3 Modelling process ...... 13 3.2.3.1 Preparation required before 3D modelling...... 13 3.2.3.2 Geological 3D modelling ...... 14 3.2.3.3 Adjustments of the 3D model...... 16 3.2.4 Tonnage estimation of the Källfallsgruvan iron-oxide deposit ...... 17 4. Results ...... 18 4.1 Description of rock types from the Riddarhyttan ore field ...... 18 4.1.1 Characteristics of collected rock samples from the Riddarhyttan ore field ...... 18 4.1.2 Characteristics of rock types proximal to the Källfallsgruvan iron-oxide deposit ...... 21 4.2 Spatial distribution of rocks at the surface ...... 25 4.3 3D model of the Källfallsgruvan iron-oxide deposit ...... 26 4.4 Structural observations ...... 29 5. Discussion ...... 34 5.1 Geometrical control of the Källfallsgruvan iron-oxide deposit ...... 34 5.2 Deformation in a regional context ...... 35 5.3 Calculated tonnage of the Källfallsgruvan iron-oxide deposit ...... 36 5.4 Implications for mineral exploration...... 37 5.5 Future work...... 38 6. Conclusions ...... 39 7. References ...... 40 8. Appendix ...... 42 8.1 Appendix A – Tables ...... 42 8.2 Appendix B – Images...... 45 1. Introduction Three-dimensional modelling is a method used to model and visualize geological volumes and structures in three-dimensional space and has for several decades been applied in the oil and gas industry but is now commonly used for geoscience and exploration purposes, as a result of constant development of the technology and methods (Royer et al. 2015). Since the modelling softwares have developed during recent decades, it is possible to create accurate 3D representations of bedrock geology, which facilitate interpretation and understanding, compared to the traditionally used 2D maps and projections (Bauer et al. 2014; Kampmann et al. 2016; Royer et al. 2015). Three-dimensional models allow for more accurate geological and structural interpretations, and can aid in exploration from district to deposit scale (Bauer et al. 2014; Kampmann et al. 2016; Royer et al. 2015).

The Bergslagen ore province hosts dominantly Zn-Pb-Ag ± Cu ± Au ± Fe sulphide and Fe- oxide ore deposits of various types and has been a key source of metal production for over a thousand years (Allen et al. 1996; Stephens & Jansson, 2020). Various studies have been performed in Bergslagen through time but there is still a lot of open research questions, which can be aided by 3D geological modelling.

A magnetic anomaly was first discovered in 1896 but it was not until after further investigation that it would become historically, one of the largest iron producing mines in Sweden, namely the Källfallsgruvan iron-oxide deposit (Geijer & Carlborg, 1923). Detailed studies have been carried out with respect to the Källfallsgruvan iron-oxide deposit resulting in a wealth of geological information such as legacy mine maps and stored drill cores.

Rare earth elements (REEs) have a great significance in the history of Sweden as many of these elements were discovered in rocks from Swedish ore deposits (Andersson et al. 2004). Recently there has been a significant spark of economic interest, as they play a big role in today’s modern society for production and improvement of green technology (Binnemans et al. 2013). Therefore, studies about the ore deposit geology of the Riddarhyttan ore field are important because of historical significance, in the context of regional geological understanding, but also due to the association with critical raw materials (e.g., REE), which sparked renewed exploration activity in the area in recent years. The REE-bearing minerals of the Riddarhyttan ore field (e.g., Allanite and cerite; Andersson et al. 2004) are situated in amphiboles such as actinolite, tremolite and anthophyllite and show association with iron deposits (Sahlström et al. 2019). The presence of magnetite associated with anthophyllite in the Källfallsgruvan iron-

1 oxide deposit (Geijer & Carlborg, 1923; Geijer & Magnusson, 1944) implies a potential for REE mineralization.

The purpose of this thesis is to generate an accurate 3D geological model of the Källfallsgruvan iron-oxide deposit. The 3D model will be used to aid interpretation and characterization of the geological structures and ore body geometries, establish a better understanding of the character and extent of the ore system and, generally, the geological evolution of the area. It is also of interest to integrate and harmonize the presented model with the result from 3D geological and geophysical modelling of the Riddarhyttan ore field by the Geological Survey of Sweden (SGU).

2. Geological background In this section, the Bergslagen ore province (regional geology, Fig. 1), mainly focused on the central structural domain (Stephens et al. 2009), as well as geological information about the Riddarhyttan ore field (semi-regional geology, Fig. 2) and the characteristics of the Källfallsgruvan iron-oxide deposit will be summarized. All maps presented in this section have the coordinate system SWEREF 99 TM, EPSG: 3006.

2.1 Regional geology of the Bergslagen ore province The Bergslagen ore province is situated in the south-central part of Sweden, in the Fennoscandian shield and is part of the Palaeoproterozoic Svecokarelian orogen (Stephens et al. 2009). The ore province hosts a large quantity of different types of metallic mineral deposits, dominantly in the west-central part and has been a considerable producer of metals for over 1000 years (Stephens & Jansson, 2020; Allen et al. 1996). Bergslagen has been divided by Stephens et al. (2009) into four main structural domains: central, northern, southern, and western structural domain.

The oldest rocks in Bergslagen are metasedimentary in nature, specifically turbiditic metagreywackes formed during sedimentation in deep water, later interrupted by a volcanic episode followed by thermal doming (Stephens et al. 2009). The volcanic episode led to the deposition of metavolcanic rocks, dominantly rhyolitic and dacitic in composition and the volcanic rocks are frequently intercalated with the carbonate metasedimentary rocks, such as marble (Stephens et al. 2009). The volcanic rocks have been affected by hydrothermal alteration on a regional and local scale during a late rift stage (Oen et al. 1982; Stephens et al. 2009). Four hydrothermal alteration types have been distinguished in the ore province: magnesium, sodium, potassium, and skarn alteration (Geijer & Magnusson, 1944; Frietsch, 1982; Jansson & Allen, 2013).

The supracrustal rocks formed during the volcanic episode, aged ≥1.91–1.89 Ga, have been intruded by different generations of plutonic rocks (Stephens et al. 2009), which by Stephens et al. (2009) have been divided into three main plutonic suites, namely: Granitoid-dioritoid- gabbroid suite (GDG suite), Granite-syenitoid-dioritoid-gabbroid suite (GSDG suite) and Granite-pegmatite suite (GP suite). The rocks in the GDG suite dominate in Bergslagen, with a composition varying from either calc-alkaline to calcic and an age of 1.90–1.87 and 1.87–1.85 Ga (Stephens et al. 2009; Stephens & Jansson, 2020). The GSDG suite has an age of 1.87–1.84 and 1.81–1.75 Ga with an alkali-calcic composition (Stephens et al. 2009). The GP suite is alkalic in composition and has an age of 1.85–1.75 Ga but differs from the GDG and GSDG

3 suite in the sense of displaying a more homogeneous composition and enrichment in both uranium and thorium (Stephens et al. 2009).

Bergslagen is a geologically complex province that has been subjected to several deformation events, with transtensional and transpressional tectonics (D1, D2 and D3), and metamorphic events (M1 and M2), ranging dominantly from greenschist to amphibolite but in some areas, up to granulite facies (Stephens & Jansson, 2020; Stephens et al. 2009; Allen et al. 1996). Beunk & Kuipers (2012) emphasized two transtensional and transpressional events occurring in

Bergslagen. Prior to D1, Bergslagen went through rifting resulting a transtensional environment, but as the rift started to close, the tectonic environment switched from a transtensional to a transpressional environment (Beunk & Kuipers, 2012), defined by Stephens et al. (2009) as D1. Eventually the same events of rifting and closure repeated itself, resulting in a transtensional environment and switching to a transpressional environment (Beunk & Kuipers, 2012), defined by Stephens et al. (2009) as D2.

The first deformation event, D1 occurred at 1.87–1.86 Ga and affected the rocks with an age of 1.91–1.87 Ga, resulting in both planar and linear fabrics, folding occurred, ranging from either isoclinal (interlimb angles ranging from 0º–10º; Fossen, 2010) or tight (interlimb angles ranging

º º from 30 –70 ; Fossen, 2010) (Stephens & Jansson, 2020). The tight–isoclinal folds (F1) have been observed by Stephens et al. (2009) concluding that the axial surface traces in the central structural domain commonly had an NNW–SSE orientation. The S1 foliation varies in strike and dip direction throughout the Bergslagen ore province (Stephens et al. 2009).

Shortly after D1, mafic underplating caused the first metamorphic event, M1 to activate at 1.86 Ga which developed a static recrystallization, with a close time-dependent relationship with the GSDG (1.87–1.84 Ga) suite and GDG (1.87–1.85 Ga) suite (Stephens & Jansson, 2020; Stephens et al. 2009).

At 1.84–1.80 Ga, the second metamorphic event occurred, M2, and was coeval with the second deformation event, D2 (Stephens & Jansson, 2020). Characteristic of D2 was a large regional scale folding, resulting in new fabrics, overprinting the earlier ones from D1 of varying degree, but completely overprinting units rich in phyllosilicate minerals (Stephens & Jansson, 2020).

The fabrics generated from D2 did not only cause overprinting of D1 fabrics but also refolding

(cross-folding) of the isoclinal-tight folds to F2 folds (Stephens et al. 2009) with varying axial surface trend, depending on the structural domain (Stephens & Jansson, 2020). Generally, in the central structural domain the orientation of the axial surface trend of F2 folds are NE–SW

4 to E–W, steeply dipping and with a vergence towards N, NNW, or NW (Stephens et al. 2009; Stephens & Jansson, 2020).

In the central structural domain, the S2 foliation tends to have a NE–SW strike, commonly steeply dipping towards SE and often transposing the S1 foliation, thus making it hard to distinguish between the two generations (Stephens et al. 2009). The dominating trend in the central structural domain of the S2 foliation may in some cases display variations (Stephens & Jansson, 2020).

The third deformation event, D3 is characterised by open folds (interlimb angles ranging from 70º–120º: Fossen, 2010), with an NNW–SSE axial surface trend (Stephens & Jansson, 2020).

Fig. 1 Geological map of Bergslagen showing the location of the Riddarhyttan ore field and the Källfallsgruvan iron-oxide deposit, modified after Stephens & Jansson, 2020.

2.2 Semi-regional geology of the Riddarhyttan ore field The Riddarhyttan ore field (Fig. 2), Skinnskatteberg municipality, is located in the western part of the Bergslagen central structural domain (Stephens et al. 2009). The iron oxide mineralization types within the Riddarhyttan ore field have been given local names with respect to the mineral association, such as Källfalls type (magnetite associated with anthophyllite, talc, biotite, and cordierite; Geijer & Magnusson, 1944), Blåkulla type (magnetite and hematite associated with diopside, garnet, etc; Geijer & Magnusson, 1944), etc. The Källfalls type is named after the Källfallsgruvan iron-oxide deposit because it is the largest mineralization of the specific Källfalls type with respect to tonnage (Geijer & Magnusson, 1944).

An early investigation of the ore field revealed that its tectonic characteristics were dominated by folding (Geijer & Carlborg, 1923), denoted by Beunk & Kuipers (2012) as the Riddarhyttan syncline. The geology constitutes mainly of volcanic rocks of rhyolitic composition, which have been subjected to magnesium and alkali alteration (Trädgårdh, 1988). Out of the two hydrothermal alterations, the alkali alteration is weaker and involved pseudomorphism and recrystallization of original feldspars to either albite or K-feldspar, thus the volcanic rocks affected by this alteration are considered the less altered ones with respect to the two kinds of hydrothermal alterations (Trädgårdh, 1988). The magnesium alteration is dominant, overprinting the alkali alteration in the form of schists with a strike of NE–SW on a semi- regional scale, characterised by destruction of feldspar and enrichment of phyllosilicates (Trädgårdh, 1988).

Furthermore, Trädgårdh (1988) found evidence of prograde and retrograde metamorphism following the hydrothermal alteration. The prograde metamorphism resulted in the following mineral association: cordierite/biotite/muscovite, cordierite/anthophyllite, andalusite/mica and garnet/andalusite (Trädgårdh, 1988). The prograde metamorphism was then followed by a retrograde phase resulting in the following mineral association: sericite/muscovite, chlorite, and epidote (Trädgårdh, 1988). The peak metamorphism reached amphibolite facies and was most likely part of M2 that has occurred in the Bergslagen ore province around 1.84–1.80 Ga (Trädgårdh, 1988; Stephens et al. 2009).

Fig. 2 Semi-regional geology of the Riddarhyttan ore field, modified (provided by SGU). 2.3 Källfallsgruvan iron-oxide deposit Out of all deposits located in the Riddarhyttan ore field, it has been stated by Ambros (1983) that the Källfallsgruvan iron-oxide deposit is the only mineralization that has been extracted

8 completely. The mining that occurred during 1897–1907, 1908–1932, 1934–1939 resulted in an estimated total amount of iron produced of c. 1 128 000 tons (Geijer & Magnusson, 1944).

The mineralization of Källfalls type has been studied by Geijer & Magnusson (1944) who concluded that the mineralization mainly consisted of magnetite associated with magnesium- rich silicates such as anthophyllite, talc, biotite, and cordierite. Proximal to the massive magnetite-rich units, the mineralogy would progressively become richer in either biotite, anthophyllite or both (Geijer & Carlborg, 1923).

3. Method In this section, the employed methods will be presented, including some limitations regarding the work process and an evaluation of the existing data used for this study. In summary, the methodology included three main steps: field work, drill core logging, and 3D geological modelling.

3.1 Existing data The significance and importance of the Källfallsgruvan iron-oxide deposit has been explained in a geological and historical context, but those arguments are not the only reasons for picking the Riddarhyttan ore field, specifically the mineralization to study thoroughly. The availability and quality of the legacy mine maps and drill cores to log have been of significance.

The legacy mine maps used for the modelling were retrieved from the database GeoLagret, administrated by SGU. A total of 51 legacy mine maps were available for 3D geological modelling with the focus on the mineralized units: massive magnetite mineralization, semi- massive and disseminated mineralization (Fig. 3A). Terms such as skarn have been used when describing the mineralization, which have been reassessed into a more modern context to: semi- massive/disseminated mineralization.

The legacy drill cores from the Källfallsgruvan iron-oxide deposit have been stored at the Swedish national drill core archive (SGU) in Malå, Sweden. On the legacy mine maps the locations of drill cores have been indicated with corresponding drill core number (Fig. 3B). The methodology for logging is presented in section 3.2.2.

Fig. 3 Legacy mine maps from the database GeoLagret (SGU). A: Legacy mine map number 10, displaying the three mineralization types and surrounding rock around the Källfallsgruvan iron-oxide deposit (in yellow), at a depth of 49 m below ground surface level. B: Legacy mine map number 24, with solely drill cores with respective drill core number pointed out, at a depth of 134 m below ground surface level. 11

3.2 New data 3.2.1 Field Work To collect new data that could be incorporated into the 3D model, field work was carried out, with the aid of a field tablet with the application Field Move (Midland Valley Exploration Ltd.) including database functionality regarding geological and structural observations.

The work involved a study area of approximately 2 km by 2 km with the Källfallsgruvan iron- oxide deposit at its centre. The field work was carried out in a systematic manner based on an additional map provided by SGU that showed locations of outcrops, which served as a base on what outcrops would be studied inside the specified study area. The outcrops would be studied in detail with regards to rock types, mineralogy, alteration features, grain size, color and, rock texture. If possible, structural measurements were taken (foliation, lineation, etc), herein reported in dip (plunge) direction/dip (plunge) format. A total of 55 structural measurements were achieved and have been summarized in table A1 (appendix A) and all foliation and lineation measurements are plotted in stereonet for further investigation.

The samples taken from the field have been studied in detail with the main purpose to characterize them mineralogically, define rock units and, in the end, serve as a basis for a geological map generated with the help of QGIS (QGIS Association).

Depending on different factors, the degree of alteration of the samples can be decided. Gifkins et al. (2005) state that the alteration intensity (subtle–weak–moderate–strong–intense; Gifkins et al. 2005) can be defined with respect to how well preserved the original minerals and textures are, characteristics of newly formed textures with respect to alteration intensity (Gifkins et al. 2005). This approach of classifying the alteration intensity has been applied in this study.

3.2.2 Drill core logging The five drillcores picked for logging were: 105, 125, 128, 135 and, 142.

When logging the drill cores, the different geological boundaries inferred were mainly based on the mineralogy but other factors such as grain size, color and variability in the mineralogy were noted as well. The minimum thickness of the geological units to be mapped was 1 metre in order to be feasible to model the unit in 3D on a deposit scale. Due to this limitation of not being able to model centimetre scale, small local variations were not possible to model and were instead merged with the sections of 1 metre scale. Samples from each geological unit have been collected and in addition the local variations as well, and are further discussed in detail in section 4.1.2.

Not all the available drill cores in Malå have been mapped in this study and many drill cores displayed on the legacy mine maps were not accessible in Malå.

3.2.3 Modelling process The 3D modelling software used was SKUA-GOCAD 19 (Paradigm Ltd.) with the addition of the SPARSE-plugin (Mira Geosciences Ltd.) to visualize the structural measurements. The geological map was incorporated into the 3D model to both visualize and model the geological bodies surrounding the mineralization. When modelling, the ground surface level was estimated to be 175 metres above sea level (Google earth, n.d).

3.2.3.1 Preparation required before 3D modelling After downloading the legacy mine maps over Källfallsgruvan iron-oxide deposit, they had to be cropped to preferable size and coordinates had to be retrieved for georeferencing purposes. Several depth measurements could be displayed on a legacy mine map with small variations, thus an average depth had to be calculated for each legacy mine map (see table A2 in appendix A). Once all the maps were imported into the 3D modelling software, they were rotated (so that the north arrow on the legacy mine maps matched with north in GOCAD) and placed at correct depth, starting at a depth of 165 metres above sea level.

An aerial photograph (Orthophoto RGB 0.5 m latest ©Lantmäteriet) and a digital elevation model (DEM, Höjddata grid 2+ 2019 ©Lantmäteriet), provided by the Swedish University of Agricultural Sciences, SLU, were imported into the 3D modelling software and the aerial photo would then be layered over the DEM to combine both the elevation data and the aerial photograph.

The length of the drill cores could be retrieved directly in GOCAD (Length On Map, LOM) but consideration must be taken once the drill cores had a plunge, in such way that the LOM would have to be re-calculated with the respect to the plunge and the new length would be called Measured Depth, MD (Fig. 4). The correct length regarding drill cores with a plunge was calculated as presented in equation (1), while equation (2) and (3) correspond to the correct calculations for both the azimuth and plunge used in equation (1).

Fig. 4 Illustrating the concept of calculating correct depth for drill cores with a plunge ˃0°.

퐿푒푛𝑔푡ℎ 푂푛 푀푎푝 (퐿푂푀) 푀푒푎푠푢푟푒푑 퐷푒푝푡ℎ (푀퐷) = (1) 퐶표푠 (푃푙푢푛𝑔푒)

퐶표푟푟푒푐푡 푝푙푢푛푔푒 = 90° − 푃푙푢푛푔푒 푟푒푡푟𝑖푒푣푒푑 푓푟표푚 푡ℎ푒 푙푒푔푎푐푦 푚𝑖푛푒 푚푎푝 (2)

퐶표푟푟푒푐푡 푎푧𝑖푚푢푡ℎ = 퐴푧𝑖푚푢푡ℎ 푟푒푡푟𝑖푣푒푑 푓푟표푚 퐺푂퐶퐴퐷 − 180° (3)

Information regarding the modelled drill cores is presented in table A3 (in appendix A).

3.2.3.2 Geological 3D modelling Once all the legacy mine maps had been correctly imported into the modelling software it was possible to conduct geological 3D modelling. Figure 5 provides an overview of the process.

2D polygons for each different mineralized unit (massive magnetite mineralization, semi- massive/disseminated mineralization) were drawn (Fig. 5A) at the specific depth of each legacy mine map. Once all the 2D polygons from each corresponding legacy mine map were made, surfaces (Fig. 5B) for each section were created and combined at each level to achieve a uniform mineralized body. Surfaces representing 3D mineralized bodies were closed by adding horizontal surfaces at their lower and upper extent, which allowed for volume calculation for each closed rock unit with the help of GOCAD.

Additional rock units not presented on the legacy mine maps have been modelled as surfaces from the geological map with a mean dip based on the structural measurements from the field work.

In addition, the axial plane, fold axis and fold limbs have been modelled for further understanding in regards of classification of the 3D model.

A surface created in GOCAD is build up by a mesh consisting of several triangles and harsh edges could in some cases occur due to the distance between the polygon nodes being too narrow and result in triangulation issues. In many cases such issue could be solved by the tool: Beautify Triangles for Equilaterality. The tool will simply retriangulate the triangles constituting the mesh to obtain maximum number of equilateral triangles, thus improving the surface overall and hopefully remove harsh edges (W. Tsoblefack, personal communication, 2021). If the tool was of no success, the node density of the polygon of interest would be decreased, making it easier to create surface and minimize triangulation issues.

A total of 100 planar features along the modelled mineralized bodies with corresponding dip direction/dip were retrieved in a systematic manner, with the purpose of plotting the points as poles in a stereonet using the Stereonet 11 application (Allmendinger et al. 2012; Cardozo & Allmendinger, 2013).

Fig. 5 3D geological modelling methodology. A: Georeferenced legacy mine map at 140 m depth below ground surface level with 2D-polygon outlining the massive magnetite mineralization. Drill core (nr 118) cross-cuts the massive magnetite mineralization and has been outlined directly based on the georeferenced legacy mine map. B: 3D-surface created from 2D-polygons in Fig. 5A.

3.2.3.3 Adjustments of the 3D model Some consideration must be taken regarding the viability, accuracy, and subjectivity of the 3D geological model. A model never perfectly reflects reality, there will always be inaccuracies. In this section, simplifications that were necessary during the modelling process are summarized.

The shape of the ore body is not consistent throughout, it varies from either being a large singular (massive) unit to several separate units. The smaller units must be modelled separately before being connected to the massive unit to preserve geological accuracy. Connecting a

16 singular massive unit to several of these smaller units directly is not the best choice from a geological point of view, as it would lead to cross-cutting. Therefore, the polygons for the smaller mineralized units would be copied and translated up to the same depth as the larger massive unit and from that, surfaces were created.

This concept of copying a polygon and moving it to a certain depth was also applied if the connection between two 2D-polygons were too complex to create a surface from.

3.2.4 Tonnage estimation of the Källfallsgruvan iron-oxide deposit The volumes of the modelled surfaces (see section 3.2.3.2) were used to estimate the total tonnage of the Källfallsgruvan iron-oxide deposit. The disseminated mineralization is not included as it has not been observed directly and it is therefore not possible to estimate the anthophyllite and magnetite abundance.

For simplicity, the massive mineralization is estimated to consist of 80% magnetite and 20% anthophyllite, while the semi-massive mineralization has a 50/50 relation between magnetite and anthophyllite.

The equations used for calculating the densities (ρ) and estimating the tonnage (T) of the massive magnetite mineralization are shown by equation (4) to (5), and (6) to (7) for the semi- massive mineralization. The volumes (V) retrieved from GOCAD and estimated tonnages are presented in table 1 (section 4.3).

3 3 3 휌푀푎푠푠𝑖푣푒 푚푎𝑔푛푒푡𝑖푡푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 = 0.8 × 5.15 푡/푚 + 0.2 × 3.21 푡/푚 ≈ 4.76 푡/푚 (4)

3 푇푀푎푠푠𝑖푣푒 푚푎𝑔푛푒푡𝑖푡푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 = 4.76 푡/푚 × 푉푀푎푠푠𝑖푣푒 푚푎𝑔푛푒푡𝑖푡푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 (5)

3 3 3 휌푆푒푚𝑖−푚푎푠푠𝑖푣푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 = 0.5 × 5.15 푡/푚 + 0.5 × 3.21 푡/푚 ≈ 4.18 푡/푚 (6)

3 푇푆푒푚𝑖−푚푎푠푠𝑖푣푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 = 4.18 푡/푚 × 푉푆푒푚𝑖−푚푎푠푠𝑖푣푒 푚𝑖푛푒푟푎푙𝑖푧푎푡𝑖표푛 (7)

Based on the values in equations (4) to (7), an estimated average Fe grade could be calculated for the massive magnetite mineralization and semi-massive mineralization, see table 1 (section 4.3).

4. Results 4.1 Description of rock types from the Riddarhyttan ore field In this section the rock samples collected from the field work (Fig. 6) and mapped drill cores (Fig. 7 and Fig. 8) will be presented. The rocks described in section 4.1.1 and 4.1.2 are interpreted to be metamorphic products of volcanic rocks that have been altered to varying degrees.

4.1.1 Characteristics of collected rock samples from the Riddarhyttan ore field All localities mentioned in this section are presented in the geological map (Fig. 9).

The Biotite-Muscovite schist (Fig. 6A) is fine-grained, has a dark grey color, and estimated biotite and muscovite content is 50% each. The Biotite-Muscovite schist is interpreted to have been strongly altered due to no feldspar and quartz being present and has a clear foliated structure.

The Quartz-Muscovite ± Biotite schist (Fig. 6B and Fig. 6C) is fine grained and has a color ranging from light grey to dark grey, depending on the biotite content. This rock unit displays variation in the muscovite and biotite content, either being equally divided or no biotite is present at all. The estimated quartz content is 70–85% and if both muscovite and biotite occur, the estimated content between muscovite + biotite is 30–15%, and if no biotite is present, the muscovite content is 30–15%. The Quartz-Muscovite ± Biotite schist is interpreted to have been strongly altered as there is quartz present, but no feldspars and it has a foliated structure.

The Quartz-Muscovite-Biotite rock (Fig. 6D and Fig. 6E) either displays a grey color or blue grey color, depending on the quartz as the quartz either had a grey or blue color. It constitutes dominantly of quartz and the biotite + muscovite content varied, either biotite dominating over the muscovite, or the opposite. With respect to the quartz and biotite + muscovite, the content is estimated to be 70–80% quartz and 30–20% biotite + muscovite. In the Quartz-Muscovite- Biotite rock unit, stretched accretionary lapilli (Fig. B1 in appendix B) have been observed. The Quartz-Muscovite-Biotite rock is interpreted to have been strongly altered as no feldspar is present but has a high quartz content and lineation and foliation was observed.

The K-feldspar-Quartz-Muscovite-Biotite rock (Fig. 6F) has a pink color (varying in intensity depending on the content of K-feldspar) and is fine grained. The unit is dominantly consisting of K-feldspar and quartz, and the concentration of muscovite and biotite varies, neither of them being dominant over the other. The estimated content of K-feldspar + quartz is 70% while the biotite + muscovite content is 30%. The K-feldspar-Quartz-Muscovite-Biotite is interpreted to

18 have been moderately altered as all K-feldspar has not been replaced but also the presence of high quartz content, and elongated biotite grains defining a foliation. It is in this study referred to the unit least altered compared to the other observed rocks.

Locally, a unit rich in Hematite-Quartz-Garnet (Fig. 6G) is observed, it is fine-grained and has a dark grey color. An estimation of mineral content is not available due to lacking information as the unit has not been of main focus in the study.

The most intensely altered rock observed in the study area was the Anthophyllite-Biotite rock (Fig. 6H), only found proximal to the Källfallsgruvan iron-oxide deposit. The estimated content between anthophyllite and biotite is 50% each. The anthophyllite show indication of growth following the deformation as it appears as elongated grains/small needles pointing in all directions. The Anthophyllite-Biotite rock unit is interpreted to have been intensely altered mainly indicated by the dominant association with anthophyllite which is not a primary mineral and the lack of primary minerals such as quartz and feldspar minerals.

Fig. 6 Optical photograph of the collection of samples representing different rock units observed in the study area. A: Sample representing the Biotite-Muscovite schist unit, sample is taken from locality 40. B: Sample representing the Quartz-Muscovite ± Biotite schist unit but with no biotite present, sample is taken from locality 43. C: Sample representing the Quartz-Muscovite ± Biotite schist unit with both muscovite and biotite present, sample taken from locality 79. D: Sample representing the Quartz- Muscovite-Biotite rock unit, same as Fig. 6E but without blue quartz, sample taken from locality 2. E: Sample representing the Quartz-Muscovite-Biotite rock unit, same as Fig. 6C, but displaying the blue quartz variation, sample taken from locality 36. F: Sample representing K-feldspar-Quartz-Muscovite- Biotite rock unit, sample taken form locality 19. G: Sample representing the Hematite-Quartz-Garnet rock unit, sample taken from locality 30. H: Sample representing Anthophyllite-Biotite rock unit, sample taken from locality 111.

4.1.2 Characteristics of rock types proximal to the Källfallsgruvan iron-oxide deposit Based on the drill core investigation, three rock units are associated with the Källfallsgruvan iron-oxide deposit: Quartz-Chlorite-Biotite-Muscovite rock (host rock ± local variations), massive magnetite rock (massive magnetite mineralization), and massive magnetite + anthophyllite rock (semi-massive mineralization). To clarify, if not specified then mineralized rock units are here referred to as both the massive magnetite mineralization and semi-massive mineralization. Distal refers to a distance ≥ 2 metres, semi-proximal ≤ 2 metres and proximal refers to a distance ≤ 1 metre.

The Quartz-Chlorite-Biotite-Muscovite rock (Fig. 7A) consisted dominantly of quartz, chlorite, biotite, and muscovite with an approximate content of quartz + chlorite of 60–70% and 40– 30% biotite + muscovite, is interpreted to be the host rock. The biotite and muscovite content varied, but often displayed an even distribution but once moving closer to a mineralized rock unit, the muscovite content would gradually decrease while the biotite content increases, until no muscovite would be present. Once no muscovite would be present anymore, the estimated content of chlorite + quartz would still be around 60–70% but the biotite content 40–30%. In some cases, once proximal to a mineralized rock unit, small anthophyllite grains could appear in very small abundance and reach a length up to 1 centimetre.

A local variation observed is a talc rich section with biotite and muscovite (Fig. 7B), with an estimated talc content of 50% and 50% biotite + muscovite. The talc rich section has so far only been observed proximal to the semi-massive mineralization.

A variation observed was the appearance of large magnetite grains, ranging from 2 millimetres up to 1.5 centimetres with small inclusions of pyrite (Fig. 7C), of a size of approximately ≤ 1 millimetre. The large magnetite grains occurred both proximal and distal to a mineralized rock unit. Similarly, another type of variation observed was enrichment in pyrite, not situated in magnetite grains (Fig. 7D). This local pyrite enrichment did not show any correlation to being

21 proximal or distal to a mineralized rock unit as it could appear in both cases, just as the magnetite grains.

Garnet (Fig. 7E) and cordierite occur locally in small abundances, and between the two minerals, garnet is the rarer one and has only been observed distal to a mineralized rock unit. Very locally, a chlorite rich section with cordierite and small calcite veins (Cc-veins, Fig. 7F) occur. The local chlorite- and cordierite-rich section (Fig. 7G) is estimated to contain 70–80% chlorite and 20–30% cordierite. The chlorite-rich section has so far only been observed semi- proximal to a mineralized rock unit.

Fig. 7 Optical photographs of the Quartz-Chlorite-Biotite-Muscovite unit ± local variations. A: Beige quartz and biotite + muscovite, from drill core 125, box 4, at 53.5 m. B: Talc rich unit with biotite and muscovite, drill core 128, box 2A, at 23.5 m. C: Large magnetite grains with inclusions of small pyrite grains, from drill core 125, box 2, at 21.6 m. D: Pyrite rich unit, from drill core 128, box 3, at 34.5 m. E: Garnet from drill core 125, box 4, at 48 m. F: Same drill core as presented in Fig. 7G displaying calcite vein (Cc-vein) and cordierite grains, from drill core 135, box 7, at 101.5 m. G: Displaying the local chlorite + cordierite rich unit, from drill core 135, box 7, at 101.5 m.

There are two types of mineralizations constituting the deposit that have been directly observed. Firstly, there is a massive magnetite unit (massive magnetite mineralization, Fig. 8A), and another unit consisting of both magnetite and anthophyllite (semi-massive mineralization, Fig. 8B). The massive magnetite unit would dominantly consist of magnetite, up to 80% with a small abundance of other minerals such as anthophyllite and biotite. The magnetite + anthophyllite unit had varying distribution of magnetite and anthophyllite but as the variations are so small, the estimated content of the minerals is 50% each.

Fig. 8 Optical photographs of the two mineralization styles observed from the Källfallsgruvan iron- oxide deposit. A: Massive magnetite (massive magnetite mineralization), from drillcore 105, box 4, at 52.33 m. B: Massive magnetite + anthophyllite (semi-massive mineralization), from drillcore 128, box 1A, at 8-12 m.

4.2 Spatial distribution of rocks at the surface This section summarizes the result achieved from the field work (section 4.1.1) and mapping of the samples taken (Fig. 6), resulting in a geological map (Fig. 9) inside the 2 km by 2 km study area. The geological map provides an updated view of the Källfallsgruvan iron-oxide deposit and its direct surroundings. To increase the accuracy of the geological map, it does not display the whole 2 km by 2 km study area (all observation points are therefore not mentioned on the geological map, see Fig. B2 in appendix B for all observation points).

From the resulting geological map, it can be observed that the K-feldspar-Quartz-Muscovite- Biotite rock unit is furthest away from the Källfallsgruvan iron-oxide deposit as it is least altered compared to the other rocks. In an easterly direction the extension of the K-feldspar-Quartz- Muscovite-Biotite rock becomes more uncertain due to the lack of observation points.

The alteration intensity increases once moving closer to the mineralization as K-feldspar is no longer present, instead a rock rich in quartz, muscovite and biotite dominates (Quartz- Muscovite-Biotite rock unit), hosting the Källfallsgruvan iron-oxide deposit. Highest degree of alteration intensity is represented by the Anthophyllite-Biotite rock unit, but it has to be noted that the rock unit doesn’t represent the host rock for the Källfallsgruvan iron-oxide deposit.

To the west of the Quartz-Muscovite-Biotite rock unit, the Quartz-Muscovite ± Biotite schist unit becomes dominant. This unit has been observed NE of the mineralization as well but due to the lack of outcrops and presence of infrastructure at the middle part of the Quartz-Muscovite ± Biotite schist unit it displays uncertainty and lack of detail.

Small local variations in both the Quartz-Muscovite-Biotite rock and Quartz-Muscovite ± Biotite schist occur with the occurrence of the Biotite-Muscovite schist, but show no relation to the Källfallsgruvan iron-oxide deposit.

The structural form line (vertical S-shaped folding) is an extrapolation of the shape of the mineralization (Fig. 10A) but also based on the S-shaped fold proximal to the mineralization (Fig. 9 and Fig. 11) and the hypothetical continuation in an NE–SW direction indicated by a magnetic anomaly (Fig. B3 in appendix B). This is the reason for the three main units (K- feldspar-Quartz-Muscovite-Biotite rock, Quartz-Muscovite ± Biotite schist and Quartz- Muscovite-Biotite rock) displaying a similar pattern.

The rock units that have been modelled from the geological map are the K-feldspar-Quartz- Muscovite-Biotite rock unit, Quartz-Muscovite-Biotite rock unit, Quartz-Muscovite ± Biotite schist and the Anthophyllite-Biotite rock unit (see section 3.2.3.2). 25

Fig. 9 Geological map inside the 2 km by 2 km study area in the Riddarhyttan ore field. For clarification, the dotted lines drawn for the Anthophyllite-Biotite rock and Fe-oxide mineralization indicate that the unit continues under a water body. See also Fig. 2 for a semi-regional overview. 4.3 3D model of the Källfallsgruvan iron-oxide deposit In this section a general overview of the 3D geological model (Fig. 10) is provided, further in- depth information of the Källfallsgruvan iron-oxide deposit is provided in section 4.4.

The 3D model constitutes of two units, massive magnetite mineralization and semi- massive/disseminated mineralization which have been described in detail in section 4.1.2 (Fig. 8). The mineralization extends to a depth of approximately 300 metres below ground surface level and dips steeply towards the southwest. The overall ore body geometry mimics an S- shaped synform (Fig. 10A-C). The disseminated mineralization has not been observed directly in the drill core logging and is therefore solely based on the observed magnetite + anthophyllite unit (semi-massive mineralization) and is in this case classified as a unit very rich in anthophyllite and little magnetite.

From the logged drill cores, it is interpreted that the Källfallsgruvan iron-oxide deposit is hosted by the Quartz-Chlorite-Biotite-Muscovite rock (± local variations, Fig. 7) which is positively supported by the Quartz-Muscovite-Biotite rock on the geological map (Fig. 9). The drillcores in Fig. 10B contain three main units: massive magnetite mineralization, semi-massive mineralization, and Quartz-Chlorite-Biotite-Muscovite rock (host rock ± local variations), which have been explained in detail in section 4.1.2.

Figure 10C shows the modelled fold limbs and axial plane of the fold controlling the ore body geometries which are used to further classify the characteristics of the fold and described in more detail in section 4.4.

Calculated volumes and tonnages for the massive magnetite and semi massive mineralization are presented in table 1.

Fig. 10 3D geological model of the Källfallsgruvan iron-oxide deposit. The modelled units from the geological map (see section 3.2.3.2 and section 4.2) are not presented as the focus is on the Källfallsgruvan iron-oxide deposit. A: View from above. B: View from a westerly direction with drill cores (not all drill cores are presented). C: View from a westerly direction with modelled fold limbs and axial plane. The modelled fold axis is not presented on the image.

Table 1: Estimated ore tonnage and Fe grade from calculated volumes of the modelled surfaces. Mineralization Total volume from Estimated tonnage Estimated average GOCAD [m3] [metric tons] Fe grade [wt%] Massive magnetite 905754 4311389 63 Semi-massive 150053 627221 45 Total: 4938610

4.4 Structural observations S-shaped asymmetric folding proximal to the Källfallsgruvan iron-oxide deposit has been observed on outcrop scale (Fig. 11), approximately 160 metres from the mineralization. The Quartz-Muscovite-Biotite rock has been folded with a vertical/sub-vertical axial plane striking NE–SW and an interlimb angle of 72º. An interlimb angle over 70° is classified as an open fold (Fossen, 2010) but the value of 72° is still close to a tight fold and can therefore be classified as a tight-open fold.

Fig. 11 Optical photograph (top view) displaying S-shaped folding of the Quartz-Muscovite-Biotite rock proximal to the Källfallsgruvan iron-oxide deposit, locality 123. Dashed lines are outlining the fold. A local part in the Quartz-Muscovite ± Biotite schist unit displayed evidence of shearing (Fig. 12). The shearing is represented by sigmoidal fragments that are dextrally sheared in an NW/NNW–SE/SSE direction and could reach a length up to 3–7 centimetres.

Fig. 12 Optical photograph (horizontal view towards W) of dextral shearing in a NW/NNW direction, locality 45. Dashed lines outlining the shape of the sigmoidal fragments. For further analysis, both structural measurements and planar features (retrieved from the 3D geological model, see section 3.2.3.2) have been plotted as poles (Fig. 13). In Fig. 13A the fold limbs and axial plane represents the ones that have been modelled in GOCAD (Fig. 10C).

The poles of the planar features retrieved from the 3D geological model (Fig. 13A) display an uneven distribution with two indicative clusters, representing the two modelled fold limbs (fold limb 1 and fold limb 2) of the Källfallsgruvan iron-oxide deposit with a strike and dip of 223/79 and 091/55. The interlimb angle was calculated to be 58° thus representing a tight fold and to a degree, similar to the interlimb angle of the S-shaped fold observed proximal to the mineralization (Fig. 11). The uneven distribution of the poles further indicates a rounded shape of the fold hinge. The modelled axial plane is striking towards NE–SW and steeply dipping 82° towards SE. By plotting a best-fit great circle and its corresponding pole, a plunge direction and plunge of 227/54 for the fold axis can be calculated, correlating to the modelled fold axis which had a plunging direction and plunge of 223/60.

The foliations measured from outcrops have been plotted as poles and the lineation measurements as points (Fig. 13B). The lineation measurements are steeply dipping towards SSW/SW, similarly as the fold axis in Fig. 13A. The plotted poles of the foliations shows varying distribution, possibly indicating a semi-regional fold. Furthermore, the pole of the best- fit great circle could then give an estimation of the fold axis of the semi-regional fold and has a plunge direction and plunge of 245/56 which correlates with the fold axis of the Källfallsgruvan iron-oxide deposit (Fig. 13A).

Fig. 13 Lower-hemisphere, equal-area stereonet projections. To emphasize the distribution density of the poles, kamb contours have been applied directly from the Stereonet application. It is a type of contour generated by first assuming that the density of poles are normally distributed (Cardozo & Allmendinger, 2013). The contours are then calculated based on how many standards deviations (sigma, σ) the density of points within them are located from the mean of a normally distributed density (Cardozo

& Allmendinger, 2013). A: Stereonet with 100 planar measurements (including kamb contour) from the 3D model, plotted as poles, and characteristics of the fold structure controlling the Källfallsgruvan iron- oxide deposit. The fold limbs and axial plane represent the ones that have been modelled in SKUA- GOCAD 19. B: Stereonet with all field foliation (plotted as poles) and lineation measurements, possibly indicating the presence of a semi-regional fold. Fleuty (1964) constructed a classification diagram for further interpretation and understanding of folds, which has later been modified by Fossen (2010). Similarly, Ramsay (1967) inferred the concept of interference patterns, which has later been modified by Fossen (2010). The modelled fold axis and axial plane (Fig. 13A) have been used to plot the fold geometrically controlling the Källfallsgruvan iron-oxide deposit and is presented in Fig. 14A. The dip of the axial plane is 82° and the plunge of the fold axis is 60° (plunge of fold axis is retrieved from the 3D model) which plots the geometrically controlling fold in the area for a steeply inclined- upright, moderately-steeply plunging fold (Fig. 14A).

Furthermore, the fold controlling the subsurface geometry of the Källfallsgruvan iron-oxide deposit resembles the result of an interference pattern of type 1 (Fig. 14B). The process shown in Fig. 14B is related to polyphase folding and was chosen to illustrate a more simplified way of creating such interference pattern, but this refolded structure can arise from progressive shearing and deformation from a single event as well (Fossen, 2010; Carreras & Druguet, 2019). The concept of polyphase folding and progressive shearing and deformation from a single event will further be discussed in section 5.1 and how it relates to the Källfallsgruvan iron-oxide deposit.

Fig. 14 Structural characteristics of the fold controlling the Källfallsgruvan iron-oxide deposit based on the dip orientation of axial plane and dip of hinge line. A: Classification of the fold structure controlling the Källfallsgruvan iron-oxide deposit with regards to the dip of the axial plane (82°) and plunge of the fold axis (60°) based on Fossen (2010). For clarification, the plunge of the fold axis has been plotted and not the plunge of the hinge line as the fold axis is a representative average. B: Schematic evolution of type 1 fold interference pattern used to describe the fold, whereas D1 represents the axial surface trace in the first deformation event, and D2 for the second deformation event and finally the result after the two deformation events (D1 + D2). The orientation of the axial planes have no specified direction in this case. Modified after Fossen (2010).

5. Discussion 5.1 Geometrical control of the Källfallsgruvan iron-oxide deposit Geijer & Carlborg (1923) stated that the geology of the Riddarhyttan ore field is characterized by intense folding. In addition, the Riddarhyttan ore field and the Källfallsgruvan iron-oxide deposit have been studied in detail with respect to mineral association and geometrical characteristics which have revealed evidence of hydrothermal alteration, metamorphism and folding (Geijer & Carlborg, 1923; Geijer & Magnusson, 1944; Beunk & Kuipers, 2012; Trädgårdh, 1988). These earlier studies with respect to folding positively agree with the results of this study with respect to geometrical controls on the Källfallsgruvan iron-oxide deposit which will be discussed in this section.

Firstly, it can be emphasized from the field work and modelling results that the Källfallsgruvan iron-oxide deposit is folded by a large scale F2 fold controlling the overall geometry of the mineralized ore body. Many factors point to the F2 fold classification being valid, as D2 has had a large influence on the Bergslagen central structural domain (Stephens & Jansson, 2020;

Stephens et al., 2009). F2 in this structural domain generally had an axial surface orientation to NE–SW (Stephens et al. 2009) which directly correlates to the Källfallsgruvan iron-oxide deposit and so does the steep dip (Fig. 10A-C). This is also supported by Fig. 13B because the foliations that have been measured are tectonic foliation (as minerals have been oriented in a specific direction), indicating that the generation is at least of S1 and is from this study interpreted to be folded by a hypothetical semi-regional fold serving as an explanation to the varying distribution in the orientation. The local F2 fold controlling the Källfallsgruvan iron- oxide deposit further supports that the semi-regional fold (Fig. 13B) could be of F2 and thus supporting the interpreted S1 foliation being folded by a semi-regional F2 fold. Because if the

F2 classification of the fold geometrically controlling the Källfallsgruvan iron-oxide deposit is correct, then the similar trend of the semi-regional fold implies that it is of the same generation. In addition, it is possible that the Källfallsgruvan iron-oxide deposit is an S-parasitic fold to the semi-regional fold as a result of progressive shearing from a single event (Fossen, 2010; Carreras & Druguet, 2019).

Figure 14B showed the formation of interference pattern of type 1, emphasized from polyphase folding but uncertainty in such statement occurs due to the lack of evidence of earlier F1 folds from this study. In addition, it is possible for complex refolding structures such as interference pattern of type 1 to occur from a single progressive deformation and shearing event (Fossen, 2010; Carreras & Druguet, 2019). Progressive shearing often occurs in high-strain shear zones

34 and results in a constant development of folds that eventually become parallel to the shear plane and as shearing continues, new folds (S- & Z-shaped) will develop in the fold limbs of the older folds (Fossen, 2010; Carreras & Druguet, 2019).

The hypothetical semi-regional F2 fold (Fig. 13B) and the S-shaped mineralization (Fig. 10A) could have been the result of progressive deformation and shearing solely related to D 2 but how such folding process relates to the decreasing volume of the mineralization in a downward direction is not well understood. Furthermore, to validate this statement, evidence of shearing is needed. From this study, a dextral shearing in an NW/NNW–SE/SSE direction has been observed (Fig. 12) but the dextral shearing in the concept of progressive shearing is most likely not related because once shearing continues, linear features such as lineations and fold axes will rotate towards parallelism with the shearing direction (Fossen, 2010). In this study the plunging direction of the fold axis of the Källfallsgruvan iron-oxide deposit and lineations are towards the SW thus not correlating to the dextral shearing direction observed. Still, progressive shearing is favourable as the Riddarhyttan ore field is located in a high-strain shear belt dominated by sinistral movement in an SW–NE direction (Beunk & Kuipers, 2012) denoted by Beunk & Kuipers (2012) as the West Bergslagen Boundary Zone, WBBZ. The sinistral shearing direction in the WBBZ (Beunk & Kuipers, 2012) correlates with the plunging direction of fold axis and lineation measurements observed from this study and could additionally also explain why the Källfallsgruvan iron-oxide deposit displays an S-shaped form.

5.2 Deformation in a regional context Multiple shifts between transtensional and transpressional tectonics in the Bergslagen lithotectonic unit (Stephens et al. 2009; Beunk and Kuipers, 2012) resulted in a ductile polyphase deformation (D1 and D2; Stephens et al. 2009) involving folding (F1 and F2; Stephens et al. 2009), and metamorphism (M1 and M2; Stephens et al. 2009). A metamorphic cycle with both the prograde and retrograde phase in the Riddarhyttan ore field has been distinguished by Trädgårdh (1988). The mineral association to respective metamorphic phase mentioned by Trädgårdh (1988) have been described in section 2.2. In a regional context, the study of the Källfallsgruvan iron-oxide deposit shows implications of supporting the existence of a transpressional tectonic regime.

This is supported by the fact that the Källfallsgruvan iron-oxide deposit is geometrically controlled by a F2 fold, either indicating progressive shearing (Fossen, 2010; Carreras &

Druguet, 2019) solely related to D2 or the presence of D1 and D2 and previously been a F1 fold that was refolded during D2 (polyphase folding). For such statement of polyphase folding

35 related to D1 and D2 to be valid, evidence of F1 folding is of importance. However, from this study F1 folding has not been observed in the field but its presence cannot be excluded.

Furthermore, whether the structural measurements are of S1, S2 or both has not been clearly established. To get a better understanding of this, as mentioned earlier the foliation measurements were plotted in a stereonet (Fig. 13B) which led to the discovery of a hypothetical semi-regional fold interpreted to be a F2 fold. This serves as an explanation of the varying orientation of the foliation measurements and therefore interpreted to be S1 foliations that have been affected by the F2 fold and thus re-oriented. This means that the S1 foliation is favourable but that the existence of a S2 foliation cannot be ruled out. Either way, the observed steeply plunging/dipping stretching lineations, foliations and evidence of shearing also argue for a transpressional tectonic regime. In addition, the stretching lineations observed have plunged moderately-steeply indicating that there have been both ductile strike-slip and dip-slip in relation to the shearing and are related to the second deformation event (D2) and are interpreted to be lineation of the second generation (L2).

Evidence of metamorphism has also been achieved from this study, indicated for example by the occurrence of anthophyllite both in the field and drill cores (cf. Trädgårdh, 1 988). The statement is valid as anthophyllite is a product from either medium- or high-grade metamorphism (Anthony et al. 1990-2003) which correlates with the peak metamorphism of amphibolite facies in the Riddarhyttan ore field stated by Trädgårdh (1988). Furthermore, the anthophyllite grew radially in all directions (Fig. 8B) indicating growth after deformation and is therefore most likely related to the second metamorphic event (M2).

5.3 Calculated tonnage of the Källfallsgruvan iron-oxide deposit It has been possible to calculate the volumes of the modelled surfaces for the massive magnetite mineralization and the semi-massive mineralization and from that calculate an estimated tonnage for the Källfallsgruvan iron-oxide deposit.

Geijer & Carlborg (1923) carefully estimated a total tonnage of the Källfallsgruvan iron-oxide deposit to 4 446 000 tons, which is close to the estimated tonnage based on this study which was 4 938 610 tons. Still, care must be taken as it is not known exactly how Geijer & Carlborg (1923) estimated the values used in their calculations, thus uncertainty exists.

As mentioned earlier, the Källfallsgruvan iron-oxide deposit has once been considered to be one of the largest iron producing mines in Sweden. It is therefore of interest to compare the estimated tonnage of the Källfallsgruvan iron-oxide deposit to another iron-oxide deposit in the

Bergslagen ore province. For example, an evaluation of the Dannemora mine by Lindholm et al. (2011) showed that the measured resources were approximately 19 745 000 tons, four times greater than the estimated tonnage of the Källfallsgruvan iron-oxide deposit. From a tonnage perspective, this implies that the Källfallsgruvan iron-oxide deposit would presently not be economically viable or close to be considered as one of Sweden’s largest iron-oxide mines. However, it may well be that REEs are present in the anthophyllite increasing the economic value, thus if a similar deposit of the Källfallsgruvan iron-oxide deposit is discovered with association to REEs, such deposit could possibly be of economic value.

5.4 Implications for mineral exploration In order to explore for similar deposits as the Källfallsgruvan iron-oxide deposit, there are some possible indicators that have been established from this study. Firstly, there exist a high magnetic anomaly (Fig. B3 in appendix B) in a NE–SW direction from the Källfallsgruvan iron-oxide deposit, possibly indicating a continuation of the mineralized horizon. In addition, the Persgruvan which is located NE of the Källfallsgruvan iron-oxide deposit, supporting the possible continuation of the mineralized horizon.

In the field, other implications exist that can be used in order to determine if the mineralization continues. It has been argued by several authors that the Riddarhyttan ore field has been strongly affected by magnesium alteration (Stephens et al. 2009; Trädgårdh, 1988). From this study it is clear that this type of mineralization occurs in highly altered rocks, making intense magnesium alteration a good indicator. Once moving closer to the mineralization, the biotite content increases, and once proximal to the mineralization both biotite and anthophyllite coexisted in a high concentration. Increasing biotite content in host rocks (cf. Geijer & Carlborg, 1923) could therefore be used as a vector towards mineralization but if biotite and anthophyllite coexist together, then the possibility of being in the vicinity of a mineralization is high. The presence of shearing could also be a possible indication as the ore field is located in high-strain shear belt (Beunk & Kuipers, 2012).

Furthermore, parasitic folding has proven to be a characteristic structural feature of the Riddarhyttan ore field as it has been observed on a local-, deposit- and possibly semi-regional scale (Fig. 9, Fig. 10A, Fig. 11, and Fig. 13B). For this reason, parasitic folding could also be an indicator for Källfalls-like deposits as it affects the altered and mineralized rocks at the Källfallsgruvan iron-oxide deposit, possibly due to competency contrasts between the rock types.

The indicators mentioned so far are established for a local scale but can be applied on a semi- regional scale and, possibly a regional scale as well.

5.5 Future work The results achieved from this thesis are a step forward in regards of information that can be achieved from 3D geological modelling and structural controls in the Riddarhyttan ore field with respect to the Källfallsgruvan iron-oxide deposit. However, in order to develop our geological understanding of this deposit to advance exploration strategies, and to clearly establish the influence of D1 and D2 or solely D2, further work needs to be performed.

Logging of the remaining drill cores at the drill core archive in Malå is important since it will lead to more data being incorporated into the 3D geological model, increasing the level of detail and accuracy. In addition, chemical analyses of the different rock types should be done to achieve more data of the composition and to evaluate the potential for REE mineralization.

Some characteristics were possible to define from this study such as geometrical controls, but uncertainties still exist of which some may be addressed by further work in the future. From a geological point of view, it is important to increase the accuracy and detail of the geological map and get a better understanding of the extension of the different geological units. In addition, more field work would result in an even more detailed overview of the Riddarhyttan ore field not only with respect to the Källfallsgruvan iron-oxide deposit, and possibly give a better understanding of the inferred mineralized horizon and broader view of the geometrical controls in the Riddarhyttan ore field. The nature and role of shear zones should be investigated in more detail to possibly get a better understanding on the observed dextral shearing (Fig. 12) or how the Riddarhyttan ore field relates to the shearing related to the WBBZ (Beunk & Kuipers, 2012).

Further understanding of S1 and S2 is of importance, which is something that can be aided by more field work. In addition, it could lead to discovery of F1 folds which could strengthen the concept of polyphase folding. More structural measurements could also give a better understanding of the proposed semi-regional scale fold and its relation to the Källfallsgruvan iron-oxide deposit.

Related to the understanding of the foliation generations and shearing would be to study samples in thin section. Thin sections could possibly ease the interpretation of S1 and S2, but also the shearing. Thin sections could also give more detailed information about the mineralogical composition of the rock samples from the field.

6. Conclusions Based upon the presented integration of geological field observations with subsurface data and 3D geological modelling of the Källfallsgruvan iron-oxide deposit it is concluded that:

• The Källfallsgruvan iron-oxide deposit is geometrically controlled by an F2 fold that is steeply inclined-upright, moderately-steeply plunging towards the southwest with an axial plane striking northeast-southwest, possibly displays an interference pattern of type 1 and results from this study favours progressive shearing and deformation from a

single event, related to D2.

• Evidence of at least one generation of transpressional tectonic regime exists which from

this study has been interpreted to be D2.

• The Fe ore tonnage of the massive magnetite mineralization (estimated average Fe grade of 63 wt%) and semi-massive mineralization (estimated average Fe grade of 45 wt%) was estimated to a total of 4 938 610 tons. A deposit of such tonnage is presently not economically viable, however if the Källfallsgruvan iron-oxide deposit or similar is determined to have a significant REE content, such deposit may then be of economic interest.

• Possible mineral exploration indicators around Källfalls-like deposits are intensely altered rocks related to magnesium alteration, consisting dominantly of quartz, biotite (increasing towards mineralization), muscovite, chlorite, anthophyllite and cordierite, affected by parasitic folding recognisable e.g., as magnetic anomalies. These possible indicators can be applied on a local-, semi-regional- and possibly regional scale.

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Ramsay, J.G. (1967). Folding and fracturing of rocks. New York: McGraw-Hill. Royer, J.J., Mejia, P., Caumon, G., & Collon, P. (2015) 3D and 4D Geomodelling Applied to Mineral Resources Exploration—An Introduction. In: Weihed P. (2015) 3D, 4D and Predictive Modelling of Major Mineral Belts in Europe. Mineral Resource Reviews. Cham: Springer, p. 73–89.

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Stephens, M. B. & Jansson, N. F. (2020). Paleoproterozoic (1.9–1.8 Ga) syn-orogenic magmatism, sedimentation and mineralization in the Bergslagen lithotectonic unit, Svecokarelian orogen, Sweden: Lithotectonic Framework, Tectonic Evolution and Mineral Resources, Stephens, M. B. & Weihed, J.B. The Geological Society, Vol. 50, p. 155–206. Stephens, M.B., Ripa, M., Lundström, I., Persson, L., Bergman, T., Ahl, M., Wahlgren, C.- H., Persson, P.-H. & Wickström, L. (2009). Synthesis of the bedrock geology in the Bergslagen region, Fennoscandian Shield, southcentral Sweden. Geological survey of Sweden Ba 58, p. 9– 259.

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8. Appendix 8.1 Appendix A – Tables Table A1: Structural measurements from the field.

Coordinates [X, Y] Locality Type of tectonic Dip/plunge) Deformation structure direction/Dip phase (plunge) 529936, 6632145 2 Foliation 225/85 D1 529936, 6632145 2 Lineation 201/69 D2 529861, 6631743 13 Foliation 125/84 D1 529831, 6631798 14 Foliation 191/60 D1 529893, 6631735 17 Foliation 242/70 D1 529956, 6631807 20 Foliation 123/85 D1 529952, 6631821 21 Foliation 111/80 D1 529893, 6631889 22 Foliation 151/65 D1 529925, 6631935 23 Foliation 121/85 D1 529951, 6632164 27 Foliation 148/75 D1 529951, 6632164 27 Lineation 191/68 D2 529951, 6632134 29 Foliation 193/59 D1 529975, 6632087 32 Foliation 185/85 D1 530220, 6632607 39 Foliation 264/73 D1 530273, 6632645 40 Foliation 264/66 D1 528425, 6632225 42 Foliation 213/82 D1 528412, 6632245 43 Foliation 221/60 D1 528397, 6632245 44 Foliation 234/70 D1 528389, 6632198 45 Foliation 220/50 D1 528389, 6632198 45 Lineation 239/60 D2 528358, 6632258 56 Foliation 191/52 D1 528350, 6632323 57 Foliation 240/60 D1 528324, 6632324 58 Foliation 191/45 D1 528337, 6632348 59 Foliation 279/65 D1 528307, 6632360 60 Foliation 251/60 D1 528358, 6632520 61 Foliation 200/60 D1 530856, 6633017 64 Foliation 314/80 D1 530870, 6633019 65 Foliation 335/75 D1 530906, 6633074 68 Foliation 284/60 D1 530889, 6633103 69 Foliation 295/50 D1 530848, 6633147 70 Foliation 340/80 D1 530909, 6633203 71 Foliation 317/68 D1 530956, 6633135 72 Foliation 311/82 D1 530989, 6633078 73 Foliation 290/60 D1 531049, 6633110 74 Foliation 323/65 D1 531022, 6633172 75 Foliation 319/65 D1 531005, 6633225 76 Foliation 305/65 D1 530979, 6633237 77 Foliation 323/80 D1 530903, 6633293 78 Foliation 303/62 D1 42

530582, 6633033 79 Foliation 324/70 D1 530611, 6632960 81 Fold axis 243/40 D2 530611, 6632960 81 Foliation 295/55 D1 530592, 6632929 82 Foliation 307/68 D1 530406, 6632782 84 Foliation 302/70 D1 530552, 6633267 85 Foliation 316/68 D1 530582, 6633425 87 Foliation 307/62 D1 530111, 6634059 89 Foliation 337/85 D1 530319, 6634169 90 Foliation 330/75 D1 530349, 6634085 91 Foliation 320/60 D1 530402, 6634053 92 Foliation 320/70 D1 530498, 6633791 95 Foliation 315/45 D1 529732, 6630621 108 Foliation 265/45 D1 529777, 6630568 109 Foliation 260/55 D1 530126, 6632401 118 Foliation 315/85 D1 530319, 6632480 120 Foliation 153/82 D1

Table A2: Depth below ground surface for each legacy mine map used in the modelling process.

Legacy mine map Depth below ground surface [m] 1 10 2 14 3 17 4 21 5 32 6 34 7 39 8 43 9 46 10 49 11 53 12 58 13 62 14 70 15 80 16 91 17 99 18 105 19 112 20 116 21 125 22 127 23 134 24 134 25 140 26 145 27 151

28 158 29 165 30 170 31 178 32 183 33 188 34 193 35 200 36 204 37 207 38 211 39 216 40 227 41 233 42 240 43 250 44 258 45 265 46 271 47 277 48 280 49 283 50 290 51 299

Table A3: Information regarding the imported drill cores in SKUA-GOCAD 19. ID Coordinates Depth Azimuth Plunge Measured Mapped [number] [X,Y] below Depth [m] in Malå ground [Yes/No] surface [m] 128 529160, 6631770 299 111 63 90 Yes 125 529160, 6631770 299 353 57 59 Yes 105 529221, 6631710 170 167 0 73 Yes 135 529381, 6632010 134 94 0 171 Yes 142 529218, 6631690 216 234 0 83 Yes 1 529391, 6631890 134 164 6 42 No 1 529229, 6631760 216 336 55 111 No 38 529176, 6631770 299 334 0 40 No 48 529222, 6631920 151 326 0 43 No 49 529224, 6631920 151 002 0 36 No 50 529245, 6631950 151 330 0 36 No 51 529202, 6631920 151 332 0 38 No 54 529214, 6631740 258 202 0 77 No 102 529248, 6631700 170 091 0 26 No 103 529245, 6631710 170 038 0 25 No 104 529217, 6631720 170 065 0 25 No

117 529255, 6631730 134 198 0 36 No 118 529261, 6631750 134 174 0 70 No 120 529245, 6631730 134 254 0 17 No 131 529236, 6631690 134 154 0 65 No 136 529335, 6631960 134 104 0 149 No 138 529335, 6631960 134 130 0 145 No 140 529218, 6631690 216 212 0 64 No 143 529190, 6631740 299 139 0 39 No 144 529190, 6631740 299 150 0 100 No 160 529415, 6631990 134 129 0 121 No 161 529524, 6631980 134 129 0 64 No 198 529469, 6631880 134 284 0 49 No Unknown 529370, 6631950 134 028 5 140.5 No Unknown 529429, 6631890 134 354 5 170.6 No 124 529160, 6631770 299 046 58 47 No data 1 529233, 6631870 170 314 0 17 No data 1 529159, 6631760 258 354 52 68 No data 2 529243, 6631840 170 308 0 29 No data 2 529184, 6631760 258 355 52 68 No data 121 529136, 6631740 299 359 56 127 No data

8.2 Appendix B – Images

Fig. B1 Optical photograph (horizontal view towards E) of stretched accretionary lapilli with plunging direction and plunge of 191/68, locality 27.

Fig. B2 Image representing all observation points from the field work in the Riddarhyttan ore field (localities) as red diamonds.

Fig. B3 Magnetic anomaly map over the Riddarhyttan area. The total magnetic field intensity map has been reduced to the geomagnetic north pole and a regional field has been subtracted (Provided by SGU).