Magnetic Signature of the Leucogranite in Örsviken

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

Magnetic signature of the

leucogranite in Örsviken

Hannah Berg Johanna Engelbrektsson

ISSN 1400-3821 B774 Bachelor of Science thesis Göteborg 2014

Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract A proton magnetometer is a useful tool in detecting magnetic anomalies that originate from sources at varying depths within the Earth’s crust. This makes magnetic investigations a good way to gather 3D geological information. A field investigation of a part of a cape that consists of a leucogranite in Örsviken, 20 kilometres south of Gothenburg, was of interest after high susceptibility values had been discovered in the area. The investigation was carried out with a proton magnetometer and a hand-held susceptibility meter in order to obtain the magnetic anomalies and susceptibility values. High magnetic anomalies were observed on the southern part of the cape and further south and west below the water surface. The data collected were then processed in Surfer11® and in Encom ModelVision 11.00 in order to make 2D and 3D magnetometric models of the total magnetic field in the study area as well as visualizing the geometry and extent of the rock body of interest. The results from the investigation and modelling indicate that the leucogranite extends south and west of the cape below the water surface. Magnetite is interpreted to be the cause of the high susceptibility values. The leucogranite is a possible A-type alkali granite with an anorogenic or a post-orogenic petrogenesis. The leucogranite shows quite similar chemical patterns to the Askim- and Kärra which may indicate that the leucogranite is associated with them. Further investigations in the study area are needed to better understand the connections between magnetic signatures and geology, which in turn can help in the interpretation process.

Keywords: Proton magnetometer, leucogranite, susceptibility, magnetic anomalies, Örsviken, magnetite, Surfer11®, Encom ModelVision 11.00.

Sammanfattning En protonmagnetometer är ett användbart verktyg för att upptäcka magnetiska anomalier som härrör från olika djup i jordskorpan. Detta gör att magnetiska undersökningar till ett bra sätt att samla geologisk 3D-information. Det var av intresse att göra en fältundersökning av en del av en udde bestående av leucogranite i Örsviken, 20 kilometer söder om Göteborg, efter att höga susceptibilitetsvärden noterats. De magnetiska anomalierna och susceptibilitetsvärdena undersöktes med hjälp av en protonmagnetometer och susceptibilitetsmätare. Höga magnetiska anomalier observerades över de södra delarna av udden samt fortsatt söder- och västerut i vattnet. Insamlad data bearbetades sedan i Surfer11® och Encom ModelVision 11.00 för att skapa 2D- och 3D- magnetiska modeller av det totala jordmagnetfältet i undersökningsområdet samt att visualisera geometrin och utsträckningen för den intressanta bergarten. Resultaten från undersökningen och modelleringen indikerar att leucogranitens utsträckning fortsätter söder- och västerut i vattnet. Magnetit är troligen orsaken till de höga susceptibilitetsvärdena som uppmätts. Leucograniten är tolkad som en A-typ alkali granit med ett anorogent eller ett post-orogent ursprung. Leucograniten har en likartad kemi som Askims- och Kärra graniten vilket kan innebära att leucograniten är associerade med dem. Ytterligare undersökningar i området är av intresse för att få en bättre förståelse mellan de magnetiska signaturerna och geologin, som i sin tur kan underlätta tolkningsprocessen.

Nyckelord: Protonmagnetometer, leucogranite, susceptibilitet, magnetiska anomalier, Örsviken, magnetit, Surfer11®, Encom ModelVision 11.00.

Contents Abstract ...... I Sammanfattning...... II 1. Introduction ...... 1 2. Background ...... 1 2.1 Geology ...... 1 2.2 Area description ...... 2 2.3 Magnetic surveying and rock magnetism ...... 3 2.4 Proton magnetometer ...... 3 3. Method ...... 4 3.1 Working hypothesis ...... 4 3.2 Preparations ...... 4 3.3 Field method ...... 4 3.4 Modelling ...... 4 4. Results ...... 5 4.1 Bedrock ...... 5 4.1.1 The granodiorite ...... 5 4.1.2 The leucogranite ...... 5 4.2 Magnetic profiles ...... 8 4.2.1 Surfer ...... 8 4.2.2 ModelVision ...... 8 5. Discussion ...... 12 6. Conclusion ...... 13 7. Acknowledgements ...... 13 8. References ...... 14 9. Appendix ...... 15

1. Introduction 2. Background During a minor field trip it was discovered that a part of a cape in Örsviken, 20 kilometres 2.1 Geology south of Gothenburg on the county border The bedrock in the Southwest Scandinavian between Halland and Västra Götaland, had higher Domain (SSD) was formed 1.8 – 0.9 Ga ago. It susceptibility values than its surroundings (Figure consists mainly of gneiss, mostly orthogneiss, 1) (E.Sturkell, personal communication, 21 which is partly veined. Parts of the SSD consist January 2013). The aim of this study is to define of rocks with sedimentary and volcanic origin the extent of that part of the cape with high (Sveriges National Atlas, 1998). susceptibility through modelling its appearance and to discuss what causes its high susceptibility The SSD is limited to its east by the Protogine values. Also, if possible to classify and interpret Zone (Figure 2) (Lundqvist, Lundqvist, the origin of the cape. Lindström, Calner, & Sivhed, 2011). The Protogine zone runs in a north–south direction To obtain the necessary information in the from Värmland to Skåne and is a deformation assigned area, a total of four profiles of and/or weakness zone. The southwest approximately 300 meters were measured using a Scandinavian province is divided in two parts, an proton magnetometer. Other data collected were east and a west segment, by the Mylonite zone. susceptibility field values and mapping of the Both the Western Segment and the Eastern bedrock. A proton magnetometer is a useful tool Segment have been deformed, in different in detecting magnetic anomalies that originates degree, by the Gothian orogeny (1.66 – 1.5 Ga) from sources at varying depths within the Earth’s and the Sveconorwegian orogeny (about 1000 crust. This makes magnetic investigations a good Ma). The Eastern Segment has been more way to gather 3D geological information. deformed and displays migmatised gneisses. West of the Mylonite zone the bedrock is less Results from “Petrological description of the deformed and veined, and consists mainly of leucogranite in Örsviken” (Engelbrektsson & gneiss and supracrustal rocks. A further shear Berg, 2013) will be taken into consideration. zone, the Göta Älv zone, cuts the Western Segment in a north-south direction from south of Gothenburg to lake Värnen (Hegardt, 2010).

Figure 1. Overview of Örsviken and the study area.

Figure 2. Major geological features of the SSD. Map modified from (Lundqvist, Lundqvist, Lindström, Calner, & Sivhed, 2011).

2.2 Area description red-greyish granite with K-feldspar augen and is The study area and its surrounding are located medium to coarse-grained. The mafic intrusions in the Western Segment north of the Mylonite associated with the granite are interpreted to be zone. The Geological Survey of Sweden (SGU) co-magmatic (Hegardt et al., 2007). East and has interpreted one part of the cape in Örsviken north of the Askim Granite the Kärra Granite is as a metamorphic intrusive rock (leucogranite) found. The Kärra Granite is also known as the and the other part as metamorphic extrusive and RA-granite (Lundqvist, Lundqvist, Lindström, intrusive rocks (granodiorite). In the leucogranite Calner, & Sivhed, 2011). It is characterised by its a xenolith is found which is interpreted to be a high uranium and thorium contents. The Kärra part of the granodiorite. The granodiorite has an granite is greyish-red to red due to its high K- age of 1.60 – 1.52 Ga (U. Bergström, personal feldspar content and usually foliated and veined. communication, 13 May 2013). Some east-west Both the Askim and Kärra granites belong to the striking dykes of ultrabasic and intermediate Kungsbacka Bimodal Suite (Hegardt, Cornell, intrusive rocks cut through the area close to the Hellström, & Lundqvist, 2007). cape (Geological Survey of Sweden (Cartographer), 2013). An aerial geophysical survey has been conducted in the area in an east-west direction Geological survey of Sweden has interpreted with a height of 30 meters (Figure 3). The map the leucogranite to belong to the Askim granite shows measured variations in the total magnetic found in the study area (U. Bergström, personal field after the Earth’s magnetic reference field communication, 13 May 2013). The Askim (DGRF 1965.0) has been subtracted (Geological granite has an age of 1336±10 Ma. It is a red to Survey of Sweden (Cartographer), 2013).

Figure 3. Aerial geophysical map over Örsviken and the surrounding area. The study area is located in the centre of the map (dashed square), which displays the magnetic field in nT. The reference system used is SWEREF 99 TM. Map modified from (Geological Survey of Sweden (Cartographer), 2013).

micas, olivines and amphiboles, have weak positive anomalies. Ferromagnetic minerals, e.g. 2.3 Magnetic surveying and rock magnetite and pyrrhotite have a large magnetic magnetism susceptibility (Aydin, Ferré, & Aslan, 2007). In magnetic surveying the aim is to 2.4 Proton magnetometer investigate subsurface geology by observing A proton magnetometer, also known as a anomalies in the Earth’s magnetic field. Only a proton precession magnetometer, is an instrument few rock types contain enough magnetic minerals that can measure small variations in the Earth’s to produce significant magnetic anomalies magnetic field and can be used in geophysical (Keary, Brooks, & Hill, 2002). investigations to detect magnetic anomalies. It

can also be used in for example archaeological When a magnetic mineral is placed within a and environmental investigations (Mussett & magnetic field, the magnetic material will Khan, 2000). produce its own magnetisation, called induced magnetisation. Remnant magnetisation on the The sensor is positioned at the top of a pole other hand is the magnetisation that remains about 2 meters above the ground (Mussett & when the applied field is removed (Clark, 1997). Khan, 2000). This removes it from potential A rock’s magnetizing character is determined by noise sources at ground level. The sensor consists its susceptibility, which in turn is dependent on of a bottle filled with a hydrocarbon fluid and is the content of magnetic minerals in the rocks. surrounded by a coil of copper wire. When a Only two mineral-geochemical groups can give polarizing current passes through the coil of such properties, the iron-titanium-oxide group copper it generates a strong magnetic field which and the iron-sulphur group. The first group has a causes the hydrogen protons to align themselves solid solution series of magnetic minerals from in the direction of the new field. When the magnetite (Fe3O4) to ulvöspinel (Fe2TiO4). current is switched off, the protons start to spin Hematite (Fe2O3) is also an iron oxide but does and wobble to realign themselves in the direction not give rise to magnetic anomalies unless a of the Earth’s field, called precession (Figure 4). parasitic antiferro-magnetism is developed. The The protons do this at a frequency that is second group provides the magnetic mineral proportional to the magnetic field and produce an pyrrhotite (Fe1-xS) (Keary, Brooks, & Hill, 2002). electromagnetic wave as they do so. The fourth most abundant element in the Earth’s Measurements of the frequency can provide a crust is iron. It occurs in a native state and in two very accurate measurement of the strength of the oxidation states: metallic (Fe0), ferrous (Fe2+) and 3+ total magnetic field, in nanotesla (nT) (Milsom & ferric (Fe ) (Clark, 1999). The most common Asger Eriksen, 2011). magnetic mineral is magnetite and it is therefore possible to classify a rock’s magnetic behaviour by its overall magnetite content (Keary, Brooks, & Hill, 2002).

Diamagnetism is a property that is present in all materials and creates a magnetic field that is in opposition to a magnetic field that is externally Figure 4. Principle of the proton magnetometer modified applied. Diamagnetic minerals, e.g. quartz, from (Mussett & Khan, 2000). feldspars and calcite, have very weak, negative susceptibilites and are commonly considered as non-magnetic in geophysical investigations (Clark, 1997). Paramagnetic minerals, such as

3. Method this report, the magnetic susceptibility is dimensionless. Also, mapping of the bedrock in 3.1 Working hypothesis the study area was done. The geodetic system By using a proton magnetometer the measured used was SWEREF99 TM. anomalies can be correlated to the extent of the To calculate the density of the leucogranite in rock body of interest. A few disrupted readings Örsviken nine samples from different sites were are expected due to magnetic and ferric objects in collected. The samples were first weighed dry. the study area. These include telephone lines, To fill all the pores and fractures the samples boatsheds and piers with poles made of magnetic were left in water over night. They were weighed materials that can generate magnetic anomalies. in water and the density was then calculated 3.2 Preparations using equation (1). The ArcGIS 10.1 software was used in advance to create a topographic map and to find (1) the most suitable profiles across the cape. Roads, built up areas and steep slopes were taken into 3.4 Modelling account when creating the profiles. In order to The data collected were processed in produce these maps with profiles data from Surfer11®, which is a program that presents 2D Sveriges Lantbruksuniversitet were used, and 3D/wireframe maps of the surface magnetic analysed and visualized (Sveriges anomalies. Besides Surfer11®, Encom Model- Lantsbruksuniversitet, 2013). Vision 11.00 was used, which is a program that enables to visualize input data such as magnetic 3.3 Field method surveys as profiles. By using Encom Field investigations were carried out along ModelVision 11.00 the geometry and extent of four profiles in a south-north direction with the rock body of interest can be interpreted and lengths of around 300 meters using a proton visualized. magnetometer (Figure 5). The sensor was aligned in a south-northerly direction to obtain the Before modelling in Encom ModelVision maximum precession and the sampling distance 11.00 information about the rock body needed to along the profiles was approximately 10 m. At be provided to the software, such as density, every measuring point three readings were taken geographic position, susceptibility values, and and used to calculate a mean value. Base station background values for the magnetic field. The readings were taken, before and after every inclination for the rock body was set to be profile, to correct for diurnal variations. between 50 and 60 degrees for all profiles, which Additionally, the mean total intensity of the is the same as the dip measured in the field. The magnetic field in the study area during the time total intensity of the magnetic field in the region of the investigation was calculated. Parts of during the time of the investigation was set to be Profile 2 and Profile 3, and the whole of Profile 4 50 390 nT, the susceptibility was set to be -5 were measured from a rowing-boat. GPS between 800 – 1000 × 10 [SI] and the density 3 coordinates for every measuring point were taken was 2.63 g/ cm . as well.

The magnetic susceptibility of rocks was measured with a hand-held magnetic susceptibility meter. To get representative susceptibility values of the area 48 readings across the leucogranite were taken. In the SI system (Système International), which is used in

values are measured where the iron content is higher (Engelbrektsson & Berg, 2013). The rock has a density around 2.63 g/cm3 and strike and dip values of about N270/60 (Figure 8). Pegmatite and quartz intrusions were seen in the study area, often close to the contact with the granodiorite, and a few of the pegmatite Figure 5. View of the cape with the four profiles and base intrusions have been folded (Figure 99 & Figure station. 1010). Furthermore, signs that might indicate a cross-bedding and gneissic banding of the 4. Results leucogranite were observed as well as mafic intrusions on the west side of the cape. Fractures 4.1 Bedrock were seen all over the leucogranite outcrop with a Two rock types dominate the cape in similar orientation as the strike and dip (Figure Örsviken, one medium to coarse-grained and one 11). fine-grained. The contact between the two divides the cape in to two main parts, one northern and one southern part. The northern part is mapped as granodiorite and the southern part leucogranite. There is a sharp visible contact between the two rock types in a west-easterly direction across the cape.

4.1.1 The granodiorite The granodiorite is reddish-grey and has a porphyric texture with small, < 1 cm K-feldspar augen. It is a medium to coarse-grained granodiorite. The magnetic susceptibility values measured for the granodiorite ranges from 0 to -5 600 ×10 [SI] (for full data see Appendix). In the Figure 6. Histogram of distribution of the magnetic contact to the leucogranite the granodiorite has a susceptibility of the leucogranite (10-5[SI]). strike and dip of around N260/60 and intrusions of pegmatite’s and quartz.

4.1.2 The leucogranite The rock is fine to medium-grained and red to greyish-red in colour. The extent of the leucogranite south and west of the cape is not observable on the surface. The magnetic susceptibility values measured ranges from 200 to 2000 ×10-5 [SI] (Figure 6 & Figure 7) (see Appendix). Higher susceptibility

Figure 7. Susceptibility values for the leucogranite given in (10-5 [SI]). Map made in ArcGIS.

Figure 8. Strike and dip values of the leucogranite. Map made in ArcGIS.

Figure 9. Contact between the leucogranite and the Figure 10. Folded pegmatite intrusion seen in the granodiorite. leucogranite.

Figure 11. Picture of the leucogranite.

4.2 Magnetic profiles 4.2.2 Encom ModelVision 11.00 The positive anomalies observed in the 4.2.1 Surfer11® profiles made in Encom ModelVision 11.00 The 2D and 3D magnetometric models correlates with the extent of the leucogranite. made in Surfer11® show the total magnetic field in the study area, with values ranging Profile 1 from 50 280 to 50 760 nT. Higher anomalies The leucogranite is modelled to crop out occur in the southern part of the cape, which between 120 m and 180 m along the profile correlates with the extent of the leucogranite and have a depth of around 65 m (Figure 15). (Figure 12, Figure 13 & Figure 14). To match the observed data the surrounding bedrock is modelled to have intruded the Profile 1 leucogranite. Values range from 50 430 to 50 760 nT. High anomalies were registered at 75 m and Profile 2 between 225 m to 250 m into the profile. The leucogranite is modelled to be between 130 m and 280 m into the profile and have a Profile 2 depth of around 50 m (Figure 16). To match Values range from 50 320 to 50 680 nT. the observed data the surrounding bedrock is High anomalies were registered between 125 modelled to have intruded the leucogranite. m and 230 m into the profile. Profile 3 Profile 3 The leucogranite is modelled to be between Values range from 50 350 to 50 620 nT. 90 m and 175 m into the profile and have a High anomalies were registered between 150 depth of around 50 m (Figure 17). To match m and 250 m into the profile. the observed data the surrounding bedrock is modelled to have intruded the leucogranite. Profile 4 Values range from 50 540 to 50 760 nT. Profile 4 High anomalies were registered between 125 The leucogranite is modelled to be between 15 m and 380 m into the profile. m and 200 m into the profile and have a depth of around 60 m (Figure 18). To match the observed data the surrounding bedrock is modelled to have intruded the leucogranite.

Figure 12. 3D model made in Surfer11® over the cape in Örsviken showing the topography in m and the total magnetic field in nT. Note that the z axis has a different scale than the x and y axes.

Figure 13. 2D model made in Surfer11® showing the total magnetic field in nT for the four profiles in Örsviken with a topographic map as background.

Figure 14. 2D model made in Surfer11® showing the total magnetic field in nT for the four profiles in Örsviken.

Figure 15. 2D model made in Encom ModelVision 11.00 displaying the magnetic anomaly in nT (Profile 1). Note the scale difference between the X and the Y axis. The leucogranite is the magnetic body used to model the observed data. The surrounding bedrock corresponds to magnetic reference level; i.e. zero magnetic susceptibility.

Figure 16. 2D model made in Encom ModelVision 11.00 displaying the magnetic anomaly in nT (Profile 2). Note the scale difference between the X and the Y axis. The leucogranite is the magnetic body used to model the observed data. The surrounding bedrock corresponds to magnetic reference level; i.e. zero magnetic susceptibility.

Figure 17. 2D model made in Encom ModelVision 11.00 displaying the magnetic anomaly in nT (Profile 3). Note the scale difference between the X and the Y axis. The leucogranite is the magnetic body used to model the observed data. The surrounding bedrock corresponds to magnetic reference level; i.e. zero magnetic susceptibility.

Figure 18. 2D model made in Encom ModelVision 11.00 displaying the magnetic anomaly in nT (Profile 4). Note the gap between the surface and the bedrock, due to the profile having been carried out on water. Also, note the scale difference between the X and the Y axis. The leucogranite is the magnetic body used to model the observed data. The surrounding bedrock corresponds to magnetic reference level; i.e. zero magnetic susceptibility.

5. Discussion The aim of this report is to define the extent of Because magnetite is one of the most common the leucogranite with high susceptibility in magnetic minerals, it is possible to classify a Örsviken and to discuss what causes the high rock’s magnetic behaviour by its overall suceptibility values, as well as to classify and magnetite content. The high susceptibility values interpret the origin of the cape if possible. obtained are therefore interpreted to be caused by The boundary between the leucogranite and magnetite. This is also confirmed in the thin the granodiorite is clearly visible on the surface sections, stone slabs and SEM as almost all the but geophysical methods are needed in order to opaque minerals found are magnetite. No try and determine the depth and extent below the apparent pattern of the magnetite distribution surface. High anomalies were not only measured across the leucogranite have been identified, when walking with the proton magnetometer neither in the field when using the magnetic across the cape but also south and west of the susceptibility meter nor in looking at thin cape when conducting the fieldwork from the sections. The magnetite appears to be scattered boat. This indicates that the leucogranite could randomly across the leucogranite extend south and west of the cape below the (Engelbrektsson & Berg, 2013). water surface. Comparing the geophysical investigation in this report with the aerial The magnetic properties of igneous intrusions geophysical survey by SGU (Figure 3) they and also the magnetic anomalies that are correlate very well, with positive magnetic associated with them, is a reflection of the bulk anomalies on the southern part of the cape. rock composition, redox state, hydrothermal alteration and metamorphism etc (Clark , French, In all the Encom ModelVision 11.00 figures Lackie, & Schmidt, 1992). The partitioning of the surrounding bedrock is modelled to have iron between oxide and silica phases, which is intruded the leucogranite. However, this intrusion influenced by oxidation ratio, is the major control has not been seen out in the field, it was merely on magnetic mineralogy and bulk magnetic done to match the observed data. According to properties (Clark, 1999). SGU the granodiorite is older than the leucogranite which makes it unlikely that the The leucogranite in Örsviken could be granodiorite would have intruded the classified as an A-type granite based on its high leucogranite. Also, the mapped xenolith SiO2 (>77%), Na2O + K2O, Fe/Mg and low CaO strengthens the idea that the granodiorite is older. (Engelbrektsson & Berg, 2013). The cross- The folded pegmatite intrusion suggests that the bedding seen in the study area could suggest that leucogranite is older than the Sveconorwegian the leucogranite in Örsviken is an H-type orogeny. In summary, the leucogranite is older (hybrid) granitoid that is a mix of both an A-type than the Svegonorwegian orogeny but younger and an S-type granite (Winter, 2010). A than the granodiorite. The leucogranite is classification of granitoids could also be done interpreted to have a depth between 40 m and 60 based on their tectonic setting, which gives an m, though in reality it may be deeper than that indication that the leucogranite has an anorogenic because the proton magnetometer is distance or a post-orogenic petrogenesis (Engelbrektsson dependent and the magnetic signal is reduced & Berg, 2013). The leucogranite in the study area with depth. show quite similar patterns and chemistry to the Askim granite (Anfinset, 1999) and Kärra granite (U. Bergström, personal communication, 24 May 2013) which might indicate that the leucogranite is associated with them.

One advantge of using the proton or a post-orogenic petrogenesis. The leucogranite magnetometer is its reliability and that it is easy is older than the Sveconorwegian orogeny but to use. However, because ferromagnetic objects younger than the granodiorite. can disrupt the readings the people conducting It is of great interest to relate the magnetic the investigation must divest itself from all mineralogy, bulk magnetic properties, ferrous objects such as keys, belt buckles, and geochemistry and petrology to the observed knives, etc. In addition, many ferrous objects can magnetic anomalies in order to develop an be found in the environment e.g. houses, fences understanding of the different geological aspects and cars. When carrying out the investigation by that controls the magnetic signatures obtained. boat it had an engine attached to it which is All this can be used to improve the interpretation believed to have given disrupted readings. Also, of magnetic surveys. it was difficult to stay completely still taking the readings due to the boat drifting away. Hence, Profile 4 should be interpreted with caution and 7. Acknowledgements may not be reliable. The disrupted readings We would like to thank our supervisor Erik measured in the field that were known to come Sturkell for help during the project and the from e.g. a car were excluded in the modelling. University of Gothenburg for funding this To obtain better representative susceptibility project. Thanks to the Wedel brothers, Per and values more than 48 measruements should have Nisse, for lending us a boat so that we could been taken and in a more systematic way. conduct our fieldwork. Also, a great thanks to Furthermore, to obtain a better accuracy of the Vera Bouvier for helping us with the modelling proton magnetometer measurements, the profiles and answering our many questions. Finally, we done should have been closer to each other and would like to give a special thanks to all our more than four. classmates, family and friends for all the support and constructive criticism during the thesis Further work is needed in the study area to writing. analyse the susceptibility values in relation to hydrothermal alteration, bulk composition, metamorphism and redox state etc. Complementary measurements of the magnetic susceptibility of the leucogranite but also of the granodiorite would be of interest in order show the difference in magnetic susceptibility across the whole cape.

6. Conclusion The results from the investigation carried out with the proton magnetometer across the cape in Örsviken indicates that the leucogranite extends south and west of the cape below the water surface. The field measurements display variations of magnetic susceptibility within the same rock type. The high susceptibility values obtained are interpreted to be caused by magnetite and appear to be scattered across the leucogranite. The leucogranite is interpreted as a possible A- type alkali granite with an anorogenic

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9. Appendix

Table 1. Minimum, maximum and mean susceptibility values for the granodiorite.

Measuring N -S E-W Susceptibility Susceptibility Susceptibility point Minimum (SI [10-5]) Maximum (SI [10-5]) Mean (SI [10-5]) 49 6384073 316002 0 0 0 50 6384075 315990 - - 0 51 6384101 315941 0 0 0 52 6384075 315984 0 0 0 53 6384077 315989 - - 0 54 6384076 316022 - - 0 55 6384081 316067 - - 0 56 6384079 316080 200 700 600 57 6384062 316165 - - 0 58 6384058 316191 - - 0 59 6384060 316235 - - 200 60 6384062 316228 - - 200 61 6384062 316333 - - 200

Table 2. Minimum, maximum and mean susceptibility values for the leucogranite.

Measuring N -S E-W Susceptibility Susceptibility Susceptibility point Minimum (SI [10-5]) Maximum (SI [10-5]) Mean (SI [10-5]) 1 6384023 316206 200 2000 500 2 6384016 316191 300 1600 600 3 6384016 316178 500 2000 800 4 6384013 316176 600 2000 1500 5 6384015 316162 400 2000 1500 6 6384017 316157 300 1500 800 7 6384013 316144 900 2500 1500 8 6384022 316129 800 2500 1500 9 6384015 316123 1500 5500 2000 10 6384031 316102 400 1500 500 11 6384023 316091 450 2000 800 12 6384038 316087 800 2000 1000 13 6384046 316089 600 1500 1000 14 6384054 316072 200 800 300 15 6384060 316047 300 1000 300 16 6384050 316030 300 500 500 17 6384061 316029 200 400 400 18 6384065 316006 200 500 400 19 6384073 316002 - 600 200 20 6384075 315984 - 400 200 21 6384077 315989 - - 400 22 6384076 316022 - - 400 23 6384072 316037 900 1500 1000 24 6384078 316040 - - 1500 25 6384082 316053 - - 500

26 6384081 316067 - - 200 27 6384079 316080 - - 0 28 6384075 316092 - - 200 29 6384065 316180 400 1500 500 30 6384060 316124 300 1500 800 31 6384063 316153 1000 2000 1500 32 6384061 316156 1000 1500 1000 33 6384062 316165 400 1000 800 34 6384048 316170 600 1500 800 35 6384058 316191 - - 600 36 6384048 316197 800 2000 1500 37 6384061 316112 - - 600 38 6384044 316214 500 4000 1500 39 6384060 316235 - - 200 40 6384041 316239 1000 2000 1500 41 6384062 316228 - - 500 42 6384051 316276 800 2000 1500 43 6384048 316298 800 2000 1000 44 6384047 316314 1000 2000 1500 45 6384062 316333 - - 500 46 6384045 316321 400 1500 600 47 6384030 316305 500 1500 800 48 6384030 316325 800 1000 800