Geothermal History of the Karoo Basin in South Africa Inferred from Magnetic Studies

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Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujdigispace.uj.ac.za (Accessed: Date).

GEOTHERMAL HISTORY OF THE KAROO BASIN IN SOUTH AFRICA INFERRED FROM MAGNETIC STUDIES

Leonie Pauline Maré

THESIS

presented in fulfillment of the requirements for the degree of

PHILOSOPHIAE DOCTOR

GEOLOGY

in the

FACULTY OF SCIENCE

of the

UNIVERSITY OF JOHANNESBURG

Supervisor Dr. Michiel O. De Kock

Co-supervisors Prof Bruce Caincross Prof Hassina Mouri

March 2015

DECLARATION

I hereby declare that this thesis submitted for the degree of Doctor of Philosophy in Geology at the Faculty of Science at the University of Johannesburg, apart from the help recognised, is my own original work, conducted under the supervisions of Dr. M.O. de Kock and co-supervised by Prof. B. Cairncross and Prof. H. Mouri and no part of this research has been submitted in the past, or is being submitted for a degree or examination at any other University.

L.P. Maré

CONTENTS

ABSTRACT ...... xx ACKNOWLEDGEMENTS ...... xxi

CHAPTER 1. INTRODUCTION AND PURPOSE OF THESIS ...... 1-1 1.1 Problem Statement ...... 1-1 1.2 Objectives of Study ...... 1-2 1.3 Methods ...... 1-3 1.3.1. Experiment 1: Baked contact test ...... 1-3 1.3.2. Experiment 2: Variation of magnetic susceptibility with repeated progressive heating ...... 1-5 1.3.3. Experiment 3: Pyrrhotite/Magnetite Geothermometer ...... 1-7 1.4 References ...... 1-8

CHAPTER 2. GENERAL GEOLOGY OF THE KAROO BASIN AND PREVIOUS STUDIES ...... 2-1 2.1 Introduction ...... 2-1 2.2 Karoo Basin ...... 2-1 2.3 Karoo Large Igneous Province ...... 2-3 2.4 Published Geothermal Related Studies on the Karoo Basin ...... 2-6 2.5 Published Palaeomagnetic Studies on the Karoo Basin ...... 2-9 2.6 References ...... 2-11

CHAPTER 3. SAMPLING METHODOLOGY AND PROCEDURES ...... 3-1 3.1 Introduction ...... 3-1 3.2 Sample collection and preparation ...... 3-1 3.2.1. Outcrop sampling ...... 3-1 3.2.2. Core sampling ...... 3-2 3.2.2.1. Kopoasfontein (G39974) - Reference borehole...... 3-3 3.2.2.2. Waterkloof (PP47) ...... 3-6 3.2.2.3. Hermon (HM1/78) ...... 3-6 3.2.2.4. Driefontein (DF1/75)...... 3-8 3.2.2.5. Goedehoop (CBC4495) ...... 3-8 3.2.2.6. Sambokkraal (SA1/66) ...... 3-8 3.2.2.7. Schietfontein (SC3/67) ...... 3-11 3.2.2.8. Groottegeluk (MY19) ...... 3-11 3.3 Method ...... 3-11 3.3.1. Sample preparation ...... 3-11

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3.3.2. Laboratory procedures ...... 3-12 3.4 References ...... 3-13

CHAPTER 4. MAGNETIC ASSEMBLAGE ...... 4-1 4.1 Introduction ...... 4-1 4.2 Identification of magnetic minerals ...... 4-1 4.2.1. Temperature Variation of Magnetic Susceptibility ...... 4-1 4.2.2. Magnetic Hysteresis ...... 4-9 4.2.3. Isothermal Remanent Magnetization (IRM) ...... 4-15 4.3 Fundamentals of rock magnetism ...... 4-21 4.3.1. Magnetic Susceptibility ...... 4-21 4.3.2. Anisotropy of Magnetic Susceptibility (AMS) ...... 4-21 4.4 Summary of magnetic fabric results ...... 4-21 4.5 References ...... 4-32

CHAPTER 5. EXPERIMENT 1 – BAKED CONTACT TEST ...... 5-1 5.1 Introduction – Baked contact test ...... 5-1 5.2 Outcrop samples (southeastern basin) ...... 5-1 5.2.1. Kommandodriftdam dyke ...... 5-2 5.2.2. Long Dyke ...... 5-8 5.3 Core samples: Boreholes with Dolerite ...... 5-12 5.3.1. Kopoasfontein Core (G39974) ...... 5-12 5.3.2. Waterkloof (PP47) ...... 5-21 5.3.3. Hermon (HM1/78) ...... 5-29 5.3.4. Driefontein (DF1/75) ...... 5-33 5.4 Core samples: Boreholes without Dolerite ...... 5-41 5.4.1. Sambokkraal (SA1/66) ...... 5-41 5.4.2. Schietfontein (SC3/67) ...... 5-48 5.4.3. Goedehoop (CBC4495) ...... 5-49 5.4.4. Groottegeluk (MY19) ...... 5-50 5.5 Discussion ...... 5-59 5.6 References ...... 5-63

CHAPTER 6. EXPERIMENT 2 – ALTERATION INDEX A40 ...... 6-1 6.1 Introduction – Variation of magnetic susceptibility with repeated progressive heating ...... 6-1 6.2 Outcrop samples (southeastern basin) ...... 6-1 6.3 Core samples: Boreholes with dolerite ...... 6-5 6.3.1. Kopoasfontein Core (G39974) ...... 6-5 6.3.2. Waterkloof Core (PP47) ...... 6-7 6.3.3. Hermon Core (HM 1/78) ...... 6-15 6.3.4. Driefontein Core (DF 1/75) ...... 6-20 6.4 Core samples: Boreholes without Dolerite ...... 6-29 6.4.1. Sambokkraal (SA 1/66) ...... 6-29 6.4.2. Schietfontein (SC 3/67) ...... 6-36 6.4.3. Goedehoop (CBC4495) ...... 6-43 6.4.4. Groottegeluk (MY19) ...... 6-46 6.5 Discussion ...... 6-49 6.6 References ...... 6-53

CHAPTER 7. EXPERIMENT 3 – PYRRHOTITE / MAGNETITE GEOTHERMOMETER ...... 7-1 7.1 Introduction ...... 7-1 7.2 Development of Pyrrhotite/Magnetite Geothermometers ...... 7-1 7.2.1. Variation of susceptibility and remanence in contact aureoles ...... 7-1 7.2.2. MagEval Low-Temperature Geothermometer ...... 7-2 7.2.3. Pyrrhotite/Magnetite Remanence Intensity Ratio ...... 7-4 7.2.4. Thermal Remagnetization Geothermometer ...... 7-5 7.3 Results from the Karoo Basin ...... 7-6 7.3.1. Variation of susceptibility and remanence in contact aureoles ...... 7-6 7.3.2. MagEval Low-Temperature Geothermometer ...... 7-12 7.3.3. Pyrrhotite/Magnetite Remanence Intensity Ratio ...... 7-13 7.3.4. Thermal Remagnetization Geothermometer ...... 7-14 7.4 Discussion ...... 7-17 7.5 References ...... 7-17

CHAPTER 8. GEOTHERMAL MODEL FOR THE KAROO BASIN AND IMPLICATIONS FOR INDUSTRY AND THE ENVIRONMENT ...... 8-1 8.1 Introduction ...... 8-1 8.2 Geothermal variation across the Karoo Basin ...... 8-1 8.3 Implications for Industry and the Environment ...... 8-10 8.3.1. Coal ...... 8-10 8.3.1.1. Parameters affecting coal rank and maturation ...... 8-10 8.3.1.2. Coalfields of South Africa ...... 8-11

8.3.2. Uranium ...... 8-12 8.3.2.1. Classification of uranium deposits ...... 8-12 8.3.2.2. Uranium Deposits in the Karoo Supergroup...... 8-15 8.3.3. Hydrocarbon Potential and Global Climate Change...... 8-16

8.3.3.1. Methane release and CO2 Degassing...... 8-19

8.3.3.2. CO2 Storage Potential ...... 8-21 8.3.3.3. Climate change ...... 8-22 8.4 Concluding Remarks ...... 8-22 8.5 References ...... 8-24

LIST OF FIGURES

Figure 1-1: Schematic diagram indicating the variation of direction of magnetization (arrows), intensity and dispersion with distance from an igneous intrusion in five possible zones (modified from Irving, 1964)...... 1-4 Figure 1-2: Example of the relationship between heating (red) and cooling (blue) curves indicate that the maximum temperature that the rock at site KAM0 underwent in it geological history was between 250–300°C...... 1-7

Figure 2-1: Simplified geology of the main Karoo Basin showing the areal distribution of the main lithostratigraphic units (modified from Johnson et al., 2006 and Lanci et al., 2013)...... 2-3 Figure 2-2: a) Simplified geological map of the Karoo Basin with the extent of dolerite intrusions indicated. b) Schematic SW-NE cross-section of the Karoo Basin showing the complexity of the dolerite sill and dyke network below the Drakensberg lavas (after Chevallier et al., 2001)...... 2-4 Figure 2-3 – Dolerite dykes of the Main Karoo Basin (after Chevallier and Woodford, 1999). Inset (a): simplified structural map showing the three structural domains. Insert (b): geodynamic interpretation of the Western and Eastern Karoo structural setting...... 2-6

Figure 3-1: Sampling location of 2008 fieldtrip in the southeastern and eastern part of the Karoo Basin. Samples were collected in the contact aureole of both dykes and sills...... 3-2 Figure 3-2: Simplified geological map showing the distribution of the Karoo Basin within South Africa. Location of sampling boreholes are indicated (red stars indicate cores with dolerite sills and blue stars indicate cores without any boreholes)...... 3-3 Figure 3-3: The core from the White Hill Formation shale is fractured significantly into small pieces...... 3-4 Figure 3-4: Exploranium KT-9 susceptibility meter used for magnetic susceptibility measurements along the core...... 3-5 Figure 3-5: Magnetic susceptibility measurements were taken on core samples at regular intervals with the Fiskars TH-15 susceptibility meter...... 3-6 Figure 3-6: Variation of magnetic susceptibility with depth along core G39974 as measured during the current study with both the Exploranium KT-9 as well as the Fiskars TH-15 magnetic susceptibility meters...... 3-7 Figure 3-7: Sample depth along borehole cores that intersected presumed dolerite sills...... 3-9 Figure 3-8: Sample depth along borehole cores without dolerite...... 3-10

Figure 4-1: Semi-quantitative separation of whole rock magnetic susceptibility into paramagnetic and ferromagnetic contributions ...... 4-2 Figure 4-2: Examples from boreholes DF1/75 (DF64), CBC4495 (GH16 and GH37) and MY19 (MY19-6) indicating the separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility of the Vryheid Formation by fitting a hyperbola offset along the y axis using the Cureval8 software (Chadima and Hrouda, 2009)...... 4-3 Figure 4-3: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM1), G39974 (LKF10) and SC3/67 (SC19) for the Tierberg Formation...... 4-4 Figure 4-4: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM49), G39974 (LKF29), SC3/67 (SC93) and SA1/66 (SA950) for the White Hill Formation or lower Ecca Group...... 4-5 Figure 4-5: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM16 and HM59), G39974 (LKF66) and PP47 (PH64) for the Prince Albert Formation...... 4-6 Figure 4-6: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes DF1/75 (DF30), HM1/78 (HM22), G39974 (LKF123) and SA1/66 (SA1000) for the Dwyka Group...... 4-7

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Figure 4-7: Temperature-dependence susceptibility measured for sample LKF75 (Borehole G39974, Prince Albert Formation) in an air atmosphere indicating an increase of susceptibility value due to the formation of more magnetic magnetite. Maximum temperature increased by 50°C after each heating (red line) / cooling cycle (blue line). (a) 100-500 °C heating/cooling curves reversible, peak at 315 °C indicating pyrrhotite (b) 550 °C cooling curve higher than heating curve indicating formation of new mineral (magnetite) (c) Peak at 580 °C indicating magnetite...... 4-9 Figure 4-8: Heavy green line: initial behavior of demagnetized specimen as applied field ramps up from zero field to a saturating field. The initial slope is the initial or low-field susceptibility χlf. After saturation is achieved the slope is the high-field susceptibility χhf which is the non-ferromagnetic contribution, in this case the paramagnetic susceptibility (because χhf is positive.) The dashed blue line is the hysteresis loop after the paramagnetic slope has been subtracted. Saturation magnetization Ms is the maximum value of magnetization after slope correction. Saturation remanence Mr is the value of the magnetization remaining in zero applied field. The field necessary to reduce the net moment to zero is defined as the coercive field (µ0Hc or Hc) and coercivity of remanence (µ0H’cr or Hcr) (after Tauxe, 2013)...... 4-10 Figure 4-9: Hysteresis loops of end-member behaviors: a) diamagnetic, b) paramagnetic, c) superparamagnetic (data for submarine basaltic glass), d) uniaxial, single domain, e) magnetocrystalline, single domain, f) “pseudo-single domain”. Hysteresis behavior of various mixtures: g) magnetite, and hematite, h) SD/SP magnetite (data from Tauxe et al. 1996), i) another example of SD/SP magnetite with a finer grained SP distribution (after Tauxe, 2013)...... 4-11 Figure 4-10: Combined hysteresis curves of selected samples from borehole G39974 (LKF) displaying ferromagnetic signatures, while borehole HM1/78 (HM) indicates dominantly paramagnetic fabric...... 4-12 Figure 4-11: Combined hysteresis curves of selected samples from boreholes PP47 (PH) and DF1/75 (DF) indicating dominantly paramagnetic fabric...... 4-13 Figure 4-12: Combined hysteresis curves of selected samples from boreholes SA1/66 (SA) and CBC4495 (GH) indicating dominantly paramagnetic fabric...... 4-14 Figure 4-13: Isothermal remanent magnetization (IRM) of selected samples from borehole G39974 (LKF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-16 Figure 4-14: Isothermal remanent magnetization (IRM) of selected samples from borehole PP47 (PH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-16 Figure 4-15: Isothermal remanent magnetization (IRM) of selected samples from borehole HM1/78 (HM) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-17 Figure 4-16: Isothermal remanent magnetization (IRM) of selected samples from borehole DF1/75 (DF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-17 Figure 4-17: Isothermal remanent magnetization (IRM) of selected samples from borehole SA1/66 (SA) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-18 Figure 4-18: Isothermal remanent magnetization (IRM) of selected samples from borehole CBC4495 (GH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 4-18 Figure 4-19: Coercivity of remanence with depth of selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF-Hcr) PP47 (PH-Hcr), HM1/78 (HM-Hcr) and DF1/75 (DF-Hcr) as well as samples from boreholes without sills, SA1/66 (SA-Hcr) and CBC4495 (GH-Hcr) ...... 4-19 Figure 4-20: S-ratios plotted as a function of depth for selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), HM1/78 (HM), PP47 (PH) and DF1/75 (DF) as well as samples from boreholes without sills, CBC4495 (GH) and SA1/66 (SA)...... 4-20 Figure 4-21: A model ternary diagram interrelating inclinations of the principal anisotropy directions (K1, K2, K3) in bedding coordinate system (after Chadima et al., 2006).

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The 30° and 60° threshold angles subdivide the diagram into nine fields. Schematic stereoplots depicting the orientation of principal directions are drawn for each field. Different types of magnetic fabrics as defined by Rochette et al. (1992, 1999) and Ferré (2002) are presented outside the diagram. Square, triangle and circle symbols represent maximum, intermediate and minimum anisotropy directions i.e., K1, K2 and K3, respectively...... 4-23 Figure 4-22: Flow fabric generation by fabric imbrication model modified after Knight and Walker (1988) ...... 4-24 Figure 4-23: Ternary AMS diagram of sedimentary rocks from boreholes intersecting dolerite sills. With the exception of PP47 that shows inverse magnetic fabric, all other borehole display normal fabric...... 4-24 Figure 4-24: Ternary AMS diagram of sedimentary rocks from boreholes not intersecting any dolerite sills. Magnetic fabric display dominantly normal fabric...... 4-25 Figure 4-25: Shape parameter, T, for boreholes intersecting dolerite sills. Sedimentary units are predominantly oblate shaped. Changes in shape from oblate to prolate are observed in contact aureoles of borehole G39974...... 4-27 Figure 4-26: Shape parameter, T, for boreholes not intersecting any dolerite sills. Sedimentary units are predominantly oblate shaped with more sandy units (e.g. Beaufort Group) tending to be more isotropic in shape. Some prolate fabric was observed along coalbeds in borehole CBC4495...... 4-28

Figure 4-27: Variation of the poles to the magnetic foliation (K3) with depth indicating minor re-alignment at sill-sediment contacts...... 4-30 Figure 4-28: Variation of the poles to the magnetic foliation (K3) with depth for boreholes without any dolerite sills...... 4-31

Figure 5–1: Kommandodriftdam dyke with sampling locations of sandstone and mudstone indicated. The total width of the dyke is 4.6m...... 5-2 Table 5-1: Palaeomagnetic results from the host rocks and the Kommandodriftdam dyke...... 5-3 Figure 5–2: Typical thermal demagnetization curves of the sandstone (KAM002C) and mudstone (KAM003B) indicating magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots...... 5-4 Figure 5–3: Typical thermal demagnetization curves of the dolerite intrusion. KA011B5 represents the dyke wall and KA016C4 represent the dyke centre. In both cases a low temperature component, either ilmenite (~205°C) or pyrrhotite (~350°C) can be identified as well as magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots...... 5-5 Figure 5–4: Equal area plot of mean magnetization directions for the sediment (pink) as well as two samples from the Kommandodrift dyke. There is no apparent correlation between the direction of the sediment and intrusion...... 5-6 Figure 5–5: Magnetostratigraphic section from Kommandodriftdam with estimated sampling location of Kommandodriftdam indicated (modified after De Kock and Kirschvink, 2004)...... 5-6 Figure 5– 6: Geology map overlayed on Google Earth indicating the KDPT sampling location of De Kock and Kirschvink (2004) as well as the Kommandodriftdam sampling location of the current study...... 5-7 Table 5-2: Palaeomagnetic results from the host rocks and the Long dyke...... 5-9 Figure 5–7: Typical thermal demagnetization curves of the sandstone (KAM005B and KAM007B) indicating magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots...... 5-10 Figure 5–8: Typical thermal demagnetization curves of the Long Dyke dolerite intrusion. KA027C8 represents the dyke wall and KA033A9 represent the dyke centre. In the case of KA027C8 a low temperature component, possibly pyrrhotite (~350°C), as well as high temperature component, magnetite (~580°C), can be identified. KA033A9 became unstable above 475°C. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots...... 5-11

Figure 5–9: Equal area plot of mean magnetization directions for the sediment (pink) as well as two dolerite samples from the Long dyke. There is no apparent correlation between the direction of the sediment and intrusion...... 5-12 Figure 5–10: Distribution of the Karoo Basin with borehole positions of cores analyzed that intersected one or more dolerite sill (modified simplified geology map of the Karoo Basin, courtesy Council for Geoscience)...... 5-13 Table 5-3: Summary of mean NRM intensity and directional data for different stratigraphic units...... 5-13 Figure 5–11: Typical demagnetization curves from the three dolerite sills. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-15 Figure 5–12: Typical demagnetization curves from the upper sedimentary units occurring in the G39974 Kopoasfontein borehole. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-16 Figure 5–13: Typical demagnetization curves from the lower sedimentary units occurring in the G39974 Kopoasfontein borehole. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-17 Figure 5–14: Typical thermal demagnetization curves from the three dolerite sills indicating magnetite (~600 °C) as the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-18 Figure 5–15: Typical thermal demagnetization curves from the upper sedimentary units occurring in borehole G39974 indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-19 Figure 5–16: Typical thermal demagnetization curves from the different sedimentary units occurring in borehole G39974 indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-20 Table 5-4: Summary of mean NRM intensity and directional data for different lithologic layers...... 5-21 Figure 5–17: Summary of the magnetic susceptibility, -intensity and variation in magnetic inclination as a function of depth in borehole G39974 with stratigraphic units superimposed. Red dots indicate only limited change in the magnetic inclinations after AF demagnetization. Thermal demagnetization (connected light blue dots) confirms the dominantly negative inclination of the dolerite with mainly positive inclination for the sediments...... 5-22 Figure 5–18: Thermal demagnetization curves of selected specimens from Shale1 in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is

in A/m; The distance of each data point from the origin indicates the total NRM intensity...... 5-24 Figure 5–19: Thermal demagnetization curves of selected specimens from the different siltstone units in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity ...... 5-25 Figure 5–20: Thermal demagnetization curves of selected specimens from Shale2 in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity...... 5-26 Figure 5–21: Typical thermal demagnetization curves of the dolerite sills in borehole PP47 indicating magnetite (~525°C) as the main magnetization component. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity...... 5-27 Figure 5–22: Summary of the magnetic intensity and variation in magnetic inclination (NRM) as a function of depth in borehole PP47 with lithologic units superimposed...... 5-28 Table 5-5: Summary of mean NRM intensity and directional data for the different stratigraphic units of core HM1/78...... 5-29 Figure 5–23: Typical thermal demagnetization curves from the upper sedimentary units occurring in the HM1/78 borehole indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-30 Figure 5–24: Typical thermal demagnetization curves from the lower sedimentary units occurring in the HM1/78 borehole indicating pyrrhotite (~350°C) to be the main magnetization component. The Prince Albert Formation however, indicates the presence of both pyrrhotite and magnetite (~580°C). (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-31 Figure 5–25: Typical thermal demagnetization curves for both the dolerite sill as well as the underlying lava of the Ventersdorp Formation indicating magnetite (580–600°C) as the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-32 Table 5-6: Variation of susceptibility and magnetic intensity for different stratigraphic units of core DF1/75...... 5-33 Figure 5–26: Summary of the magnetic susceptibility, -intensity and variation in magnetic inclination as a function of depth in borehole HM1/78 with stratigraphic units superimposed. Black diamonds indicate direction before demagnetization (NRM). Directions after thermal demagnetization are indicated by yellow squares (low temperature component), purple dots (intermediate temperature component), and red triangles (high temperature component). Thermal demagnetization confirms a dominantly negative inclination for both the dolerite as well as the sediments...... 5-34

Figure 5–27: Typical thermal demagnetization curves from the top sedimentary units occurring in borehole DF1/75 indicating a low temperature component, possibly pyrrhotite (350–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~580°C), observed. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-35 Figure 5–28: Typical thermal demagnetization curves from the Pietermaritzburg Formation indicating a low temperature component, possibly pyrrhotite (400–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~580°C), observed. Sample DF27B are one of the exceptions where magnetite is the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-36 Figure 5–29: Typical thermal demagnetization curves from the Dwyka Group and basement occurring in borehole DF1/75 indicating a low temperature component, possibly pyrrhotite (~400°C), to be the main magnetization component with minimal contribution from the high temperature component, magnetite (~580°C). (a) Equal- area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-37 Figure 5–30: Typical thermal demagnetization curves from the two dolerite sills. (a) Equal- area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-39 Figure 5–31: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination (NRM) as a function of depth in borehole DF1/75 with stratigraphic units indicated...... 5-40 Figure 5–32: Distribution of the Karoo Basin with borehole positions of core analyzed that do not intersect any dolerite (modified simplified geology map of Karoo Basin, courtesy Council for Geoscience)...... 5-41 Table 5-7: Susceptibility variation between different stratigraphic units of core SA1/66...... 5-42 Figure 5–33: Typical thermal demagnetization curves from the Bokkeveld and Table Mountain Groups occurring below the Karoo Supergroup in borehole SA1/66. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-43 Figure 5–34: Typical thermal demagnetization curves from the Karoo sedimentary units occurring in borehole SA1/66 indicating a low temperature component, possibly pyrrhotite (350–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~500°C), observed. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-44 Figure 5–35: Typical thermal demagnetization curves from the lower sedimentary units occurring in borehole SA1/66 indicating a low temperature component, possibly pyrrhotite (350–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~500°C), observed. (a)

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Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-45 Figure 5–36: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole SA1/66 with stratigraphic units indicated...... 5-46 Figure 5–37: Seismic profile with overlay of borehole SA1/66 stratigraphy indicating folding from the Cape Fold Belt effecting the Beaufort and upper Ecca groups (Lindeque et al., 2011)...... 5-47 Table 5-8: Variation of susceptibility and magnetic intensity between different stratigraphic units of core SC3/67...... 5-48 Figure 5–38: Summary of the magnetic susceptibility and intensity of magnetization as a function of depth in borehole SC3/67 with lithologic units superimposed...... 5-49 Table 5-9: Variation of susceptibility and magnetic intensity between different stratigraphic units of core CBC4495...... 5-49 Table 5-10: Variation of susceptibility and magnetic intensity between different stratigraphic units of core CBC4495...... 5-50 Figure 5–39: Typical thermal demagnetization curves from different lithologic units in the Vryheid Formation (core CBC4495) indicating both low temperature (200–400 °C) as well as high temperature (580–600°C) magnetization components. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-51 Figure 5–40: Typical thermal demagnetization curves from the Dwyka tillite (GH71C) (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-52 Figure 5–41: Typical thermal demagnetization curves from a sedimentary unit below the Dwyka presumed to belong to the Transvaal Supergroup (GH74B). Sample GH36C is an example of an unresolved unblocking temperature that becomes unstable above 200°C. Sandstone GH64C near the base of the Vryheid Formation indicate a stable endpoint a ~400°C suggesting anoxic conditions. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-53 Figure 5–42: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole CBC4495 with stratigraphic units indicated...... 5-54 Figure 5–43: Typical thermal demagnetization curves for the Beaufort Group shale in borehole MY19. Samples became unstable at temperatures above 300°C. (a) Equal- area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-55 Figure 5–44: Typical thermal demagnetization curves for the Vryheid Formation shale in borehole MY19. Stable endpoint was reached at an unblocking temperature 375– 400°C. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-56

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Figure 5–45: Typical thermal demagnetization curves for the Vryheid Formation sandstone in borehole MY19. Stable endpoint was reached at an unblocking temperature 375°C–400°C. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results...... 5-57 Figure 5–46: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole MY19 with stratigraphic units indicated...... 5-58 Figure 5–47: Variation in magnetic inclination with depth indicating the limited degree of remagnetization due to dolerite intrusions. Different symbols are associated with different magnetic phases, i.e. High Temperature (HT), Medium Temperature (MT) and Low Temperature (LT) components...... 5-61 Figure 5–48: Variation in magnetic inclination with depth for boreholes without any dolerite intrusions. Blue symbols indicate NRM inclination while red symbols indicate inclination after thermal demagnetization. In all three boreholes the inclination became intermediate to steeply negative after thermal demagnetization...... 5-62

Table 6-1: Values of A40 alteration index after individual steps for investigated samples (after Hrouda et al., 2003) ...... 6-2

Figure 6-1: Change in the average A40 alteration index for individual heating/cooling runs after Hrouda et al. (2003)...... 6-2

Figure 6-2: Change in the average A40 alteration index for samples at the Golden Valley sill. KAM000 is located below the sill and KAM001 is located above the sill. A maximum geothermal temperature of 200–250°C is observed...... 6-3

Figure 6-3: Change in the average A40 alteration index for samples at the Kommandodriftdam dyke. A maximum geothermal temperature of 200–225°C is observed...... 6-3

Figure 6-4: Change in the average A40 alteration index for samples at the Long Dyke. Samples KAM005 and KAM007 is samples on either side of the dyke and is located on the farm Denmark. Sample KAM006 is located several kilometers north of Denmark. A maximum geothermal temperature at Denmark of ~300°C is observed, while a slightly lower maximum temperature of ~250°C is observed at site KAM006...... 6-4

Figure 6-5: Change in the average A40 alteration index for samples at the Taylor’s Koppie dyke in the Insizwa vicinity. A maximum geothermal temperature of ~275°C is observed...... 6-5

Figure 6-6: Variation of alteration index (A40) for the different stratigraphic units indicating maximum acquired temperatures, where individual curves show first significant increase or decrease (indicated by ellipses), to be above 250 °C...... 6-6 Figure 6-7: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different sedimentary units. Total loss of susceptibility occurs at the Curie temperatures of the main magnetic carriers. The White Hill Formation is dominated by pyrrhotite (b) and the Dwyka Group by magnetite (f). The Tierberg (a) and Prince Albert Formations (c-e) host both pyrrhotite and magnetite. Red line indicates heating curve and blue line the cooling curve...... 6-8

Figure 6-8: Correlation of magnetic alteration (A40) acquired Tmax temperatures for borehole G39974 plotted with depth with the modelled maximum temperatures calculated by Aarnes et al. (2011) from metamorphic minerals...... 6-9

Figure 6-9: Variation of alteration index (A40) for the different shale units indicating maximum acquired temperatures (indicated by ellipses) to range between 390°C and 540°C...... 6-10

Figure 6-10: Variation of alteration index (A40) for the different shale units indicating maximum acquired temperatures (indicated by ellipses) to range mainly between 490°C and 590°C. Two samples from siltstone 2 also indicate a lower temperature between 190–240°C...... 6-11 Figure 6-11: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different lithologic units in borehole PP47. Total

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loss of susceptibility occurs at the Curie temperatures of the main magnetic carriers. Red line indicates heating curve and blue line the cooling curve. Both shale (a) & (b) as well as the siltstone (c) – (e) samples indicate magnetite as the main carrier of magnetism with a Curie temperature of ~580°C...... 6-13

Figure 6-12: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole PP47 plotted with depth...... 6-14 Figure 6-13: Variation in magnetic susceptibility with increasing temperature for sample HM01 from the Tierberg Formation in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve...... 6-15 Figure 6-14: Variation in magnetic susceptibility with increasing temperature for sample HM49 from the White Hill Formation in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve...... 6-16 Figure 6-15: Variation in magnetic susceptibility with increasing temperature for sample HM53 from the Prince Albert Formation in borehole HM 1/78 located above the dolerite sill. Red line indicates heating curve and blue line the cooling curve...... 6-17 Figure 6-16: Variation in magnetic susceptibility with increasing temperature for sample HM16 from the Prince Albert Formation in borehole HM 1/78 below the dolerite sill. Red line indicates heating curve and blue line the cooling curve...... 6-17 Figure 6-17: Variation in magnetic susceptibility with increasing temperature for sample HM22 from the Dwyka Group in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve...... 6-18

Figure 6-18: Variation of alteration index (A40) for the different stratigraphic units in borehole HM 1/78 indicating maximum acquired temperatures (indicated by ellipses) to be above 340°C...... 6-19

Figure 6-19: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole HM 1/78 plotted with depth...... 6-20

Figure 6-20: Variation of alteration index (A40) for the different stratigraphic units in borehole DF 1/75 indicating maximum acquired temperatures (indicated by ellipses) for the Volksrust Formation to be above 350°C, Vryheid Formation between 500- 550°C, Pietermaritzburg Formation between 450–600°C and the Dwyka Group above 450°C...... 6-21 Figure 6-21: Variation in magnetic susceptibility with increasing temperature for sample DF41 from the Volksrust Formation in borehole DF 1/75. Rapid increase in susceptibility of the heating curve above 430°C (inset) indicates the formation of possibly titanomagnetite. The observed Curie temperature at 580°C and lower cooling curve after heating to 590°C with susceptibility returning to its previous value suggests no additional mineral transformation. Red line indicates heating curve and blue line the cooling curve...... 6-22 Figure 6-22: Variation in magnetic susceptibility with increasing temperature for two samples from the Vryheid Formation above sill 1 in borehole DF 1/75. From sample DF64 (a) both pyrrhotite and magnetite was observed while in sample DF70 (b) only magnetite was formed after heating to temperatures above 540°C. Red line indicates heating curve and blue line the cooling curve...... 6-23 Figure 6-23: Variation in magnetic susceptibility with increasing temperature for two samples from the Vryheid Formation below sill 1 in borehole DF 1/75. Sample DF103 (a) indicate both pyrrhotite and magnetite while sample DF08 (b) only indicated magnetite after heating to temperatures above 590°C. Red line indicates heating curve and blue line the cooling curve...... 6-24 Figure 6-24: Variation in magnetic susceptibility with increasing temperature for sample DF17 from the Pietermaritzburg Formation above sill 2 in borehole DF 1/75. Pyrrhotite as primary mineral is transformed into magnetite above temperatures 590°C. Red line indicates heating curve and blue line the cooling curve...... 6-25 Figure 6-25: Variation in magnetic susceptibility with increasing temperature for two samples from the Pietermaritzburg Formation below sill 2 in borehole DF 1/75. Sample DF25 (a) indicate oxidation of weakly magnetic phases into magnetite above applied temperatures of 640°C. Primary pyrrhotite in sample DF27 (b) is transformed into magnetite at temperatures above 540°C. Red line indicates heating curve and blue line the cooling curve...... 6-26

Figure 6-26: Variation in magnetic susceptibility with increasing temperature for sample DF30 from the Dwyka Group in borehole DF 1/75 indicating initial formation of magnetite [1] at temperatures above 490°C that is gradually dissolved again [2] into the host rock until the heating and cooling curve become reversible again above 600°C [3]. Red line indicates heating curve and blue line the cooling curve...... 6-27

Figure 6-27: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole DF 1/75 plotted with depth...... 6-28

Figure 6-28: Variation of alteration index (A40) for the different stratigraphic units in borehole SA 1/66 indicating maximum acquired temperatures (ellipses)...... 6-30 Figure 6-29: Variation in magnetic susceptibility with increasing temperature for two samples from the Lower Beaufort Group in borehole SA 1/66. Both samples show rapid increase in magnetic susceptibility above 440°C, but indicate reabsorption and lowering of susceptibility to its original value above 540–590°C. Red line indicates heating curve and blue line the cooling curve...... 6-31 Figure 6-30: Variation in magnetic susceptibility with increasing temperature of sample SA829 from the Upper Ecca Group in borehole SA 1/66. Gradual increase in magnetic susceptibility occurs only at temperatures above 540°C with a Curie temperature of 680°C indicating that hematite was formed. Red line indicates heating curve and blue line the cooling curve...... 6-32 Figure 6-31: Variation in magnetic susceptibility in borehole SA 1/66 for samples SA950 and SA956 from the Upper Dwyka (White Hill and Prince Albert formations). Rapid increase in magnetic susceptibility occurs at temperatures above 440°C with a variety of peaks observed in both the heating and cooling curves indicating the formation of a wide mixture of magnetic minerals. Red line indicates heating curve and blue line the cooling curve...... 6-33 Figure 6-32: Variation in magnetic susceptibility with increasing temperature of sample SA1000 from the Dwyka Group in borehole SA 1/66. Gradual increase in magnetic susceptibility occurs only at temperatures above 540°C indicating oxidation magnetite and hematite. Red line indicates heating curve and blue line the cooling curve...... 6-34

Figure 6-33: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SA 1/66 plotted with depth...... 6-35

Figure 6-34: Variation of alteration index (A40) for the different stratigraphic units in borehole SC 3/67 indicating maximum acquired temperatures (ellipses) to vary between 290°C (White Hill and Prince Albert formation) and 590°C (Teekloof Formation)...... 6-36 Figure 6-35: Variation in magnetic susceptibility with increasing temperature of samples from the Lower Beaufort Group (Teekloof Formation) in borehole SC 3/67. Oxidation only started above 540°C and the Curie temperatures indicate magnetite and hematite as the oxidation products. Red line indicates heating curve and blue line the cooling curve...... 6-38 Figure 6-36: Variation in magnetic susceptibility with increasing temperature of samples from the Upper Ecca Group in borehole SC 3/67. First oxidation started above 290°C (SC72) with a gradual increase in magnetic susceptibility. Heating and cooling curves are nearly reversible for this sample. Red line indicates heating curve and blue line the cooling curve...... 6-39 Figure 6-37: Variation in magnetic susceptibility with increasing temperature of samples from the Lower Ecca Group (White Hill and Prince Albert formations) in borehole SC 3/67. Both pyrrhotite and magnetite are observed in sample SC89, while only magnetite occurs in sample SC98. Red line indicates heating curve and blue line the cooling curve...... 6-40 Figure 6-38: Variation in magnetic susceptibility with increasing temperature from the Dwyka Group in borehole SC 3/67. Both pyrrhotite and magnetite are observed. Red line indicates heating curve and blue line the cooling curve...... 6-40 Figure 6-39: Variation in magnetic susceptibility with increasing temperature of samples from the Bokkeveld Group in borehole SC 3/67. Both pyrrhotite and magnetite are observed with heating and cooling curves near completely reversible. Red line indicates heating curve and blue line the cooling curve...... 6-41

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Figure 6-40: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SC3/67 plotted with depth...... 6-42

Figure 6-41: Variation of alteration index (A40) for different lithologies within the Vryheid Formation in borehole CBC4495 indicating maximum acquired temperatures (indicated by ellipses) to be above 390°C. SD = sandstone; MS = mudstone-siltstone; TM = sandstone-mudstone-carbonaceous; SD80-MS-20 = 80% sandstone and 20% mudstone-siltstone; ZG90-MX10 = 90% sandstone-gridstone and 10% mudstone- carbonaceous-siltstone...... 6-43 Figure 6-42: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different lithologic units in borehole CBC4495. Red line indicates heating curve and blue line the cooling curve. Most samples start to oxidize above 390°C. Heating curves above 490°C suggest the formation of possibly titanomagnetite which transforms into magnetite at higher temperatures. The wide peaks of nearly all the cooling curves after progressive heating to 540°C suggest a mixture of magnetic phases...... 6-44

Figure 6-43: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole CBC4495 plotted with depth...... 6-46

Figure 6-44: Variation of alteration index (A40) for the Beaufort Group and Vryheid Formation in borehole MY19 indicating maximum acquired temperatures (indicated by ellipses) of between 390°C above and 360°C below the coal seam...... 6-47 Figure 6-45: Variation in magnetic susceptibility with increasing temperature for sample MY19_3 from the Beaufort Group in borehole MY19. Curves are reversible up to 390°C whereafter a rapid increase in the heating and cooling curves indicate the formation of magnetite or titanomagnetite. The wide ‘hump’ at ~350°C suggests the formation of pyrrhotite during cooling. Red line indicates heating curve and blue line the cooling curve...... 6-48 Figure 6-46: Variation in magnetic susceptibility with increasing temperature for sample MY19_8 from the Vryheid Formation in borehole MY19. Curves are reversible up to 390°C whereafter an increase in the cooling curves indicate the formation of new minerals. Red line indicates heating curve and blue line the cooling curve...... 6-48

Figure 6-47: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SC3/67 plotted with depth...... 6-49 Figure 6-48: Variation in maximum geothermal temperatures reached by Karoo sediments with depth indicating the extent of the thermal influence within the contact aureoles. Expected extent of contact aureoles (50% of sill thickness) are indicated by red bars. ..6-50 Figure 6-49: Variation in maximum geothermal temperatures with depth reached by Karoo sediments in boreholes not intersecting any dolerites...... 6-52

Figure 7-1: Time vs. temperature contours calculated for pyrrhotite (Fe7S8) and magnetite (Fe3O4). Blocking or unblocking temperatures of magnetization for a particular ensemble of single-domain grains can be determined as a function of time by following one of the contours. Below the pyrrhotite Curie point, Tc = 320°C, magnetite and pyrrhotite contours defined by laboratory thermal demagnetization data intersect to give a unique determination of the remagnetization time and temperature in nature (taken from Dunlop et al., 2000)...... 7-6 Figure 7-2: Isothermal remanent magnetization (IRM) of selected samples from borehole G39974 (LKF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-7 Figure 7-3: Isothermal remanent magnetization (IRM) of selected samples from borehole PP47 (PH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-8 Figure 7-4: Isothermal remanent magnetization (IRM) of selected samples from borehole DF1/75 (DF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-8 Figure 7-5: Isothermal remanent magnetization (IRM) of selected samples from borehole HM1/78 (HM) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-9

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Figure 7-6: Isothermal remanent magnetization (IRM) of selected samples from borehole SA1/66 (SA) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-9 Figure 7-7: Isothermal remanent magnetization (IRM) of selected samples from borehole CBC4495 (GH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction...... 7-10 Figure 7-8: Coercivity of remanence with depth of selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), PP47 (PH), HM1/78 (HM) and DF1/75 (DF) as well as samples from boreholes without sills, SA1/66 (SA) and CBC4495 (GH)...... 7-11 Figure 7-9: Day plots of hysteresis parameters for selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), PP47 (PH), HM1/78 (HM) and DF1/75 (DF) as well as samples from boreholes without sills, SA1/66 (SA) and CBC4495 (GH). With the exception of borehole LKF, the grain sizes of all samples fall within the multi-domain (MD) range, often increasing toward the sill contacts...... 7-12 Figure 7-10: Low-temperature magnetic properties of White Hill Formation shale along a 64 m profile above an 83 m thick sill. Distances of samples from the dyke contact are indicated...... 7-13

Figure 7-11: Ratio of remanence intensity RPYR/MAG along depth for the Prince Albert (Vryheid) Formation...... 7-14 Figure 7-12: Examples of variation in vector plots of thermal demagnetization data for samples within the contact aureole of the White Hill Formation (Borehole G39974). Differently coloured vectors in the Zijderveld diagrams indicate different pyrrhotite phases...... 7-15 Figure 7-13: Determining the remanence conditions under which the White Hill Formation pyrrhotite overprint was acquired due to the Karoo LIP by extrapolating from laboratory data using the contours of Dunlop et al. (2000)...... 7-16

Figure 8-1: Laterally observed variation of temperature across the Karoo Basin. Red stars indicate boreholes that intercepted dolerite sills, while blue stars indicated boreholes that did not intercept any dolerite. Green dots indicate additional Soekor boreholes (modified simplified geological map of Karoo Basin, courtesy Council for Geoscience). . 8-2 Figure 8-2: Comparative subsea depths of stratigraphic units from selected boreholes (see text) across the main Karoo Basin as derived from borehole logs, with the upper occurrences of the Ecca Group (Ecca), Ecca Shales (ES), Dwyka tillite (DT) and Table Mountain Group (TM) indicated as interconnecting dashed lines. Relative depths of logged dolerite sills within each of these boreholes are indicated. Water temperatures measured at selected depths within boreholes KA, CR and WE are indicated (modified after Scheiber-Enslin et al., 2014) ...... 8-3 Figure 8-3: Thermal modelling of borehole G39974 using a thermal gradient of 25°C/km and an intrusion temperature of 1150°C for the dolerite sills. Within 2000 years the combined temperature has decreased to below the Curie temperature for magnetite. .... 8-4 Figure 8-4: Laterally observed variation in temperature across the Karoo Basin with location of proposed magma sources indicated (red stars) and numbered according to text (modified simplified geological map of Karoo Basin, courtesy Council for Geoscience)...... 8-6 Figure 8-5: Laterally observed variation in temperature across the Karoo Basin in relation to the borders of the Kaapvaal craton. CA = Colesburg Anomaly; TML = Thabazimbi- Muchison Lineament (modified simplified geological map of Karoo Basin, courtesy Council for Geoscience)...... 8-8 Figure 8-6: Thermal modelling of borehole G39974, replacing shale host rock with sandstone, to demonstrate the change in heat flow for a higher thermal conductivity. The combined temperature has decreased to below the Curie temperature for magnetite in half the time (1000 years) compared to shale...... 8-9 Table 8-1: Uranium deposit classification based on depositional environment according to Lally and Bajwah, (2006)...... 8-13

xviii

Figure 8-7: Volcanic associated uranium deposits in relation to magmatic source, mixing of high temperature fluids and meteoric waters, and temperature of mineralizing solutions (Gandhi and Bell, 1995)...... 8-14 Figure 8-8: Hydrocarbon formation temperatures (after Tissot and Welte, 2008)...... 8-17 Figure 8-9: Relative thermogenic gas yield from organic matter, (a) Sapropelic source and (b) Humic source, buried in fine-grained sediments as a function of temperature. C2H6+ represents “wet gas” (hydrocarbon gases heavier than methane) (after Hunt, 1996)...... 8-18

xix

ABSTRACT

GEOTHERMAL HISTORY OF THE KAROO BASIN IN SOUTH AFRICA INFERRED FROM MAGNETIC STUDIES by L.P. MARÉ The Karoo succession has economic significance through the exploitation of extensive coal deposits and in recent years has seen significant international interest due to potentially large shale gas resources. The thermal history of sedimentary basins affects the genesis of hydrocarbon deposits and it is therefore essential to model and reconstruct the geothermal variation across the Karoo Basin before evaluation of the hydrocarbon resources can take place. The main scientific questions related to the thermal history of the Karoo Basin are whether the emplacement of large volumes of magma was preceded by a large-scale low- grade thermal doming as proposed for continental rift settings. Alternatively, was the Karoo thermal event restricted to the contact aureole of intrusives, as well as the question whether the intrusion of dolerite resulted in large-scale CO2 or CH4 degassing from coalbeds and carbonaceous shales based on similarities to other large igneous provinces? Magnetic techniques provide an alternative to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. Three experiments using existing magnetic and palaeomagnetic methods were conducted to determine the peak temperatures reached by Karoo sedimentary rocks before and after the Karoo magmatic event. These experiments include the classic palaeomagnetic baked contact tests (magnetostratigraphy), analyses of the variation of magnetic susceptibility during repeated progressive heating (alteration index method) as well the variation of relative concentrations of fine grained pyrrhotite and magnetite in sedimentary strata relative to their distance from an intrusive (pyrrhotite/magnetite geothermometer). Additionally various magnetic fabric analyses were performed including a study of the variation in anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in delineating the extent of the contact aureoles, only the alternating index (A40) had the ability to give estimated peak temperatures. Results indicate a general elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining between these intrusions. A general increase in the palaeotemperatures from southwest to northeast across the basin was observed. This is mainly due to differences in thermal conductivity of the various lithologies across the basin from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone in the northeast.

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to the following people and organizations:

• The Council for Geoscience for supporting individual development and allowing me to use the facilities of the Petrophysical Laboratory for my research. • The National Research Fund for financial support for two years (grant TTK20110805000023453). • A special word of thanks to the staff at the National Core Library who made an effort to supply me with the requested cores for sampling even when no forklift was available to assist them. • To Petro du Pisani at Anglo American for donating cores from the Goedehoop mine for research purposes. • To Claris Dreyer from Exxaro for donating ten samples from the Groottegeluk mine for research purposes. • To my supervisor (Dr. M.O. de Kock) and co-supervisors (Prof. B. Cairncross and Prof. H. Mouri) for your patience and continuous motivation, guidance and support. • To the technical assistants of the Petrophyscial Laboratory, Doreth Kruger and Reuben Mantsha for all your hard work during sample collection and analyses. • To Prof. E.C. Ferré for offering to perform hysteresis and room-temperature IRM measurements on a selection of samples, free of charge, at Southern Illinois University, Carbondale. • To Dr. M. Jackson who offered to perform low-temperature IRM and AARM measurements on a selection of samples, free of charge, at the Institute for Rock Magnetism, Minneapollis. • To Dr. M.O. de Kock for allowing me access to the UJ Palaeomagnetic Laboratory where samples from one of the cores were analysed. • I am grateful to my different managers, Patrick Cole and Christo Craill, who supported me through the whole duration of this study. • Thank you to my co-workers at the Council for Geoscience for your friendship and continuous encouragement. • A special word of thanks to my mother and late father, my whole family and friends for your encouragement, love and support. • And last but not the least, a very special word of thanks to my husband, Frans, and two daugthers, Annie and Lindie, for your patience, encouragement, love and support through all the years when I could not give you all the time and attention you deserved.

xxi CHAPTER 1

CHAPTER 1. INTRODUCTION AND PURPOSE OF THESIS

1.1 Problem Statement

The Late Carboniferous to Middle Jurassic Karoo Supergroup covers large areas of

Southern Africa, South America, the Falkland Islands, Madagascar, India, Australia and

Antarctica, together forming part of what was the Gondwana supercontinent. The Main Karoo

Basin in South Africa covers an area of approximately 700 000 km2 with the bulk of the

sediments being supplied by source areas located to the south of the basin (Catuneanu et al.,

1998, 2002; Johnson et al., 2006). Deposition was terminated with the emplacement of the

Karoo Igneous Province at approximately 183 Ma (Jourdan et al., 2005; Svensen et al.,

2012).

The Karoo succession has economic significance through the exploitation of extensive

coal deposits (Bredell, 1987; Johnson et al., 2006) and in recent years has seen significant

international interest due to potentially large shale gas resources (Cole et al., 2011; Kuuskraa

and Moodhe, 2013; Wait and Rossouw, 2014). The thermal history of sedimentary basins

affects the genesis of hydrocarbon deposits and therefore need to be understood and

modelled before any resource calculations can be made.

The main scientific questions related to the thermal history of the Karoo Basin are:

1) Was the emplacement of large volumes of magma preceded by a large-scale low-

grade thermal doming as proposed for continental rift settings (e.g., Huismans et al., 2001)?

Or, alternatively, was the Karoo thermal event restricted to the contact aureole of intrusives?

2) Did the intrusion of basalts result in large-scale CO2 or CH4 degassing from coalbeds

as suggested by Svensen et al. (2007) and Aarnes et al., (2011) for the Karoo Basin based

on similarities to other large igneous provinces (Svensen et al., 2004 and Ganino and Arndt,

2009)?

Traditionally the low-grade thermal history of sedimentary rocks is constrained by either

illite crystallinity (IC) in the range 160-380°C (Ji and Browne, 2000; Jaboyedoff et al., 2001) or

coal vitrinite reflectivity (Rm) in the 25-325°C range (Hower and Davis, 1981; Bray et al.,

1992; Boudou, 1984). Both of these techniques, however, have significant uncertainty in

terms of estimated temperature.

1–1 CHAPTER 1

The alteration of magnetic fabric due to low-grade metamorphism was observed and described decades ago (Everitt and Clegg, 1962; Irving, 1964; McElhinny, 1973). More recently the use of magnetic techniques to study the metamorphic effect of igneous intrusions on sedimentary rocks were focused on the variable occurrence of pyrrhotite and magnetite in limestone and claystone (Schill et al., 2002; Gillett, 2003; Wehland et al., 2005). These studies highlighted the applicability of using rock magnetic properties and their response to heating and cooling as a tool to study the geothermal history of sedimentary basins.

This study will discuss results from different rock magnetic and palaeomagnetic experiments applied to samples from several boreholes and from outcrop within the Karoo

Basin, South Africa, to evaluate the effective use of these methods in determining peak temperatures reached by the Karoo sedimentary rocks during large-scale magma intrusions.

1.2 Objectives of Study

This study proposes to conduct several experiments using existing magnetic and palaeomagnetic methods to determine the peak temperatures reached by Karoo sedimentary rocks. These experiments include:

1) Palaeomagnetic baked contact tests (Everitt and Clegg, 1962) that will be conducted around specific intrusives of known thickness to constrain pre-intrusion temperatures at various heights in the Karoo Basin stratigraphy.

2) Variation of magnetic susceptibility during repeated progressive heating following the method of Hrouda et al. (2003), will be investigated to evaluate sedimentary rocks peak palaeotemperatures away from an intrusive and at decreasing distance from contacts.

3) The Pyrrhotite/Magnetite Geothermometer (Schill et al., 2002; Aubourg and Pozzi,

2009), which is based on the variation of relative concentrations of fine grained pyrrhotite and

magnetite in sediments due to changes in the temperatures relative to the distance from an

intrusive.

This project will contribute key information in modeling the potential impact of the Karoo

large igneous province (LIP) on release of greenhouse gases from baked organic-rich

sediments. This process of degassing has been shown (e.g., Svensen et al., 2004) to have

important implications for global climate change as characterized by major perturbation of the

1–2 CHAPTER 1

global carbon cycle. Thermal modeling of the Karoo basin also has very relevant and

important economic implications for hydrocarbon exploration in South Africa by identifying the

most probable areas where shale gas resources might be found.

1.3 Methods

The response of a magnetic substance to temperature depends strongly on the physical properties of the material (Tauxe, 2002). Several methods of rock magnetic analysis can be used to identify the ferromagnetic assemblage of rocks, including thermal variation of magnetic susceptibility (to distinguish between paramagnetic and ferromagnetic fractions and to identify magnetic minerals from their Curie temperatures), magnetic hysteresis loops (give indication of mineralogy and grain size) and isothermal remanent magnetization (IRM) (to determine magnetic saturation and coercivity spectra). A study of the rock magnetic fabric will be essential to understand and interpret the results from the proposed experiments.

1.3.1. Experiment 1: Baked contact test

The fundamental concept underlying the palaeomagnetic baked contact test is conductive heat transfer, a model developed by Jaeger (1957). In a seminal review by Jaeger

(1964) on the thermal effects of intrusions in adjacent country rock, the influence of different parameters such as the initial temperature, thickness, and thermal properties of the magma; the temperature, conductivity, and thickness of the cover as well as the thermal effect of water or metamorphism in the country rock was studied. It was shown mathematically that the effect of the intrusion on the country rock becomes progressively less as the distance from the contact increases.

The intruding magma heats the surrounding host rock, which upon cooling will acquire a magnetic remanence in the same magnetic field as that in which the intrusive rock becomes magnetized (i.e. geomagnetic field). Unless the country rock is of similar age (and composition) as the intrusive rock, there will generally be directional differences which can be used as indication of the stability of the magnetization of the intrusion (McElhinny, 1973). This also applies to the baked rock within the contact aureole and the changes in properties with distance from the baked contact correspond to the diminishing heating effects of the intrusion

(Everitt and Clegg, 1962).

1–3 CHAPTER 1

Irving (1964) defined three gradational zones outside an igneous body (Figure 1-1) namely the metamorphic zone (extensive changes in the magnetic minerals), the adjacent heated zone (small changes in magnetic minerals), and the warmed zone (temperature never rose to the Curie temperature of the magnetic minerals and only a partial TRM is induced). In both the metamorphic and heated cases the rock will have acquired a total thermal remanent magnetization (TRM). Outside the warmed zone is the unheated country rock.

Often, as is the case in the current study, drill cores are not azimuthally oriented.

However, by using the knowledge of the top/bottom of the core the palaeomagnetic inclination can still be used as indicator of polarity. Plotting the variation in inclination with depth along a stratigraphic section (polarity log) is one of several steps towards the creation of a magnetostratigraphic section. As discussed above, when a sedimentary sequence is intruded by a younger igneous rock of different geomagnetic polarity, a gradual change in the

1–4 CHAPTER 1

observed magnetic properties and more specific the magnetic inclination of the host rock

would be observed with distance from the contact aureole. Combining the polarity log with the

results from baked contact tests can thus be used as a tool to determine the extent of

magnetic overprinting caused by younger magmatic intrusions on a sedimentary sequence. A

positive contact test would indicate that the host rock was heated to temperatures above the

unblocking temperature of the remanence carriers and subsequently remagnetized in the

contact aureole to reflect the younger intrusive magnetization direction.

A complementary approach to magnetostratigraphy is the measurement of the

anisotropy of low-field magnetic susceptibility (AMS). The AMS of any rock is dependent on

the intrinsic magnetic susceptibility, volume fraction and degree of preferred orientation of the

individual rock-constituent minerals (Tarling and Hrouda, 1993; Chadima et al., 2006), and

therefore not only reflects the differences in rock fabric, but also any mineralogical differences

within the studied rock (Robion et al., 1995). Significant errors in magnetostratigraphic

interpretations may occur if these mineralogical variations are caused by the presence of

different types of ferromagnetic minerals (sensu lato; e.g. magnetite, goethite, hematite) (e.g.,

Lehman et al., 1996) as each of these minerals show different behavior in different geological

situations and when occurring within the same sample could display complex interaction and

overlapping coercivity sprectra.

It is important that the rock magnetic fabric of the samples collected in the Karoo Basin is

determined to ensure a clear understanding of the observed magnetic responses in relation to

the ferromagnetic mineralogy of these rocks.

1.3.2. Experiment 2: Variation of magnetic susceptibility with repeated progressive

heating

The peak palaeotemperature away from the intrusive can be examined using the method described by Hrouda et al. (2003). With this method the relationship between the heating and

cooling curves of magnetic susceptibility measurements made during individual progressive

heating and cooling runs are evaluated. According to Hrouda et al. (2003), if the heating and

cooling curves of a particular run are very near each other, it is likely that the rock previously

1–5 CHAPTER 1 experienced the maximum run temperature in its geological history, and that the laboratory heating does not result in any changes of the magnetic phases (magnetic phases equilibrated). If, however, the heating and cooling curves differ substantially, it is likely that the rock did not previously experience temperature conditions to the maximum run temperature.

This can then be used as a palaeotemperature indicator (Figure 1-2). The observed increase in susceptibility with increasing temperature (Figure 1-2) indicates that the magnetic mineralogy of this sample is dominantly ferromagnetic (senso lato).

The magnetic susceptibility of powdered as well as rock fragments from each sedimentary unit taken in varying intervals from mafic intrusions were measured while progressive increasing the temperature according to a method described by Hrouda et al.

(2003). The maximum run temperature for the first borehole studied was increased in intervals of 25°C from a minimum of 90°C up to a maximum temperature of 690°C. For the remaining boreholes the temperature intervals was set at 50°C.

The changes in magnetic susceptibility due to heating were evaluated quantitatively by the A40 alteration Index, hereafter referred to as A40, as described by Hrouda et al. (2002):

100 −= / KKkA 40 ( 40 40 ) 40 ,

where k40 and K40 are the magnetic susceptibilities on the cooling and heating curves at

40°C, respectively.

The result of this experiment will indicate the change in peak temperature with depth in sampled drill core, as well as the change in temperature experienced at various distances from an intrusion.

1–6 CHAPTER 1

Figure 1-2: Example of the relationship between heating (red) and cooling (blue) curves indicate that the maximum temperature that the rock at site KAM0 underwent in it geological history was between 250–300°C.

1.3.3. Experiment 3: Pyrrhotite/Magnetite Geothermometer

During early diagenesis in anoxic sedimentary environments, magnetic iron sulphides and iron oxides can form as a result of temperature elevation (Kars et al., 2012). The neoformation of magnetic minerals promotes chemical remanent magnetization with associated differences in magnetic signatures. The variation of magnetic mineral assemblages can thus give an indication of the temperature conditions and/or burial depth.

However, neoformation of magnetic minerals may also originate from other mechanisms such as fluid circulation (Evans et al., 2000; Elmore et al., 2001), pyrite alteration (Gillett,

2003), deformation (Lewchuk et al., 2003), maturation of organic matter (Banjeree et al.,

1997) and contact metamorphism (Aubourg et al., 2014).

Several palaeotemperature studies in sedimentary basins have made use of the variation in magnetic mineral assemblages as geothermometers. Wehland et al. (2005) studied the variation in rock magnetic and palaeomagnetic signals associated with pyrrhotite- bearing limestones from different contact metamorphic settings; Aubourg and Pozzi (2010)

1–7 CHAPTER 1

identified different concentrations of heat-induced pyrrhotite and magnetite assemblages in

claystones in the 50–250 °C range and together with Abdelmalak et al. (2012) studied the

associated impact on determining maturity and burial temperature; Schill et al. (2002) studied

the systematic variations in the ferrimagnetic content of metacarbonates along a profile of

increased metamorphism. These variations were detected by the ratio of remanence intensity

of pyrrhotite to magnetite, derived from natural remanent magnetization and saturation

magnetization. The temperature gradient and ranges suggested by this method was

successfully correlated with results from the non-magnetic calcite twin lamellae

geothermometry method. Dunlop et al. (2000) developed a pyrrhotite time-temperature (T-t)

contour diagram to be used in conjunction with the Pullaiah et al. (1975) magnetite T-t contour

diagram to estimate palaeotemperatures from observed pyrrhotite remagnetization

temperatures.

Taking the above mentioned approaches into consideration, the variation in

pyrrhotite/magnetite ratios within sedimentary rocks of the Karoo Basin as well as within

contact aureoles will be studied with the aim of determining the geothermal temperature

variation within the Karoo Basin.

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Schill, E., Appel, E. and Gautam, P., 2002. Towards pyrrhotite/magnetite geothermometry in

low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola, Central

Nepal). Journal of Asian Earth Sciences, 20, 195–201.

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Svensen, H., Planke, S., Malthe-Sorenssen, A., Jamtveit, B., Myklebust, R., Rasmussen

Eidem, T. and Rey, S.S., 2004. Release of methane from a volcanic basin as a

mechanism for initial Eocene global warming. Nature, 429(6991), 542–545.

Svensen, H., Planke, S. and Chevallier, L., Malthe-Sorenssen, A., Corfu, F. and Jamtveit, B.,

2007. Hydrothermal venting of greenhouse gases triggering Early Jurassic global

warming. Earth and Planetary Science Letters, 256, 554–566.

Svensen, H., Corfu, F., Polteau, S., Hammer, Ø. and Planke, S., 2012. Rapid magma

emplacement in the Karoo Large Igneous Province. Earth and Planetary Science

Letters, 325–326, 1–9.

Tauxe, L., 2002. Paleomagnetic principles and practice. Kluwer Academic Publishers,

London, 299 pp.

Wait, R. and Rossouw, R., 2014. A comparative assessment of the economic benefits from

shale gas extraction in the Karoo, South Africa. Southern African Business Review,

18(2), 1–34.

Wehland, F., Alt-Epping, U., Braun, S. and Appel, E., 2005. Quality of pTRM acquisition in

pyrrhotite bearing contact-metamorphic limestones: possibility of a continuous record of

Earth magnetic field variations. Physics of the Earth and Planetary Interiors, 148, 157–

173.

1–12 CHAPTER 2

CHAPTER 2. GENERAL GEOLOGY OF THE KAROO BASIN AND

PREVIOUS STUDIES

2.1 Introduction

The Karoo Basin contains a succession of clastic sedimentary strata that range in palaeoenvironmental setting from glacial at its base, followed by shallow to deep marine sediments, paralic, fluvial-deltaic and fluvial, ultimately terminated by continental fluvial, lacustrine and semi-arid to arid settings. Closure of the basin is reached with the emplacement of the Karoo Igneous Province and outpouring of vast volumes of continental flood basalt (CFB). According to Johnson et al. (1997), the highly asymmetrical Karoo foreland Basin has a maximum cumulative thickness of some 12 km in the southern extremities, thinning dramatically towards the north. The Karoo CFB and its associated dolerite intrusions constitute a major part (~106 km3) of the stratigraphic section within the

Karoo Basin. The increased abundance of pyroclastic sediments in the south-eastern part of

the basin and decrease in thickness of the Drakensberg Formation volcanic pile to the west

suggests a volcanic source (thermal anomaly), some 150–200 km off the present southeast

coast of South Africa within the offshore limit for Karoo volcanism (Griffiths and Campbell,

1990). The metamorphic effects of this magmatic event on the sedimentary rocks of the

Karoo Basin remain poorly constrained. Ballard et al. (1986) proposed pervasive sediment remagnetization due to the emplacement of the Karoo Igneous Province, a viewpoint at odds with the low thermal maturity inferred from the presence of light-coloured tetrapod bones in several localities (Smith, 1995). The palaeomagnetic results of De Kock and Kirschvink

(2004) suggest that the thermal imprint was only significant in proximity to Karoo intrusives.

Brown et al. (1994) came to a similar conclusion after investigating fission tracks in detrital zircons.

2.2 Karoo Basin

The main Karoo Basin has been described by Dickinson (1974) and later discussed in great detail by Catuneanu et al. (1998, 2002). The Karoo Basin constitutes a retro-arc foreland basin and is one of the most complete of a series of basins developed on

2–1 CHAPTER 2

southwestern Gondwana between the Carboniferous and the Jurassic (Smellie, 1981;

Johnson et al., 1996; Catuneanu et al., 1998). Basin fill, which comprises sedimentary rocks

of the Karoo Supergroup, attains a maximum cumulative thickness of some 12 km in the

southern extremities of the basin (Johnson et al., 1997), but this thins dramatically towards

the north, to produce a highly asymmetrical foreland basin. This asymmetry reflect maximum

down-warping along a linear belt (the so-called Karoo Trough) situated along the southern

edge of the basin adjacent to the Cape Fold Belt during basin development, which gradually

shifted northward as the basin filled from the Late Carboniferous onwards via gradual

subsidence of essentially cratonic crust (Figure 2-1). The Cape Fold Belt formed while

sedimentation of at least the upper Karoo units were still in progress and resulted in intense

deformation of the underlying Cape Supergroup and basement, as well as the lower units of

the Karoo Supergroup, along the southern basin edge (Johnson et al., 2006).

Catuneanu et al. (1998, 2002) described the Karoo Basin as an under filled foreland system that was controlled by cyclic loading and unloading of the Cape Fold Belt. Contrasting tectonic histories across a flexural hinge line of the foreland system account for the differences in stratigraphy between the southern (proximal) and northern (distal) regions of the basin.

The palaeoenvironmental setting of basin fill ranges from glacial at the base (Dwyka

Group), transitions through carbonaceous marine and sub-marine (Hodgson et al., 2006; Flint et al., 2007; Hodgson, 2009) into fluvial-deltaic coal-bearing strata (Ecca Group) in the north, succeeded by red and greenish terrestrial fluvio-lacustrine strata (Beaufort Group), and ultimately terminates as fluvial and aeolian sandstones deposited in a more arid climate

(Smith, 1990; Smith et al., 1993) (i.e., the Molteno, Elliot and Clarens formations, also referred to as the Stormberg Group). Sedimentation in the main Karoo Basin was terminated during the Middle Jurassic with the outpouring of at least 1400 m of basaltic lavas

(Drakensberg Group). The Karoo CFB and its associated dolerite intrusions constitute a major part of the stratigraphic section within the Karoo Basin (Duncan and Marsh, 2006).

2–2 CHAPTER 2

Figure 2-1: Simplified geology of the main Karoo Basin showing the areal distribution of the main lithostratigraphic units (modified from Johnson et al., 2006 and Lanci et al., 2013).

2.3 Karoo Large Igneous Province

During the Jurassic, dolerite dykes and sills intruded the Karoo Basin in a period of extensive magmatic activity that took place over almost the entire Southern African subcontinent during one of the phases in the break-up of Gondwanaland. These intrusives represent the roots and the feeders of the extrusive Drakensberg basalts and were recently dated using the U-Pb zircon and baddeleyite method ranging between 183 ± 0.5 and 182.3 ±

0.6 Ma (Svensen et al., 2012). 40Ar-39Ar geochronology of the intrusives and lavas suggests a

slightly longer, but still very brief duration of magmatism (Jourdan et al., 2005). The total

volume of the magma extruded onto Southern Africa has been estimated at 106 km3 (White,

1997). Large-scale erosion of the main Karoo Basin has revealed the deeper portions of the

2–3 CHAPTER 2 intrusive system and a tectonic complexity not encountered in other CFB provinces in the world (e.g. Jourdan et al., 2006) (Figure 2-2).

Figure 2-2: a) Simplified geological map of the Karoo Basin with the extent of dolerite intrusions indicated. b) Schematic SW-NE cross-section of the Karoo Basin showing the complexity of the dolerite sill and dyke network below the Drakensberg lavas (after Chevallier et al., 2001).

The Karoo intrusive rocks consist of interconnected and coalescing dolerite sills and dykes, with many individual sill bodies being saucer shaped (e.g., Chevallier and Woodford,

1999; Chevallier et al., 2001; Malthe-Sørenssen et al., 2004; Polteau et al., 2008). Chevallier and Woodford (1999) reported that most of the igneous bodies are preferentially emplaced at contacts between the Dwyka and Ecca groups and between the upper Ecca and lower

Beaufort groups. A sharp decrease in the amount of intrusions was observed by the authors at the boundary between the lower Ecca Group and the upper Ecca Group. This boundary corresponds to the appearance of the first major sandstone units in the Karoo Basin. The bulk of the intrusions is stratabound and concentrated between the upper Ecca Group and sandstones of the Beaufort Group. This suggests that the dolerite might have propagated laterally (subhorizontally) along strike, and not vertically (Chevallier and Woodford, 1999).

2–4 CHAPTER 2

The increased abundance of pyroclastic rocks in the southeastern part of the basin and decrease in thickness of the Drakensberg volcanic pile to the west suggests a volcanic source (thermal anomaly), some 150-200 km off the present southeast coast of South Africa within the offshore limit for Karoo volcanism (Griffiths and Campbell, 1990). Chevallier and

Woodford (1999) suggested that the magma source corresponds to a triple junction of three rift zones located east of the town of East London.

Based on dyke distribution, three major structural domains have been identified in the main Karoo Basin (Figure 2-3) by Chevallier and Woodford (1999):

1) The Western Karoo domain extends from Calvinia to Middelburg and is characterized by two distinctive structural features. These are an E–W trending zone of long and thick dykes associated with right lateral shear deformation (assuming all dykes are of the same age), and

NNW-SSE trending dykes;

2) The Eastern Karoo domain extends from Middelburg to East London and comprises two major dyke swarms, namely a major curvi-linear E-W to NW-SE swarm of extensive and thick dykes diverging from a point offshore of East London, and a minor NNE-SSW trending dyke swarm (see D2 in Figure 2-3 a));

3) The Transkei-Lesotho-Northern Karoo domain consists of two swarms. NW-SE trending dykes in the Transkei region, curving to become E-W in the Free State, and NE-SW trending dykes mainly occurring within and alongside the Drakensberg basalt in Lesotho.

The sills and saucer-shaped complexes in the Karoo Basin have the same geographic distribution as the dykes, and to a large extent control the geomorphology of the landscape. Chevallier and Woodford (1999) suggested inherent structural control in the intrusive event such as jointing associated with initial uplift just prior to the magmatic intrusions. The dolerite sills and dykes form a complex intrusive network that was probably acting as a shallow magma storage system (Chevallier and Woodford, 1999).

2–5 CHAPTER 2

Figure 2-3 – Dolerite dykes of the Main Karoo Basin (after Chevallier and Woodford, 1999). Inset (a): simplified structural map showing the three structural domains. Insert (b): geodynamic interpretation of the Western and Eastern Karoo structural setting.

2.4 Published Geothermal Related Studies on the Karoo Basin

No discussion on the geothermal history of the Karoo would be complete without mentioning the seminal work by Rowsell and De Swardt (1976) that summarized the results of many years of investigation by SOEKOR (Southern Oil Exploration Corporation) aimed at proving/disproving the existence of oil and gas in South Africa, and whose conclusions still largely stand today. According to Rowsell and De Swardt (1976) a suite of diagenetic indicators (which included amongst others illite crystalinity or IC, and vitrinite reflectance) show a general increase in diagenesis from north to south across the Karoo Basin as a whole, with palaeotemperature estimates ranging from 150-170°C in the north to 270-300°C in the south. The degree of diagenesis of Karoo rocks is related mainly to the depth of burial,

2–6 CHAPTER 2 and also to the effect of dolerite intrusions in the central part of the Karoo Basin (Rowsell and

De Swardt, 1976). It is the effect of the dolerite intrusions, which is of particular interest for this thesis. Recent work (summarized below) has hinted at variably complex thermal effects that these intrusions have on the Karoo sedimentary strata. The effects of intrusions range from being regionally extensive to being restricted locally, depending on hydrothermal activity associated with intrusions, intrusion thickness and vertical spacing between multiple intrusions.

Brown et al. (1994) made a quantitative assessment of the effects of magmatism on the thermal history of the Karoo sedimentary sequence. These authors used quantitative numerical modelling to investigate the thermal effects of both deep sub-crustal heating, and shallower effects of high-level intrusions compared to the level of erosion of the Basin. These models were tested against maximum palaeotemperature estimates derived from vitrinite reflectance profiles and zircon fission track data. From their models the authors concluded that the transfer of excess heat of both increased burial depth and high level intrusions could not have occurred by thermal conductivity alone. Fission track data from detrital zircons from two sedimentary-hosted uranium ore deposits implies that these deposits experienced maximum palaeotemperatures of at least 250 ± 50°C subsequent to deposition during the period of Karoo magmatism (Brown et al., 1994). This suggests the existence of magmatically driven hydrothermal systems within the Karoo Basin, and their probable genetic association with uranium mineralization. Hydrothermal activity ensured that the excess heat was dissipated over a more extensive thickness of the stratigraphic sequence than would have occurred under a system controlled by conductive heat transfer alone.

A large number of hydrothermal vent complexes have been identified in the Karoo

Basin (Jamveit et al., 2004; Svensen et al., 2006; Van der Walt, 2012). Svensen et al. (2006) suggests that these pipe-like structures originate in contact aureoles around sill intrusions, where heating and expansion of host rock pore fluids results in rapid pore pressure build-up and phreatic eruptions. Svensen et al. (2006) further suggests that these hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, and that they slightly predate the onset of the main phase of flood volcanism.

Jamveit et al. (2004) suggests from theoretical arguments that the extent of fluid-pressure

2–7 CHAPTER 2

build-up is dependent on the relative rate of heat and fluid transport and that, in the case of

the Karoo Basin, high fluid fluxes towards the surface were sustained by boiling of aqueous

fluids near the sill. These authors suggested that both sill bodies and hydrothermal vent

complexes represent major perturbations of the permeability structure of the sedimentary

basin. Jamveit et al. (2004) concluded that the fluid pressure in the Karoo Basin must have

been very high during the initial vent formation to accommodate roof lifting and ultimately the

disaggregation and partial fluidization of the overlying porous wedges.

Svensen et al. (2007) reported results from several breccia pipes in the Ecca and

lower Beaufort groups. As with the hydrothermal vent complexes these pipes are also shown

to originate within the contact aureoles around sill intrusions. Vitrinite reflectance

measurements of metamorphosed shale and breccia adjacent to the sills show the severe

thermal effects of the intruded magma on the organic material. Metamorphosed shale in a

stratigraphic borehole (i.e., borehole G39974 near Calvinia) ~700 m west of such a breccia

pipe, shows vitrinite reflectance values of 2.4 to 8.2% Ro (Svensen et al., 2007). These

values are well above the threshold value of 1.5% Ro corresponding to temperature regimes

where gas is normally produced (Hunt, 1996), and demonstrate a very high degree of

transformation of the original organic matter.

Aarnes et al. (2011) collected samples from the same borehole (G39974), as well as

from boreholes KL1/78 and DP1/78 near Hopetown. They calculated the bulk H2O content of the Tierberg, White Hill and Prince Albert formations (Ecca Group) based on mineral equilibria as a function of pressure and temperature. From these results as well as petrographic analysis of two samples from the Tierberg and White Hill formations showing the highest degree of metamorphism, it was clear that temperatures of at least 400°C were reached within the contact aureole of the ~10 m thick sill separating these formations. Aarnes et al.

(2011) studied the effects of multiple sill intrusions on the contact metamorphic devolatilization of shales in the Karoo Basin. These authors quantified the methane gas production potential of single and multiple sill intrusions as well as the effect that vertical spacing between simultaneously emplaced sills has on gas generation potential. This quantification was based on an integration of numerical modelling with organic and inorganic geochemistry. Some of the major conclusions from their study are that the consequence of

2–8 CHAPTER 2

intrusive heating is a major loss of organic carbon, increased maturity, metamorphic mineral

reactions and generation of overpressure and fluid flow out of the aureoles. They also

suggest that there is a critical relationship between the thickness and vertical separation of

simultaneous multiple-layer intrusions and the increased total gas generation, and that the

high occurrence of sills of variable thicknesses in the Ecca Group suggests that hydrocarbon

production occurred throughout the entire sedimentary section (i.e., it was not restricted to the

contact aureoles).

Gröcke et al. (2009) used vitrinite reflectance (Ro) as a palaeo-geothermometer on

two coal transects intruded by dykes in the Highveld Coalfield, Karoo Basin. Their study

indicated that background temperatures of ~100°C increasing to >300°C close to the dyke

contacts ( less 0.3 m from contact). Gröcke et al. (2009) observed no significant changes in

δ13C and suggested that the low vitrinite and liptinite contents of the Highveld coals in part

explains the modest decreases in volatile matter adjacent to dykes. This combined with a

relative narrow metamorphic aureole surrounding the intrusions and the likelihood that at least

some of the volatiles generated by the intrusion were trapped as coalbed methane (CH4) or condensed as pyrolytic carbon, led the authors to conclude that only limited CH4 release was

likely. In addition Gröcke et al. (2009) suggested that based on the original estimates of

moisture contents of the Highveld coals and the depth at time of intrusion (1–2 km) the dykes

would have lost most of their energy by the heating and evaporation of water, thus having

little remaining energy to generate thermogenic CH4.

2.5 Published Palaeomagnetic Studies on the Karoo Basin

Palaeomagnetic studies traditionally have the aim to demonstrate the preservation of primary magnetism in rocks, which usually requires a very low thermal history. By studying the nature and timing of secondary magnetic overprints, however, palaeomagnetic studies can be used to make broad inferences on the thermal history of a basin.

In the past, palaeomagnetic studies from the Karoo Basin have struggled to separate the mineralogical effects of modern day weathering from the thermal effects of Jurassic magmatism and the combined chemical and thermal effects of deformation in response to the development of the Cape Fold Belt. Recent palaeomagnetic studies aided by more highly

2–9 CHAPTER 2 sensitive equipment and improved methodology, however, have increasingly shown that the magnetic and thermal effects of Jurassic magmatism on sedimentary rocks are restricted to within close proximity of intrusions (References to follow).

Ballard et al. (1986) proposed pervasive sediment remagnetization due to the Karoo igneous event after the youngest folding pulse of 230 Ma in the Cape Fold Belt (Halbich et al.,

1983). This viewpoint is at odds with the low thermal maturity deduced from the presence of light-coloured tetrapod bones in several localities (Smith, 1995). The palaeomagnetic directions published by Ballard et al. (1986) and others referred to in their paper, were obtained from sedimentary strata from the lower and upper Beaufort Group at unblocking temperatures less than 350°C.

Kirschvink and Ward (1998) also reported partial or complete remagnetization of

Beaufort strata by Jurassic dolerite intrusions. At Lootsberg Pass, Kirschvink and Ward

(1998) were able to demonstrate a positive baked contact test, thereby establishing that their inferred Permian-Triassic direction is primary and that this direction was only later overprinted by a dolerite intrusion. These authors further suggested that this remagnetisation was not widespread, as suggested by Ballard et al. (1986), but rather narrowly associated with the specific intrusion at Lootsberg Pass. Similarly De Kock and Kirschvink (2004) suggested from palaeomagnetic results in the Komandodriftdam area that the thermal imprint is only significant in proximity to Karoo intrusives. These authors obtained unblocking temperatures from the Palingkloof Member of the Balfour Formation (Beaufort Group) at 350°C and ca.

550°C, tentatively attributed to the presence of maghemite and magnetite. The low-coercivity present-field component observed in the Palingkloof member was suggested to be due to the oxidation of magnetite to maghemite in the modern near-surface environment. A primary magnetic component is carried by magnetite and previous studies (summarized by Ward et al., 2005) suggested a change in the magnetic reversal (R-N-R) across the Permian-Triassic boundary interval.

Lanci et al. (2013) carried out a magnetostratigraphic and geochronological study in the western Karoo Basin on late Permian sediments across the upper Ecca Group (Waterford

Formation) to lower Beaufort Group (Abrahamskraal Formation) contact. The palaeomagnetic results from this study also revealed partial overprint that was tentatively ascribed to the

2–10 CHAPTER 2 emplacement of the Karoo CFB. The authors did, however, obtain a stable primary direction at temperatures higher than 450°C, supported by a positive reversals test for this dual polarity direction, and suggested the main carrier of the palaeomagnetic remanence to be magnetite/maghemite. Lanci et al. (2013) used the presence of both normal and reversed polarity zones to suggest that deposition occurred after the end of the Kiaman Superchron.

The magnetostratigraphic pattern of the Ouberg Pass section correlates with the composite

Illawarra sequence of Steiner (2006), as well as the latest time-scales from Shen et al. (2011) and Gradstein et al. (2012). This clearly suggests that these rocks have not been completely magnetically overprinted by the younger intrusions.

2.6 References

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devolatilization of shales in the Karoo Basin, South Africa, and the effect of multiple sill

intrusions. Chemical Geology, 281, 181–194.

Ballard, M. M., Van der Voo, R. and Halbich, I. W., 1986. Remagnetizations in Late Permian

and Early Triassic rocks from southern Africa and their implications for Pangea

reconstructions. Earth and Planetary Science Letters, 79, 412–418.

Brown, R., Gallagher, K. and Duane, M., 1994. A quantitative assessment of the effects of

magmatism on the thermal history of the Karoo sedimentary sequence. Journal of

African Earth Sciences, 18(3), 227–243.

Catuneanu, O., Hancox, P.J., Cairncross, B. and Rubidge, B.S., 2002. Foredeep submarine

fans and forebulge deltas: orogenic off-loading in the underfilled Karoo Basin. Journal

of African Earth Sciences, 35, 489–502.

Catuneanu, O., Hancox, P.J. and Rubidge, B.S., 1998. Reciprocal flextural behaviour and

contrasting stratigraphies: a new basin development model for the Karoo retroarc

foreland system, South Africa. Basin Research, 10, 417–439.

Chevallier, L. and Woodford, A., 1999. Morpho-tectonics and mechanism of emplacement of

the dolerite rings and sills of the western Karoo, South Africa. South African Journal of

Geology, 102, 43–54.

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Chevallier, L., Goedhart, M. and Woodford, A.C. (Eds.), 2001. The influences of dolerite sill

and ring complexes on the occurrence of ground water in Karoo fractured aquifers: A

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De Kock, M. O. and Kirschvink, J. L., 2004. Paleomagnetic constraints on the Permian-

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African Earth Sciences, 16, 143–169.

Steiner, M.B., 2006. The magnetic polarity timescale across the Permian-Triassic boundary.

In: Lucas, S.G., Cassinis, G., Schneider, J.W. (Eds.), Non-Marine Permian

Biostratigraphy and Biochronology. Geological Society of London, London, 15–38.

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Svensen, H., Corfu, F., Polteau, S., Hammer, Ø. and Planke, S., 2012. Rapid magma

emplacement in the Karoo Large Igneous Province. Earth and Planetary Science

Letters, 325–326, 1–9.

Svensen, H., Jamtveit, B., Planke, S. and Chevallier, L., 2006. Structure and evolution of

hydrothermal vent complexes in the Karoo Basin, South Africa. Journal of the

Geological Society, London, 163, 671–682.

Svensen, H., Planke, S. and Chevallier, L., Malthe-Sorenssen, A., Corfu, F. and Jamtveit, B.,

2007. Hydrothermal venting of greenhouse gases triggering Early Jurassic global

warming. Earth and Planetary Science Letters, 256, 554–566.

Van der Walt, B., 2012. The petrology, petrography and geochemistry of anomalous borehole

core sequences in the Highveld coalfield, South Africa: a case study for diatreme

activity. M.Sc Dissertation, University of Johannesburg, 265 pp.

Ward, P.D., Botha, J., Buick, R., De Kock, M.O., Erwin, D.H., Garrison, G., Kirschvink, J. and

Smith, R., 2005. Abrupt and gradual extinction among late Permian land vertebrates in

the Karoo Basin, South Africa. Science, 307 (5710), 709–714.

White, R.S., 1997. Mantle plume origin for the Karoo and Ventersdorp flood basalts, South

Africa: South African Journal of Geology, 100, 271–282.

2–15 CHAPTER 3

CHAPTER 3. SAMPLING METHODOLOGY AND PROCEDURES

3.1 Introduction

To achieve the aims of the thesis (Chapter 1) a wide spatial distribution of samples was collected from different stratigraphic heights and from various geographic parts of the Karoo

Basin. Samples originate from both outcrop as well as drill core. Sampling allows for investigating the thermal effects of the dolerite intrusions, which constitute a major part of the up to 12 km thick stratigraphic section within the Karoo Basin. In particular, the boreholes containing dolerite intrusions were sampled along a roughly SE-NW distributed profile spanning the main Karoo Basin. This is contrasted with boreholes without dolerite intrusion intersections from the main Karoo Basin as well as from the Ellisras sub-basin. In addition to these samples that focus on the effects of dolerite intrusions, a separate sample set was collected from outcrops well south of the area affected by intrusions (i.e., south of the so- called “dolerite line”). These were affected by deformation of the Cape Fold Belt. The set of

samples thus allows for an investigation of temperature variation due to intrusions,

deformation and diagenesis throughout the basin.

3.2 Sample collection and preparation

3.2.1. Outcrop sampling

During the 2008 field season three dykes and two sills from varying stratigraphic levels were sampled in a preliminary study. The author collected samples from the country rock in the immediate vicinity (within the contact aureole) of the intrusions, while students from

Southern Illinois University, Carbondale (SIU) collected traverse samples from the dolerite dykes and sills respectively. Due to the preliminary nature of the study at that stage, no samples were collected outside of the contact aureole.

Hand samples were collected and oriented in situ with GPS and magnetic compass. Site locations were determined from 1: 50 000 scale topographic sheets and a GPS (Figure 3-1).

3–1 CHAPTER 3

Figure 3-1: Sampling location of 2008 fieldtrip in the southeastern and eastern part of the Karoo Basin. Samples were collected in the contact aureole of both dykes and sills.

3.2.2. Core sampling

During 2010 the core logs of all available boreholes drilled into the Karoo basin and stored at the CGS National Core Library were studied. The depths of intersected dolerite sills were noted as well as the width of the baked zones (if noted by the geologist). Cores that reached the base of the Karoo Basin were identified. A set of eight boreholes was selected on a cross section from southwest to northeast across the basin to represent as wide a distribution as possible. The stratigraphic distribution of rocks intersected by these boreholes ranges from Dwyka, Ecca to lower Beaufort groups. They are thus representative of the lower part of the basin fill, from the units in the Karoo Supergroup that have wide regional distribution. This allows a comparison to be made of the rock magnetic properties within the same lithostratigraphic unit for the whole basin.

It is important to note that the stratigraphic cores stored at the National Core Library are not oriented. Palaeomagnetic study of these cores is limited to the variation in magnetic susceptibility, magnetic intensity and magnetic inclination.

A total of eight boreholes were studied of which four intersect dolerite sills (Figure 3-2).

The aim is to determine the temperature variations associated with the Karoo Large Igneous

3–2 CHAPTER 3

Province (LIP) event on a basin-wide scale. Furthermore the hypothesis of a mantle plume situated SE of the coast near East London as source for the Karoo LIP (Griffiths and

Campbell, 1990) will be tested, as it is assumed that there would be a decrease in the influence of magma temperature with increasing distance from the source. A short summary of the eight boreholes follows.

Figure 3-2: Simplified geological map showing the distribution of the Karoo Basin within South Africa. Location of sampling boreholes are indicated (red stars indicate cores with dolerite sills and blue stars indicate cores without any boreholes).

3.2.2.1. Kopoasfontein (G39974) - Reference borehole

Location: 31°29’25”S; 19°53’50”E; elevation 1044 m.

The first core selected for analysis is situated in the Calvinia district on the farm

Kopoasfontein, on the western edge of the Karoo Basin. This stratigraphic borehole was drilled during 1996 for the Department of Water Affairs and the borehole log was prepared by

Dr. L. Chevallier. The log indicates the intersection of three dolerite sills (9 m, 83 m and

114 m thick), each separating a different stratigraphic unit (Tierberg Formation, White Hill

Formation and the Dwyka Group). This borehole is thus situated below the Beaufort Group

3–3 CHAPTER 3 within the Ecca Group. The Beaufort Group is considered to be the main stratigraphic horizon where most of the dolerite sills in the Karoo Basin occur.

Borehole G39974 was chosen as reference core for the rest of the study by testing the viability of the proposed magnetic methods (Maré et al., 2014) and comparing results with the detailed geochemical, petrographic and vitrinite reflectivity data already published by Svensen et al. (2007) and Aarnes et al. (2011).

Magnetic susceptibility values were measured at the National Core Library on the cores from approximately 35 m above the first sill to approximately 40 m below the lower most sill using the KT-9 hand-held magnetic susceptibility meter. Measuring intervals varied from

0.25 m within the adjacent sediments to several meters apart within the thicker dolerite sills.

Smaller intervals were used at the contact zones with the dolerite. Unfortunately the shale and mudstone cores were very fragmented and sampling areas were limited to larger samples (Figure 3-3).

Figure 3-3: The core from the White Hill Formation shale is fractured significantly into small pieces.

3–4 CHAPTER 3

The top ~375 m of core has a diameter of 110 mm and the Exploranium KT-9 susceptibility meter was used for preliminary analysis (Figure 3-4). Four measurements were taken at each depth, taken roughly every quarter around the core, and the data averaged.

Figure 3-4: Exploranium KT-9 susceptibility meter used for magnetic susceptibility measurements along the core.

From ~375-575 m the core diameter was 63.5 mm allowing additional measurements to be taken with the Fiskars TH-15 magnetic susceptibility meter as well (Figure 3-5). Below

575 m the core diameter was 47.6 mm. Similarly to measurements with the KT-9, four measurements were taken at each depth, at 90° from each other, and averaged. Magnetic susceptibilities of selected samples were afterwards also measured in the laboratory using the MFK1-FA Kappabridge.

A total of 306 depth measurements were taken and the mean susceptibility values plotted with depth (Figure 3-6). The logging data from the two instruments correlate generally well with laboratory measurements. The observed differences that could possibly be explained due to pre-set core diameter settings for the two portable instruments resulting in incorrect volume adjustments (see Maré, 2010).

The magnetic susceptibility data were used to select samples for further rock magnetic analysis at the Council for Geoscience’s Petrophysical Laboratory. A total of 125 quarter core samples were collected and cut for further analysis (Figure 3-7). Samples were collected to

3–5 CHAPTER 3 include material macroscopically showing high sulphide content as well as material with visible hydrothermal alteration in order to determine the maximum acquired temperatures.

Figure 3-5: Magnetic susceptibility measurements were taken on core samples at regular intervals with the Fiskars TH-15 susceptibility meter.

3.2.2.2. Waterkloof (PP47)

Location: 30°17’24”S; 25°14’24”E; elevation unknown.

This borehole was drilled to a depth of 1405 m and intersected five dolerite sills (86 m,

18 m, 67m, 49 m, and 199 m thick). Sampling of only the Karoo Supergroup rocks was conducted by Dr. M.O. de Kock from the University of Johannesburg. The log does not specify which formations were intersected except for the occurrence of the Dwyka Group near the bottom. A total of 115 palaeomagnetic samples (25 mm diameter) were drilled horizontally into the core at various depths (Figure 3-7).

3.2.2.3. Hermon (HM1/78)

Location: 28°21’44”S; 25°19’17”E; elevation 1241 m.

The borehole occurs in the Boshof district and is 110.78 m deep. The borehole intersects the Tierberg, White Hill and Prince Albert formations as well as the Dwyka Group. One dolerite sill (~19 m thick) intruded the Prince Albert Formation. The core was terminated when possible Ventersdorp lava was intersected. This core is significantly weathered and only a

3–6 CHAPTER 3

small number of samples (28) could be collected. In order to get a continuous dataset for this

borehole 32 additional samples (~50 cm3) consisting of rock chips from the broken shale were collected. Figure 3-7 indicates the relative depths at which samples were collected.

Figure 3-6: Variation of magnetic susceptibility with depth along core G39974 as measured during the current study with both the Exploranium KT-9 as well as the Fiskars TH-15 magnetic susceptibility meters, including selected measurements with the MFK10FA Kappameter.

3–7 CHAPTER 3

3.2.2.4. Driefontein (DF1/75)

Location: 27°21’25”S; 29°46’09”E; elevation 1719 m.

This SOEKOR borehole intersects the Volksrust and Vryheid formations as well as the

Dwyka Group before terminating into the granitic basement. Two thick dolerite sills were also encountered between 159 m and 343 m and between 534 m and 582 m. Two thin coal seams

(0.5 and 5 m thick) were intersected between depths 390 m to 460 m. In total, 123 samples were collected along the core at various depths (Figure 3-7).

3.2.2.5. Goedehoop (CBC4495)

Location: 25°58’27”S; 29°25’55”E; elevation 1605 m.

This core was kindly donated by Anglo American for research purposes. The borehole is situated in the northeastern part of the Karoo Basin and intersects several coal seams but no dolerite sills. The borehole log does not specify the stratigraphic units, but it is assumed that in this region the Vryheid Formation of the Ecca Group was intersected. A total of 74 samples were collected (Figure 3-8) including 18 coal samples.

3.2.2.6. Sambokkraal (SA1/66)

Location: 32°40’40”S; 21°19’45”E; elevation 741 m.

This core is ~4100 m deep and was drilled in 1966 by SOEKOR and preliminary susceptibility measurements were made in situ at the National Core Library with the

Exploranium KT9 hand-held magnetic susceptibility meter.

Borehole logs still contain old geological descriptions and subdivisions between different stratigraphic units are not very clear. The depth was measured in Cape feet and there is some controversy over the correct method to convert Cape feet to meters (Lindeque et al.,

2011). The borehole does not intersect any dolerite. Samples from the Karoo stratigraphy were collected roughly every 20-30 m increasing to ~150-200 m in the Dwyka, Bokkeveld and

Table Mountain groups. A total of 117 samples were collected (Figure 3-8).

3–8 CHAPTER 3

Figure 3-7: Sample depth along borehole cores that intersected presumed dolerite sills.

3–9 CHAPTER 3

Figure 3-8: Sample depth along borehole cores without dolerite.

3–10 CHAPTER 3

3.2.2.7. Schietfontein (SC3/67)

Location: 32°46’25”S; 24°18’00”E; elevation 797 m.

Borehole SC3/67 was drilled in the 1960’s by SOEKOR to a depth of ~5560 m. This

borehole does not intersect any dolerite. The core is kept at the National Core Library

although only rock chips are available for roughly every 10 feet (~3 m). A total number of 122

samples, each with volume of approximately 40 cm3, were collected every 40 feet (~12 m) along the core (Figure 3-8).

3.2.2.8. Groottegeluk (MY19)

Location: 23°38’42”S; 27°29’45”E.

Selected samples along this core were kindly donated by Exxaro for research purposes.

The borehole is situated in the northern part of the Karoo Basin and intersects a coal seam

but no dolerite. Five samples above and five samples below the Groottegeluk Formation were

studied (Figure 3-8).

3.3 Method

3.3.1. Sample preparation

A total of 1529 quarter core samples and 153 crushed samples (volume 40-50 cm3) from the eight different boreholes spread across the Karoo basin were collected.

Quarter core samples were marked to indicate the bottom (down dip) orientation of the core. This method was used to determine any variation in geomagnetic inclination with depth as well as within the contact aureole of observed dolerite sills. Each ‘oriented’ core sample was cut into at least three oriented 20 mm cubic specimens. Care was taken to transfer the orientation markings onto each specimen.

Thin slices for thin-section analysis were retained. Small chips of the remaining samples were collected for thermomagnetic analysis. The rest of the samples are being kept in labeled bags for future XRD analysis.

Crushed samples were transferred into small plastic sample containers for magnetic susceptibility and magnetic intensity measurements.

3–11 CHAPTER 3

3.3.2. Laboratory procedures

The bulk magnetic susceptibility, anisotropy of low field magnetic susceptibility (AMS) as well as temperature variations of magnetic susceptibility in normal atmosphere was measured using MFK1-FA Kappabridge with the CS3 furnace from AGICO (Brno). Measurements were conducted in a peak field of 200 A/m and operating frequency of 976 Hz. The sensitivity of the instrument at 976 Hz and a field of 400 A/m peak is 2 x 10-8 SI units. The accuracy of the absolute calibration is 3%. The AMS parameters were analysed with the Windows-based software Anisoft 4.2 (Chadima and Jelinek, 2009) while the thermomagnetic curve browser

Cureval 8.0 (Chadima and Hrouda, 2009) was used to analyse the temperature variation of magnetic susceptibility.

The magnetic intensity and polarity of the specimens were measured using the AGICO

JR6 dual speed spinner magnetometer. The measuring range JR6 is 0–12.5 X 103 A/m with

sensitivity of 2.4 X 10-6 A/m. Progressive thermal demagnetization of selected samples were performed in intervals of 50°C up to a maximum of 600°C using the MMTD80 thermal demagnetiser from Magnetic Moment Pty (Ltd). This instrument is capable of reaching temperatures up to 800°C. Samples were progressively heated at intervals of 25°C and cooled a field free environment. Components of magnetization of oriented field samples were isolated by examining at the orthogonal vector plots (Zijderveld, 1967) and by the use of principal component analysis (Kirschvink, 1980). The Remasoft 3.0 for Windows data browser

written Chadima and Hrouda (2007) was used for this purpose. Samples from core PP47

were analysed at the palaeomagnetic laboratory of the University of Johannesburg (UJ) using

their cryogenic system and the data analysed using the PaleoMag V3.1 software. Results

from the UJ Palaeomagnetic Laboratory are comparable with measured data from the CGS

Petrophysical Laboratory.

As mentioned in Chapter 1 the core samples are not azimuthally oriented and thus

normal vector analysis of the data are not possible. Therefore, only the variation in inclination

relative to depth was considered for both AMS and magnetostratigraphic purposes. Arason

and Levi (2010) have developed a robust maximum likelihood method for estimating the

unbiased mean inclination from such data.

3–12 CHAPTER 3

Samples from a traverse in one contact aureole from each of the four boreholes that intersect sills were selected for hysteresis and room temperature isothermal remanent magnetization (IRM) analysis. Additional samples from two boreholes without sills were selected for control purposes. These samples were sent to Prof. Eric Ferré at Southern Illinois

University (SIU) for magnetic hysteresis and IRM analysis. Another six sample fragments from borehole G39774 were sent to the Institute for Rock Magnetism in Minneapolis for low temperature IRM analysis.

3.4 References

Aarnes, I., Svensen, H., Polteau, S. and Planke, S., 2011. Contact metamorphic

devolatilization of shales in the Karoo Basin, South Africa, and the effects of multiple sill

intrusions. Chemical Geology, 281, 181–194.

Arason, P. and Levi, S., 2010. Maximum likelihood solution for inclination-only data in

palaeomagnetism. Geophysical Journal International, 182, 753–771.

Chadima, M. and Hrouda, F., 2007. Paleomagnetic data browser and analyser, Remasoft 3.0

for Windows. Beta version. AGICO, Inc., Brno, Czech Republic at www.agico.com.

Chadima, M. and Hrouda, F., 2009. Thermomagnetic curve browser, Cureval 8.0 for

Windows. Beta version. AGICO, Inc., Brno, Czech Republic at www.agico.com.

Chadima, M. and Jelinek, V., 2009. Anisotropy data browser, Anisoft 4.2 for Windows. Beta

version. AGICO, Inc., Brno, Czech Republic at www.agico.com.

Griffiths, R.W. and Campbell, I.H., 1990. Stirring and structure in mantle starting plumes.

Earth and Planetary Science Letters, 99, 66–78.

Johnson, M.R., Van Vuuren, C.J., Visser, J.N.J. Cole, D.I., de V. Wickens, H., Christie,

A.D.M. and Roberts, D.L., 1997. The foreland Karoo Basin, South Africa. In: Selley,

R.C. (Ed.), African Basins. Elsevier, Amsterdam, 269–317.

Kirschvink J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic

data. Geophysical Journal of the Royal Astronomical Society, 62, 699-718.

Lindeque, A., de Wit, M.J., Ryberg, T., Weber, M. and Chevallier, L., 2011. Deep crustal

profile across the southern Karoo Basin and Beattie Magnetic Anomaly, South Africa:

3–13 CHAPTER 3

an integrated interpretation with tectonic implications. South African Journal of

Geology, 114 (3-4), 265–292.

Maré, L.P., 2010. Comparison of results from different magnetic susceptibility instruments.

Council for Geoscience, Pretoria, Report 2010-0233, 16 pp. (unpublished).

Maré, L.P., De Kock M.O, Cairncross, B. and Mouri, H., 2014. Application of magnetic

geothermometers in sedimentary basins: an example from the western Karoo Basin,

South Africa. South African Journal of Geology, 117(1), 1–14.

Svensen, H., Planke, S., Chevallier, L., Malthe-Sorenssen, A., Corfu, F. and Jamtveit, B.,

2007. Hydrothermal venting of greenhouse gases triggering Early Jurassic global

warming. Earth and Planetary Science Letters, 256, 554–566.

Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: Analysis of results. In: Collinson,

D.W., Creer, K.M. and Runcorn, S.K. (eds.), Methods in Palaeomagnetism, Elsevier,

Amsterdam, 254–286.

3–14 CHAPTER 4

CHAPTER 4. MAGNETIC ASSEMBLAGE

4.1 Introduction

As discussed in Chapter 1 knowledge of the magnetic fabric of a rock is important for proper interpretation of the magnetic responses of the materials. This chapter will discuss results on magnetic experiments on the Karoo sedimentary rocks and dolerite intrusions including thermal variation of magnetic susceptibility, magnetic hysteresis properties and isothermal remanent magnetization (IRM) as well as anisotropy of low-field magnetic susceptibility (AMS).

4.2 Identification of magnetic minerals

4.2.1. Temperature Variation of Magnetic Susceptibility

The variation of magnetic susceptibility with temperature depends on the magnetic mineralogy (Grommé et al., 1969; Hrouda et al., 2003). Diamagnetic minerals (e.g. quartz, feldspars, calcite) show no variation in susceptibility with temperature. The magnetic susceptibility versus temperature curves of paramagnetic minerals (e.g. olivine, pyroxene, amphibole, mica), on the other hand, is always hyperbolic (Hrouda et al., 2003). In ferromagnetic (sensu lato) minerals such as magnetite, titanomagnetite, hematite and pyrrhotite, the magnetic susceptibility vs temperature curve is more complex, but the

Curie/Neél temperatures can always be observed when the magnetic susceptibility decreases sharply as the mineral changes from ferromagnetic sensu lato into a paramagnetic state

(Dunlop and Özdemir, 1997; Hrouda et al., 2003).

For this study the AGICO CS3 furnace was used to determine the temperature dependence of magnetic susceptibility. Analysing the temperature curves using the Cureval8

software, allows for the semi-quantitative resolution of the room temperature magnetic

susceptibility into its paramagnetic component using the hyperbola-fitting method, and the

ferromagnetic component by fitting a straight line with non-zero slope (Hrouda, 1994 and

Hrouda et al., 1997) where the sum of the two components is equal to the whole rocks

magnetic susceptibility (Figure 4-1).

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CHAPTER 4

Figure 4-1: Semi-quantitative separation of whole rock magnetic susceptibility into paramagnetic and ferromagnetic contributions

Paramagnetic susceptibility is related to the presence of transition metal ions, typically iron. At low concentrations of the metal ions, the magnetic susceptibility follows a Curie law

(Morrish, 1965), Kpara = C/T, where Kpara is the mass magnetic susceptibility, C is the Curie constant and T the absolute temperature. At high concentrations of metal ions, magnetic interactions with neighboring cations can no longer be neglected and the Curie-Weiss law,

Kpara = C/(T-Ɵ) applies, where Ɵ is the paramagnetic Curie temperature. Plots of inverse

magnetic susceptibility versus temperature give the paramagnetic Curie temperature, Ɵ. If Ɵ

is negative, the material is antiferromagnetic; if positive, it is ferromagnetic; Ɵ is zero for paramagnetic materials (Dunlop and Özdemir, 1997).

Temperature variations of magnetic susceptibility from room temperature up to 700 °C were measured in cycles of increasingly higher temperature (Tmax) on selected samples from each borehole. All measurements were performed on rock fragments in an uncontrolled atmosphere (i.e., not in a special atmosphere such as argon atmosphere), with a heating rate of 8.3°C/min and cycle increments of 50°C. No pattern with respect to the paramagnetic and ferromagnetic components was observed within the same lithologies between the different boreholes (Figure 4-2 to Figure 4-6) and ferromagnetic contribution range from 10-100%. The local (micro) mineralogies as well as the grade of metamorphism of individual samples due to their relative distances from any dolerite intrusions might play a significant role in the results. 4-2

CHAPTER 4

Figure 4-2: Examples from boreholes DF1/75 (DF64), CBC4495 (GH16 and GH37) and MY19 (MY19-7) indicating the separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility of the Vryheid Formation by fitting a hyperbola offset along the y axis using the Cureval8 software (Chadima and Hrouda, 2009). The reciprocal magnetic susceptibility (1/k) in the insets indicating the linear relationship with temperature.

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CHAPTER 4

Figure 4-3: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM1), G39974 (LKF10) and SC3/67 (SC19) for the Tierberg Formation. The reciprocal magnetic susceptibility (1/k) in the insets indicating the linear relationship with temperature.

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CHAPTER 4

Figure 4-4: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM49), G39974 (LKF29), SC3/67 (SC93) and SA1/66 (SA950) for the White Hill Formation or lower Ecca Group. The reciprocal magnetic susceptibility (1/k) in the insets indicating the linear relationship with temperature.

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CHAPTER 4

Figure 4-5: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes HM1/78 (HM16 and HM59), G39974 (LKF66) and PP47 (PH64) for the Prince Albert Formation. The reciprocal magnetic susceptibility (1/k) in the insets indicating the linear relationship with temperature.

4-6

CHAPTER 4

Figure 4-6: Separation of paramagnetic and ferromagnetic contributions to magnetic susceptibility for examples from boreholes DF1/75 (DF30), HM1/78 (HM22), G39974 (LKF123) and SA1/66 (SA1000) for the Dwyka Group. The reciprocal magnetic susceptibility (1/k) in the insets indicating the linear relationship with temperature.

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CHAPTER 4

Even though there is some variation between different formations and even laterally across the

Karoo Basin, the heating/cooling cycles are generally almost reversible from room temperature to temperatures lower than 250°C (e.g., Figure 4-7; see also Chapter 6), which suggests that no mineralogical changes occur below this temperature. Above 250°C two different behaviors are observed. For some samples the magnetic susceptibility gradually increases while in others it decreases. The cooling curves are most commonly higher, while less commonly some samples can be characterized by lower cooling curves. Higher cooling curves indicate the formation of new magnetic minerals, e.g. pyrrhotite or magnetite, while lower cooling curves indicate mixing of magnetic minerals with the less magnetic host mineral (Grommé et al.; 1969 Graham et al., 1987;

Hrouda, 2003).

Two main magnetic minerals, namely pyrrhotite and magnetite, were identified from observed decreases in magnetic susceptibility at their respective Curie temperatures. In most cases the magnetite was not primary but formed due to oxidation in the uncontrolled atmosphere during the heating process of this test. This secondary oxidation can be tested by scanning the samples before and after the test using a scanning electron microscope (SEM). Figure 4-7 illustrates the chemical changes that take place during the heating process in sample LKF75 from the Prince Albert

Formation in borehole G39974. Initially the heating and cooling curves are reversible with peak values at ~315°C indicating the presence of pyrrhotite in the rock (Dekkers, 1989). As the temperature is increased further a gradual increase in the magnetic susceptibility suggests the formation of a new mineral (magnetite) due to oxidation. This is confirmed by the sudden drop in magnetic susceptibility at the Curie temperature of magnetite of 580°C. A shrinking of the pyrrhotite peak in the heating curve and disappearance thereof in the cooling curve after heating to the maximum of 600°C indicates a complete transformation of the pyrrhotite into magnetite. The elevated magnetic susceptibility values (initial magnetic susceptibility was ~7.5x10-5, while the final magnetic susceptibility was 12.0x10-5) can be ascribed to the fact that the magnetic susceptibility of magnetite is significant higher than that of pyrrhotite. However, the observed increase in magnetic susceptibility of 4.5x10-5 corresponds to only a very small quantity of magnetite forming by oxidation of pyrrhotite. A considerably higher final magnetic susceptibility would be expected if most of the initial magnetic susceptibility was indeed due to pyrrhotite.

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CHAPTER 4

4.2.2. Magnetic Hysteresis

Hysteresis loops are diagrams displaying the response of a magnetic substance to an applied magnetic field. The shape of a hysteresis loop depends strongly on the physical properties of the material (mineralogy and grain size) and could thus be used to constrain the magnetic component of a given rock specimen (e.g., Tauxe, 2002, 2013). Figure 4-8 illustrates a theoretical magnetic hysteresis curve with commonly used terms indicated. Two of the most commonly used hysteresis ratios are Mr∕Ms and Hcr∕Hc. These are sensitive to remanence state (e.g., single versus multi- domain state) and the source of magnetic anisotropy (e.g., crystal defects). Magnetic hysteresis hence reveals something about grain size and shape of the magnetic minerals. For a random assemblage of single domain uniaxial particles, Mr∕Ms = 0.5 (Tauxe, 2002, 2013).

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CHAPTER 4

Figure 4-8: Heavy green line: initial behavior of demagnetized specimen as applied field ramps up from zero field to a saturating field. The initial slope is the initial or low-field susceptibility χlf. After saturation is achieved the slope is the high-field susceptibility χhf which is the non- ferromagnetic contribution, in this case the paramagnetic susceptibility (because χhf is positive.) The dashed blue line is the hysteresis loop after the paramagnetic slope has been subtracted. Saturation magnetization Ms is the maximum value of magnetization after slope correction. Saturation remanence Mr is the value of the magnetization remaining in zero applied field. The field necessary to reduce the net moment to zero is defined as the coercive field (µ0Hc or Hc) and coercivity of remanence (µ0H’cr or Hcr) (after Tauxe, 2013).

Similar to IRM acquisition curves (see section 4.2.3), hysteresis loops are the sum of all the individual magnetic particles in the specimen (Tauxe, 2013). Tauxe (2013) discusses the basic types of loops observed in geological materials. Figure 4-9a shows the negative slope typical of diamagnetic material such as calcite or quartz, while Figure 4-9b shows a paramagnetic slope. Such slopes are common when the specimen has little ferromagnetic material and is rich in non-magnetic iron-bearing phases such as biotite or clay minerals (Tauxe, 2013). When grain sizes are very small

(~10 nm), a specimen can display superparamagnetic “hysteresis” behavior (Figure 4-9c). Figure 4-

9d shows a loop characteristic of specimens whose remanence stems from SD magnetite with uniaxial anisotropy. Figure 4-9e shows data from specular hematite whose anisotropy ought to be magnetocrystalline in origin (hexagonal within the basal plane). Finally, Figure 4-9f shows a loop that has lower Mr∕Ms ratios than single domain, yet some stability. Loops of this type have been characterized as pseudo-single domain (PSD). In geological materials, however, mixtures of several magnetic phases and/or domain states are encountered (Tauxe, 2013). Such mixtures can lead to

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CHAPTER 4 distorted loops, such as those shown in Figure 4-9g-i. In Figure 4-9g, Tauxe (2013) shows a mixture of hematite plus SD-magnetite. The loop is distorted in a manner referred to as ‘goose-necked’.

Another commonly observed mixture is SD plus SP magnetite, which can result in loops that are either ‘wasp-waisted’ (see Figure 4-9h) or ‘pot-bellied’ (see Figure 4-9i).

Figure 4-9: Hysteresis loops of end-member behaviors: a) diamagnetic, b) paramagnetic, c) superparamagnetic (data for submarine basaltic glass), d) uniaxial, single domain, e) magnetocrystalline, single domain, f) “pseudo-single domain”. Hysteresis behavior of various mixtures: g) magnetite, and hematite, h) SD/SP magnetite (data from Tauxe et al. 1996), i) another example of SD/SP magnetite with a finer grained SP distribution (after Tauxe, 2013).

The hysteresis curves of selected samples from six boreholes in the Karoo Basin are given in

Figure 4-10 to Figure 4-12. It is clear that in all cases except borehole G39974 (Figure 4-10a) the dominant magnetic fabric is paramagnetic in nature with very little ferromagnetic minerals. The hysteresis loops in Figure 4-10a range from uniaxial single domain to a goose-necked mixture of more than one magnetic mineral (probably pyrrhotite and magnetite) as well as pot-bellied.

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BOREHOLE# SAMPLE# DEPTH(m) DISTANCE TO SILL (m) DISTANCE TO SILL ABOVE (m) Stratigraphy LKF27 352.15 63.85 8.45 LKF30 378.73 37.27 35.03 LKF35 409.00 7.00 65.30 LKF39 413.35 2.65 69.65 White Hill Formation G39974 LKF42 414.40 1.60 70.70 LKF43 414.90 1.10 71.20 LKF46 415.85 0.15 72.15 Sill below 83 m thick. Sill above 9.25 m thick.

BOREHOLE# SAMPLE# DEPTH(m) DISTANCE TO SILL BELOW(m) DISTANCE TO SILL ABOVE (m) Stratigraphy HM1 25.50 57.50 25.00 Tierberg Formation HM3 36.00 47.00 35.50 White Hill Formation HM4 64.95 18.05 64.45 HM6 67.12 15.88 66.62 HM1/78 HM8 69.50 13.50 69.00 Prince Albert Formation HM10 74.00 9.00 73.50 HM13 83.00 0.00 82.50 Sill below 11.5 m thick. Sill above to surface, original thickness unknown

Figure 4-10: Combined hysteresis curves of selected samples from borehole G39974 (LKF) displaying ferromagnetic signatures, while borehole HM1/78 (HM) indicates dominantly paramagnetic fabric.

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BOREHOLE# SAMPLE# DEPTH(m) DISTANCE TO SILL BELOW (m) Stratigraphy PH33 440 54.64 PH36 454.5 40.14 PH37 465 29.64 Lower Ecca Group (no White PP47 PH39 474.5 20.14 Hill identified) PH41 482.3 12.34 PH43 492 2.64 Sill 85.7 m thick

BOREHOLE# SAMPLE# DEPTH(m) DISTANCE TO SILL (m) Stratigraphy DF44 76.95 82.05 Volksrus Formation DF50 85.3 73.7 DF55 101.4 57.6 DF1/75 Vryheid Formation DF60 131.6 27.4 DF66 158.45 0.55 Sill 191.3 m thick

Figure 4-11: Combined hysteresis curves of selected samples from boreholes PP47 (PH) and DF1/75 (DF) indicating dominantly paramagnetic fabric.

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BOREHOLE# SAMPLE# DEPTH(m) Stratigraphy SA541A 1409.4 SA631A 1703.7 Lower Ecca Group SA761A 2139 SA1/66 SA920A 2622 White Hill & Prince Albert SA981A 2925.8 No Sills in this borehole

BOREHOLE# SAMPLE# DEPTH(m) Stratigraphy GH17 32.8 GH19 36.5 GH23 40.1 Vryheid Formation CBC4495 GH33 50.6 GH40 64.9 GH43 69.2 No Sills in this borehole

Figure 4-12: Combined hysteresis curves of selected samples from boreholes SA1/66 (SA) and CBC4495 (GH) indicating dominantly paramagnetic fabric.

When comparing the variation in magnetization values relative to the sample distances from the sill contacts (see tables in Figure 4-10 to Figure 4-12), a general decrease in magnetization towards the contact aureole is observed. In a few selected cases the values were also influenced by the lithology (e.g. HM1A and HM3 in Figure 4-10 and DF44 in Figure 4-11).

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4.2.3. Isothermal Remanent Magnetization (IRM)

Short-term exposure to strong magnetization fields at constant temperature such as lightning strikes or the fields produced during hysteresis experiments, produces a secondary remanent magnetization that is referred to as isothermal remanent magnetization (IRM). The IRM is acquired by ferromagnetic grains with coercive force less than the applied field (Butler, 1992). The acquisition of IRM curves through the application of stepwise-increasing uniaxial fields to a rock-magnetic sample therefore provides an important non-destructive tool for the investigation of magnetic coercivity spectra (Dunlop and Ozdemir, 1997).

According to Robertson and France (1994), the individual magnetic mineral phases all contribute to the bulk IRM curve. Each mineral phase can be described using three parameters, namely the applied field at which the mineral phases acquire half of its saturation IRM (B½, measure of mean coercivity of population), the magnitude of the phases distribution (Ms, indication of contribution to bulk IRM curve) and the dispersion parameter.

The isothermal remanent magnetization (IRM) of selected samples collected in the contact aureoles above sill intrusions of four boreholes (G39974 (LKF), PP47 (PH), HM1/78 (HM) and

DF1/75 (DF)) as well as two reference boreholes with no sills (SA1/66 (SA) and CBC4495 (GH)), was acquired progressively in fields up to 0.5 T using a MiocroMag 2900/3900 VSM at the Southern

Illinois University, Carbondale. The observed acquisition of IRM of the Karoo samples vary relative to distance from the contact aureole. The magnetic saturation (Ms) closest to the sill contacts reached between 1.09 x 10-7 Am2/kg and 2.98 x 10-7 Am2/kg, while further away saturation is reached between

9.77 x 10-7 Am2/kg and 167.00 x 10-7 Am2/kg. Borehole G39974 is the exception with much higher magnetic saturation reached between by 0.15 x 10-3 Am2/kg and 4.83 x 10-3 Am2/kg. (Figure 4-13 to

Figure 4-18).

Backfield demagnetization of the saturation IRM (SIRM) revealed the coercivity of remanence,

Hcr, from the selected samples to range between 20-120 mT (Figure 4-13 to Figure 4-18 and Figure

4-19). In general, the lowest values occur closest to the sill contacts indicating an increase in concentration of low coercivity minerals such as magnetite.

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Figure 4-13: Isothermal remanent magnetization (IRM) of selected samples from borehole G39974 (LKF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 4-14: Isothermal remanent magnetization (IRM) of selected samples from borehole PP47 (PH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

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Figure 4-15: Isothermal remanent magnetization (IRM) of selected samples from borehole HM1/78 (HM) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 4-16: Isothermal remanent magnetization (IRM) of selected samples from borehole DF1/75 (DF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

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Figure 4-17: Isothermal remanent magnetization (IRM) of selected samples from borehole SA1/66 (SA) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 4-18: Isothermal remanent magnetization (IRM) of selected samples from borehole CBC4495 (GH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

The S-ratio is a rock magnetic parameter employed to provide a relative measure of the contributions of low and high coercivity material to a sample’s saturation isothermal remanent magnetization (SIRM) (Heslop, 2009). S-ratios lie in the range from -1 (only high coercivity material) to 1 (only low coercivity material). With the exception of samples from borehole G39974 (Figure 4-20,

LKF-*S) all samples are positive with S-ratios less than 0.2. The S-ratio from borehole G39974

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CHAPTER 4 changes from positive to negative approximately 5 m above the sill contact indicating an increase in low coercivity material closer to the sill contact.

Figure 4-19: Coercivity of remanence with depth of selected samples from the contact aureoles above sills (green bars) in each of boreholes G39974 (LKF-Hcr) PP47 (PH-Hcr), HM1/78 (HM-Hcr) and DF1/75 (DF-Hcr) as well as samples from boreholes without sills, SA1/66 (SA-Hcr) and CBC4495 (GH-Hcr).

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Figure 4-20: S-ratios plotted as a function of depth for selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), HM1/78 (HM), PP47 (PH) and DF1/75 (DF) as well as samples from boreholes without sills, CBC4495 (GH) and SA1/66 (SA).

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4.3 Fundamentals of rock magnetism

4.4 Summary of magnetic fabric results

The magnetic fabric analysis demonstrated that the sedimentary rocks of the Karoo Basin have mostly normal fabric and are oblate in nature with minor re-alignment close to the contact aureoles.

Magnetic susceptibility variations with temperature identified the main magnetic minerals as pyrrhotite and magnetite, while variable contributions from paramagnetic and ferromagnetic components were suggested. However, hysteresis studies on representative samples from several boreholes indicated that paramagnetism dominates the sedimentary rocks of the Karoo Basin. This discrepancy suggests a possible weakness or limitation in either of the methods to separate the contribution of magnetization components from the whole. Isothermal remanent magnetization (IRM) indicates a decrease in coercivity towards the contact aureoles suggesting alteration of the magnetic assemblage of the host rock due to the heating effect of the dolerite intrusions.

4.4.1. Magnetic Susceptibility

The magnetic susceptibility (k) is defined as k=M/H, the ratio of the induced magnetization (M) to the applied magnetic field (H). In the International System [SI] magnetizations and magnetic fields are both measured in A/m, therefore k is dimensionless although its magnitude is commonly referred to as [SI].

In polymineralic rocks, the magnetic susceptibility is the sum of the magnetic susceptibilities of all rock-forming minerals. These may include diamagnetic (negative and typically small magnitude on the order of -10-5 [SI]), paramagnetic (positive and typically on the order of 10-5 to 10-4 [SI]) or ferromagnetic species (in the 10-3 [SI] range).

4.4.2. Anisotropy of Magnetic Susceptibility (AMS)

In most rocks the magnetic susceptibility is anisotropic, i.e. varies with direction of the applied field (review in Tarling and Hrouda, 1993). Paramagnetic minerals display a magnetocrystalline anisotropy caused by the interaction of spins within the crystal lattice. Ferromagnetic grains exhibit dominant magnetostatic anisotropy, directly proportional to the shape of the grains, and negligble magnetocrystalline anisotropy (except for hematite and pyrrhotite). In addition, ferrimagnetic grains

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CHAPTER 4 may also display distribution anisotropy due to their alignment although in this case the magnitude of the moments is unequal (Stephenson, 1994; Hargraves et al., 1997).

The anisotropy of magnetic susceptibility (AMS) can be approximated by a symmetric second rank tensor, which is represented as an ellipsoid with three principal axes (k1 ≥ k2 ≥ k3), with principal orientation directions K1, K2 and K3. The measurements procedures are described in Collinson et al.

(1967) and Tarling and Hrouda (1993). In the framework of magnetic fabric, the maximum direction

(K1) defines a magnetic lineation while the plane perpendicular to the minimum direction (K3) and containing maximum and intermediate directions (K1, K2) defines a magnetic foliation.

In sedimentary rocks unaffected by tectonic ductile deformation, “normal” magnetic fabric is usually observed. The normal magnetic fabric is characterized by magnetic foliation oriented parallel to bedding, the magnetic lineation being roughly parallel to the near-bottom water current direction or, in special cases, perpendicular to it (Hamilton and Rees, 1970). The magnitude of the sedimentary magnetic fabric is relatively low and the fabric is distinctly oblate. Despite the predominant occurrence of normal magnetic fabrics, “inverse” magnetic fabric are observed in the sedimentary rocks containing minerals with an inverse relationship between magnetic axes and shape and/or crystallographic axes (Rochette et al., 1988). The origin of inverse and intermediate magnetic fabrics was reviewed and modeled by Rochette et al. (1992, 1999) and Ferré (2002). The terms normal, inverse, intermediate and anomalous magnetic fabric were defined in these models according to the relationship of the principal anisotropy directions to the principal directions of structural features.

Therefore “normal” magnetic fabric refers to K1 being parallel to structural lineation (either flow, current, stretching direction, or apparent lineation), while K3 is perpendicular to a structural foliation

(e.g. plane flattening, flow or bedding plane). In “inverse” magnetic fabric the maximum and minimum anisotropy axes are inverted relative to normal fabric (Rochette et al., 1992; Ferré, 2002). Figure 4-21 illustrates the theoretical ternary diagram interrelating inclinations of the principal anisotropy directions (K1, K2, and K3) in bedding coordinate system after Chadima et al. (2006).

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Figure 4-21: A model ternary diagram interrelating inclinations of the principal anisotropy directions (K1, K2, K3) in bedding coordinate system (after Chadima et al., 2006). The 30° and 60° threshold angles subdivide the diagram into nine fields. Schematic stereoplots depicting the orientation of principal directions are drawn for each field. Different types of magnetic fabrics as defined by Rochette et al. (1992, 1999) and Ferré (2002) are presented outside the diagram. Square, triangle and circle symbols represent maximum, intermediate and minimum anisotropy directions i.e., K1, K2 and K3, respectively.

Quantitatively, the magnetic anisotropy is presented in terms of the arithmetic mean of the low field susceptibility, k, along the three principal axes K1, K2 and K3. The corrected degree of

()ln2 −Σ kk 2 anisotropy is given by P' = exp i (i=1 to 3, k is the arithmetic mean susceptibility) and the ellipsoid shape is given by the parameter T = [(2lnk2 - lnk1 - lnk3) / (lnk1 - lnk3)] (Jelinek, 1978).

P’ is a measure of the degree to which the AMS ellipsoid deviates from a sphere. In rocks having no preferred mineral orientation P’ = 1. The magnetic ellipsoid is oblate (pancake shaped) for 0 ≤ T ≥ 1 and prolate (pencil shaped) for -1 ≤ T ≤ 0.

In igneous rocks, mineral grains generally align parallel to magmatic flow and therefore, the rock

AMS is considered a proxy for magmatic flow, provided it originates from primary minerals (e.g.,

Borradaile and Henry, 1997; Bouchez, 1997). While the magma is flowing in a dyke or sill, elongated particles become imbricate against the chilled margins (Knight and Walker, 1988). If the heat from the intrusion sufficiently elevates the ambient temperature of the host rock, re-alignment of the magnetic minerals within the contact aureole could occur (Figure 4-22).

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Figure 4-22: Flow fabric generation by fabric imbrication model modified after Knight and Walker (1988)

In the case of core data, such as in the current study, the shape and attitude of intersected intrusives cannot generally be inferred from core logs alone. A thorough description of the dip angles of contacts, as well as other structural elements is necessary to evaluate whether an intrusive is a sill, a dyke, or an inclined sheet. In some cases, however, the internal fabric (AMS) may provide additional insight.

As discussed above, magnetic fabric (AMS) of horizontally layered shale is normally more or less coaxial with the bedding. Ternary diagrams for the Karoo strata from all the boreholes are given in Figure 4-23 and Figure 4-24 illustrating, with the exception of borehole PP47, the generally expected “normal” magnetic fabric.

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Figure 4-23: Ternary AMS diagram of sedimentary rocks from boreholes intersecting dolerite sills. With the exception of PP47 that shows inverse magnetic fabric, all other borehole display normal fabric.

Figure 4-24: Ternary AMS diagram of sedimentary rocks from boreholes not intersecting any dolerite sills. Magnetic fabric display dominantly normal fabric. MY19 display intermediate fabric at depth (triangles).

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CHAPTER 4

Typical for undeformed horizontally layered sedimentary units, the shape parameters (T) for the four boreholes that intersected one or more dolerite sills (Figure 4-25), indicate dominantly oblate fabrics with depth while isotropic to prolate fabric dominated the intrusive rocks. Some re-alignment or re-crystallization of magnetic minerals in the sedimentary rocks can be observed in contact aureoles (e.g. borehole G39974), however, since no azimuthal information for these boreholes were available, proper evaluation of re-alignment is not possible.

The shape parameters with depth for the three boreholes that do not intersected any dolerite

(Figure 4-26) indicate the mainly isotropic nature of the lower Beaufort Group sandstone (borehole

SA1/66) with the shale of the Ecca Group displaying oblate fabric. The Vryheid Formation sandstones display more isotropic fabric as the grain sizes increase. A number of samples from borehole CBC4495, however, displayed prolate fabric. These prolate samples occur interlayered with logged coal seams and can possibly be associated with lenses of sulphide mineralization that were observed in some of these coal seams.

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CHAPTER 4

Figure 4-25: Shape parameter, T, for boreholes intersecting dolerite sills. Sedimentary units are predominantly oblate shaped (>0). Changes in shape from oblate (>0) to prolate (<0) are observed in contact aureoles of borehole G39974.

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Figure 4-26: Shape parameter, T, for boreholes not intersecting any dolerite sills. Sedimentary units are predominantly oblate shaped with more sandy units (e.g. Beaufort Group) tending to be more isotropic in shape. Some prolate fabric was observed along coalbeds in borehole CBC4495.

As mentioned previously the magnetic fabric of undisturbed layered sedimentary rocks is very

simple with a dominantly horizontal magnetic lineation (K1) and vertical poles to the magnetic foliation (K3). The magnetic foliation results of the sedimentary units with depth for the four

boreholes intersecting several dolerite sills (Figure 4-27) display, as would be expected for

normal horizontally layered sediments, near vertical K3 values in all the boreholes except core

PP47. Core PP47 displays shallow (<30°) K3 values that might either indicate inverse

magnetic fabric or complete remagnetization of the magnetic minerals. Two samples from this

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borehole were sent to the Institute for Rock Magnetism to verify the observed inverse

magnetic fabric with anisotropy of anhysteretic remanence (AARM). Only one of the samples

confirmed inverse magnetic fabric. Hysteresis tests on the two samples indicated that both

samples are dominantly paramagnetic and while the measured AMS is overwhelmingly due to

the paramagnetic mineral fabric, the ARM anisotropy is due to preferred orientations of

ferromagnetic grains, which are limited or absent in these samples.

Core DF1/75 displayed several randomly oriented K3 directions that could be explained

from descriptions in the borehole logs that indicate signs of bioturbation suggesting the

complete destruction of bedding at various locations throughout the borehole (Figure 4-27).

The observed effect of the dolerite intrusions on the sediment is most prominent only at short

distances (less than half sill thickness) from the sill-sediment contact where a rapid change in

the foliation direction can be observed (best observed in core G39974).

The intrusive rocks in most of the boreholes also indicated steep K3 values suggesting

horizontal intrusion typical of sills (Figure 4-27). The samples with moderate K3 values suggestive of inclined sheets, are however also those with isotropic shape parameters (Figure

4-25), making it difficult to determine the true attitude of the intrusion. For the sake of simplification, all intersected intrusions from the current study will be refered to as sills.

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CHAPTER 4

Figure 4-27: Variation of the poles to the magnetic foliation (K3) with depth indicating minor re-alignment at sill-sediment contacts.

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CHAPTER 4

The reference boreholes without any dolerite (Figure 4-28) in general produced dominantly steeply plunging poles to the magnetic foliation (K3). The shallow K3 values of the lower Beaufort Group in borehole SA1/66 are due to the triaxial nature of mainly sandstone grains (that is uniformly shaped quartzitic grains). The shallowing of K3 below 50 m in core

CBC4495 is yet unexplained. It is suggested that that the shallow K3 of the Vryheid Formation

in MY19 is either due to the more triaxial nature of coarser-grained sandstone (Swartrand

Formation), early slope deposition within the half graben flattening out at higher levels or syn-

sedimentary deformation.

Figure 4-28: Variation of the poles to the magnetic foliation (K3) with depth for boreholes without any dolerite sills.

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4.5 References

Borradaile, G.J. and Henry, B., 1997. Tectonic applications of magnetic susceptibility and its

anisotropy. Earth-Science Reviews, 42, 49–93.

Bouchez, J.L., 1997. Granite is never isotropic: an introduction to AMS studies of granitic

rocks. In: Bouchez, J.L., Hutton, D., Stephens, W.E., (Eds). Granite: from Segregation

of Melt to Emplacement Fabrics. Dordrecht, Kluwer Academic, 95–112.

Butler, R.F., 1992. Palaeomagnetism. Blackwell Scientific Publications, Boston, 319 pp.

Chadima, M., Pruner, P., Slechta, S., Grygar, T. and Hirt, A.M., 2006. Magnetic fabric

variations in Mesozoic black shales, Northern Siberia, Russia: possible paleomagnetic

implications, Tectonophysics, 418, 145–162.

Collinson, D.W., Creer, K.M. and Runcorn, S.K., 1967. Methods in Palaeomagnetism.

Elsevier, Amsterdam, 609 pp.

Dekkers, M.J., 1989. Magnetic properties of natural pyrrhotite. II. High and low temperature

behaviors of Jrs and TRM as a function of grain size. Physics of the Earth and

Planetary Interiors, 57, 266–283.

Dunlop, D.J. and Özdemir, O., 1997. Rock Magnetism: Fundamentals and Frontiers,

Cambridge University Press, Cambridge, 596 pp.

Ferré, E.C., 2002. Theoretical models of intermediate and inverse AMS fabrics. Geophysical

Research Letters, 29 (31), 1–4.

Graham, J., Bennett, E.G. and van Riessen, A., 1987. Oxygen in pyrrhotite: Thermomagnetic

behaviour and annealing of pyrrhotites containing small quantities of oxygen. American

Mineralogist, 72, 599–604.

Grommé, C.S., Wright, T. and Peck, D., 1969. Magnetic properties and oxidation of iron-

titanium oxide minerals in Alae Makaupuki lava lakes, Hawaii. Journal of Geophysical

Research, 74, 5277–5293.

Hamilton, N. and Rees, A.I., 1970. Magnetic fabric of sediments from the shelf at La Jolla,

California. Marine Geology, 9, M6–M11.

Hargraves, R.B., Rehacek, J., and Hooper, P.R., 1997. Palaeomagnetism of the Karoo

igneous rocks in southern Africa. South African Journal of Geology, 100, 195–212.

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Heslop, D., 2009. On the statistical analysis of the rock magnetic S-ratio. Geophysical Journal

International, 178, 159–161.

Hrouda, F., 1994. A technique for the measurement of thermal changes of magnetic

susceptibility of weakly magnetic rocks by the CS-2 apparatus and KLY-2 Kappabridge.

Geophysical Journal International, 118, 604–612.

Hrouda, F., 2003. Indices for numerical characterization of the alteration processes of

magnetic minerals taking place during investigation of temperature variation of

magnetic susceptibility. Studia Geophysica et Geodaetica, 47(4), 847–861.

Hrouda, F., Jelínek, V. and Zapletal, K., 1997. Refined technique for susceptibility resolution

into ferromagnetic and paramagnetic components based on susceptibility temperature-

variation measurement. Geophysical Journal International, 129, 715–719.

Hrouda, F., Muller, P. and Hanak, J., 2003. Repeated progressive heating in susceptibility vs.

temperature investigation: a new palaeotemperature indicator? Physics and Chemistry

of the Earth, 28, 653–657.

Knight, M.D. and Walker, G.P.L., 1988. Magma flow directions in dikes of the Koolau

Complex, Oahu, determined from magnetic fabric studies. Journal of Geophysical

Research, 93(B5), 4301–4319.

Morrish, A.H., 1965. The physical principles of magnetism. Wiley, New York, 680 pp.

Robertson, D.J. and France, D.E., 1994. Discrimination of remanence-carrying minerals in

mixtures, using isothermal remanent magnetisation acquisition curves, Physics of the

Earth and Planetary Interiors, 84, 223–234.

Rochette, P., 1988. Inverse magnetic fabric in carbonate-bearing rocks. Earth and Planetary

Science Letters, 90, 229–237.

Rochette, P., Jackson, M. and Aubourg, C., 1992. Rock magnetism and the interpretation of

anisotropy of magnetic susceptibility. Reviews of Geophysics, 30, 209–226.

Rochette, P., Aubourg, C. and Perrin, M., 1999. Is this magnetic fabric normal? A review and

case studies in volcanic formations. Tectonophysics, 307, 219–234.

Stephenson, A., 1994. Distribution anisotropy: two simple models for magnetic lineation and

foliation. Physics of the Earth and Planetary Interiors, 82, 49–53.

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Tarling, D. and Hrouda F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall,

London, 217 pp.

Tauxe, L., 2002. Paleomagnetic principles and practice. Kluwer Academic Publishers,

London, 299 pp.

Tauxe, L., 2013. Essentials of Paleomagnetism: Second Web Edition. Scripps Institution of

Oceanography, http://magician.ucsd.edu/Essentials_2/ WebBook2.html

Tauxe, L., Mullender, T. and Pick, T., 1996. Potbellies, wasp-waists and superparamagnetism

in magnetic hysteresis. Journal of Geophysical Research, 101, 571–583.

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CHAPTER 5. EXPERIMENT 1 – BAKED CONTACT TEST

5.1 Introduction – Baked contact test

During a preliminary study (Maré, 2010), a limited number of outcrop samples were collected within the contact aureoles of two dykes (Kommandodriftdam and Long dykes) in the southeastern Karoo Basin (Figure 3-1). Dolerite samples from the centers as well as from the margins of these two dykes were collected and supplied by C. Ranaweera from SIU for baked contact testing while L. Maré collected samples from baked sedimentary rocks next to the same dykes. Unfortunately no unbaked samples were collected at a distance from these intrusive rocks. However, enough published data exist to serve as a database of unbaked sedimentary rock magnetic behavior at the relevant stratigraphic levels within the Karoo

Basin. Although the limited samples from the two dykes and surrounding sediments are nowhere near enough to constrain the magnetic direction/behavior from these sites, the results will be briefly discussed.

The main study focused on evaluating the magnetostratigraphy of a selection of boreholes spread across the Main Karoo Basin. Of these, four boreholes intersect one or more dolerite sills each and the baked contact test was used to study the heating effect of the intrusive rocks on the magnetic fabric within the baked zones. Four additional borehole cores were sampled that do not intersect any dolerite. The latter were used to construct a magnetostratigraphic signature for un-baked Karoo sediments. As no azimuthal information is available for all of these cores, only the inclination data were used.

5.2 Outcrop samples (southeastern basin)

A limited number of outcrop samples from two dolerite dykes and adjacent sedimentary rocks in the southeastern part of the basin were collected as a preliminary assessment of the viability of more widescale baked contact tests in the Karoo Basin. The limitations of collecting too few samples at a site include the inability to identify anomalous or lightning affected data as well as poor statistics.

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CHAPTER 5

5.2.1. Kommandodriftdam dyke

The Kommandodriftdam dyke is located west of the Kommandodrift dam within a riverbed (32°07’33.6”S, 26°01’34.57”E). Stratigraphically the site is located within the Balfour

Formation of the Beaufort Group, possibly within the Elandsberg member (unconfirmed). Two

sedimentary rock samples (sandstone and mudstone) were collected at the

Kommandodriftdam site within 0.78m from each other and at a distance of 0.7m west of the

dyke contact. The dyke width is 4.6 m (Figure 5–1). Due to alluvial cover it was not possible to

collect more samples within or outside of the contact aureole. Two oriented cubic specimens

from the dyke, one from the center and one from the western edge, were obtained from C.

Ranaweera for the baked contact test.

Figure 5–1: Kommandodriftdam dyke with sampling locations of sandstone and mudstone indicated. The total width of the dyke is 4.6m.

One specimen each of the mudstone and sandstone samples were demagnetized using

alternating field (AF) demagnetization up to 120 mT, while four sandstone and one mudstone

specimen were thermally demagnetized up to temperatures of 700°C. Both dolerite

specimens were subjected to thermal demagnetization. Typical demagnetization curves for

the sedimentary rocks and given in Figure 5–2 and Figure 5–3. The rapid decrease in

normalized intensity at 580°C (Figure 5–2C) is interpreted as the Curie temperature of Ti-poor

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magnetite indicating this as the main remanence carrier. The normalized intensity curves from

the dolerite specimens (Figure 5–3C) show a sharp decrease between 200-350°C,

suggesting the presence of pyrrhotite with a Curie temperature of ~350°C. The mean

magnetization direction for the sediments was calculated using the stable high temperature

components of each specimen, which is carried by magnetite. This mean direction was

compared with that of the dyke centre and edge (Table 5-1, Figure 5–4). The directional data

of the sediment have no correlation with the dyke edge, but since only one specimen was

available for analysis, the possibility of an orientation error cannot be ruled out. A better

correlation is observed with respect to the dyke centre, however due to the limited number of

samples the possibility cannot be ruled out that the sedimentary rocks and dyke represent the

same population of magnetic directions.

The paleolatitude for the sedimentary rock samples has been calculated as ±37°, while

for the dolerite it is calculated as ±32°, which suggests either that little continental movement

took place between the time of deposition and the intrusion, or confirms the suggested

remagnetization of the host rock.

Table 5-1: Palaeomagnetic results from the host rocks and the Kommandodriftdam dyke.

SITE DESCRIPTION N R DEC INC k α95 VGPlat VGPlon Dp Dm

0.7m west of KAM2&3 Kommandodriftdam 4 3.92 333.1 -54.8 37.64 15.2 -67.4 100.5 15.2 21.5 dyke

KA011B5 Dyke edge 1 - 315.2 50.4 - 7.4 13.8 347.6 6.7 9.9

KA016C4 Dyke centre 1 - 37.2 -52.0 - 15.5 -58.7 307.1 14.5 21.2

According to De Kock and Kirschvink (2004) reversed magnetization (positive inclination)

of the sediments at the Kommandodriftdam site (current study) is to be expected if no

magnetic contamination took place (see estimated position of Kommandodriftdam samples

relative to their published magnetostratigraphic log (Figure 5–5). It is however clear from the

results in Table 5-1 as well as Figure 5–4 that the magnetization directions of the sedimentary

rocks at the Kommandodriftdam site were indeed partially remagnetized (negative inclination).

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Figure 5–2: Typical thermal demagnetization curves of the sandstone (KAM002C) and mudstone (KAM003B) indicating magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots.

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Figure 5–3: Typical thermal demagnetization curves of the dolerite intrusion. KA011B5 represents the dyke wall and KA016C4 represent the dyke centre. In both cases a low temperature component, either ilmenite (~205°C) or pyrrhotite (~350°C) can be identified as well as magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots.

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Figure 5–4: Equal area plot of mean magnetization directions for the sediment (pink) as well as two samples from the Kommandodrift dyke. There is no apparent correlation between the direction of the sediment and intrusion.

Figure 5–5: Magnetostratigraphic section from Kommandodriftdam with estimated sampling location of Kommandodriftdam indicated (modified after De Kock and Kirschvink, 2004).

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The samples from the current study were collected within a meter of a dyke while the sampling site of De Kock and Kirschvink (2004) was reportedly not affected by any intrusive rocks.

Figure 5– 6: Geology map overlayed on Google Earth indicating the KDPT sampling location of De Kock and Kirschvink (2004) as well as the Kommandodriftdam sampling location of the current study (star).

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5.2.2. Long Dyke

The Long Dyke is a prominent, up to 30 m wide, vertical dolerite dyke, which can be followed for over 100 km between Cradock and Middelburg in the Eastern Cape Province,

South Africa (Marsh and Mndaweni, 1998). Marsh and Mndaweni (1998) suggested that the dyke might represent a fissure that fed effusions of Karoo basalt into the Karoo Basin. The

Long Dyke cuts through the Balfour Formation (Beaufort Group).

Two sandstone samples were collected on either side of the Long Dyke on the farm

Denmark (32°03’17.21”S, 25°21’33.62”E). Sample KAM007 was collected 2.45 m to the east of the dyke contact and sample KAM7 ~2 m to the west of the dyke contact. The dyke is approximately 12 m wide at this location. Another sample was collected several kilometers to the north of this site from the same dyke (31°40’22.23”S, 25°08’40.02”E). Sample KAM6 was collected ~1 m to the east of the dyke possibly on an unconfirmed E–W trending fault. Two oriented cubic specimens from the Long Dyke, one from the center and one from the eastern edge, were again obtained from C. Ranaweera (SIU).

One sandstone specimen was demagnetized using alternating field (AF) demagnetization up to 120 mT, while three sandstone specimens and two dolerite specimens were thermally demagnetized up to temperatures of 700°C. Typical demagnetization curves for the sediments are given in Figure 5–7. The normalized intensity curves (Figure 5–7C) indicate an unblocking temperature of ~580°C, suggesting magnetite as the main magnetization mineral.

The normalized intensity curves from the dolerite specimen KA027C8 (Figure 5–8C) show a sharp decrease between 200–350°C, which indicates the possible presence of

pyrrhotite. Another sharp decrease in magnetization between 580°C– 600°C suggests

magnetite as high temperature component. The projection of the directional information of

specimen KA027C8 on an equal area stereoplot (Figure 5–8A) follows a great circle that

intersects the magnetization direction of specimen KA033A9. Specimen KA033A9 became

unstable during demagnetization at temperatures above 475°C. (Figure 5–8C).

The palaeomagnetic results from the individual specimens are given in Table 5-2 and the

directional data displayed on equal area stereoplot in Figure 5–9. The directional data of the

sedimentary specimens have no correlation with that of the dyke or with each other. This is

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probably a result of the limited number of specimens analyzed. Again it is a bit worrying that

the dyke specimens have different polarity, although in both cases (Kommandodriftdam and

Long Dyke) the dyke edges seem to have positive inclinations and the dyke centres negative

inclination. This suggests that the dykes experienced protracted cooling that spans at least

one geomagnetic field reversal. This is not uncommon for large igneous complexes like the

Bushveld Complex, but unlikely in thin dykes that should cool quite quickly. This phenomenon

should be investigated further by additional sampling.

Table 5-2: Palaeomagnetic results from the host rocks and the Long dyke.

SITE DESCRIPTION N R DEC INC k α95 VGPlat VGPlon Dp Dm KAM5B 2.45 m east of dyke 1 - 308.0 -18.0 - 12.4 -36.9 128.9 6.7 12.9 Several kilometers north along same dyke. Sample KAM6B 1 - 17.0 -57.3 - 7.4 -74.7 323.9 7.9 10.8 on possible fault cutting dyke.

KAM7B ~2.0 m west of dyke 1 - 338.1 -21.0 2.8 -60.7 156.8 1.5 2.9

KA027C8 Dyke edge 1 - 39.6 53.9 - 2.9 46.8 342.8 3.7 7.3

KA033A9 Dyke centre 1 - 175.1 -39.6 - 4.2 35.3 19.8 3.0 5.0

The paleolatitude for the sediment at the farm Denmark has been calculated as ±10°,

while at site KAM6 it is calculated as ±38°, which is similar to the Kommandodriftdam site.

The paleolatitude of the dyke edge was calculated as ±5° and ±23° for the dyke centre. It

must be kept in mind that the specimen from the dyke centre became unstable below the

Curie temperature of pure magnetite, which could explain the differences observed.

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Figure 5–7: Typical thermal demagnetization curves of the sandstone (KAM005B and KAM007B) indicating magnetite (~580°C) as the main magnetization component. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots.

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Figure 5–8: Typical thermal demagnetization curves of the Long Dyke dolerite intrusion. KA027C8 represents the dyke wall and KA033A9 represent the dyke centre. In the case of KA027C8 a low temperature component, possibly pyrrhotite (~350°C), as well as high temperature component, magnetite (~580°C), can be identified. KA033A9 became unstable above 475°C. (A) Equal area projection, (B) Zijderveld digram, (C) normalized intensity plots.

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5.3 Core samples: Boreholes with Dolerite

Four boreholes that intersected dolerite sills at different stratigraphic heights spread across the

Karoo Basin were sampled (Figure 5–10). The aim was to determine the temperature variations associated with the Karoo LIP event on a basin-wide scale. Furthermore the hypothesis of a mantle plume situated SE of the coast near East London as source for the Karoo LIP (Griffiths and Campbell,

1990) will be tested, as it is assumed that there would be a decrease in magma temperature with increasing distance from the source.

5.3.1. Kopoasfontein Core (G39974)

The variation in magnetization with depth along the Kopoasfontein core was measured and is summarized in Table 5-3 according to different stratigraphic units. ‘Mean’ inclination data were calculated by excluding the obvious variations due to sill contacts or hydrothermal alteration.

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Figure 5–10: Distribution of the Karoo Basin with borehole positions of cores analyzed that intersected one or more dolerite sill (modified simplified geology map of the Karoo Basin, courtesy Council for Geoscience).

Table 5-3: Summary of mean NRM intensity and directional data for different stratigraphic units. Depth No. samples NRM [A/m] INC Formation (m) (specimens) mean stdev median (°)

Tierberg Formation 309.00-334.30 13 (42) 2.382 2.572 1.329 0.77

Sill1 334.45-343.70 6 (21) 0.324 0.192 0.386 -22.67

White Hill Formation 344.10-415.85 27 (80) 1.393 1.291 0.223 -24.89

Sill 2 416.00-499.00 13 (40) 0.246 0.156 0.223 -35.57

Prince Albert Formation 499.40-572.15 26 (79) 0.944 3.101 0.002 -6.48

Sill 3 572.27-686.70 25 (76) 0.369 0.484 0.206 -41.49

Dwyka Group 687.25-720.20 15 (46) 0.005 0.013 0.002 -11.46

The NRM intensities of the different stratigraphic units are clearly distinguishable. There is a gradual increase of intensity as the sediments become younger (towards the top of the core) which correlates with a gradual increase in observed sulphide mineralization. With the exception of two peaks the NRM intensities of the Prince Albert Formation are distinctly lower than the other units and ranges between

0.0003 A/m and 15.715 A/m with mean 0.944 A/m (0.001 A/m excl. peaks) and median 0.002 A/m. The

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The NRM intensities of the Tierberg Formation range from 0.001 A/m to 7.142 A/m with a mean of

2.382 A/m and median of 1.329 A/m. Zones of high sulphide content account for the measured maximum intensity values. The White Hill formation ranges between 0.020 A/m and 5.610 A/m with a mean of

1.393 A/m and median of 0.223 A/m. The magnetic intensities of the three dolerite sills are very similar.

Sill 1 ranges between 0.059 A/m and 0.578 A/m with mean 0.324 A/m and median 0.386 A/m. Sill 2 ranges between 0.038 A/m and 0.574 A/m with mean of 0.246 A/m and median of 0.223 A/m. Sill 3 ranges between 0.001 A/m and 1.906 A/m with mean of 0.369 A/m and median of 0.206 A/m. The very low intensity samples of Sill 3 coincide with a zone of logged gas release.

A preliminary set of 38 specimens spread across the different units were progressively demagnetized using alternating field (AF) demagnetization in gradually increasing steps of 2.5 mT, 5 mT,

10 mT and 20 mT up to a maximum applied field of 120 mT. Most of the samples were completely demagnetized and became unstable after an applied field of only 30 mT. Figure 5–11 displays typical demagnetization curves for the three dolerite sills, while Figure 5–12 and Figure 5–13 display typical demagnetization curves for the sedimentary units.

Stepwise thermal demagnetization was applied 63 specimens to determine what magnetic phases dominate. The temperature was progressively increase by 25°C from 50°C up to a maximum applied temperature of 600°C. Figure 5–14 displays typical thermal demagnetization curves for the three dolerite sills, while Figure 5–15 and Figure 5–16 display typical thermal demagnetization curves for the sedimentary units. The main magnetization component identified in all three dolerite sills is primarily magnetite with unblocking temperature around 580°C. The unblocking temperatures of the magnetization component dominating within the sedimentary units range between 350°C and 400°C indicating either pyrrhotite or titanomagnetite. Considering the anoxic deposition environment of these deposits, however, it is more likely to be pyrrhotite.

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Figure 5–11: Typical demagnetization curves from the three dolerite sills. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–12: Typical demagnetization curves from the upper sedimentary units occurring in the G39974 Kopoasfontein borehole. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–13: Typical demagnetization curves from the lower sedimentary units occurring in the G39974 Kopoasfontein borehole. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–14: Typical thermal demagnetization curves from the three dolerite sills indicating magnetite (~600 °C) as the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–15: Typical thermal demagnetization curves from the upper sedimentary units occurring in borehole G39974 indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–16: Typical thermal demagnetization curves from the different sedimentary units occurring in borehole G39974 indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Polarity distribution

As no azimuthal information of the core is available only the magnetic inclination has significance as a polarity indicator. Figure 5–17 illustrates a plot of the magnetic susceptibility, intensity of magnetization as well as the magnetic inclination (both before and after demagnetization) with depth. A negative inclination dominates throughout the borehole. The isolated AF inclination data with depth do not vary significantly from the NRM inclinations (red dots overlaid on NRM inclinations). Shallowing of the inclination occurs at the sediment-sill contacts, while reversed polarity appears to be confined to specimens further away from the contact aureole

(i.e. top few samples from the Tierberg Formation as well as the bottom samples collected in the

Dwyka Group). This suggests that remagnetization is limited to short distances within the contact aureole.

5.3.2. Waterkloof (PP47)

No clear distinction of stratigraphic units could be made of core PP47. The lithostratigraphy consists of 2 shale layers and three siltstone layers intruded by 5 dolerite sills of varying thicknesses with evidence of significant carbonate alteration.

Table 5-4 summarizes the NRM data for the different stratigraphic units. ‘Mean’ inclination data were calculated by excluding the obvious variations due to sill contacts or hydrothermal alteration.

Table 5-4: Summary of mean NRM intensity and directional data for different lithologic layers. Depth No. NRM [mA/m] INC Formation (m) samples mean stdev (°)

Shale1 240.70-428.00 31 37.12 4.373 -54.81

Siltstone1 433.5-492.00 12 0.782 2.265 -57.29

Sill1 494.00-580.00 6 400.709 289.894 -63.23

Siltstone2 585.00-709.00 17 0.302 0.103 -50.46

Sill2 684.00-700.00 3 202.849 122.318 -63.43

Shale2 714.00-831.00 17 0.875 0.779 -45.27

Siltstone3 836.00-1070.00 18 3.760 9.788 -38.24

Sill3 865.00-928.00 3 40.392 49.777 -59.70

Sill4 959.00-1004.00 3 52.894 42.375 -65.45

Sill5 1071.00-1148.00 5 405.812 203.543 -50.35

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Figure 5–17: Summary of the magnetic susceptibility, -intensity and variation in magnetic inclination as a function of depth in borehole G39974 with stratigraphic units superimposed. Red dots indicate only limited change in the magnetic inclinations after AF demagnetization. Thermal demagnetization (connected light blue dots) confirms the dominantly negative inclination of the dolerite with mainly positive inclination for the sediments.

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The NRM of the first shale layer is significantly higher than the other sedimentary layers.

The reason for this is possibly related to the occurrence of pyrrhotite reported on the borehole log. Although the dolerite as well as the sediment has predominantly negative NRM inclinations, the values for the sediment are significantly shallower than the intrusive rocks.

The magnetic intensities of the dolerite sills are significantly higher than that of the sediment and similar in value to the Kopoasfontein core, however, other than the values of the

Kopoasfontein core the NRM inclinations of the Waterkloof dolerite sills correlate well with the publish mean inclination for the Karoo Dolerite Suite at -56° as incorporated into the Global

Paleomagnetic Database. This is however also very close to the present axial dipole at -64°.

Samples from the Waterkloof core were demagnetized at the University of Johannesburg using a cryogenic system. A set of 57 specimens spread across the core length were demagnetized with progressive AF demagnetization in steps of 25 mT up to 100 mT, where after the same samples were subjected to stepwise thermal demagnetization up 525°C.

Although stable endpoints were not always reached at this temperature, the directions of magnetization became unstable above ~475°C.

Figure 5–18 displays typical demagnetization curves from the first shale unit, indicating a range of unblocking temperatures. The dominant magnetization component in specimen

PH7A is pyrrhotite (~350°C). Both specimens PH15A and PH31A indicate the presence of pyrrhotite as well as possibly titanomagnetite (400-500°C), however, the dominance of either differs in the two specimens as is evident from the decrease in normalized intensity plots (a) in Figure 5–18. Specimen PH31A was sampled from a part of the core indicating numerous thin medium grained sills thus not excluding the possibility of mineral contamination of the shale.

Figure 5–19 displays typical demagnetization curves for the different Siltstone units.

Specimens PH35A, PH57A and PH97A all indicate the presence of pyrrhotite (~350°C) as well as possibly titanomagnetite (400-500°C), with the change in magnetization directions for these components visible from Figure 5–19(c). Specimen PH43A indicates the main magnetization component to be dominantly titanomagnetite.

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Figure 5–18: Thermal demagnetization curves of selected specimens from Shale1 in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity.

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Figure 5–19: Thermal demagnetization curves of selected specimens from the different siltstone units in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity

Figure 5–20 displays typical demagnetization curves from the second shale unit, again indicating a range of unblocking temperatures. For both specimens, however, the dominant magnetization component is pyrrhotite with unblocking temperature ~350°C.

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Figure 5–20: Thermal demagnetization curves of selected specimens from Shale2 in borehole PP47 indicating the variation of magnetization components. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal-area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity.

Figure 5–21 displays typical demagnetization curves for the dolerite sills. With the exception of sill 3 that become random above 350°C, all the sills displayed unblocking temperatures at approximately 500°C. This is much lower than the expected unblocking temperature of magnetite (~580°C) and could possibly be ascribed to the presence of small amounts of titanium (to be confirmed), which would lower the Curie temperature.

Determination of individual Fe-Ti oxide grains can be done by petrography or with electron microprobe techniques. In contrast to these methods, which require special sample preparation, Raman spectroscopy can be done with minimal preparation.

Polarity distribution

Figure 5–22 illustrates a plot of the vertical distribution of the magnetic susceptibility, intensity of magnetization as well as the inclination before and after thermal demagnetization with depth. No significant change in the inclination took place after demagnetization, with only a few sedimentary specimens tending to become shallower and even positive. This would suggest than remagnetization did not completely over print the sediments.

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Figure 5–21: Typical thermal demagnetization curves of the dolerite sills in borehole PP47 indicating magnetite (~525°C) as the main magnetization component. (a) Normalized magnetic intensity curve of progressive demagnetization results; (b) Equal- area projection of the change in direction of magnetization; (c) Zijderveld plot with blue representing the vertical plane and red the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity.

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Figure 5–22: Summary of the magnetic intensity and variation in magnetic inclination (NRM) as a function of depth in borehole PP47 with lithologic units superimposed.

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5.3.3. Hermon (HM1/78)

Borehole HM1/78 is relatively shallow (110m) and significantly weathered resulting in large sampling intervals. Additional chipped samples were collected from weathered sections for susceptibility analysis. Table 5-5 summarizes the NRM data for the different stratigraphic units.

Table 5-5: Summary of mean NRM intensity and directional data for the different stratigraphic units of core HM1/78.

Depth No. NRM [mA/m] INC Formation (m) samples mean stdev (°)

Tierberg Formation 0.50-32.00 3 0.762 0.146 -19.75

White Hill Formation 32.00-43.00 2 0.837 0.153 5.67

Prince Albert 43.00-100.00 12 5.101 9.853 1.07 Formation

Dolerite sill 73.50-92.00 12 89.894 241.847 1.59

Dwyka Formation 100.00-105.50 4 2.253 1.166 11.82

Lava (Ventersdorp?) 105.50-110.75 3 377.756 73.598 63.14

A total of 33 specimens from borehole HM1/78 were thermally demagnetized in steps of

25°C up to 600°C. Figure 5–24 displays typical thermal demagnetization curves for the

sedimentary units, while Figure 5–25 displays typical thermal demagnetization curves for the

single dolerite sill intersected in this borehole as well as the mafic lava (presumably

Ventersdorp Supergroup) at the base of the core.

Polarity distribution

Figure 5–26 illustrates a plot of the magnetic susceptibility, intensity of magnetization as

well as the inclination with depth. Significant changes in the directions of magnetization before

demagnetization (black diamonds) and after demagnetization (yellow squares for low

temperature; purple dots for intermediate temperature; and red triangles for high temperature

components) can be observed. Thermal demagnetization confirms a dominantly negative

inclination for both the dolerite as well as the sedimentary rocks. The inclination values of the

sedimentary rocks are, however, slightly shallower than that of the dolerite, while the

presumed Ventersdorp lava displays a positive inclination.

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Figure 5–23: Typical thermal demagnetization curves from the upper sedimentary units occurring in the HM1/78 borehole indicating pyrrhotite (~350°C) to be the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–24: Typical thermal demagnetization curves from the lower sedimentary units occurring in the HM1/78 borehole indicating pyrrhotite (~350°C) to be the main magnetization component. The Prince Albert Formation however, indicates the presence of both pyrrhotite and magnetite (~580°C). (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–25: Typical thermal demagnetization curves for both the dolerite sill as well as the underlying lava of the Ventersdorp Formation indicating magnetite (580–600°C) as the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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5.3.4. Driefontein (DF1/75)

Borehole DF1/75 starts in the Volksrust Formation extends through the Vryheid and

Pietermaritzburg formations as well as the Dwyka Group terminating in granitic basement.

Two dolerite sills of varying thicknesses were also intersected. Table 5-6 summarized the

NRM data for the different stratigraphic units ‘Mean’ inclination data for Table 5-6 were thus calculated by excluding possible disturbed samples. It is clear that the dolerites in borehole

DF1/75 are predominantly positively inclined, while the sedimentary rocks are mostly negative.

Table 5-6: Variation of susceptibility and magnetic intensity for different stratigraphic units of core DF1/75.

Depth No. NRM [mA/m] INC Formation (m) samples mean stdev (°)

Volksrust Formation 1.00-76.00 6 0.894 0.914 -40.71

Vryheid Formation 76.00-514.00 65 7.207 36.268 -30.06

Pietermaritzburg 514.00-597.00 13 14.566 27.024 -30.51 Formation

Dolerite sill 1 159.00-344.00 22 267.152 209.774 64.50

Dolerite sill2 534.00-582.00 7 564.400 268.271 25.36

Dwyka Group 597.00-598.00 2 1.759 0.022 8.48

Basement granite 598.00-612.00 3 1.216 0.121 -66.55

A total of 66 specimens where thermally demagnetized up to 600°C. Figure 5–27 to

Figure 5–29 display typical thermal demagnetization curves for the sedimentary units as well

as basement samples from borehole DF1/75. Figure 5–30 displays typical thermal

demagnetization curves for the two dolerite sills intersected within the borehole.

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Figure 5–26: Summary of the magnetic susceptibility, -intensity and variation in magnetic inclination as a function of depth in borehole HM1/78 with stratigraphic units superimposed. Black diamonds indicate direction before demagnetization (NRM). Directions after thermal demagnetization are indicated by yellow squares (low temperature component), purple dots (intermediate temperature component), and red triangles (high temperature component). Thermal demagnetization confirms a dominantly negative inclination for both the dolerite as well as the sediments.

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Figure 5–28: Typical thermal demagnetization curves from the Pietermaritzburg Formation indicating a low temperature component, possibly pyrrhotite (400–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~580°C), observed. Sample DF27B are one of the exceptions where magnetite is the main magnetization component. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–29: Typical thermal demagnetization curves from the Dwyka Group and basement occurring in borehole DF1/75 indicating a low temperature component, possibly pyrrhotite (~400°C), to be the main magnetization component with minimal contribution from the high temperature component, magnetite (~580°C). (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Two magnetization components were identified namely a low temperature (LT) as well as a high temperature (HT) component. The main magnetization component identified in all three dolerite sills is primarily magnetite with unblocking temperature around 580°C. The unblocking temperatures of the magnetization components dominating within the sedimentary units range between 350°C and 400°C suggesting pyrrhotite.

Polarity distribution

Figure 5–31 illustrates a plot of the vertical component (INC) with depth. The magnetic inclinations for the sedimentary rocks were rather erratic but can be attributed to bioturbation activity that was logged along the core. In contrast with the other boreholes, dolerite sill 1 is positively inclined. If widespread overprinting did take place due to the dolerite intrusions as suggested by Ballard et al. (1986) the directions of the sedimentary rocks would have re- aligned parallel to the direction of the dolerite. Since this was not the case, we conclude that the heating effect of the intrusion was only minor and not above the unblocking temperature of the magnetic minerals contained in the sedimentary rocks.

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Figure 5–30: Typical thermal demagnetization curves from the two dolerite sills. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–31: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination (NRM) as a function of depth in borehole DF1/75 with stratigraphic units indicated.

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CHAPTER 5

5.4 Core samples: Boreholes without Dolerite

Four (4) boreholes were selected on the outer edge of the Karoo Basin that does not intersect any dolerite sill (Figure 5–32). The aim of these boreholes was to determine background values for the Karoo sedimentary rocks without the heating effect of any dolerite intrusions.

Figure 5–32: Distribution of the Karoo Basin with borehole positions of core analyzed that do not intersect any dolerite (modified simplified geology map of Karoo Basin, courtesy Council for Geoscience).

5.4.1. Sambokkraal (SA1/66)

This is the longest borehole studied with a sample collected for laboratory analysis every

30m up to a depth of 4100m. Initial in situ susceptibility measurements were taken every 1m with a KT9 kappameter. Table 5-7 summarizes the magnetic intensity and NRM inclination data for the different stratigraphic units.

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CHAPTER 5

Table 5-7: Susceptibility variation between different stratigraphic units of core SA1/66. Depth No. samples NRM [mA/m] INC Formation (m) mean stdev (°)

Lower Beaufort Group 0.00-1267.00 48 1.554 1.382 -52.75 (Teekloof Formation?) Ecca Group 1267.00-2754.00 43 0.803 0.749 -53.38

Upper Dwyka (White Hill and Prince Albert 2754.00-2947.00 18 0.142 0.106 -40.67 Formations)

Dwyka Tillite Formation 2947.00-3587.00 4 2.678 1.810 -37.83

Bokkeveld Group 3587.00-4104.00 3 1.695 1.224 -28.00

Table Mountain 4104.00-4175.00 1 0.631 - -25.00 Sandstone Formation

A total of 44 specimens where thermally demagnetized up to 600°C. Figure 5–33 to

Figure 5–35 display typical thermal demagnetization curves for the sedimentary units from borehole SA1/66. Unblocking temperatures between 350°C and 475°C were observed with only the Beaufort Group indicating minor magnetite at ~580°C.

Polarity distribution

Figure 5–36 illustrates a plot of the magnetic susceptibility, intensity of magnetization as well as the magnetic inclination (both before and after demagnetization) with depth. Although the inclination before demagnetization was very variable, especially for the lower Beaufort

Group, after thermal demagnetization the dominant direction of magnetization is steeply negative. This is completely unexpected for the Kaiman reverse polarity superchron. Possible remagnetization due to thermotectonic heating from the Cape Fold Belt might explain the consistent normal polarity of this borehole. Lindeque et al. (2011) interpreted deep seismic data from the southern Karoo Basin in the vicinity of borehole SA1/66 and displayed the stratigraphy from this borehole on top of the seismic section, clearly showing the folded layers of the upper Ecca and Beaufort groups (Figure 5–37).

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CHAPTER 5

Figure 5–33: Typical thermal demagnetization curves from the Bokkeveld and Table Mountain Groups occurring below the Karoo Supergroup in borehole SA1/66. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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CHAPTER 5

Figure 5–34: Typical thermal demagnetization curves from the Karoo sedimentary units occurring in borehole SA1/66 indicating a low temperature component, possibly pyrrhotite (350–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~500°C), observed. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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CHAPTER 5

Figure 5–35: Typical thermal demagnetization curves from the lower sedimentary units occurring in borehole SA1/66 indicating a low temperature component, possibly pyrrhotite (350–450°C), to be the main magnetization component with only minor occurrences of high temperature component, magnetite (~500°C), observed. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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CHAPTER 5

Figure 5–36: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole SA1/66 with stratigraphic units indicated.

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CHAPTER 5

Figure 5–37: Seismic profile with overlay of borehole SA1/66 stratigraphy indicating folding from the Cape Fold Belt effecting the Beaufort and upper Ecca groups (Lindeque et al., 2011).

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CHAPTER 5

5.4.2. Schietfontein (SC3/67)

Table 5-8 summarizes the NRM data for the different stratigraphic units. As samples from this borehole consisted only of rock chips, the baked contact test could not be performed.

Table 5-8: Variation of susceptibility and magnetic intensity between different stratigraphic units of core SC3/67.

Depth No. NRM [mA/m] Formation (m) samples mean stdev

Lower Beaufort Group (Teekloof 0.00-2286.00 50 203.75 241.608 Formation?) Upper Ecca Group (Abrahamkraal and Waterford 2286.00-3125.00 19 56.304 81.660 Formations?) Lower Ecca Group (Tierberg, Ripon and Collingham 3125.00-3972.00 18 393.70 435.48 Formations?) Upper Dwyka (White Hill and 3972.00-4539.00 12 365.80 359.98 Prince Albert Formations) Dwyka Tillite Formation 4539.00-5258.00 14 313.64 243.75

Bokkeveld Group 5258.00-5559.55 9 1062.53 740.43

Figure 5–38 illustrates a plot of the magnetic susceptibility and intensity of magnetization

with depth. There is no clear distinction between different lithologic units apart from a gradual

increase in magnetic intensity with depth. The data are quite variable, and might indicate the

effect of increased weathering due to enhanced exposure to oxidized atmosphere of the

chipped samples during storage.

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CHAPTER 5

Figure 5–38: Summary of the magnetic susceptibility and intensity of magnetization as a function of depth in borehole SC3/67 with lithologic units superimposed.

5.4.3. Goedehoop (CBC4495)

This borehole is situated in the northeastern part of the Karoo Basin and intersects several coal seams but no dolerite. The borehole log does not specify the stratigraphic units, but it in this part of the Basin the Vryheid Formation of the Ecca Group dominates (Johnson et al, 2006). For the purpose of this thesis the mean magnetic results of the coal seams will be given separate from the stratigraphic units (Table 5-9).

Table 5-9: Variation of susceptibility and magnetic intensity between different stratigraphic units of core CBC4495. Depth NRM [mA/m] Formation No. samples (m) Mean stdev

Sandstone and mudstone (Vryheid 0.00-92.25 51 0.566 0.425 Formation) Dwyka Tillite Formation 92.25-92.85 1 0.476 -

Sandstone/gridstone (Transvaal 92.85-95.39 3 1.726 0.658 Supergroup?) Coal 18 0.338 0.287

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CHAPTER 5

A total of 50 specimens where thermally demagnetized up to 600°C. Figure 5–39 to Figure 5–41 display typical thermal demagnetization curves for the sedimentary lithologies from borehole

CBC4495.

Polarity distribution

Figure 5–42 illustrates a plot of the magnetic susceptibility, intensity of magnetization as well as magnetic inclination before and after thermal demagnetization with depth. Similar to borehole SA1/66 the magnetic inclination after demagnetization of core CBC4495 tends to be steeply negative. Unlike core SA1/66 no impact from the Cape Fold Belt is expected. Possible remagnetization due to secondary heating from hydrothermal fluids associated with distant intrusions could be investigated.

Although the borehole logs from CBC4495 as well as CBC4496 nearby indicate no evidence of intrusions, the Witbank Coalfield is known for widespread dolerite occurences, for example the Ogies

Dykes situated north of the Goedehoop Collery (Du Plessis, 2008)

5.4.4. Groottegeluk (MY19)

This borehole is situated in the far northern Ellisras sub-basin and consists of shale from the

Beaufort Group above the 70 m thick Groottegeluk Formation interlayered coal seams and the

Vryheid Formation shale and sandstone below the coal. No dolerite was intercepted. Underlying the

Karoo sediments is the Letaba basalt. Only a limited number of samples above and below the coal seam were available for analysis, courtesy Exxaro Groottegeluk Coal Mine. Table 5-10 summarizes the mean magnetic results of the different lithologic units.

Table 5-10: Variation of susceptibility and magnetic intensity between different stratigraphic units of core CBC4495.

Depth No. NRM [mA/m] INC Formation (m) samples mean stdev (°)

Beaufort Group (shale) 30.92 – 51.85 5 0.560 0.114 51.00

Vryheid Formation (shale) 140.00 – 148.73 2 1.631 0.972 -18.32

Vryheid Formation 155.17 – 163.62 3 0.979 0.363 -21.67 (sandstone)

Ten specimens where thermally demagnetized up to 600°C. Figure 5–43 displays the typical thermal demagnetization curves for the Beaufort Group shale while Figure 5–44 displays the typical

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CHAPTER 5 thermal demagnetization curves for the Vryheid Formation shale and Figure 5–45 for the Vryheid

Formation sandstone.

Figure 5–39: Typical thermal demagnetization curves from different lithologic units in the Vryheid Formation (core CBC4495) indicating both low temperature (200–400 °C) as well as high temperature (580–600°C) magnetization components. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–40: Typical thermal demagnetization curves from the Dwyka tillite (GH71C) (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

5-52 CHAPTER 5

Figure 5–41: Typical thermal demagnetization curves from a sedimentary unit below the Dwyka presumed to belong to the Transvaal Supergroup (GH74B). Sample GH36C is an example of an unresolved unblocking temperature that becomes unstable above 200°C. Sandstone GH64C near the base of the Vryheid Formation indicate a stable endpoint a ~400°C suggesting anoxic conditions. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

5-53 CHAPTER 5

Figure 5–42: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole CBC4495 with stratigraphic units indicated.

5-54 CHAPTER 5

Figure 5–43: Typical thermal demagnetization curves for the Beaufort Group shale in borehole MY19. Samples became unstable at temperatures above 300°C. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

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Figure 5–44: Typical thermal demagnetization curves for the Vryheid Formation shale in borehole MY19. Stable endpoint was reached at an unblocking temperature 375–400°C. (a) Equal-area projection of the change in direction of magnetization; (b) Zijderveld plot with blue representing the vertical plane and green the horizontal plane; The scale on the axes is in A/m; The distance of each data point from the origin indicates the total NRM intensity; (c) Normalized magnetic intensity curve of progressive demagnetization results.

5-56 CHAPTER 5

5-57 CHAPTER 5

Figure 5–46: Summary of the magnetic susceptibility, intensity of magnetization and variation in magnetic inclination as a function of depth in borehole MY19 with stratigraphic units indicated.

5-58 CHAPTER 5

Polarity distribution

Figure 5–46 illustrates a plot of the magnetic susceptibility, intensity of magnetization as well as magnetic inclination before and after thermal demagnetization with depth. Although the NRM inclination of the Groottegeluk Formation was positive before thermal demagnetization it converges to -38° after thermal demagnetization. With the exception of sample P1039 (Figure 5–45) that displays a shallow inclination, both the Vryheid Formation shale and sandstone display a steep mean inclination of -66°. This is nearly indistinguishable from the present axial dipole. It is suggested that remagentization was caused either due to heating by the underlying Lataba basalt, or due to elevated crustal heat flow in a predominant rift setting.

5.5 Discussion

Remagnetization of baked rock within the magma-sediment contact aureole is a well- known occurrence (McElhinny, 1973) and the changes in properties with distance from the baked contact correspond to the diminishing heating effects of the intrusion (Everitt and

Clegg, 1962). Ballard et al. (1986) suggested extensive remagnetization of the Karoo sediments due to the intrusion of the Karoo LIP. To confirm the extent of remagnetization in the Karoo Basin the variation in magnetic inclination with depth as well as laterally across the

Basin were investigated.

Many authors have reported dual polarity for the Karoo dolerite suite (Graham and

Hales, 1957; van Zijl et al., 1962a,b; Kosterov and Perrin, 1996; Hargraves et al., 1997;

Prevot et al., 2003) with up to 6 consecutive flows identified in the Lesotho sequence. The published mean inclination for the Karoo dolerite suite as incorporated into the Global

Paleomagnetic Database is -56°. Published mean inclination data from Dwyka Group diamictite in South Africa is +69° (Opdyke et al., 2001), although three sites from the upper

Dwyka Group yielded normal polarities. Lanci et al. (2013) carried out a magnetostratigraphic study of the uppermost Ecca Group (upper Waterford Formation) and the lowermost Beaufort

Group (Abrahamskraal Formation) and observed 3 normal and 4 reversed magnetozones with a combined inclination for the transition from upper Ecca to lower Beaufort groups of -

53° (ranging between -51° and +53°). De Kock and Kirschvink (2004) as well as Kirschvink and Ward (1998) studied the Permian Triassic (PT) boundary that occurs within the Beaufort

5-59 CHAPTER 5

Group and identified a R-N-R polarity pattern for the Karoo PT boundary interval. The combined inclination for the Beaufort Group is -62° (ranging between +65° and -57°).

As discussed previously, the drill cores from the current study are not azimuthally oriented, and the palaeomagnetic inclination is thus the only indicator of polarity. It was assumed that drilling took place without deviation from the vertical and data are given without consideration of possible bedding tilt and the effect that it could have on the inclination values.

The author is limited to evaluating the directional variation adjacent to each intrusion and associated aureole individually. Figure 5–47 displays the inclination data with depth for the four boreholes that intersected dolerite, while Figure 5–48 displays the inclination data from three boreholes that did not intersect any dolerite.

The mean magnetic inclination of the dolerite sills range from -35° for borehole G39974 in the western part of the Basin to -60° for both HM/78 and PP47 in the center of the Basin to

+50° (sill1) and ±60° (sill2) for DF1/75 in the northeast.

It is clear that the magnetic inclination of the positively inclined sediment in borehole

G39974 was not significantly altered due to the sill intrusions with variation limited to very short distances from the sill-sediment contact. In a similar but inversed fashion the sediments in borehole DF1/75 are mostly negatively inclined while the sill intrusions are positively inclined. The centrally situated two boreholes (HM1/78 and PP47), however, appear to have been completely overprinted by the intrusive rocks with little distinction between the inclination values of the dolerite and sediment layers. Even the inclination of the presumed Allanridge

Formation (Ventersdorp Supergroup) at the base of core HM1/78 appears to be overprinted

(published inclination for Allanridge is +60.7°, Strik et al., 2007).

However, considering the published reports of dual polarity directions for the Karoo sediments and taking into account the directional uncertainty associated with collecting samples from unoriented archived core samples, proving any pervasive overprint would thus be very difficult.

5-60 CHAPTER 5

Figure 5–47: Variation in magnetic inclination with depth indicating the limited degree of remagnetization due to dolerite intrusions. Different symbols are associated with different magnetic phases, i.e. High Temperature (HT), Medium Temperature (MT) and Low Temperature (LT) components.

5-61 CHAPTER 5

Figure 5–48: Variation in magnetic inclination with depth for boreholes without any dolerite intrusions. Blue symbols indicate NRM inclination while red symbols indicate inclination after thermal demagnetization. In all three boreholes the inclination became intermediate to steeply negative after thermal demagnetization.

5-62 CHAPTER 5

The magnetic inclination from the boreholes that did not intersect any dolerite (Figure 5–

48) all became intermediate (MY19 and CBC4495) to steep negatively inclined (SA1/66) after thermal demagnetization. This is in contradiction to the expected reversed magnetization for the Kiaman Superchron. It is suggested that remagnetization of core SA1/66 occurred due to heating from thermotectonic processes related to the Cape Orogeny, while possible increased heating from the Letaba basalt in the Ellisras Basin might explain the inclination pattern from borehole MY19. The normal polarity observed in CBC4495 is yet unexplained.

5.6 References

Ballard, M. M., Van der Voo, R. and Halbich, I. W., 1986. Remagnetizations in Late Permian

and Early Triassic rocks from southern Africa and their implications for Pangea

reconstructions. Earth and Planetary Science Letters, 79, 412–418.

De Kock, M. O. and Kirschvink, J. L., 2004. Paleomagnetic constraints on the Permian-

Triassic boundary in terrestrial strata of the Karoo Supergroup, South Africa:

implications for causes of the end-Permian extinction event. Gondwana Research, 7(1),

175–183.

Du Plessis, G.P., 2008. The relationship between geological structures and dolerite intrusions

in the Witbank Highveld coalfield, South Africa. MSc thesis, University of the Free

State, Bloemfontein, 164 pp.

Everitt, C. W. F. and Clegg, J. A., 1962. A field test of paleomagnetic stability. Geophysical

Journal of the Royal Astronomical Society, 6, 312–319.

Graham, K.W.T. and Hales, A.L., 1957. Palaeomagnetic Measurements on Karoo Dolerites.

Philosophical Magazine Supplement, 6(22), 149–161.

Griffiths, R.W. and Campbell, I.H., 1990. Stirring and structure in mantle starting plumes.

Earth and Planetary Science Letters, 99, 66–78.

Hargraves, R.B., Rehacek, J., and Hooper, P.R., 1997. Palaeomagnetism of the Karoo

igneous rocks in southern Africa: South African Journal of Geology, 100, 195–212.

Johnson, M.R., Van Vuuren, C.J., Visser, J.N.J. Cole, D.I., de V. Wickens, H., Christie,

A.D.M., Roberts, D.L. and Brandl, G., 2006. Sedimentary rocks of the Karoo

Supergroup. In: Johnson, M.R., Anhaeusser, C.R. and Thomas, R.J. (Eds.), The

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Geology of South Africa. Geological Society of South Africa, Johannesburg/Council for

Geoscience, Pretoria, 461–499.

Kirschvink, J.L. and Ward, P.D., 1998. Magnetostratigraphy of the Permian/Triassic boundary

sediments in the Karoo of South Africa. Journal of African Earth Sciences, 27(1A),

Special Abstract Issue Gondwana 10, Events Stratigraphy of Gondwana, 124 pp.

Kosterov, A.A. and Perrin, M., 1996. Paleomagnetism of the Lesotho Basalt, Southern Africa,

Earth and Planetary Science Letters, 139, 63–78.

Lanci, L., Tohver, E., Wilson, A. and Flint, S., 2013. Upper Permian magnetic stratigraphy of

the lower Beaufort Group, Karoo Basin. Earth and Planetary Science Letters, Earth and

Planetary Science Letters, 375, 123–134.

Lindeque, A., de Wit, M.J., Ryberg, T., Weber, M. and Chevallier, L., 2011. Deep crustal

profile across the southern Karoo Basin and Beattie magnetic anomaly, South Africa:

An interpretation with tectonic implications. South African Journal of Geology, 114,

265–292.

Maré, L.P., 2010. Thermal History of the Karoo Basin in South Africa Inferred from Magnetic

Studies: Progress Report. Council for Geoscience, Pretoria, Report 2010-0061, 29 pp

(unpublished).

Marsh, J. S. and Mndaweni, M. J., 1998. Geochemical variations in a long Karoo dyke,

Eastern Cape. South African Journal of Geology, 101(2), 119–122.

McElhinny, M., 1973. Paleomagnetism and Plate Tectonics. Cambridge University Press,

368 pp.

Opdyke, N.D., Mushayandebvu, M. and de Wit, M.J., 2001. A new palaeomagnetic pole for

the Dwyka System and correlative sediments in sub-Saharan Africa. Journal of African

Earth Sciences, 33(1), 143–153.

Prevot, M., Roberts, N., Thompson, J., Faynot, L., Perrin, M. and Camps, P., 2003. Revisiting

the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa).

Geophysical Journal International, 155, 367–378.

Strik, G., de Wit, M.J. and Langereis, C.G., 2007. Palaeomagnetism of the Neoarchaean

Pongola and Ventersdorp Supergroups and an appraisal of the 3.0–1.9 Ga apparent

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polar wander path of the Kaapvaal Craton, Southern Africa. Precambrian Research,

153, 96–115.

Van Zijl, J.S.V., Graham, K.W.T. and Hales, A.L., 1962a. The palaeomagnetism of the

Stormberg lavas of South Africa 1: evidence for a genuine reversal of the Earth’s field

in Triassic-Jurassic times. Geophysical Journal of the Royal Astronomical Society, 7,

23–39.

Van Zijl, J.S.V., Graham, K.W.T. and Hales, A.L., 1962b. The palaeomagnetism of the

Stormberg lavas of South Africa 2: the behaviour of the magnetic field during a

reversal. Geophysical Journal of the Royal Astronomical Society, 7, 169–182.

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CHAPTER 6. EXPERIMENT 2 – ALTERATION INDEX A40

6.1 Introduction – Variation of magnetic susceptibility with repeated

progressive heating

Powdered as well as rock chip samples were subjected to measurements of magnetic susceptibility following the method described by Hrouda et al. (2003) using a CS3 furnace attached to an Agico MFK1-FA Kappabridge (see also Chapter 1 for introduction to the method). The changes in magnetic susceptibility due to heating in normal atmosphere were evaluated quantitatively by the A40 alteration index as described by Hrouda et al. (2002). The values of the A40 alteration index calculated after individual heating/cooling runs on samples collected at different depths along eight different boreholes spread across the Karoo Basin as well as a few surface samples will be used to evaluate the thermal effect of the dolerite intrusions on the sedimentary strata.

6.2 Outcrop samples (southeastern basin)

Preliminary samples from the southeastern Karoo Basin were collected next to dolerite dykes as well as below and above dolerite sills. Progressive measurements of the magnetic susceptibility were made while heating the samples in intervals of 25°C up to 400 °C, there after increasing the interval temperatures to 50°C up to a maximum temperature of 600 °C. It is clear from Table 6-1and Figure 6-1 that the values of the A40 index are very low at temperatures less than 200°C in the case of site KAM3 and 250°C for site KAM0, but increase substantially at higher temperatures. This may indicate that the maximum palaeotemperature that the eastern Karoo Basin has reached during the intrusion of the

Karoo large igneous event ranged between 200–250 ± 50°C. Figure 6-2 up to Figure 6-5 focus on specific sampling locations.

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CHAPTER 6

Table 6-1: Values of A40 alteration index after individual steps for investigated samples (after Hrouda et al., 2003) Temperature KAM0 KAM1 KAM2 KAM3 KAM4 KAM5 KAM6 KAM7 KAM8

100 1.52 1.09 1.51 0.69 0.48 0.82 3.26

125 -0.18 0.84 0.30 1.90 1.97 0.15 2.97 0.78

150 -0.18 1.06 0.73 1.51 -0.41 3.68 0.54 2.58 2.43

175 -0.43 0.75 0.52 0.71 2.01 -0.42 5.11 3.54

200 -0.57 1.19 1.19 2.30 0.27 4.31 0.73 3.04 7.91

225 -0.40 1.19 0.97 3.53 2.03 0.15 3.68 2.20

250 0.25 1.64 1.43 2.75 2.68 2.13 1.31 1.41 3.10

275 0.94 2.23 2.15 3.65 6.48 4.70 1.95

300 1.18 2.26 3.12 3.73 4.17 3.44 0.54 4.91 4.86

325 2.48 3.03 3.03 4.93 6.28 4.71 6.21 5.27

350 3.72 3.84 5.24 6.10 7.42 8.48 5.60 6.36 4.03

375 4.87 7.29 5.81 7.04 3.89 8.50 3.99

400 5.00 8.40 6.15 9.34 5.73 4.49 9.14 7.28

450 9.24 16.05 20.74 11.22 14.00 13.66 18.29 7.11

500 13.70 16.48 18.87 12.90 5.81 17.45 27.33 9.48

550 11.12 9.46 11.65 10.16 -7.02 15.29 22.76 3.21

600 15.21 17.38 1.55 12.49 16.64 15.83 24.74 -6.38

Figure 6-1: Change in the average A40 alteration index for individual heating/cooling runs after Hrouda et al. (2003). 6–2

CHAPTER 6

The Golden Valley sill is a saucer shaped sill located within the Burgersdorp Formation of the Tarkastad Subgroup (Beaufort Group). Figure 6-2 shows the change in A40 with increased temperature for a sample below (KAM000) as well as above (KAM001) the Golden

Valley sill. There seems to be a ~75°C difference in the maximum attained temperature between the top (~175°C) and bottom (250°C) of the sill. A possible explanation for this could be that the sill acted as both a source of heat as well as a barrier preventing heat transferred to the sedimentary strata below the sill from escaping. The total thickness of the sill is estimated to be 40 m.

The Kommandodriftdam dyke intrudes into the Balfour Formation, Adelaide Subgroup

(Beaufort Group). The maximum temperature that the host rock attained due to the intrusion of the 4 m wide Kommandodriftdam dyke was calculated to be 200–225°C (Figure 6-3).

Figure 6-3: Change in the average A40 alteration index for samples at the Kommandodriftdam dyke. A maximum geothermal temperature of 200–225°C is observed.

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CHAPTER 6

Stratigraphically the 12 m wide Long Dyke also cuts through the Balfour Formation

(Beaufort Group) at the Denmark site, although approximately ~100 m higher in the stratigraphic sequence than the Kommandodriftdam dyke. The maximum host rock temperature calculated is 300°C (Figure 6-4), while still another 200 m higher in the stratigraphic sequence (site KAM006) the maximum temperature is only 250°C.

The Taylor’s Koppie dyke is approximately 22 m meters wide and stratigraphically approximately 100 m above the Demark site, although further east near the Insizwa complex.

Some authors have suggested the Taylor’s Koppie dyke to be a feeder of the Insizwa

Complex (Marsh et al., 2003). The maximum temperature attained by the sediment adjacent to this dyke has been calculated to be ~275–300°C (Figure 6-5).

6–4

CHAPTER 6

Figure 6-5: Change in the average A40 alteration index for samples at the Taylor’s Koppie dyke in the Insizwa vicinity. A maximum geothermal temperature of ~275°C is observed.

Although all three dykes mentioned were sampled within the Adelaide Subgroup, the lower most sampling site produced the lowest attained maximum heat.

6.3 Core samples: Boreholes with dolerite

6.3.1. Kopoasfontein Core (G39974)

The variation in magnetic susceptibility with temperature was determined for rock chips from selected samples from each sedimentary unit of the Kopoasfontein Core (G39974)

(Hrouda et al., 2003). The maximum run temperature was increased in intervals of 50°C from

100°C up to 700°C using a CS3 furnace attached to an Agico MFK1-FA Kappabridge.

Samples were selected as to provide an overview of the change in temperature with depth as well as at varying distances from the dolerite sills within each of the sedimentary units.

The combined A40 values for selected samples from the different sedimentary units were calculated after individual heating and cooling runs (Figure 6-6).

6–5

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6–6

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The variation in susceptibility curves for the Tierberg Formation (Figure 6-7a) indicates the presence of both pyrrhotite (320°C) and magnetite (600°C). The A40 diagram (Figure 6-6a), however, indicates a maximum acquired field temperature of 400°C suggesting the observed magnetite to be a secondary product from the oxidization of pyrrhotite. This increased A40 temperature can possibly be explained by hydrothermal heat dissipating away from Sill 1 forming carbonate layers up to 20 m above the upper contact of the sill

(Aarnes et al., 2011).

The alteration (A40) curves of all the samples from the White Hill Formation shows rapid increase at

~250°C (Figure 6-6b), the temperature at which pyrrhotite starts to oxidize to magnetite (Evans and Heller,

2003). This suggests that the maximum geothermal temperature was reached and the A40 test was terminated. The presence of pyrrhotite is confirmed in Figure 6-7b where the susceptibility curve drops to a minimum at a temperature of ~330°C.

The low magnetic content of the Prince Albert Formation made interpretation of the alteration index (A40) diagram more difficult. Even though the magnetic signature of some samples within the Prince Albert

Formation are also dominated by pyrrhotite (Figure 6-7c), the calculated A40 temperatures for this formation are mostly elevated, ranging between 450–600°C (Figure 6-6c) depending on the distance from the contact aureole. This coincides with observations by Aarnes et al. (2011) of high metamorphic conditions of the

Prince Albert Formation. The susceptibility variation curves of the Prince Albert Formation give variable signatures (Figure 6-7c-e) with most of the pyrrhotite altered into magnetite due to the elevated temperatures.

The susceptibility variation curves of the Dwyka Group indicate magnetite as the main magnetization mineral, while the A40 diagram suggests a maximum geothermal temperature of ~500–650°C (Figure 6-6d).

The A40 derived maximum geothermal temperatures (Tmax) of individual sedimentary samples are plotted with depth (Figure 6-8) and can be correlated with the modeled maximum temperatures of Aarnes et al. (2011) obtained from metamorphic minerals. It is clear that Tmax generally increases closer to the dolerite sills.

6.3.2. Waterkloof Core (PP47)

Rock chips from selected samples from each sedimentary unit of the Waterkloof Core (PP47) were subjected to stepwise thermal magnetic susceptibility using temperature increments of 50°C from 90°C up to

690°C using a CS3 furnace attached to an Agico MFK1-FA Kappabridge. The lower limit of the CS3 furnace

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CHAPTER 6 for this test is 90°C. Similar to all other boreholes, samples were selected to get an overview of the change in temperature with depth as well as at varying distances from the dolerite sills within each of the sedimentary units.

Figure 6-7: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different sedimentary units. Total loss of susceptibility occurs at the Curie temperatures of the main magnetic carriers. The White Hill Formation is dominated by pyrrhotite (b) and the Dwyka Group by magnetite (f). The Tierberg (a) and Prince Albert Formations (c-e) host both pyrrhotite and magnetite. Red line indicates heating curve and blue line the cooling curve.

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The combined A40 values for selected samples from the different lithologic units were calculated after individual heating and cooling runs (Figure 6-9 and Figure 6-10).

Figure 6-9: Variation of alteration index (A40) for the different shale units indicating maximum acquired temperatures (indicated by ellipses) to range between 390°C and 540°C.

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The low magnetic content of the sediment (shale and siltstone) in borehole PP47

(magnetic intensity typically less than 1 mA/m) made interpretation of the alteration index

(A40) diagrams more difficult. The calculated maximum acquired temperatures (Tmax) of the shale samples vary between 390°C and 540°C. The A40 values for the different siltstone units showed limited variation with increased temperature while the calculated Tmax varied between elevated temperatures of 490°C and 590°C.

Figure 6-11 displays typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different lithologic units in borehole PP47. The magnetic signature of all the samples selected for variable temperature analysis indicate magnetite with curie temperature ~580°C as the dominant magnetic phase (Figure 6-11, a-e).

However, the slightly lower calculated A40 temperatures, especially of the shale, would suggest other magnetic minerals (e.g. titanomagnetite) to be present in small amounts as well. The thermal demagnetization data (Chapter 5, sub-section 3.2) indicated the presence of pyrrhotite in the shale samples even though it is not apparent from the susceptibility data under discussion here. A possible explanation could be the continuous transformation of fine grained pyrite into pyrrhotite, titanomagnetite (given the individual chemical components do occur in the rock) and finally magnetite during heating similar to observations by Li and Zhang

(2005) and Minyuk et al. (2013).

The A40 derived maximum geothermal temperatures (Tmax) of individual sedimentary samples are plotted with depth (Figure 6-12) indicating a mean Tmax above 400°C. A possible explanation for the high temperature could be the large number of dolerite sills intersected in this borehole. The numerous recorded carbonaceous bands indicate high fluid content, which would have acted as heat conveyor for a much wider area around these sills.

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Figure 6-11: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different lithologic units in borehole PP47. Total loss of susceptibility occurs at the Curie temperatures of the main magnetic carriers. Red line indicates heating curve and blue line the cooling curve. Both shale (a) & (b) as well as the siltstone (c) – (e) samples indicate magnetite as the main carrier of magnetism with a Curie temperature of ~580°C.

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Figure 6-12: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole PP47 plotted with depth.

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6.3.3. Hermon Core (HM 1/78)

Rock chips from selected samples from each sedimentary unit of the relatively shallow borehole, HM1/78, were subjected to stepwise thermal magnetic susceptibility using temperature increments of 50°C from 90°C up to 690°C. Similar to all other boreholes, samples were selected to get an overview of the change in temperature with depth as well as at varying distances from the dolerite sills within each of the sedimentary units.

Figure 6-13 to Figure 6-17 display typical responses of magnetic susceptibility to variation in temperature for samples from the different stratigraphic units in borehole HM 1/78.

Figure 6-13: Variation in magnetic susceptibility with increasing temperature for sample HM01 from the Tierberg Formation in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve.

The variable temperature analysis from the Tierberg Formation (Figure 6-13) shows a rapid increase in magnetic susceptibility above 350°C (inset Figure 6-13) indicating the presence of pyrrhotite. Upon further heating the pyrrhotite reacts to form magnetite with significantly higher susceptibility than at the start of the heating cycle. The White Hill

Formation progressively transforms from pyrrhotite to an intermediate mineral with a susceptibility peak between 400°C and 450°C followed by the formation of magnetite (Figure

6-14). 6–15

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Figure 6-14: Variation in magnetic susceptibility with increasing temperature for sample HM49 from the White Hill Formation in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve.

The Prince Albert Formation also displays characteristic peaks for pyrrhotite and magnetite. However, the susceptibility values above the 18.5 m thick dolerite sill is much lower than below it. No pyrrhotite was observed above the sill (Figure 6-15), and the shapes of the heating and cooling curves above and below the sill differs significantly. The broad

“hump” below the sill (Figure 6-16) indicates a complex mixture of magnetic minerals coexisting at a wide range of temperatures. The sample from the Dwyka Group (HM22,

Figure 6-17) shows a dominant pyrrhotite “hump” with a small peak for magnetite or possibly titanomagnetite (lower Curie temperature) after heating above 550°C.

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Figure 6-15: Variation in magnetic susceptibility with increasing temperature for sample HM53 from the Prince Albert Formation in borehole HM 1/78 located above the dolerite sill. Red line indicates heating curve and blue line the cooling curve.

Figure 6-16: Variation in magnetic susceptibility with increasing temperature for sample HM16 from the Prince Albert Formation in borehole HM 1/78 below the dolerite sill. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-17: Variation in magnetic susceptibility with increasing temperature for sample HM22 from the Dwyka Group in borehole HM 1/78. Red line indicates heating curve and blue line the cooling curve.

The combined A40 values for selected samples from the different stratigraphic units were calculated after individual heating and cooling runs and are displayed in Figure 6-18. The calculated maximum acquired temperatures for the Tierberg and White Hill formations as well as the Dwyka Group is 340°C, while for the Prince Albert Formation there is a rapid increase towards the sill contact with values ranging from 390°C up to 540°C. Figure 6-19 displays the

A40 derived maximum geothermal temperatures (Tmax) of individual samples with depth demonstrating the rapid increase of Tmax within the contact aureole. The observed background temperature is an elevated 340°C.

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Figure 6-18: Variation of alteration index (A40) for the different stratigraphic units in borehole HM 1/78 indicating maximum acquired temperatures (indicated by ellipses) to be above 340°C.

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Figure 6-19: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole HM 1/78 plotted with depth.

6.3.4. Driefontein Core (DF 1/75)

Rock chips from selected samples from the Volksrust, Vryheid and Pietermaritzburg formations as well as the Dwyka Group were subjected to stepwise thermal magnetic susceptibility measurement. Similar to all other boreholes, samples were selected to get an overview of the change in temperature with depth as well as at varying distances from the dolerite sills within each of the sedimentary units.

Figure 6-20 displays calculated A40 responses of samples from the different stratigraphic units in borehole DF 1/75 during variation in temperature analysis. The maximum acquired temperatures for the Volksrust Formation is above 350°C, for the Vryheid Formation it ranges between 500–550°C, while for the Pietermaritzburg Formation maximum temperatures between 450–600°C were calculated. The calculated Tmax for the Dwyka Group is above

450°C.

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Figure 6-21 displays the change in magnetic susceptibility of sample DF41 from the

Volksrust Formation to variation in temperature. A rapid increase in susceptibility is observed in the heating curve above 430°C (Figure 6-21 inset). This indicates that Tmax was exceeded

(as suggested in Figure 6-20) and new magnetic minerals are formed (possibly titanomagnetite). The observed Curie temperature at 580°C as well as the lower cooling curve after heating to 590°C and the observed susceptibility returning to its previous value suggests that no additional mineral transformation took place.

Figure 6-21: Variation in magnetic susceptibility with increasing temperature for sample DF41 from the Volksrust Formation in borehole DF 1/75. Rapid increase in susceptibility of the heating curve above 430°C (inset) indicates the formation of possibly titanomagnetite. The observed Curie temperature at 580°C and lower cooling curve after heating to 590°C with susceptibility returning to its previous value suggests no additional mineral transformation. Red line indicates heating curve and blue line the cooling curve.

The Vryheid Formation (Figure 6-22 and Figure 6-23) confirmed the elevated A40 temperatures with near reversible heating and cooling curves up to temperatures of 540°C.

The presence of pyrrhotite (Curie temperature of 350°C) was observed in some of the

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CHAPTER 6 samples but the reversibility of the heating and cooling curves indicated that these were not newly formed minerals.

Figure 6-22: Variation in magnetic susceptibility with increasing temperature for two samples from the Vryheid Formation above sill 1 in borehole DF 1/75. From sample DF64 (a) both pyrrhotite and magnetite was observed while in sample DF70 (b) only magnetite was formed after heating to temperatures above 540°C. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-23: Variation in magnetic susceptibility with increasing temperature for two samples from the Vryheid Formation below sill 1 in borehole DF 1/75. Sample DF103 (a) indicate both pyrrhotite and magnetite while sample DF08 (b) only indicated magnetite after heating to temperatures above 590°C. Red line indicates heating curve and blue line the cooling curve.

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Above heating temperatures of 590°C rapid increases in the susceptibility values of the cooling curves can be observed. This increase in susceptibility together with the small peaks observed at ~580°C suggests the transformation of the pyrrhotite into magnetite at elevated temperatures.

The Pietermaritzburg Formation (Figure 6-24 and Figure 6-25) also indicates primary pyrrhotite that transforms into magnetite at elevated temperatures. Heating and cooling curves are reversible up to temperatures of 490°C above sill 2 (Figure 6-24) and as high as

590°C below sill 2 (Figure 6-25a) which correlates with the calculate A40 temperatures for this formation (Figure 6-20).

The variation in magnetic susceptibility curves for sample DF30 from the Dwyka Group

(Figure 6-26) rapidly increase above 490 °C indicating initial formation of magnetite [1] that is gradually dissolved again [2] into the host rock until the heating and cooling curve become reversible again above 600°C [3]. The maximum acquired geothermal temperature for the

Dwyka Group in borehole DF1/75 is thus less than 490°C which correlates with the calculated

A40 temperature (Figure 6-20).

Figure 6-24: Variation in magnetic susceptibility with increasing temperature for sample DF17 from the Pietermaritzburg Formation above sill 2 in borehole DF 1/75. Pyrrhotite as primary mineral is transformed into magnetite above temperatures 590°C. Red line indicates heating curve and blue line the cooling curve. 6–25

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Figure 6-25: Variation in magnetic susceptibility with increasing temperature for two samples from the Pietermaritzburg Formation below sill 2 in borehole DF 1/75. Sample DF25 (a) indicate oxidation of weakly magnetic phases into magnetite above applied temperatures of 640°C. Primary pyrrhotite in sample DF27 (b) is transformed into magnetite at temperatures above 540°C. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-27 displays the A40 derived maximum geothermal temperatures (Tmax) of individual samples with depth indicating an elevated background temperature above 400°C with rapid increase of Tmax within the contact aureole of sill 2.

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Figure 6-27: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole DF 1/75 plotted with depth. 6–28

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6.4 Core samples: Boreholes without Dolerite

In order to distinguish between elevated geothermal temperatures due to sill and dyke intrusions as opposed to possible heating due to other tectonic events (Cape Fold Belt or crustal doming prior to the intrusions), the analysis were repeated on samples from four borehole cores that did not intersect any dolerites.

6.4.1. Sambokkraal (SA 1/66)

This is a very deep borehole (4175 m) and samples were collected at relatively large intervals (20–30 m). For the A40 test at least 4 to 5 samples from the main stratigraphic units were selected for analysis.

Figure 6-28 displays calculated A40 responses of samples from the different stratigraphic units during variation in temperature analysis. The maximum acquired temperatures for the

Lower Beaufort Group is approximately 390°C, for the Upper Ecca Group it ranges between

440–590°C, while for the White Hill and Prince Albert formations (Lower Ecca) the maximum temperatures range between 340–440°C. The calculated Tmax for the Dwyka Group is above

490°C and for the underlying Bokkeveld and Table Mountain groups the maximum temperature is approximately 390°C. These temperatures are much higher than expected considering that no dolerite sills were intersected.

The variation in magnetic susceptibility curves for two samples (SA10 and SA89) from the Lower Beaufort Group are displayed in Figure 6-29. A rapid increase in magnetic susceptibility is observed above 440°C indicating the creation of new magnetic minerals.

However, above 540°C the susceptibility values gradually decrease again to its original value indicating reabsorption of the newly formed minerals into the host rock. Sample SA829

(Figure 6-30) represents the Upper Ecca Group. A gradual increase in magnetic susceptibility occurs only at temperatures above 540°C with a Curie temperature of 680°C observed indicating that hematite was formed. The variation in magnetic susceptibility for samples

SA950 and SA956 from the Upper Dwyka (White Hill and Prince Albert formations) indicate a rapid increase in magnetic susceptibility at temperatures above 440°C. A variety of peaks can be observed in both the heating and cooling curves indicating the formation of a wide mixture

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CHAPTER 6 of magnetic minerals. The final intensity of magnetic susceptibility for this stratigraphic unit is significantly higher than the rest of the borehole.

Figure 6-28: Variation of alteration index (A40) for the different stratigraphic units in borehole SA 1/66 indicating maximum acquired temperatures (ellipses).

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Figure 6-29: Variation in magnetic susceptibility with increasing temperature for two samples from the Lower Beaufort Group in borehole SA 1/66. Both samples show rapid increase in magnetic susceptibility above 440°C, but indicate reabsorption and lowering of susceptibility to its original value above 540–590°C. Red line indicates heating curve and blue line the cooling curve. 6–31

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Figure 6-30: Variation in magnetic susceptibility with increasing temperature of sample SA829 from the Upper Ecca Group in borehole SA 1/66. Gradual increase in magnetic susceptibility occurs only at temperatures above 540°C with a Curie temperature of 680°C indicating that hematite was formed. Red line indicates heating curve and blue line the cooling curve.

Similar to samples from the Upper Ecca Group, sample SA1000 from the Dwyka Group only started to oxidize at relatively high applied temperatures (>540°C). This indicates a lack of sulphide minerals in these samples with the source of the iron probably contained in clay minerals such as siderite.

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Figure 6-31: Variation in magnetic susceptibility in borehole SA 1/66 for samples SA950 and SA956 from the Upper Dwyka (White Hill and Prince Albert formations). Rapid increase in magnetic susceptibility occurs at temperatures above 440°C with a variety of peaks observed in both the heating and cooling curves indicating the formation of a wide mixture of magnetic minerals. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-32: Variation in magnetic susceptibility with increasing temperature of sample SA1000 from the Dwyka Group in borehole SA 1/66. Gradual increase in magnetic susceptibility occurs only at temperatures above 540°C indicating oxidation magnetite and hematite. Red line indicates heating curve and blue line the cooling curve.

Figure 6-33 displays the A40 derived maximum geothermal temperatures (Tmax) of individual selected samples with depth indicating an elevated background temperature above

400–450°C. This is much higher than expected as no dolerite was intersected. This brings to mind the suggestion that the thermal effect of the Cape fold Belt might have played a far greater role than initially suspected. A classic fold test on samples from the Laingsburg area will be discussed in Chapter 8.

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Figure 6-33: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SA 1/66 plotted with depth. 6–35

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6.4.2. Schietfontein (SC 3/67)

Borehole SC3/67 is very deep (~6000 m) and since only crushed core were available sample were collected at intervals of roughly every 150 feet (45.72 m) for analysis. For the

A40 test 2 to 3 samples from the main stratigraphic units were selected for analysis.

Figure 6-34 displays calculated A40 responses of samples from the different stratigraphic units during variation in temperature analysis.

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The maximum acquired temperatures for the Lower Beaufort Group (Teekloof

Formation) vary considerably (340–590°C) with one sample spiking at 190°C. The Upper

Ecca Group also indicates both a low temperature around 190°C followed by 340–440°C. The

Lower Ecca Group show limited change at a temperature of 390°C. The Upper Dwyka (White

Hill and Prince Albert formations) ranges between 290-490°C, while the Dwyka Group started to oxidize above 390°C. The underlying Bokkeveld Group produced a maximum temperature of approximately 390°C.

The variation in magnetic susceptibility curves for the Lower Beaufort Group (Teekloof

Formation) is displayed in Figure 6-35. Oxidation only started above heating temperature of

540°C and the observed Curie temperatures indicate both magnetite and hematite as the oxidation products. The Upper Ecca samples show a significant variation in oxidation patterns during heating and cooling (Figure 6-36) with the first non-reversible curve observed after heating to only 290°C (sample SC72) with a gradual increase in magnetic susceptibility.

Heating and cooling curves are nearly reversible for this sample (SC72) while sample SC84 gradually decrease in magnetic susceptibility indicating magnetite being dissolved back into the host rock. The Lower Ecca Group, which consists mainly of the White Hill and Prince

Albert formations, shows a rapid increase in magnetic susceptibility at higher temperatures with clear peaks at the Curie temperature for magnetite (Figure 6-37). The Dwyka Group sample (Figure 6-38) indicates the complete transformation of pyrrhotite to magnetite above

540°C. The near reversible heating and cooling curves as well as the gradual decrease in magnetic susceptibility indicate the dissolving of the newly formed magnetite and reabsorption into the host rock. Both pyrrhotite and magnetite peaks can be observed in the

Bokkeveld Group samples with the pyrrhotite completely transformed into magnetite at higher temperatures. As with the Dwyka Group example, the heating and cooling curves of the

Bokkeveld samples are completely reversible.

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Figure 6-35: Variation in magnetic susceptibility with increasing temperature of samples from the Lower Beaufort Group (Teekloof Formation) in borehole SC 3/67. Oxidation only started above 540°C and the Curie temperatures indicate magnetite and hematite as the oxidation products. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-36: Variation in magnetic susceptibility with increasing temperature of samples from the Upper Ecca Group in borehole SC 3/67. First oxidation started above 290°C (SC72) with a gradual increase in magnetic susceptibility. Heating and cooling curves are nearly reversible for this sample. Red line indicates heating curve and blue line the cooling curve. 6–39

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Figure 6-37: Variation in magnetic susceptibility with increasing temperature of samples from the Lower Ecca Group (White Hill and Prince Albert formations) in borehole SC 3/67. Both pyrrhotite and magnetite are observed in sample SC89, while only magnetite occurs in sample SC98. Red line indicates heating curve and blue line the cooling curve.

Figure 6-38: Variation in magnetic susceptibility with increasing temperature from the Dwyka Group in borehole SC 3/67. Both pyrrhotite and magnetite are observed. Red line indicates heating curve and blue line the cooling curve. 6–40

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Figure 6-39: Variation in magnetic susceptibility with increasing temperature of samples from the Bokkeveld Group in borehole SC 3/67. Both pyrrhotite and magnetite are observed with heating and cooling curves near completely reversible. Red line indicates heating curve and blue line the cooling curve.

Figure 6-40 displays the calculated A40 maximum geothermal temperatures of individual selected samples with depth. Similar to borehole SA1/66 an elevated background temperature above 400°C is observed.

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Figure 6-40: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SC3/67 plotted with depth. 6–42

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6.4.3. Goedehoop (CBC4495)

The log of borehole CBC4495 give a detailed description of the change in small scale lithology with depth. This gives the opportunity to determine if there is any relation between rock type and maximum acquired temperatures. Rock chips from the Vryheid Formation as well as presumed Transvaal Supergroup were subjected to stepwise thermal magnetic susceptibility.

Figure 6-41 displays calculated A40 responses during variation in temperature analysis of samples from the different lithologies within the Vryheid Fromation as well as the Transvaal

Supergroup. Figure 6-42 displays typical responses of the variation in magnetic susceptibility with increasing temperature for selected samples from the different lithologic units in borehole

CBC4495.

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Figure 6-42: Typical responses of the variation in magnetic susceptibility with increasing temperature for samples from the different lithologic units in borehole CBC4495. Red line indicates heating curve and blue line the cooling curve. Most samples start to oxidize above 390°C. Heating curves above 490°C suggest the formation of possibly titanomagnetite which transforms into magnetite at higher temperatures. The wide peaks of nearly all the cooling curves after progressive heating to 540°C suggest a mixture of magnetic phases.

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Most samples only started to oxidize after being heated to 390°C (Figure 6-41 and

Figure 6-42) suggesting that the previous heating cycle to 340°C indicate the maximum acquired temperature (Tmax) of the Vryheid Formation in this part of the basin. The samples labeled as sandstone (SD) displayed rapid increase in magnetic susceptibility above 390°C continuously increasing until reaching a peak at ~520°C suggesting the formation of magnetite. During cooling some of the magnetite is transformed into iron sulphides

(pyrrhotite).

The sandstone-mudstone-carbonaceous (TM) sample displays an additional peak at

420°C which disappears at higher temperatures into a magnetite peak. This either suggests that the addition of carbonaceous material to the rock lowered the temperature at which magnetite started to form, or might indicate the intermediate formation of titanomagnetite before being transformed into magnetite.

The sample labeled MS (mudstone siltstone) has a more distinct separation between presumed titanomagnetite and magnetite upon heating to 540°C. The wide peaks of nearly all the cooling curves after progressive heating to 540°C suggest a mixture of magnetic phases.

The coarse grained gritstone (GH58B) indicated no significant oxidation to create magnetic minerals (Figure 6-42) with very low to negative magnetic susceptibility, suggesting a diamagnetic composition.

Samples with low mudstone or siltstone content (<20%) and high sandstone content

(70+ %) only started to oxidize above 490 °C with the associated cooling curves producing wide peaks at ~200°C suggesting the formation of ferrimagnetic hexagonal pyrrhotite (Minyuk et al., 2013).

Figure 6-43 displays the calculated A40 maximum geothermal temperatures of individual selected samples with depth. The mean calculated background temperature for this part of the Karoo Basin is elevated to 340°C and above.

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Figure 6-43: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole CBC4495 plotted with depth.

6.4.4. Groottegeluk (MY19)

Although the Ellisras Basin is situated further north from the main Karoo Basin in a half- graben, the opportunity arouse to compare the geothermal signature from this basin with that of the main Karoo Basin. Only ten samples from borehole MY19 were available for analysis.

Five samples were collected above the ~70 m thick Groottegeluk coal seams and another five in the Vryheid Formation below. Unfortunately a detailed log is not available and stratigraphic grouping are assumed from differences in observed properties.

Figure 6-44 displays calculated A40 responses of samples from the different stratigraphic units during variation in temperature analysis. The observed maximum temperatures range between 390°C above and 360°C below the coal seam.

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The variation in magnetic susceptibility with increasing temperature for sample MY19_3 from the Beaufort Group above the Groottegeluk Formation is displayed in Figure 6-45. The heating and cooling curves are reversible up to 390°C, thereafter a rapid increase in the magnetic susceptibility indicates the possible formation of magnetite or titanomagnetite during heating while the wide ‘hump’ at ~350°C during the cooling process indicate the formation of pyrrhotite. Figure 6-46 show a similar pattern for sample MY19_8 from the Vryheid Formation.

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However, in the susceptibility values from the Vryheid Formation is significantly lower than the

Beaufort Group and no pyrrhotite was observed.

Figure 6-45: Variation in magnetic susceptibility with increasing temperature for sample MY19_3 from the Beaufort Group in borehole MY19. Curves are reversible up to 390°C whereafter a rapid increase in the heating and cooling curves indicate the formation of magnetite or titanomagnetite. The wide ‘hump’ at ~350°C suggests the formation of pyrrhotite during cooling. Red line indicates heating curve and blue line the cooling curve.

Figure 6-46: Variation in magnetic susceptibility with increasing temperature for sample MY19_8 from the Vryheid Formation in borehole MY19. Curves are reversible up to 390°C whereafter an increase in the cooling curves indicate the formation of new minerals. Red line indicates heating curve and blue line the cooling curve.

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Figure 6-47 displays the calculated A40 maximum geothermal temperatures of the samples with depth. Similar to the other boreholes without dolerite an elevated background temperature of approximately 400°C was observed.

Figure 6-47: Variation of temperature (Tmax) calculated from magnetic alteration (A40) values for borehole SC3/67 plotted with depth.

6.5 Discussion

The peak palaeotemperatures away from the intrusive sills were examined by quantitatively studying the variation of magnetic susceptibility with increasing temperature in an normal atmosphere as described by Hrouda et al. (2002, 2003).

Figure 6-48 displays the measured A40 peak temperatures with depth for the four boreholes that intersected dolerite sills. According the Aarnes et al. (2010) most aureole thicknesses vary between 30-200% of the sill thickness with thinner intrusions having smaller aureoles. Assuming a constant contact aureole of 50% of the sill thickness, the extent of the contact aureoles around each dolerite sill are displayed as red bars next to each core in

Figure 6-48. 6–49

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Figure 6-48: Variation in maximum geothermal temperatures reached by Karoo sediments with depth indicating the extent of the thermal influence within the contact aureoles. Expected extent of contact aureoles (50% of sill thickness) are indicated by red bars.

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Results from core G39974 indicated maximum acquired temperatures (Tmax) to vary between 200°C and 650°C with the highest temperatures limited to short distances within the contact aureole. These results correlated very well with modeled results published by Aarnes et al. (2011) obtained from vitrinite reflectance as well as a study of the metamorphic mineralogy. Core HM1/78 follow a similar pattern with steep increase in temperature limited to short distances within the contact aureole. The lower observed temperature, however, in this case is above 350°C. Core PP47 indicates elevated temperatures above 400°C, while observed temperatures in core DF1/75 are above 450°C. The multiple sills intersected by core PP47 have overlapping contact aureoles below a depth of 900 m that could explain the increased palaeotemperature greater than 550°C below this depth. The overlapping aureoles in G39974, however, did not have the same super heating effect. This suggests that other factors also play a role in the dissipation of heat from sill intrusions.

Aarnes et al. (2010) highlighted that the most important factors influencing the extent of a contact aureole is the depth of sill emplacement as well as the geothermal gradient in the basin, while other factors to be considered include pore-water volatilization of the host rock

(Wang, 2012; Wang and Song, 2012).

Considering results from outcrop samples in the southeastern part of the basin (see section 2 above), it is clear that burial alone cannot explain the observed increase in temperature of up to 100°C. The difference in sampling elevation between the three dykes from this area is approximately 300 m that would produce an increase in burial heat of only ~

10°. It is therefore suggested that the width of dykes together with fluid content of the host rock, play a far greater geothermal role than stratigraphic height alone (Brown et al., 1994 and

Svensen et al., 2006).

Figure 6-49 displays the measured A40 peak temperatures with depth for the four boreholes that do not intersected any dolerite sills. Surprisingly the calculated maximum acquired temperatures for all four boreholes, with the exception of two individual samples, are above 300°C. The question now is what could be the cause of this elevated background temperature. It cannot be purely due to the impact of the Cape Fold Belt since boreholes

CBC4495 and MY19 are located far outside the mapped margin of the Cape Fold Belt.

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Figure 6-49: Variation in maximum geothermal temperatures with depth reached by Karoo sediments in boreholes not intersecting any dolerites.

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The Groottegeluk core (MY19) may show elevated temperatures due to higher crustal heat flow in the rift setting or else due to heating effect of the suggested thick basaltic lava that Reid et al., (1997) reported capped the sequence. However, these high values are in contrast to the suggested maximum temperature of 100°C from Faure et al. (1996) as derived from the presence of the smectite montmorillonite. The temperatures of the Goedehoop core

(CBC4495) seems to be inverted with the lowest temperatures observed near the bottom of the core. This could possibly be explained by influences from hydrothermal fluids as observed in other basins by Hower and Gayer (2002). Another possibility is an increased heat flow due to crustal uplift (bulging) before the emplacement of the magma intrusions. However, since the lowest observed maximum temperature in the main Karoo Basin (current study) is 200°C from borehole G39974, there is still an additional 100°C currently not accounted for.

6.6 References

Aarnes, I., Svensen, H., Connolly, J.A.D. and Podladchikov, Y.Y., 2010. How contact

metamorphism can trigger global climate changes: Modeling gas generation around

igneous sills in sedimentary basins. Geochimica Cosmochimica Acta, 74 (24), 7179–

7195.

Aarnes, I., Svensen, H., Polteau, S. and Planke, S., 2011. Contact metamorphic

devolatilization of shales in the Karoo Basin, South Africa, and the effects of multiple sill

intrusions. Chemical Geology, 281, 181–194.

Ballard, M. M., Van der Voo, R. and Halbich, I. W., 1986. Remagnetizations in Late Permian

and Early Triassic rocks from southern Africa and their implications for Pangea

reconstructions. Earth and Planetary Science Letters 79, 412–418.

Brown, R., Gallagher, K. and Duane, M., 1994. A quantitative assessment of the effects of

magmatism on the thermal history of the Karoo sedimentary sequence. Journal of

African Earth Sciences, 18(3), 227–243.

Evans, M.E. and Heller, F., 2003. Environmental Magnetism. Academic Press, 312 pp.

Hrouda, F., Chlupacova, M. and Novak, J., 2002. Variation in magnetic anisotropy and

opaque minerals along a kilometre deep profile within a vertical dyke of the

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CHAPTER 6

syenogranite porphyry at Cinovec (Czech Republic). Journal of Volcanology and

Geothermal Research, 2359, 1–12.

Hrouda, F., Müller, P. and Hanák, J., 2003. Repeated progressive heating in susceptibility vs.

temperature investigation: a new palaeotemperature indicator? Physics and Chemistry

of the Earth, 28, 653–657.

Li, H-Y. and Zhang, S-H., 2005. Detection of mineralogical changes in pyrite using

measurements of temperature-dependence susceptibilities. Chinese Journal of

Geophysics, 48(6), 1454–1461.

Marsh, J. S., Allen, P. and Fenner, N., 2003. The geochemical structure of the Insizwa lobe of

the Mount Ayliff complex with implications for the emplacement and evolution of the

complex and its Ni-sulphide potential. South African Journal of Geology, 106, 409–428.

Minyuk, P.S., Tyukova, E.E., Subbotnikova, T.V., Kazansky, A. Yu., and Fedotov, A.P., 2013.

Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia.

Russian Geology and Geophysics, 54, 464–474.

Svensen, H., Jamtveit, B., Planke, S. and Chevallier, L., 2006. Structure and evolution of

hydrothermal vent complexes in the Karoo Basin, South Africa. Journal of the

Geological Society, London, 163, 671–682.

Wang,D., 2012. Comparable study on the effect of errors and uncertainties of heat transfer

models on quantitative evaluation of thermal alteration in contact metamorphic

aureoles: thermophysical parameters, intrusion mechanism, porewater volatilization

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Wang, D. and Song, Y., 2012. Influence of different boiling points of pore water around an

igneous sill on the thermal evolution of the contact aureole. International Journal of

Coal Geology, 104, 1–8.

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CHAPTER 7

CHAPTER 7. EXPERIMENT 3 – PYRRHOTITE / MAGNETITE

GEOTHERMOMETER

7.1 Introduction

Several paleo-temperature studies in sedimentary basins have made use of the variation in magnetic mineral assemblages as geothermometers (Dunlop et al., 2000; Schill et al.,

2002; Gillet, 2003; Wehland et al., 2005; Aubourgh and Pozzi, 2009, 2010; Abdelmalak et al.,

2012). This chapter will give a short summary of the different approaches followed as well as its application to the sediments of the Karoo Basin.

7.2 Development of Pyrrhotite/Magnetite Geothermometers

7.2.1. Variation of susceptibility and remanence in contact aureoles

Wehland et al. (2005) studied the variation in rock magnetic and palaeomagnetic signals

associated with pyrrhotite bearing limestones for different contact metamorphic settings.

Samples were demagnetized both thermally and by alternating field (AF) demagnetization.

The AF-treated samples were then subjected to isothermal remanent magnetization (IRM)

analysis while the susceptibility was monitored during thermal treatment. Both

thermomagnetic analyses as well as Thellier experiments were conducted on the samples.

The upper stability limit of the pyrrhotite window was determined by thermomagnetic

measurements under highly oxidizing conditions with a KLY3 Kappabridge with a CS

temperature unit. Wehland et al. (2005) found that alteration of pyrrhotite towards magnetite

does not start below 440°C, and considering the lower oxygen fugacity f(O2) during metamorphism in the presence of carbon, the authors suggested that the upper boundary of the pyrrhotite window to be ~450°C. The lower limit is set by the assumption that pyrrhotite forms during desulphidation reactions or by the breakdown of primary pyrite and magnetite under reducing conditions above 200°C.

Wehland et al. (2005) observed that magnetic susceptibility values gradually increase

with increasing distance from intrusions. This was ascribed by the authors to the fact that

magnetite has a higher susceptibility than pyrrhotite, thereby suggesting the formation of

7–1 CHAPTER 7

pyrrhotite at the expense of magnetite in the vicinity of the contact. However if primary

magnetite was not originally present the pyrrhotite/magnetite ratio cannot be determined. The

formation of pyrrhotite in contact-metamorphic limestones is thus reported to dependent not

only on the metamorphic temperature, but also on the composition and amount of

metamorphic fluids.

7.2.2. MagEval Low-Temperature Geothermometer

Aubourg and Pozzi (2010) investigated the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation: immature

(burial temperature ~40°C) of Bure Callovo–Oxfordian claystones in the Basin of Paris

(France); early mature (burial temperature ~85°C) of Opalinus Lower Dogger claystones from the Mont Terri anticline in front of the Jura fold belt (Switzerland); and mature to overmature

(burial temperature <170°C) of Chartreuse Callovian–Oxfordian claystones from Chartreuse

Sub-Alpine chains. A low temperature (10K–300K) investigation of an isothermal remanent magnetization was performed to determine the nature of the magnetic assemblage. Initially the authors imparted a chemical remanent magnetization (CRM) at 95°C on Bure and

Opalinus claystones for several weeks. Thermal demagnetization of the CRM revealed that while magnetite was formed by heating the Opalinus claystones, an assemblage of magnetite and iron sulphide was formed in the Bure claystones. Auborg and Pozzi (2010) further documented the appearance of a magnetic transition at ~35K in Bure claystones after heating, which were related to the formation of fine-grained pyrrhotite (Fe7S8). This transition was named the P-transition by Auborg and Pozzi (2010), but here it is referred to as the

Besnus transition as suggested by Rochette et al. (2011) following the first description of this low temperature transition by M.J. Besnus in her Ph.D. thesis (1966). The Besnus transition was also detected in the early mature to mature Opalinus and Chartreuse claystones.

Aubourg and Pozzi (2010) conducted experimental heatings of natural Opalinus claystones:

(1) short-term heating (1 hour) from 100°C to 200°C to investigate of the effect of short-lived heating processes in geology, and (2) Burial-like experiments were performed by heating

Opalinus claystones from 150°C to 250°C for several weeks under a pressure of 100 MPa. In both experiments, Aubourg and Pozzi (2010) observed a correlative diminution of the pyrrhotite signature at 35K with increasing temperature. They interpreted this trend as the

7–2 CHAPTER 7

appearance of magnetite. A parameter PM was derived from the warming curve of a

saturated isothermal remanent magnetization acquired at 10K after zero field cooling (ZFC). A

consistent evolution of PM with temperature in the range of 40°C to 250°C was reported,

including natural samples, heated samples at 95°C, and burial-like heated samples. From

these observations Aubourg and Pozzi (2010) suggested that the magnetic properties of

pyrrhotite–magnetite claystones can be used to infer palaeotemperatures and named this

geothermometer MagEval.

Abdelmalak et al. (2012) investigated the magnetic assemblage of Cretaceous claystones and also suggested that the magnetic minerals could be used as indication of burial temperature. Vitrinite reflectance (R0) and Rock-Eval pyrolysis (Tmax) was used to classify the maturity of the organic matter in the claystones. The magnetic assemblage of the same samples was then determined by low-temperature and high temperature properties of isothermal remanent magnetization (IRM). Abdelmalak et al. (2012) found that the magnetic minerals goethite, greigite and some magnetite dominated in immature claystones, while for immature to early mature claystones the magnetic assemblage resides in stoichiometric magnetite and fine-grained pyrrhotite.

Continued research by Aubourg et al. (2013), using the MagEval method on the thermal aureole of a metric size dyke in immature to early mature claystones, found that in settings where larger than micron size pyrrhotite occur, the MagEval geothermometer is not appropriate. The reason for this is that contrary to nano-sized pyrrhotite, the micron-sized pyrrhotite is carrying a remanence at room temperature and has the capability to retain a natural remanent magnetization. Rochette (1987) suggested that the formation of micron- sized pyrrhotite in sediments is related to the temperature elevation in anoxic conditions. In burial conditions, Crouzet et al. (2001) and Schill et al. (2002) suggested that the occurrence of pyrrhotite may be an indication of burial temperature above 200°C. In non-standard conditions of elevated temperature, such as within the thermal aureole of a magmatic intrusion, several studies reported the formation of pyrrhotite within the thermal aureole

(Gillet, 2003; Wehland et al., 2005; Ledevin et al., 2012 and Aubourg et al., 2013).

Low-temperature magnetic investigations by Aubourg et al. (2013) on a selection of samples extending from 5 cm to 300 cm of a 1 m thick dyke displayed a variable drop in

7–3 CHAPTER 7

remanence at temperatures below 40K. Aubourg et al. (2013) noticed that the drop in

remanence is a function of the distance to the dyke, being largest for the sample closest to

the dyke. The authors suggested that this indicates that the concentration of the fine fraction

of pyrrhotite is larger near the dyke. Aubourg et al. (2013) indicated that the absence of a

Verwey transition (120K) for samples near the dykes suggested that magnetite is not

detectable, and that pyrrhotite was formed at the expense of magnetite as proposed by Gillet

(2003). Gillet (2003) studied the formation of pyrrhotite from magnetite and pyrite in limestone

under submetamorphic conditions. The author suggested that the formation of pyrrhotite

within the contact aureole of impermeable metalimestone is not triggered by external intrusion

fluids, but, assuming thermodynamic equilibrium implies, more locally by hydrous phases,

perhaps merely a film on individual grains, allowing mass transfer over millimetric distances.

This reaction is favored in reducing conditions. Aubourg et al. (2013) however, also

suggested that dehydration of clays can act as the trigger for the formation of micron-sized

pyrrhotite.

7.2.3. Pyrrhotite/Magnetite Remanence Intensity Ratio

Schill et al. (2002) developed a method that can be used as geothermometer for

T≤300°C in low-grade metamorphic carbonates. This method makes use of the

pyrrhotite/magnetite ratio determined from temperature-related formation of pyrrhotite at the

expense of primary magnetite.

These authors studied the systematic variations in the ferrimagnetic content of meta-

carbonates along a profile of increased metamorphism. These variations were detected by

the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent

magnetization (RPYR/MAG) and saturation magnetization (SPYR/MAG). The temperature gradient and ranges suggested by this method was successfully correlated with results from the non- magnetic, and therefore independent calcite twin lamellae geothermometry.

The intensities of the characteristic components (ChRM) of pyrrhotite and magnetite for each samples was obtained with principal component analysis (PCA) between 250–360°C

(IPYR for pyrrhotite) and between 430–580°C (IMAG for magnetite) from thermal

demagnetization of both NRM and SIRM. The ratios of remanence intensities RPYR/MAG for

NRM and SPYR/MAG for SIRM is calculated by:

7–4 CHAPTER 7

I PYR PYR + II MAG

This method, however, requires that initial conditions of all the samples be the same (i.e. limited variation in content of constituent minerals), as to ensure that variation in the pyrrhotite/magnetite ratio is due to temperature variations only.

7.2.4. Thermal Remagnetization Geothermometer

The remagnetization geothermometer first published by Pullaiah et al. (1975) is based on the Néel theory of magnetic single-domain grains.

1 n = C −µ κ s 000 β 2/)( kTTMVH τ

9 10 -1 -7 Where C=10 – 10 s , µ0 = 4π x 10 H/m, V and Hκ are grain volume and coercive force, β(T) is the normalized T dependence of spontaneous magnetization Ms, n depends on the mechanism of anisotropy, and k is Boltzmann’s constant. The changes in relaxation time τ for thermally activated magnetization in weak fields can thus be calculated for different blocking temperatures TB. Pullaiah et al. (1975) created theoretic blocking curves (T-t contour

plots) for magnetite and hematite. The curves can be used to predict how remanent

magnetization in rocks is gradually lost during the heating accompanying burial and

metamorphism, and can be replaced by new secondary magnetization during uplift and

cooling.

Dunlop et al. (2000) highlighted two associated problems of the magnetite T-t contours:

(1) The T-t contours for magnetite are gently sloping below the Curie temperature,

subsequently large corrections has to be made between the overprint demagnetization

temperature (TL) measured in the laboratory and the remagnetization temperature (Tr) measured in nature.

(2) The typical grain size of magnetite is multidomain with anomalously high TL values compared to single-domain TL values assumed in the remagnetization theory. This in turn makes palaeotemperatures Tr estimates anomalously high without an easy method to correct the values.

Dunlop et al. (2000) studied the time-temperature relations for single-domain pyrrhotite that occurs much more commonly in nature. The typical range of remagnetization

7–5 CHAPTER 7

temperatures is close to the pyrrhotite Curie temperature leading to steep T-t contours and

thus smaller TL to Tr corrections. The main difficulty in using the remagnetization method is to find suitably overprinted formations, where the thermoviscous overprint is carried by single- domain pyrrhotite. Dunlop et al. (2000) removed any multidomain influences from Tr by additionally pre-treating samples to low temperature demagnetization (LTD).

Single-domain pyrrhotite yields an independent estimate of palaeotemperatures if geologically reasonable estimates of the heating time can be made. Furthermore, Dunlop et al. (2000) suggested that if data from both magnetite and pyrrhotite are combined (Figure 7-1) palaeotemperatures can be estimated without geological input.

7.3 Results from the Karoo Basin

7.3.1. Variation of susceptibility and remanence in contact aureoles

A selection of samples collected within the contact aureoles above sill intrusions in 4 boreholes (G39974 (LKF), PP47 (PH), HM1/78 (HM) and DF1/75 (DF)) as well as two

7–6 CHAPTER 7

reference boreholes with no sills (SA1/66 (SA) and CBC4495 (GH)), were sent to the

Southern Illinois University (SIU), Carbondale for isothermal remanent magnetization (IRM)

as well as hysteresis analyses.

The changes in intensity of magnetization, Ms, and coercitvity, Hcr, of the Karoo samples

as observed from these analyses varies relative to distance from the contact aureoles. The

stratigraphic information of the selected samples discussed here can be found in Chapter 4

(Figures 4-10 to 4-12). The magnetic saturation (Ms) generally increases further away from

the intrusions (Figure 7-2 to Figure 7-7), similar to observations made by Wehland et al.

(2005). This suggests, similar to findings by Wehland et al. (2005), that pyrrhotite formed during desulphidation reactions or by the breakdown of primary pyrite and magnetite under reducing conditions above 200°C.

Figure 7-2: Isothermal remanent magnetization (IRM) of selected samples from borehole G39974 (LKF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction. Ms and Hcr decreases towards the contact aureole (inset).

7–7 CHAPTER 7

Figure 7-3: Isothermal remanent magnetization (IRM) of selected samples from borehole PP47 (PH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 7-4: Isothermal remanent magnetization (IRM) of selected samples from borehole DF1/75 (DF) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 7-5 appears not to show this pattern, however, considering that the samples from this borehole represent three different formations (Tierberg, White Hill and Prince Albert formations), it is clear that within each of the individual formations the Ms decreases towards the intrusion, and that in this case differences in composition might play a role. Figure 7-6 and

Figure 7-7 represent samples collected from two reference boreholes without any intrusive

7–8 CHAPTER 7

rocks. The Ms of these samples are very low with the small observed variations not being

related to depth.

Figure 7-5: Isothermal remanent magnetization (IRM) of selected samples from borehole HM1/78 (HM) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

Figure 7-6: Isothermal remanent magnetization (IRM) of selected samples from borehole SA1/66 (SA) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

7–9 CHAPTER 7

Figure 7-7: Isothermal remanent magnetization (IRM) of selected samples from borehole CBC4495 (GH) acquired progressively in fields up to 0.5 T and backfield IRM acquired in opposite direction.

The coercive parameters derived from hysteresis curves for bulk samples of the same samples are displayed in Figure 7-8 as a function of depth (see also discussion in Chapter 4, paragraph 2.3). Nearly all the profiles show a significant decrease in coercivity for the samples closest to the sill contacts (all profiles were collected above dolerite sills).

This correlates well with the expected increase in grain size normally associated with metamorphic recrystallization close to a heat source. This increase in grain size is confirmed by Day plots (Day et al., 1977) of selected samples from the different boreholes indicating an increase in multi-domain (MD) characteristics towards the intrusive contact (Figure 7-9).

7–10 CHAPTER 7

Figure 7-8: Coercivity of remanence with depth of selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), PP47 (PH), HM1/78 (HM) and DF1/75 (DF) as well as samples from boreholes without sills, SA1/66 (SA) and CBC4495 (GH).

7–11 CHAPTER 7

Figure 7-9: Day plots of hysteresis parameters for selected samples from the contact aureoles above sills in each of boreholes G39974 (LKF), PP47 (PH), HM1/78 (HM) and DF1/75 (DF) as well as samples from boreholes without sills, SA1/66 (SA) and CBC4495 (GH). With the exception of borehole LKF, the grain sizes of all samples fall within the multi-domain (MD) range, often increasing toward the sill contacts.

7.3.2. MagEval Low-Temperature Geothermometer

Seven samples collected within the White Hill Formation at distances 0.15 m, 1.10 m,

1.60 m, 2.65 m, 7.00 m, 37.27 m and 63.85 m, above the contact of an 83 m thick sill in borehole G39974, were sent to the Institute for Rock Magnetism in Minneapolis, USA for low- temperature IRM analyses. These are the only samples from all the cores being studied that hysteresis analyses indicated not to be dominated by paramagnetic minerals (see Chapter 4, paragraph 2.2).

All the samples showed a drop in remanence below 40K indicating the presence of pyrrhotite (Figure 7-10). The Verwey transition, indicating magnetite at 120K, was not

7–12 CHAPTER 7

detectable in any of the samples suggesting complete transformation of magnetite to

pyrrhotite. Similar to findings by Aubourg et al. (2013), I observed a relationship between the

drop in remanence in my samples and the distance to the dyke, being largest for the sample

closest to the dyke (Figure 7-10). According to Aubourg et al. (2013) this indicates that the

concentration of fine fraction of pyrrhotite is larger near the dyke.

Figure 7-10: Low-temperature magnetic properties of White Hill Formation shale along a 64 m profile above an 83 m thick sill. Distances of samples from the dyke contact are indicated.

7.3.3. Pyrrhotite/Magnetite Remanence Intensity Ratio

The ratio method of Schill et al. (2002) was tested on samples from the Prince Albert

Formation and associated Vryheid Formation, assuming limited variation in content of

constituent minerals as is required for this method to be successful. Ratios of measured

remanence intensities RPYR/MAG (for NRM) from each of the four boreholes with dolerites were used to determine the variation of pyrrhotite content adjacent to sill intrusions. However, from

Figure 7-11 no clear pattern can be distinguished for any of the boreholes.

According to Schill et al. (2002) the RPYR/MAG ratio (for NRM) and SPYR/MAG for SIRM are mainly controlled by small single- (SD) and pseudo-single domain (SPD) particles of magnetite. Day plots from selected samples in six boreholes, however, have indicated that

7–13 CHAPTER 7

the magnetization is mainly carried by multi-domain (MD) minerals (Figure 7-9), thereby

making this method unsuitable for geothermometer studies in the Karoo Basin.

Figure 7-11: Ratio of remanence intensity RPYR/MAG along depth for the Prince Albert (Vryheid) Formation.

7.3.4. Thermal Remagnetization Geothermometer

Thermal overprinting of selected samples from the White Hill Formation was determined from laboratory thermal demagnetization. The dominant carrier of the thermal overprint is pyrrhotite (Figure 7-12) with unblocking temperatures between 250°C and 400°C. With no clear magnetite component in the vector plots, an attempt was made to identify different pyrrhotite overprints using the vector plots in conjunction with the normalized intensity plots.

This along with an estimated geological heating time is required for this method to be successful.

7–14 CHAPTER 7

Figure 7-12: Examples of variation in vector plots of thermal demagnetization data for samples within the contact aureole of the White Hill Formation (Borehole G39974). Differently coloured vectors in the Zijderveld diagrams indicate different pyrrhotite phases.

7–15 CHAPTER 7

However, distinguishing between the NRM carrier and subsequent overprints have proven to be very difficult. The different components do not have sharp changes in magnetization vectors as can be seen from the curved Zijderveld diagrams (Figure 7-12).

These overlapping signatures can be explained due to the gradual change in unblocking temperature spectra (or coercivity spectra) and thus in chemical composition of the different pyrrhotite grains carrying the components of magnetization. Only sample LKF38C indicates two clearly distinguishable components between 250°C and 400°C with a change in inclination of 25°. It has thus been even more difficult to observe the subtle differences in temperature expected from samples at varying distances from the contact aureole.

For demonstrative purposes, if a pyrrhotite overprint with thermal values between 250–

320°C is assumed and by using the suggested duration of ~0.8 Ma for the intrusion of the

Karoo LIP (Svensen et al., 2012), then our estimated palaeotemperature is approximately

260°C (Figure 7-13). This value correlates well with the palaeotemperatures determined using the alteration index (A40) method (see Chapter 6, paragraph 3.1).

Figure 7-13: Determining the remanence conditions under which the White Hill Formation pyrrhotite overprint was acquired due to the Karoo LIP by extrapolating from laboratory data using the contours of Dunlop et al. (2000).

7–16 CHAPTER 7

7.4 Discussion

All the methods discussed in this Chapter indicated that there is a compositional variation in magnetite/pyrrohite of sedimentary rocks to be expected within the contact aureole of intrusives. The limitations of these methods, however, include the necessity of a significant, although variable amount of magnetite and pyrrhotite in the rocks, as well as the additional requirement that the magnetic minerals be single-domain in grain size.

Results from IRM and hysteresis data were able to identify chemical changes within the contact aureoles of most of the boreholes. However, due to the lack of significant amounts of magnetic minerals, and their multi-domain nature, in these boreholes the derivation of conclusive geothermal temperatures for the Karoo Basin proofed unsuccessful.

7.5 References

Abdelmalak, M.M., Aubourg, C., Geoffroy, L. and Laggoun-Défarge, F., 2012. A new oil-

window indicator? The magnetic assemblage of claystones from the Baffin Bay

volcanic margin (Greenland). AAPG bulletin, 96(2), 205–215.

Aubourg, C. and Pozzi, J-P., 2010. Toward a new <250 °C pyrrhotite–magnetite

geothermometer for claystones. Earth and Planetary Science Letters, 294, 47–57.

Aubourg, C., Techer, I., Geoffroy, L., Clouer, N. and Baudin, F., 2013. Detecting thermal

aureole of a magmatic intrusion in immature to mature sediments: a case study in the

East Greenland Basin (73°N). Geophysical Journal International, 196(1), 160–174.

Besnus M.J., 1966. Propriétés magnétiques de la pyrrhotine naturelle. PhD Thesis University

of Strasbourg.

Crouzet, C., Rochette, P. and Ménard, G., 2001. Experimental evaluation of thermal recording

of polarity reversals during metasediments uplift. Geophysical Journal International,

145, 771–785.

Day, R., Fuller, M. and Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: Grain

size and composition dependence. Physics of the Earth and Planetory Interiors, 13,

260–267.

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Dunlop, D.J., Özdemir, Ö. Clark, D.A. and Schmidt, P.W., 2000. Time-temperature relations

for the remagnetization of pyrrhotite (Fe7S8) and their use in estimating

palaeotemperatures. Earth and Planetary Science Letters, 176, 107–116.

Gillet, S.L., 2003. Paleomagnetism of the Notch Peak contact metamorphic aureole, revisited:

Pyrrhotite from magnetite+pyrite under submetamorphic conditions. Journal of

Geophysical Research, 108, B9, 2446, doi:10.1029/2002JB002386.

Ledevin, M., Arndt, N., Cooper, M.R., Earls, G., Lyle, P., Aubourg, C. and Lewin, E., 2012.

Intrusion history of the Portrush Sill, County Antrim, Northern Ireland: Evidence for

rapid emplacement and high-temperature contact metamorphism. Geological

Magazine, 149(1), 67–79.

Pullaiah, G., Irving, E., Buchan, K.L. and Dunlop, D.J., 1975. Magnetization changes caused

by burial and uplift. Earth and Planetary Science Letters, 28, 133–143.

Rochette, P., 1987. Metamorphic control of the magnetic mineralogy of black shales in the

Swiss Alps: toward the use of “magnetic isograds”. Earth and Planetary Science

Letters, 84, 446–456.

Rochette, P. Fillion, G. and Dekkers, M.J., 2011. Interpretation of Low-Temperature Data

Part 4: The Low-Temperature Magnetic Transition of Monoclinic Pyrrhotite. The IRM

Quarterly, 21(1), 1–10.

Schill, E., Appel, E. and Gautam, P., 2002. Towards pyrrhotite/magnetite geothermometry in

low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola, Central

Nepal). Journal of Asian Earth Sciences, 20, 195–201.

Svensen, H., Corfu, F., Polteau, S., Hammer, Ø. And Planke, S., 2012. Rapid magma

emplacement in the Karoo Large Igneous Province. Earth and Planetary Science

Letters, 325–326, 1–9.

Wehland, F., Alt-Epping, U., Braun, S. and Appel, E., 2005. Quality of pTRM acquisition in

pyrrhotite bearing contact-metamorphic limestones: possibility of a continuous record of

Earth magnetic field variations. Physics of the Earth and Planetary Interiors, 148, 157–

173.

7–18 CHAPTER 8

CHAPTER 8. GEOTHERMAL MODEL FOR THE KAROO BASIN AND

IMPLICATIONS FOR INDUSTRY AND THE

ENVIRONMENT

8.1 Introduction

This project contributed key information regarding the local impact of igneous intrusions on the Karoo sedimentary strata. This again highlighted the probability of release of greenhouse gases from baked organic-rich sediments. Important implications of this type of process for global climate change have already been suggested by other authors. New constraints on regional-scale geotherms were identified using magnetic thermal modeling with associated economic implications for hydrocarbon and uranium exploration.

8.2 Geothermal variation across the Karoo Basin

During the current study several rock magnetic and palaeomagnetic experiments were conducted on samples from eight boreholes spread across the Karoo Basin as well as on surface samples collected adjacent to dolerite dykes in the vicinity of the town of Cradock

(Maré, 2010) in the southeastern part of the basin. Four of these boreholes intercepted one or more dolerite sills and the thermal impact of these on the sedimentary host rock was studied.

Low field anisotropy of magnetic susceptibility (AMS), magneto-stratigraphy using the classic baked contact test (Everitt and Clegg, 1962), as well as the Alteration Index (A40) method

(Hrouda et al., 2002; 2003) indicated that the heating effect occurred no wider than half of the sill thicknesses.

Temperatures calculated by the A40 method (minimum observed values) indicated a general increase from southwest to northeast in the thermal effect of the intrusions on the

Karoo sediments (Figure 8-1). This correlates with the reported increased maturity of Karoo

Basin coal deposits from west to east (Sullivan, 1995). However, according to Rowsell and De

Swardt (1976) a suite of diagenetic indicators (which included IC and vitrinite reflectance, amongst others) show a general increase in diagenesis from north to south across the Karoo

Basin as a whole, with palaeotemperature estimates ranging from 150–170°C in the north to

270–300°C in the south. The degree of diagenesis of Karoo rocks is related mainly to the

8–1 CHAPTER 8 depth of burial, and also to the effect of dolerite intrusions in the central part of the Karoo

Basin (Rowsell and De Swardt, 1976).

Figure 8-2 represents a comparative west to east profile plot of boreholes QU1/65,

AB1/65, KA1/66, VREDE, CR1/68, WE1/66 and SW1/67 (Figure 8-1) indicating the abundance of dolerite sills intercepted with depth. The down-hole temperatures recorded by

Soekor at selected depths of three of these boreholes are indicated. These temperatures indicate the geothermal gradient to range between 25°C/km and 30°C/km with no significant variations associated with the numerous magma intrusions (Figure 8-2). It is suggested that the basin temperature has stabilized and that the intrusions cooled to the present equilibrium.

8–2 CHAPTER 8

Figure 8-2: Comparative subsea depths of stratigraphic units from selected boreholes (see text) across the main Karoo Basin as derived from borehole logs, with the upper occurrences of the Ecca Group (Ecca), Ecca Shales (ES), Dwyka tillite (DT) and Table Mountain Group (TM) indicated as interconnecting dashed lines. Relative depths of logged dolerite sills within each of these boreholes are indicated. Water temperatures measured at selected depths within boreholes KA, CR and WE are indicated (modified after Scheiber-Enslin et al., 2014)

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The state of thermal equilibrium was reached within 2000 years from intrusion, as demonstrated by thermal modelling of borehole G39974 using the Raven Applications software Thermos, Version 2.0 designed and developed by Feodor Walraven (Cawthorn and

Walraven, 1998). The intrusion temperature of the dolerite was chosen at 1150°C (Toplis and

Carroll, 1995) and the thermal gradient as 25°C/km (Brown et al., 1994). Reiter and Tovar

(1982) published the thermal conductivity of dolerite as 2.1 W/mK, while Jones (2003)

reported values for the Karoo sandstone at 3.14±0.38 W/mK and 1.88±0.25 W/mK for the

shale. It is clear from Figure 8-3 that the combined heat of all three intercepted sills in

borehole G39974 has decreased to below the Curie temperature of magnetite within 2000

years. The combined heating effect of the two sills bordering the Prince Albert Formation,

however, increased the temperature of this formation over a period of 100 year to a maximum

of 700°C and kept it at this temperature for at least another 900 years, thus explaining the

near complete transformation and removal of organic carbon from this stratigraphic unit.

Figure 8-3: Thermal modelling of borehole G39974 using a thermal gradient of 25°C/km and an intrusion temperature of 1150°C for the dolerite sills. Within 2000 years the combined temperature has decreased to below the Curie temperature for magnetite.

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During the current study, samples were also collected from boreholes that did not intercept any dolerite sills. The aim was to determine a background temperature for the basin before the main magmatic event took place. The calculated temperatures for these boreholes were however much higher than expected (Figure 8-1) suggesting heating mechanisms other than magmatism playing a role in the basin.

Possible explanations for the observed lateral geothermal variation across the intruded section of the Karoo Basin include:

1) Increased number of sills towards the northeast.

Winter and Venter (1970) reported on the distribution of dolerite sills within the Karoo

Basin and suggested an increased intensity of sills towards the northeastern part of the

Basin. Cawthorn (2012) re-visited the boreholes used by Winter and Venter (1970) and

determined the range of thicknesses of dolerite sills as well as the number of sills and

their total thickness within each borehole. Cawthorn (2012) reported that although the

maximum thickness of dolerite occurs near Barkley East (900 m), the next thickest

occurrence is near Beaufort West (600 m) and considering that an unknown thickness of

Karoo sediments was eroded away at this location, the original thickness of dolerite could

have been much greater. The Insizwa Complex southwest of Kokstad reaches at least

1000 m thereby negating the suggestion that the observed increase in temperatures are

related to number or thickness of sills, unless the impact is localized around each

intrusion (see Figure 8-4 for relative locations).

2) Location of magma source.

Hastie et al. (2014) evaluated the melt sources of different dyke swarms of the Karoo

LIP. Points (a) and (b) from the following discussion refers extensively to references from

their paper and a more detailed discussion can be found in Hastie et al. (2014). Five

basic regional magma flow hypotheses have been suggested (Figure 8-4): (a) Karoo

triple junction at Mwenezi, southeastern Zimbabwe (Burke and Dewey, 1973; Campbell

and Griffiths, 1990; Cox, 1992; Storey, 1995; Ernst and Buchan, 1997; Storey and Kyle,

1997;White, 1997; Storey et al., 2001); (b) Weddell Sea triple junction east of East

London (Chevallier and Woodford, 1999; Elliot and Fleming, 2000, 2004; Leat, 2008;

Luttinen et al., 2010); (c) plume head beneath Namibia based on geochemistry (Le Roex

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and Lanyon, 1998); (d) magma flow driven by subduction-related drag of a mantle plume

(Rapela et al., 2005); and (e) passive melting due to a lack of cooling rather than active

heating by a plume. It potentially implicates internal heating of the upper mantle in Karoo

magmatism, as opposed to a mantle plume (Heinonen et al., 2010). Burke et al. (2008)

on the other hand indicated that the majority of eruption sites of LIP for the past 300 Ma

lie vertically above 1% slow shear wave velocity contours bounding the African and

Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core-mantle boundary

(CMB).

Figure 8-4: Laterally observed variation in temperature across the Karoo Basin with location of proposed magma sources indicated (red stars) and numbered according to text (modified simplified geological map of Karoo Basin, courtesy Council for Geoscience).

An AMS study by Geismann et al. (2011) on one of the lower most sills in the Ecca

Group spanning most of the Main Karoo Basin indicated magma flow fabric supporting

both hypotheses (c) and (d). Chevallier et al. (2001) suggested that the dolerite sills in

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the Karoo Basin propagated laterally rather than vertically based on stratabound

concentration of dykes and sills in the Upper Ecca and Beaufort sandstone. Thus if

hypotheses (a) or (b) are true, it would imply that the heating effect on the host rock

would be least furthest from the source, i.e. cooler towards the western Karoo Basin.

Similarly if hypothesis (c) is true the lowest aureole temperatures would be found

towards the south of the Basin. If (d) is true, the maximum heating influence would be

closest to the Cape Fold Belt. Hypothesis (e) suggests that the accumulation and/or

generation of heat and subsequent magmatism beneath the supercontinent of

Gondwana would have weakened the crust, making continental break-up more likely as

opposed to necessarily being a direct result of the emplacement of a plume beneath the

supercontinent (Storey et al., 1992; White, 1997; Hawkesworth et al., 1999).

Our data do not support (a) and (b), but do support (c) and (d) with the lowest

temperatures observed along most of the southern border of the basin (Figure 8-1),

correlating with the findings of Geismann et al. (2011).

3) On-craton versus off-craton heat flow.

The highest observed ‘minimum’ temperatures (Figure 8-5) are situated on the

Kaapvaal Craton and the possibility of different heat flow on-craton versus off-craton was investigated. Ballard and Pollack (1987) reported the surface heat flow in the interior of

Archean cratons to be much lower than in surrounding younger terrains. This would only play a role if the dolerite intruded vertically through the craton instead of laterally along stratigraphic layers as suggested by Chevallier et al. (2001). On the other hand, the thermal conductivity of sandstone is higher than that of shale (Beardsmore and Cull,

2001) suggesting that depositional setting and associated lithofacies might play an important role in heat transfer.

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Figure 8-5: Laterally observed variation in temperature across the Karoo Basin in relation to the borders of the Kaapvaal craton. CA = Colesburg Anomaly; TML = Thabazimbi-Muchison Lineament (modified simplified geological map of Karoo Basin, courtesy Council for Geoscience).

4) Marine versus lacustrine host rock.

Reeckmann and Mebberson (1984) suggested that hydrothermal activity was

responsible for lateral and vertical transfer of excess heat released by cooling igneous

intrusions. The sedimentary strata of the Karoo Basin transgresses from a mainly marine

setting in the south and southwest (Ecca Group), to a more lacustrine and terrestrial

setting towards the northeast (Beaufort Group). Further east-northeast a fluvial rather

than lacustrine setting dominated. Lithologically there is a transgression from tight low

porosity marine shales in the south and southwest towards coarser-grained mudstone

and porous sandstone towards the northeast. Jones (2003) determined that although

heat capacity of different rock types does not differ significantly, there are distinct

variations in the thermal conductivities. The primary factors controlling the thermal

properties are mineral composition, degree of metamorphism and porosity. Although

Jones’ study focused on rocks from the Witwatersrand gold mining areas, he published

mean thermal conductivity (K) values for 15 sandstone and 25 shale samples from the

Ecca Group. A distinct difference was observed in the thermal conductivity of these two

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rock types, with 3.14±0.38 W/mK for the sandstone and 1.88±0.25 W/mK for the shale.

Using these heat conductivity values for Ecca shale and sandstone, and the formulas for

heat diffusivity for clay and sand from the ocean floor (Goto and Matsubayashi, 2008) it

was calculated that the heat in sandstone will conduct at a rate of 40.68 m2/year, while

shale will spread at a rate of only 21.47 m2/year. Therefore it is clear that heat from an

intruding sill will travel a shorter lateral distance into a shale layer than in sandstone. That

is why the heating effect observed in the boreholes is more widespread between sills in

predominantly sandstone host than in shale hosts. The heat lingers longer around sills

intruded in shale, therefore, a higher metamorphic effect of the shale should be observed

closer to the sill than would be observed in sandstone for the same distance. To

demonstrate this, the thermal modelling of core G39974 (Figure 8-3) was repeated by

replacing the thermal conductivity of shale (1.88 W/mK) with that of sandstone (3.14

W/mK). It is clear from Figure 8-6 that the heat dissipated away from the sills in half the

time it took in a tight shale hosted environment (1000 years to decrease below 600 °C,

and 2500 years to decrease below 350 °C, Figure 8-3).

Figure 8-6: Thermal modelling of borehole G39974, replacing shale host rock with sandstone, to demonstrate the change in heat flow for a higher thermal conductivity. The combined temperature has decreased to below the Curie temperature for magnetite in half the time (1000 years) compared to shale.

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8.3 Implications for Industry and the Environment

8.3.1. Coal

8.3.1.1. Parameters affecting coal rank and maturation

Coal rank increases with depth due to the natural increase in geothermal gradient expressed as °C/100m and is determined by the heat flow and the heat conductivity of rocks.

Normal heat flow density is 40–100 mW/m2 with the estimated mean value on continents around 57 mW/m2 (Chapman, 1985). Heat conductivities of rocks depend mainly on their

mineral composition and their porosity. Buntebarth (1980) published that sandstone has

higher heat conductivity (2–3 W/m °C) than claystone (1–2 W/m°C), while Rybach (1984) suggested that in general the heat flow and geothermal gradient are inversely proportional to the depth of the basement. Van Heek et al. (1971) found that during contact metamorphism

(high heating rates), to attain a given rank the temperatures need to be higher as opposed to the low heating rates required during gradual subsidence. Typical coalification temperatures for bituminous coals is 100–170°C, while for anthracites it ranges between 170–250°C. The effect of duration on coalification is much debated, and Gretener and Curtis (1982) concluded that the effect of temperature on coalification (contact metamorphism) is exponential whereas the effect of time (burial) is linear and only noticeable in the range of 70–100°C. Barker (1989) compared normal burial coalification with coalification caused by extremely high temperatures and concluded that vitrinite reflectance stabilizes after 1–10 Ma of burial diagenesis; after about 10 ka in geothermal systems; and after only one year or less in contact metamorphism by intrusives. Pressure on the other hand promotes physic-structural coalification with a decline in porosity and a decrease of moisture with depth. However, pressure retards chemical coalification because the removal of liquid and gaseous coalification products is also retarded. Consequently Cecil et al. (1977) concluded that, based on chemical considerations, hydrocarbon generation should be retarded by an increase of pressure. Teichmüller and

Durand (1983) performed Rock-Eval analyses on a rank series of coals and showed that the temperature of maximal hydrocarbon release (Tmax) increases gradually with increasing rank.

Hower and Gayer (2002) proposed that flow of hydrothermal fluids through the coals can also cause coal metamorphism. This can often explain the occurrence of inverted rank gradients

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and can be confirmed by elevated CFL as an indicator of brine fluids, isotopic evidence for

hydrothermal fluids, and vein and cleat mineral assemblages.

8.3.1.2. Coalfields of South Africa

In South Africa the Early Permian coals occur in the Vryheid Formation of the Ecca

Group (Johnson et al., 1996). The Vryheid Formation represents deltaic and fluvial deposits.

Peat swamps typically cap upward-coarsening deltaic cycles which were then later modified and eroded away by superimposed fluvial systems (Cairncross and Cadle, 1988a,b). The peat-formation was terminated by periods of marine transgression as evidenced by glauconitic sandstone and siltstone in the hanging wall of some seams (Cairncross, 1989).

The coals in the northern Karoo basin are seldom more than 100 m below surface and with the exception of disturbances by dolerite sills and dykes, have been tectonically undisturbed since their deposition. In contrast the outlying basins (e.g. Ellisras and Pafuri basins) are structurally controlled intra and intercratonic grabens and half-grabens that can be intensely faulted (Fourie et al., 2014). The 70 m thick Groottegeluk Formation in the Waterberg coalfield, for example, occurs in some places over 500 m below surface. Faure et al. (1996) reported that no additional heat sources in the Waterberg (Ellisras) Basin existed to influence the coal rank, as no dolerite dykes or sills were identified and suggested that a chemical change in the environment rather than burial depth was responsible for changes in the clay ratios. However, Reid et al., (1997) reported that the sequence is capped by thick basaltic lava similar in age to the Drakensberg Group. Dolerite intrusions have been associated with changes in coal rank and seam distribution (e.g. Majuba Colliery, De Oliveira and Cawthorn,

1999). The South African coals are primarily high- to medium-volatile bituminous with some occurrences of anthracite in the eastern sections of the Karoo Basin as well as in the Limpopo area (Falcon and Ham, 1988; Snyman and Barclay, 1989). Sullivan (1995) identified an increase in the vitrinite content and rank of the South African coals from west to east across the coalfields. This west to east increase in rank was ascribed by Snyman and Barclay (1989) to greater geothermal gradients in the east where the lithosphere was relatively thinner, coupled with intrusions of dolerite during the subsequent break-up of Gondwana. Similar tectonic control on coal rank has been observed in the northern basins of South Africa where coal seams occurring in basins positioned within intercratonic belts and rifted grabens have

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been exposed to greater heat flow from below due to thinner crust (Cairncross, 2001). Gröke

et al. (2009) used vitrinite reflectance (Ro) as a palaeo-geothermometer on two coal transects associated with dykes in the Highveld Coalfield, Karoo Basin. Their study indicated background temperatures of ~100°C increasing to >300°C close to the dyke contacts.

Although during the current study thermal analysis was not conducted directly on coal samples, the magnetically acquired geothermal information from surrounding host rock in three boreholes drilled in coalfields (DF1/75, CBC4495 and MY19) confirm the suggested high temperatures (350–400°C, see also Chapter 6). The highest minimum temperature

(400°C) was observed in core DF1/75, ascribed mainly to the influence of two thick dolerite sills (184 m and 48 m). This is the only borehole with both coal and dolerite, although not in direct contact with each other. Borehole DF1/75 is located the furthest east of the three mentioned boreholes. However, due to the relative small difference in longitudinal position between these three boreholes, the regional increase in geothermal gradient from west to east, as suggested by increasing coal ranks (Sullivan, 1995) cannot be properly tested.

8.3.2. Uranium

8.3.2.1. Classification of uranium deposits

Uranium deposits are generally classified based on host rock, structural setting, and mineralogy of the deposit. The most widely used classification scheme was developed by the

International Atomic Energy Agency (IAEA) and subdivides deposits into 15 categories.

These are, on the basis of geological setting and in order of economic importance the classes of uranium deposits: (1) unconformity related (33% of world uranium resources), (2) sandstone (18% of world uranium resources), (3) quartz-pebble conglomerate (13% of world uranium resources), (4) breccia complex, (5) veins, (6) intrusive (Alaskites), (7) phosphorite,

(8) collapse breccia pipe, (9) volcanic, (10) surficial (calcrete) (4% of world uranium resources), (11) metasomatite, (12) metamorphic, (13) lignite, (14) black shale, and (15) other deposits (IAEA, 1996).

Similar processes may, however, form many deposit types in different geological settings and an alternate scheme was developed (Lally and Bajwah, 2006) to group the above deposit types based on their environment of deposition (Table 8-1).

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Sandstone deposits are generally contained within medium- to coarse-grained sandstones deposited in a continental fluvial or marginal marine sedimentary environment.

Impermeable shale or mudstone units are interbedded in the sedimentary sequence and often occur immediately above or below the mineralized horizon (WNA, 2010). Uranium is mobile under oxidizing conditions and precipitates under reducing conditions, and thus the presence of a reducing environment is essential for the formation of uranium deposits in sandstone

(McKay and Meiitis, 2001). Reducing agents include carbonaceous material (e.g., detrital plant debris, amorphous humate, marine algae), sulphides (e.g., pyrite, H2S), hydrocarbons

(e.g., petroleum), and interbedded basic volcanics with abundant ferro-magnesian minerals

(e.g., Chlorite).

Table 8-1: Uranium deposit classification based on depositional environment according to Lally and Bajwah, (2006). Uranium Deposit Classification Uranium Transport / Precipitation Deposit Type Conditions Surficial deposits Quartz-pebble conglomerate deposits Surface Processes / synsedimentary Phosphorite deposits Lignite

Black shales Diagenetic Sandstone deposits Unconformity-related deposits Diagenetic – Hydrothermal? Vein deposits Collapse breccia pipe deposits Breccia complex deposits Volcanic deposits Magmatic – Hydrothermal? Metasomatic deposits Vein deposits Intrusive deposits Metamorphic – Hydrothermal? Metamorphic deposits

Primary mineralization consists of pitchblende and coffinite, with weathering producing secondary mineralization. Sandstone deposits constitute about 18% of world uranium

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resources. Ore bodies of this type are commonly low to medium grade (0.05–0.4% U3O8) and individual ore bodies are small to medium in size (WNA, 2010).

Hydrothermal deposits involves the precipitation of uranium from migrating groundwater and includes examples such as collapse-breccia pipe uranium deposits; lignite uranium deposits where precipitation occurs by absorption on organic matter, which acts as a reductant; as well as uraniferous black shales thought to have formed by adsorption of uranium onto organic matter or phosphatic nodules from seawater. Figure 8-7 illustrates the role of magmatic source, mixing of high temperature fluids and meteoric waters, as well as temperature of mineralizing solutions in volcanic associated uranium deposits (Gandhi and

Bell, 1995).

Many deposits represent combinations of these types.

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8.3.2.2. Uranium Deposits in the Karoo Supergroup

Cole (1998, 2009) discussed the different uranium deposits in South Africa in great detail and the two types of uranium deposits found in the Karoo Supergroup will be briefly summarized here. The first type consists of fluvially-deposited, sandstone-hosted, peneconcordant, tabular deposits in the Adelaide Subgroup (lower Beaufort Group) as well as the Molteno and Elliot Formations within the main Karoo Basin. The second type of uranium deposits occur in the Springbok Flats Basin as coal-hosted lignite type deposits.

The sandstone-hosted ore bodies are normally 1 m thick, but can reach up to 7 m in places and Cole (1998) reported that where these bodies are vertically stacked, a combined thickness of up to 20 m has been observed. They occur elongated along the paleochannel thalweg within the lower portion of the enclosing fluvial sandstone body. Calcareous nodules and layers are commonly found in mudstone interbedded with the sandstone bodies. The dark greenish grey, dark greyish red and maroon mudstone was deposited in a flood-basin environment with suspension-settling. The reddish colours indicate oxidation of the mud taking place during subaerial exposure and the calcareous nodules and layers have been interpreted as pedogenic calcrete that formed in a semi-arid environment (Smith, 1990).

Uranium in the ore bodies is hosted by the mineral coffinite and less abundant uraninite with associated sulphides such a molybdenite, pyrite, arsenopyrite and chalcopyrite.

Metallogenesis is thought to have been dependent upon three factors, namely uranium source, paleoclimate and availability of a reductant. Turner (1985) and Le Roux and Toens

(1986) suggested the uranium source to originate from basement granite and volcanic ash located west, southwest and south of the main Karoo Basin. The uranium was transported in solution as uranyl carbonate complexes, precipitated in relatively sparse reduced zones that contained carbonaceous debris in the basal part of the sandstone below a low palaeo-water table (Cole and Wipplinger, 2001). In the Northern Cape Province the concentration of uraniferous ferruginous mudrock from the White Hill and the Tierberg formations (Ecca

Group) is attributed to remobilization in the supergene environment before being absorbed onto haematite (Wipplinger 1987; Cole et al., 1991) while enrichment in Co, Mo, Cu and V in the mudrock occurred as a result of transportation and redistribution of metal ions in pore

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waters as a result of thermal metamorphism associated with dolerite intrusions (Wipplinger,

1987).

In the Springbok Flats Basin uranium is hosted by coal in the uppermost part of the

Hammanskraal Formation. The uppermost part of the Hammanskraal Formation consists of

interbedded carbonaceous shale and coal (also referred to as the Coal Zone) attributed to a

lacustrine environment with suspension-transported mud in low-energy fluvial systems and

windblown dust being the main sources of clastic sediment (Christie, 1989). Uranium is

concentrated over a vertical interval of 1 m in the upper part of the Coal Zone. However,

Christie (1989) reported that on the flanks and shoulders of palaeovalleys in the vicinity of

Bushveld Complex granite bedrock, the entire Coal Zone is uraniferous. Kruger (1981)

identified coffinite, oyamalite and auerlite as uranium-bearing minerals and suggested the

granite of the Bushveld Complex as probable source. A high proportion of uranium is held in

organo-metallic compounds that were formed as a results of the uranium being mobilized

from the granite by oxidizing groundwater and transported to the Coal Zone where it was

absorbed by the lignite under reducing and slightly acidic conditions (Hambleton-Jones,

1980).

A study of a limited number of geophysical logs from the main Karoo Basin indicated no

correlation between the location of observed uranium anomalies and dolerite intrusions. Cole

(1977) concluded that uranium mineralization in the Beaufort Group mudstone and siltstone is

epigenetic hosted in natural levee deposits. In the argillaceous siltstone and silty mudstone

the low permeability prevented migration of uranium-rich fluids, and thus represent in situ

syngenetic uranium deposits.

8.3.3. Hydrocarbon Potential and Global Climate Change

Hydrocarbons begin to be generated above the temperature threshold of 60°C. This process, where peats and lignites become dehydrated and lose other volatiles and kerogen splits into its four distinctive types is known as the ‘carbonization jump’. The ‘oil window’ lies

between temperatures of ~60–120°C and the gas window between ~120–150°C (Figure 8-8).

At temperatures greater than 150°C, the organic matter is said to be post mature and is no

longer reactive to the development of hydrocarbons. Metagenesis occurs from 150–200 °C

(vitrinite reflectance between 2-5%) above which the organic compounds are reduced to

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graphite and methane along with other non-hydrocarbon gases such as CO2, N2 and H2S.

Hunt (1996) illustrated the relative thermogenic gas yields from organic matter buried in fine- grained sediments as a function of temperature (Figure 8-9). Petroleum gases are generated by abiogenic reactions during late diagenesis, catagenesis and metagenesis (Doule, 2001) and generally consist of methane (>70%) and carbon dioxide (<10%) with minor ethane

(<10%), propane (<5%) and n-butane (<2%). The most active period of thermogenic gas generation is during catagenesis and depends on the H/C ratio of the source kerogen. During late categenesis, methane is generated by cracking of carbon-carbon bonds in kerogen, while during metagenesis it is generated from the small amount of hydrogen remaining in the kerogen matrix and from cracking of previously generated hydrocarbons (Doyle, 2001).

Figure 8-8: Hydrocarbon formation temperatures (after Tissot and Welte, 2008).

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Figure 8-9: Relative thermogenic gas yield from organic matter, (a) Sapropelic source and (b) Humic source, buried in fine-grained sediments as a function of temperature. C2H6+ represents “wet gas” (hydrocarbon gases heavier than methane) (after Hunt, 1996).

The results from the current study indicate a general elevation of the palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas (>200°C). Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon

(methane) resources remaining between these intrusions.

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8.3.3.1. Methane release and CO2 Degassing

Ganino and Arndt (2009) listed the inputs that must be considered when evaluating the environmental impact of LIP emplacement:

• Basalt and granitoid do not release abundant volatiles.

• In most sandstone, the main volatile is water, whose release has little effect on global climate.

• Pure limestone contains large amounts of CO2, but the thermal decomposition of limestone into CaO and CO2 takes place at high temperatures (>950°C) that are rarely

reached in contact aureoles.

• Impure limestone releases large amounts of CO2 (up to 29 wt%) during the formation of calc-silicates at moderate temperatures of ~450–500°C.

• Gypsum and anhydrite in evaporite release abundant SO2 (up to 47 wt%). This reaction

normally happens at high temperatures (1400°C) that are not observed in contact aureoles,

but the reaction proceeds at temperature as low as 615°C for impure anhydrite (West and

Sutton, 1954).

• Contact metamorphism of salt releases halocarbons.

• Sulphidic sediments release abundant SO2 at low temperature.

• Organic carbon-rich shale and carbonate release methane and hydrocarbon when heated at relatively low temperature (<300°C).

• Coal releases abundant CO2 if ignited.

Svensen et al. (2007) evaluated the carbon production potential of the Karoo Basin and suggested that the total mass of carbon produced in contact aureoles, WC, can be converted

to equivalents of methane (=WC*1.34) and carbon dioxide (=WC*3.66). Hunt (1996) reported that the produced thermogenic methane has a depleted carbon isotope ratio (δ13C=−35 to

−50‰). Compilation of characteristic total organic carbon (TOC) values from the Karoo Basin shows that un-metamorphosed shale has 2–8 wt.% TOC in the White Hill Formation and 0.5–

4 wt.% TOC in the Prince Albert Formation. Thus the gas generation potential is significant.

Also, the actual aureole data from borehole G39974 supports that a significant fraction of the

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organic matter was transformed to carbon gas during metamorphism, as increasing % Ro

values correlate with decreasing TOC contents.

The thickness of the Ecca Group varies considerably on a basin scale, and there is

limited information available about the full extent of contact metamorphism and the details of

TOC profiles. Nevertheless, maturity parameters and diagenetic mineralogy show that the sill

emplacement had a significant effect on the Ecca Group sediments on a basin-wide scale

(Rowsell and De Swardt, 1976; Cole and McLachlan, 1994), thus supporting widespread gas

generation in contact aureoles. This would suggest significant implications for climate.

Svensen et al. (2007) calculated the potential for gas generation in aureoles is between 1870

and 7490 Gt C or the equivalent 6850 and 27,400 Gt CO2. This estimate does not include the contribution from metamorphism of coal beds in the Ecca Group of the Eastern Karoo Basin or the metamorphism around sills in the Beaufort and Stormberg formations.

Gröke et al. (2009) used vitrinite reflectance (Ro) as a palaeo-geothermometer on two

coal transects associated with dykes in the Highveld Coalfield, Karoo Basin. Their study

indicated background temperatures of ~100°C increasing to >300°C close to the dyke

contacts. Gröcke et al. (2009) observed no significant changes in δ13C and suggested that the

low vitrinite and liptinite contents of the Highveld coals in part explains the modest decreases

in volatile matter adjacent to dykes. This combined with a relative narrow metamorphic

aureole surrounding the intrusions and the likelihood that at least some some of the volatiles

generated by the intrusion were trapped as coalbed methane (CH4) or condensed as pyrolytic carbon, led the authors to conclude that only limited CH4 release was likely. In addition

Gröcke et al. (2009) suggested that based on the original extimates of moisture contents of the Highveld coals and the depth at time of intrusion (1000–2000 m) the dykes would have lost most of their energy heating and evaporating water, thus having little remaining energy to generate thermogenic CH4.

The opposing conclusions of Svensen et al. (2007) and Gröcke et al. (2009) regarding

green-house gas release caused by intrusions could be due to the difference in heat transfer

around a dyke as apposed to a sill. That is, the temperature of the host-rock on either side of

a dyke would be the same, where as it would be different in the footwall and hanging wall of a

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sill. Sills also tend to be emplaced along the contact between two different lithologies that

might have different thermal conductivities.

8.3.3.2. CO2 Storage Potential

CO2 storage in deep saline formations of hydrocarbon reservoirs is generally accepted to

take place at depths below 800 m, or where ambient pressures and temperatures will result in

the CO2 being in a supercritical state, i.e. in a dense fluid form where it is neither gas nor liquid. In the supercritical state the CO2 is able to dissolve more readily in formation water and react with cations to form stable mineral compounds (Cloete, 2010).

Shale, the most common type of sedimentary rock, usually contains some organic material, which provides an adsorption substrate for CO2 storage similar to CO2 storage in

coal seams. Because of the very low permeability of shale, however, achieving economically

viable injection rates is currently not possible. There is some highly disaggregated potential

for the storage of CO2 associated with enhanced coalbed methane (ECBM) recovery from

deep presently unmineable coal seams, should a coalbed methane industry develop in South

Africa (Cloete, 2010).

The safe storage of injected CO2 requires the area to be overlain by appropriate caprocks to prevent contamination of drinking water at shallower zones. Geological processes such as diagenesis and metamorphism, however, will diminish rock porosity and permeability, whereas tectonism and magmatism will lead to jointing, faulting and cross-cutting relationships which will generally compromise secure storage.

Saghafi et al. (2008) studied the CO2 storage potential of South African coals as well as

gas entrapment enhancement due to igneous intrusion. These authors found that the

adsorption and porosity properties of metamorphosed coals at the Middelbult Colliery in the

Secunda region have been enhanced thus increasing their gas storage capacity. Their results

showed that the maximum adsorption capacity of coals presented by the Langmuir volume

3 3 parameter VL vary from 27–52 m /t for CO2 gas and 8–23 m /t for the CH4 gas. The larger adsorption capacity was exhibited by coals closer to dyke intrusions. Diffusivity results from

-6 -6 2 -6 the same study revealed values of 2.9 x 10 – 12.9 x 10 cm /s for CH4 and 3.7 x 10 –

-6 2 15.1 x 10 cm /s for CO2 (higher values close to the dyke), while measurements of the dyke

-6 2 -6 2 showed a CO2 diffusivity of 5.45 x 10 cm /s and a CH4 diffusivity of 5.23 x 10 cm /s. In

8–21 CHAPTER 8

comparison to metamorphosed coal in the vicinity of the dyke, these values are less than half

of the diffusivity of gas in coal. Therefore, the study showed that dolerite intrusions would act

as enclosures and gas traps for storage of gas.

8.3.3.3. Climate change

Svenson et al. (2007) reiterated the fact that proxy data demonstrate a major perturbation of the carbon cycle during the Pliensbachian–Toarcian, about 183 Ma. The

Karoo dolerite age is indistinguishable from the lower Toarcian interval where a negative carbon excursion and an oceanic anoxic event are located (Hesselbo et al., 2000; McElwain et al., 2005). This period of climate change was characterized by a global warming of ~6 °C, anoxic conditions in the oceans, and extinction of marine species (Jenkyns, 1988; Little and

Benton, 1995; Pálfy and Smith, 2000; Pálfy et al., 2002; Kemp et al., 2005;). Svenson et al.

(2007) suggested that the Toarcian greenhouse would have been further accelerated by the metamorphism of coal deposits in Antarctica, long term lava degassing from the Karoo–Ferrar province, astronomical climate forcing (Kemp et al., 2005), and by feedback mechanisms such as melting of gas hydrates (Hesselbo et al., 2000; McElwain et al., 2005).

Contrary to the western Karoo Basin, the Highveld Basin showed only modest decreases in volatile matter adjacent to dykes. This combined with a relative narrow metamorphic aureole surrounding the intrusions (mainly dykes) led Gröcke et al. (2009) to conclude that only limited CH4 release was likely. The geothermal data obtained from magnetic sample analysis (current study) indicated basin wide increased temperatures up to levels were hydrocarbons are converted into gas. Elevated temperature in boreholes that did not intercept any dolerite, however suggests that other factors such as the lithological setting also played a significant role in the dispersal of heat and subsequently on the metamorphic effect of intrusions on organic matter.

8.4 Concluding Remarks

This study aimed to determine the peak temperatures reached by the Karoo sedimentary strata as a result of heating by the Karoo LIP. Due to the uncertainties of estimated temperatures associated with more traditional techniques (illite crystallinity and vitrinite reflectance) the viability of using magnetic fabric analysis as geothermometers in sedimentary

8–22 CHAPTER 8

basins was investigated. Three magnetic techniques were applied to 8 boreholes spread

across the Karoo Basin. These included the classic palaeomagnetic baked contact test;

variation in magnetic susceptibility during progressive thermal heating (alteration index

method); as well as a study of the variation of magnetite-pyrrhotite ratios within the contact

aureoles. Although all three techniques were successful in delineating the extent of the

contact aureoles, only the alternating index (A40) had the ability to give estimated peak temperatures. The accuracy of these calculated temperatures can be increased by decreasing the temperature intervals of progressive heating and measurement, with associated increase in the output time. This technique is however limited to a minimum applied temperature of 90°C which unfortunately impairs the ability to determine very low- grade metamorphic effects often encountered in sedimentary basins not affected by magmatic intrusions.

A general increase in the calculated peak temperatures from west to east were observed and several possible scenarios investigated. The most probable explanation relates to the different environmental settings that prevailed in the western and eastern parts of the basin during magma intrusion. The heat flow occurred much slower and for shorter distances in the low permeable marine shale from the Ecca Group in the western part of the basin compared to the porous coarser-grained sandstone and siltstone towards the east and northeast where the heat was quickly dispersed over a much wider distance.

Although some mention was made to the concentration of uranium due to intrusion induced remobilization, this is limited to the Northern Cape Province and none of the other uranium deposits were influenced by the Karoo LIP.

The current study confirms a general increase in the geothermal temperature of the

Karoo Basin to levels where hydrocarbons are converted into gas; it is however not yet proven beyond doubt whether the majority of the gas was released into the atmosphere or whether it was re-absorbed for future exploitation.

8–23 CHAPTER 8

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