Dolerite control on the distribution of high-grade iron 

orebodies at Assen Iron Ore mine, South Africa 

 

Elekanyani Shadrack Negwangwatini 

1986493 

A thesis submitted to the Faculty of Science, University of the 

Witwatersrand, in fulfilment of the requirements for the degree of 

Master of Science in Economic Geology 

 

University of the Witwatersrand, School of Geoscience, 2020 

 

 

 

01 November 2021 

 
 

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i 
 

Declaration 
 
I declare that this research report is my own original work, conducted under the supervision of Prof. 

Glen Nwaila. It is submitted in fulfilment of the requirements for the Master of Science in Economic 

Geology at the University of Witwatersrand, School for Geoscience.  

 

………………………………………………………………………………… 

Elekanyani Negwangwatini 01-11-2021 

 

 

 

  

  



ii 
 

Abstract 
 
The Assen Iron Ore deposit is located in the Crocodile River Inlier which lies within alkali rocks of the 
2.05 Ga Bushveld Igneous Complex (BIC). The host rocks to the Assen Iron Ore deposit are contained 
in the lower portion of the Penge Formation of the Transvaal Supergroup, close to the contact with 
the underlying dolomite of the Malmani Subgroup. This study presents a detailed geological 
mapping of the Assen Iron Ore deposit. Petrographic and mineral characteristics of different iron ore 
facies and the dolerite intrusions from three iron ore mineralised portions named west, middle, and 
east orebody were conducted. The study investigated whether the intrusions control the 
mineralisation of high-grade iron orebodies. The different types of hematite and dolerite intrusions 
were sampled from the Assen Iron Ore mine's current operating pits and analysed using X-ray 
powder diffraction (XRD) and optical microscopy. Existing X-ray fluorescence (XRF) from the 
previously drilled diamond cores were used to compare and correlate different oxides with different 
iron ore facies. 
  
The banded iron formations (BIFs) that constitute the Penge Formation in the Assen area comprise 
several stratigraphically conformable rocks, namely shale, limestone, and mineralised units 
represented by calcitic hematite, high-grade hematite ore (i.e., laminated, and massive hematite 
ore), and goethite-rich hematite ore. The shale forms a basal layer consisting mainly of chert-rich BIF 
lenses with pyroclastic and tuffaceous sections. The intermediate layer with micro-and meso-
layering consists of hematite and calcite, typical of carbonate facies. A high-grade hematite-rich ore 
directly overlies the calcitic ore. Lying above the high-grade hematite is atypical goethite-rich ore 
with grades between 50 to 55 % Fe.  Detrital iron ore is located at the base of the southern slope of 
the Assen ridge. This ore is unconsolidated and occurs as small, rounded cobbles and pebbles of 
iron-rich sandy sediment from 1 m to 2 m thick.  
 
The geological mapping and exploration drilling indicate that the orebodies are structurally 
controlled along the dolerite sills that intruded the BIF sequence of that Transvaal supergroup. At 
places, goethite-rich and laminated hematite are found above or on the hanging wall of the dolerite 
sills. Massive hematite tends to be underlain the dolerite.  Calcitic hematite is always located in the 
stratigraphy below massive and laminated hematite and is underlain by either the dolerite on the 
edge of iron formation or shale.  At Assen Iron Ore mine, dolerite sills intruded the BIF of the Penge 
Formation. Post-deposition and intrusion, both the dolerite intrusions and BIF were affected by the 
three stages of folding (F1, F2, F3-folds) and faulting which caused deformation and partial 
metamorphism. Thermal metamorphism is associated with the BIC which surrounds the Inlier and 
resulted in alteration of primary minerals, change in bulk rock chemistry, and visual appearance of 
the rocks. 
  
The XRF analysis showed the downward increases in % iron (Fe), downward decrease in silica (SiO2) 
in both BIF, goethite, laminated and massive hematite. The calcitic hematite showed a downward 
decrease in iron (Fe) and a downward increase in calcium (Ca). The laminated hematite (55-59 % Fe) 
and high-grade Fe in BIF (50-55 % Fe) ore types showed some clear removal of almost all silica in 
between hematite bands to partial removal of silica in the high Fe BIF. Both high-grade BIF and 
laminated hematite remained with the preserved primary layers of hematite bands that compacted 
through time due to pressure and folding to form a high-grade bedded iron ore. The space that was 
occupied by silica is at places filled with thin and scattered recrystallised quartz or goethite matrix. 
The goethite-rich, friable soft hematite, sometimes in a dark grey powder was formed by the 
weathering and dissolution of Fe from the BIF by the descending surface or meteoric water. The 
spatial relationships between the dolerite intrusions and the high-grade hematite ore suggest that 
the dolerite intrusions caused changes in mineral texture and ore grade. The XRF analysis from the 
drilled cores showed clear evidence that at closer contacts, hematite orebodies in contact with sills 



iii 
 

underwent enrichment in Fe and reduction in SiO2. This result shows that aside from secondary 
supergene enrichment, tertiary Fe enrichment can occur due to igneous activity such as dolerite 
intrusion. 
 
  



iv 
 

Acknowledgements 
 
Firstly, I would like to acknowledge the University of Witwatersrand for accepting my application as 

an MSc candidate and granting me the opportunity to study with them to obtain my MSc in 

Economic Geology. The university has also granted me access to use their laboratories and their 

facilities and assisted in preparing thin sections and analysing the thin sections using a petrographic 

microscope for the project.  

I would like to thank my Professors for all the knowledge impartation, academic support, and advice 

throughout my studies, without them this research report wouldn’t be possible. My deepest 

gratitude goes to my internal project supervisor Prof. Glen Nwaila for the sterling guidance, 

assistance, and supervision throughout the project. The constant steering and tireless effort to my 

research report were endearing and I was honored to have his supervision throughout the project. 

Prof. Glen Nwaila also assisted in suggesting important publications to read and get us some 

research specialists to impart knowledge to us during our biweekly meetings.  My sincere gratitude 

goes to Prof. Judith Kinnard for the much-needed suggestions and valuable ideas for my research 

proposal. I would also like to thank Prof. Paul Nex who gave guidance in mineral correlation and all 

the support throughout my research. 

My heartfelt gratitude to Mr. Matodzi Nesongozwi, the CEO of Manngwe Mining Company who 

approved the financial support for the project and providing necessary permissions and facilities to 

access the Assen Iron Ore mine exploration boreholes data necessary for the project. Mr. Matodzi 

Nesongozwi also granted me the time to focus on my studies while working for him. His leniency 

allowed me to be flexible in my schedule to complete my masters and continue working 

concurrently. Manngwe Mining Company also helped me in using their geological modelling 

software for me to produce the 3D geological model and ArcGIS software to produce some 2D 

geological maps. I would like to acknowledge MQA for their financial support in funding my studies 

by granting me a bursary. Assen Iron Ore mine provided access to the pits for sampling and 

geological mapping and provide the required drill cores.  

 

I would like to thank Mr. Ernst Makupula and Mr. Vhusafheli Ramanugu for their help in the 

geological modelling of the orebody using Leapfrog and Datamine.  I would also like to thank Mr. 

Katekani Musisinyani, who is not just a colleague but is also my friend who assisted me with 

collecting the samples, sample preparation, and his input to my research report. I am grateful to 

Thapelo Sialo from the South African Bureau of Standards (SABS) laboratory for doing XRF analysis 

for some of the Iron Ore samples and his team for their abundant and helpful support. 

 

Thank you to my Fiancé, Nompumelelo Mokgethoa for the love, support, prayers, and 

encouragement throughout my research. Nompumelelo is a qualified geologist who assisted me in 

my research work, the editing of my research report, and the input required to complete my 

research report. Finally, to my parents, thank you for the constant prayers, support, love, and most 

importantly patience, and understanding. I have spent most of my time away from home due to my 

studies that even during festive seasons and holidays I would not visit while I would be chasing the 

deadline for my research report.  

 



v 
 

TABLE OF CONTENTS 
Declaration .............................................................................................................................................. i 

Abstract .................................................................................................................................................. ii 

Acknowledgements ............................................................................................................................... iv 

List of Figures ........................................................................................................................................ vii 

List of Tables .......................................................................................................................................... ix 

List of figures of the appendices: ............................................................ Error! Bookmark not defined. 

List of tables of the appendices: ........................................................................................................... ix 

List of abbreviations and acronyms ....................................................................................................... x 

List of abbreviations of rock types, minerals and elements ................................................................. x 

CHAPTER 1 .............................................................................................................................................. 1 

1. Introduction .................................................................................................................................... 1 

1.1 Background ............................................................................................................................. 1 

1.2 Aims and the objectives of the research. .............................................................................. 2 

1.3 Scope of the study .................................................................................................................. 2 

1.4 Previous Work ........................................................................................................................ 3 

1.4.1 The depositional model of the BIF ................................................................................. 3 

1.4.2 High-grade iron ore genetic models .............................................................................. 4 

1.5 Statement of the Problem ..................................................................................................... 7 

1.6 Project Methodology ............................................................................................................. 7 

1.6.1 Detailed geological mapping .......................................................................................... 7 

1.6.2 Chemical analysis using X-ray fluorescence .................................................................. 8 

1.6.3 Quantitative X-ray powder diffraction analysis ............................................................ 8 

1.6.4 Reflected light microscopic study. ................................................................................. 8 

1.7 Research structure ................................................................................................................. 8 

CHAPTER 2 .............................................................................................................................................. 9 

2. Literature review ............................................................................................................................ 9 

2.1. Iron ore deposits and their classification schemes ............................................................... 9 

2.1.1. Iron ore deposits ............................................................................................................ 9 

2.1.2. Classification of iron ore deposits ................................................................................ 10 

2.2. Regional Geology .................................................................................................................. 13 

2.2.1. Kaapvaal Craton ........................................................................................................... 13 

2.2.2. Transvaal Supergroup .................................................................................................. 14 

2.3. Deposit Geology ................................................................................................................... 18 

2.3.1. Lithostratigraphy .......................................................................................................... 18 

2.3.2. Mineralisation Style ..................................................................................................... 20 

2.3.3. Geological structures .................................................................................................... 21 

2.4. Iron ore and gangue mineralogy ...................................................................................... 23 



vi 
 

CHAPTER 3 ............................................................................................................................................ 25 

3. Samples and methodology........................................................................................................... 25 

3.1. Sample descriptions ............................................................................................................. 25 

3.2. Methods ................................................................................................................................ 26 

3.2.1. Geological mapping ...................................................................................................... 26 

3.2.2. Chemical analysis using X-ray fluorescence ................................................................ 27 

3.2.3. Petrography and Mineralogy ....................................................................................... 27 

CHAPTER 4 ............................................................................................................................................ 30 

4. Detailed geological mapping ........................................................................................................ 30 

4.1. Regional-scale observations ................................................................................................ 30 

4.2. Deposit-scale observations .................................................................................................. 33 

4.3. The lithostratigraphy of the Assen deposit ......................................................................... 37 

4.4. 3D implicit modelling of the Assen iron deposit ................................................................. 38 

CHAPTER 5 ............................................................................................................................................ 51 

5. Mineralogy and Petrography ....................................................................................................... 51 

5.1. Calcitic hematite ................................................................................................................... 51 

5.2. Massive hematite ................................................................................................................. 53 

5.3. Laminated hematite ............................................................................................................. 53 

5.5. Dolerite intrusions ................................................................................................................ 55 

5.6. The XRF geochemistry of iron ore ........................................................................................ 56 

CHAPTER 6 ............................................................................................................................................ 60 

6. Discussion ..................................................................................................................................... 60 

6.1. Paragenesis of different iron ore types ............................................................................... 60 

6.2. The relationship between the dolerite and hematite ores ................................................ 61 

6.3. The relationship between the Faults and folds with hematite ores .................................. 63 

6.4. Fe grade distribution in the Assen Iron Ore deposit ........................................................... 63 

CHAPTER 7 ............................................................................................................................................ 64 

7. Conclusions ................................................................................................................................... 64 

References ............................................................................................................................................ 66 

Appendices ........................................................................................................................................... 71 

Appendix A. 1. Quality assurance and quality control analysis ...................................................... 71 

Appendix A. 2. Certified reference materials in different samples ................................................ 72 

Appendix A. 3. Accuracy of CRM’s ................................................................................................... 73 

Appendix B. 1. Block’s validation statistics ..................................................................................... 74 

Appendix C. 1. Semi-Variograms for east, middle, and west orebody ........................................... 76 

Appendix D. 1. Histograms of elements distribution in both east, west, and middle orebody. ... 84 

 
  



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List of Figures  
Figure 1: Classification of major types of high-grade hematite ore deposits and their model of 

enrichment (Beukes et al., 2002). ........................................................................................................... 5 

Figure 2: Distribution of the abundance of Iron Formations through time (After Klein, 2005). .......... 10 

Figure 3: The genetic model for the major BIF-hosted iron ores of the world showing the 

transformation from BIF to different iron ore types (modified from Morris, 1998). ........................... 12 

Figure 4: The geological map of the Kaapvaal Craton showing the terrain boundaries and the location 

of different greenstone belts (From Poujol et al., 2003). ..................................................................... 14 

Figure 5: Generalized stratigraphy of the Transvaal Supergroup in the Northwest Province and 

Crocodile River Inlier Stratigraphy (after Hartzer, 1995). ..................................................................... 15 

Figure 6: Regional Geological map of the Transvaal basin of the Transvaal Supergroup illustrating the 

Assen Iron Ore deposit in the Crocodile River Inlier (Modified after Eriksson et al., 2006; Netshiozwi, 

2002). .................................................................................................................................................... 16 

Figure 7: Schematic N-S cross-section through the Assen ridge showing different lithologies (From 

Kevin, 2015). ......................................................................................................................................... 19 

Figure 8: The stratigraphy of Assen Iron Ore deposit showing different lithologies (Mineral 

Corporation, 2012). ............................................................................................................................... 20 

Figure 9: Geology of the Crocodile River Inlier showing the east-northeast trending syncline through 

the western portion of the Assen Iron Ore mining license - marked in red. (from Hartzer, 1989). ..... 21 

Figure 10: Tectonic development of the Crocodile River Inlier. In (A) shows deposition of the Pretoria 

Group and folding along pre-existing structural lineaments. In (B) Bushveld Complex intrusion 

deformed the Inlier. In (C) two major faults caused the Inlier doming against the surrounding rocks 

(after Hartzer, 1995). ............................................................................................................................ 22 

Figure 11: The methods and procedures followed in conducting the studies. .................................... 25 

Figure 12: Regional geological map of Crocodile River fragment showing the location of Assen area 

and the geological structures within the area (Modified after Hartzer, 1995). ................................... 30 

Figure 13: Three synform where a large volume of hematite ore is concentrated at Assen Iron Ore 

deposit. ................................................................................................................................................. 33 

Figure 14: The geological map of the Assen iron ore deposit showing three mineralised portions at 

Assen. .................................................................................................................................................... 36 

Figure 15: The lithostratigraphy of the Assen Iron Ore deposit the Penge Iron formation rocks 

intruded by the dolerite sills. ................................................................................................................ 37 

Figure 16: Boreholes showing the alternating lithologies of the drill holes that created an alternating 

succession. ............................................................................................................................................ 40 

Figure 17: The 3D implicit lithological model showing distinctive hematite mineralisation trends at 

Assen Iron Ore deposit. The hematite ores are parallel to bedding and located close to the contact of 

dolerite and shale in both west, middle, and east orebody. ................................................................ 40 

Figure 18: Univariate statistics and histograms of major, minor elements, and specific gravity of high-

grade hematite at the west orebody .................................................................................................... 43 

Figure 19: East-west view of Fe (%) distribution on east and middle block model all Domains .......... 45 

Figure 20: Isometric view of geological domains in east and middle orebody ..................................... 45 

Figure 21: East to west view of Fe (%) distribution - West orebody block model of all domains ........ 46 

Figure 22: Omni-directional Variogram - Fe- high-grade hematite domain at the west orebody ....... 47 

Figure 23: Field observations from the Pit1 and Pit 2 at the middle orebody of the Assen Iron Ore 

deposit. A) Approximately 2.5 m of shallow dipping hematite layer underlain by the dolerite. B) 



viii 
 

North-South cross-section of the outcropping shale, hematite, and dolerite dipping to the north at 

an angle of approximately 60ᵒ at pit 2 in the middle orebody. C) Pit 1 benches affected by the 

Northeast-southwest fault and the wavy hematite located above the dolerite. ................................. 50 

Figure 24: Photographs of representative rock samples from the west, middle, and east orebody at 

the Assen Iron Ore deposit representing the available iron ore facies. The ore types gradually grade 

from one ore type to another. The Carbonaceous facies is represented by A, B, and C. The sample A 

and B shows the alternating bands of hematite and calcite. Sample C is very rich in calcite having 

large crystals of calcite and some patches of hematite in between the crystals.  BIF is represented by 

samples D and E, where BIF D upgrades to BIF E, with increasing Fe and decreasing SiO₂. Sample F is 

a Friable goethite-rich hematite. Samples G and H are laminated hematite, micro-banding in H while 

G has thick bands of 0.5 mm hematite with goethite and Chert matrix between the hematite bands. 

Massive hematite is I, which is a bluish-grey, shiny, and flinty fine-grained hematite. ....................... 52 

Figure 25: Semi-quantitative mineralogy in Vol.% of selected iron ore and dolerite samples from the 

diamond drilled hole. ............................................................................................................................ 53 

Figure 26: The reflected light image of different iron ore types (A-G): BIF (A, B, and C), calcitic 

hematite (D), goethite (E), laminated hematite (F), massive hematite (G) and dolerite (H). .............. 54 

Figure 27: Element’s correlation graphs in different iron ore lithologies representing: A. Goethite-rich 

hematite, B. Laminated hematite, C. Massive hematite, and D. Calcitic hematite. ............................. 58 

Figure 28: Dolerite Vs hematite graph showing the high Fe concentration towards the dolerite 

contacts. Red dots indicate the dolerite, green dots indicate the dolerite/hematite transaction, blue 

dots indicate hematite. Hematite (% Fe) shows an increase when it is near the dolerite and 

decreases as it moves further away from the dolerite. ........................................................................ 59 

Figure 29: The stratigraphy of different boreholes located at Assen Iron Ore showing the location of 

hematite (red) in relation to the dolerite (green). ASNRC 001, the hematite is located on the dolerite 

footwall, PB 006- high-grade hematite is in the hangwall of the dolerite, In SH 015 and AMI 001- The 

hematite is located both at the hangingwall and footwall of the dolerite sill branches, in A 061 thin 

hematite is associated with shale and in A61, no hematite intersected. ............................................. 62 

 

 

 

  



ix 
 

List of Tables 
Table 1: The Assen logged drill hole rock units and their relative time scale in relation to 

corresponding regional geological units. The logged lithologic units belong to the Chuniesport Group 

of the Transvaal Supergroup and the dolerite sills of the post karoo age. Table compiled from 

different sources such as Button (1981); Eriksson et al. (2005; 2006; 2011); Fockema (1945); 

Hammerbeck et al. (1976); Hartzer (1987;1989;1995). ........................................................................ 32 

Table 2: Table providing a summary on geo domains modelled in this project. .................................. 41 

Table 3: Block model variables used for both west, east, and middle orebody ................................... 42 

Table 4: The variogram modelling parameters. .................................................................................... 44 

Table 5: Block validation statistics: Block estimates vs Composite’s statistics in west orebody high-

grade hematite domain ........................................................................................................................ 48 

Table 6: Block validation statistics: Block estimates vs Composite’s statistics in west orebody low-

grade hematite domain ........................................................................................................................ 49 

Table 7: Block validation statistics: Block estimates vs Composite’s statistics in west orebody calcitic 

hematite ................................................................................................................................................ 49 

Table 8: The exploration borehole SH 015 XRF analysis for different types of hematite with depth. . 57 

 

List of tables of the appendices: 
Table A. 1. Assen Iron Ore compliance to the QAQC basic principles .................................................. 71 

Table B. 1: Block validation statistics: Block estimates vs Composite’s statistics in west orebody high-

grade BIF domain .................................................................................................................................. 74 

Table B. 2: Block validation statistics: Block estimates vs Composite’s statistics in east orebody high-

grade hematite domain ........................................................................................................................ 74 

Table B. 3: Block validation statistics: Block estimates vs Composite’s statistics in middle orebody 

high-grade hematite domain ................................................................................................................ 74 

Table B. 4: Block validation statistics: Block estimates vs Composite’s statistics in middle/east 

orebody calcitic domain ........................................................................................................................ 75 

Table B. 5: Table: Block validation statistics: Block estimates vs Composite’s statistics in middle 

orebody high-grade BIF domain ........................................................................................................... 75 

 

 

  



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List of abbreviations and acronyms  
 
Avg. dist Average anisotropic distance to samples 

BIC 

BIF 

Bushveld Igneous Complex 

Banded Iron Formation 

Block var Block variance 

CRM Certified Reference Material 

CSIR Council for Scientific and Industrial Research 

EDS Energy-Dispersive Spectroscopy 

EDX Energy-Dispersive X-rays 

EMPA Electron Microprobe Analysis 

Fm Formation 

g/t grams per metric ton 

GIF Granular Iron Formation 

GPS Global Positioning System 

HG High-grade 

krig eff Kriging efficiency 

krig var Kriging variance 

Lg Lagrange multiplier 

LG Low-grade 

Neg weight Number of negative weights 

Num dh Number of drill holes 

Num samp Number of samples 

QAQC Quality Assurance and Quality Control 

RIF Rapitan Iron Formation 

SABS South African Bureau of Standard 

SANAS South African National Accreditation System 

SG Specific Gravity 

USGS United States Geological System 

wt.% Weight Percentage 

XRD X-ray Diffraction 

XRF X-ray Fluoresces  

List of abbreviations of rock types, minerals and elements  
 

Am Amphibole 

BIF Banded Iron Formation 

Al2O3 Aluminium Oxide 

Ca Calcium  

CaCl2 Calcium Chloride 

Cal Calcitic hematite 

CaO Calcium Oxide 

Chl Chlorite 

Db Dolerite 



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Dol Dolomite 

Fe Iron 

Fe2O3 Hematite 

Fe3O4 Magnetite 

FeOOH Goethite 

Fsp Feldspars 

H2 Hydrogen 

H2S Hydrogen Sulfide 

K2O Potassium Oxide 

Ls Limestone 

Mg Magnesium  

MgO Magnesium Oxide 

Mn Manganese 

MnO Manganese Oxide 

NaCl Sodium Chloride 

P Phosphorus 

Qtz Quartz 

Sh Shale 

  

 

 

 

 

 



1 
 

CHAPTER 1 

1. Introduction 
 

1.1 Background  
 
Banded Iron Formation (BIF) hosts the major world high-grade iron ore deposits, primarily in the 

form of hematite (Smith and Beukes, 2016). To date, hematite remains the main source of iron ore 

for the world’s steel industries, contributing approximately 98 % in steelmaking (Shaltami et al., 

2020). The world steel demand increased by 3.9 % in 2019 and was expected to increase by 1.7 % in 

2020 mainly due to growth and high demand in developed and developing economies (USGS, 2019). 

The grade of iron ore deposits ranges from 20 to 30 % for low-grade deposits and 60 to 70 % for 

high-grade deposits (Slack and Comtois, 2016). The acceptable iron ore grade recommended globally 

in steel industries for making pig iron ranges from 58 % to 65 % iron for countries having high-grade 

iron ore reserves of 50 % Fe and above. The depletion of high-grade hematite iron ore reserves and 

declining ore grade in the United States of America and high demand for iron ore in China have led 

to low-grade iron ore processed. In 2019, Australia was the largest iron ore producer contributing 37 

% of world supply, followed by Brazil at 16 % and China at 12 %. South Africa is ranked the 7th in the 

world, contributing an average of 3 % per annum to the global output (USGS, 2020). 

 

Known iron ore mining activities in South Africa started between 1 200 AD and 15 000 AD and was 

led by the Khoisan people in Northern Cape and Thabazimbi area (Cairncross et al., 1997). South 

Africa host and produce iron ore from Palaeoproterozoic iron formations (approximately 2.2 to 2.0 

Ga) of the Transvaal Supergroup in the Griqualand West and the Transvaal basins. The major 

deposits occur in Asbesheuwels Subgroup iron formation, Rooinekke Iron Formation of the Koegas 

Subgroup, the Hotazel Iron Formation of the Voëlwater Subgroup, the Penge Iron Formation of 

Chuniespoort group, and Kuruman Iron Formation of the Asbesheuwels Subgroup (Beukes et al., 

2003). In 2018, South Africa had approximately 770 million tons of known iron ore reserves which 

were approximately 0.9 % of global reserves. In the same year, South Africa produced approximately 

81 000 million metric tons of iron ore. The current study focuses on the Assen Iron Ore which has a 

total of 30 million tons of iron ore, and it is currently the 3rd Iron ore producer in South Africa. The 

deposit falls under a class of Superior-type Iron Formation, and it is part of the Transvaal Basin, 

located in the Northwest Province ± 60 km south of Thabazimbi mining district. 

 

It is now well known that the Superior-type represents one of the best-preserved, chemically 

precipitated iron formations formed in marine continental shelves and in shallow basins (Smith and 

Beukes, 2016). This class of iron formations is commonly interlayered with other sedimentary or 

volcanic rocks such as shale and tuff. Most Superior-type BIFs formed during the Paleoproterozoic 

age, between 2.5 and 1.8 Ga years ago. In South Africa, the Penge Formation is widely distributed 

within the Transvaal Basin. This basin is developed on the Kaapvaal Craton which has been 

tectonically stable for at least 2.5 Ga.  The Transvaal Supergroup has an age between 2.1 and 2.3 Ga 

of years which is comparable to the formation age of Lake Superior iron ore deposits.  The economic 

iron ore occurs in the basal Penge Formation, immediately above the lowest shale unit, a 10 m thick 



2 
 

chert-rich band that immediately overlies the thick dolomite and chert succession of the Malmani 

dolomite. 

 

All high-grade hematite iron orebodies in the world have similar characteristics in terms of origin and 

mode of formation. The differences in qualities and quantities of hematite are due to the intensities 

of supergene modification and hydrothermal fluid interaction with the host rock BIF (Smith and 

Beukes, 2016).  The BIF that constitutes the Penge Formation in the Assen Iron Ore Deposit 

comprises several stratigraphically conformable lithological subdivisions. 

 

In this study, detailed geological and structural mapping over the Assen Iron Ore deposit with the aid 

of drill hole data to produce an integrated 3D geological model encompassing the orientation and 

geometry of the iron ore deposit was undertaken. The mineralogical and geochemical characteristics 

of different iron ore facies were determined using Thermo Fisher’s ARL Advant'X family of X-ray 

fluorescence (XRF) sequential spectrometers, Quantitative X-ray powder diffraction (XRD), and 

reflected light microscopy. Lastly, lithostratigraphic and litho-geochemical correlations between iron 

ore grade and dolerite intrusions are done on the premise of localised secondary Fe enrichment. 

 

 

1.2 Aims and the objectives of the research.   
 

The Primary aim of this study is to investigate the relationship between dolerite intrusions and iron 

ore grade. Although much of the Fe enrichment was brought by supergene enrichment, it is 

hypothesised that the intrusion of dolerite dykes and sills led to localised post-depositional 

alteration which upgraded the Fe grade. 

 

In order to fulfil the aim of this study, the following objectives were addressed:  

• Detailed field mapping in order to acquire field relation data between dolerite intrusions and 

hematite ore. 

• Determination of Fe and associated major elements distribution on pristine ore and proximal to 

the dolerite intrusions; and 

• Petrographic and mineralogical studies of both dolerite and Fe ores in order to understand their 

composition, association, and paragenesis.  

 

1.3 Scope of the study 
 

The Assen Iron Ore deposit occurs in the Crocodile River Fragment, an inlier of Transvaal Sequence 

rocks surrounded by younger alkali rocks of the felsic Lebowa Granite Suites (2.052 Ga) of the 2.060 

and 2.050 Ga Bushveld Igneous Complex. The mafic phase of the Bushveld Complex is not in direct 

contact with the inner inliers, and this is confirmed by geophysical data indicating that the Crocodile 

River Inlier is not underlain by the Bushveld Complex (Hartzer, 1989). The Assen Iron Ore deposit has 

three adjacent and disconnected hematite-bearing lenses which are developed along the crest on 

the farms Assen 140 JQ, Assen 161 JQ, Buffelspoort 149 JQ, Doornkloof 141 JQ, and Swarthoek 10 JQ 

halfway between Brits in the Northwest Province and Thabazimbi in the Limpopo Province of South 



3 
 

Africa. The deposit is along the east-west trending ridge of hills southeast of the confluence of the 

Pienaars and Crocodile River. 

 

Commonly, oxygen isotopes are used to differentiate between the supergene and hypogene iron 

ores because the δ18O composition of hydrothermal iron ore deposits range between +0.9‰ to -

7.3‰ while in supergene iron ore deposits the δ18O composition range between +2.0‰ and -3.9‰ 

(Gutzmer et al., 2006). In this study, stable isotopes are excluded and fall outside the scope of the 

study. The microthermometric studies of fluid inclusions to distinguish the type of fluids involved in 

the iron formation were also beyond the scope of this work. The research covers the mineralogy, 

geochemistry, and textural characteristics of both the iron formations and the dolerite intrusions. 

Conventional XRF geochemistry, scanning electron microscopy studies, X-ray diffraction (XRD) were 

used to analyse the chemistry, mineralogy, and texture. The geochemical studies focused on 

analysing major elements in both Iron Formations facies and the dolerite intrusions. The trace 

elements and Rare Earth Elements (REE) were not covered in this study. 

 

 

1.4 Previous Work 
 

The host rocks of the Assen Iron Ore deposit are contained in the lower portion of the Penge 

Formation of the Transvaal Supergroup, close to the contact with the underlying dolomite of the 

Malmani Subgroup. The iron ore deposit is found in the northern margin of Transvaal Basin, ± 60 km 

south of Thabazimbi mining district and the geological sequence lies within the Crocodile River Inlier. 

Several genetic models have been proposed by different authors as having been responsible for the 

formation of high-grade iron ore deposits from the chemical sedimentary rocks, namely, supergene 

(Wagener, 1921), hydrothermal (Netshiozwi, 2002; Gutzmer et al., 2015; Beukes et al., 2003) and 

supergene-modified hydrothermal (Van Deventer et al, 1986; Strauss, 1964; Du Preez, 1944) and 

magmatic model (De Villiers, 1944). This research will attempt to evaluate the possible link between 

the high-grade hematite and dolerite intrusions, the geochemistry of both the iron ore facies and 

dolerite intrusions, and the process that lead to the transformation and enrichment of low-grade BIF 

to high-grade hematite. 

 

1.4.1 The depositional model of the BIF 
 

The genesis of BIF in the Transvaal Supergroup in South Africa and the Dales Gorges Member of the 

Brockman Iron Formation at Hamersley Group in Australia was studied recently using the Fe and Si 

isotope composition of coexisting mineral phases by Steinhoefel et al. (2010). Their finding from the 

samples taken suggests that there was a high concentration of Fe and Si in seawater, ± 6 to 10 ppm 

ferrous iron (Fe+2) (Klemm, 2000) and these elements precipitated in a steady-state large ocean 

basin. They also suggested that chert may also be coming from Precambrian marine basins as 

depicted by heavy Si isotopes and Fe sink containing heavy Fe isotopes might have been pelagic 

sediments.  

 

Lascelles (2007) came with the model for the origin of BIFs and suggested that hydrothermal fluids 

enriched with iron and silica inflow into the oceans as black smokers to mid-ocean ridges and 

continental margins were after cooling and spreading ferrous iron (Fe2+ ) in solution and hydroxide 



4 
 

(OH− ) particles, reacts with dissolved silica to form hydrous Al-poor iron-rich silicates ((Fe,Mg)SiO₃), 

and together with excess iron precipitated as hydrous oxides form mounds around vent chimneys. 

Bekker et al. (2010) proposed that submarine volcanism was responsible for generating a large 

amount of ocean and basin-scale anoxia by venting more fluxes of reductants hydrogen (H₂), 

hydrogen sulfide (H₂S), iron II (Fe+2), and manganese III (Mn+3) in BIFs formed during Precambrian 

and Phanerozoic times.  

 

1.4.2 High-grade iron ore genetic models 
 

According to Hagemann et al. (2016), the BIF of 30 to 35 % Fe is enough to result in high-grade 

hematite and goethite-rich deposits that can contain 55 % Fe and above. In their report, they stated 

that BIFs may be transformed in the process of interaction with hot fluids circulating and channelled 

along geological structures such as faults and more permeable, interbedded horizons such as 

dolomite during deformation. This may remove large volumes of silica, resulting in a concentration 

of iron. Iron, in the form of microplaty hematite, can also crystallise in structurally controlled sites 

such as fold hinges and along detachment faults. In the Crocodile Inlier, the intrusion of the Bushveld 

Complex may have been a driver in providing heat to circulating fluids within the Transvaal 

Supergroup. 

 

1.4.2.1 Supergene enrichment  
 

Fluids responsible for the formation of high-grade BIF-hosted iron ore are classified into 1. Silica 

undersaturated alkaline fluids for dissolving chert from BIF and to form metasomatic carbonate and 

2. oxidized fluids that result in the oxidation of magnetite to hematite (Hagemann et al., 2016). The 

two processes that lead to the formation of supergene mimetic and supergene lateritic iron ore 

deposits were defined by (Ramanaidou and Morris, 2015). To differentiate the two, Ramanaidou and 

Morris (2015) stated that the supergene mimetic process happens where goethite pseudomorphs, 

the primary gangue minerals conserving all the original BIF textures and supergene lateritic process 

disbands the BIF constituents in a destructive and subtractive process that removes the gangue and 

reprecipitates the iron mostly as aluminous goethite and hematite with original BIF textures lost. 

 

According to Smith et al. (2016), supergene iron ore deposit occurs because of chert leaching from 

the BIF during supergene alteration. A classic example is found in the Transvaal Supergroup in the 

Northern Cape Province within the Maremane Dome, South Africa. Strauss (1964) suggested that 

metasomatic alteration of BIF was followed by supergene processes which resulted in the formation 

of the iron-rich deposits (Fig. 1). An earlier study by Van Deventer (1985) suggested that the ores 

reflect initial chemical sedimentation, followed by metamorphism and supergene enrichment of the 

iron content of the ores. If there is an association of the alteration of the Penge Formation with the 

placement of the underlying Bushveld Igneous Complex, this has not been established. The 

orebodies are intruded by dolerite sills like those identified at Assen.  

 



5 
 

 
Figure 1: Classification of major types of high-grade hematite ore deposits and their model of enrichment (Beukes et al., 

2002). 

 

1.4.2.2 Hydrothermal processes 
 

The Thabazimbi Iron ore deposit at the Penge Iron Formation in South Africa is known to be a 

hydrothermally enriched BIF-hosted iron ore. The orebody originated from the interaction between 

BIF with a hydrothermal fluid of meteoric origin at temperatures of not more than 160°C to 200°C 

(Netshiozwi, 2002). Cooling of hydrothermal fluid is thought to have resulted in the formation of 

hematite, as the process does not require higher oxygen fugacity compared to magnetite formation. 

Ore formed from silica leaching from chert, dissolution, and recrystallisation of ferrous iron minerals 

like siderite, ankerite, and iron silicates into microplaty hematite, magnetite, and martite (Beukes et 

al., 2003). Netshiozwi (2002) concluded that the high-grade hematite mineralisation is due to 

hydrothermal processes as verified by ore petrography, fluid alteration, fluid composition, and 

stable isotope geochemistry. The hydrothermal alteration zonation, high and low temperature, and 

salinity of fluid inclusions in hematite analysed samples in some major iron ore deposits around the 

world implies that fluid channelled through geological structures such as Faults and hydrothermal 

processes resulted in the alteration and conversion of BIF to high-grade hematite (Hagemann et al., 

2007). 

 

Recent studies done by Basson and Koegelenberg (2016) showed that structural control on 

hydrothermal or hypogene hematite mineralisation at the Penge Iron Formations within the 

Thabazimbi Iron Ore deposit resulted in the high-grade hematite at 60 % Fe.  The study showed that 

the high-grade hematite mineralisation is mostly limited to sheared detachment zones at the 

contacts between Malmani dolomite, shale, and BIF (Fig. 1). These detachments acted as a structural 

control on fluid flow where mineralisation occurs, especially in fold limbs and along the main east-

west to east-northeast trending fold and thrust Belt, conjugate shear joints, normal and reverse 

faults, and thrusts. Dolerite sills and dykes also intrude and crosscut the iron formations at Assen 

Iron Ore mine. Stratigraphically controlled hematite occurs along dolerite dykes and sills and along 

black shale in Penge Iron Formation where the Assen Iron Ore deposit is located and in the 

Hamersley Province (Taylor et al., 2001). Iron, in the form of microplaty hematite, can also crystallize 



6 
 

in structurally controlled sites such as fold hinges and along detachment faults. In the Crocodile Inlier 

where the Assen Iron Ore is located, the intrusion of the Bushveld Complex may have been a driver 

in providing heat to circulating fluids within the Transvaal Supergroup (Netshiozwi, 2002). 

 

1.4.2.3 Supergene-modified hydrothermal 
 

Supergene modifications make the size and quality of the iron orebodies to be large, but not 

important as compared to the hydrothermal ore-forming process (Gutzmer et al., 2015). According 

to Beukes et al. (2003), magnetite and carbonate mineralisation resulted at the initial stage of iron 

ore deposits where the alkaline hydrothermal fluid carried Caᶧᶧ and Mgᶧᶧ. This environment resulted 

in the leaching of silica. High-grade hematite of Penge Formation owes their origin to a Late 

Paleoproterozoic event of extensive oxidative carbonate metasomatism and Gutzmer et al. (2015) 

described its genetic as the process of calcification where ferrous iron in BIF oxidised and residually 

enriched. Hydrothermal fluids are transported along steep, parallel bedding, and fault structures. 

Alteration effective where basal shale was in direct contact with microbanded carbonate-magnetite 

iron formation. This chert-poor but carbonate-rich iron formation facies was very conducive to 

chemical corrosion and enrichment hard, high-grade martite and martite-microplaty hematite ore 

formed in close association with low-grade carbonate-rich ore in this process. 

 

1.4.2.4 Magmatic model 
 

Although outdated, a study by De Villiers (1944) regarding the Thabazimbi iron ore deposit genetic 

model did not agree with the theory that the iron was leached out of the BIFs or in any association 

with the underlying dolomite. He concluded that the Iron ore deposit is of magmatic origin and 

connected genetically with the Bushveld Igneous Complex granite intrusion and the deposition of 

high-grade hematite took place when ascending Fe-rich magmatic waters mixed with oxygenated 

meteoric water. The evidence brought forward was the presence of high-temperature manganese 

minerals and the chemical properties of the mineralizing solutions. Studies on the hydrothermal 

controls on iron and lead mineralisation on the northern side of Thabazimbi Iron ore district by 

Möller et al. (2014) showed that high salinity NaCl-CaCl₂-rich brines occurrence showed that the 

fluids originated from deep-buried sedimentary sequence where fluids interacted with host rocks 

and further reacted with the carbonate wall rocks during iron and lead deposition. This ruled out the 

magmatic fluid influx model due to low temperatures, the absence of boiling assemblages, the 

absence of crystals, and the stable isotope signatures of the fluids. 

 

The high-grade deposit model of ore formation remained not well understood and unclear due to 

the overprinting that happened as a supergene modification and alteration. The influence of dolerite 

dykes and sills on iron ore mineralisation and their local relationships to the distribution of iron ores 

needs further investigation. It looks like where some dykes and sills are in contact with the ore, the 

iron ore grades increase. 

 

 

 

 

 



7 
 

1.4.2.5 Importance and benefits of the study 
 

Evaluating the geologic, geochemical, and spatial relationship between the dolerite intrusions and 

the high-grade iron ore and the effects of dolerite intrusions on iron ore grade will give guidance in 

the exploration of new iron ore deposits. According to Netshiozwi (2002), the dolerite dykes and sills 

crisscross the Penge iron formation BIF. The phenomenon where dolerites crisscross the BIF 

occurred also at Crocodile River Inlier where the Assen Iron Ore deposit is located.  

 
The genetic relationship between the dolerite intrusions and the high-grade iron ore will help in 

reviewing the epigenetic ore-forming processes and some genetic models of the high-grade BIF-

hosted ores of the Transvaal Supergroup. The relationship between the two will require a different 

exploration strategy to get the new and undiscovered iron ore reserves. Detailed mineralogical and 

ore genesis studies of different iron ores coupled by X-ray diffraction, scanning electron microscopy 

energy dispersive spectroscopy are critical in developing, processing, and beneficiation mechanisms 

to get the desired iron ore product qualities. 

 

1.5 Statement of the Problem  
 

The research done by Netshiozwi (2002) and Chisonga (2012) on Penge Iron Formation from 

Thabazimbi district and Taylor et al. (2001) at Hammersley iron ore province in Australia detailed 

well the dolerite intrusions association with the hematite, the origin of based on the petrographic 

analysis, fluid inclusion and stable isotope geochemistry studies. Further studies characterised the 

high-grade hematite are associated with shale, normal faults, and folds (Mukhopadhyay et al., 2008; 

Gutzmer et al. 2008). While Hartzer (1987), Hartzer (1989), and Hartzer (1995) documented well the 

geology and tectonic deformation of Crocodile River Inlier where Assen Iron Ore deposit occurs.  

Although the literature on petrogenesis and geochemistry of the major iron ore deposits in South 

Africa are well known, there is no published information on the genetic relationship and 

geochemistry of iron ore facies and dolerite intrusions found in Crocodile River Fragment.  

 

Despite genetic models proposed to have resulted in the formation of high-grade iron ore deposits 

around the world, the processes of high-grade mineralisation vary from deposit to deposit and at the 

Assen Iron Ore mine, the processes of enrichment are not clearly understood. 

 

1.6 Project Methodology  
 

1.6.1 Detailed geological mapping  
 
Geological mapping of the Assen Iron Ore deposit covering the farms Assen 140 JQ, Assen 161 JQ, 

Buffelspoort 149 JQ, Doornkloof 141 JQ, and Swarthoek 10 JQ was undertaken as part of this study. 

All lithological contacts, strikes, and dips of different rock types and all geological structures within 

the area were mapped. Diamond drilling was done within the farms to correlate different lithological 

units and geological structures within the study area. Parallel traverse lines that pass through the 

west, middle and east orebody were done, and geological observations were plotted and 

extrapolated within the mining pit. Detailed 3D geological and structural models of the study area 

were produced to establish any relationship between dolerite intrusions and hematite ore. 



8 
 

 

1.6.2 Chemical analysis using X-ray fluorescence  
 
The existing geological database from diamond drilling core data and some selected reverse 

circulation (RC) was accessed to retrieve chemical assets. A total of 206 holes have already been 

drilled within the middle, east, and west orebody. A total of 196 holes are diamond holes with cores 

that have already been logged. A total of 12 holes are RC drilled holes analysed from the Anglo 

research laboratory and South African Bureau of Standards (SABS) lab. XRF analyses have been done 

for the following major elements: SiO₂, Al₂O₃, Fe, Fe₂O₃, TiO₃, CaO, MgO, K₂O, MnO, P, S. These 

analyses were used to create the Fe assay model at the Assen Iron Ore deposit and to determine 

spatial iron and associated major elements distribution on pristine ore and proximal to the dolerite 

intrusions. 

 

1.6.3 Quantitative X-ray powder diffraction analysis  
 
A total of Twenty-four representative samples were sampled from both west, middle, and east 

orebodies from the mine pits and trenches for both XRF and XRD analysis. The samples were four 

BIF, six dolerite samples, four laminated hematite, six massive hematite, and four calcitic hematite 

samples. Samples of hard massive and laminated hematite, BIF, calcitic hematite, and dolerite 

intrusions were collected from different freshly exposed pits, trenches, and outcrops to understand 

their mineral composition and association. 

 

1.6.4 Reflected light microscopic study. 
 
A different type of BIF and high-grade hematite (hard bluish-grey massive hematite, laminated 

hematite, friable goethite rich hematite, calcitic hematite) and dolerite intrusion were sampled from 

the pits, trenches, and exposed outcrops and sent for petrographic thin sections. They were studied 

under the reflected light in the optical microscope to identify the different types of minerals and 

paragenesis of different iron ore facies, their crystal shapes, and mineral associations.  

 

1.7 Research structure 
 

The research report has six chapters. Chapter 1 is the introductory chapter and gives the context 

that gives rise to the study, the main aim, and objectives of the research. Chapter 2 describes the 

different types of iron ores and their classification, regional geology of Kaapvaal Craton and 

Transvaal Supergroup, and local geology of Penge Iron Formation. Chapter 3 detailed geological and 

structural mapping of the Assen Iron Ore deposit. Chapter 4 covers methods, steps, and analytical 

techniques applied in the petrographic study of different iron ore facies and dolerite intrusions and 

their mineralogy. It also presents the geochemical results. Chapter 5 contains the findings and 

analysis of the results from the techniques applied. Chapter 6 discusses the effects of dolerite 

intrusions on ore mineralisation, the geologic and geochemical character of the iron ore, and the 

concluding remarks of this study. 

 
 



9 
 

CHAPTER 2 
 

2. Literature review 
 

2.1. Iron ore deposits and their classification schemes  
 

2.1.1. Iron ore deposits  
 

Iron ore has been mined for many years from igneous, metamorphic, and sedimentary rocks. The 

most important sedimentary hosted iron ore deposits include the Precambrian BIFs.  The 

Precambrian BIFs occur as an alternating layering of iron and chert or quartz, black bands and clay 

band ore, bog iron ores, and laterites (Mohanta, 2007). Sedimentary-hosted deposits dominated by 

hematite or magnetite bedded in metamorphosed sedimentary BIF account for most of the current 

world production and resources of high-grade iron ore (Clout and Simonson, 2005). Other iron ore 

deposits that contribute 10 % of the current iron ore produced worldwide include magmatic 

magnetitic ore type, volcanic-hosted type, skarns type, hydrothermal type whereas in some other 

deposits, iron is produced as a bi-product (Yiming et al., 2014).  

 

Currently, the Precambrian metamorphosed sedimentary hosted iron deposits (BIF) are the world’s 

major source of hematite and main contributors in terms of production at approximately 90 %.  The 

BIF on its own is not economically viable due to low % Fe (20–35 %) and high SiO₂ (40–50 %) 

(Trendall and Blockley, 1970) as compared to the hematite that they host (52–67 % Fe).  The BIF-

hosted hematite reserves are at approximately 60 % and constitute 70 % of high-grade ore ranging 

from 52–67 %. This type of sedimentary hosted deposit is hosted by chemical sedimentary iron-rich 

rocks. They have deposited in Precambrian time at 3.8 to 2.5 Ga where it reaches a peak and ceased 

at 1.8 Ga (Fig. 2). The process continued again from 0.8 to 0.6 Ga (Klein, 2005). Examples of these 

types of deposits include the Thabazimbi and Sishen iron ore districts in South Africa, the 

Quadrilatero Ferrifero district and Carajas in Brazil, Hamersley iron ore province in Australia, Kursk 

deposit in Russia, Central Province of India, and Anshan-Benxi in China (Yiming et al., 2014). 

 

Iron ore products are classified into four types; namely: 1. High-grade direct shipping ores that 

requires only crushing and grinding and then feed to blast furnace with grades ranging in % Fe from 

55–65 %. 2. Concentrates magnetite ores with 30 % Fe that have undergone a process of crushing, 

grinding, and magnetic separation which upgrades the ore to > 60 % Fe. 3. Pellets where the fines 

resulting from the concentration process or ground ore are agglomerated, mixed with a bentonite 

binder, and burnt in a grate-kiln 4. Sinter, an agglomerated product formed by firing iron ore fines 

and coke plus limestone to produce as a furnace feed. Three sizes of the iron ore products are Lump 

ranging in size from 6.3–31.7 mm and is directly fed to blast furnace, the fine product sinter ranging 

from 0.15–6.3 mm and agglomeration by sintering to be fed in blast furnaces and the third one is the 

Pellets which are < 0.15 mm in size formed by agglomeration and pelletising to be fed to blast 

furnace. 

 

 



10 
 

 

 
Figure 2: Distribution of the abundance of Iron Formations through time (After Klein, 2005). 

 

2.1.2.  Classification of iron ore deposits  
 

This study is focused on Iron Formation-hosted iron ore deposits. The classification of iron ore 

deposits evolved over time as more and more studies were done. One of the earliest classifications 

was done for North American iron ore deposits by Gross (1980) and Gross et al. (1983) where they 

classified them into the Lake Superior type, the Algoma Iron Formation, and Rapitan Iron Formation. 

Hagemann et al. (2016) simplified this classification of Iron Formations and noted that the 

classification is dependent on their textural characteristics and depositional time and geotectonic 

settings of the host Iron formation; namely, BIFs, Granular Iron Formation (GIF), and Rapitan Iron 

Formation (RIF) (Gutzmer and Beukes, 2009). High-grade iron ore deposits are also classified 

according to the genetic model, depositional settings, their textural characteristic, geochemistry, and 

mineralogy with varying grades and physical characteristics (James, 1992; Netshiozwi, 2002; Smith 

and Beukes, 2016; Beukes et al., 2003). 

 

Iron formations are divided into two groups of textures which are BIFs which occurred mostly 

between Archean and earliest Paleoproterozoic time and Granular Iron Formation (GIF) that 

occurred later in Paleoproterozoic successions (Trendall, 2002). Based on the depositional 

environments and associated rock types, BIF is further subdivided into two types: Lake Superior and 

the Algoma Iron Formations (Hagemann et al., 2016). The Lake Superior type is unmetamorphosed 

and undeformed (Lascelles, 2007) and is stratigraphically associated and interlayered with quartzite, 

dolomite, and black shales in near shore continental shelf environments (Hagemann et al., 2016). 

The Hamersley-type BIF host the world’s largest known iron ore deposit. An example of this type of 

Iron Formation in South Africa is found in the Neoarchean to Paleoproterozoic Transvaal Supergroup 

which hosts approximately 2.5 Ga Asbesheuwels and Penge Iron Formations (Smith, 2018). The 

Algoma Iron Formations are highly deformed and metamorphosed (Lascelles, 2007) and in South 

Africa are hosted by Meso- to Neoarchean granite-greenstone belts (Smith, 2018). Their deposition 

is associated with greywacke and volcanic rocks in deep-seated faults and fractures, rift zones, and 

volcanic arcs and may also be formed by exhalative hydrothermal processes (Gross, 1980).  

 



11 
 

The Granular Iron Formations which were deposited between 1.8 and 2.1 Ga are characterised by 

coarse layering and granules of chert and iron oxides. Clout and Simonson (2005) noted three 

primary textural components in Granular Iron Formations which are a framework of clasts, fine-

grained matrix, and cement of minerals filling interstitial voids. Rapitan Iron Formations are limited 

to Neoproterozoic geologic time between 0.715 to 0.58 Ga. In Southern Africa, this type of Iron 

Formation is found in the Gariep Belt of the Northern Cape Province in South Africa, southern 

Namibia, and Damara and Otavi Belts of northern Namibia (Smith, 2018). These deposits are 

believed to have been deposited during the global Sturtian glaciation and are associated with 

glaciogenic sedimentary successions (Hagemann et al., 2016). The iron enrichment was due to either 

supergene or hydrothermal processes that leached SiO₂ and oxidised all Fe-bearing minerals.  

 

Beukes et al. (2003) studied different BIF-hosted deposits around the world which include Sishen-

Beeshoek and Thabazimbi iron ore deposits in South Africa, Quadrilatero Ferrifero and Carajas 

deposits in Brazil, Noamundi and Dalli-Rajhara Iron ore in India, and Hamersley iron ore deposits in 

Australia. From their studies, they classified the deposits into three genetic models which are 

supergene, hydrothermal, and supergene-modified hydrothermal deposits. They stated that in 

supergene iron ore deposits, mineralisation decreases from top to bottom into unmineralised BIF 

and this is represented by the Asbesheuwels Subgroup-hosted deposits at Sishen, Khumani, 

Beeshoek, and Kolomela in the Northern Cape Province of South Africa (Smith, 2018). Hydrothermal 

deposits increase the grade from bottom to top into low-grade BIF and an example is the Penge Iron 

Formations-hosted deposit at Thabazimbi in the Limpopo Province of South Africa. The Quadrilatero 

Ferrifero and Carajas deposits in Brazil have bulk friable saprolitic ores that originated from 

supergene enrichment of earlier hydrothermally and altered hematite and they are a representation 

of supergene-modified hydrothermal deposit (Beukes et al., 2003).  

 

According to Clout and Simonson (2005), for high-grade hematite deposits to form, there must be an 

iron-rich BIF with sufficient composition, right textures, and structural architecture such as faults.  

The major and minor faults must be coupled with regional deformation to permit the fluid flow. The 

enrichment to high-grade hematite from BIF is controlled by the geological structures that channel 

the upward and downward fluid flow through the structures and hypogene alteration leading to the 

deposition and precipitation of various iron ore types.  The enrichment process starts with the 

leaching of silica to form magnetite and carbonate, then followed by the oxidation of magnetite to 

form hematite (Fig. 3) and further dissolving gangue chert, quartz, and iron silicates to form martite, 

microplaty hematite, specular hematite, and carbonate (Hagemann et al., 2016). Although the 

physical characteristics of iron ore vary considerably from deposit to deposit, Netshiozwi (2002) 

studied the mineralogical and physical characteristics of Thabazimbi Iron ore deposit and classified 

iron ores into three types which are hard hematite ore, carbonate ore, and supergene modified ores. 

Like Assen Iron Ore deposit, carbonate hematite occurs at the bottom of the Iron Formations and 

increases the grade to the overlying massive hematite and supergene ore formed closer to the 

surface.  

 

Ramanaidou and Wells (2014) classified the BIF-hosted iron ores into two main types which are the 

bedded iron deposits and detrital iron ore deposits. They noted that bedded iron deposits are strata-

bound iron ore deposits. The detrital iron deposits formed due to weathering, erosion, and 

deposition on the valley or foot of the mountain. Bedded iron ore deposits are further subdivided 



12 
 

into three classes: unenriched primary Iron Formation (30 to 45 % Fe), residual iron ore (56-63 % Fe), 

martite-goethite ores (56-63 % Fe), and hard hematite ores (60-68 % Fe) which include martite and 

microplaty (Beukes et al., 2002; Clout and Simonson, 2005).  The low-grade iron ore mining in China 

is an example of unenriched primary iron formation and includes both magnetite and hematite-rich 

Iron Formation. Iron minerals in unenriched primary iron formations vary in composition and 

mineralogy representing sedimentary variations through geologic time. 

 

Figure 3: The genetic model for the major BIF-hosted iron ores of the world showing the transformation from BIF to 
different iron ore types (modified from Morris, 1998). 

The residual iron ore form because of supergene weathering of BIF (Smith, 2018) that result in iron 

upgrading and generating high quality by dissolving the unstable minerals or gangue by lateritic 

alteration mostly by groundwater to give residual concentrates of Fe oxides also known as blue dust 

ores (Fig. 3) (Morris, 1998) and reprecipitation of secondary iron minerals in the upper part of a 

formation with the original textures changed. The ore quality ranges from a silica-rich (40 %) friable 

BIF to a high-grade above 60 % residues of martite (Morris, 1980). Unlike the residual iron ore, the 

martite-goethite ore is formed by supergene processes, where hydrous iron oxides are upgraded to 

above 60 % Fe (Clout, 2013) with the ore preserve the original texture of the BIF such as banding and 

goethite pseudomorphed microtextures (Morris, 1998) remain unchanged and conserved during the 

process of weathering or metamorphism. The mineral hematite remains unchanged, and the 

magnetite is oxidised (Fig. 3) to martite, and impurities like chert, silicates, and iron-rich carbonates 

are replaced or leached out. Al-rich silicate is also leached out and replaced by clay to form shale 

bands, and phosphorus levels range from around a moderate 0.07 % to an extremely high 0.17 % P. 

 

The hard high-grade hematite which includes martite-hematite and microplaty hematite has greater 

than 60 % Fe with less than 15 % goethite and originated from hypogene or metamorphic process 

overprinted by supergene enrichment (Clout, 2013). Hagemann et al. (2016) classified the hematite 



13 
 

and magnetite-based iron ores into microplaty hematite with little or no goethite, martite hematite, 

martite-goethite, granoblastic hematite, specular hematite, and magnetite into magnetite-martite, 

magnetite-specular hematite, and magnetite-amphibole. The microplaty hematite is composed of 

microplates of hematite with variable porosity and hardness (Clout and Simonson, 2005).  Primary 

BIF relationships and enrichment to different high-grade hematite were summarised by Clout and 

Simonson (2005) as follows chert or carbonate layers are replaced by microplaty hematite to form 

microplaty hematite ores, magnetite layers by martite hematite, and Al silicate bands by shale or 

clays. Martite-goethite ore occurs when BIF chert or carbonate layers are replaced by goethite 

disseminated martite, magnetite layers by martite, and Al silicate by shale. Other types of hematite 

ores such as itabirite and friable hematite are formed by the leaching of chert or carbonate layers 

from BIF due to residual accumulation of friable hematite-residual quartz, magnetite, hematite, and 

Al silicate layers. 

 
Iron ore is also classified in terms of the ore texture of which the main textural types are massive, 

laminated, brecciated and conglomeratic, and carbonate hematite (Smith and Beukes, 2016). All 

these petrographic characteristics can be found or associated with all the genetic models discussed 

above. The laminated ore forms from both supergene and hydrothermal enrichment from 

microbanded BIF while massive ores are enriched from clastic textured muddy BIF called lutite 

(Beukes and Gutzmer, 2008b) and the three main textural occurrences are microplaty hematite, 

patchy hematite, and hematite pseudomorphs of magnetite (Smith and Beukes, 2016). 

 

The detrital iron ore is formed by physical weathering, erosion, and the deposition of colluvial iron 

ore at the valley or foot of the mountain slopes. It can also be the deposition of scree material in an 

escarpment or syncline sourced from the iron formation. The top part of detrital iron ore is mostly 

unsorted detritals which comprise BIF clasts underlain by well sorted and rounded to subrounded 

detritals which consists of hematite. Detrital iron ore also occurs as unconsolidated or is cemented 

as a hematite conglomerate with pebbles ranging from coarse to boulder-sized material 

(Ramanaidou and Wells, 2014). 

 

2.2. Regional Geology  
 

2.2.1. Kaapvaal Craton  
 

The Kaapvaal Craton is the most stable, oldest, and well-preserved Archaean continental crust 

and covers a larger portion of the northern part of South Africa, Swaziland, and a portion of eastern 

Botswana (Bumby et al., 2011). The Craton covers an area of 10⁶ Km² (Fig. 4) and is surrounded by 

The Proterozoic Namaqua-Natal Mobile Belt and the Lebombo monocline which form the eastern 

and southern border, the Kheis overthrust belt form the western boundary while the Mozambique 

Belt boarders the northern side and is also separated from the Zimbabwean Craton by the Limpopo 

Belt (De Wit et al., 1992). The Kaapvaal Craton is made up of a 3.6–2.6 Ga granite-greenstone 

basement overlain by 3.1–2.6 Ga Neoarchaean to Palaeoproterozoic volcano-sedimentary cover 

sequences covering a larger portion of the craton. The Craton evolved between 3.0 and 1.8 Ga 

resulting in the development of five orderly stratigraphic successions of sedimentary basins which 

are the Dominion Group, the 3.1–2.8 Ga placer gold-bearing, and the oldest known depository 



14 
 

Witwatersrand (Eriksson et al., 2009), Ventersdorp, Transvaal and Waterberg Supergroup 

respectively.  

 
Figure 4: The geological map of the Kaapvaal Craton showing the terrain boundaries and the location of different 

greenstone belts (From Poujol et al., 2003). 

Eglington and Armstrong (2004) showed that the development of new crust occurred in the south-

eastern, eastern, northern and the central zone of the Limpopo Mobile belt of the Kaapvaal Craton 

are connected by granitoid intrusions between 3.25 Ga and 3.1 Ga. The numerous igneous activities 

along Colesberg lineament and Thabazimbi-Murchison lineament at approximately 3.1 Ga to 2.8 Ga 

acted as suture zones (Anhaeusser, 2006) where younger domains were accumulated and 

amalgamated during the formation of the Kaapvaal Craton. By approximately 3 Ga, the lithosphere 

was stable, allowing the deposition of Dominion, Witwatersrand, and Pongola sedimentary basins. 

The massive volcanism and granitoid throughout the Craton followed together with the deposition 

of Ventersdorp. At approximately 2.6 Ga the sedimentation of the overlain Transvaal Supergroup 

occurred, and the volcano-sedimentary rocks of this basin acted as floor rocks for the Rooiberg 

felsites which resulted from the massive magnetic event of the Bushveld Igneous Complex (2.06 Ga).  

the latter was followed by the deposition of sediments comprising the Waterberg and Soutpansberg 

Groups (Poujol et al., 2002; 2003).  

 

2.2.2. Transvaal Supergroup  

 
The 2.7 Ga to 2.05 Ga Transvaal Supergroup rocks (Eriksson et al., 2005) cover approximately 

250 000 km² (Button, 1981) which is exposed and preserved in three separate basins which are the 

Transvaal basin in the northern side of Kaapvaal Craton, the Griqualand West basin found in the 

south-western part of the Kaapvaal Craton and Kanye basin in the western part of the Transvaal 

Supergroup in Botswana (Eriksson et al., 2006). The Transvaal basin and the Griqualand West basin 

are separated by major basement high, north-northwest trending Vryburg arch (Moore, 2001). The 

Transvaal succession’s thickness is up to 15 km (Button, 1981) of unmetamorphosed volcanic, clastic, 

and chemical sedimentary rocks (Eriksson and Clendenn, 1990). This Supergroup is underlain by 



15 
 

Witwatersrand and Ventersdorp Supergroup.  The Griqualand West basin is further subdivided into 

Ghaap Plateau and Prieska sub-basins. 

 

Eriksson et al. (2001, 2006) detailed the stratigraphy of the Supergroup and classified it into four 

stratigraphic units (Fig. 5), from bottom to top: 1. Protobasinal rocks comprise volcanic and 

immature siliciclastic sedimentary rocks 2. Black Reef Formation of approximately 30-60 m (Eriksson 

et al., 2011) made of thin and sheet sandstones formation (Eriksson and Reczko, 1995) 3. 

Chuniespoort Group is less than 2 km thick and composed of a chemical sedimentary succession of 

dolomites with chert in some areas and limestone of Malmani Subgroup carbonates, Penge 

Formation BIFs, and shaley Duitschland Formation.  4. Pretoria Group is composed of interbedded 

mudrocks, quartzite, and sandstones with some basaltic-andesitic lavas (Eriksson et al., 2005).  

 

 
Figure 5: Generalized stratigraphy of the Transvaal Supergroup in the Northwest Province and Crocodile River Inlier 

Stratigraphy (after Hartzer, 1995). 

The number of structural domains has been observed and studied (Button, 1981). In the Transvaal 

basin five rocks are surrounded by approximately 2.06 Ga intrusive Bushveld Complex and their 

occurrences are known as the Rooiberg, Crocodile River, Stavoren, Marble Hall, and Dennilton Inliers 

(Hartzer, 1995).  The floor-attached domes and roof pendants-the Crocodile River and Marble Hall-

Dennilton inliers are the attached structures that have remained attached to the floor of the 

Transvaal Basin (Hartzer, 1995). Their deformation pattern reflects regional deformation patterns 

observed outside of the Bushveld Complex, which were distorted by the emplacement of the 



16 
 

Bushveld Complex, and the Stavoren and Rooiberg fragments which were detached and are 

underlain by Bushveld Complex granites (Eriksson et al., 2006). Only the Carbonate-BIF succession of 

the Chuniespoort Group, the Crocodile River Inlier, and the Penge Iron Formation which host the 

Assen Iron Ore deposit will be discussed in this section:  

 

2.2.2.1. The Crocodile River inlier 
 

The Crocodile River inlier is located at the western part of the preserved Transvaal basin (Hartzer, 

1995). The Assen Iron Ore deposit occurs in the Crocodile River Fragment, an inlier of Transvaal 

Sequence rocks surrounded by younger alkali rocks of the felsic Lebowa Granite Suites of the 

Bushveld Igneous Complex (Fig. 6). The Bushveld Complex intrudes into the Transvaal Supergroup as 

bimodal intrusive magmas of mafic and felsic characteristics, with the Magaliesberg Formation of 

the Pretoria Group forming the floor of the Complex in most areas. The mafic phase of the Bushveld 

Complex is not in direct contact with the inner inliers, and this is confirmed by geophysical data 

indicating that the Crocodile River Inlier is not underlain by the Bushveld Complex (Hartzer, 1989). 

The Crocodile River, Marble Hall, and Dennilton Inliers consist of highly deformed, lower Transvaal 

strata that were subjected to low-grade metamorphism. The domes were formed by interference 

folding that was accentuated by the intrusion of the Bushveld Complex. They acted as physical 

barriers to the emplacement of the mafic rocks of the Bushveld Complex in the centre of the 

Transvaal Basin. The Crocodile River dome is located at the intersection of two major anticlines with 

an east-northeast and a northwest striking fold axis, respectively. 

 

Figure 6: Regional Geological map of the Transvaal basin of the Transvaal Supergroup illustrating the Assen Iron Ore 

deposit in the Crocodile River Inlier (Modified after Eriksson et al., 2006; Netshiozwi, 2002).                                                           

 



17 
 

2.2.2.2. Chuniespoort Group 
 

Stratigraphically the Chuniespoort Group consists of the lower Campbellrand-Malmani carbonate 

succession and the Penge Iron Formation and the upper Duitschland Formation (Eroglu et al., 2015). 

The Chuniespoort group comprises the chemical sedimentary rocks, are exposed throughout the 

Transvaal basin, and varies in thickness up to 3500 m (Button, 1981). A similar stratigraphy is found 

in the Griqualand West basin where these types of rocks occur in Campbell and Griquatown groups.  

 

2.2.2.3. The Malmani Subgroup 
 

The dolomite succession which forms the bottom of the Chuniespoort Group known as Malmani 

dolomite also occurs in Campbell Group in the Northern Cape. The Malmani dolomite is subdivided 

into several units depending on physical and chemical properties such as colour, texture, and 

chemistry of the dolomite and the concentration of chert, shale, and limestone while Grunerite is 

restricted to the Transvaal basin due to metamorphism caused by intrusive Bushveld Complex 

(Button, 1981). A basal Malmani Subgroup has five formations (Fig. 5), namely: Oaktree (10–200 m), 

Monte Cristo (300–500 m), Lyttelton (100–200 m), Eccles (400–600 m), and Frisco (300–400 m) 

(Catuneanu and Eriksson, 1999). The Malmani dolomites had a gradual transition to the overlying 

iron-rich facies of the Penge Formation. The thickness of Penge varies from area to area and has a 

maximum thickness of 2100 m in the Crocodile River Inlier (Eriksson and Reczko, 1995). The Oaktree 

Formation, which is at the bottom, is a transition zone between the Black Reef Formation clastic 

sedimentary rocks and the overlying carbonate rocks. 

 

2.2.2.4. Penge Iron Formation  

 

The rocks of the Penge Iron Formation host economic deposits of both hematite, crocidolite, and 

amosite and are exposed at the contact metamorphic aureole of the Bushveld Complex in the 

eastern Transvaal basin of the Transvaal Supergroup (Miyano and Beukes, 1997) and its thickness is 

approximately 640 m in the Transvaal basin and approximately 2000 m in Northern Cape. The same 

stratigraphic section and correlation occur in the Griquatown Group of the Asbesheuwels and 

Koegas formations in the Northern Cape (Dulski, 1996). 

 

Penge Iron Formation is highly affected by regional contact metamorphism due to the Bushveld 

Complex intrusion resulting in magnetite-quartz concentrations in BIF (Dulski, 1996). Hartzer (1989) 

subdivided the lithofacies of the Penge Formation into four zones and the lithofacies cycle grunerite-

microbanded chert→grunerite–magnetite rhythmite→magnetite–grunerite rhythmite with the 

bottom part of Penge being chert-rich and increases in oxides facies upwards. The BIF depict macro, 

meso, and micro-BIFs (Button, 1981) interbedded with carbonaceous mudstones. The major 

minerals are quartz, magnetite, hematite, stilpnomelane, riebeckite, minnesotaite, grunerite, and 

carbonates minerals of siderite, ankerite, dolomite, and calcite (Beukes, 1973). 

 

Thabazimbi Iron Ore mine is located 60 km to the north-northwest of Assen and has been mining 

iron ore from the same stratigraphy as that of Assen Iron Ore mine. The base of the Penge 

Formation comprises sporadic chert-rich shale, ± 1–2 m thick, which is frequently brecciated due to 

the karstification of the underlying dolomites. This is succeeded by approximately 350 m of overlying 



18 
 

iron-rich rocks. The iron orebodies occur in the basal 80 m thick iron oxide BIF of the Penge 

Formation. The ore is defined as having over 60 % Fe and less than 15 % SiO₂. The ore occurs as 

irregular, tabular bodies over a strike length of 12 km. This directly correlates to the stratigraphy of 

the Assen Iron Ore deposit. With depth, the orebodies pass laterally into a carbonate-hematite rock 

which also directly correlates with the calcitic facies found at the Assen Iron Ore mine. This facies is 

often underlain by a basal shale unit of the Penge Formation which has also been recorded at Assen 

(Van Deventer et al., 1986). 

 

The calcitic iron ore, in all aspects like that, found at Thabazimbi, is also found on Pylkop 26 JQ ± 20 

km from the Assen Iron Ore mine. The ore is situated at the base of the iron formation and is 

underlain by dolomites. It forms a high hill with a steep dip-slope on its southern margin: strike 

length is 240 m; the average width is 45 m, and the reserves are estimated at ± 1.5 Mt per 30 m 

down dip. The ore is finely banded and is composed of alternating layers of hematite and white 

calcite, with practically no chert (Hammerbeck et al., 1976). No high-grade hematite ore and calcitic 

ore are present in a prominent ridge of BIF extending eastwards from Doornkloof 141 JQ to 

Buffelspoort 149 JQ along the common boundary with Assen 140 JQ. A composite sample assayed by 

Mineral Corporation (2012) contained 42.3 % Fe, 35.2 % CaCO₃, and 3.1 % SiO₂. According to Hartzer 

(1987), the carbonate rocks in the Crocodile River Inlier contain mineral assemblages of low to 

medium grade metamorphism with abundant tremolite, occurring in association with talc, calcite, 

quartz, and dolomite. 

 

2.3. Deposit Geology  
 

2.3.1. Lithostratigraphy  
 

The host rocks to the Assen Iron Ore deposit are contained in the lower portion of the Penge 

Formation of the Transvaal Supergroup, close to the contact with the underlying Malmani Subgroup 

dolomite. The geological sequence lies within the Crocodile River Inlier. 

 

The Penge Formation in the Assen area comprises several subdivisions: An upper layer, comprising a 

more typical siliceous BIF (Fig. 7) with a layering of chert and goethite-hematite, partially 

metasomatised and containing primary magnetite in unaltered sections. The upper layer appeared 

visually like amphibolite and contains crocidolite, asbestos, and nontronite mineralisation (Mineral 

Corporation, 2012). These mineralogical observations render the mineralisation like the talc ore 

reported at Khumba Iron Ore in Thabazimbi. The calcitic hematite rock is mostly brecciated in 

appearance and forms prominent outcrops at Assen. This rock type may be a metasomatised 

product of the magnetite-carbonate BIF. Its mineralogical composition appears to be quite 

consistent between boreholes. The low-grade hematite facies is typical of a distal facies iron-rich 

chemical sediment with layers of chert or silica 5–30 mm thick and laminated at millimetre or sub-

millimetre scale. This occurrence has been described by Hartzer (1995) as being analogous to 

Superior-type mineralisation. 

 



19 
 

 
Figure 7: Schematic N-S cross-section through the Assen ridge showing different lithologies (From Kevin, 2015). 

Shales from the Penge Iron Formation are ferruginous indicating that shale deposition occurred in 

iron-rich waters (Dulski, 1996). A poorly exposed basal shale layer consists mainly of chert-rich and 

BIF lenses with pyroclastic and tuffaceous sections. Fockema (1945) stated that shale overlies 

dolomitic limestones and mostly thin layers of fine-grained black shale. At the Assen Iron Ore mine, 

the shale layer becomes amphibole-rich near the basal contact with the Malmani dolomite. 

Concordant to transgressive intrusives of dolerite composition with altered upper zones have been 

observed throughout the area in borehole cores and reverse circulation drilling chips. 

The dolomite presents itself as an irregularly eroded karst sub-surface floor (Fig. 7) upon which a 

well-sorted and winnowed detrital hematite bed has been deposited (Kevin, 2013). The Malmani 

dolomite extends in an undulatory manner two-thirds of the way to the crest of the ridge becoming 

progressively grainier upwards to sandy marl at the contact with the overlying ironstone formations 

belonging to the lower portion of the Penge formation of the Transvaal Supergroup. The dolerite 

dykes and sills crosscut the Iron Formations (Kevin, 2015). Observation in the extraction pits shows 

intense shear lamination parallel to the dolerite intrusion margins with epidote-amphibolite veining, 

as well as blister pitting of the hematite in places giving the material a pig iron appearance. Green 

serpentinite dykes were also encountered within the high-grade hematite zone which contain traces 

of copper, nickel, lead, zinc mineralisation (Kevin, 2013). According to Fockema (1945), Banded 

ironstones and shales in the Crocodile River Inlier are intruded by dolerite sills of thickness less than 

50 m. The sills are altered to hornblende-plagioclase because of metamorphism. 

 
 
 



20 
 

2.3.2. Mineralisation Style 
 

Three types of iron mineralisation have been observed at the Assen Iron Ore deposit. The high-grade 

ore of greater than 60 % Fe, the medium grade ore of 50–60 % Fe is hematite rich with varying 

degrees of goethite oxidation. Massive mineralisation is either laminated (soft and crumbly) or blue-

grey (hard and flinty). These mineralisation types of outcrops close to the crust of the hill. The 

calcitic hematite rock entirely composed of hematite and calcite contains up to 50 % Fe and forms 

prominent outcrops along the steep southern slope of the crest. A magnetite carbonate rock is 

observed at depth only and is considered the protore at Assen. There is a notable absence of 

magnesium content in the deposit. 

 

 
Figure 8: The stratigraphy of Assen Iron Ore deposit showing different lithologies (Mineral Corporation, 2012). 

Three distinct mineralisation types occur in the Assen Iron Ore deposit which is strata-bound to the 

specific host rock types, namely:  

1. Magnetite carbonate,  

2. High and medium grade hematite ore which is goethite/ hematite rich,  

3. The calcitic hematite  

 

In the Mineral Corporation Report (2012), the geological mapping and field observations (Fig. 8) are 

as follows: 

Lying above the high-grade hematite is sedimentary BIF which is goethite or hematite rich and 

typically grades between 35 to 50 % Fe. This facies is termed by the mine as the low-grade BIF. A 



21 
 

high-grade hematite ore directly overlies the calcitic hematite ore.  The mineralisation is either 

laminated or blue-grey in colour which is very hard and flinty. This facies is referred to as high-grade 

hematite ore and typically assays of this ore are greater than 60 % Fe.  The lower unit of the 

ironstone formation comprises heavily calcite-banded hematite which is very hard and resistant to 

weathering (Petzer, 2015). This carbonate rock is the protore at Assen and consists of magnetite, 

hematite, talc, quartz, calcite, and amphibole and is referred to by the mine as calcitic facies. This 

zone is generally intruded by dolerite with parallel shearing.  According to Hammerbeck et al. (1976), 

the Crocodile River Fragment which lies south of Thabazimbi has BIF enriched to high-grade 

hematite ore. On Boschkop 138 JQ three unconnected lenses, 60 to 230 m long and 2.5 to 9 m thick 

contain ore with 65–68 % Fe. The reserves are between 2 and 4 Mt. The presence of high-grade 

hematite orebodies on the ridge is more than 30 m thick. 

 

2.3.3. Geological structures  
 

Folding resulted in the stratigraphic sequence to dip at an angle between 40˚ to 60˚ northwards and 

formations strike east-west at the Assen lease area. This forms the southern limb of an east-

northeast plunging syncline. The more resistant BIF contained iron mineralisation and form the 

prominent ridge along the top of the hills. The Transvaal Supergroup on the Assen Lease area is 

dominated by a large southwest-northeast tending synform (Fig. 9). The entire ironstone unit is 

folded, flexed along the crest of the ridge (Mineral Corporation, 2012).  

 

Figure 9: Geology of the Crocodile River Inlier showing the east-northeast trending syncline through the western portion of 

the Assen Iron Ore mining license - marked in red. (from Hartzer, 1989). 

 

The general geological structures of the Crocodile River Inlier are well documented by Hartzer 

(1989), Hartzer (1987), and Hartzer (1995) as follows: 

The Crocodile River Inlier consists of highly deformed, lower Transvaal strata that were subjected to 

low-grade metamorphism (Fig 9). The domes were formed by interference folding that was 



22 
 

accentuated by the intrusion of the Bushveld Complex. They acted as physical barriers to the 

emplacement of the mafic rocks of the Bushveld Complex in the centre of the Transvaal Basin. The 

Crocodile River dome is located on the intersection of two major anticlines with an east-northeast 

and a northwest striking fold axis respectively. 

  

A major east-northeast striking syncline dominates the central part of the Crocodile Inlier. This is 

manifested as an east-northeast striking ridge comprised of the Penge Formation on the Assen Lease 

area. The metamorphic temperature in the Crocodile Inlier has been estimated by Hartzer (1987) to 

be in the range 410-510° C (Fig. 9). The carbonate rocks in the Inlier contain mineral assemblages of 

low- to medium-grade metamorphism with abundant tremolite, occurring in association with talc, 

calcite, quartz, and dolomite. Hartzer (1989) mapped the two well-known faults (F1 and F2) that are 

striking south-east and north-west and are marked by breccia, Iron Formation, and dolomite. The 

faults displaced the Bushveld Complex and Transvaal Supergroup rocks and can be traced to the 

south as far as Brits Graben. The stratigraphic displacement of the F2 fault is up to 4000 m in the 

south and the amount of throw on both faults decrease northwards. There are also minor faults 

associated with the folding and the strike-slip faults in the southern side of the Fragment. The 

western side of the Fragment is dominated by numerous folds striking to the north-west and the 

basin-shaped structures caused by refolding. The intrusion of the Nooitgedacht Complex into the 

Crocodile River Inlier caused thrusting and faulting resulting in the duplication of the Penge 

Formation. 

 

 
Figure 10: Tectonic development of the Crocodile River Inlier. In (A) shows deposition of the Pretoria Group and folding 

along pre-existing structural lineaments. In (B) Bushveld Complex intrusion deformed the Inlier. In (C) two major faults 

caused the Inlier doming against the surrounding rocks (after Hartzer, 1995). 

Several models for the tectonic development (Fig. 10) of the inliers have been proposed by various 

authors ranging from thrusting (De Waal, 1970), xenolithic emplacement, roof pendants (Verwoerd, 

1963) by the process of cone-fracturing, and even as points of impact from extraterrestrial bodies. 

The rocks of the Penge Iron Formation have been plastically deformed, partially metamorphosed, 



23 
 

metasomatised, and recrystalised due to the proximity to the Bushveld Complex which surrounds 

the Inlier. This has altered the original mineralogy, geochemistry, and visual appearance of the rocks. 

Hartzer (1989) stated that the three stages of folding and major faulting trending north-west to 

south-east caused the complexity in Penge Formations. He recorded that the first stage of folds (F1-

folds) has an axial trend and varies from the northeast in the northern side of the inlier to the 

northwest in the southern portion of the inlier. The refolding of F1 folds occurred along the axis (F2-

folds) and striking east-northeast and resulted in the major syncline in the eastern-central part of 

the fragment. Stage three of folding (F3-folds) is represented by north-west to south-east folds in 

the western part of the inlier and is depicted by major anticline in the south. 

 

2.4. Iron ore and gangue mineralogy 
 

The three most common iron ore minerals are magnetite (Fe₃O₄), hematite (Fe₂O₃), and goethite 

(FeOOH). Magnetite which occurs as fine to coarse-grained euhedral crystals is from the 

sedimentary and magmatic origin, hematite resulting from oxidation of magnetite in the near-

surface environment. The brown and yellow goethite is iron oxyhydroxide and the most common 

iron ore mineral in altered metasedimentary near-surface iron ore deposits. Other Iron ore minerals 

include Maghemite, Kenomagnetite, Martite, Hydrohematite (Bekker et al., 2014). 

The most common gangue mineral in an unweathered iron ore deposit is chert and crystalline quartz 

in metamorphosed deposits (Bekker et al., 2014). The silicate minerals are minnesotaite and 

stilpnomelane while kaolinite and gibbsite dominate altered supergene and surficial iron ore 

deposits. Other silicate impurities include amphiboles and chlorites. Carbonates facies are 

dominated by siderite, ankerite, calcite, and dolomite dominates, sulphides facies gangue minerals 

include pyrite, and in oxides facies is pyrolusite.  

According to Ramanaidou and Wells (2014), the high-grade iron ores required by steel industries 

should have above 55 % Fe, 5 % or less of SiO₂ and Al₂O₃, and less than 0.075 % P with no chlorine 

and sulphur. Phosphorus is one of the most ruinous impurities in iron ore as it forms iron phosphides 

in blast furnaces that cause steel to be brittle. A high concentration of phosphorus has a negative 

impact on the price of iron in the steel industry (Smith, 2018) and the acceptable % is less than 0.08 

wt % P on the iron feed. Ore that forms because of supergene metasomatic enrichment of BIF like 

goethite ore mostly has high levels of phosphorus, Silica, and aluminium and it is difficult to separate 

it during ore processing discarding valuable ore (Pownceby et al., 2019). 

 

The mineralogy of the Precambrian Iron Formations varies depending on three metamorphic grades 

which are diagenetic to very low, medium-grade, and high-grade metamorphism. Regional 

metamorphic grade plays a major role and an increase in metamorphic grade causes the 

mineralogical changes because of high temperature and burial pressure. High temperature is 

associated with minerals such as amphiboles, almandine, pyroxene, and olivine (Ramanaidou and 

Wells, 2014). The mineralogy of Iron Formations is mostly dominated by silica (SiO₂) and various Fe-

rich and Al-poor minerals. The magnetite or hematite forms an alternating band of several 

millimetres with silica microcrystalline bands (Bekker et al., 2014).  

 



24 
 

There are four lithological Iron facies based on the dominant iron mineral present (James, 1992): 

silicate, carbonate, oxide, and sulphide. Oxide facies is the most dominant, largest, most economic 

BIF facies (Lascelles, 2007) and comprises magnetite subfacies or hematite subfacies. The carbonate 

facies consist of siderite or ankerite with minor silicate mesobands and minor oxide laminae. The 

silicate facies Iron Formation is mostly dependent on the degree of metamorphism where low-grade 

metamorphism is associated with minerals like biotite, greenalite, minnesotaite, stilpnomelane, 

chamosite, ripidolite, riebeckite, and ferri-annite and high-grade metamorphism minerals are 

cummingtonite, grunerite, pyroxene, garnet, and fayalite. Sulphide facies are pyritic carbonaceous 

shales or slates resulting from seafloor hydrothermal, iron-rich exhalites, and sulfidic cherts (Bekker 

et al., 2014). 

 

 



25 
 

CHAPTER 3 

3. Samples and methodology 
 

Figure 11: The methods and procedures followed in conducting the studies. 

Figure 11 above summarises the work done which involved the geological mapping fieldwork and 

sampling, the petrographic analysis and geochemical techniques used in the study. The final product 

was to produce a 3D geological, 3D structural model from drilled boreholes. The geochemical and 

petrographic studies were done to give field relationship between dolerite intrusions and hematite 

ore and to determine the Fe and associated major elements distribution on pristine ore 

and proximal to the dolerite intrusions; and to understand their composition, association, and 

paragenesis.  

 

3.1. Sample descriptions 

Five sampling positions were selected in three different pits comprising 24 samples which were 

sampled in the current operating and old pit for different iron ore facies and dolerite intrusions 

perpendicular to the stratigraphic section. The samples were collected in a profile where the full 

stratigraphy from bottom to top is exposed and includes shale, dolerite, calcitic hematite, high-grade 

hematite, and BIF.  The samples were packaged and sent to XRD – Analytical & Consulting for the 

identificat