Effect of Sericitization on the Engineering Properties of the Miango Granite ...
Complete Research[FULL VERSION]
1. SCHOOL OF ENVIROMENTAL SCIENCES
DEPARTMENT OF MINING AND ENVIRONMENTAL GEOLOGY
INVESTIGATION OF THE MODE OF MAGNESITE MINERALISATION WITHIN THE
HOST ROCKS AT FOLOVHODWE MINE, LIMPOPO PROVINCE, SOUTH AFRICA
BY
NAME: MBEDZI ADAM
STUDENT NUMBER: 11602169
A MINI-DISSERTATION SUBMITTED TO THE DEPARTMENT OF MINING AND
ENVIRONMENTAL GEOLOGY, UNIVERSITY OF VENDA, IN PARTIAL
FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF
EARTH SCIENCES IN MINING AND ENVIRONMENTAL GEOLOGY
SUPERVISOR: PROF. J.S. OGOLA
DEPARTMENT OF MINING AND ENVIROMENTAL GEOLOGY
UNIVERSITY OF VENDA
APRIL 2014
2. ii
DECLARATION
I, Mbedzi Adam, do hereby declare that this mini-dissertation for Bachelor of Earth
Sciences in Mining and Environmental Geology Degree at the University of Venda,
hereby submitted by me, is my own original work; has not been previously submitted
for degree work at this or any other University; and, that all reference material
contained therein has been duly acknowledged.
Undersigned,
Student’s signature……………………………………… Date……………………....22/04/2014
3. iii
ACKNOWLEDGEMENTS
Firstly, I would like to give my sincere and deepest gratitude to God Almighty for
guiding and protecting me all the days of my life especially providing me with
strength that I needed during the compilation of this mini-dissertation.
Secondly, I would like to thank Prof. J. S. Ogola, my supervisor for his wise advice,
encouragement and enduring sense of humor that motivated me to carry on with the
research. May the good Lord open doors of the heavens to bless you and may His
salvation be upon you.
I am most appreciative of the support from GSSA and the NRF for sponsoring this
study, and making it possible for me to complete this research without any difficulty.
The appreciation extends to the management of the Folovhodwe Mine for giving me
an opportunity to carry out this research at their mine. May the God Almighty bless
them.
I also give thanks to Dr. F. A. Dacosta, who is the Head of Department of Mining and
Environmental Geology, and my lecturers especially Mr. C. Muzerengi, Ms. H. R.
Mundalamo and Mr. S. E. Mhlongo who took their time to support me academically
and socially.
I would also like to thank all my friends especially my dearest friends and classmates
for being there for me all the time and supporting me spiritually, socially and
academically when I was doing my mini-dissertation.
Last but not least I would like to thank my parents and my siblings who have always
been there by my side while carrying out this research, supporting me spiritually,
socially, financially. May the God Almighty bless you.
4. iv
ABSTRACT
This research focused on the mode of magnesite mineralisation within the hostrocks
in Folovhodwe deposit. Two hostrocks and the hanging wall were sampled from
Folovhodwe Mine. The investigation was undertaken to establish the way magnesite
was formed within the hostrocks. The study established the following rock types:
basalt (hostrock), dolerite sill (hostrock) and sandstone (hanging wall), as well as the
magnesite ore occurrence in the study area.
Petrographic studies and geochemical analysis were used to establish the mode of
magnesite mineralisation within basalt and dolerite sill. The major rock-forming
minerals of basalt are plagioclase and clinopyroxene, and of dolerite are olivine,
orthopyroxene, clinopyroxene and plagioclase. Petrographic studies have shown
evidence of alterations of the hostrocks through the presence of sericite, smectite,
talc and serpentine.
The geochemical analysis of the hostrocks showed lower concentrations of MgO in
basalt (3.61%) as compared to dolerite (30.75%). The analysis also indicated 51%
SiO2 in basalt and 41.14% SiO2 in dolerite. Geochemical analysis also showed that
the magnesite ore hosted in dolerite comprised 65% MgCO3 and in basalt it was
16% MgCO3.
The magnesite mineralisation took place along the joints, cracks and crevices within
weathered basalt and dolerite, resulting from the carbonation of forsterite and
enstatite that were altered to amphibole, serpentine and talc in the presence of CO2
and H2O to produce magnesite, smectite, palygorskite and sepiolite. Forsterite and
enstatite were the primary sources of the Mg2+
ion.
The high content of olivine and pyroxene in dolerite led to the abundance of the Mg2+
ion during alteration, hence, the dominance of magnesite in dolerite. However,
further work is recommended to ascertain the role of temperature and pressure in
the formation of magnesite within basalt and dolerite in the study area.
5. v
______________________________________________________________________
Declaration……………………………………………………………………………...…ii
Acknowledgements…………………………………………………………………..….. iii
Abstract………………………………………………………………………………….…iv
List of Figures…………………………………………………………………………….. vii
List of Plates……………………………………………………………………….……... ix
List of Tables………………………………………………………………………….…...x
1. Chapter One: Introduction……………………………………………………..… 1
1.1.Background………………………………………………………………………….1
1.2.Study Area………………………………………………………………………….. 1
1.2.1. Location………………………………………………………………………. 1
1.2.2. Climate………………………………………………………………………...2
1.2.3. Topography and Drainage………………………………………………….. 3
1.2.4. Vegetation……………………………………………………………………. 3
1.3.Problem Statement…………………………………………………………………3
1.4.Justification………………………………………………………………………….4
1.5.Hypothesis…………………………………………………………………………. 4
1.6.Objectives…………………………………………………………………………...4
2. Chapter Two: Literature Review…………………………………………………..5
2.1.Tshipise Basin of the Karoo Supergroup…………………………………………5
2.1.1. Dwyka Group………………………………………………………………… 6
2.1.2. Ecca Group…..………………………………………………………………. 6
2.1.3. Beaufort Group………………………………………………………………. 7
2.1.4. Stormberg Group……………………………………………………………. 8
2.1.5. Lebombo Group………………………………………………………………8
2.2. Structural Geology and Tectonic Setting………………………………………. 9
2.3. Mineralisation…………………………………………………………………….. 12
3. Chapter Three: Materials and Methods………….……………………………...13
3.1. Desktop Study……………………………………………………………………. 14
3.2. Preliminary Survey………………………………………………………………..14
3.3. Fieldwork………………………………………………………………………….. 14
Contents Pages
6. vi
3.3.1. Sampling………………………………………………………………………..14
3.4. Laboratory Work…………………………………………………………………..15
3.4.1. Sample Preparation……………………………………………………………15
3.4.1.1. Preparation of Thin-sections ………………………………………….. 15
3.4.1.2. Milling of Samples ……………………………………………………... 16
3.4.2. Petrographic Studies…………………………………………………………..16
3.4.3. Whole Rock Geochemical Analysis………………………………………….21
4. Chapter Four: Data Analysis and Interpretation……………………………… 26
4.1. Textural and Grain Size Analysis………………………………………………. 26
4.2. Mineralogical Analysis…………………………………………………………… 26
4.3. Geochemical Data Analysis ……………………………………………………. 28
4.3.1. Analysis of Major Oxides……………………………………………………...28
4.3.2. Analysis of Trace Elements………………………………………………….. 32
5. Chapter Five: Discussion, Conclusions and Recommendations…………. 37
5.1. Discussion………………………………………………………………………...37
5.2. Conclusions………………………………………………………………………. 39
5.3. Recommendations………………………………………………………………..39
References………………………………………………………………………………..40
Appendices
7. vii
List of Figures
Figure 1.1: Topographic map showing Folovhodwe Mine (Nedawaila, 2009)………. 2
Figure 1.2: Average climatic condition of Folovhodwe (Chinoda et al., 2009)…….... 4
Figure 2. 1: The Karoo Supergroup in southern Africa and the Tshipise Basin
(Johnson et al., 1996)………………………………………………………... 6
Figure 2. 2: Stratigraphic section of Tshipise Basin (Johnson et al., 2006)………….. 7
Figure 2. 3: Stratigraphy and correlation of Tshipise Basin with the Main Karoo
Basin (Johnson et al., 2006)………………………………………………... 10
Figure 2. 4: Distribution of faults in the post-Soutpansberg, pre-Karoo period
(Barker et al., 2006)………………………………………………………….. 11
Figure 2. 5: Distribution of faults within the Tshipise Basin and the Soutpansberg
Group (Mtimkulu, 2009)……………………………………………………... 11
Figure 2. 6: Simplified model outlining the tectonic evolution of the Limpopo Area
and the Lebombo Monocline (Bordy, 2000)……………………………….. 12
Figure 2. 7: Tectonic setting during each stage of deposition in the Tshipise Basin
(dashed arrow indicates forebulge migration along flexural profile of the
foreland system) (Bordy, 2000)……………………………………………... 13
Figure 3. 1: Flow-chart showing a summary of the methods and procedures applied
in the study……………………………………………………………………. 14
Figure 3. 2: Satellite image showing position of the last quarry in Folovhodwe Mine
premises (https://maps.google.co.za/)..................................................... 15
Figure 3. 3: Grinding in thin section preparations………………………………………. 17
Figure 3. 4: Sandstone under crossed polars (10-x magnification)…………………... 19
Figure 3. 5: Basalt with an intergranular texture under 10x magnification cross-
polarized light…………………………………………………………………. 20
Figure 3. 6: Dolerite minerals under microscope (cross-polarized light 10x
magnification)…………………………………………………………………. 21
Figure 3. 7: Altered dolerite under cross-polarized light (10x magnification)………… 21
Figure 4. 1: Distribution of magnesite grade distribution in hostrocks………………... 27
Figure 4. 2: Dolomite grade distribution in the hostrocks…………………………….... 28
Figure 4. 3: Dolerite (SiO2 = 41.14 and 43.90 wt%) and basalt (SiO2 = 50.25 and
51.00 wt%)……………………………………………………………………. 29
Figure 4. 4: ACF ternary diagram showing basalt and dolerite samples……………... 30
Figure 4. 5: AKF plot showing proportions of basalt and dolerite samples…………... 31
Figure 4. 6: AFM plot showing proportions of basalt and dolerite samples………….. 31
Figure 4. 7: Relationship between MgO and MgCO3 in hostrocks……………………. 32
8. viii
Figure 4. 8: Distribution of selected trace elements in volcanic rocks from
Folovhodwe Mine. The rocks include dolerite (SiO2 = 41.14% &
43.90%); basalt (SiO2 = 50.25% & 51%). In this system, Ni, Cr, Sr and
Ba are compatible, whereas Zr is incompatible…………………………… 33
Figure 4. 9: A plot showing rare earth elements concentrations in hostrocks
samples (normalised by chondrite values sourced from Sun and
McDonough, 1989)…………………………………………………………… 34
Figure 4.10: Ce/Zr vs. Ce/Nb showing different degrees of mantle partial melting
(normalised by mid-oceanic ridge basalt sourced from Thompson
(1982))…………………………………………………………………………. 34
Figure 4.11: Rare earth elements distribution in magnesite (normalised by chondrite
values sourced from Sun and McDonough, 1989)……………………….. 35
Figure 4.12: Close relationship between magnesite and hostrocks based on
mercury analysis……………………………………………………………… 36
Figure 5. 1: Veins of magnesite in a weathered dolerite……………………………….. 37
9. ix
List of Plates
Plate 3.1: Hand specimen of sandstone from Folovhodwe Mine……….………….. 17
Plate 3.2: Hand specimen of basalt from Folovhodwe Mine…………………….….. 18
Plate 3.3: Hand specimen of dolerite from Folovhodwe Mine………………………. 19
10. x
List of Tables
Table 3.1: Sandstone properties and description………………………………………... 17
Table 3.2: Mineral concentrations within different samples analysed by XRD
measured in wt%......................................................................................... 22
Table 3.3: Major oxides in samples of dolerite and basalt in weight percent…………. 23
Table 3.4: Trace elements in different samples in part per million (ppm)……………... 24
Table 3.5: Concentrations of mercury (Hg) in different samples measured in part per
billion (ppb)………………………………………………………………………. 25
Table 4.1: Textural classification of rocks from the Folovhodwe Mine………………… 26
Table 4.2: Relationship between hostrocks and magnesite mineralisation…………… 27
Table 4.3: Calculated proportions of minerals in normative weight per cent (wt%
norm)……………………………………………………………………………… 29
11. 1
1. CHAPTER ONE: INTRODUCTION
1.1 Background
This research is focused on the investigation of the mode of magnesite (MgCO3)
mineralisation within the hostrocks at Folovhodwe magnesite deposit. Magnesite is
the natural form of magnesium carbonate and the major source for the commercial
production of magnesium compounds. Magnesite may occur either as crystalline or
cryptocrystalline with a range of colours including white, grey, yellow, brown, orange,
light pink and even colourless (Strydom, 1998).
Other important sources of magnesium include dolomite [CaMg(CO3)2], brucite
[Mg(OH)2], olivine [(Fe,Mg)2SiO4], the various salts of sea water, brines and
evaporites. Economically viable magnesite deposits in South Africa occur as a
weathering product of rocks with high magnesium contents. The Folovhodwe
magnesite deposit occurs within the Tshipise Magnesite Field (Wilson and
Anhaeusser, 1998).
The Tshipise Magnesite Field extends about 50km east-northeast from Tshipise
consisting of large deposits of amorphous magnesite occurring in weathered sheets
of olivine dolerite, intrusive into the basal portion of the Karoo Supergroup and into
the limburgitic/basalts of the Letaba Formation (Karoo Supergroup) (Strydom, 1998).
Exposures of both the intrusive and extrusive volcanic rocks are controlled by three
major east-northeast striking fault systems, namely the Klein Tshipise, the Tshipise
and Bosbokpoort Faults (Strydom, 1998).
The magnesite deposit under this study occurs in the Fallershall 74 MT farm and
within the decomposed and altered olivine dolerite sills dipping between 12o
and 18o
to the north. Most of the magnesite occurs as concordant veinlets varying in width
from a few millimetres to about 12 cm (Strydom, 1998).
1.2 Study Area
1.2.1 Location
Folovhodwe Mine is located in the Limpopo Province of the Republic of South Africa
and operates as open-cast magnesite mine. The mine is located about 20 km east of
Tshipise, just north of the R525 provincial road and about 60 km south-east of
Musina town which is situated just next to the border between the Republic of South
Africa and Zimbabwe.
Folovhodwe mine is located between latitude 22o
30‟22” South and longitude
30o
22‟18” East, at an elevation of about 517 m above sea-level, the map is
presented in Figure 1.1.
12. 2
1.2.2 Climate
The Folovhodwe area normally receives about 368 mm of rain per year, as shown in
figure 1.2 in page 3, with most rainfall occurring during mid-summer. It receives the
lowest rainfall in August and the highest in January. The rainy season is
predominantly from November to March when about 83% of the total annual rainfall
occurs. The driest months are from May to September, when less than 7 mm of rain
per month is recorded (Van Rooyen, 2008).
Figure 1.1: Topographic map showing Folovhodwe Mine (Nedawaila, 2009).
13. 3
Figure 1.2: Average climatic condition of Folovhodwe (Chinoda et al., 2009).
1.2.3 Topography and Drainage
The area is situated in a relatively flat to gentle slope terrain of the Soutpansberg
mountain ranges, geographically stretching in the general north-east direction from
Tropic of Capricorn to the Beit-Bridge border post, with evenly distributed rocky hills
(Brandl, 1981). The altitude or elevation ranges from 500 – 670 m above mean-sea-
level, with a slight dipping towards the north-eastern direction, and the area is
dominated by non-perennial rivers which forms tributaries of the Nwanedzi River
which drains into the Limpopo River, forming a dendritic drainage pattern (Brandl,
1981).
1.2.4 Vegetation
The Mopane shrubs is the dominating plant cover, with dense-medium grass over
relatively flat terrenes and highly scattered Baobab trees within the greyish soil-
coverage (Tshisikhawe, 2002). Repeatedly, this appears mainly on the gentle and
flat terrains, while thorn shrubs appear only on the flat clay soils and sandy soil along
the stream channels on the non-perennials (Palgrave, 2002).
1.3 Problem Statement
Despite previous studies that were conducted to understand the general geology and
mineral occurrence in the area, the mode of magnesite mineralisation is not clearly
understood.
0
10
20
30
40
50
60
70
80
90
Maximum temperature
Minimum temperature
Precipitation (in millimeters)
14. 4
1.4 Justification
This research will try to decipher the mode of magnesite mineralisation. Previous
studies also recommended further studies on the mode of magnesite mineralisation
within the hostrocks in the area. Results of the present study will help in further
improving techniques for evaluation, estimation of ore reserves and the development
of a clear understanding of the way magnesite was formed within the deposit.
1.5 Hypothesis
Magnesite can be formed through talc carbonate metasomatism of peridotite
and other ultrabasic rocks;
Magnesite is formed through carbonation of olivine in the presence of water
and carbon dioxide, and is favoured at moderate temperatures and pressures
typical of greenschist facies.
Magnesite can be formed through the carbonation of magnesian serpentine
(lizardite) through the following reaction: serpentine + carbon dioxide → talc +
magnesite + water.
Magnesite can also be formed from metasomatism in skarn deposits, in
dolomitic limestones, associated with wollastanite, periclase, and talc.
1.6 Objectives
The main objective is to establish the way magnesite was formed within the
hostrocks of the Folovhodwe deposit. In order to achieve the main objective, the
following specific objectives were considered:
To compare and contrast the mineralogical composition of hostrocks in thin-
section through the use of a petrographic microscope.
To determine differences in magnesium oxide (MgO) content in hostrocks
through X-ray fluorescence major oxides analysis.
To establish path-finder elements for magnesite through trace elements
analysis.
To determine the grade distribution of magnesite in hostrocks through the use
of X-ray diffraction analysis.
15. 5
2. CHAPTER TWO: LITERATURE REVIEW
2.1 Tshipise Basin of the Karoo Supergroup
The Tshipise Basin is located between the Soutpansberg Group and the Central
Zone of the Limpopo Belt as shown in Figure 2.1. According to Johnson et al.,
(2006), the location and shape of the basin were controlled by the ENE – WSW
faults that follow the trend of the Limpopo Belt. They further states that, the original
basin was much larger than the extent of the present outcrops, which are preserved
in fault blocks. The basin is made up of the sedimentary and igneous rocks of the
Karoo Supergroup (Johnson et al., 2006).
The Tshipise basin in South Africa and partly in Zimbabwe, together with the Tuli
basin in South Africa, Zimbabwe, and Botswana and the Nuanetsi basin in
Zimbabwe represent the so-called Limpopo area Karoo-age basins (Bordy, 2000).
Vail et al. (1969) and Burke and Dewey (1973) further state that the Limpopo area
forms the western arm of a failed rift triple junction, which later extended in a north-
south direction, from the Save basin in Zimbabwe to the Lebombo „Monocline‟ in
South Africa and Mozambique; and the genesis of the rift was associated with the
Gondwana break-up. The Tshipise basin is divided into five groups: Dwyka, Ecca,
Beaufort, Stormberg and Lebombo Groups as Figure 2.2 in page 6.
Figure 2.1: The Karoo Supergroup in southern Africa and the Tshipise Basin
(Johnson et al., 1996).
16. 6
2.1.1 Dwyka Group
The glacial beds of the Dwyka Group are represented by poorly sorted
conglomerates (diamictites of the Tshidzi Formation) which attain a thickness of 20 m
Van der Berg (1980). According to Bordy (2000), the formation consists of fragments
of all shapes and sizes in an argillaceous to sandy matrix. Johnson et al. (2006)
states that at Tshipise basin the Tshidzi formation is approximately 5 m thick whilst
Bordy (2000) describes the Tshidzi formation as the most sub-angular, poorly sorted
fragments with sizes up to 2 m and are set in a light-coloured sandy or quartzitic
matrix. The diamictites and interbedded sandstones generally reflect glacial and
fluvioglacial (braided stream) environments (Johnson et al., 2006). According to van
der Berg (1980), the fluvioglacial sediments were transported in an E-ENE to W-
WSW direction.
Figure 2.2: Stratigraphic section of Tshipise Basin (Johnson et al., 2006).
2.1.2 Ecca Group
Studies conducted by Bordy (2000) reports the Ecca Group within the Tshipise Basin
being represented by three formations; the Fripp Sandstone Formation in the upper
part, the Mikambeni Formation in the middle and the Madzaringwe Formation in the
lower part of the group.
The Fripp Sandstone Formation consists of white feldspathic, trough-cross bedded,
fine-grained and very course-grained sandstones with thin pebble horizons and
occasional thin silty bands (Brandl, 1981). Few conglomerates containing pebbles of
17. 7
vein quartz, pegmatites, sandstone, quartzite and volcanic rocks have been reported
to exist within the formation (Brandl, 1981).
Johnson et al. (2006), reports the thickness of the formation to reach a maximum of
about 110 m in the northeastern part of the basin. The sandstones were probably
deposited by braided streams flowing towards the northwest and west. They further
noted that the plant fossils (which involve Dicroidium) indicate that this unit is
probably a time-equivalent of the Molteno Formation in the Main Karoo Basin.
According to Bordy (2000), the Mikambeni Formation comprised a series of massive
dark to pale mudstones and black shales with few thin laminated sandstone layers
towards the base. Johnson et al. (2006) report the maximum thickness of the
formation to be about 150 m. They divided the formation into three units: an upper
unit, 60 to 70 m thick, comprising dark to grey mudstone with plant fragments and
occasional seams of bright coal; a middle unit, 50 m thick, comprising black,
carbonaceous shale with occasional bright coal seams; and a lower unit, 15 to 20 m
thick, comprising alternating black shale and grey, feldspathic sandstone.
According to McCourt and Brandl (1980), these beds were formed in a shallow –
water lacustrine environment while Johnson et al. (2006) state that the overall fine-
grained character of the rocks points to deposition on the distal floodplains of
meandering rivers.
According to Johnson et al. (2006), the Madzaringwe Formation consists of up to
200 m of alternating sandstone, siltstone and shale, the later containing thin coal
seams. The sandstone is feldspathic, usually micaceous and commonly cross-
bedded (Johnson et al., 2006). McCourt and Brandl (1980) noted that the best
developed coal seam is 3.9 m thick and Johnson et al. (2006) report the main coal
seam to be developed between 85 and 100 m above the carbonaceous zone. The
Madzaringwe Formation appears to have been largely laid down by meandering
rivers flowing from the northwest; and sandstones probably represent point bar,
levee and crevasse splay deposits (Johnson et al., 2006).
2.1.3 Beaufort Group
According to Bordy (2000), the Beaufort Group within the Tshipise Basin is
represented by the Solitude Formation. Johnson et al. (2006) describe the Solitude
Formation to be comprised of purple mudstones and grey shales. Johnson et al.
(2006) further state that, in the Tshipise basin the Beaufort Group comprise 30 m of
grey shale is overlain by 80 m of alternating purple and grey mudstone with three
intercalated siltstone units whilst elsewhere, the bottom part of the lower unit may
consist of black shale with occasional bands of bright coal.
Greenish or reddish, fine-to coarse-grained sandstones, up to 5 m thick are reported
to occur in places and the formation is said to have a maximum thickness of about
170 m (Bordy, 2000; Brandl, 1981; and Johnson et al., 2006).
According to Johnson et al. (2006), the Solitude Formation presumably represents
the overbank deposits of meandering rivers with extensive floodplains, and the dark
18. 8
shales with associated coals accumulated in flood basins and marshes under
reducing conditions.
2.1.4 Stormberg Group
The Stormberg Group is made up of the uppermost formations of the Karoo
Supergroup sedimentary rocks (Johnson et al., 2006). Bordy (2000) reports that the
upper part of the Stormberg Group within the Tshipise basin is represented by the
Clarens Formation. According to Johnson et al. (2006), Clarens Formation within the
Tshipise Basin is divided into the Red Rocks Member comprising very fine and fine-
grained, light red argillaceous sandstone with irregular patches or occasional layers
of cream coloured sandstone, and commonly with calcareous concretions.
Based on borehole information, Johnson et al. (2006), state that the member attains
a maximum thickness of about 150 m, but in some areas, this member has been
reported to be absent (Bordy, 2000). The Tshipise Member forms the uppermost part
of the Clarens Formation and comprised fine-grained, well-sorted, white or cream
coloured sandstone with large scale cross bedding and calcareous concretions at
the base (Johnson et al., 2006).
The middle part of the Stormberg Group within the Tshipise Basin is reported to be
made up of the Bosbokpoort Formation (Bordy, 2000). According to Johnson et al.
(2006), this formation comprised dominantly the red lithologies varying from about 60
m of dark-red mudstone to about 40 m of very fine-grained sandstone and both of
these units are considered to contain numerous calcareous concretions.
The lower part of the group is made up of Klopperfontein Formation (Bordy, 2000).
He further states that the Klopperfontein (Sandstone) Formation corresponds to the
Elliot Formation of the main Karoo Basin. The Klopperfontein Formation dominates
the central part of the basin and attains a maximum thickness of about 20 m, and
where the formation is absent the Bosbokpoort Formation makes contact to Solitude
Formation (Johnson et al., 2006). The Klopperfontein Formation comprised course
sandstone (with ubiquitous trough cross-bedding) and subordinate conglomerate
(Johnson et al., 2006). According to Bordy (2000), the fairly braided river channel
environment is indicated by the association of the overall litho-facies.
2.1.5 Lebombo Group
The Lebombo Group is part of the Karoo Igneous Province and constitutes the
uppermost units of the Tshipise Basin (Duncan and Marsh, 2006). Due to the
systematic succession of the various igneous rock types, it was subdivided into a
number of formations: Letaba, Mashikiri, Sabie River, Movene, Mbuluzi and Jozini
formations (Johnson et al., 2006). According to Duncan and Marsh (2006), the
Letaba Formation is composed of a sequence of picritic (olivine-rich) lavas and
dominate almost entirely the whole basin with the nephelinite lavas of the Mashikiri
Formation forming relatively tin unit (≤170 m) at the base of these volcanic
sequence. The Letaba Formation is also considered to have dominantly high
titanium and zirconium (Johnson et al., 2006).
19. 9
According to studies by Bordy (2000), there is no evidence of other formations of the
Lebombo Group occurring within the Tshipise Basin. According to Duncan and Marsh
(2006), the Sabie River Formation is evident near Shingwidzi and extends to the
southern end of the Lebombo Group west of Richards Bay, while Jozini, Mbuluzi and
Movene formations are confined to parts of Mozambique, Swaziland and the Kwa-
Zulu Natal Province of South Africa.
2.2 Structural Geology and Tectonic Setting
The Tshipise Basin reflects the reactivation of structures within the Limpopo Belt
(Cox, 1970). The main structural lines within the basin strike in an ENE to WSW
direction (Bordy, 2000) shown in Figure 2.4. Many of these structures developed
over long periods and there is a wide range in the age of faults (Cox, 1970).
According to Cox et al. (1965), the geology of the pre-Karoo rocks shows that the
line of the Limpopo Belt was intensely reactivated in the post Soutpansberg (1.7 Ga),
pre-Karoo period. Truster (1945), further states that after the faulting of the
Soutpansberg rocks, a long period of tectonic quiescence and erosion ensued before
the deposition of the Karoo strata.
Figure 2.3: Stratigraphy and correlation of Tshipise Basin with the Main Karoo Basin
(Johnson et al., 2006).
20. 10
Figure 2.4: Distribution of faults in the post-Soutpansberg, pre-Karoo period (Barker
et al., 2006).
According to Cox (1970), the syn-Karoo faults are represented by those that cut the
Karoo sedimentary rocks but which displace only the lower basalt flows or fail to
displace the basalts at all, with the best example being the Shurungwe Fault that
marks the northern boundary of the Bubye Coalfield in the north-eastern extension of
the Tshipise Basin in Zimbabwe. The major faults exist within the Tshipise Basin and
the Soutpansberg Group are shown in Figure 2.5.
Figure 2.5: Distribution of faults within the Tshipise Basin and the Soutpansberg
Group (Mtimkulu, 2009).
The Karoo outcrops of the Tshipise Basin are separated from both the Tuli and
Nuanetsi basins by the highly deformed Messina Block which may have acted as a
consistently positive area during the accumulation of the Karoo strata (Cox, 1970).
According to Cox (1970), the majority of the faults affecting the Karoo formations are
21. 11
of normal type, and no reverse faulting has been noted, thus there is no evidence for
compressive tectonic phases in the Phanerozoic history of this area (Fig.2.6).
In contrast to Cox (1970), a study by Bordy (2000) indicates that the formation of the
Karoo Supergroup in southern Africa occurred in two contrasting tectonic regimes:
an initial compressive system which was replaced by an extensive regime related to
the Gondwana break-up. According to Catuneanu et al. (1998), the compressive
foreland system that existed north of the Cape Fold Belt developed in response to
the Late Paleozoic – Early Mesozoic subduction of the palaeo-Pacific plate below the
Gondwana plate. Catuneanu et al. (1998) further states that, due to the flexural
warping of the lithosphere, three distinct areas developed across the warped profile
of the system: foredeep, forebulge and back-bulge as shown in Figure 2.7. The main
Karoo Basin preserves sediments that accumulated in both foredeep and forebulge
flexural provinces (Catuneanu et al., 1998).
The orientation of this postulated ENE to WSW striking back-bulge basin is
consistent with the measured and reported palaeo-current directions in the Dwyka
and Ecca Groups of the Tshipise Basin (McCourt and Brandl, 1980). According to
Johnson et al. (1997) within the Tshipise Basin, the paraglacial outwash fans and
fluvio-deltaic deposits of the lower Ecca Group were built out from a southeasterly
source.
Figure 2.6: Simplified model outlining the tectonic evolution of the Limpopo Area and
the Lebombo Monocline (Bordy, 2000).
Bordy (2000) adds that, these strata may be identified as deposits of the northerly
inclined foredeep slope which were derived from the northerly situated mountainous
of the forebulge as shown in Figure 2.7.
22. 12
Figure 2.7: Tectonic setting during each stage of deposition in the Tshipise Basin
(dashed arrow indicates forebulge migration along flexural profile of the foreland
system) (Bordy, 2000).
2.3 Mineralisation
In a zone extending some 50 km east-northeast from Tshipise, large deposits of
amorphous magnesite are known in weathered sheets of olivine dolerite, intrusive
into the basal portion of the Karoo Supergroup and into the limburgitic or basalts of
the Letaba Formation (Karoo Supergroup) (Strydom, 1998). Strydom (1998) further
state that the exposures of both the intrusive and extrusive (volcanic) rocks are
controlled by three major east-northeast-striking fault systems, namely Klein
Tshipise, the Tshipise and the Bosbokpoort Faults.
The Karoo sediments and lavas strike east-northeast and dip at between 12o
and 18o
to the north (Strydom, 1998). According to Strydom (1998) the magnesite deposits
occur in fairly decomposed and altered northerly dipping, olivine dolerite sills.
Strydom (1998) further noted that the magnesite capping in some areas was
completely covered by recent deposits of red sand and gravel.
The occurrence of coal in the Tshipise Basin has also been noted by Johnson et al.
(2006). According to Snyman (1998) the Pafuri-Tshipise-Mopane sub-basin is
severely faulted with most faults striking between E10o
N and E35o
N. according to
van der Berg (1980) the coal seams in Tshipise Basin consists of alternating bands
of coal and mudstone, and the coal bands exhibit same trend of decreasing vitrinite
content (from 90 to 80%) with increasing depth. Snyman (1998) adds that, the raw
coal (excluding easily distinguished shale and mudstone bands) has an ash content
of 25%.
23. 13
3. CHAPTER THREE: MATERIALS AND METHODS
This chapter describes the materials and methods used in this study. Figure 3.1
below illustrates the methods and procedures that were applied in this study.
Figure 3.1: Flow-chart showing a summary of the methods and procedures applied in
the study.
24. 14
3.1 Desktop Study
This was conducted to acquire relevant information before the fieldwork. This
consisted of the collation, evaluation and integration of the project-relevant
information already available and was used for preliminary assessment of the site
conditions and project conceptual design. The specific areas of reference and
sources of relevant information included books, previous technical reports, journals,
topographical maps, geological maps, aerial photographs and internet information.
3.2 Preliminary Survey
Preliminary work was conducted in the study area before conducting the actual
fieldwork. This helped to gain more useful information about the study area, in terms
of changes in actual area which were not presented on topo-sheets and aerial
photos and this was also helpful for appropriate design of fieldwork.
3.3 Fieldwork
3.3.1 Sampling
Fieldwork was conducted through sampling at selected sites. Sampling sites were
selected based on the difference of hostrocks in the last quarry of the Folovhodwe
Mine (Fig.3.2). Both the hostrock and the hosted magnesite were sampled. Sixteen
(16) samples were collected and marked clearly using a permanent marker. Samples
were collected for petrographic studies and whole rock geochemical analysis. Items
of equipment used include a digital camera, geological hammer and a sledge
hammer.
Figure 3.2: Satellite image showing position of the last quarry in Folovhodwe Mine
premises (https://maps.google.co.za/).
25. 15
3.4 Laboratory Work
3.4.1 Sample Preparation
Following laboratory tools and apparatus were used to prepare samples for analysis:
Diamond saw
Petro-trim machine (trim saw)
Struers (Accutom-50)
Milling machine (Retch RS200)
Drying oven (Vacutec)
Direct Mercury Analyzer (DMA-80)
Petro-bond (bonding fixture/bonding jig)
Polishing machine (RotoPol-35)
Glass slides
Micrometer gauge
Silicon carbide powder (#120, #220, #400 and #800)
3.4.1.1 Preparation of Thin-sections
Four of the representative samples of different hostrocks were selected for thin-
sections and they were cut into rectangular blocks of 50 x 80 mm in size using a
diamond saw. The rectangular blocks were trimmed to 30 x 40 mm sizes using
petro-trim machine and samples were then polished using silicon carbide grit in
sequence of 120, 220, 400 and 800 grit sizes until samples were smooth for bonding
process.
The polished samples were then cleaned by washing in ultrasonic bath before they
were dried at 80o
C for 15 minutes in the drying oven. After drying, samples were
cooled to room temperature before they were bonded. The bonding agent (epoxy)
was prepared by mixing hardener and resin using a ratio of 2:15 (2 parts hardener
and 15 parts of resin) and samples were bonded on frosted glass slides using the
epoxy.
The samples were then placed on a bonding jig overnight for effective bonding. The
bonded samples were placed on Accutom-50 cutting machine and were reduced to a
thickness of 10 mm in a cutting process that lasted for 13 minutes per each sample.
The Accutom-50 cutting machine was further used for the grinding of samples to a
thickness of 50 micrometers in a process lasting 30 minutes (Fig.3.3).
During the process of cutting, the instrument was programmed to cut and stop
automatically. The ground samples were then placed onto a RotoPol-35 polishing
machine and polished using a polishing disk for 2 hours. Once polished, the
samples‟ thickness was checked using a micrometer gauge to confirm the required
less than 30 micrometers thickness for petrographic studies.
26. 16
Figure 3.3: Preparation of thin sections.
3.4.1.2 Milling of Samples
All samples were dried overnight in the drying oven at 30o
C to release any moisture
before milling so to avoid the jamming of the milling pots due to moisture. The
reason to dry samples at 30o
C was so to avoid volatile elements from the samples
while effective enough to remove the moisture.
Samples were crushed into smaller particles using a sledge hammer and were milled
in the milling machine on manual operation mode for five minutes to particle size of
about 75 µm. The samples were then transferred to sample bags and weighed. The
milling of the samples was done to ensure homogeneity and liberation of mineral
grains in the sample.
3.4.2 Petrographic Studies
Sandstone
The hand rock specimen shown in Plate 3.1 represents the sandstone that formed
the hanging wall of the Folovhodwe magnesite body. The properties and descriptions
are summarized in Table 3.1.
27. 17
Table 3.1: Sandstone properties and description
Rock properties Description
Competency The rock is not competent (no difficulty in breaking).
Colour The dominance of quartz grains, carbonates, and other light
coloured minerals have influenced the sandstone to be almost
white to cream-white with dark spots as a result of dark
minerals.
Mineral
composition
The rock was dominated by sand size quartz grains with
carbonate as cementing materials. The carbonate as cement
was confirmed by a hydrochloric acid test that was conducted in
the laboratory. Also biotite and clay minerals are included in the
cementing material of the rock.
Texture It is coarse to medium textured rock dominated by sugary sand
size particles (clearly visible under a hand lens).
Fabric Randomly oriented.
Plate 3.1: Hand specimen of sandstone from Folovhodwe Mine.
In thin-section, the rock was found to be dominantly composed of a variation of
angular grains of quartz comprising about 95% of the rock. Magnesite, Biotite and
smectite were also present within cementing materials dominated by magnesite; and
together constitute the remaining 5% of the rock (Fig.3.4).
Biotite and Clay minerals
Quartz
28. 18
Figure 3.4: Sandstone with a clastic texture under crossed-polars (10-x
magnification).
Amygdaloidal Basalt
This rock is composed of clearly the quartz amygdoloids, pyroxenes, olivine and the
plagioclase feldspars. The rock appears to be dark to grey in colour, influenced by
the dominance of mafic minerals as opposed to quartz amygdoloids which are much
dispersed (Plate 3.2). It occurs as one of the hosting rocks of the Folovhodwe
magnesite deposit.
Plate 3.2: Hand specimen of amygdaloidal basalt from Folovhodwe Mine.
In thin-section, the rock is composed of about 60% plagioclase feldspar with
abundance of albite twinning, almost 20% clinopyroxenes, 10% of opaque minerals
(probably oxides) and the remaining 10% is composed of quartz and smectite
(Fig.3.5). The rock is very fine grained and unfortunately due to the cutting during
thin-section preparation, the amygdoloids were not present on the thin section slide.
Quartz amygdoloids
Quartz
Smectite
Hematite coatings
Magnesite
29. 19
Figure 3.5: Basalt with an intergranular texture under 10x magnification cross-
polarized light.
Dolerite
This rock is dominated by a rusty color with some dark to grey areas suggesting the
presence of pyroxenes, and other mafic minerals (Plate 3.3). The rusty colour proves
to be equivalent to oxides such as ilmenite, limonite, and magnetite. The rock has a
medium to coarse texture.
Plate 3.3: Hand specimen of dolerite from Folovhodwe Mine.
Oxides
Orthopyroxene
Albite twinning
Plagioclase
Clinopyroxene
30. 20
In thin section, the rock is composed of about 45% pyroxenes (Orthopyroxenes =
25% and clinopyroxenes = 20%), about 25% of plagioclase and the remaining 30%
is comprised of mica (dominantly some traces of sericite), serpentine and smectite
(Fig.3.6). A mineral such as sericite appears to be as a result of alteration of other
minerals, typically plagioclase feldspar.
Figure 3.6: Dolerite minerals with allotriomorphic texture under microscope (cross-
polarized light 10x magnifications).
Altered dolerite samples show high proportion of pyroxenes (both orthopyroxene and
clinopyroxene) to about 50% while olivine and plagioclase feldspar combined
together constitute about 40% of the rock (Fig.3.7). The rock has opaque minerals
which are oxides constituting about 8%, and both serpentine and smectite occurs as
alteration of other minerals constituting only 2% of the rock.
Figure 3.7: Altered dolerite with porphyritic texture under cross-polarized light (10x
magnification).
Olivine
Clinopyroxene
Orthopyroxene
Serpentine
Sericite
Plagioclase
Olivine
Serpentine
Sericite
Oxides
Orthopyroxene
Clinopyroxene
31. 21
3.4.3 Whole Rock Geochemical Analysis
Eight samples (i.e. two samples from each of the two different hostrock types and
four samples of different magnesite occurring within the different hostrocks) were
selected from different samples that were collected from the field. Selected samples
were prepared and then sent to the Council for Geoscience Laboratories in Pretoria
for mineralogical analysis by X-ray Diffraction (XRD), major oxides and trace
elements using X-ray fluorescence (XRF) spectrometry, and for mercury using Direct
Mercury Analysis (DMA-80).
X-ray Diffraction (XRD002)
X-ray diffraction was used to detect minerals that could not be identified by physical
analysis of hand specimen and those which could not be clearly distinguished
through petrographic studies. According to Nesse (2012), in a conventional X-ray
diffratometer, X-rays are generated in a cathode ray tube. Free electrons produced
by heating a filament called the cathode (similar to the filament in a conventional
incandescent electric light bulb) to a high temperature and these negatively charged
electrons are accelerated to high energy by applying a voltage of several tens of
kilovolts between the filament and the target or anode.” Results of the analysis are
presented in Table 3.2.
X-ray Fluorescence
The X-ray fluorescence spectrometer functions in a similar way to the electron
microprobe, but with two significant differences. Firstly, the characteristic X-ray
spectra of the elements in the sample are excited by the high-energy continuous
spectrum of an electron beam, and this minimizes the amount of continuous
spectrum produced by the sample. The second is that heterogeneous samples such
as rocks must be either finely ground or fused before being analyzed (Nesse, 2012).
XRF uses primary radiation from an X-ray tube to excite secondary X-ray emission
from the sample and the radiations emerging from the sample include the
characteristic X-ray peak of the major and trace elements present in the samples.
The dispersion of these secondary X-rays into spectrum by the X-ray diffraction,
allows the identification of the elements present in the samples.
The XRF005 was used for major elements while XRF006 was used for trace
elements. Two dolerite samples and two basalt samples were analyzed for major
oxides and trace elements, but magnesite samples were only analyzed for trace
elements. Major oxides are presented in Table 3.3 and trace elements in Table 3.4.
33. 23
Table 3.3: Major oxides in samples of dolerite and basalt in weight percent
Rock type Dolerite Basalt
Sample ID/major oxides sample 1 sample 2 sample 1 sample 2
SiO2 41.14 43.90 50.25 51.00
TiO2 1.43 1.59 4.30 4.26
Al2O3 4.16 6.76 14.24 13.92
Fe2O3 15.08 13.32 13.11 12.90
FeO 0.52 0.25 0.31 0.00
MnO 0.170 0.158 0.119 0.123
MgO 30.75 22.47 3.54 3.61
CaO 2.98 5.31 6.30 7.10
Na2O 0.79 0.73 2.92 2.77
K2O 1.05 1.22 3.12 2.76
P2O5 0.246 0.646 0.763 0.733
Cr2O3 0.451 0.281 0.012 0.010
LOI 1.23 3.37 1.01 0.82
Total 100.00 100.00 100.00 100.00
Direct mercury analysis (DMA-80)
DMA-80 is a direct mercury analyzer which uses the principle of thermal
decomposition, amalgamation and atomic absorption. All mercury is released from
the sample through thermal decomposition, and mercury was then trapped in a
separate furnace through gold amalgamation. The amalgamation furnace was then
heated and mercury was released, flown by carrier gas into unique block
arrangement of spectrophotometer where mercury was quantitatively measured.
Mercury analysis results are important for relating the alteration of the hostrocks with
the grade hosted magnesite (Winter, 2010).
The mercury test was repeated twice and the average concentration was considered
for each sample (Table 3.5). Two samples were tested per each hostrock and also
the magnesite samples associated with each hostrock sample.
35. 25
Table 3.5: Concentrations of mercury (Hg) in different samples measured in part per
billion (ppb)
Rock type/
mineral
Sample ID Hg (µg/kg) Hg (µg/kg) repeat Average
Hg (µg/kg)
Dolerite Sample 1 0.4 0.3 0.4
Sample 2 0.5 0.3 0.4
Basalt Sample 1 0.8 0.4 0.5
Sample 2 0.4 0.3 0.4
Magnesite
Sample 1 0.9 0.4 0.6
Sample 2 0.5 0.4 0.4
Sample 3 0.7 0.5 0.6
Sample 4 0.3 0.6 0.5
36. 26
4. CHAPTER FOUR: DATA ANALYSIS AND INTERPRETATION
4.1 Textural and Grain Size Analysis
All the hostrocks together with the sandstone which forms the hanging wall were
analysed by their grain sizes. For the hostrocks, textural and grain size analysis was
done to classify them as intrusive, hypabyssal or extrusive, however, for the
sandstone it was to relate the grain size and textural maturity with the permeability of
the rock.
The sandstone comprised sharp-edged quartz grains of medium to very fine grains.
The basalt was found to be of aphanitic texture, hence a volcanic rock. The basalt
was composed of subhedral plagioclase crystals, anhedral pyroxene, and anhedral
quartz crystals.
The dolerite comprised subhedral pyroxene crystals, anhedral plagioclase and
anhedral olivine crystals, and it was devoid of any quartz. It was also noted during
the analysis that the dolerite present much a texture that suggest a hypabyssal
environment rather than just volcanic, and this correlates with the literature review
where they are said to occur as dikes and sills (Strydom, 1998). The textural
classifications of the above rocks are presented in Table 4.1.
Table 4.1: Textural classification of rocks from the Folovhodwe Mine
Rock type Colour Texture
Sandstone Light Very fine to medium sand (medium
texture)
Basalt Grey Aphanitic texture
Dolerite Rusty to grey Aphanitic texture
4.2 Mineralogical Analysis
Mineralogical analysis was done only on the hostrocks excluding the sandstone
which makes up the hanging wall. Mineralogical analysis was done through thin-
sections, and X-ray diffraction. This analysis was to classify the hostrocks according
to their mineralogical compositions and to determine the grade distribution of
magnesite. Petrographic studies have shown that, the dolerite is rich in olivine and
enstatite.
It was also observed during petrographic studies that, mica occurs as an alteration
product of plagioclase in the form of sericite. Smectite (clay mineral) is indicative of
alteration of the hostrocks including talc and serpentine. Mineralogical analysis by X-
ray diffraction was conducted so that every mineral within the hostrocks will be
considered in all analysis. A summary of hostrock alteration and associated
magnesite mineralisation is presented in Table 4.2.
37. 27
Table 4.2: Relationship between hostrocks and magnesite mineralisation
Hostrocks Major minerals Minor minerals Degree of
alteration
Magnesite
mineralisation
Basalt Plagioclase
Clinopyroxene
Ilmenite
Goethite
Quartz
Smectite
Sericite
Moderate Moderate
Dolerite Olivine
Orthopyroxene
Clinopyroxene
Plagioclase
Serpentine
Smectite
Talc
Sericite
High High
The grade of magnesite is higher in dolerite than in basalt but in all these hostrocks,
magnesite concentration showed a close relationship with the alteration that has
occurred. Figure 4.1 shows the distribution and concentration of magnesite in
dolerite and basalt.
Figure 4.1: Distribution of magnesite grade distribution in hostrocks.
Dolerite contains high grade magnesite while basalt contains low grade. Magnesite
occurs together with dolomite in different proportions. The abundance of anorthite in
basalt favours the formation of dolomite while the calcium deficiency in dolerite
favours occurrence of magnesite. Dolomite grade distribution is presented in Figure
4.2.
0
10
20
30
40
50
60
70
Dolerite Basalt
MgCO3(wt%)
Hostrocks
sample 1
sample 2
38. 28
Figure 4.2: Dolomite grade distribution in the hostrocks.
4.3. Geochemical Data Analysis
Chemical composition in igneous rocks is known to be the most distinguishing
feature. This is so because chemical composition reflects composition of the magma,
and therefore provides information about the source of the rock. It is also much
common among other factors to consider chemical composition of the magma to be
the main determinant of which minerals to crystallize and their proportions (Philpotts
and Ague, 2009). As a result graphs, ternary diagrams and CIPW norm calculations
are used to facilitate comparison between the hostrocks.
4.3.1 Analysis of Major Oxides
Based on the total alkalies [Na2O + K2O] plotted against the silica [SiO2], dolerite is
more mafic than basalt (Fig.4.3). Any igneous rock with silica content less than 45
weight per cent is classified as ultramafic; while any igneous rock with silica content
between 45 to 52 weight per cent is classified as mafic (Goldschmidt, 1937). The
more mafic nature of dolerite makes it to be more susceptible to chemical weathering
than basalt.
0
10
20
30
40
50
60
70
80
90
Dolerite Basalt
Ca,Mg(CO3)2(wt%)
Hostrocks
sample 1
sample 2
39. 29
Figure 4.3: Dolerite (SiO2 = 41.14 and 43.90 wt%) and basalt (SiO2 = 50.25 and
51.00 wt%).
In order to determine which minerals and proportions can crystallize under „normal
conditions‟ from magmas with the same chemical composition as the hostrocks, the
CIPW norm calculation spread sheet program (Appendix A) was used to obtain the
normative weight per cent presented in Table 4.3.
Table 4.3: Calculated proportions of minerals in normative weight per cent (wt%
norm)
Rock type Dolerite Basalt
Normative
Minerals/
sample ID
Sample 1 Sample 2 Sample 1 Sample 2
Quartz - - 6.41 8.22
Plagioclase 11.36 17.89 41.23 40.75
Orthoclase 6.55 8.24 19.10 16.95
Diopside 5.11 5.87 - -
Hypersthene 16.22 33.25 8.87 9.02
Olivine 41.19 15.23 - -
Sphene 1.62 2.80 7.34 9.61
Apatite 0.58 1.54 1.78 1.71
Hematite 15.23 13.74 13.20 12.96
Ilmenite 1.48 0.95 0.92 0.28
Chromite 0.54 0.31 0.01 -
Rutile - - 0.85 0.22
Zircon 0.04 0.04 0.12 0.12
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
40.00 42.00 44.00 46.00 48.00 50.00 52.00
Na2O+K2O(wt%)
SiO2 (wt%)
40. 30
The norm calculations included nickel, barium, strontium, chromium and zirconium
trace elements values to accommodate other minerals that contain these elements in
their variations such as plagioclase. The purpose of norm calculations was to fully
distinguish minerals in the hostrocks that occurred as alteration products and the
primary minerals. Serpentine, sericite, smectite and talc were found to be alteration
products of olivine, pyroxene and plagioclase.
Analysis by ACF (Al2O3 – CaO –FeO) ternary shows that, basalt is richer in CaO and
Al2O3 than dolerite, hence the abundance of plagioclase in basalt than dolerite. FeO
in the ternary groups together FeO, MgO and MnO, and these are more abundant in
dolerite than in basalt. Dolerite contains more olivine and pyroxene, and therefore
more mafic than basalt. Calculations are shown in Appendix B and proportions of
CaO – Al2O3 – FeO in both basalt samples and dolerite samples are shown in Figure
4.4.
Figure 4.4: ACF ternary diagram showing basalt and dolerite samples.
The AKF (Al2O3 – K2O – FeO) ternary also used to analyse for potassium feldspar.
Dolerite contains less than 5% potassium feldspar and basalt contains more than
10% potassium feldspar. According to AKF ternary plot the FeO group components
in dolerite are greater than 60%, calculations are shown in Appendix C. The AKF plot
is shown in Figure 4.5. But both the ACF and AKF plots groups the FeO and MgO,
cannot clearly indicate with component is greater than the other. To compensate for
this problem, an AFM (Al2O3 – FeO – MgO) plot was used.
41. 31
Figure 4.5: AKF plot showing proportions of basalt and dolerite samples.
AFM plot indicates that both basalt and dolerite are FeO deficient, but dolerite is
much richer in MgO and basalt is richer in Al2O3 (calculations shown in Appendix D).
Figure 4.6: AFM plot showing proportions of basalt and dolerite samples.
42. 32
The magnesite grade distribution follows the concentration of MgO in the hostrocks;
MgO is higher in dolerite than basalt. Alteration is also a major controlling factor for
magnesite; highly altered hostrocks contain high magnesite. The relationship
between MgO concentration and magnesite enrichment in the hostrocks is shown in
Figure 4.7. Both magnesite and MgO decrease with increasing silica (SiO2).
Figure 4.7: Relationship between MgO and MgCO3 in hostrocks.
4.3.2 Analysis of Trace Elements
Igneous rocks can be classified on the basis of modal mineralogy or major element
composition and it has been noted that the trace elements concentrations of some
magma types are sufficiently distinctive that they can be used as an aid to
classification (Blatt et al., 2006). As indicated by most of the hypothesis of this study,
some degree of alteration of the hostrocks is expected, and therefore Blatt et al.
(2006) recommends the use of trace elements to aid to classification and this is
because some trace elements are generally considered „immobile‟ under these
circumstances, and thus their concentrations may better reflect those of the
unaltered rock.
Trace elements are usually classified into compatible (prefer to be in mineral) or
incompatible (prefer to be in liquid), and furthermore, the incompatible elements are
classified into mobile (very easily leached by water-rich fluids) and immobile (not
easily leached by water-rich fluids) (Misra, 2012). Based on SiO2 versus trace
elements (Fig.4.8), compatible elements decrease with increasing silica content and
while incompatible elements increase with silica content, Ni is more partitioned into
olivine, Cr in pyroxene while Sr and Ba are partitioned in plagioclase.
0
10
20
30
40
50
60
70
41 43 45 47 49 51
measuredinwt%
SiO2 (wt%)
MgO
Magnesite
43. 33
Figure 4.8: Distribution of selected trace elements in volcanic rocks from Folovhodwe
Mine. The rocks include dolerite (SiO2 = 41.14% & 43.90%); basalt (SiO2 = 50.25% &
51%). In this system, Ni, Cr, Sr and Ba are compatible, whereas Zr is incompatible.
Magnesite increases with increasing Ni and Cr while Sr and Ba increase with
decreasing magnesite enrichment in the hostrocks. Zr generally increases with
increasing silica and is most partitioned in zircon. Rare earth elements were also
analysed and basalt had higher concentration of rare earth elements than dolerite.
This is because rare earth elements are mobile and are highly partitioned with
increasing silica.
In the rare earth elements, compatibility increases with increasing atomic number,
but the partitioning of these elements is governed by Goldschmidt‟s rules: “when two
ions possessing the same charge but different radii compete for a particular lattice
site, the ion with the smaller radius would be incorporated preferentially because the
smaller ion forms a stronger ionic bond;” and “ions whose charges differ by one unit
may substitute for one another provided electrical neutrality of the crystal is
maintained by coupled (or compensatory) substitution” (Goldschmidt, 1937).
La, Ce, Nd, Sm, and Yb are all strongly partitioned into clinopyroxene, slightly into
plagioclase and rarely into orthopyroxene and olivine. They were used to indicate
clinopyroxene-plagioclase phases within their respective rocks. The abundance
pattern of these elements indicates abundance of clinopyroxene and plagioclase in a
rock. Clinopyroxene and plagioclase are highly portioned in basalt than dolerite, and
therefore lower rare earth elements concentrations are associated with high
magnesite enrichment. Different concentrations of rare earth elements are shown in
Figure 4.9.
0
500
1000
1500
2000
2500
41.14 42.14 43.14 44.14 45.14 46.14 47.14 48.14 49.14 50.14
Traceelement(ppm)
SiO2 (wt%)
Sr
Cr
Ni
Zr
Ba
44. 34
Figure 4.9: A plot showing rare earth elements concentrations in hostrocks samples
(normalised by chondrite values sourced from Sun and McDonough, 1989).
With the help of Ce/Zr and Ce/Nb ratios (Fig.4.10), dolerite has shown higher degree
of partial than basalt (normalising values are shown in Appendix E).
Figure 4.10: Ce/Zr vs. Ce/Nb showing different degrees of mantle partial melting
(normalised by mid-oceanic ridge basalt sourced from Thompson (1982)).
0
50
100
150
200
250
300
350
57 58 59 60 61 62 63 64 65 66 67 68 69 70
Sample/Chondrite
REE in increasing atomic number
La Ce Nd Sm Yb
Basalt
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3 3.5 4
Ce/Nb
Ce/Zr
45. 35
Winter (2010) notes that the degree of partial melting increases with a decreasing
proportion of incompatible element as they are progressively diluted. Therefore, the
extent of partial melting increases from upper left to lower right along the mantle
partial melting curve. The positive slopes shown by the samples indicate light rare
earth elements enrichment (Fig.4.11).
Figure 4.11: Rare earth elements distribution in magnesite (normalised by chondrite
values sourced from Sun and McDonough, 1989).
High grade magnesite has low Ce and Nd concentrations while low grade has high
La and Yb. Sm concentration is generally constant throughout all grades of
magnesite. The general concentration of rare earth elements in magnesite are highly
influenced by their concentration in the hostrocks. To further analyze how magnesite
is related to the hostrocks, Hg analysis was undertaken.
The Hg concentrations comparison of the hostrocks and magnesite shows that the
Hg concentration in magnesite is influenced by alteration of the hostrocks. Magnesite
that is hosted in highly altered hostrock has higher Hg concentrations and is of high
grade. This is because mercury is a highly mobile element and can be easily
affected by alteration of the hostrocks. Figure 4.12 plots the concentrations of
mercury in both the hostrocks and the associated magnesite against the grade of
magnesite. Magnesite has generally higher mercury concentrations as compared to
the hostrocks.
The pattern of concentration in magnesite follows the pattern of concentration in
hostrocks, hence indicative of close relationship between them. A dolerite sample
without significant (low) alteration showed equal partitioning of the mercury
concentration between magnesite and the hostrock (Fig.4.12). Therefore, the
0
20
40
60
80
100
120
57 58 59 60 61 62 63 64 65 66 67 68 69 70
Sample/Chondrite
REE in increasing atomic number
Sample 1
Sample 3
Sample 4
Sample 2
La Ce Nd Sm Yb
46. 36
mercury concentration within magnesite increase with increasing alteration of the
hostrock, hence, the difference in concentration between magnesite and the
associated hostrock (Fig.4.12) is indicative of the degree of alteration the hostrock
has undergone. In this case, alteration in basalt is generally constant throughout and
is moderate, while in dolerite it varies from one point to another and varies from very
low to very high. In dolerite, the grade of magnesite increases with the degree of
alteration.
Figure 4.12: Close relationship between magnesite and hostrocks based on mercury
analysis.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
7 17 27 37 47 57
Hg(ppb)
MgCO3 (wt%)
Hostrocks
Magnesite
=0.2
DoleriteBasalt
47. 37
5. CHAPTER FIVE: DISCUSSION, CONCLUSIONS, AND RECOMMENDATIONS
5.1 Discussion
According to Strydom (1998), magnesite in the Tshipise magnesite field occurs in
weathered sheets of olivine dolerite and limburgitic or basalts of the Letaba
Formation of the Karoo Supergroup. This magnesite is cryptocrystalline to
microcrystalline in texture and occurs as veins within the hostrocks (Fig.5.1).
Figure 5.1: Veins of magnesite in a weathered dolerite.
The major rock forming minerals in the dolerite are olivine, orthopyroxene,
clinopyroxene and plagioclase in that order of abundance, while in basalt they
include plagioclase and clinopyroxene. The AFM plot depicts that all the hostrocks
are iron deficient but rich in magnesium, and therefore allows the consideration that
each group of mafic minerals consist most entirely of the Mg end-member and rarely
Fe end-member i.e. forsterite and enstatite are the most abundant olivine and
pyroxene in the hostrocks respectively.
Nedawaila (2009) suggested that the magnesite at Folovhodwe deposit was
enriched by hydrothermal fluids rich in CO2. The mercury analysis showed a close
relationship between magnesite and the hostrocks (Fig.4.12). The fluid responsible
48. 38
to establish such a relationship was meteoric water where carbon dioxide is usually
dissolved during rainfall as shown in the following equation:
H2O + CO2 → H+
+ HCO3
-
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (1)
Another source of CO2 is the sedimentary CO2 which was trapped within sandstone
that forms the hanging wall. Due to permeability and porosity of sandstone, dissolved
CO2 was able to flow throughout and to infiltrate further down into basalt and
dolerite, causing chemical weathering. The chemical weathering occurred in the form
of carbonation and was controlled by the state of aqueous fluid. According to Möller
(1989), the state of the aqueous fluid is controlled by pH:
H2O + CO2 → H2CO3 [low pH] - - - - - - - - - - - - - - - - - - - - - - - - (2)
H2CO3 → H+
+ HCO3
-
[mid pH] - - - - - - - - - - - - - - - - - - - - - - - (3)
HCO3
-
→ H+
+ CO3
2-
[high pH] - - - - - - - - - - - - - - - - - - - - - - -(4)
It is more obvious that reaction (4) will be favoured in basic rocks due to high pH and
more carbonate ion will be available to form magnesite. More magnesite is favoured
in a more basic rock than lesser basic rock, and this is the reason why dolerite has
high grade magnesite than basalt. Magnesite is formed through the following
simplified reaction:
Mg2+
+ CO3
2-
→ MgCO3 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -(5)
The actual reactions are more complex and involve freeing the Mg2+
ion from Mg-rich
minerals, and the formation of other minerals as by-products. The following reaction
shows how the Mg2+
is freed from enstatite to form talc and Mg2+
ion:
Mg8Si8O24 + 4H+
→ Mg6Si8O20(OH)4 + Mg2+
- - - - - - - - - - - - - - - - - - - - - - - -(6)
Olivine was altered into enstatite and further into serpentine and talc; sepiolite,
palygorskite and smectite are by-products that formed after all the available
carbonate ion had been used up. The higher amount of CaO compared to MgO in
basalt favoured the formation of dolomite rather than magnesite, and smectite as by-
product. In dolerite, the higher concentration of MgO favoured formation of
magnesite, and palygorskite and sepiolite as by-products.
49. 39
5.2 Conclusions
Magnesite within the hostrocks was formed through carbonation of forsterite
and enstatite. Forsterite and enstatite were the primary sources of the Mg2+
ion in which they were altered into amphibole, serpentine and talc in the
presence of water (equation 6). The high content of olivine and pyroxene in
dolerite led to the abundance of the Mg2+
ion during alteration, hence, the
dominance of magnesite in dolerite.
The magnesite formed within the hostrocks was dependant on the availability
of the carbonate ion within the aqueous fluid. The released Mg2+
ion reacted
with the CO3
2-
to form magnesite (equation 5). In the absence of carbonate
ion, the Mg2+
ion reacted with other ions to form sepiolite, smectite
palygorskite (Appendix F).
The availability of carbonate ion was highly influenced by the pH of the
aqueous fluid. Therefore, high pH (equation 4) favoured higher grade
magnesite formation. Dolerite was more mafic than basalt, hence, reactions in
dolerite favoured high pH conditions than in basalt.
5.3 Recommendations
This research recommends the following:
Highly altered dolerite contains high grade magnesite, thus mining should
target highly altered dolerite.
The role of temperature and pressure during the formation of magnesite was
not clearly understood, thus there is need to undertake further studies along
this line.
50. 40
References
Barker, O. B., Brandl, G., Callaghan, C. C., Eriksson, P. G., and van der Neut, M.,
2006. The Soutpansberg and Waterberg Groups and the Blouberg formation. In
Johnson, M. R., Anhaeusser, C. R. and Thomas, R. J. (Eds.): The Geology of South
Africa. Geological Society of South Africa, Johannesburg/Council for Geoscience,
Pretoria, pp.301 – 308.
Blatt, H., Tracy, R. J., and Owens, B. E., 2006. Petrology: Igneous, sedimentary and
metamorphic. 3rd ed., W. H. Freeman and Company, New York, pp.65 – 115.
Bordy, E. M., 2000. Sedimentology of the Karoo Supergroup in the Tuli Basin,
Limpopo River Area, South Africa. Unpublished M.Sc. Thesis, Rhodes University,
Grahamstown, pp.45 – 89.
Brandl, G., 1981. The geology of the Messina area. Explanation of the Sheet 2230.
Geol. Surv. S. Afr., pp.1 – 35.
Brandl, G., 1992. Geological map of the Limpopo Belt and its Environs. Contribution
to “A filed workshop on granulites and deep crustal tectonics, 1990” scale 1:500 000.
Geol. Surv. S. Afr., pp.5 – 10.
Bristow, J. W., 1982. Geology and structure of Karoo volcanic and sedimentary rocks
of the northern and central Lebombo. Trans. Geol. Soc. S. Afr., 85, pp.167 – 178.
Burke, K. C. A. and Dewey, J. F., 1973. Plume-generated triple junctions: key
indicators in applying plate tectonics to old rocks. J. Geol. 86, pp.406 – 433.
Cadle, A. D.; Cairncross, B.; Christie, A. D. M. and Roberts, D. L., 1993. The Karoo
Basin of south Africa: type basin for the coal bearing deposits of the southern
African. Int. J. Coal Geol., 23, pp.117 – 157.
Campbell, S. D. G.; Oesterlen, P. M.; Blenkinsop, T. G.; Pitfield, P. E. J. and
Munyanyiwa, H., 1991. A provisional 1:2 500 000 scale tectonic map and the
tectonic evolution of Zimbabwe. Ann. Zimb. Geol. Surv. XVI, 31 – 52 (p.44).
Catuneanu, O.; Beumont, C. and Waschbusch, P. 1997. Interplay of static loads and
subduction dynamics in foreland basin: Reciprocal stratigraphies and the “missing”
peripheral bulge. Geology, 25, pp.1087 – 1090.
Catuneanu, O.; Hancox, P. J. and Rubidge, B. S. 1998. Reciprocal flexural behavior
and contrasting stratigraphies: a new basin development model for the Karoo
retroarc foreland system, South Africa. Basin Research, 10, pp.417 – 439.
Chinoda, G., Moyce, R., Matura, N. and Owen, R. 2009. The Geology of the
Limpopo Basin Area: Integrated Water Resource Management for Improved Rural
Livelihoods. Mineral Resource Centre. pp. 10-36.
51. 41
Cox, K. G. 1970. Tectonics and volcanism of the Karoo period. In: T. N. Clifford and I.
G. Gass (eds.): African magmatism and tectonics. Edinburgh: Oliver and Boyd,
pp.211 – 236.
Cox, K. G. 1992. Karoo igneous activity, and the early stages of the break-up of
Gondwanaland. In B. C. Storey; T. Alabaster and R. J. Pankhurst (eds.): magmatism
and causes of continental break-up. London; Geol. Soc. Spec. Publ., 68, pp.137 –
148.
Cox, K. G. and Bristow, J. W. 1984. The Sabie River basalt Formation of the
Lebombo Monocline and south-east Zimbabwe. Spec. Publ. Geol. Soc. S. Afr., 13,
pp.125 – 147.
Cox, K. G.; Johnson, R. L.; Monkman, L. J.; Stillman, C. J.; Vail, J. R. and Wood, D.
N. 1965. The geology of Nuanetsi Igneous Province Phil. Trans. Royal. Soc. Ser. A.,
257, pp.71 – 218.
Deer, W. A.; Howie, R. A. and Zussman, J. (1992). An introduction to the rock
forming minerals, 2nd
Ed., Pearson Prentice Hall, London, pp.53 -77.
Duncan, R. A.; Hooper, P. R.; Rehacek, J.; Marsh, J. S. and Duncan, A. R., 1997.
The timing and duration of the Karoo igneous event, southern Gondwana. J.
Geophys.Res., 102, No.B8 pp.18127 – 18138.
Duncan, A. R., and Marsh, J. S., 2006. The Karoo Igneous Province. In Johnson, M.
R., Anhaeusser, C. R. and Thomas, R. J. (Eds.): The Geology of South Africa.
Geological Society of South Africa, Johannesburg/Council for Geoscience, Pretoria,
pp.501 – 504.
Goldschmidt, V. M., 1937. The principles of distribution of chemical elements in
minerals and rocks. Journal of the Chemistry Society of London, 1937, pp.665 – 673.
Hawkesworth, C.; Kelly, S.; Turner, S.; le Roex, A. and Storey, B., 1999. Mantle
processes during Gondwana break-up and dispersal. J. Afr. Earth Sci., 28, pp.239 –
261.
Hess, F. L., 1908. The magnesite deposits of California. Washington Government
Printing Office, U.S.A, pp.23 – 34.
Johnson, M. R.; Van Vuuren, C. J.; Hegenberger, W. F.; Key, R. and Shoko, U.,
1996. Stratigraphy of the Karoo Supergroup in southern Africa: an overview. J. Afr.
Earth Sci., 23(1), pp.3 – 15.
Johnson, M. R.; Van Vuuren, C. J.; Visser, J. N. J.; Cole, D. I.; Wickens, H. de V.;
Christie, A. D. M. and Roberts, D. L., 1997. The foreland Karoo Basin, South Africa.
In: R. C. Selly (ed.): African Basins. Sedimentary basins of the world, 3. Amsterdam:
Elsevier Science B. V, pp.269 – 317.
Johnson, M. R.; van Vuuren, C. J.; Visser, J. N. J.; Cole, D. I.; Wickens, H. de V.;
Christie, A. D. M.; Roberts, D. L. and Brandl, G., 2006. Sedimentary rocks of the
Karoo Supergroup. In Johnson, M. R.; Anhaeusser, C. R. and Thomas, R. J. (eds.):
52. 42
The Geology of South Africa, Johannesburg/Council for Geoscience, Pretoria,
pp.461 – 494.
Kadir, S., Kolayli, H. and Eren, M., 2012. Genesis of sedimentary- and vein-type
magnesite deposits at Kop Mountain, NE Turkey. Turkish Journal of Earth Sciences.
21, pp.09 – 015.
Kramers, J. D., McCourt, S. and van Reenen, D. D., 2006. The Limpopo belt. In
Johnson, M. R., Anhaeusser, C. R. and Thomas, R. J. (Eds.): The Geology of South
Africa. Geological Society of South Africa, Johannesburg/Council for Geoscience,
Pretoria, pp.209 – 227.
McCourt, S. and Brandl, G., 1980. A lithostratigraphic subdivision of the Karoo
sequence in the north-eastern. Transvaal Annuals Geol. Surv. S. Afr., 14(1), pp.51 –
56.
McDonough, W. F., and Sun, S. S., 1995. The composition of the earth. Chemical
Geology, 120, pp.223 – 253.
Misra, K. C., 2012. Introduction to geochemistry: Principles and applications, 2nd
ed., Wile-Blackwell, New York, pp.75 – 94.
Möller, P., 1989. Nucleation processes of magnesite. Mineral Deposits, 28, pp.287–
292.
Mtimkulu, M. N., 2009. A provisional basinal study of the Waterberg-Karoo, South
Africa. Unpublished M.Sc. Thesis, University of Pretoria, Pretoria, pp.21 – 63.
Nedawaila, L. D., 2009. Geology and mineralisation of magnesite deposit in
Folovhodwe, Limpopo, South Africa. Unpublished Honors thesis, University of
Venda, Thohoyandou, pp.4 – 29.
Nesse, W. D., 2012. Introduction to mineralogy, 2nd
Ed., Oxford University Press,
New York, pp.63 – 78.
Olafsson, M. and Eggler, D.H., 1983. Phase relations of amphibole, amphibole-
carbonate, and phlogopite-carbonate peridotite: Petrologic constraints on the
asthenosphere Earth and Planetary Science letters, 64, pp.305 – 31.
Palgrave, K.C., 2002. Trees of Southern Africa. Struik Publishers, Cape Town, pp.23
– 67.
Perkins, D. and Henke, K. R., 2004. Minerals in thin sections, 2nd
Ed, Pearson
Education, Inc. USA, pp.44 – 73.
Philpotts, A. R., and Ague, J. J., 2009. Principles of igneous and metamorphic
petrology, 2nd Ed., Cambridge University press, New York, pp.182 – 300.
Rust, I. C., 1975. Tectonic and sedimentary framework of Gondwana basins in
southern Africa. In Campbell, K. S. W. (ed.): Gondwana geology. Canberra:
Australian National University Press, pp.537 – 564.
53. 43
Snyman, C. P., 1998. Coal in The Mineral Resources of South Africa (M. G. C.
Wilson and C. R. Anhaeusser, eds.): Handbook, Council for Geoscience, 16, pp.145
– 192.
Strydom, J. H., 1998. Magnesite. In Wilson, M. G. C. and Anhaeusser, C. R. (eds.):
The Mineral Resources of South Africa: Handbook, Council for Geoscience, 16,
p.447 – 448.
Sun, S. S., and McDonough, W. F., 1989. Chemical and isotopic systematics of
ocean basalts: Implications for mantle composition and process. In Saunders, A. D.
and Norry, M. J. (eds.): Magmatism in the ocean basins. Geological Society of
London Special Publication, 42, pp.313 – 345.
Thompson, R. N., 1982. Magmatism of the British Tertiary volcanic province. Scottish
Journal of Geology, 18, pp.49 – 107.
Truster, F. C., 1945. The geology of a post-Karoo fault trough in the Zoutpansberg
District, Transvaal. Geol. Soc. S. Afr., 48, pp.143 – 159.
Tshisikhawe, M.P., 2002. Trade of indigenous medicinal plants in the Northern
Province, Venda region: their ethnobotanical importance and sustainable use. M.Sc.
dissertation, University of Venda for Science and Technology, Thohoyandou, South
Africa, pp.15 – 30.
Vail, J. R.; Hornung, G. and Cox, K. G., 1969. Karoo basalts of the Tuli syncline,
Rhodesia Bull. Volcan., 33, pp.398 – 418.
Van der Berg, H. J., 1980. Die sedimentology van die Soutpansberg-Steenkoolveld
met spesiale verwysing na Steenkool vorming. Unpublished M.Sc. Thesis, University
of Orange Free State, Bloemfontein, pp.64 – 84.
Wilson, M. G. C., 1989. A preliminary appraisal of the mineral potential of Venda
based on a reconnaissance geochemical soil sampling survey and literature review.
Unpublished M.Sc. Thesis, Rhodes University, Grahamstown, pp.45 – 68.
Wilson, M. G. C. and Anhaeusser, C. R., 1998. The Mineral Resources of South
Africa: Handbook, Council for Geoscience, 16, p. 447 – 448.
Winter, J. D., 2010. Principles of igneous and metamorphic petrology, 2nd ed.,
Prentice Hall, New York, pp.69 – 102.
Zhang, R. Y. and Liou, J. G., 1994. Significance of magnesite paragenesis in
ultrahigh-pressure metamorphic rocks. American mineralogist, 79, pp.397 – 400.
https://maps.google.co.za/folovhodwe_mine, satellite image of Folovhodwe Mine,
Limpopo, South Africa, 2013.
55. Appendix A: Excel-plugin norm calculation program
Appendix B: ACF ternary calculations
A = [Al2O3 + Fe2O3] - [Na2O + K2O]
C = [CaO] - 3.33[P2O5]
F = [FeO + MgO + MnO]
Basalt sample 1: A = [14.24 + 13.11] – [2.92 + 3.12]
= 27.35 – 6.02
= 21.31
C = [6.30] – 3.33[0.763]
= 3.75921
= 3.76
56. F = [0.31 + 3.54 + 0.119]
= 3.969
= 3.97
Total [A + C+ F] = 29.04
Plotting values:
A = (21.31/29.04) x 100%
= 73.38%
C = (3.76/29.04) x 100%
= 12.95%
F = (3.9729.04) x 100%
= 13.67%
Basalt sample 2:
A = [13.92 + 12.90] – [2.77 + 2.76]
= 21.29
C = [7.10] – 3.33[0.733]
= 4.66
F = [0.00 + 3.61 + 0.123]
= 3.73
Total [A + C + F] = 29.68
Plotting values:
A = (21.29/29.68) x 100%
= 71.73%
C = (4.66/29.68) x 100%
57. = 15.70%
F = (3.73/29.68) x 100%
= 12.57%
Dolerite sample 1:
A = [4.16 + 15.08] – [0.79 + 1.05]
= 17.40
C = [2.98] – 3.33[0.246]
= 2.16
F = [0.52 + 30.75 + 0.170]
= 31.44
Total [A + C + F] = 51
Plotting values:
A = (17.40/51) x 100%
= 34.12%
C = (2.16/51) x 100%
= 4.24%
F = (31.44/51) x 100%
= 61.65%
Dolerite sample 2:
A = [6.76 + 13.32] + [0.73 + 1.22]
= 22.03
C = [5.31] – 3.33[0.646]
= 3.16
58. F = [0.25 + 22.47 + 0.158]
= 22.88
Total [A + C + F] = 48.07
Plotting values:
A = (22.03/48.07) x 100%
= 45.83%
C = (3.16/48.07) x 100%
= 6.57%
F = (22.88/48.07) x 100%
= 47.60%
59. Appendix C: AKF ternary calculations
A = [Al2O3 + Fe2O3] - [Na2O + K2O + CaO]
K = [K2O]
F = [FeO + MgO + MnO]
Basalt sample 1:
A = [14.24 + 13.11] – [2.92 + 3.12 + 6.30]
= 15.01
K = 3.12
F = [0.31 + 3.54 + 0.119]
= 3.97
Total (A + K + F) = 22.1
Plotting values:
A = (15.01/22.1) x 100%
= 67.92%
K = (3.12/22.1) x 100%
= 14.12%
F = (3.97/22.1) x 100%
= 17.96%
Basalt sample 2:
A = [13.92 + 12.90] – [2.77 + 2.76 + 7.10]
= 14.19
K = 2.76
F = [0.00 + 3.61 + 0.123]
60. = 3.73
Total (A + K + F) = 20.68
Plotting values:
A = (14.19/20.68) x 100%
= 68.62%
K = (2.76/20.68) x 100%
= 13.35%
F = (3.73/20.68) x 100%
= 18.04%
Dolerite sample 1:
A = [4.16 + 15.08] – [0.79 + 1.05 + 2.98]
= 14.42
K = 1.05
F = [0.52 + 30.75 + 0.170]
= 31.44
Total (A + K + F) = 46.91
Plotting values:
A = (14.42/46.91) x 100%
= 30.74%
K = (1.05/46.91) x 100%
= 2.24%
F = (31.44/46.91) x 100%
= 67.02%
61. Dolerite sample 2:
A = [6.76 + 13.32] – [0.73 + 1.22 + 5.31]
= 12.82
K = 1.22
F = [0.25 + 22.47 + 0.158]
= 22.88
Total (A + K + F) = 36.92
Plotting values:
A = (12.82/36.92) x 100%
= 34.72%
K = (1.22/36.92) x 100%
= 3.30%
F = (22.88/36.92) x 100%
= 61.97%
62. Appendix D: AFM ternary calculations
A = [Al2O3]
F= [FeO]
M = [MgO]
Basalt sample 1:
A = 14.24
F = 0.31
M = 3.54
Total (A + F + M) = 18.09
Plotting values:
A = (14.24/18.09) x 100%
= 78.72%
F = (0.31/18.09) x 100%
= 1.71%
M = (3.54/18.09) x 100%
= 19.57%
Basalt sample 2:
A = 13.92
F = 0.00
M = 3.61
Total (A + F + M) = 17.53
Plotting values:
A = (13.92/17.53) x 100%
63. = 79.41%
F = 0%
M = (3.61/17.53) x 100%
= 20.59%
Dolerite sample 1:
A = 4.16
F = 0.52
M = 30.75
Total (A + F + M) = 35.43
Plotting values:
A = (4.16/35.43) x 100%
= 11.74%
F = (0.52/35.43) x 100%
= 1.47%
M = (30.75/35.43) x 100%
= 86.79%
Dolerite sample 2:
A = 6.76
F = 0.25
M = 22.47
Total (A + F + M) = 29.48
Plotting values:
A = (6.76/29.48) x 100%
64. = 22.93%
F = (0.25/29.48) x 100%
= 0.85%
M = (22.47/29.48) x 100%
= 76.22%
65. Appendix E: Normalising values
Trace elements Thompson (1982) Mid-Oceanic Ridge Basalt values
Sr 120
K2O 0.15
Rb 2
Ba 20
Th 0.2
Ta 0.18
Nb 3.5
Ce 10
P2O5 0.12
Zr 90
Hf 2.4
Sm 3.5
TiO2 1.5
Y 30
Yb 3.4
Sc 40
Cr 250
Trace elements McDonough and Sun (1995) chondrite values
Ba 6.9
Rb 0.35
Th 0.042
K 120
Nb 0.35
Ta 0.02
La 0.329
Ce 0.865
Sr 11.8
Nd 0.63
P 46
Sm 0.203
Zr 6.84
Hf 0.2
Ti 620
Tb 0.052
Y 2
Tm 0.034
Yb 0.22
