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COMPOSITION AND PROVENANCE OF QUARTZITES OF THE MESOARCHEAN WITWATERSRAND SUPERGROUP, SOUTH AFRICA
by
Craig Harry Blane
Dissertation Submitted in fulfillment of the requirements for the degree MAGISTER SCIENTAE in GEOLOGY in the FACULTY OF SCIENCE at the UNIVERSITY OF JOHANNESBURG
Supervisor: Prof. N. J. Beukes Co-Supervisor: Prof. J. Gutzmer
September 2013
Table of Contents
Chapter 1 : Introduction ...... 1
1.1 Motivation ...... 1
1.2 Aims...... 2
1.3 Basic Concepts ...... 2
1.4 Methods ...... 3
Chapter 2 : Geological Overview ...... 5
2.1 Location and Description ...... 5
2.2 Lithostratigraphy ...... 5
2.3 Geochronology ...... 8
2.4 Metamorphic History ...... 8
2.5 Tectonic history ...... 9
2.6 Eustatic and Tectonic controls on sedimentary infill ...... 11
2.7 Correlation with the Mozaan Group ...... 11
2.8 Existing Provenance Studies ...... 11
Chapter 3 : Lithofacies and Stratigraphic Setting of Samples ...... 14
3.1 Major Lithofacies ...... 14
3.2 Sampling ...... 15
3.3 Hospital Hill Subgroup: West Rand Group ...... 21
3.3.1 Orange Grove Formation ...... 21
3.3.2 Parktown Formation ...... 21
3.3.3 Brixton Formation ...... 22
3.3.4 Bonanza Formation ...... 23
3.4 Government Subgroup: West Rand Group ...... 23
3.4.1 Promise Formation ...... 23
3.4.2 Coronation Formation ...... 27
3.4.3 Tusschenin Formation ...... 27
3.4.4 Palmietfontein Formation ...... 29
3.4.5 Afrikander Formation ...... 29
3.5 Jeppestown Subgroup: West Rand Group ...... 30
3.5.1 Koedoeslaagte Formation ...... 30
3.5.2 Rietkuil Formation ...... 31
3.5.3 Babrosco Formation ...... 31
3.5.4 The Crown Lava Formation ...... 33
3.5.5 Roodepoort Formation ...... 33
3.5.6 Maraisburg Formation ...... 33
3.6 Johannesburg Subgroup : Central Rand Group ...... 34
3.6.1 Blyvooruitzicht Formation ...... 34
3.6.2 Main Formation ...... 34
3.6.3 Randfontein Formation ...... 34
3.6.4 Luipaardsvlei Formation ...... 35
3.6.5 Krugersdorp Formation ...... 35
3.6.6 Booysens Formation ...... 35
3.7 Turffontein Subgroup: Central Rand Group ...... 37
3.7.1 Kimberley Formation ...... 37
3.7.2 Elsburg Formation ...... 37
3.7.3 Mondeor Formation ...... 37
3.8 Summary of Stratigraphic Variation and Samples Taken ...... 38
Chapter 4 : Mineralogy ...... 41
4.1 Introduction ...... 41
4.2 Allogenic/Monomineralic Minerals and Composite Rock Particles ...... 41
4.2.1 Monomineralic grains ...... 41
4.3 Allogenic Rock Particles ...... 44
4.3.1 Lithoclasts ...... 44
4.4 Accessory Allogenic Minerals...... 45
4.5 Authigenic Minerals ...... 47
Chapter 5 : Petrography ...... 50
5.1 Introduction ...... 50
5.2 Inner shelf orthoquartzites ...... 50
5.2.1 Current-dominated Orthoquartzites ...... 51
5.2.2 Wave-dominated Orthoquartzites ...... 52
5.2.3 Controls and Composition by Depositional Environment ...... 53
5.3 Middleshelf Argillaceous Quartzite ...... 53
5.4 Fluvial Quartzite ...... 55
5.4.1 Immature Fluvial Quartzites ...... 55
5.4.2 Mature Fluvial Quartzites ...... 56
5.5 Diamictite ...... 58
5.6 Stratigraphic Petrographic Variations ...... 59
5.6.1 Hospital Hill Subgroup ...... 59
5.6.2 Government Subgroup ...... 60
5.6.3 Jeppestown Subgroup ...... 62
5.6.4 Johannesburg Subgroup ...... 63
5.6.5 Turffontein Subgroup ...... 64
Chapter 6 : Heavy Mineral Analysis ...... 65
6.1 Introduction ...... 65
6.2 Mineral Liberation Analyser Results ...... 65
6.3 Point Counting Analyses and Broad Stratigraphic Variations ...... 67
6.4 Chromite to Zircon Ratio ...... 71
Chapter 7 : Chromite Mineral Chemistry ...... 74
7.1 Introduction ...... 74
7.2 Zonation in Chromite Grains ...... 76
7.3 Chromite Variation ...... 76
7.4 Tectonic Setting ...... 78
7.5 Conclusions ...... 81
Chapter 8 : Geochemistry ...... 82
8.1 Major Element Geochemistry ...... 82
8.1.1 Basic Data Set ...... 82
8.2 Relationship between Chemical and Mineralogical composition ...... 83
8.2.1 Influence of feldspars, sericite and muscovite ...... 83
8.2.2 Effect of Chlorite concentration ...... 83
8.2.3 Effect of Sphene and Leucoxene Concentration ...... 85
8.2.4 Effect of Quartz Concentration ...... 85
8.2.5 Effect of Sorting ...... 86
8.3 Major Element Chemostratigraphy ...... 86
8.3.1 Hospital Hill Subgroup ...... 86
8.3.2 Government Subgroup ...... 89
8.3.3 Jeppestown Subgroup ...... 89
8.3.4 Johannesburg Subgroup ...... 89
8.3.5 Turffontein Subgroup ...... 90
8.3.6 Lateral variation of quartzite chemical composition ...... 90
8.3.7 Summary of Major Element Variations ...... 90
8.4 Trace Element Geochemistry ...... 91
8.4.1 Background ...... 91
8.4.2 Trace Element Composition of Rocks ...... 92
8.4.3 Evaluation of Factors Influencing Trace Element Composition ...... 95
8.4.4 Determining Provenance ...... 100
8.4.5 Stratigraphic Variation in Provenance Indicators ...... 106
8.4.6 Rare Earth Elements as Provenance Discriminant ...... 110
8.4.7 Tectonic Discrimination ...... 112
Chapter 9 : Summary and Conclusion ...... 113
9.1 Characteristics of the Witwatersrand Supergroup Source Rocks...... 113
9.2 Evolution of the Greater Witwatersrand Basin ...... 117
9.3 Conclusion ...... 120
References Cited ...... 121
Appendices
Appendix I : XRF Major Element Geochemistry
Appendix II : XRF Trace Element Geochemistry
Appendix III : INNA Trace Element Geochemistry
Declaration
I declare that this research in my own work and that it was conducted under the supervision of Prof. N. J. Beukes and Prof J. Gutzmer. No part of this research has been submitted in the past, or is being submitted, for a degree at another university.
C. H. Blane
Acknowledgements
I would like to thank Prof. Nic Beukes for the opportunity, the constant guidance, patience and help throughout this research project. This thesis would not be concluded without your resolute motivation.
Thanks go out to Oom Hennie, Tannie Elsa and all the staff in the sample preparation facility and in the analytical facility at UJ.
Bradley Guy and Justin Cochrane are thanked for their support and friendship.
The National Research Foundation of South Africa and the Department of Geology at the University of Johannesburg are thanked for their financial support.
The staff at Spectrau is also acknowledged for contributing their extensive equipment and knowledge.
AngloGold-Ashanti, the Council of Geosciences and Goldfields are thanked for granting access to their drill cores which provided samples for this study.
Finally, I would like to thank my family and girlfriend for the continued support and encouragement.
Abstract
COMPOSITION AND PROVENANCE OF QUARTZITES OF THE MESOARCHEAN WITWATERSRAND SUPERGROUP, SOUTH AFRICA by Craig H. Blane University of Johannesburg, Department of Geology, Auckland Park 2006, South Africa ______
The Mesoarchean Witwatersrand Supergroup is a remarkably well preserved siliciclastic dominated cratonic platform succession located on the Kaapvaal Craton in South Africa. The vast gold resources which have been mined since 1886 make it relevant for study.
The study aimed to identify significant provenance shifts throughout the depositional life of the basin which should be reflected in the in heavy mineral populations and the geochemical composition of the siliciclastic rocks. The study identified major changes in the source rock compositions through the basin lifespan and inferred major tectonic events during the life of the basin.
It was found that the mechanical effects of sorting in different depositional environments tended to obscure provenance shifts, but with careful evaluation of the various factors in play significant provenance shifts could be identified. It was found that these provenance shifts corresponded closely with major unconformity sequence boundaries identified by Beukes (1995). These major provenance shifts are a record of a major tectonic event during the development of the basin.
The Hospital Subgroup records a passive trailing margin, fed by a combination of felsic and ultra-mafic source rocks. Within the Hospital Hill Subgroup, there is a trend of increasing ultramafic components in the source area with increasing stratigraphic height. This trend is believed to reflect progressive unroofing of tonalite and greenstone belt complexes over the life of the Hospital Hill Subgroup.
At the base of the Promise Formation a basin wide unconformity is present, which marks a shift from mature shallow marine and outer shelf sediments of the Hospital Hill Subgroup to immature fluvial quartzites for the Government and Jeppestown Subgroups (Beukes, 1995). In addition to the major change in depofacies that was recognised by Beukes (1995), this study found evidence for a shift in provenance to generally more fractionated source rocks, that
were heterogeneous, but well mixed. The presence of lithoclasts indicates a possible metamorphic component was also present in the source area. This is consistent with a source area containing granitoid batholiths, and granite plutonism which is associated with early subduction tectonics and volcanic arc formation during the deposition of the Government and Jeppestown Subgroups (Wronkiewicz and Condie, 1987 and Poujol, et al., 2003, Kositcin and Krapez, 2004).
Another important basin wide unconformity is present at the base of the Johannesburg Subgroup, and marks another major provenance change. These rocks are chemically more mature than the Government and Jeppestown Subgroups and represent a shift to an immature fluvial depositional setting related to basin closure (Beukes, 1995). A shift to moderate Th:Sc and La:Sc suggests a less fractionated mix of source rocks. The disappearance of the lithoclasts indicates that the metamorphic source rocks no longer supplied material to the basin. A small increase in the chromite to zircon ratio also suggests that some unfractionated source rocks were present. The narrow range in Th:Sc, La:Sc, Nb:Y ratios suggests that a homogeneous source area is present, but this is contradicted by the highly variable zircon ages measured by Kositcin and Krapez (2004), so the narrow spread might indicate that the rocks are very well mixed. Zircon populations measured by Kositcin and Krapez (2004) suggest that source terrain of the Johannesburg Subgroup probably consisted of a mixture of the granitoid batholiths from which the Government and Jeppestown Subgroups are a derived as well as some intermediate igneous material with ages of 3000-2870 ma. This would reflect incorporation of syntectonic granitoid plutons into the source areas, Kositcin and Krapez, (2004).
The Turffontein Subgroup rocks are very coarse and chemically mature, but they display poor to moderate sorting and rounding. The rocks were deposited in a fluvial environment but marine quartzites are not uncommon. It is believed that these rocks were transported in a high energy environment, but the duration of transportation was short. This allows for effective winnowing but insufficient time for physically mature rocks with well-rounded grains to develop, explaining the mature chemical composition but immature physical composition. The source rocks of the Turffontein Subgroup were probably the same as the Johannesburg Subgroup with the higher energy mode of transportation responsible for the observed increase in Zr:Ti ratio. It would also explain the scarcity of feldspars and chlorite in the Turffontein Subgroup. Th:Sc and Nb:Y ratios suggest highly fractionated source rocks, but
care must be taken because the mature nature and coarse grainsize of these rocks make trace element analyses unreliable. The zircon population indicates the presence of 3090-3060ma (Kositcin and Krapez, 2004) granite batholiths, as well as 3000-2870 Ma (Kositcin and Krapez, 2004) syntectonic granite plutons, as well as ancient granitoid gneiss (Kositcin and Krapez, 2004) in the source area.
This study has provided new support for a foreland basin origin of the Witwatersrand Supergroup, proposed by Beukes (1995), Beukes and Nelson (1995) and Nhleko (2003), resulting from orogenic collision of the Witwatersrand and Kimberley blocks along the western margin of the Witwatersrand block. The Amalia, Kraaipan and Madibe greenstone belts and Colesberg Magnetic Anomaly are probably the only remaining remnants of this orogeny today.
Chapter 1 : Introduction
1.1 Motivation
The Mesoarchean Witwatersrand Supergroup (Fig. 1) is a 7000m (Tankard et al, 1982) thick siliciclastic dominated cratonic platform succession located on the Kaapvaal Craton in South Africa. The Witwatersrand is correlative to the Mozaan Group of the Pongola Supergroup (Beukes and Cairncross, 1991), and were deposited in the period 2960 ma – 2840 ma (Gutzmer, et al, 1999). Together the Greater Witwatersrand-Mozaan basin represents one of the best preserved and earliest continental depositional basins known. Considering its age, the Witwatersrand Basin has been exposed to relatively minor metamorphic events and the resulting low metamorphic grade makes its study important to understanding surface processes on Earth during the Mesoarchean. In addition, the Witwatersrand succession hosts the largest gold province ever found and accounts for over 40% of all gold ever mined (Sanders et al, 1994). This fact rather than its academic importance explains the long history of research on the Witwatersrand Supergroup rocks since the discovery of gold in 1886. Even with such interest in the succession, there is much that is unknown or poorly understood. In the past research has mostly focused on the auriferous conglomerate beds with less attention on the intervening quartzite and shale. Especially with regards to the provenance of the sedimentary fill very little work has been published (Wronkiewicz and Condie, 1987; Kositcin and Krapez, 1996; Holdsworth, 1996). It is believed that a comprehensive study of the heavy mineral populations in the succession as a whole, and their geochemical composition, may result in a better understanding of the basin fill and possible shifts or changes in the composition of the source areas or provenance of the sediment.
1 Introduction
1.2 Aims
The primary aim of this study is the identification of provenance shifts in the Witwatersrand succession using petrographic, heavy mineral and bulk rock geochemical analyses of mostly coarse to fine grained quartzite units sampled over the entire preserved stratigraphy.
1.3 Basic Concepts
A vast number of processes and events influence the characteristics of a sedimentary rock (Taylor and McLennan, 1985; Johnsson, 1993; McLennan et al., 1990). These include The composition of the source area Weathering which is predominantly controlled by climate Sorting of grains, as well as chemical and mechanical breakdown of grains, during transportation and deposition. Diagenesis and Metamorphism Tectonic setting, which has a strong influence on the composition of the source area, and some aspects of transportation and deposition Transport slope and the subsidence in the basin.
These processes are all interrelated. In order to understand the history of a sediment and especially the composition of the source terrain, a complicated process of individually evaluating the effect of each process is necessary, so that its influence can be understood. In some cases one process can so severely alter the characteristics of the sediment so that all other information on earlier processes that played a role in the formation of the rock is lost. With a goal of understanding the original source composition, detailed petrographic and geochemical studies of the rock units have to be undertaken in order to try and evaluate the influence of each of these processes on the rock. Only once the influence of these processes is understood, valid interpretations can be made as to the characteristics of source area.
2 Introduction
1.4 Methods
The study is based on a set of 103 samples collected from fresh drill core which represents a complete section through the Witwatersrand Supergroup. The techniques used to achieve the mentioned aims include: Logging of drill core Petrography: Optical microscopy, scanning electron microscopy and X-ray diffraction analysis of the samples. Bulk Rock Geochemistry: X-ray Fluorescence and Neutron Activation analyses Heavy Mineral Analysis: A standard dense medium separation was performed and when required, electromagnetic separations were carried out. Point Counting: Point counting of heavy mineral separates was carried out under a binocular microscope aided by a mechanical counter. Phases which could not be identified were picked from the sample and an EDS analysis was done to aid identification. Mineral Chemistry: Microprobe analyses were carried out on chromite grains.
A total of 140 thin sections from 103 samples were prepared for petrographic studies, which were undertaken at the Department of Geology, University of Johannesburg. Equipment used included a Leica DMLP reflected light research microscope and a JOEL JSM-5600 Scanning Electron Microscope which is equipped with a Noran X-ray detector and Noran Vantage Software. These studies were supplemented with X-Ray diffraction (XRD) analysis of milled sample on a Phillips PW 1710 diffractometer. X-rays were generated with a cobalt tube at 40kV.
Samples for geochemical analyses were crushed and milled to a fine powder. Major and trace element analyses were performed by XRF at Spectrau (the Central Analytical Facility of the Faculty of Science at the University of Johannesburg). XRF analyses were carried out on a Magix X-ray fluorescence spectrometer operated at 4kV, from Panalytical. To prepare pressed powder pellets for XRF analyses, 8 grams of sample powder and 4 grams of binder, composed of 90% cellulose and 10% wax, were used. Glass beads were prepared by fusing sample powder into a borate glass. One gram of sample powder was mixed with 1g flux (50% Li meta-borate and 50% tetra borate). Samples were heated in an induction-heated Pt- crucible. Based on the results of XRF analyses 68 samples were selected for trace element
3 Introduction analysis using instrumental neutron activation analysis (INAA) at ACME Analytical Laboratories, Vancouver, Canada.
Heavy mineral separates were obtained by crushing samples in a jaw crusher and milling in a swing mill until the sample would pass through a 400 µm sieve. The fines were washed off with water in a glass beaker. Samples were then washed with acetone and dried in an oven to prevent metal fragments derived from crushing and milling from rusting. The sample was placed in vessel containing bromoform and the dense minerals were tapped off and recovered in filter paper. The heavy minerals were washed with acetone to remove any bromoform and dried in an oven. A hand magnet was used to remove any magnetic particles from the separate. Observation under a binocular microscope revealed that the only magnetic particles were steel mill fragments. When deemed necessary, a magnetic separation was performed on the Frantz isodynamic magnetic separator at 0.3amps and 0.7amps, and samples were weighed on an electronic micro-balance. Point counting was performed on the magnetic separate using a binocular microscope and a mechanical counter. Zircon, chromite and rounded pyrite grains were counted and occurrences of other minerals exhibiting rounded morphologies were noted. The accuracy of the point counting technique was evaluated with a Mineral Liberation Analyser. 10 sample concentrates were mounted in resin using the bottle- mount technique.
A selection of chromite grains from particularly chromite-rich samples were hand-picked and mounted in epoxy blocks for chemical analyses. The instrument used was a Jeol Superprobe 733 equipped with an EDS and ibex Spectrometer, which is housed at Spectrau, University of
Johannesburg. TiO2 concentrations were measured with an IBEX wavelength dispersive spectrometer at an accelerating voltage of 15kv and a beam current of 100 na. The concentrations of other elements were determined by energy dispersive X-ray spectrometry (EDX).
4
Chapter 2 : Geological Overview
2.1 Location and Description
The Mesoarchean Witwatersrand Supergroup is crescent shaped, mainly preserved in a structurally controlled erosional “basin” located on the Kaapvaal craton. It outcrops in Gauteng, Mpumalamga and the Northwestern Province of South Africa (Fig 2-1) with large parts of the preserved succession covered by younger rock successions in for example the Free State. The basin fill is greater than 7000 km thick and is predominantly composed of siliciclastic rocks which are preserved over 45000 km2 (Fig 2.1) (Tankard et al, 1982). The structurally preserved basin trends in a northeast direction and is asymmetric in cross section, with maximum sediment accumulation in the northwest.
The erosional relict of the Witwatersrand Basin is bound to the north and west by large faults (Fig 2.1). Syndepositional displacement on this fault system during deposition of the Turffontein Subgroup is responsible for the preservation of the Witwatersrand Basin (Myers et al., 1990). As a result the basin is considered a structurally preserved erosional remnant of the original depository that was most probably much larger than what is currently preserved.
2.2 Lithostratigraphy
The Witwatersrand Supergroup unconformably overlies Archean basement granite- greenstone terrane and the strata of the volcano-sedimentary Dominion Group. It is in turn unconformably overlain by the Ventersdorp Supergroup.
The Witwatersrand Supergroup is subdivided into two fundamentally different groups. The lower West Rand Group is composed of shale and fine immature marine
5 Geological Overview shelf wackestones, interbeded with shallow marine quartzites (Fig. 2.2). Less mature fluvial quartzite and conglomerate become more abundant towards the top of the West Rand Group. The Central Rand Group contains rare shale and wackestone, and is dominated by immature fluvial quartzite and conglomerate (Fig. 2.2) (Beukes, 1995).
Figure 2.1 : The Location of the Witwatersrand Basin on the Kaapvaal Craton. W. Witwatersrand Basin, P-Pietersburg Greenstone Belt, M-Murcherson Greenstone Belt, G - Giyani Greenstone Belt, B- Barberton Greenstone Belt, K-Kraaipan Group, A-Amalia Greenstone Belt.
The West Rand Group is further subdivided into three Subgroups and the Central Rand Group into two. These subgroups are all bounded by major basin wide scale unconformities or disconformities (McCarthy et al., 2000, Beukes 1995) (Fig 2.2).
6 Geological Overview
Figure 2.2 : Cross section through the Witwatersrand Supergroup after Beukes 1995
7 Geological Overview
2.3 Geochronology
The age of the Witwatersrand Supergroup is broadly bracketed by an age of 3074±6 Ma (Armstrong et al., 1991) for underlying Dominion Group lavas and ages of 2709±4 Ma and 2714±8 Ma (Armstrong, et al, 1991) for felsic lavas of the overlying Ventersdorp Supergroup. The maximum age of deposition is further constrained to 2985±15 Ma which is the youngest concordant zircon in the Orange Grove Formation (Kositcin and Krapez, 2004). The minimum age for the Witwatersrand Supergroup is 2764±5 Ma, based on the age of authigenic xenotime in the Central Rand Group (England et al., 2001). The internal age of the Witwatersrand Supergroup is poorly constrained. The only age available is one of 2194±8 Ma on the Crown Lava in the Jeppestown Subgroup (Armstrong et al., 1991). However, it has been suggested that this age may be unreliable as it may represent a xenocrystic zircon population (Armstrong et al., 1991, Kositcin and Krapez, 2004). A maximum age of 2894 Ma for the Johannesburg Subgroup is suggested by Poujol et al. (1999), based on detrital zircons from the Krugersdorp Formation.
The age of the Witwatersrand Supergroup can be further refined using correlations with the coeval Pongola Supergroup, which Beukes and Cairncross (1991) correlated, based on stratigraphic similarities. A pre-folding quartz porphyry which intrudes the upper part of the Mozaan Group yields an age of 2837±5 Ma (Gutzmer et al, 1999). This upper part of the Mozaan Group correlates with the Booysens Formation (Beukes and Cairncross, 1991). It is thus considered by Gutzmer et al (1999) to define the upper age limit for the Johannesburg Subgroup. In summary it can be stated that the Witwatersrand succession was deposited between about 2960 Ma and 2840 Ma.
2.4 Metamorphic History
The Witwatersrand Supergroup has undergone a long history of metamorphic and alteration events. Phillips and Law (1994) suggest that the Witwatersrand Supergroup was affected by a regional scale lower greenschist facies metamorphism, with peak conditions of 350 ± 50°C at 2-3kb.
8 Geological Overview
The central area of the preserved Witwatersrand Basin in the Vredefort area has experienced the highest grade of metamorphism, up to amphibolite facies (Gibson and Wallmach, 1995). This has been attributed to the formation of the 2.02 Ma Ventersdorp dome (Bisschoff, 1982), However, more recent studies in this area have shown that amphibolite-grade metamorphic rocks of the Vredefort area may simply represent burial of the sediments by the Transvaal Supergroup under conditions of high geothermal gradients associated with Bushveld Complex magmatism (Gibson and Walmach, 1995).
A later event, of slightly lower grade, related to the formation of the Vredefort Dome (2.02 Ma) has also been identified (Gibson and Reimold, 1999). This dome is thought to have formed following impact by a meteorite (Reimold and Gibson, 2006).
Abundant sericite-chlorite alteration from hydrothermal fluids has been recognised in the sedimentary rocks of the Witwatersrand Basin (Frimmel, 1997). Events responsible for alteration, include burial by Transvaal strata, potassium metasomatism associated with intrusion of the Bushveld Complex and hydrothermal alteration associated with the formation of the Vredefort Dome (Frimmel, 1997). The relative importance of each of these events varies in different parts of the basin (Frimmel, 1997).
2.5 Tectonic history
The Kaapvaal craton represents the amalgamation of granite greenstone terrains, formed by tectonic accretion of crustal fragments from subduction related processes in a manner that resembles modern plate tectonic processes (De Witt, et. al, 1992). A series of lineaments defined by shear zones and greenstone belts are present on the Kaapvaal craton. These are oriented in a west-east direction, and include the Thabazimbi fault zone and the Murchison, Pietersburg and Gyani greenstone belts (Fig 2.1). An additional set of structures trending in a north-south direction are also present, and include the Kraaipan Group, Amalia greenstone belts and the Colesberg magnetic anomaly (Fig.2.1) (Schmitz et al, 2004). Based on these structures the Kaapvaal craton divided into two new areas, i.e. eastern Witwatersrand block and western Kimberley block (Fig. 2.1).
9 Geological Overview
Early models described the upper part of the Witwatersrand Basin as an intra- continental half-graben with a normal faulted northwestern margin (Brock & Pretorius, 1964; Vos, 1975). In the early 1980’s, observations of the basin cross- sectional asymmetry, characteristics of the sediment infill and evidence of synsedimentary compression tectonics, led to popularity of models involving compressive tectonics and an Andean style foreland basin, which formed in response to lithospheric flexure (Van Biljon, 1980; Burke et al. 1986; De la Rey Winter, 1987; Stanistreet and McCarthy, 1991; Coward et al., 1995; Catuneanu, 2001; Kositcin and Krapez, 2004). Older models suggest compression normal to the north-east, south- west axis (Coward et al, 1995), but more recent models suggest subduction and collision along the northern and western margins, with the Zimbabwe craton and Kimberly block respectively (Stanistreet and McCarthy, 1991; Catuneanu, 2001; Schmitz et al., 2004). The age of collision between the Zimbabwe and Kaapvaal cratons, resulting in the Limpopo orogeny, is highly controversial, but is believed to have occurred at 2.68 Ma (Treloar and Blenkinsop, 1995), well after the formation of the Witwatersrand deposits. Catuneanu, (2001) showed that subduction of oceanic lithosphere beneath the Kaapvaal craton is sufficient to cause flexure of the lithosphere, which may be responsible for the formation of the basin prior to continental collision. Accretion of the Kimberly block onto the western margin of the Kaapvaal craton is believed to have occurred between 2.93-2.88 Ma (Schmitz et al., 2004).
Thermal subsidence related to late stage rifting of the Dominion Group, and passive margins are commonly cited as important for formation of the lower West Rand Group. The transition between thermal subsidence/passive margin and foreland basin tectonics varies between authors (Burke et al, 1986; Stanistreet and McCarthy, 1991; Coward et al, 1995; Kositcin and Krapez, 2004).
Beukes (1995), Camden-Smith (1980) and Minter and Loen (1991) have shown than sediment input into the Witwatersrand Basin occurred from a northern and western basin margins, and at least during early West Rand times, the basin was open to the ocean. The analysis of palaeocurrent directions as well as fining directions and isopach mapping by Beukes (1995)¸ has revealed that a shift from a dominantly northern source to a western and north western source may be related increased
10 Geological Overview influence from compressional tectonics along the western to northwestern margin of the basin.
2.6 Eustatic and Tectonic controls on sedimentary infill
There are two competing ideas about the eustatic and tectonic controls on deposition of the Witwatersrand succession. Myers et al. (1990) and Stanistreet and McCarthy (1991) suggest that deposition was controlled by relative tectonic movement of various blocks of the Kaapvaal craton. These basement blocks respond individually to regional tectonic forces by rotation and tilting, which causes the complex syndepositional folding and faulting observed in the rocks of the Central Rand and Upper West Rand Groups (Myers et al, 1990). In contrast, Beukes (1995), and Beukes and Nelson (1995), prefer a predominantly eustatic control on sedimentation, but palaeo-highs and areas of increased subsidence are recognised and probably represent variable subsidence in the floor of the depository.
2.7 Correlation with the Mozaan Group
The Mozaan Group forms the upper portion of the Pongola Supergroup. It is a well preserved siliciclastic sequence located to the southeast of the Witwatersrand Basin (Fig 2.1). Correlations of the Mozaan Group with the lower portion of the Witwatersrand Supergroup by Beukes and Cairncross (1991), indicates that these two sequences are part of the same depository. A foreland basin model for the greater depository, with the Mozaan and Witwatersrand Basins separated by a forebulge seems likely (Nhleko, 2003; Catuneanu, 2001).
2.8 Existing Provenance Studies
A long history of research of the rocks of the Witwatersrand succession is recorded in the literature. A number of different techniques have been employed to try and understand the details of the source rocks from which the Witwatersrand Supergroup was derived.
11 Geological Overview
Wronkiewicz and Condie (1987) conducted whole rock analyses on shales from seven intervals through the Witwatersrand Supergroup. The study showed that the Witwatersrand source rocks contained larger mafic and ultramafic components relative to what is characteristic of more recent Phanerozoic crust. With increased stratigraphic height, increasing proportions of granitic and greenstone sources fed the basin at the expense of tonalitic sources. The Orange Grove Formation is the exception to this trend. Shales of the Orange Grove Formation are enriched in incompatible elements which requires dominantly granitic source rocks.
Hallbauer and Barton (1994) analysed quartz pebbles from auriferous conglomerates for trace elements. Quartz pebbles from conglomerates of the older Central Rand Group (i.e. Steyn, Vaal and Carbon Leeder conglomerates) are enriched in the immobile compatible trace elements Co, Cr and Sc relative to the younger Ventersdorp contact reef at the base of the Ventersdorp Supergroup. The older reefs also display well developed negative europium anomalies. This is interpreted as a shift to more fractionated source rocks for the Ventersdorp Contact Reef.
Perhaps the most convincing provenance studies are those based on zircon geochronology. The most thorough study of detrital zircons age populations in the Witwatersrand Basin was conducted by Kositcin and Krapez (200)4. The study measured the ages of at least 60 detrital zircon grains per sample for 14 samples collected from the Witwatersrand Basin. The study showed that zircons modes of ~3090 Ma-3060 Ma were dominant in the West Rand Group rocks, while Central Rand Group rocks contain both younger (~3000-2870 Ma) and older (>3100 Ma) zircon modes which are not present in the West Rand Group rocks. The zircon ages of the Central rand Group are far more diverse that the narrow range of zircon ages exhibited by the West Rand Group. This indicates that the West Rand Group did not undergo tectonic rejuvenation and supports the thermal subsidence and passive margin origins of the West Rand Group. The sources of Central Rand Group zircons are more diverse than the West Rand Group and the zircon age modes increase in complexity up-section. This in interpreted to indicate the inclusion of more diverse sources by unroofing of successively younger granitoids from tectonic rejuvenation (Kositcin and Krapez, 2004). These findings of Kositcin and Krapez (2004) are
12 Geological Overview broadly similar to those of Wronkiewicz and Condie (1987) and form a good base for further provenance studies.
13
Chapter 3 : Lithofacies and Stratigraphic Setting of Samples
3.1 Major Lithofacies
The common rocks observed in the Witwatersrand Supergroup are divided into six distinct lithofacies, namely: Immature quartzite and conglomerate, Orthoquartzite, Argillaceous quartzite, Diamictite, Shale Iron formation and magnetic mudstone
The immature quartzites are generally medium to coarse grained and rather poorly sorted. They frequently contain scattered small pebbles and minor argillaceous matrix. Grains are typically rounded to subangular. conglomerate, and gritstone commonly occur as subordinate beds within immature quartzite units. Immature quartzite units are typically composed of stacked fining-upward facies assemblages with a scour surface at the base. Sedimentary structures include trough and planar cross-bedding. Palaeocurrent directions as measured by Beukes (1992) and Beukes and Nelson (1992), are mostly unimodal. These deposits are thought to have formed in a braided fluvial environment (Beukes, 1995). In some cases the immature quartzite may be chemically mature, but the term is retained to distinguish these rocks from the finer grained, better sorted orthoquartzite.
The orthoquartzites are typically light coloured rocks with a variety of grain sizes. They are composed almost exclusively of rounded quartz grains which are well
14 Lithofacies and Stratigraphic Setting of Samples sorted. Beukes and Buxton (1992) differentiated current-dominated orthoquartzite, which are supermature usually with trough cross bedding as the dominant sedimentary structure present. Wave-dominated orthoquartzite contains hummocky cross stratification which in some instances is draped by fine silty bands. These two orthoquartzite varieties are formed by different processes in different depositional environments, but have in general similar lithological characteristics. Palaeocurrent assemblages measured by Beukes (1995) and Beukes and Nelson (1992), indicate bimodal and polymodal paleocurrent distributions.
The term argillaceous quartzite is used here to describe units of fine to very fine- grained wackestone, sandstone, shales and siltstone. Two different types are present. The first is graded wackestones that are fine grained rocks composed of argillaceous or silty matrix and usually contain fine sand-sized grains. They were most probably deposited on middle shelf environments by storm-wave action (Beukes, 1995). The second type of argillaceous quartzite which is commonly observed is finely interlaminated siltstone mudstone and fine quartzite. These rocks are thought to have been deposited by bottom currents in the transition zone between storm-wave base and deep sub-wave base distal shelf environments (Beukes, 1995).
Diamictites are dark in colour and are composed of coarse grained clasts in a dark argillaceous matrix. They are composed of stacked sequences which are weakly upward fining with erosive bases.
Shale, banded iron formation and magnetic mudstone are common facies in the Witwatersrand Supergroup especially the lower West Rand Group. They represent muddy outer shelf deposits, and starved shelf deposits (Beukes, 1995; Beukes and Cairncross, 1992). The reader is referred to Smith (2007) for more detailed descriptions of the iron-rich magnetic mudstone and iron formation lithofacies.
3.2 Sampling
The study is based on 91 samples of quartzite and 3 of diamictite collected from fresh diamond drill core (Table 3.1), covering the entire Witwatersrand succession (Fig.3.1 and 3.2).
15 Lithofacies and Stratigraphic Setting of Samples
A chip sampling technique was employed to ensure that samples are representative of an entire lithological unit. This involved collecting a small sample at approximately two meter intervals. The samples were then mixed to achieve a representative sample of the unit.
Cores that were sampled in the proximal part of the depository in the Klerksdorp area include TF1, BAB1 and JY8 (Figs. 3.1 and 3.2). TF1 is located approximately 100 km north of Klerksdorp at Mazista. This core is the only available intersection of the Hospital Hill Subgroup, and is stored by the Council for Geoscience at Donkerhoek near Pretoria. The lower Government Subgroup was sampled from core BAB1 stored by Goldfields at Klerksdorp. The upper Government and Jeppestown Subgroups were sampled from core JY8, stored by AngloGold-Ashanti at the Carletonville coreshed. The Central Rand Group in the ‘proximal’ area of the basin was sampled from core DK12 supplied courtesy of Wits Gold at the Carletonville coreshed of Goldfields South Africa.
Figure 3-1 : The Location of Boreholes from which samples were collected. Note, sediment input directions are known only from the Central Rand Group.
16 Lithofacies and Stratigraphic Setting of Samples
Table 3.1 Location and description of samples studied
Sample Drill Core Sample Location Sample Description Interpreted Depofacies Number
TF1 LOG BH-TF1 Lower Orange Grove Formation Mature Medium grained Orthoquartzite (was this not gritty) Inner Shelf Shallow Marine TF1 OGM1 BH-TF1 Middle Orange Grove Formation Fine grained Argillaceous Quartzite Intermediate Shelf Marine Mature Fine Grained Orthoquartzite with occasional silty TF1 OGM2 BH-TF1 Middle Orange Grove Formation Inner Shelf Shallow Marine interlaminations TF1 OGU1 BH-TF1 Upper Orange Grove Formation Mature Fine-Medium grained Orthoquartzite Inner Shelf Shallow Marine Mature Medium grained Orthoquartzite with occasional TF1 OGU2 BH-TF1 Upper Orange Grove Formation Inner Shelf Shallow Marine Argillaceous bands TF1 OGU3 BH-TF1 Upper Orange Grove Formation Fine grained Mature Orthoquartzite Inner Shelf Shallow Marine TF1 Bul1 BH-TF1 Bullskop Marker Mature Medium Grained Orthoquartzite, Feldspathic Inner Shelf Shallow Marine TF1 Bul2 BH-TF1 Bullskop Marker Mature Medium Grained Orthoquartzite, Feldspathic Inner Shelf Shallow Marine TF1 Bul3 BH-TF1 Bullskop Marker Mature Medium Grained Orthoquartzite, Feldspathic Inner Shelf Shallow Marine TF1 Rip1 BH-TF1 Ripple Marker Mature Medium-Coarse Orthoquartzite Inner Shelf Shallow Marine TF1 Rip2 BH-TF1 Ripple Marker Immature Silty Argillaceous Quartzite Intermediate Shelf Marine TF1 Spec1 BH-TF1 Speckled Marker Mature Coarse grained Orthoquartzite Inner Shelf Shallow Marine TF1 Spec2 BH-TF1 Speckled Marker Immature Silty Argillaceous Quartzite Intermediate Shelf Marine TF1 VER 1 BH-TF1 Versterkop Member Mature Medium Grained Orthoquartzite Inner Shelf Shallow Marine Mature Medium to Fine grained Quartzite mixed with TF1 VER 2 BH-TF1 Versterkop Member Inner Shelf Shallow Marine argillaceous material TF1 WIT 1 BH-TF1 Witkop Member Mature Medium grained Orthoquartzite Inner Shelf Shallow Marine TF1 WIT 2 BH-TF1 Witkop Member Mature Medium to Fine Grained Orthoquartzite Inner Shelf Shallow Marine TF1 RV BH-TF1 Range View Member Mature Medium to Coarse Grained Quartzite Braided Fluvial TF1 TOP BH-TF1 Brixton(Uncertain) Mature Medium to Coarse Grained Quartzite Braided Fluvial BH- Thick mature medium grained quartzite which is well sorted BAB1 S1 Bonanza Member Inner Shelf Shallow Marine BAB1 but composed of multiple upward fining sequences. BH- Mature Very Coarse to Gritty, poorly sorted but well rounded, Braided Fluvial possibly Shallow BAB1 S2 Bonanza Member BAB1 Glassy Quartzite. Marine BH- BAB1 S3 Bonanza Member Immature Fine Grained Slightly Argillaceous Quartzite Braided Fluvial BAB1 BH- BAB1 S4 Bonanza Member Coarse Grained Immature Quartzite Braided Fluvial BAB1 BH- BAB1 S5 Promise Formation Medium Grained Highly Altered Inner Shelf Shallow Marine BAB1 BH- BAB1S6 Breaunanda Member Immature Coarse Quartzite Inner Shelf Shallow Marine BAB1 BH- BAB1 S6B Breaunanda Member Fine grained Argillaceous Quartzite Intermediate Shelf Marine BAB1 BH- BAB1 S7 Hamberg Member Immature Medium Grained Quartzite Braided Fluvial BAB1 BH- BAB1 S9 Tusschenin Formation Immature Medium to Coarse Grained Quartzite Braided Fluvial BAB1 JY8 S2 BH-JY8 Elangslaagte Formation Gritty Bands from the base of the Elandslaagte Formation Braided Fluvial Mature Medium Grained Orthoquartzite bands from base of JY8 S3 BH-JY8 Elangslaagte Formation Mature Braided Fluvial the Elandslaagte Formation. Defiantly some marine influence. JY8 S4 BH-JY8 Elangslaagte Formation Immature Medium Grained Quartzite Braided Fluvial JY8 S5 BH-JY8 Afrikander Formation Interlaminated fine grained quartzite and siltstone Intermediate Shelf Marine JY8 S6 BH-JY8 Afrikander Formation Immature Medium Grained Quartzite Inner Shelf Marine Immature Medium grained Quartzite represents the average JY8 S7 BH-JY8 Afrikander Formation Braided Fluvial of the Noyedale Member Orthoquartzite collected from the tops of fining upwards JY8 S8 BH-JY8 Afrikander Formation Braided Fluvial sequences in the Noyedale Member Mature Gritty Quartzite collected from the top of the Noyedale JY8 S9 BH-JY8 Afrikander Formation Braided Fluvial Member JY8 S10 BH-JY8 Koedoeslaagte Formation Mature Fine to Medium grained Orthoquartzite Inner Shelf Shallow Marine Braided Fluvial could also be an JY8 S11 BH-JY8 Rietkuil Formation Immature Medium Grained Quartzite Shallow Shelf Marine Sand Selection of Mature Medium Grained Orthoquartzite from the JY8 S12 BH-JY8 Babrosco Formation Inner Shelf Marine Sand base of the Babrosco Formation JY8 S13 BH-JY8 Babrosco Formation Immature Medium Grained Flat Laminated Quartzite Inner Shelf Marine sand Immature Medium to Coarse Grained Trough Cross-Bedded JY8 S14 BH-JY8 Babrosco Formation Braided Fluvial Quartzite JY8 S15 BH-JY8 Roodepoort Formation Immature Medium Grained Quartzite Inner shelf marine Immature Medium to Coarse Grained Trough Cross Bedded JY8 S16 BH-JY8 Roodepoort Formation Inner Shelf marine influence Quartzite JY8 S17 BH-JY8 Roodepoort Formation Immature Medium to Coarse Grained Flat Bedded Quartzite Inner Shelf Marine – CHECK Transitional Shelf Shallow JY8 S18 BH-JY8 Roodepoort Formation Immature Fine grained Argillaceous Quartzite Marine
17 Lithofacies and Stratigraphic Setting of Samples
Sample Drill Core Sample Location Sample Description Interpreted Depofacies Number
JY8 MAR BH-JY8 Maraisburg Formation Immature Medium to Coarse Grained Quartzite Braided Fluvial DK12 S19 BH-DK12 Blyvooruitzicht Formation Mature Medium to Coarse Grained Quartzite Braided Fluvial DK12 S20 BH-DK12 Main Formation Immature Coarse Grained Fluvial Quartzite Braided Fluvial DK12 S18 BH-DK12 Randfontein Formation Immature Medium Grained Quartzite Braided Fluvial DK12 S17 BH-DK12 Randfontein Formation Immature Coarse Grained to Gritty Quartzite Braided Fluvial DK12 S16 BH-DK12 Randfontein Formation Immature Medium Grained Quartzite Braided Fluvial DK12 S15 BH-DK12 Luipardsvlei Formation Immature light coloured Feldspathic bands Braided Fluvial DK12 S14 BH-DK12 Luipardsvlei Formation Immature Medium Grained Quartzite Braided Fluvial DK12 S13B BH-DK12 Luipardsvlei Formation Immature Gritty Quartzites Braided Fluvial DK12 S13 BH-DK12 Krugersdorp Formation Immature Medium Grained Quartzite Braided Fluvial DK12 S12 BH-DK12 Krugersdorp Formation Mature Medium to Coarse grained Quartzite Braided Fluvial DK12 S11 BH-DK12 Booysens Formation Mature Fine to Medium Grained Orthoquartzite Inner Shelf Shallow Marine DK12 S10 BH-DK12 Booysens Formation Immature Fine grained Quartzite with silty intercalations Intermediate Shelf Marine DK12 S9 BH-DK12 Booysens Formation Immature Fine grained Quartzite with silty intercalations Intermediate Shelf Marine DK12 S7 BH-DK12 Kimberly Formation Immature Medium to Coarse grained Quartzite Braided Fluvial DK12 S8 BH-DK12 Kimberly Formation Immature Medium to Coarse grained Quartzite Braided Fluvial DK12 S3 BH-DK12 Elsburg Formation Mature Fine to Medium grained Orthoquartzite Inner Shelf Shallow Marine DK12 S4 BH-DK12 Elsburg Formation Mature Coarse Grained Quartzite Braided Fluvial DK12 S5 BH-DK12 Elsburg Formation Mature Fine to Medium grained Orthoquartzite Braided Fluvial DK12 S6 BH-DK12 Elsburg Formation Mature Fine to Medium grained Orthoquartzite Braided Fluvial DK12 S6A BH-DK12 Elsburg Formation Mature Coarse grained Orthoquartzite Braided Fluvial DK12 S2 BH-DK12 Mondeor Formation Mature Gritty Quartzite Braided Fluvial DK12 S1 BH-DK12 Mondeor Formation Mature Gritty Quartzite Braided Fluvial AM1 4 BH-AM1 Promise Formation Immature Fine grained argillaceous Quartzite Intermediate Shelf Marine Immature Fine-Medium grained Argillaceous Quartzite with Inner Shelf to Transitional Shelf AM1 5 BH-AM1 Corronation Formation thin siltstone intercalations Shallow Marine AM1 6 BH-AM1 Tusschenin Formation Mature Fine grained quartzite Braided Fluvial AM1 7 BH-AM1 Tusschenin Formation Immature Medium Grained slightly Argillaceous Quartzite Braided Fluvial AM1 8 BH-AM1 Palmietfontein Formation Mature Medium Grained Orthoquartzite Inner Shelf Shallow Marine AM1 9 BH-AM1 Palmietfontein Formation Fine Grained Quartzite Inner Shelf Shallow Marine AM1 10 BH-AM1 Palmietfontein Formation Immature silty argillaceous Quartzite Intermediate Shelf Marine AM1 11 BH-AM1 Elandslaagte Formation Coarse to Gritty immature Feldspathic, Quartzite Braided Fluvial AM1 12 BH-AM1 Elandslaagte Formation Mature Coarse grained Quartzite Braided Fluvial AM1 13 BH-AM1 Elandslaagte Formation Immature Medium grained Orthoquartzite Braided Fluvial AM1 14 BH-AM1 Afrikander Formation Mature Medium Grained Quartzite Inner Shelf Shallow Marine AM1 15 BH-AM1 Afrikander Formation Selection of Immature Medium Grained Quartzites Inner Shelf Shallow Marine AM1 16 BH-AM1 Afrikander Formation Selection of Fine Grained Mature Quartzites Inner Shelf Marine Quartzite AM1 S17 BH-AM1 Koedoeslaagte Formation Mature Medium Grained Orthoquartzite Inner Shelf Marine Quartzite AM1 S18 BH-AM1 Babrosco Formation Mature Medium Grained Orthoquartzite Inner Shelf Shallow Marine Inner Shelf to Intermediate Shelf AM1 S19 BH-AM1 Babrosco Formation Fine-Medium Grained argillaceous quartzite Shallow Marine AM1 S20 BH-AM1 Roodepoort Formation Mature Medium grained Orthoquartzites Shallow Shelf Marine AM1 S21 BH-AM1 Roodepoort Formation Immature Very Fine grained Silty Quartzite Intermediate Shelf Marine AM1 S22 BH-AM1 Roodepoort Formation Immature Quartz Wackestone Intermediate Shelf Marine Intermediate to Shallow Shelf AM1 S23 BH-AM1 Roodepoort Formation Medium Grained argillaceous Quartzite Marine AM1 S25 BH-AM1 Maraisburg Formation Immature Medium to Coarse Grained Quartzite Braided Fluvial AM1 S26 BH-AM1 Main Formation Mature Medium Grained Quartzite Braided Fluvial AM1 S27 BH-AM1 Main Formation Mature Coarse Grained Orthoquartzite Braided Fluvial BH- BAB1 Prom Promise Formation Reworked Diamictite Diamictite BAB1 BH- BAB1 Corr Corronation Formation Diamictite Diamictite BAB1 BH- AM1 Lag Afrikander Formation Diamictite Diamictite BAB1
18 Lithofacies and Stratigraphic Setting of Samples
A comparative distal basinal interval was also sampled from core AM1 close to Edenville in the Free State (Fig. 3.1). This core intersects the succession from the base of the Central Rand Group in the Maraisburg Formation to the low part of the West Rand Group in the Promise Formation of the Government Subgroup (Fig. 3.2). Core AM1 also contains the only intersection of the Lagerspoort diamictite (Fig. 3.2).
19 Lithofacies and Stratigraphic Setting of Samples
Figure 3-2 – Borehole Intersections from which samples were collected.
20 Lithofacies and Stratigraphic Setting of Samples
3.3 Hospital Hill Subgroup: West Rand Group
3.3.1 Orange Grove Formation
The Orange Grove Formation was sampled in core TF1 (Figs. 3.1 and 3.2). Only argillaceous quartzites and orthoquartzite units were sampled (Fig. 3.2). The orthoquartzite typically rests with sharp conformable contacts on underlying shale units and grade rapidly upward into argillaceous quartzite (Fig.3.3).
The Orange Grove Formation orthoquartzites are generally well sorted and composed of rounded grains. However, the basal quartzite is slightly different, in that it contains gritty bands and is generally less mature than the other units higher up in the succession. The structures observed in these rocks included flat bedding and small scale trough cross bedding with polymodal palaeocurrent directions (Beukes, 1995). These features suggest a marine inner shelf depositional environment according to Beukes (1995) and Beukes and Cairncross (1992).
3.3.2 Parktown Formation
The Parktown Formation was also sampled in core TF1. In this core it is composed of stacked coarsening upward shale and siltstone successions, occasionally capped by orthoquartzite (Fig.3.2). Two prominent banded iron formations are present. Of specific interest to this study, is the presence of three thin but laterally persistent orthoquartzitic beds interbedded with the essentially shale formation. The beds typically have sharp erosive basal contacts and fine upwards to a sharp gradational contact with siltstone or mudstones above. The beds are known as the Bullskop, Ripple and Speckled Marker Beds (Fig. 3.3).
The Bullskop bed is composed of medium to coarse grained mature orthoquartzite and is developed directly above the Watertower Iron Formation. The intersection of this unit in the TF1 drill core is somewhat unusual. The unit is composed of three distinct orthoquartzite beds, each separated by siltstones and shales, whereas in other parts of the basin the Bullskop bed is a single orthoquartzitic unit located a few meters above the Watertower Iron Formation (Beukes, 1995). The unit is texturally very mature, composed of rounded and well sorted grains. However, it also contains quite
21 Lithofacies and Stratigraphic Setting of Samples abundant feldspar grains. The Ripple Marker Bed is medium to coarse grained orthoquartzite, with distinctive wavy lamination and trough cross bedding. It is overlain by a fine grained laminated siltstone and argillaceous quartzite and shale. The Speckled Marker is composed of interbeded medium grained orthoquartzite and flat-laminated fine argillaceous quartzite, arranged in upward coarsening increments of sedimentation (Fig. 3.3).
Beukes (1995) measured bimodal paleocurrent directions in these rocks, which are interpreted as marine shallow shelf sands. The shales present in the Parktown Formation represent outer shelf muds and the fine grained argillaceous quartzites and siltstones are transitional between these two depositional environments (Beukes, 1995).
3.3.3 Brixton Formation
Samples from the Brixton Formation were collected from core TF1 (Fig. 3.3) The succession comprises three thick orthoquartzite units separated by shale and siltstone. The orthoquartzites are mostly medium grained and well sorted, with well-rounded quartz grains.
The Versterkop Member is the basal quartzite of the Brixton Formation (Fig. 3.3). It overlies the argillaceous quartzites and shale of the Parktown Formation with a sharp but gradational contact. Although the Versterkop Member is fairly homogeneous, three weakly defined fining upward sequences with thin gritty beds at the base are present. Small scale trough cross bedding is common in the rocks.
Overlying the Versterkop Member is the Blinkpoort Member, which is composed of shale with three small coarsening upward sequences from shale to quartzite and a magnetic mudstone (Beukes, 1995). In TF1, drill core this unit is not present due to loss on a fault located at the base of the overlying Witkop Member (Fig. 3.3).
The Witkop and Rangeview Members both form the upper parts of coarsening upward facies units from shale, interlaminated silt-fine sand outer shelf sediments to mature medium grained orthoquartzite. Polymodal palaeocurrent directions were measured for the Versterkop and Witkop Members and unimodal palaeocurrent directions were measured for the Range View Member (Beukes, 1995). The Versterkop and Witkop
22 Lithofacies and Stratigraphic Setting of Samples
Quartzites are mature and are classified as marine shallow shelf sand deposits (Beukes, 1995). The Range View Quartzite is less mature and is classified as a braided fluvial quartzite (Beukes, 1995). All of these units are somewhat transitional and contain features of braided fluvial systems and marine shallow shelf sands.
3.3.4 Bonanza Formation
The Bonanza Formation is a thick sequence of mostly coarse grained immature quartzites which disconconformably overlies the Brixton Formation (Beukes, 1995). In core TF1 only the most basal part of the formation was intersected (Fig. 3.3). The intersection is composed of mature quartzite. However, the correlation is somewhat uncertain and therefore core BAB1 (Fig 3.1), which also intersects the Bonanza Formation (Fig. 3.4), was used for sampling. This core intersects all of the Bonanza Formation. In this core it is composed of coarse to very coarse moderately sorted quartzite. Some of the quartzite are argillaceous and contain gritty beds. These features suggest a braided stream depositional environment (Beukes, 1995). However some subordinate fine grained, very well sorted bands are also present which are most likely shallow shelf marine origin.
3.4 Government Subgroup: West Rand Group
The Government Subgroup in the proximal area of the basin and was sampled in cores BAB1 and in JY8 (Fig.3.4). Core AM1 (Fig. 3.1) in the Free State area provided an intersection of the Government Group in the more distal part of the depository.
3.4.1 Promise Formation
The Promise Formation unconformably overlies the Bonanza Formation. This is a major unconformity present across the entire basin (Fig. 3.2). A diamictite is present at the base of the Promise Formation and represents a distinctive marker bed along the base of the Government Subgroup.
The lower argillaceous part of the Promise Formation is the Witfontein Member. It is composed of flat laminated siltstone, diamictite and three thin upward coarsening quartzite units. The package is capped by another orthoquartzite unit (Fig. 3.4).
23 Lithofacies and Stratigraphic Setting of Samples
Figure 3.3 Borehole log of the Borehole TF1 drilled in the Klerksdorp Area showing sampling intervals.
24 Lithofacies and Stratigraphic Setting of Samples
In core BAB1 the diamictite units in the Witfontein Member are poorly developed. They are mostly represented by argillaceous silty units with fine immature sandy bands. Occasional matrix-supported clasts are present in the sandy bands. In other parts of the basin, the Promise diamictite is well developed, where it is a thick unit of mudstone with abundant pebble sized clasts. Thin quartzite beds forming the tops of small coarsening upward successions in the Witfontein Member, was sampled for this study The quartzites are medium grained and are moderately to well sorted, so they probably represent deposits from a shallow marine depositional environment (Beukes, 1995).
The Breaunda Member is mainly composed of medium-coarse quartzite with some gritty bands. It forms the upper part of a coarsening upward facies succession, which overlies the siltstones and wackestones of the Witfontein Member (Fig. 3.4). The Breaunda quartzites are composed of moderately sorted grains but no evidence of argillaceous material is present. The quartzites are therefore mature and were most probably deposited in a shallow marine environment. The top of the Breaunda Member gradually fines upwards to siltstone, which was deposited in an intermediate shelf environment, during a transgressive event (Beukes, 1995).
The Hamberg Member overlies the Breaunda Member (Fig. 3.4). It is composed of three coarsening upward facies successions, but is coarser grained than the Breaunda Member, and not as well sorted, therefore it is considered an immature quartzite deposited in a braided fluvial depositional environment (Beukes and Nelson, 1995)(Fig. 3.4).
The lower part of core AM1 in the Free State area intersects the Promise Formation, but the rocks are highly faulted and as a result large sections of the succession are missing. The Hamberg Member is better preserved and was sampled in the core (Fig. 3.5). It consists of fine grained slightly argillaceous quartzite, considered to have been deposited in a shallow marine shelf environment (Fig. 3.5).
25 Lithofacies and Stratigraphic Setting of Samples
Figure 3-4 : Borehole log of Borehole BAB1 drilled in the Klerksdorp area showing sampling interval. See Figure 3-3 for legend
26 Lithofacies and Stratigraphic Setting of Samples
3.4.2 Coronation Formation
The base of the Coronation Formation is represented by a prominent unit of rather massive diamictite with large pebble-sized clasts floating in a sandy matrix. This unit is well developed in core BAB1. It is composed of several stacked weakly-defined upward coarsening increments of sedimentation. The base of the diamictite unit is sharp but the upper contact is gradational. The size and frequency of clasts gradually decreases over a few meters in the upper part of the unit to form an interbedded shale and siltstone succession. A magnetic shale or iron formation is sometimes present in this interval (Smith, 2007), but is not developed in the BAB1 drill core. The unit was sampled from the BAB1 drill core (Fig. 3.4).
Like the Promise Formation, much of the Coronation Formation is missing in the AM1 drill core due to faulting. The unit contains a coarsening upwards facies succession from shale to medium-fine grained quartzite with silty intercalations. The Kensington Diamictite overlies this, but is poorly developed in the AM1 drill core. It consists of a very thin mudstone bed with scattered small clasts. This sequence is capped by an iron formation known as the Silverfields Member (Fig. 3.5).
3.4.3 Tusschenin Formation
The Tusschenin Formation (Fig. 3.2) is a thick succession of coarse, slightly argillaceous, poorly sorted quartzite. It is composed of four or five stacked upward- fining successions with gritty bases and more mature tops in BAB1 (Fig.3.4). Most beds are massive but some large planar cross sets are recognized and it is considered to represent braided fluvial depositions (Fig. 3.4).
In core AM1 of the Free State area the quartzites of the Tusschenin Formation are fine grained and flat bedded. The sequence contains a basal gritty bed which is believed to be correlative of the Coronation Reef (Fig. 3.5 and 3.4). The lower half of the succession is composed of medium grained quartzites, which are slightly argillaceous and contain some planar crossbedding. These quartzite beds fine upwards and become more mature flat-bedded quartzites to the top of the succession. A braided fluvial depositional environment is interpreted for the sequence (Fig. 3.5), but the increasing maturity may indicate an upwards shift to a marine shallow shelf depositional environment.
27 Lithofacies and Stratigraphic Setting of Samples
Figure 3-5 : Borehole log of Borehole AM1 drilled in the Edenville Area, showing sampling interval See Figure 3-3 for legend
28 Lithofacies and Stratigraphic Setting of Samples
3.4.4 Palmietfontein Formation
The Palmietfontein Formation overlies the Tusschenin Formation with a sharp contact (Fig. 3.2). It is intersected in both cores BAB1 and JY8 (Figs. 3.4 and 3.6). Only the very basal part is intersected in core BAB1 (Fig. 3.4). The best intersection is in core AM1 (Fig. 3.5) where it is composed of a coarsening upwards facies succession from shale and siltstones to the mature orthoquartzite of the Townhouse Member (Fig. 3.5). The Townhouse quartzite is overlain by a succession of shale, interlaminated shale and siltstone, siltstone, fine argillaceous wackestone and magnetic mudstone in the AM1 core. In core JY8 the succession is not well preserved because of the presence of three thick diabase sills. The core only intersects the succession down to the top of the Townhouse Member (Fig. 3.5). The Townhouse quartzite is normally very mature (Beukes, 1995). However, in AM1 it is quite feldspathic and contains many siltstone partings. The shales and siltstones are considered to have formed in outer and intermediate shelf depositional environments with the quartzites in an inner shallow marine shelf environment. Samples were taken of all major quartzite units of the Palmietfontein Formation in core AM1 (Fig 3.5). Elandslaagte Formation
The quartzite-rich Elandslaagte Formation (Figs. 3.2, 3.5 and 3.6) contains a mature gritty to conglomerate basal unit, informally known as the Government Reef (Fig. 3.6). Overlying this is two stacked scour-based fining upwards successions of coarse- medium immature quartzite. The entire unit fines and matures upwards. A mature braided fluvial depositional environment is interpreted for the Elandslaagte Formation, with the introduction of a shallow marine component near the top of the formation (Figs. 3.5 and 3.6). The succession intersected in core AM1 is almost identical to that in core JY8 (Fig 3.6), apart from the fact that in AM1 the upper part of the unit contains more silty beds and some wavy bedding. A wave-dominated inner shelf marine depositional environment is interpreted for this upper part of the unit in core AM1 (Fig. 3.5). Quartzites and conglomerates from the Elandslaagte Formation were sampled in both AM1 (Fig 3.5 and HY8 Fig 3.6).
3.4.5 Afrikander Formation
The fining upwards facies succession in the upper part of the Elandslaagte Formation grades upwards into the overlying Afrikander Formation, which is composed of
29 Lithofacies and Stratigraphic Setting of Samples interlaminated siltstone and quartzite with a fine wackestone at the top. The wackestone is overlain by a coarsening upwards facies succession from wackestone to a cream-grey coloured, medium grained quartzite with distinctive small scale trough cross-bedding. This latter succession is interpreted as a prograding shallow marine shelf unit (Figs. 3.5 and 3.6).
The Noycedale Member is a prominent quartzite unit which overlies this coarsening upwards facies succession described above (Figs. 3.5 and 3.6). It has a sharp basal contact and is composed of an immature quartzite arranged in a succession of stacked fining upwards successions with gritty bases and in some cases orthoquartzite tops (Figs. 3.5 and 3.6). The Noycedale Member is truncated by an erosional unconformity in core JY8 (Fig. 3.6). This surface represents the regional unconformity at base of the overlying Jeppestown Subgroup (Figs. 2.2 and 3.6). The Steynskraal Member is considered mature inner shelf quartzite deposits in AM1 (Fig. 3.5).
The Lagerspoort Member is represented by a thick succession of diamictite which overlies the Steynskraal Member with sharp erosional base. It is composed of angular clasts which range in size from coarse sand to cobbles. The clasts are supported in a fine sandy and silty matrix. It is overlain with sharp erosive contact by the basal Koedoeslaagte Formation of the Jeppestown Subgroup. In core AM1 a diabase sill intrudes along this contact (Fig. 3.6). The Afrikander Formation was sampled in both AM1 (Fig 3.5) and JY8 (Fig 3.6).
3.5 Jeppestown Subgroup: West Rand Group
Complete successions of the Jeppestown Subgroup were intersected and sampled in both cores AM1 (Fig. 3.5) and JY8 (Fig. 3.6).
3.5.1 Koedoeslaagte Formation
The Koedoeslaagte Formation is a supermature orthoquartzite unit at the base of the Jeppestown Subgroup. A basin-wide low angle disconformity is present at its base. This basal unconformity surface is directly overlain by a thin gritty to conglomeratic unit known as the, Buffelsdoorn Reef of the Koedoeslaagte Formation (Fig.3.5). This conglomeratic basal unit grades upwards into fine grained mature orthoquartzite that
30 Lithofacies and Stratigraphic Setting of Samples characterizes the Koedoeslaagte Formation (Figs. 3.5 and 3.6). It is considered a shallow inner shelf marine deposit. The Koedoeslaagte Formation was sampled in both AM1 (Fig 3.5) and JY8 (Fig 3.6).
3.5.2 Rietkuil Formation
The Rietkuil Formation is a fine argillaceous unit that gradationally overlies the Koedoeslaagte Formation (Figs. 3.5 and 3.6). It is composed of fine immature quartzite, argillaceous quartzite, siltstone and shale. The central part of the unit is formed by shale which coarsens downward into the quartzite of the Koedoeslaagte Formation and in turn coarsens upward into a mature quartzite at the base of the Babrosco Formation in core JY8 (Fig. 3.6). In core AM1 , the succession is composed of only the basal fining upwards unit (Fig.3.5). Only one sample was collected from an immature quartzite bed in the Rietkuil Formation in JY8 (Fig 3.6).
3.5.3 Babrosco Formation
The Babrosco Formation in core JY8 (Fig. 3.6) has a fine grained orthoquartzite at its base which is overlain by a succession of medium, immature flat-laminated quartzite, with occasional orthoquartzite interbeds. An immature, medium-coarse, trough cross- bedded quartzite overlies the flat bedded quartzite. The trough cross-bedded quartzite is composed of multiple stacked thin fining upwards units with gritty bases, which is indicative of a braided fluvial depositional environment (Fig. 3.6). With the exception of the upper trough cross-bedded immature quartzite, an inner shelf marine depositional environment is interpreted for the Babrosco Formation in core JY8 (Fig. 3.6).
In core AM1 the Babrosco Formation contains the same mature basal orthoquartzite, but overlying this is a fine grained slightly argillaceous quartzite, with small scale trough cross bedding, interpreted as shallow shelf to intermediate shelf depositional environment (Fig. 3.5). The fluvial upper unit of the formation recognized in core JY8 (Fig. 3.6) is thus absent in core AM1 (Fig. 3.5). The Babrosco Formation was sampled in both AM1 (Fig 3.5) and JY8 (Fig 3.6).
31 Lithofacies and Stratigraphic Setting of Samples
Figure 3-6 : Borehole Log of the JY8 borehole drilled in the Klerksdorp area showing sampling Interval. See Figure 3-3 for legend
32 Lithofacies and Stratigraphic Setting of Samples
3.5.4 The Crown Lava Formation
The Crown Lava overlies the Babrosco Formation with a sharp contact in both cores JY8 and AM1. Abundant calcite filled vesicles are presents which points to a subaerially erupted basaltic lava flow (Armstrong, et al, 1991).
3.5.5 Roodepoort Formation
The Roodepoort Formation is composed of four distinct units, namely basal quartzite (Lower Roodepoort Member), a lower shaley unit, a middle unit of fine quartzite and wackestone (Central Roodepoort Member) and an upper unit of shale (Figs. 3.2). A similar succession is intersected in both cores AM1 (Fig. 3.5) and JY8 (Fig. 3.6).
In core JY8 (Fig. 3.6) the Lower Roodepoort Member is a fine quartzite with siltstone., It grades upwards through quartzwacke into shale. This is followed by coarsening upwards facies succession of shale, wackestone, interlaminated quartzite and siltstone to quartzite, representing the Central Roodepoort Member. An upwards fining facies succession, similar to the one above the Lower Roodepoort Member, then follows and end in a prominent shale unit (Figs. 3.5 and 3.6).
The Middle Roodepoort quartzite contains a lower trough cross-bedded unit and an upper flat bedded unit, which were sampled separately (Fig. 3.6). A shallow shelf and intermediate shelf marine depositional environments are interpreted for the Roodepoort Formation in core JY8 (Fig. 3.6).
In core AM1 the succession is rather similar to that in core JY8 (Fig. 3.7). However, in AM1 the quartzite units are slightly finer grained and contain more silty lamina than in core JY8. These fine quartzites are believed to have formed in an intermediate shallow marine shelf environment.
3.5.6 Maraisburg Formation
The top of the Maraisburg Formation is considered as the base of the Central Rand Group because it is gradational to underlying shale of the Roodepoort Formation, it is here preferred to classify it with the West Rand Group.
As mentioned above, the Maraisburg Formation forms part of an upwards coarsening facies succession, which originates in the upper Roodepoort Shale. In itself the
33 Lithofacies and Stratigraphic Setting of Samples
Maraisburg Formation also coarsens upwards from medium to coarse grained quartzite. The quartzites are typically slightly argillaceous. The unit is composed of stacked fining upwards assemblages with gritty erosive bases. Most of the unit, except for the more mature quartzite in its lower part, is considered to have formed in a braided fluvial environment. Only the coarser fluvial quartzites were sampled in both cores JY8 (Fig. 3.6) and AM1 (Fig. 3.5).
3.6 Johannesburg Subgroup : Central Rand Group
3.6.1 Blyvooruitzicht Formation
An unconformity marked by the Ada May reef marks the base of the Central Rand Group and the Blyvooruitzicht Formation (Fig. 2.2). In core DK12 only the quartzite above the Ada May Reef is intersected. This quartzite is upward coarsening from fine to medium and coarse grained and contains gritty bands near the top. It is truncated by the overlying Main Formation. In core AM1 the Blyvooruitzicht Formation is absent as it has been truncated by the Main Reef which rests directly on quartzite of the Maraisburg Formation quartzite. Quartzites of the Blyvooruitzicht Formation is considered to have formed in a braided fluvial environment (Fig. 3.8). The quartzite was sampled in core DK12 (Fig. 3.8).
3.6.2 Main Formation
The Main Formation contains the Main Leeder Reef conglomerate at its base. This conglomerate is overlain by a mature but coarse upward fining quartzite in core DK12. The formation has rather similar character in core AM1, with the exception that a gritty bed marks the top. Two composite samples were taken in core AM1 (Fig. 3.5) and one sample in DK12 (Fig.3.8).
3.6.3 Randfontein Formation
The Randfontain Member is a thick succession of immature quartzite. The base is the Middlevlei Reef which erodes into the Main Formation (Fig. 3.5). The succession consists of mostly medium to coarse grained immature quartzite. The Johnstone Member is an immature gritty quartzite, with poorly developed conglomerate at the
34 Lithofacies and Stratigraphic Setting of Samples base. It is located in about the middle of the Randfontein Formation (Fig. 3.8). Three composite samples were taken from core DK12 (Fig. 3.8).
3.6.4 Luipaardsvlei Formation
The Luipardsvlei Formation is an upwards fining succession of coarse quartzite, with the Livingstone reef at the base. The succession contains a basal unit of coarse sedimentary rocks, composed of conglomerates, pebbly quartzites and gritty lags, in the form stacked fining upwards assemblages (Fig. 3.8). The upper part of the Luipardsvlei Formation is a fining upward succession from coarse to medium quartzite. Quartzites are immature and typically contains feldspathic interbeds. Two composite samples were taken from the DK12 drill core (Fig. 3.8).
3.6.5 Krugersdorp Formation
The Bird Reef marks the base of the Krugersdorp Formation (Figs 3.2 and 3.8). The Krugersdorp Formation and the overlying Booysens Formation form a fining upwards facies succession. The lower part of the Krugersdorp Formation contains immature pebbly quartzite and conglomerate. The sequence then fines upwards to a medium grained trough-cross bedded immature quartzite (Fig. 3.8). The latter unit further fines upwards into argillites of the Booysens Formation (Fig. 3.8).
3.6.6 Booysens Formation
The base of the Booysens Formation is a fine grained quartzite with shale intercalations and reflects the progression of the fining upwards facies succession observed in the Krugersdorp Formation. This succession continues through interlaminated sand and silt facies to a wacke and finally shale. A similar facies succession occurs in the top of the sequence, but here it is upward coarsening (Fig. 3.8). A prominent fine mature orthoquartzite occurs at the top of the formation known as the Doornkop Member (Fig. 3.8). Samples were taken of argillaceous quartzite in the lower and upper part of the formation and also of the upper orthoquartzite (Fig. 3.8). The argillaceous rocks are considered intermediate marine shelf deposits and the orthoquartzite at the top a shallow marine inner shelf deposit (Fig. 3.8).
35 Lithofacies and Stratigraphic Setting of Samples
Figure 3.7 : Borehole Log of the DK12 borehole drilled in the Klerksdorp area showing sampling interval. See Figure 3-3 for legend
36 Lithofacies and Stratigraphic Setting of Samples
3.7 Turffontein Subgroup: Central Rand Group
3.7.1 Kimberley Formation
The Kimberley Formation has the Kleinfontein Reef at its base. It is a gritty quartzite composed of stacked fining upwards successions with erosive bases along which conglomeratic beds are occasionally developed. The sequence is split by a thin shale unit with an erosive base. The shale is therefore considered a channel infill. A braided fluvial depositional environment is interpreted for this succession (Fig. 3.8). Two composite samples, one above and one below the channel fill, were taken from core DK12 (Fig. 3.8).
3.7.2 Elsburg Formation
The Elsburg Formation contains a thin conglomerate overlain by a fine mature orthoquartzite at its base (Fig. 3.8). It is overlain by a succession of medium moderately mature quartzite composed of stacked fining upwards units with erosive bases (Fig.3.8). This succession is in turn overlain by two thick upwards fining successions with coarse to gritty immature bases and fine to medium orthoquartzite tops. The entire formation is capped by a coarse but mature quartzite bed. The Elsburg Formation quartzites are overall mature, but the coarse grainsizes and moderate sorting suggest deposition in a braided fluvial environment rather than shallow marine shelf environment. One composite sample was taken of the basal Denny’s Marker and three of the overlying coarse quartzite from core DK12 (Fig. 3.5).
3.7.3 Mondeor Formation
The Mondeor Formation is composed of multiple stacked fining upward quartzite successions in an overall upward coarsening unit (Fig. 3.8). The rocks are very coarse, mostly gritty and pebbly quartzites and conglomerates. The formation contains two reefs, the Elandsrand at the base, and the Deelkraal Reef in the upper part (Fig. 3.8). The sediments are poorly sorted and likely formed in a braided fluvial environment (Fig. 3.8). One composite sample was taken of the entire unit in core DK12 (Fig 3.8).
37 Lithofacies and Stratigraphic Setting of Samples
3.8 Summary of Stratigraphic Variation and Samples Taken
The Hospital Hill Subgroup is composed of shale, argillaceous quartzite, and some orthoquartzite units which become more common near the top of the subgroup (Fig 3.2). The Government and Jeppestown Subgroups are composed mostly of argillaceous quartzite and immature quartzite and some orthoquartzite units. The Johannesburg and Turffontein Subgroups are composed almost entirely of immature quartzite and conglomerate. In very broad terms the Witwatersrand Supergroup can be considered an upwards coarsening sequence, from shale, siltstone and wackestone in its lower part and to coarse immature quartzites and conglomerates to the top. This results from the progressive filling of the basin. The transition from dominantly marine to dominantly non-marine sedimentation takes place at the top of the West Rand Group (Fig. 3.2).
The majority of the variation in sediment grainsize is controlled by relative variations in sea level and by subsidence. Transgressive sequences contain erosive bases and fining upwards facies assemblages. The Versterkop Member is a good example of a transgressive sequence in the underfilled basin stage, resulting in deposition of shallow shelf and outer shelf depofacies rocks (Fig. 3.2). In the overfilled stage of the basin a transgressive sequence results in the Luipardsvlei Formation which is a fining upwards sequence composed of mostly braided fluvial facies sediments arranged in smaller fining upwards sequences. In a similar manner regressions form coarsening upwards facies successions. Condensed sections like is observed in the Booysens Formation represents rapid relative sea level falls. Marine flooding surfaces like Range View Formation cause outer shelf deposits to directly overlying terrestrial deposits. It is clear that eustatic variations control the facies distribution and fining sequences observed in the rocks. For more detailed description on the Genetic and sequence stratigraphy see Beukes 1995, Beukes and Nelson, 1995.
With reference to lateral variations within the basin, the strata intersected in core AM1 are generally finer grained and contain more matrix than comparable intervals in the JY8 and BAB1 drill cores (Fig. 3.7). In some cases the sedimentary facies are entirely different, as occurs in the Noyedale Member which was deposited in a shallow marine shelf environment in core AM1 in the Free State but is represented by
38 Lithofacies and Stratigraphic Setting of Samples fluvial deposits in core JY8 of the Klerksdorp area (Fig. 3.8). This is also the case in the Babrosco Formations and Lower Roodepoort Member (Fig. 3.8). These depositional facies variations are consistent with a more distal basinal setting for the AM1 drill core relative to a more proximal setting for the JY8 core in the Klerksdorp area (Fig. 3.8).
The upper part of the Afrikander Formation has also been removed by erosion at the base of the Koedoeslaagte Formation in core JY8 in the Klerksdorp area relative to AM1 in the Free State area (Fig. 3.8). In core JY8 Steynskraal and Laagerspoort Members were removed by erosion. Thicknesses of formations in the Free State area are also less than in the Klerksdorp area, which suggests less subsidence has occurred in this part of the basin (Fig. 3.8). It is clear that the sedimentation was controlled by basin wide eustatic variations with local tectonic influences an important factor controlling the characteristics of the rocks (Beukes, 1995).
39 Lithofacies and Stratigraphic Setting of Samples
Figure 3-8 : Correlation between Lithologies in the Klerksdorp and Edenville Areas. See Figure 3-3 for legend
40
Chapter 4 : Mineralogy
4.1 Introduction
Minerals related to deposition of the quartzites of the Witwatersrand succession can be classified as allogenic (Detrital) that includes allogenitic composite mineral rock particles and authigenic (diagenetic to low grade metamorphic) minerals.
Major allogenic monomineralic grains are represented by quartz, feldspar, chlorite and muscovite. Allogenic rock particles include polycrystalline quartz particles, lithoclasts, chert and devitrified volcanic glass particles. Accessory allogenic minerals include leucoxene, pyrite, zircon, chromite, epidote, allanite and some rare minerals. Common authigenic minerals are represented by quartz, sericite, chlorite, pyrophyllite, sulphides, sphene, clinozoite, glauconite, calcite and chloritoid.
4.2 Allogenic/Monomineralic Minerals and Composite Rock Particles
4.2.1 Monomineralic grains
4.2.1.1 Quartz Monomineralic quartz grains are common in all samples. It occurs in a variety of sizes and shapes according to the sorting processes responsible for formation of the quartzite. In all samples, monocrystalline quartz is far more common than polycrystalline quartz. Polycrystalline quartz contents are highly variable, accounting for between 0 and 10% of framework grains, but it is usually less than 5%. The most common polycrystalline quartz grains observed contained only two or three subgrains, but metamorphic quartz grains with many fine smaller subgrains, frequently with sutured
41 Mineralogy textures, are present but are not frequently encountered (Figs. 4-1 A and B). Polycrystalline quartz grains are typically larger and more often than not are better rounded than monocrystalline quartz grains. Quartz with undulose and normal extinction are present in all samples. However, no relationship could be established between the size and shape of quartz grains and their extinction character.
4.2.1.2 Feldspar Three types of feldspar are present in the Witwatersrand quartzites, namely multiple twinned plagioclase feldspars, which is most common, checkerboard twinned microcline and partially altered untwinned orthoclase. The latter is especially abundant in the lower parts of the West Rand Group of the Witwatersrand succession. SEM and XRD analyses indicate that the plagioclase feldspar is mostly sodium-rich and approach the composition of albite. This is consistent with feldspar compositions measured by Watchorn (1981) and Holdsworth (1996), who suggest that albitization of feldspars took place. The size of feldspar grains are usually slightly larger than that of associated quartz grains in any specific sample. In most cases they also display marginally better rounding than quartz grains. Feldspar grains are frequently partially or completely altered to sericite.
4.2.1.3 Chlorite and Muscovite Chlorite is common in the immature quartzite, siltstones and shale. It occurs as flakes, amongst quartz grains. Some flakes are bent around quartz grains, which may indicate compaction. In some cases the edges of the flakes are slightly rounded, which suggests a detrital origin for them. Authigenic overgrowth of chlorite on existing chlorite grain margins is present in some instances. Quartz appears to be consumed during the reaction to form the authigenic chlorite. The Mg-rich variety, clinochlore, is highly pleochroic and is easily identified by anomalous blue interference colours. Chlorite with weak pleochroism and green interference colours is also present. This is the iron-rich end member, chamosite. Both phases occur as flakes in the quartzites and in some cases both minerals are present in the same chlorite flake. This can be attributed to epigenetic growth of chlorite on an existing grain or replacement of magnesium with iron by hydrothermal fluids.
42 Mineralogy
Figure 4-1 : Common framework components of Witwatersrand Quartzites. Scale bars 0.5mm. A-Large well rounded polycrystalline quartz grain surrounded by monocrystalline quartz and sericite pseudomatrix. B-Large strained quartz grain from a metamorphosed source area. C-Large Well rounded sericite mass in coarse grained mature quartzite represents an altered feldspar. D-Alteration of Multiple twinned plagioclase grain to sericite forming sericite pseudomatrix. E-Highly altered siltstone lithoclasts of intrabasinal origin. F- Altered lithoclasts composed of finely intergrown chlorite and iron oxide minerals under plain light.
43 Mineralogy
In the finer grained quartzites, chlorite is present as a matrix mineral. This variety will be discussed under the heading of authigenic minerals. These chlorites are metamorphic in origin.
Most samples that contain chlorite flakes also contain muscovite flakes. The muscovite flakes typically have the same texture as chlorite flakes. Chlorite and muscovite flakes are only present in immature, slightly argillaceous to argillaceous quartzite.
4.3 Allogenic Rock Particles
4.3.1 Lithoclasts
Lithoclasts are generally rare or not present at all in quartzite samples. Exceptions are samples from the Government and Jeppestown Subgroups. Like feldspar, lithoclasts are rarely unaltered. They are usually dark brown grains which display high adsorption and are composed of finely intergrown sericite and hydrous iron oxides. Their origins are uncertain but they probably represent altered shale or volcanic lithoclasts. Sedimentary lithoclasts composed of fine quartzite and siltstones are also present but very rare.
4.3.1.1 Chert Chert grains are present in almost all samples. It occurs as well rounded grains, even in immature rocks, and is commonly much larger than other framework grains (Fig. 4- 2D). The abundance of chert is highly variable and does not show much difference between different rock types.
4.3.1.2 Devitrified Glass Shards Brown glass shards, which contain one bulbous end are present in the rocks of the Jeppestown Subgroup. These are interpreted as volcanic glass shards which indicates volcanic eruption.
44 Mineralogy
Figure 4-2 : Common framework components of Witwatersrand Quartzites. A-Rounded muscovite grain. B-Anomalous Blue interference colours of detrital clinochlore and quartz in a sericite pseudomatrix. C- Chamosite and quartz grains in a sericite pseudomatrix. D-Large rounded chert grain
4.4 Accessory Allogenic Minerals
4.4.1.1 Leucoxene Leucoxene is a commonly observed allogenic mineral. It is composed of fine bladelike grains which are located in a matrix of quartz sericite and chlorite. They are typically moderately rounded and are typically located with other heavy minerals.
Two distinctly different types of leucoxene are present. The most abundant type is composed of more irregularly shaped rutile laths or bulbs. The second type that is rare contains a trellis texture of rutile lamellae in three directions.
45 Mineralogy
It is important to note that although leucoxene might be considered authigenic, it is discussed here with allogenic minerals because their abundance reflects the presence of original Fe-Ti oxide minerals in the quartzites. The reason for that is that they probably represent the original presence of illmenite in the sediments.
4.4.1.2 Pyrite Rounded detrital pyrite grains are common in most samples. They are typically between 50-100um in diameter and show variable roundness, from poorly rounded pyrite cubes with rounded edges to well-rounded grains. The abundance and size of pyrite grains appears to increase from the West Rand Group into the Central Rand Group. In immature quartzites, rounded pyrites appear to concentrate in heavy mineral lags, but are more sporadically distributed in more mature quartzite samples.
4.4.1.3 Zircon Zircon is a common accessory mineral in Witwatersrand quartzites. It is present in a variety of sizes, from <50um to >150um, and shapes. Smaller grains of 50um or less tend to display low sphericity and poor rounding, whereas grains in the 50-100um range are typically highly spherical. Large grains, >150um show moderate sphericity but are rounded. In any one sample a variety of sizes and shapes of zircon are normally present.
4.4.1.4 Chromite Witwatersrand chromite grains are typically large (100-150um) and well rounded. They are most common in the Hospital Hill Subgroup, and are rare in the Central Rand Group. Chromites tend to concentrate with other heavy minerals especially zircon and pyrite.
4.4.1.5 Epidote and Allanite Large well rounded epidote grains are present in surprising abundances in the quartzite of the Jeppestown Subgroup. The grains are typically large, 100um – 200um, and are recognised by weak yellow green pleochroism and second order interference colours. Allanite is commonly present in samples that contain epidote. Allanite is best recognised under the SEM where its high REE content is diagnostic.
46 Mineralogy
4.4.1.6 Other Rare Minerals Other allogenic minerals which are present but are rarely observed include, monazite, apatite, sphene, urananite and rutile. These minerals are only observed in heavy mineral concentrates. They are present in rare quantities throughout the succession.
4.5 Authigenic Minerals
Quartzites of the Witwatersrand Supergroup contain a large variety of authigenic minerals which reflects a long history of alteration and metamorphism of the succession.
4.5.1.1 Sericite Sericite occurs as fine grains, which replace mostly feldspar. It is present to some degree in all samples from trace to abundant amounts. The abundance of sericite appears to reflect both the abundance of feldspar that was originally present in the sample, and also the degree of alteration of the samples. Mature samples with few feldspars contain isolated masses of sericite which are interpreted as the remains of feldspar. Highly feldspathic samples may contain partially altered feldspars or they may contain large amounts of sericite pseudomatrix, which is interpreted as complete alteration of highly feldspathic rocks.
In some highly altered samples small sericite blades grow into quartz grain margins. Such quartz grain margins are typically highly embayed. This is taken to indicate that quartz was consumed during the growth of the sericite.
4.5.1.2 Authigenic Quartz Authigenic quartz cement is only present in mature quartzite (especially orthoquartzite) samples. This is probably so because growth of quartz was inhibited in samples with sericitic matrix. No typical cement textures were observed in the authigenic quartz, but moderately recrystallised quartz grains suggest that porosity was filled with quartz.
47 Mineralogy
4.5.1.3 Chlorite Authigenic chlorite is present in fine wackestone, siltstone and argillaceous quartzite. Chlorite appears to replace the matrix component that was probably composed of a mixture of clay minerals and quartz. Authigenic chlorite also grows on the margins of existing detrital chlorite flakes. In addition it tends to overgrow quartz grains.
4.5.1.4 Pyrophyllite Pyrophyllite is difficult to identify under the petrographic microscope, due to the fine grained nature of the mineral and the similar optical characteristics to sericite. However, it is easily identified using the SEM and XRD. SEM imaging showed that it occurs as fine grained flakes in chlorite or sericite matrix.
4.5.1.5 Sulfides Sulfides make up a large portion of the authigenic minerals. The most common sulfides are pyrite, pyrrhotite and chalcopyrite. Many other sulfides such as galena, arsenopyrite and pentlandite, gerdorfite and millerite are present but are relatively rare. These sulfides show many different forms, and multiple phases of sulfide growth are present. The detailed study of these falls outside of the scope of this project.
Post diagenetic subhedral pyritic overgrowths are the most common pyrites present in the Witwatersrand quartzites. They appear to be distributed evenly between mature and immature quartzites and high concentrations are observed when there is evidence of fluid flow (e.g. hydrothermal veining). Most pyrite appears to post-date the growth of sericite.
4.5.1.6 Sphene Sphene is one of the most common authigenic minerals. It occurs as anhedral crystals. It is abundant in immature samples but scarce in mature quartzite samples. This suggests that authigenic sphene grows by redistribution of Ti-bearing minerals in the rocks, probably from the components released by dissolution of Fe-Ti oxides.
4.5.1.7 Clinozoite Anhedral clinozoite crystals occur in immature quartzites which are lightly or moderately altered and especially in fine grained varieties. Quartzites containing
48 Mineralogy clinozoite occur in the Jeppestown and Johannesburg Subgroups. Clinozoite is recognised by weak pleochroism and distinctive second order yellow and blue interference colours, and commonly occurs as overgrowths on rounded epidote grains.
4.5.1.8 Glauconite Subhedral glauconite overgrowths are common in supermature quartzites from the Hospital Hill Subgroup, especially the mature quartzite markers in the Parktown Formation. They are easily recognised as distinctive green minerals in otherwise mature quartzites.
4.5.1.9 Calcite Calcite is a fairly common authigenic mineral in the Jeppestown quartzites. It occurs as large anhedral grains and probably results from calcium released during albitization of feldspar.
4.5.1.10 Chloritoid Chloritoid is a common authigenic mineral throughout the succession. It occurs as twinned tabular overgrowths, frequently with multiple crystals radiating outwards from the same point.
4.5.1.11 Other Minerals Anhedral rutile occurs in most samples, but crystals are usually very small and account for a very small proportion of the total mineral assemblage. Rutile typically occurs as clusters of fine needles, which overgrow quartz, and also as fine blades which commonly cluster in detrital chlorite laths. Fuchsite, associated with detrital chromite, is present in the orthoquartzites of the Hospital Hill Subgroup.
49
Chapter 5 : Petrography
5.1 Introduction
Extensive sericitisation of samples makes it difficult to accurately distinguish between lithoclasts and feldspars. This makes traditional modal point analysis according to the Gazzi-Dickinson method (Dickenson and Suczek, 1979, Dickenson et. al. 1983) virtually impossible (Law et al., 1990). Therefore the data presented in this section is based on visual estimation of the abundance of components after the method of Shvetsov (1955). In some cases textural evidence is used as an aid to identify feldspar composition in sericitised sediments. In mature quartzites, feldspars are usually completely surrounded by quartz grains and the presence of discrete sericite masses is indicative of original feldspar grains. In immature quartzite, where feldspar contents may be high, sericitic alteration results in the formation of a sericite pseudo-matrix, which makes the estimation of feldspar contents very difficult. Samples from the Jeppestown Subgroup in cores JY8 and AM1 show the least alteration and feldspars are readily identifiable. In order to describe the rocks also from a genetic point of view, they were classified into four major groups, namely inner shelf marine orthoquartzite, middle shelf fine quartzite (wacke and siltstone i.e. argillaceous fine quartzite) fluvial quartzite and glaciogenic diamictite (Table 4.1).
5.2 Inner shelf orthoquartzites
Orthoquarzites which formed in a marine inner shelf depositional facies are divided into two groups, namely current dominated orthoquartzites and wave dominated orthoquartzites following the genetic stratigraphic subdivision of Beukes and Nelson (1995).
50 Petrography
5.2.1 Current-dominated Orthoquartzites
Current-dominated orthoquartzites display a wide variety of grain sizes from fine to coarse grained sand and can be gritty in some cases. They are always well sorted and are mostly composed of well-rounded grains. However, some sub-rounded and sub- angular grains may also be present. The rocks virtually contain no matrix and as a result are grain supported.
Quartz accounts for the bulk of framework component, usually between 90-95% of framework grains (Fig 5-1). Sporadic masses of sericite occur, and are interpreted as altered feldspars. It is possible that these grains represent the altered remains of lithoclasts, but a feldspathic origin is preferred, because lithoclasts are unlikely to survive, given the intense transportation processes indicated by the rounded grain shapes. Feldspar components are highly variable and can account for up to 30% of framework minerals in some cases (Fig. 5-1). Lithoclasts and detrital chlorite grains are either not present or are very rare in these rocks. Monocrystalline quartz grains are the most common quartz type present, chert and polycrystalline quartz only make up a few percent of samples.
Q Q Current Dominated Orthoquartzites Wave Dominated Orthoquartzites
C F L Figure 5-1 : QFL and QFC plots of Orthoquartzites. Q = Total Quartz, F = Interpreted original Feldspar, L = Lithoclasts, C = Detrital Chlorite Laths.
51 Petrography
The more durable heavy minerals such as zircons and chromites are most common in these rocks, but leucoxene, pyrite and monazite are also present. Common authigenic minerals include pyrite, glauconite, chloritoid and muscovite. Sericite is present as minor alteration product. No authigenic sphene was observed in these rocks.
Many of the current-dominated orthoquartzites exhibit an equigranular texture which is the product of silicification of pore spaces followed by compaction and moderate recrystallisation. In some samples much of the original texture is missing, but there are always some grains which have retained their original rounded shape and size information.
These rocks are classified as quartz arenites and sub-arkoses. They are physically and chemically mature. This indicates that the sediment spent a long time in the depositional environment being reworked, and matrix components have been removed by winnowing. Variable grain sizes are believed to reflect the strength of currents responsible for forming these deposits, which in some cases must have been significant.
5.2.2 Wave-dominated Orthoquartzites
Wave-dominated orthoquartzites (Table 3.1) are typically fine sandy sediments with silty intercalations. The sandy component is much larger than the silty components. The sandy portion of these rocks is composed of fine sand and is usually moderately to well sorted and grains are commonly rounded. Small amounts of matrix may be present in some cases, but is difficult to identify due to sericitic alteration.
Quartz and feldspar are the most common framework components (Fig 5.1). Quartz accounts for between 50-80% and feldspars between 10-40% of framework grains. Intrabasinal shale lithoclasts are present in some samples, but account for a maximum of 10% of framework grains. Detrital chlorite and mica flakes are present in all wave- dominated samples and account for 10-30% of framework grains. Monocrystalline quartz is most common but small amounts of chert and polycrystalline quartz is present. Detrital accessory minerals observed include zircon. chromite, epidote and pyrite, and common authigenic minerals include sericite, pyrite, chloritoid, clinozoite and sphene.
52 Petrography
The wave-dominated orthoquartzites contain some sericite pseudomatrix, especially the feldspar-rich samples. The rocks contain units of alternating mature quartzite and siltstone. In some instances silty material is mixed with the quartz grains, most likely by wave action. Chlorite is present in such units, but higher concentrations are found next to the silty bands. The rocks are classified as subarkoses and arkoses.
5.2.3 Controls and Composition by Depositional Environment
The different environments in which these orthoquartzites were deposited influence their petrographic characteristics. The currents responsible for deposition of the current-dominated orthoquartzites result in the variable grain sizes observed and cause winnowing of clay and silty material and chlorite grains, and results in better sorting. Only weak currents may have been present at times during the deposition of the wave- dominated orthoquartzites. Mature well-sorted sand in the wave-dominated units must have moved into the inner shelf environment, where storms caused the wavy bedding observed in these deposits. During calm periods, silty material is draped over the storm generated ripples. In contrast the current-dominated deposits lack siltstone drapes and probably formed at a shallower depth and can be considered more proximal.
Both the current and wave-dominated orthoquartzites are mature sediments, and the detrital quartz particles must have been transported over a long distance to achieve the rounding and sorting characteristics that are present.
5.3 Middleshelf Argillaceous Quartzite
Argillaceous quartzite comprises, as defined earlier, of wackestones and finely interlaminated siltstone-shale-quartzite. Wackestones and finely interlaminated siltstone-shale-quartzites are fine grained sediments, deposited in transitional or middleshelf environments. The largest components in these rocks are silty and very fine sandy material, but mud and fine sands are also present.
Wackestone and siltstone samples are composed of very fine sand-sized and silt-sized grains in a matrix of finely intergrown sericite and chlorite. The matrix content is generally in the order of about 20 volume percent. However, accurate estimation is
53 Petrography not possible due to sericitic alteration of matrix and feldspars. The grains are usually moderately to well sorted, which means that there are no medium or coarse sandy grains present, but there is usually some mixing of silty and fine sandy grains. Grain shapes are highly variable and tend to be represented by a mixture of angular and rounded grains with subrounded grains the most common.
The framework grain components have a similar composition to those of the wave- dominated quartzites. Quartz grains account for between 50%-80% of framework grains. Feldspar and detrital chlorite grains each account for between 10%-30% (Fig 5.2). Lithoclasts are rare. Detrital chlorite grains were distinguished from chloritic matrix using textural parameters like rounding versus intergrowth of particles.
The detrital accessory minerals in these rocks include leucoxene and zircon. Rounded pyrite and chromite is present but rare. The authigenic mineral assemblage is distinctly different from the other rocks. Sulphides are particularly abundant and include pyrite, chalcopyrite, pyrrhotite and occasionally sphalerite and galena. Authigenic clinozoite is common and authigenic sphene is particularly abundant.
The major differences between wackestone and interlaminated shale-siltstone- quartzite lies in the texture of units. The wackestone units are massive or are composed if thin graded beds. In contrast, interlaminated siltstone-shale-quartzite are composed of a well-defined alternating laminae of quartzite, siltstone and mudstone (shale).
Wackestone are considered wave deposits formed in the distal environment above normal wave base (Beukes, 1995). Interlaminated units are formed in more distal environments below normal wave base. They are of similar origin to the wave dominated orthoquartzites, but are finer grained. Despite the different depositional environments, the rocks are fairly similar in composition.
54 Petrography
Q Q Wackestones, Siltstones and Interlaminated Quartzites and Siltstone
C F L Figure 5-2 : QFL and QFC plots of Transitional Shelf Facies Sediments. Q = Total Quartz, F = Interpreted original Feldspar, L = Total Lithoclasts, C = Detrital Chlorite Laths.
5.4 Fluvial Quartzite
The term “fluvial quartzite” is used to distinguish the rocks that were deposited in a braided fluvial environment from those more mature orthoquartzites which were deposited in a shallow marine environment. Petrographic study of quartzites deposited in a braided fluvial system has revealed that these rocks can be divided into two distinct groups, namely mature and immature quartzites.
5.4.1 Immature Fluvial Quartzites
Immature fluvial quartzites are typically composed of moderately to poorly sorted grains. Both rounded and angular grains are present. The fabric is grain-supported, and in some instances matrix is present, but only in minor amounts. Grain size varies dramatically, often even on thin section scale, but mostly they are medium to coarse- grained.
Estimation of framework mineral composition is difficult because these rocks frequently contains between 30-50% sericite pseudomatrix (Fig 5.3). This is thought to reflect high initial feldspar abundance, but the contribution of lithoclasts cannot be discounted.
55 Petrography
The abundance of quartz in these samples is relatively low, between 40-70% (Fig 5.3). Feldspars account for 10-30%, and lithoclasts for 5-20%. The most common lithoclasts are the hydrous iron oxide variety, which probably represent altered shale rock fragments. Detrital chlorite and muscovite laths are common in all immature quartzite samples, accounting for 5-20% of framework grains.
Q Q
Mature Quartzites Immature Quartzites
C F L Figure 5-3 : QFL and QFC plots of Immature quartzites. Q = Total Quartz, F = Interpreted original Feldspar, L = Lithoclasts, C = Detrital Chlorite Laths.
Zircon, pyrite and leucoxene are the most common heavy minerals. Allanite, epidote and monazite are also present. Rare sphene, pyrite, chalcopyrite, chloritoid, and occasionally clinozoite complete the authigenic mineral assemblage.
The immature fluvial quartzites are classified as arkose, subarkoses and lithic arkoses according to QFL and QFC classification diagrams (Fig. 4.3). They are physically immature and chemically immature, which indicates that the detritus was not highly reworked before final deposition.
5.4.2 Mature Fluvial Quartzites
Mature fluvial quartzites are generally coarse to granular and slightly coarser than immature quartzites. They are generally moderately sorted and are mostly composed of subrounded grains. Rounded and angular grains are, however, also present. This
56 Petrography indicates that they are physically somewhat more mature than the immature quartzites.
The framework components of these quartzites are highly variable (Fig 5.3). A simple summary is shown in table 5.1. Immature quartzites contain less quartz, similar amounts of feldspar and fewer lithoclasts than immature quartzites. Mature quartzites contain no detrital chlorite or muscovite flakes, which is expected in more mature sediments because these are non-durable minerals.
Table 5-1 : Comparison of approximate ranges of Framework components for Immature and Mature Quartzites. Immature Mature Component Quartzites Quartzites Quartz 40-70% 70-90% Feldspar 10-30% 10-30% Lithoclasts 5-20% 0-5% Detrital Chlorite and 5-20% None Mica Flakes
The detrital accessory mineral population contains zircon, chromite, pyrite and leucoxene. Authigenic accessory minerals are sericite, pyrite, chalcopyrite, chloritoid and occasionally sphene.
QFL plots (Fig. 5.3), comparing mature quartzites to immature quartzites show that both mature and immature quartzites are arkoses, subarkoses and lithic arkoses. A few of the mature quartzites actually fall in the field of quartz arenites. The immature and mature samples are not drastically different. Grains in mature quartzites are slightly more rounded, better sorted and contain fewer non-durable minerals like chlorite and lithoclasts. This indicates that these rocks are rather similar to immature fluvial quartzites. They were just somewhat better sorted in the fluvial system. However, the texture of the mature fluvial quartzites is different to that of the immature quartzites. In comparison to immature quartzite, grains are no longer medium to poorly-sorted, but rather well-sorted. Sericite, which may in part reflect initial feldspar content, is a significant part of the rocks, but is usually present in distinct masses rather than as
57 Petrography pseudomatrix as in the immature fluvial quartzite. This implies that the mature sands initially had less clay matrix than the immature fluvial sands.
Although less mature than the orthoquartzites, these samples are still considered mature overall, and are interpreted to have spent a long time in the depositional system. The rocks are generally coarser so the energy levels are interpreted to be higher. And although feldspars abundance is higher than in the mature quartzites, it is still low and further supports the idea that the material underwent much reworking before deposition. Lithoclasts are rare, but when they are present, they can normally be considered intrabasinal clasts, eroded from immediately underlying mudstones.
5.5 Diamictite
The samples collected from the Promise diamictite in the BAB1 drill core appear to be reworked by fluvial processes. The sample is composed of a fine- to medium- grained quartzite with moderately sorted subrounded grains. The sample contains 20% feldspar, 1% lithoclasts and 80% quartz.
The Corronation diamictite is composed of mostly angular clasts supported by a fine argillaceous matrix. Some rounded clasts are also present. The matrix is partially overgrown with fine grained sericite needles. The clasts are generally quite similar to those observed in the quartzites. Monocrystalline quartz and feldspar grains dominate. Lithoclasts are more common in the diamictite. The most common types are fine- grained quartz arenite and shale lithoclasts which appear to be derived from within the basin. However, some endoclastic plutonic lithoclasts, including granite and gabbro, are also present. Most interesting the diamictite also contains some large, 2-3 mm diameter, kerogen nodules. Replacive zoned subhedral calcite crystals are also present.
The Lagerspoort diamictite is very similar in composition to the Corronation diamictite. The diamictites is also composed of angular clasts, but they tend to be grain supported and the matrix is fine sand- and silt-sized material composed of quartz and feldspar mainly.
58 Petrography
5.6 Stratigraphic Petrographic Variations
There are some rather well-delineated stratigraphic variations in the petrographic composition of arenaceous rocks of the Witwatersrand Supergroup. These are presented below.
5.6.1 Hospital Hill Subgroup
The Orange Grove, Parktown and Brixton Formations, are composed mostly of mature current dominated orthoquartzites, and very fine grained wackestones. Near the top of the Brixton Formation, less mature quartzites with characteristics of braided fluvial depositional systems become common, and this trend continues into and through the Bonanza Formation.
The Hospital Hill Subgroup sediments are petrographic ally very mature with very few lithoclasts and minor detrital chlorite grains (Fig. 5.4). The rocks are physically mature, well sorted and contain well rounded grains. This suggests that the reason for the depletion of lithoclasts and chlorite is due to sorting processes during transportation and deposition, rather than the absence of these minerals in the source rocks. Three samples contain higher concentration of matrix chlorite but this is because they represent samples of slightly deeper water fine quartzite in the transition between inner- and middle shelf shallow marine environments. Accessory mineral phases common in these rocks are well rounded chromite and zircons, glauconite and authigenic pyrite.
In order to deposit supermature quartzites like these in the Hospital Hill Subgroup, the sediment must either consist of recycled sedimentary rocks or the duration of sedimentary transport must have been long. Recycling of older sediments from the Dominion Group and greenstone belts is a possibility, but the Witwatersrand Supergroup appears to have been deposited for the most part on granitic basement with only a small component of older sediments available in the source. Supermature sediments like those of the Hospital Hill Subgroup will develop in basins which are not very tectonically active, low rates of subsidence coupled with little upliftment will result in low sediment supply, and long periods of sediment reworking within the
59 Petrography basin. Under such conditions less durable minerals will be broken down by chemical and mechanical processes.
Figure 5-4 : The Framework Components of Quartzites from the Hospital Hill, Government and Jeppestown Subgroups in the Klerksdorp area.
5.6.2 Government Subgroup
One outstanding characteristic of the comparison of petrographic composition of quartzites of the Government Subgroup is the high degree of alteration, especially in core BAB1. The samples collected from the Government Subgroup are mostly mature and immature fluvial quartzites but some current-dominated shallow marine facies and transitional shelf and facies, rocks are also present. The immature quartzites in this subgroup and more specifically the Afrikander and Elandslaagte Formations, contain abundant chlorite grains and lithoclasts. The lithoclasts are of the “hydrous iron sillicate” variety and are interpreted as altered shale rock fragments, but altered volcanic rock fragments cannot be discounted. Due to the highly altered nature of the
60 Petrography rocks, the feldspar component is impossible to quantify, but despite this, the abundance of chlorite and lithoclasts is clearly visible.
The maturity of the quartzite varies, to a certain extent, according to the environment in which the rocks were deposited. However, overall the quartzites are less mature than the samples from the Hospital Hill Subgroup. The rocks are mostly arkose and lithic arenite (Fig. 5.5) and are moderately sorted and contain subangular and subrounded grains. Some mature inner shelf marine sediments similar to the Hospital Hill Subgroup are present. A change in the nature of sedimentation on the basin thus apparently took place at the base of the Government Subgroup.
The comparable interval in core AM1 displays a different character (Fig. 5.5). Like in the Klerksdorp area, the distal samples in AM1 are mostly represented by wave- and current-dominated shallow marine orthoquartzites, and immature fluvial quartzites. However, in AM1, all the units are generally somewhat finer grained. The quartzites from core AM1 are also enriched in chlorite, but the strong enrichment in lithoclasts observed in samples from Klerksdorp in core BAB1 is absent. The enrichment in chlorite can be ascribed to the general fining of quartzites, i.e. a facies control. Similarly the absence of lithoclasts can be ascribed to a more distal setting, i.e. again a facies control.
Samples of the Government Group from both the Klerksdorp and the Edenville (Free State) areas are all physically and mineralogically less mature than those of the underlying Hospital Hill Group. Because they also contain a higher feldspar content (Fig. 5.4), a possible recycled orogenic provenance is interpreted for quartzites of the Government Subgroup. Such an interpretation would be supported by the abundance of lithoclasts in proximal samples of the Government Subgroup in the Klerksdorp area. These could be of volcanic and/or shale clast origin. This is in contrast to a basic cratonic granitic source terrane that could tentatively be ascribed to the quartzite of the underlying Hospital Hill Subgroup. Overall the Government Subgroup thus appear to mark the onset of active tectonics in the source area of the basin, in contrast to the stable cratonic setting of the Hospital Hill Subgroup.
61 Petrography
Figure 5-5 : The Framework Components of quartzites collected from the AM1 Drill core.
5.6.3 Jeppestown Subgroup
Inner marine shelf quartzites, braided fluvial quartzites and transitional shelf wackestones are present in the Jeppestown Subgroup. These sedimentary rocks are frequently less altered than other parts of the Witwatersrand succession. There is a trend towards poorly sorted, highly feldspathic and chlorite rich quartzites in the Jeppestown Subgroup. Lithoclasts are present but they are not as abundant as in the Government Subgroup in the Klerksdorp area (Fig. 5.4).
Epigenetic calcite and clinozoite is common, but of interest is the appearance of detrital epidote and allanite in the mineral assemblage in the Klerksdorp area in BAB1, which is not present anywhere else in the succession. Detrital epidote and allanite is indicative of granitic source rocks. A subaerially extruded volcanic source is indicated by the presence of devitrified volcanic glass shards in the Roodepoort Formation. It is possible that these glass shards result from the same event that caused the eruption of the Crown Lava Formation.
62 Petrography
Similar to the Government Subgroup, the rather immature nature of the quartzites suggests active tectonics In the basin and source area. The presence of lithoclasts may indicate a recycled orogenic provenance. This is also a period of volcanic activity in the basin.
In comparison to the Klerksdorp area, the samples of the Jeppestown Subgroup from core AM1 is less enriched in feldspars and lithoclasts (Fig 5.5). However, chlorite abundance remain similar. Some samples in the more distal part of the basin contain detrital epidote which indicates that this detrital component was widespread.
5.6.4 Johannesburg Subgroup
The quartzites from the Johannesburg Subgroup are mostly mature and immature braided fluvial quartzite with subordinate shallow marine orthoquartzites. The quartzites are medium to coarse grained, moderately or poorly sorted and are composed of subrounded and subangular grains. These are some of the most immature quartzites of all of the Witwatersrand succession. The Johannesburg quartzites contain pervasive sericite pseudo-matrix, suggesting that they were feldspathic. The framework composition is enriched in detrital chlorite and muscovite, but the lithoclasts which are characteristic of the Government Subgroup are rare (Fig. 5.6).
Detrital pyrite and zircon are the most common detrital accessory minerals, and authigenic pyrite and sphene are the most common authigenic minerals.
The feldspathic nature of the original sediments suggests a plutonic continental provenance. The immature textures observed are similar to those of the Jeppestown Subgroup, which indicates that the sediments formed under active tectonic conditions like the Jeppestown and Government Subgroups. No devitrified glass shards are present, which indicates that the period of volcanic activity observed in the Jeppestown Subgroup, ceased before the deposition of the Johannesburg Subgroup.
63 Petrography
Figure 5-6 : The Framework Components of quartzites collected from the Central Rand Group in the Klerksdorp Area.
5.6.5 Turffontein Subgroup
The Turffontein Subgroup is composed of coarse to gritty quartzites which are poorly or moderately sorted and are composed of mostly subangular and subrounded grains. They are believed to have been deposited by a braided fluvial system. These samples are physically immature, but the lack of detrital chlorite and muscovite and lithoclasts in the framework assemblage (Fig. 5.6) indicates that these rocks are mineralogically mature. This is a strange combination that probably results from the high energy conditions required to transport the gritty quartzites and conglomerates. This does not explain the lack of feldspars or evidence for the presence of such grains in the original sediment. A highly altered source rock may explain the disparity between chemical and physical maturity. It is also possible that mature sediments contributed to the source rocks.
64
Chapter 6 : Heavy Mineral Analysis
6.1 Introduction
A vast amount of literature describing the mineralogy and heavy minerals of the Witwatersrand auriferous conglomerates exists. The information is well summarised by Feather and Koen (1975) and Phillips and Myers (1987). The common detrital heavy minerals present in the reefs include, pyrite, zircon, chromite and in some cases monazite (Table 6.1). Although Fe-Ti Oxide minerals are not present pseudomorphs of rutile and leucoxene indicate that they were present in the rock and the abundance of leucoxene indicates that they were common. Other heavy minerals which have been recorded but are rare include xenotime, garnet, cassiterite, apatite, corundum, rutile (also authigenic), tourmaline (mostly authigenic), arsenopyrite (also authigenic), cobaltite, and urananite (Phillips and Myers, 1987, Feather and Koen, 1975). However little is known about the heavy minerals present in the quartzite units of the succession. This chapter is to focus on the heavy mineral assemblage in these rocks.
6.2 Mineral Liberation Analyser Results
During this study analysis of 8 dense medium heavy mineral separates of quartzites were undertaken with a Mineral Liberation Analyser (MLA). The results of those analyses are presented in Table 6-2. The results reveal that pyrite and Ti-bearing phases (leucoxene and rutile) make up a large proportion of the separate, averaging 31% and 10% respectively. Zircon, epidote and chromite account for between 1% and 2% of the samples and the other heavy minerals which are known to exist in the Witwatersrand reefs, are not present in these samples. Many of these rare heavy minerals were observed in the petrographic studies, so it is believed that they are present in quartzites, but concentration of these minerals in conglomerates by placer
65 Heavy Mineral Analysis
Table 6-1 contains a summary of the Witwatersrand Reef components, modified after Phillips and Myers, 1987). Minerals marked by an asterisk generally account for more than 99% of reef constituents(Feather and Koen, 1975). Detrital Authigenic
Silicates *Quartz X X *Feldspar X *Muscovite X *Pyrophyllite X *Chlorite X X *Chloritoid X *Zircon X Garnet X Tourmaline X X Sphene
Sulfides *Pyrite X X *Arsenopyrite X X *Pyrrhotite X *Cobaltite X *Gersdorffite X X *Galena X Chalcopyrite X Sphalerite X Molybdenite X
Oxides *Chromite X *Urananite X *Branerite X *Leucoxene X X *Rutile X X Corundum X Cassiterite X
Phosphates Monazite X Apatite X Xenotime X
66 Heavy Mineral Analysis forming processes explains the greater abundance and diversity of heavy minerals encountered in the reef studies.
The abundance of leucoxene is not accurately indicated because the leucoxene grains tend to break apart in the milling process and are therefore overestimated. Leucoxene is a common reef component, it is regarded as the second most abundant heavy mineral after pyrite (Feather and Koen, 1975). The “other” mineral class (Table 6-2) contains mostly grains where a mixed spectrum was collected and it is impossible to classify the grain. When authigenic and detrital minerals contain the same EDS spectrum, they cannot be differentiated with the MLA. This is the case for epidote and pyrite where both authigenic and detrital minerals are known to be present in the rocks. Quartz contents are higher than expected at about 15% of the samples. Observation with the MLA revealed that rutile inclusions and incomplete liberation heavy mineral were the main causes for high quartz contents.
6.3 Point Counting Analyses and Broad Stratigraphic Variations
The heavy mineral populations derived from point counting dense medium separates on a large set of samples are presented in Table 6.3. Common minerals which were encountered are zircon, rounded pyrite, leucoxene, chromite, epidote and allanite. Some of the rare heavy minerals mentioned earlier in the section on mineralogy and petrography were encountered, but only in trace amounts. Only minerals which are known to be detrital or which displayed a detrital morphology in the point counts were recorded. Leucoxene is known to be most probably severely underestimated because grains are fractured in the milling process. The few grains that were recorded displayed rounded morphologies.
A wide variety of zircon grains are present. Major variations in size occur, which appears to vary according to the average grain size of the rock. The smallest zircons present are small elongated euhedral prismatic grains, with the short axis less than 30µ and long axes less than 70µm. These zircons may be of volcanic origin. The most common size fraction of zircon grains contain long axes of between 50-100µm. These
67 Heavy Mineral Analysis
zircons are probably from a plutonic source and they display variable morphologies. The largest zircons measure >200 µm long axes, are well rounded, but are quite rare. No systematic stratigraphic variation in zircon morphology was observed, the average grain size of the rock appears to be the controlling factor.
Table 6-2 : MLA Modal Mineralogy Analyses. Data are grain counts expressed as percentages. *The average is calculated excluding sample JY8 16. Sample BAB1 BAB1 DK12 DK12 JY8 7 JY8 TF1 TF1 Average* 5 (%) 7 (%) 2 (%) 8 (%) (%) 16 (%) OGU2 VER1 (%) (%) (%) Pyrite 14.66 48.50 23.14 51.42 46.81 22.85 18.47 17.74 31.53 Mica 31.42 7.27 15.16 2.55 8.39 5.10 36.18 18.45 17.06 Quartz 8.15 11.40 20.78 13.37 5.13 3.58 5.30 46.92 15.87 Leucoxene and 15.85 8.17 20.39 11.31 13.87 7.46 1.99 2.15 10.53 Rutile Others 9.25 5.70 3.14 2.69 11.37 8.15 21.71 2.72 8.08 Chlorite 7.33 6.70 0.78 0.20 3.61 2.67 4.39 0.43 3.35 Carbon 2.52 3.56 3.14 3.67 0.90 0.99 1.36 1.72 2.41 Feldspar 3.76 3.23 2.35 1.22 3.54 7.62 1.05 1.57 2.39 Pyrophyllite 0.09 0.67 2.09 9.50 0.55 0.38 0.35 1.14 2.06 Zircon 1.01 1.85 4.18 1.91 0.49 0.53 1.08 1.00 1.65 Epidote and 2.66 1.05 0 0.05 4.23 39.07 1.15 0 1.30 Allanite Chromite 0.09 0.43 0.39 0.73 0 0.15 2.06 4.86 1.22 Apatite 1.37 0.81 2.75 0.44 0.14 0.69 0.87 0.29 0.95 Chalcopyrite 0.18 0.38 0.26 0.34 0.14 0.30 1.74 0 0.44 Brannerite 0.64 0.29 0.65 0.05 0.42 0 0.38 0.43 0.41 Carbonates 0.18 0 0 0 0.07 0.38 1.57 0 0.26 Uraninite 0.09 0 0.52 0.24 0.35 0 0 0.57 0.25 REE Phosphate 0.09 0 0.13 0.29 0 0 0.35 0 0.12 Sphalerite 0.55 0 0 0 0 0.08 0 0 0.08 Millerite 0 0 0.13 0 0 0 0 0 0.02 Galena 0.09 0 0 0 0 0 0 0 0.01 Arsenopyrite 0 0 0 0 0 0 0 0 0 Gersdorffite 0 0 0 0 0 0 0 0 0 Total 100 100 100 100 100 100 100 100 100 n 1091.5 2105 765 2042 1442 1313 573.8 699
68 Heavy Mineral Analysis
Chromite grains are generally rounded to well-rounded spherical grains. Their size varies between 50µm and 250µm according to the average grain size of the rock. Chromite grains are usually similar in size to zircon grains in the same sample. It has been noted that the samples collected from the distal Edenville area contain chromites which are smaller on average than the chromites from the Klerksdorp area. This probably reflects the overall finer grain sizes of the more distal rocks in the Edenville area.
The size and morphology of pyrite is complicated. In general the grains can be divided into four different morphologies. These are: Very fine pyrite roughly 20 µm in size. This is disseminated late authigenic pyrite Larger anhedral grains between 50-400µm in size is considered authigenic pyrite Pyrite cubes between 50-250µm that may represent later hydrothermal epigenetic pyrite Rounded pyrite between 50-250µm, represent detrital grains. Pseudomorphs after rounded Fe-Ti oxide grains is another possibility for these grains, but associations with other known detrital minerals suggests that the majority of rounded pyrite grains are detrital in nature (refer to Guy et al, 2010 : 2012 for more complete description of pyrite in the Witwatersrand Succession).
Detrital pyrite is much more abundant in the Central Rand Group quartzites than in those of the West Rand Group (Table 6.3). Detrital pyrite is also quite rare in samples collected from the Edenville area in core AM1. This may be the result of sorting processes in the finer grained rocks.
Epidote is largely limited to the Upper West Rand Group, and is most commonly found in the Babrosco, Roodepoort, and Maraisburg Formations (Table 6.3). Allanite occurs sporadically through the sequence, occasionally in significant numbers (Table 6.3). A number of other heavy minerals were identified, but the minerals generally display very low concentrations in quartzites.
69 Heavy Mineral Analysis
Table 6-3 :Results of point counting heavy mineral populations. Cr – Chromite, Zr – Zircon, Py – Rounded Pyrite, Ep – Epidote, Al – Allanite, Le – Leucoxene, Tot - Total Formation Sample Number Cr Zr Py Ep Al Le Other Tot Cr:Zr Samples collected from the Klerksdorp area Orange Grove TF1 OGL1 76 32 16 124 2.4 Orange Grove TF1 OGU3 87 91 12 190 1.0 Parktown TF1 BUL3 35 424 1 460 0.1 Parktown TF1 SPEC 1 63 50 2 115 1.3 Brixton TF1 VER 1 237 113 72 8 Sphene 430 2.1 Brixton TF1 VER 2 213 86 0 299 2.5 Brixton TF1 WIT 1 275 16 92 Arsenopyrite 383 17.2 Bonanza BAB1 S2 29 316 96 10 451 0.1 Bonanza BAB1 S3 62 241 6 309 0.3 Promise BAB1 S4 43 251 9 303 0.2 Promise BAB1 Prom 2 198 29 1 2 232 0.0 Promise BAB1 S7 15 140 27 5 187 0.1 Corronation BAB1 Corr 0 269 92 2 1 3 367 0.0 Tusschenin BAB1 S9 27 131 125 3 Garnet 286 0.2 Elangslaagte JY8 S4 97 200 66 5 4 372 0.5 Afrikander JY8 S7 72 140 142 2 2 2 Monazite 360 0.5 Koedoeslaagte JY8 S10 165 195 1 1 8 370 0.8 Rietkuil JY8 S11 37 152 13 7 Arsenopyrite 209 0.2 Babrosco JY8 S12 43 98 322 2 465 0.4 Roodepoort JY8 S16 25 168 27 138 4 362 0.1 Maraisburg JY8 MAR 26 246 6 33 311 0.1 Randfontein DK12 S16 6 224 65 Monazite 295 0.0 Randfontein DK12 S17 3 197 96 1 Iridosmene 297 0.0 Krugersdorp DK12 S12 3 68 157 17 245 0.0 Kimberly DK12 S8 6 135 74 2 217 0.0 Elsburg DK12 S6 26 156 162 1 345 0.2 Elsburg DK12 S6A 73 319 3 395 0.2 Mondeor DK12 S2 2 154 95 4 255 0.0 Mondeor DK12 S1 10 76 298 Rutile 384 0.1 MLA Data points Orange Grove TF1 OGU2 11.8 6.2 - - - - - 17 1.9 Brixton TF1 VER1 34 7 - - - - Urananite 41 4.9 Promise BAB1 5 1 11 - - - - Urananite 12 0.1 Promise BAB1 7 9 39 - - - - - 48 0.2 Afrikander JY8 7 7 - - - - Urananite 7 0.0 Roodepoort JY8 16 2 7 - - - - - 9 0.3 Kimberley DK12 8 15 39 - - - - Urananite 54 0.4 Mondeor DK12 2 3 32 - - - - Urananite 35 0.1 Samples collected from the Edenville area Corronation AM1 S5 119 247 5 0 371 0.5 Tusschenin AM1 S6 51 258 3 2 314 0.2 Palmietfontein AM1 S8 45 255 3 20 1 324 0.2 Palmietfontein AM1 S9 59 203 0 262 0.3 Elandslaagte AM1 S12 108 208 0 12 328 0.5 Elandslaagte AM1 S11 54 133 1 5 193 0.4 Afrikander AM1 S14 29 141 1 Fe-Mn Oxide 171 0.2 Afrikander AM1 S15 83 190 0 29 302 0.4 Afrikander AM1 S16 122 274 0 5 Fe-Mg Oxide 401 0.4 Afrikander AM1 LAG 3 397 0 2 Hematite, Urananite 402 0.0 Koedoeslaagte AM1 S17 142 178 2 5 327 0.8 Babrosco AM1 S18 175 128 0 30 333 1.4 Babrosco AM1 S19 225 182 0 0 407 1.2 Roodepoort AM1 S21 138 275 3 23 8 Rutile 447 0.5 Roodepoort AM1 22 66 136 2 75 50 329 0.5 Roodepoort AM1 S23 125 238 0 26 15 404 0.5
70 Heavy Mineral Analysis
6.4 Chromite to Zircon Ratio
The relative abundance of chromite and zircon could be an important indicator of the composition of the source area of sedimentary successions. Chromite is derived from mafic and ultramafic rocks, most likely from greenstone belts whereas zircon is derived from granitic rocks. Chromite and zircon are both durable and stable phases which makes them ideal indicators of the relative abundance of granitic and mafic rocks in the source area. Zircon and chromite also have similar specific gravities and grains are of similar sizes in the samples, suggesting that they were transported in hydraulic equilibrium and their relative abundances are not influenced by hydraulic sorting processes.
The Cr:Zr ratio display very interesting stratigraphic variations in core samples from the Klerksdorp area (Fig. 6.1). Samples from the Hospital Hill Subgroup contain elevated Cr:Zr ratios greater than 1 (Fig. 6.1). The Upper quartzite of the Orange Grove Member and the Bullskop Bed near base of the Parktown Formation are exceptions, displaying Cr:Zr ratios < 1 (Fig. 6.1). The Bullskop Bed contains abundant small zircon grains (<50µm) but it appears that chromite grains of this size are rare, which suggests that the very low Cr:Zr ratio results from the absence of a specific size fraction of chromite, rather than an absence of mafic material in the source terrain.
The Brixton Formation is highly enriched in large well-rounded chromites. Mafic rocks in the source area are required to contribute this material. The transition into the Bonanza Formation is marked by a rapid decrease in the Cr:Zr ratio, which most probably indicates a reduction of mafic material in the source area (Fig. 6.1). An increase in the ratio is observed in the upper Government and lower Jeppestown Subgroups (Fig. 6.1). This signifies the introduction of mafic material into the basin, but the ratios are lower than those from the Hospital Hill Subgroup.
The samples collected from the Edenville area show a rather similar stratigraphic pattern to that of the Klerksdorp area (Fig. 6.2). Most characteristic is the increase in Cr:Zr ratios in the lower part of the Jeppestown Subgroup (Fig. 6.2).
71 Heavy Mineral Analysis
Figure 6-1 : Chromte:Zircon ratio of samples collected in the Klerksdorp area. See Figure 3-3 for legend
72 Heavy Mineral Analysis
Figure 6-2 : Chromite:Zircon ratio of samples collected in the Edenville area. See Figure 3-3 for legend
73
Chapter 7 : Chromite Mineral Chemistry
7.1 Introduction
Chromite is a common constituent of sandstone heavy mineral assemblages, it is durable, resistant to chemical alteration. Chromite composition can thus be used as a provenance and petrogenetic indictor (Ara1, 1992, Barnes and Roeder, 2001 and Kamenetsky et al, 2001). In order to be able to obtain such information, microprobe analyses were undertaken on 82 individual chromite grains, hand-picked from seven samples from the Witwatersrand succession. The results are presented in Table 7.1.
Table 7-1 : Electron microprobe analyses of single detrital chromite grains in the Witwatersrand Supergroup.
Al2O3 Cr2O3 FeO MgO MnO TiO2 V2O5 ZnO Sum Sample Formation (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) Comp TF1 LOG Orange Grove 12.5 57.2 15.9 13.6 - 0.1 - - 99.8 TF1 LOG Orange Grove 11.7 57.8 15.2 14.1 - - - - 99.5 TF1 LOG Orange Grove 20.7 43.9 25.0 8.0 0.5 0.5 - - 98.9 TF1 LOG Orange Grove 7.9 61.5 20.2 10.3 - 0.2 - - 100.7 TF1 LOG Orange Grove 11.6 51.7 29.4 5.9 0.8 0.6 - - 100.3 TF1 LOG Orange Grove 11.8 50.0 33.3 2.4 0.8 0.4 - 0.5 99.4 TF1 LOG Orange Grove 26.4 34.6 34.0 0.6 1.0 0.3 0.3 - 97.5 TF1 LOG Orange Grove 29.4 31.3 34.6 0.8 1.1 0.6 0.4 - 98.4 TF1 LOG Orange Grove 11.4 52.1 27.6 6.5 0.9 0.4 0.3 - 99.3 TF1 LOG Orange Grove 9.0 45.2 41.4 0.9 1.0 0.7 - 0.4 98.9 TF1 LOG Orange Grove 15.8 55.0 13.3 14.7 - 0.2 - - 99.4 TF1 LOG Orange Grove 9.0 43.1 43.7 - 1.2 1.2 - 0.8 99.3 TF1 LOG Orange Grove 18.6 45.5 27.5 6.9 0.5 0.6 - - 100.2 TF1 LOG Orange Grove 23.8 45.2 13.4 16.1 - 0.2 - - 99.2 TF1 LOG Orange Grove 23.8 45.2 13.4 16.1 - 0.2 - - 99.2 TF1 LOG Orange Grove 15.7 56.2 17.6 10.6 0.8 0.1 - - 101.2 TF1 LOG Orange Grove 10.9 58.2 17.9 12.0 0.3 0.1 - - 99.6 TF1 VER1 Brixton 9.2 55.2 35.1 0.3 - 0.1 - - 100.3 TF1 VER1 Brixton 6.8 55.0 36.6 0.7 - 0.5 - - 100.2 TF1 VER1 Brixton 7.2 59.4 21.7 9.6 - 0.4 - - 99.0 TF1 VER1 Brixton 9.9 55.9 20.8 10.0 0.4 0.1 - - 97.3 TF1 VER1 Brixton 7.9 58.2 28.7 4.2 0.4 0.2 - - 99.9 TF1 VER1 Brixton 10.6 60.0 18.7 10.7 - 0.1 - - 100.5 TF1 VER1 Brixton 7.2 55.3 35.4 0.7 0.3 0.3 - 0.4 99.7 TF1 VER1 Brixton 10.6 54.7 29.3 4.3 0.4 0.1 - - 99.6 TF1 VER1 Brixton 11.5 57.7 18.1 12.1 - 0.1 - - 99.7 TF1 VER1 Brixton 10.1 55.0 31.1 2.1 0.7 0.1 - 0.4 99.6
74 Chromite Mineral Chemistry
Table 7-1 Continued
Al2O3 Cr2O3 FeO MgO MnO TiO2 V2O5 ZnO Sum Sample Formation (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) Comp
TF1 VER1 Brixton 7.6 59.5 24.1 7.7 - 0.2 - - 99.6 TF1 VER1 Brixton 10.6 57.8 18.7 11.7 - 0.1 - - 99.3 TF1 VER1 Brixton 9.2 58.7 23.3 8.8 - 0.1 - - 100.4 TF1 VER1 Brixton 11.3 56.4 26.0 5.6 0.5 0.1 - - 99.9 TF1 VER1 Brixton 8.8 60.5 18.5 11.6 - 0.2 - - 100.2 TF1 VER1 Brixton 8.2 57.5 25.6 8.7 - 0.1 - - 100.6 TF1 VER1 Brixton 10.2 55.1 29.8 4.4 0.4 - - - 100.1 TF1 VER1 Brixton 11.0 59.1 17.6 12.9 - 0.1 - - 101.1 TF1 VER1 Brixton 10.1 55.6 28.4 5.4 - 0.2 - - 100.2 TF1 WIT 1 Brixton 9.0 54.9 32.0 2.9 0.4 - - - 99.7 TF1 WIT 1 Brixton 9.6 53.8 32.1 3.2 0.6 0.2 - - 99.6 TF1 WIT 1 Brixton 9.3 52.9 33.5 2.2 0.5 0.1 - 0.3 99.0 TF1 WIT 1 Brixton 9.7 54.7 30.9 2.9 0.6 0.1 - - 99.2 TF1 WIT 1 Brixton 9.6 51.6 36.2 1.1 0.7 0.1 - 0.3 99.8 TF1 WIT 1 Brixton 9.7 54.5 30.0 3.9 0.5 0.1 - - 99.0 TF1 WIT 1 Brixton 9.1 53.4 32.7 2.2 0.4 0.1 - - 98.3 TF1 WIT 1 Brixton 11.1 44.0 40.6 - 0.7 1.3 - - 98.4 TF1 WIT 1 Brixton 11.4 51.3 34.4 1.3 0.7 0.5 - 0.4 100.1 TF1 WIT 1 Brixton 11.1 51.6 33.3 1.4 0.7 - - 0.3 98.9 TF1 WIT 1 Brixton 9.5 52.9 35.1 - 0.5 0.1 - 0.5 99.1 TF1 WIT 1 Brixton 13.1 46.5 36.8 0.9 0.7 0.7 - 0.5 99.3 TF1 WIT 1 Brixton 8.0 56.4 29.9 3.8 0.5 0.1 - - 98.8 TF1 WIT 1 Brixton 10.9 50.9 35.0 0.6 0.6 0.1 - 0.4 98.8 TF1 WIT 1 Brixton 9.2 54.5 31.5 2.7 0.6 - - 0.4 99.0 TF1 WIT 1 Brixton 4.2 55.5 36.6 - 0.8 0.3 - - 98.0 TF1 WIT 1 Brixton 10.1 51.6 35.8 0.5 0.5 0.2 - - 99.2 TF1 WIT 1 Brixton 9.0 53.7 33.7 1.4 0.6 - - - 99.0 TF1 WIT 1 Brixton 7.6 55.4 31.9 2.9 0.5 0.1 - - 98.7 TF1 WIT 1 Brixton 6.2 55.2 35.4 0.5 0.6 0.3 - - 98.7 TF1 WIT 1 Brixton 10.5 53.6 36.1 0.3 0.7 0.6 - - 102.3 BAB1 7 Promise 8.9 57.8 23.0 9.1 0.4 0.2 - - 99.5 BAB1 7 Promise 9.9 55.5 21.9 10.6 0.4 0.5 - - 99.0 BAB1 7 Promise 17.1 49.0 18.3 13.0 - 0.7 - - 98.7 BAB1 7 Promise 10.8 50.1 28.7 7.3 0.5 0.7 - - 98.6 BAB1 7 Promise 9.8 52.4 26.2 9.3 - 0.9 - - 99.2 BAB1 7 Promise 14.2 49.7 22.1 11.2 - 0.8 0.5 - 98.8 BAB1 7 Promise 10.7 54.3 21.8 11.1 - 0.7 - - 99.0 BAB1 7 Promise 11.4 43.8 33.5 6.9 0.6 1.2 - - 97.9 BAB1 7 Promise 6.0 60.2 20.0 10.8 - 0.3 - - 97.8 BAB1 7 Promise 13.7 46.7 25.7 11.1 0.4 0.9 - - 98.9 BAB1 7 Promise 14.2 52.6 20.9 11.3 - 0.3 0.3 - 99.9 BAB1 7 Promise 16.6 49.7 18.6 13.7 - 0.7 - - 99.9 BAB1 7 Promise 13.7 46.2 25.5 10.4 - 2.4 0.6 - 99.3 BAB1 7 Promise 10.2 55.4 26.6 4.3 1.4 0.1 - 0.7 98.7 BAB1 9 Tusschenin 9.4 57.4 20.5 10.7 - 0.2 - - 98.9 BAB1 9 Tusschenin 16.3 48.3 19.1 13.5 0.4 0.7 0.4 - 98.8 BAB1 9 Tusschenin 16.3 48.3 19.1 13.5 0.4 0.7 0.4 - 98.8 BAB1 9 Tusschenin 9.9 55.6 21.9 10.9 - 0.2 - - 99.0 JY8 7 Afrikander 7.7 61.2 18.8 11.5 - 0.3 - - 100.2 JY8 7 Afrikander 5.9 60.3 21.9 11.0 0.4 0.4 - - 100.1 JY8 7 Afrikander 16.5 51.0 17.1 14.7 - 0.2 - - 100.0 JY8 16 Roodepoort 11.9 54.9 19.9 11.0 - 0.3 0.5 - 98.8 JY8 16 Roodepoort 9.8 46.1 36.0 3.6 0.4 0.9 0.3 - 97.3 JY8 16 Roodepoort 12.0 41.5 38.5 3.7 0.5 2.5 0.4 0.6 99.6 JY8 16 Roodepoort 8.0 50.1 32.7 4.9 0.7 0.7 - 0.6 98.0
75 Chromite Mineral Chemistry
7.2 Zonation in Chromite Grains
Line scans across the grain from the core to the rim revealed that compositional zoning is present in at least some of the chromites (Fig. 7.1). In one of the grains analysed for example there is an antipathetic relationship between FeO and Al2O3 contents. The center and margin of the grain also contains highest Cr contents (Fig 7.1). Zoning is a response to the availability of cations during crystallisation. Substitution of bivalent cations Fe2+ and Mg2+ and trivalent cations Cr3+ and Al3+ is common. It is important to understand that some variation in mineral chemistry may be the result of zoning. Therefore, analyses on the 82 grains were done as close to the center of each grain as possible.
Figure 7-1 : Compositional Zoning in a Chromite grain, as determined by a microprobe line scan.
7.3 Chromite Variation
The composition of chromite can be highly variable. In order to illustrate the chemical composition of chromite the spinel prism is normally used (Barnes and Roeder, 2001). Most of the samples of the Witwatersrand succession are tightly clustered in the 90% probability field (Barnes and Roeder, 2001) (Fig 7.2B). Samples from the Orange Grove Formation
76 Chromite Mineral Chemistry
A
B West Rand Group Chromite
Figure 7-2 : Composition of chromite grains from different stratigraphic intervals of the West Rand group (A) and a comparison of chromites from the Bushveld Complex and greenstone belt komatiite (B), and. Both figures are plotted on the Spinel prism after Barnes and Roeder (2001).
77 Chromite Mineral Chemistry contain low Fe3+ concentrations and are compositionally distinct. They plot along the Cr-Al trend (Fig. 7.2A). Samples from the Brixton Formation plot in a tight cluster near the Cr end member (Fig. 7.2A). Samples from the Promise, Tusschenin and Afrikander Formations are somewhat more variable. They contain higher Fe3+ concentrations than the Brixton Formation chromites but contain lower Al concentrations than the chromites from the Orange Grove Formation (Fig. 7.2A).
Bushveld Complex chromite analyses by Schurmann (1993), which formed much later than the deposition of the Witwatersrand succession, appear to overlap only partly in composition to those of the Witwatersrand (Fig. 7.2B).
A comparison with chromites from komatiites from the 3.3 Ga Mendon Formation in the Barberton Greenstone Belt (Byerly, 1999) is also included in Figure 7.2B. All samples, with the exception of the Orange Grove Formation, are similar to chromites present in komatiites of the Barberton greenstone belt (Fig7.2B).
7.4 Tectonic Setting
Kamenetsky et al. (2001) showed that TiO2 and Al2O3 contents in chromites are good indicators of parental melt composition. This allows the identification of the tectonic environment in which the chromites formed. Kamenetsky et al. (2001) also showed that the rate of cooling can influence the composition of the chromite. Following principles outlined by Kamenetsky et al. (2001) it can be said that chromites from the West Rand Group with
TiO2 concentrations >0.2wt % cooled rapidly and were therefore derived mostly from extrusive volcanic rather than deeper crustal source rocks (Fig.7.3A). This would conform to the idea that the chromites were derived from lavas in greenstone belt environments. Chromite from the Central Rand Group which were analysed by Viljoen and van der Walt
(1973) contains an additional low TiO2 population (Fig. 7.3B), Kamenetsky et al. (2001) suggests that chromites with low Al2O3 and low TiO2 are typically derived from peridotites formed from suprasubduction zones.
Two types of chromites derived from primitive source rocks, and another broad group of chromites which may be derived from more evolved source rocks, possibly MORB, are
78 Chromite Mineral Chemistry present in the Orange Grove Formation (Fig. 7.5). This suggests that diverse mafic and ultramafic source rocks were exposed during the deposition of the Orange Grove Formation.
Figure 7-3 : Al2O3 vs. TiO2 discrimination plots modified after Kamenetsky et al. 2001 to demonstrate the volcanic origin of chromite from the West Rand group(A) and combined volcanic and plutonic origins of chromite from the Central Rand Group (B).
Figure 7-4 : Cr# vs. TiO2 discrimination plots modified after Arai 1992 to demonstrate the addition of MORB / Island Arc derived chromites with low Cr numbers in the Central Rand Group (A) which are not present in the West Rand Group (B).
The Brixton Formation chromites are tightly clustered (Fig 7.6). They contain low Al2O3 and
TiO2 and high Cr-numbers which suggests that chromites are derived from a primitive source. The chromite derived from the more evolved source observed in the Orange Grove Formation are not present in the Brixton Formation (Fig 7.6).
79 Chromite Mineral Chemistry
The high TiO2 concentrations and low Al2O3 concentrations in chromites observed in Promise, Afrikander and Roodepoort Formations indicates the persistence of a primitive source rock (Fig. 7.7). The more evolved source recognised in the Orange Grove Formation is not present, but the samples tend to contain slightly higher TiO2 concentrations.
Figure 7-5 : Al2O3 vs. TiO2 discrimination plot modified after Kamenetsky et al, 2001 and Cr-number vs. TiO plots after Arai, 1992 for chromites from the Orange Grove Formation
Figure 7-6 : Al2O3 vs. TiO2 discrimination plot modified after Kamenetsky et al, 2001 and Cr-number vs. TiO plots after Arai, 1992 for chromites from the Brixton Formation
Two interesting grains with compositions indicating an oceanic intra-plate source are present in the Promise and Roodepoort Formations (Fig. 7-7), The volcanic arc derived chromites are quite abundant in the upper formations of the West and Group (Fig. 7.6).
80 Chromite Mineral Chemistry
7.5 Conclusions
The chromite mineral chemistry supports a primitive mantle source similar in composition to greenstone belts for most of the West Rand Group. An additional more evolved source rock, is indicated by the composition of chromite grains in the Orange Grove Formation of the
Figure 7-7 : Al2O3 vs. TiO2 discrimination plot modified after Kamenetsky et al, 2001 and Cr-number vs. TiO plots after Arai, 1992 for chromites from the upper part of the West Rand Group
Hospital Hill Subgroup, but is not present in the Brixton Formation. Two grains indicate the presence of chromite from an intra-plate basalt in the Promise and Roodepoort Formation (Fig. 7-7) which persists into the Central Rand Group (Fig. 7.4 B) (Viljoen and van der Walt, 1973).
81
Chapter 8 : Geochemistry
8.1 Major Element Geochemistry
8.1.1 Basic Data Set
The mean major element chemical compositions of different quartzites and diamictites of the Witwatersrand succession are presented in Table 8-1, and the complete data set is presented in Appendix I. The rocks are all highly enriched in silica and are depleted in most other element oxides. The current-dominated marine orthoquartzite facies and mature fluvial quartzite facies are most enriched in silica, with average concentrations of 93 wt% SiO2, and around 5 wt% Al2O3. As mentioned earlier, these rocks are mineralogically similar but are texturally very different in composition. The wave-dominated orthoquartzite facies and immature fluvial quartzite facies contain slightly less silica, at 82 wt% and 85 wt% on average respectively, and more Al2O3 at 8.8 wt% and 9.6 wt% respectively on average. These rocks also contain FeO, MgO, K2O, Na2O and
CaO concentrations in the order of 1-2 wt% (Table 8.1). The lowest silica and highest Al2O3 concentrations occur in the finely interlaminated sandstone, siltstone and shale, and in the wackestones. In these rocks, FeO and MgO start to account for larger portions of the chemical composition of the rocks. The diamictite samples are similar in composition to the interlaminated sandstone-siltstones-shale facies (Table 8.1).
Chemical cross-plots give a good indication of the geochemical variation between different lithofacies (Fig. 8.1). The compositional variations appear to reflect changes in the mineralogy in the rock. The common minerals that are present include quartz, feldspars, chlorite, sericite, muscovite and to a lesser extent pyrite and calcite. Only the effects of the most abundant minerals is reflected in the major element geochemistry.
82 Geochemistry
Table 8-1 : Average major element chemical compositions of rock types, from XRF Analysis.
SiO2 Al2O3 FeO MnO MgO TiO2 CaO Na2O K2O P2O5 S (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (Wt%) (W%) Current Dominated 93 4.6 1.2 0.02 0.5 0.1 0.4 0.5 1.4 0.02 0.3 Orthoquartzite Wave Dominated 82 9.6 2.3 0.03 1.1 0.3 1.1 1.9 1.8 0.03 0.3 Orthoquartzite Immature Fluvial 85 8.8 2.0 0.05 0.9 0.3 0.6 1.2 1.9 0.04 0.3 Quartzite Mature Fluvial 93 5.3 1.0 0.03 0.4 0.1 0.3 0.5 1.2 0.03 0.2 Quartzite Finely Laminated Siltstones and 70 13.6 6.3 0.01 3.3 0.6 1.3 2.1 2.1 0.02 0.3 Wackestones
Diamictite 74 11.4 5.6 0.05 2.5 0.4 1.6 1.5 2 0.04 0.3
8.2 Relationship between Chemical and Mineralogical composition
8.2.1 Influence of feldspars, sericite and muscovite
Weak correlations exist between Na2O and K2O, with Al2O3 (Figs. 8.1A and B). These components are hosted by feldspars and muscovite. The poor correlations are probably related to the fact that authigenic sericite grains formed at the expense of mostly feldspars and matrix, and also by the redistribution of mobile elements by fluids, such as albitisation of feldspars. Small variations in the concentration of these elements in orthoquartzite and mature quartzite facies, causes large variations due to low abundances. This accounts for the scatter observed in these rocks.
8.2.2 Effect of Chlorite concentration
The rather strong correlations between FeO and MgO with Al2O3 (Fig 8.1E and F) most probably results from the inclusion of these components in chlorite minerals. The correlation between Fe2O3 and Al2O3 is not as good as between MgO and Al2O3. This could be explained by the presence of pyrite in many samples. The presence of different chlorite minerals may also have some effect. The highest chlorite concentrations occur in the interlaminated quartzite-siltstone-shale facies and in wackestones. Chlorite is also a rather common component of the wave-dominated marine orthoquartzites facies and immature fluvial quartzite facies. Most of the transitional shelf facies i.e. greywackes and interlaminated quartzite-siltstone-shale units are strongly enriched in MgO and FeO which causes them to plot as a separate group to the quartzites (Figs 8.1E and F). This
83 Geochemistry
A B 10 10
1
1 K2O(Wt%) Na2O(Wt%) 0.1
0.1 0.01 1 10 100 1 10 100 Al2O3(Wt%) Al2O3(Wt%)
C D 10 100 95
90
85
1 80 SiO2(Wt%)
CaO(Wt%) 75
70 65
0.1 60 1 10 100 0 5 10 15 20 Al2O3(Wt%) Al2O3(Wt%)
E F 10 100
10
1 MgO(Wt%) FeO(Wt%) 1
0.1 0.1 1 10 100 1 10 100 Al2O3(Wt%) Al2O3(Wt%)
Caption and legend overleaf
84 Geochemistry
G H 1 10
0.1 1 TiO2(Wt%) MgO(Wt%)
0.01 0.1 1 10 100 0.1 1 10 100 FeO(Wt%) Al2O3(Wt%)
Figure 8-1 : Bivariate plots of major element oxides of lithofacies in the Witwatersrand succession.
relationship is very well illustrated in separate cross-plots of MgO and FeO vs. Al2O of the different rock groups (Fig. 8.1 E and F).
8.2.3 Effect of Sphene and Leucoxene Concentration
Petrographic evidence combined with the chemical data suggests that TiO2 concentrations reflect the abundance of authigenic sphene and leucoxene in the rocks. It is believed that the growth of authigenic sphene is related to the release of TiO2 during the breakdown of Fe-Ti oxides, as is suggested by the textures present in the leucoxene (see chapter 4). Excellent correlations exist between TiO2 and Al2O3 (Fig. 8.1G).
8.2.4 Effect of Quartz Concentration
The SiO2:Al2O3 ratio is a convenient indicator of the relative abundance of quartz and chert, and aluminosilicates such as chlorite, feldspar and sericite. Orthoquartzites and mature quartzites show higher SiO2:Al2O3 ratios which reflects the high quartz content. Similarly low ratios for finely laminated siltstones and wackestones indicates the relative abundance of
85 Geochemistry aluminosilicate minerals in these rocks (Fig.8.1D). However, it is so that correlation breaks down in fine rocks (Fig. 8.1D).
8.2.5 Effect of Sorting
The dominant trend in the dataset is higher abundances of all major elements with a decrease in SiO2 content (Table 8.1). This is caused by the dilution of all other elements by quartz, which is dependent on the sorting processes which formed the rock. The importance of the effect of sorting on chemical composition of the various facies is discussed in more detail under the section on trace element composition of rocks.
8.3 Major Element Chemostratigraphy
8.3.1 Hospital Hill Subgroup
The Hospital Hill Subgroup is composed of mature orthoquartzites, which have high SiO2 concentrations, and low Al2O3 concentrations (Fig. 8.2). Fine grained transitional shelf facies rocks occur between the orthoquartzites. These contain relatively low SiO2 and high Al2O3 concentrations (Fig. 8.2). The quartzites of the Hospital Hill Subgroup contain low concentrations of FeO, MgO and TiO2, (Figs. 8.2 and 3) which is expected for mature orthoquartzites. Some samples from the Orange Grove Member, like the Speckled Marker
Bed and especially the Bullskop Marker Bed contain high levels of K2O (Fig. 8.3), which probably reflects the original presence of abundant alkali feldspars.
Quartzites from the Brixton and Bonanza Formations, show a trend of decreasing SiO2, and increasing Al2O3, which is associated with increasing TiO2 upwards in the succession (Fig. 8.2). This is the result of decreasing maturity and more fluvial characteristics of the rocks in the Bonanza Formation. Over the same interval an increase in K2O and corresponding decrease in Na2O occurs (Fig. 8.3). These less mature quartzites contain a significant feldspar component, which is dominated by albite. Sericite replaces the albite and is probably responsible for the increase in K2O observed. FeO and MgO concentrations are low which reflects the low abundance of chlorite (Fig. 8.3). Samples TF1, WIT-2 and TF1RV contain, higher than average MgO, and indicates that clinochlore is present in these samples (Fig 8.3).
86 Geochemistry
Figure 8-2 : Stratigraphic variation of major element geochemistry in the Klerksdorp area for SiO2, Al2O3, Fe2O3 and
TiO2.See Figures 3-3 and Figure 8-1 for legends
87 Geochemistry
Figure 8-3 : Stratigraphic variation of the major element geochemistry in the Klerksdorp area for MgO, MnO, CaO,
Na2O and K2O. See Figures 3-3 and Figure 8-1 for legends
88 Geochemistry
8.3.2 Government Subgroup
The Government Subgroup is mainly composed of immature fluvial quartzite interbedded with transitional shelf facies wackestones and siltstones, shale and also diamictites. These rocks display lower SiO2, and higher Al2O3 and TiO2 concentrations than the Hospital Hill Subgroup (Fig. 8.2), all consistent with slightly less mature rocks. FeO and MgO concentrations are elevated (Fig. 8.3) and consistent with what is expected for immature and mature quartzites.
All samples collected from the BAB1 drill core, including samples from the Bonanza Formation of the Hospital Hill Subgroup contain low Na2O concentrations and high K2O concentrations (Fig. 8.3). This results from the replacement of feldspar by sericite. It is important to again note that samples collected from core BAB1 all display a higher degree of sericitic alteration than samples collected from other drill cores.
8.3.3 Jeppestown Subgroup
The Jeppestown Subgroup is composed of mostly immature fluvial quartzite and wave- dominated orthoquartzite marine quartzite rocks, with SiO2 concentrations of between 80-90% and high Al2O3 of 5-15%. Fine-grained transitional shelf sediments are also present and are correspondingly enriched in Al, Fe, Mg, Ti and depleted in Si (Figs. 8.2 and 8.3). Samples collected from the Babrosco, Roodepoort and Maraisburg Formation contain relatively high CaO concentrations of between 1 - 1.75% (Fig. 8.3). This is associated with the presence of authigenic epidote and calcite.
Increasing Na2O concentration with stratigraphic height and relatively constant K2O concentrations over the same interval (Fig. 8.3) are interpreted as resulting from moderate sericitic alteration and increasing proportion of Na-plagioclase feldspars in the quartzites.
8.3.4 Johannesburg Subgroup
The Johannesburg Subgroup is a sequence of immature fluvial quartzite with a shale sequence unit, the Booysens Formation, near the top. A trend of decreasing SiO2 and increasing Al2O3 and TiO2, FeO and MgO occurs, which suggest that the quartzites become less mature up section and incorporate more chlorite (Figs. 8.2 and 8.3). The quartzites of the Johannesburg Subgroup are similar to those of the Government and Jeppestown Subgroups, but they are more homogenous without interbeds of orthoquartzite and argillaceous mature beds and shales. High levels of K2O in these samples indicates that they are sericitic. Samples DK12-20, DK12-9 and DK12-15 contain a
89 Geochemistry different mineral assemblage as they are finer grained (Fig. 8.3). Tourmaline, clinozoite, feldspars and chlorites are much more abundant in these rocks, which explains variations in Na2O, CaO and
K2O concentrations in these samples (Fig. 8.3). Samples DK12 DK12-13B and DK12-19 contain high Na2O concentrations as a result of Na-feldspar in the rocks (Fig. 8.3).
8.3.5 Turffontein Subgroup
The Turffontein Subgroup is composed of chemically mature fluvial quartzites which display high
SiO2 and low Al2O3, Fe2O3, MgO and TiO2 concentrations. A trend of increasing K2O and decreasing Na2O upwards in the succession reflects replacement of feldspars by sericite (Fig. 8.3).
8.3.6 Lateral variation of quartzite chemical composition
A comparison of the compositions of quartzites in the distal Government and Jeppestown Subgroups in the core AM1 (Fig. 8.4) with correlative units in the proximal Klerksdorp area (Fig.
8.2) indicates that they are rather similar. The main difference appears to be slightly lower SiO2 and higher Al2O3 concentrations in samples from the distal area in core AM1 (Compare Figs 8.2 and 8.4). This is attributed to the finer grain size and larger chlorite component in these rocks.
8.3.7 Summary of Major Element Variations
The major conclusion that can be drawn from the major element geochemistry is that it reflects the variation of the major mineral assemblages associated with different sedimentary facies and depositional environments. Major element composition thus does not reveal any information on the characteristics of the source rocks, but it does record some of the processes which have influenced the sediment, such as sorting and maturity. Understanding these factors will prove important for reaching the ultimate aim of this study, namely the nature of source terranes or provenance of the Witwatersrand succession.
90 Geochemistry
Figure 8-4 : Stratigraphic variation of major element geochemistry in the AM1 bore hole in the Edenville area. See Figures 3-3 and Figure 8-1 for legends
8.4 Trace Element Geochemistry
8.4.1 Background
Trace element geochemistry is a highly versatile technique which can help solve many different geological problems. This chapter will focus on the application of trace element geochemistry to
91 Geochemistry reveal information on the source rocks of the Witwatersrand Supergroup. Certain trace elements, especially REES, and HFSE are regarded as robust indicators of source rock composition (McLennan, 1985; McLennan et al., 2003). However, these techniques are typically applied to shale and wackestone samples sets. In order to apply them to a quartzite sample set, the impact of the processes which influence sedimentary rocks composition must be well understood.
The factors which influence the trace element geochemical composition of sedimentary rocks are highly varied, but as mentioned earlier the most important factors are as follows according to Morton and Hallsworth, (1999): Post depositional redistribution of trace elements by chemical processes such as diagenesis, metamorphism and alteration Hydraulic sorting during transportation and deposition Mechanical and chemical breakdown of minerals during transportation
These processes tend to mask or overwrite the record of the source terrane stored in the rocks. Therefore they must be well understood in order to evaluate the composition of the source terrane.
8.4.2 Trace Element Composition of Rocks
The average trace element compositions of different arenaceous lithofacies of the Witwatersrand succession are presented in Table 8.2 and the complete dataset is presented in Appendix III. Analyses of the data via an Archean Upper Crust (AUC) (McLennan, et al., 1980) normalized distribution diagram, indicate that the trace element distribution varies quite significantly between different lithofacies and depositional settings (Fig. 8.5). In the element distribution diagram K, Ba and Rb represent mobile incompatible elements (MIE), and Ti, Yb, V, Co and Si immobile compatible elements (ICE) (Fig. 8.5).
Large-ion lithophile elements, K, Ba and Rb, are typically contained in feldspars and micas. They are considered mobile and variations in K, Ba and Rb concentration usually reflects the presence of micas and feldspars in the sample. These elements are enriched in slightly argillaceous wave- dominated orthoquartzites, immature fluvial quartzites and transitional facies sediments relative to AUC (Fig. 8.5). The reason is that these rocks contain greater feldspar and sericite components than the current dominated and mature fluvial quartzite. These elements are strongly influenced by metamorphism, alteration and by hydraulic sorting.
92 Geochemistry
Table 8-2 : Average trace element compositions of different arenaceous lithofacies of the Witwatersrand succession
1 3 3 3 2 3 2 3 3 2 2 2 2 3 3 Element K2O Ba Rb Th La Ce Zr Hf Sm Y TiO2 Yb V Sc Co
Unit % ppm ppm ppm ppm ppm ppm ppm ppm ppm % ppm ppm ppm ppm
Current dominated 1.30 228 47 3.0 13.0 30.5 151 4.4 1.7 4.8 0.08 0.60 21 2.0 8.2 orthoquartzite
Wave dominated 2.12 400 75 4.5 18.3 44.2 176 5.0 2.8 9.3 0.22 0.96 39 7.0 18.4 orthoquartzite
Laminated siltstone and 2.13 400 76 5.3 17.8 46.1 178 5.3 3.4 15.7 0.36 1.61 66 13.5 32.1 wackestone
Immature fluvial 1.97 426 62 4.8 16.9 45.6 117 3.4 2.6 7.6 0.16 0.69 34 5.1 14.4 quartzite
Mature fluvial 1.22 228 41 4.3 11.8 32.2 109 3.2 1.9 6.6 0.09 0.67 21 2.2 8.1 quartzite
AUC (McLennan et 1.80 265 50 5.7 20.0 42.0 125 3.0 4.0 18.0 0.50 2.00 195 14.0 25.0 al, 1980)
1 2 3 ANALYTICAL METHOD XRF FUSION BEAD XRF PRESSED PELLET NEUTRON ACTIVATION
The immobile incompatible elements (Th, La, Ce, Zr and Hf) concentrations are rather similar to that of Average Archean Upper Crust (AUC) abundances, but slight depletions in Th and La and enrichment in Zr and Hf are observed (Fig. 8.5). This could be the result of the durability of zircon that hosts Zr and Hf. The average La:Th ratio is 3.5 which is consistent with that of Archean rocks (McLennan, et al., 1980). Primary igneous and metamorphic minerals which host compatible elements are typically very unstable at near surface temperatures and pressures, and weather easily to form clay minerals.
Ti is mainly hosted by leucoxene, and authigenic sphene in the Witwatersrand succession. The depletion of Ti relative to AUC, especially in mature orthoquartzite lithofacies (Fig. 8.5), could be related to the non-durable nature of most Ti bearing minerals, which are rapidly altered during weathering and are sorted into fine sediments. Ti should be concentrated in the shale beds of the Witwatersrand succession rather than the arenaceous rocks investigated in this study. This is
93 Geochemistry illustrated by the fact that Ti is most enriched in the argillaceous transitional facies relative to the coarser grained quartzite (Fig. 8.5).
10
Current Dominated Orthoquartzites
Wave Dominated 1 Orthoquartzite
Laminated siltstones and Wackestones
Norm AUC Norm Immature Fluvial 0.1 Quartzites
Mature Fluvial Quartzites
0.01 K Ba Rb Th La Ce Zr Hf Sm Y Ti Yb V Sc Co
Figure 8-5 : Trace element distribution between different sedimentary lithofacies.
The immobile compatible elements Ti, Yb, V, Co and Sc all show moderate to strong correlation coefficients (Table 8.3). This is an indication that they are incorporated in minerals sorted in the matrix. All of these elements would normally be sorted into clays and their depletion relative to AUC in the arenaceous rocks of the Witwatersrand succession (Fig. 8.5) is thus to be expected. With reference to Figure 8.5 it is seen that V is highly depleted relative to AUC. This can be explained by the fact that V is commonly hosted by magnetite and Fe-Ti oxide minerals. Dissolution of Fe-Ti oxides by reducing fluids, resulting in the formation of leucoxene, causes the depletion of vanadium relative to AUC. A strong correlation between Sc, Ti, Co and V (Table 8.3) indicates that remaining vanadium is present with the matrix minerals together with these elements. It is thus clear that the sorting and maturity processes, unique to the sedimentary facies strongly influence the trace element composition of the rocks.
94 Geochemistry
Table 8-3 : Pearsons correlation coefficients for selected elements.
Th La Ce U Zr Hf Nb Ti Y Yb V Co Sc Th 1 La 0.70 1 Ce 0.71 0.99 1 U 0.75 0.54 0.54 1 Zr 0.63 0.49 0.50 0.63 1 Hf 0.64 0.49 0.51 0.63 1.00 1 Nb 0.40 0.39 0.45 0.18 0.38 0.35 1 Ti 0.42 0.49 0.55 0.27 0.43 0.39 0.91 1 Y 0.21 0.37 0.43 0.01 0.09 0.05 0.72 0.76 1 Yb 0.53 0.49 0.55 0.42 0.64 0.65 0.84 0.87 0.77 1 V 0.29 0.38 0.44 0.18 0.30 0.26 0.86 0.97 0.78 0.82 1 Co 0.42 0.48 0.53 0.41 0.52 0.53 0.85 0.93 0.74 0.89 0.90 1 Sc 0.24 0.37 0.44 0.11 0.18 0.19 0.88 0.95 0.88 0.78 0.98 0.87 1
8.4.3 Evaluation of Factors Influencing Trace Element Composition
8.2.3.1 Post depositional Redistribution of Mobile Elements Chemical processes, such as weathering, diagenesis, alteration and metamorphism, especially processes involving warm or hydrothermal fluids, have the ability to alter the chemical composition of the rock. It is impossible to establish the precise effects of these chemical processes on the trace element composition of the rock, but it is well known that the Witwatersrand Supergroup rocks are altered and have experienced greenschist facies metamorphism (Phillips and Law, 1994). In order to avoid incorrect interpretations, arising from analysis of rocks modified by chemical processes, this analysis is limited to elements which are least affected by chemical processes.
Taylor and McLennan (1985) showed that elements with low sea water abundance and high residence time in sea water are immobile and are more likely to record the composition of the source rocks. These elements are Al, Th, Y, Zr, Ti, REEs, Sc, Co, V, Nb and Hf. Although these elements are generally considered immobile their possible mobility from data obtained during this study, will be evaluated in a later section.
95 Geochemistry
8.4.3.1 Mineral Fractionation during Hydraulic Sorting Hydraulic sorting during transportation and deposition is highly complicated and operates in many different ways. It involves the separation of sediment components, according to grain size and density.
The most commonly observed effect of hydraulic sorting is separation of grains according to grain size. Silt- and clay-sized material is separated from sandy material usually resulting in elevated silica content in the sandy fraction and an increase in the content of clay particles, and with that the contents of most other elements, in argillaceous sedimentary rocks. Incompatible trace elements (Sc, Co, V, Ti) are typically concentrated in the finer grained fraction. Hydraulic sorting can also have a strong influence on the heavy mineral population present in a sediment, this in turn strongly influences on the concentrations of some trace elements. Entrainment and winnowing are two common processes which can cause fractionation of heavy minerals in a sediment. The concentrations most likely to be influenced by heavy mineral fractionation are Zr, Cr, Hf, Nb, Ta, Sn, B, Sr, REE and Th (McLennan et al 1990, Taylor and McLennan, 1985). Table 8.4 present a list of the common host heavy minerals of some trace elements. Important is to note the zircon hosts mainly Zr and Hf and that Y is hosted by apatite for example (Table 8.4).
Table 8-4 : The common host minerals of some trace elements. Data from Rollingson(1993) and Green(1989) in Best and Christiansen(2001), Phillips and Myers(1987) and own observations. Element Compatibility Typical Host Minerals Zr, Hf Highly Incompatible Zircon Th Highly Incompatible Monazite La, Ce Highly Incompatible Allanite, Monazite Y Incompatible Apatite, Xenotime, Monazite, Garnet Nb Incompatible Sphene, Magnetite, Amphibole V Compatible Fe-Ti Oxides Yb Compatible Garnet, Xenotime Sc Compatible Garnet, Amphiboles, Pyroxene, Fe-Ti Oxides Co Highly Compatible Arsenopyrite, Cobaltite, Pyroxene, Spinel, Fe-Ti Oxides Ni Highly Compatible Gersdorffite, Millerite, Niccolite, Olivine
A chip sampling technique of core intersections was specifically implemented during the study in order to reduce the impact of hydraulic sorting on trace element concentrations. This technique should reduce the impact of small scale concentration of heavy minerals, such as lags, on the
96 Geochemistry composition of the sample. In addition comparisons of different lithofacies based on element ratios can be implemented to reduce the impact of mineral sorting (Taylor and McLennan, 1985).
In extreme situations, hydraulic sorting may separate components of the sediment so that the composition of a rock is no longer representative of the source rocks from which it was derived. Therefore, it is important to recognize when mineral fractionation by hydrodynamic sorting has taken place. One way of doing that is to compare trace element concentrations in different lithofacies, using bivariate plots (Fig. 8.6) and trace element ratios (Table 8.5). Scatter in the plot indicates that redistribution by chemical processes or fractionation by sorting has taken place. Strong linear correlations indicate that the sediment is well mixed and trace element concentrations are not influenced by sorting or element mobility.
A selection of the ratios which are least influenced by sorting and maturity are presented in figure 8-6. Correlations between compatible and incompatible elements (Th:Sc, La:Sc, Th:Y, Zr:Co, Th:Co) do exist, but hydraulic sorting is present to some degree. It is believed that overall trends in these ratios reflect variations in the source terrain, but some variations are attributed to hydraulic sorting.
Strong correlations between elements of similar compatibility are observed (Figs. 8.6 H,I and J). This indicates that these element ratios are not influenced by sorting and reflect changes in the source rock composition.
Transitional shelf facies sediments tend to plot on different trends to the rest of the sample set (Fig. 8.6). They are also generally more enriched in compatible elements. This indicates that the sorting processes which operated during the deposition of these rocks are different to those of the sandy quartzites, so the trace element ratios of sandy sediments and fine grained transitional shelf facies sediments cannot be compared.
8.4.3.2 Mineral Durability and Maturity Maturity is closely related to sorting. It involves the breakdown of less durable minerals by chemical weathering and by physical abrasion. The Ti:Zr ratio is a commonly recognized indicator of maturity (Taylor and McLennan, 1985). Titanium bearing phases have lower durability than zircon, which causes them to break down more rapidly than zircon during sediment transport, which
97 Geochemistry
A B C 12.0 20.0 45.0 18.0 40.0 10.0 16.0 35.0 14.0 8.0 30.0 12.0 25.0
Th 6.0
Th 10.0 La 8.0 20.0 4.0 6.0 15.0 4.0 10.0 2.0 2.0 5.0 0.0 0.0 0.0 0.0 5.0 10.0 15.0 20.0 0.0 5.0 10.0 15.0 20.0 0.0 10.0 20.0 30.0 Sc Sc Y
D E 20.0 F 500 18.0 3.0 450 16.0 2.5 400 14.0 350 12.0 2.0 300
Th 10.0 Zr
250 Yb 1.5 8.0 200 6.0 150 1.0 4.0 100 2.0 0.5 50 0.0 0 0.0 0.0 10.0 20.0 30.0 40.0 50.0 0.0 10.0 20.0 30.0 40.0 50.0 0.0 10.0 20.0 30.0 Co Co Y
G H I 60 90 50 80 45 50 70 40 35 40 60 30
50 25 Ni 30 Co V 40 20 20 30 15 10 20 10 5 10 0 0 0 0 5 10 15 20 0 5 10 15 20 0 10 20 30 Y Sc Sc
J K L 60 12.0 45 40 50 10.0 35 40 8.0 30 25
Ni 30 6.0 La Th 20 20 4.0 15 10 10 2.0 5 0 0.0 0 0 10 20 30 40 50 0.0 10.0 20.0 30.0 40.0 50.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Co La Yb Mature and supermature marine orthoquartzites Immature fluvial quartzite Immature wackestones Chemically mature fluvial quartzite Immature argillaceous quartzite and arkose
Figure 8-6 : Bivariate plots of some trace elements. Strong correlations indicate that rocks are well mixed and are not influenced by post depositional redistribution .
98 Geochemistry results in the depletion of Ti relative to Zr in sandy sediments. The titanium that is lost from the sandy sized sediment is incorporated into the fine fraction and results in an elevated Ti:Zr ratios in silty rocks. Unfortunately, the initial concentrations of zircon and Ti bearing phases in the source rocks also contributes to the Ti:Zr ratio measured in a rock (Taylor and Maclennan, 1985). The only manner to determine if the Ti:Zr ratio is the result of initial concentrations in the source rocks or the result of the maturity of a sediment is to contrast the petrographic characteristics which indicate maturity with the measured Zr:Ti ratio.
Plots of Zr versus Ti from rocks of the Witwatersrand Supergroup indicate that mature samples without matrix contain relatively high Zr:Ti ratios and less mature finer grained samples with matrix contain abundant fine grained rutile and sphene and consequently have low Zr:Ti ratios. Therefore it can be concluded that Zr:Ti ratio is dependent on the maturity of the rock. This is seen in figure 8-7 where mature quartzites are enriched in Zr and siltstones and shales are enriched in Ti. A number of samples do not follow this trend and this is attributed to inclusion of abundant zircon grains where sand grains are in hydraulic equilibrium with a specific zircon population. With the important conclusion that the Zr:Ti ratio depends on comparison of maturity of a lithofacies is established, a trace element ratio to Zr:Ti ratio will reveal how that ratio is influenced by maturity.
Figure 8-7: Ti vs. Zr bivariate plot reveals that Zr:Ti is dependent on maturity. Chemically mature quartzites rocks have higher Zr:Ti ratios than chemically immature siltstones and shales. See Figure 8-6 for legend
99 Geochemistry
The element ratios composed of incompatible and compatible elements (Th:Sc, La:Sc, Zr:Co and Th:Co) display a good correlation with Zr:Ti (Fig. 8.8). This then strongly suggest that that these trace element ratios are also strongly influenced by maturity. Recognising this is of critical importance because many techniques for determining provenance assume that specific element ratios reflects source composition and is not influenced by sorting and/or maturity.
Maturity has the greatest influence on trace elements ratios when the difference between the durability of the dominant host minerals is large. For example, Th:Zr ratios are not influenced by maturity as much as Zr:Sc because the minerals which host Zr (i.e. zircon) and Th (i.e. monazite) are much more durable than the minerals which host Sc (i.e. amphiboles and pyroxenes), for example. Therefore, normalising element ratios, which are strongly influenced by maturity, to the Zr:Ti ratio will reduce the effect on sorting, if the relative durability of the host minerals is similar to that of zircon and the mineral phase hosting Ti.
The normalisation function is (Th:Sc)N = ((Th:Sc)/(Zr:Ti)) x constant. The constant is included to bring the ratios to an appropriate range after normalization. The normalization constant is selected so that the mean remains the same. By applying this normalization factor it is seen that (Th:Sc)N, (La:Sc)N, (Zr:Co)N no longer show good correlation with Zr:Ti (Fig. 8.9). The ratios Th:Y, V:Y, Th:La, Y:Yb, Ni:Co also do not display any good correlations with Zr:Ti. This indicates that they are not influenced by the maturity of the rock, either because they are hosted by the same mineral or because the dominant host minerals display similar durability.
8.4.4 Determining Provenance
Ratios of highly incompatible and compatible elements are regarded as reliable indicators of provenance (McLennan et al, 1990, Fralick, 2003, McLennan et al 2003, Taylor and McLennan, 1985). The ratios of Th:Sc, La:Sc, and Nb:Y are particularly useful because these elements usually are not strongly partitioned into one specific mineral, but into several (Table 8.4). This makes them less susceptible to influence from hydraulic sorting during transportation (McLennan et al, 1990). Although pioneered on shales, these techniques have been successfully applied to quartzite samples (Bhatia and Crook 1986; Van Staden et al, 2006), but the possible effects of sorting and maturity must be well understood and always be kept in mind .
100 Geochemistry
7 25
6 20 5
4 15
Th:Sc 3
(Th:Sc)N 10 2 5 1
0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Zr:Ti Zr:Ti
30 100 90 25 80 20 70 60 15 50 La:Sc
10 (La:Sc)N 40 30 5 20 0 10 0.0 0.1 0.2 0.3 0.4 0.5 0 0.0 0.1 0.2 0.3 0.4 0.5 Zr:Ti Zr:Ti
35 140 30 120 25 100 20 80
Zr:Co 15 60 (Zr:Co)N 10 40 5 20 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Zr:Ti Zr:Ti
2.5 1.6 1.4 2 1.2 1 1.5 0.8 Ni:Co Y:Nb 1 0.6 0.4 0.5 0.2 0 0 0.0 0.2 0.4 0.0 0.1 0.2 0.3 0.4 0.5 Zr:Ti Zr:Ti
Figure 8-8 : Correlations between trace element ratios and Zr:Ti indicates influence from maturity of arenaceous rocks of the Witwatersrand succession. See Figure 8-6 for legend
101 Geochemistry
The following ratios (Th:Sc)N, (La:Sc)N, Nb:Y, (Zr:Co)N, (Th:Co)N, Th:Y, V:Y, Th:La, Y:Yb, Ni:Co, Zr:Hf (Table 8.5) show very little influence from sorting, but will only function as accurate indicator of provenance when the difference in compatibility between elements is high. Therefore Th:Sc, and Zr:Sc are most commonly used as provenance indicators (Table 8.5). However, in Witwatersrand quartzites, these ratios are strongly influenced by mineral sorting and therefore do not accurately record the characteristics of the source rocks. Instead, based on analyses shown above, the following ratios have been selected to most accurately represent variation in the composition of Witwatersrand source rocks, namely
(Th:Sc)N, (La:Sc)N, and Y:Nb (Table 8.5).
Following this decision it is seen that when (Th:Sc)N and (La:Sc)N ratios are plotted against each other on bivariate diagrams, all samples plot along a linear trend. this is the average crustal compositional trend. The transitional shelf facies wackestones and siltstones virtually plot along the average crustal composition trend, and are clustered around the composition of the Archean upper crust (AUC) (Fig. 8.9A). These rocks are least influenced by sorting, and indicate a clear Archean upper crustal affinity. Three samples which plot below the AUC would suggest that a mafic component is present in the rocks (Fig. 8.9A). One sample from the Johannesburg Subgroup demonstrates a more fractionated composition, nearer Post Archean upper crust (Fig. 8.5A).
The quartzite samples of the West Rand Group plot further along the compositional trend towards a more fractionated granitic composition (Fig. 8.9B). This is due to a reduction in Sc concentration which is removed along with matrix material during sorting. The implication of this is that that position of the sample on the plot is not indicative of the source rock, but the relative position of one sample to another is indicative of the relative importance of source composition. Three distinct groupings are present along the average crustal compositional (AUC) trend (Fig. 9.9B). The Hospital Hill Subgroup samples plot in all three of the groupings, this indicates mixing of sediment derived from fractionated and less fractionated source rocks. Therefore it can be concluded that Hospital Hill Subgroup source rocks were very heterogeneous. Most of the Hospital Hill Subgroup samples plot lower on the trend than the Government, Jeppestown and Johannesburg, samples which suggests that the source terrain was less fractionated (i.e. more primitive Archean granites and greenstones).
102 Geochemistry
Table 8-5 :Average trace element ratios in various lithofacies of the Witwatersrand succession.
Area Subgroup Zr:Ti (La:Sc)N (Th:Sc)N Zr:Co (Zr:Co)N Nb:Y La:Yb Y:V Y:Yb Zr:Nb Ni:Co Ni:V Th:Y Th:La Zr:Hf Zr:Y Y:Yb Zr:Nb Klerksdorp Hospital Hill 0.24 5.04 1.00 14.56 10.69 0.84 17.79 0.25 9.88 20.86 0.95 0.32 0.37 0.19 34.89 16.92 9.88 20.86 Klerksdorp Government 0.20 6.90 1.69 14.08 12.35 1.03 31.97 0.21 9.25 30.78 0.73 0.30 0.87 0.26 33.86 21.31 9.25 30.78 Klerksdorp Jeppestown 0.14 7.47 1.39 10.16 11.24 0.78 42.16 0.23 13.10 18.89 0.81 0.28 0.58 0.18 39.83 14.24 13.10 18.89 Klerksdorp Johannesburg 0.13 5.90 1.22 8.28 11.32 0.78 30.00 0.23 10.78 20.34 0.81 0.33 0.57 0.20 33.45 16.13 10.78 20.34 Klerksdorp Turffontein 0.19 7.17 1.96 15.73 11.69 0.80 21.37 0.33 11.00 15.60 1.00 0.25 0.49 0.27 34.60 12.41 11.00 15.60 Edenville Jeppestown 0.18 9.54 1.60 12.91 11.01 0.80 43.40 0.26 11.36 23.71 0.82 0.31 0.64 0.17 37.11 18.98 11.36 23.71 Edenville Johannesburg 0.17 7.21 1.10 11.99 11.31 0.55 29.65 0.42 10.85 23.00 0.93 0.32 0.42 0.15 32.73 12.98 10.85 23.00 Edenville Turffontein 0.13 8.48 1.93 8.10 12.56 0.59 38.33 0.26 12.33 17.90 0.80 0.29 0.69 0.22 44.55 10.63 12.33 17.90
103 Geochemistry
A Siltstones and Wackestones from the Klerksdorp area
Granite Hospital Hill
Government AUC
Jeppestown (Th:Sc)N
Johannesburg
Komatiite
(La:Sc)N
B Quartzites from the Klerksdorp area
Granite Hospital Hill
AUC Government (Th:Sc)N Jeppestown
Komatiite
(La:Sc)N
C Quartzites in the Klerksdorp area
Granite Johannesburg
AUC
(Th:Sc)N Turrfontein
Komatiite
(La:Sc)N
Figure 8-9 : (La:Sc)N vs. (Th:Sc)N plots of quartzites and Siltstones/Wackestones from the Klerksdorp Area. Archean Upper Crust(AUC) after Brown and Rushmer, 2006 , and rock types after Wronkiewicz and Condie, 1990.
104 Geochemistry
Unlike the Hospital Hill Subgroup, samples of the Government and Jeppestown Subgroups plot mostly in the most fractionated groups, but influence from mafic source areas is still present as is indicated by a few less fractionated samples (Fig. 8.9B).
The pattern observed in the Johannesburg Subgroup is very similar to that of the Government and Jeppestown Subgroups, but the subgroup contains a larger apparent component of fractionated source rocks (compare figure 8.9B with 8.9C). The Turffontein Subgroup contains only a tight grouping of the highest (La:Sc)N (Th:Sc)N ratios, which indicates a rather homogeneous source area that was fractionated (Fig. 8.9C). Important to note is the overall trend of increasingly fractionated and more homogeneous source terrain stratigraphically upwards in the Witwatersrand succession through time as the basin evolved.
The Nb:Y vs. Zr:Ti discrimination diagram originally developed by Winchester and Floyd, 1977 to discriminate highly altered and metamorphosed volcanic rocks is also suitable for discrimination of sandstones (Fralick, 2003; Van Staden et al., 2006). Nb:Y is a useful ratio to indicate the contribution of alkaline rocks to the source area, as well as an indicator of the overall degree of fractionation of a sample. If significant abundance of alkaline source rocks are present in the source rocks, elevated Nb:Y ratios are expected.
The samples of the Witwatersrand succession plot in a highly fractionated field, which suggests granitic source rocks only (Fig. 8.10). However, this is highly unlikely as has been shown by other provenance indicators. The loss of Ti in quartzites during sorting results in the excessively fractionated position. The relative position of samples is important as it will indicate changes in the source rocks.
The majority of the Hospital Hill Subgroup samples display a narrow range of Nb:Y ratios (0.7 – 0.9), in a strong linear trend (Figure 8.10A). These rocks contain variable Ti and Zr concentrations, but this probably reflects facies variation. The narrow range of Nb:Y ratios indicates that these rocks contained no material derived from alkaline rocks and probably represents mixing of mafic and felsic source rocks. The samples which fall outside of the narrow range of Nb:Y ratios (Fig. 8.10) are very mature and have very low trace element abundances which causes some variation in the ratio.
The Government Subgroup displays a much larger range of Nb:Y ratios (Fig. 8.10A) which suggests mixing of fractionated and alkaline source rocks from a heterogeneous source
105 Geochemistry terrain. Nb:Y ratios in the Jeppestown Subgroup are similar to the Government Subgroup but contain less alkaline source rocks (Fig. 8.10A)
The Johannesburg and Turffontein Subgroups show two distinct clusters. No strong linear trends are present in the plot (Fig. 8.10B). This indicates multiple source rocks are mixed. The clusters are tight (Fig. 8.10B) which indicates that the source composition did not change dramatically. Johannesburg Subgroup is a little more immature than the Turffontein Subgroup, as is indicated by lower Zr:Ti ratios, but the Nb:Y ratios are very similar which suggests that the source rocks were similar (Fig 8.10B).
In the Edenville area the Government Subgroup (Fig 8.10C) displays no linear trends in Nb:Y ratios. It is tightly clustered like the Johannesburg Subgroup in the Klerksdorp area and indicates mixing of diverse source rocks. The source rocks seem to be similar to those found in the Klerksdorp area, except with a smaller mafic component. The Jeppestown Subgroup in the Edenville area, is identical to the rocks in the Klerksdorp area. A relatively wide range in Zr:Ti ratios indicates that rocks of different maturity is present, but large variation in Nb:Y points to an alkaline source rock (Fig 8.10C). Only two samples from the Johannesburg Subgroup in the Edenville area were analysed. They show characteristics identical to the Johannesburg Subgroup in Klerksdorp (Fig 8.10C), but there are not enough data points for meaningful conclusions.
8.4.5 Stratigraphic Variation in Provenance Indicators
Chemostratigraphic plots of specific trace element ratios from the Klerksdorp area are presented in figures 8.11 A and B. When plotted against the stratigraphic profiles, the position where major changes in the trace element ratios occur, are much easier to recognise. The ratios (Th:Sc)N, (La:Sc)N and (Th:Co)N all show similar patterns, but it is important to verify trends with more than one ratio.
The Hospital Hill Subgroup displays highly variable Zr:Ti ratios (Fig 8.11A), much of this variability is facies dependant. The orthoquartzites such as Orange Grove Quartzite, Bullskop Maker and speckled marker contain high Zr:Ti ratios (>0.2). Petrographic study of these samples has revealed that they display characteristics of supermature sediments. Samples with Zr:Ti ratios less than 0.2 are finer grained and contain Ti-rich argillaceous material. It is
106 Geochemistry
A
B
C
Figure 8-10 : Zr:Ti vs Nb:Y and (Th:Sc)N vs Nb:Y plots. A – Samples collected from the West Rand Group in the Klerksdorp area, B - Samples collected from the Central Rand Group in the Klerksdorp area, C – Samples collected from the Edenville Area.
107 Geochemistry
Figure 8-11 : Trace element ratio chemostratigraphic profile of the Witwatersrand Supergroup in the Klerksdorp Area. See Figure3-3 for legend
108 Geochemistry
Figure 8-12 : Trace element ratio chemostratigraphic profile of the Witwatersrand Supergroup in the Klerksdorp Area. See Figure 8-6 for legend
109 Geochemistry believed that the relatively high Zr:Ti ratios (Fig 8.11) are the result of the maturity of the sediment, and do not indicate a zircon enriched source.
(Th:Sc)N and (La:Sc)N, progressively decrease up section in the Hospital Hill Subgroup (Fig 8.11A), indicating incorporation of progressively less fractionated source material. Low (Th:Co)N and high Ni:Co ratios for the Parktown and Brixton Formations (Fig 8.12) supports a less fractionated source, and the highest mafic input appears to take place during Brixton Formation times. In addition, the Nb:Y ratio increases progressively over this interval (Fig 8.11B), which contradicts (Th:Sc)N, so it is likely that the elevated Nb:Y ratio reflects an additional source.
A shift to higher Th:Sc and La:Sc ratios across the Hospital Hill-Government sequence boundary (Fig 8.11) indicates a shift to a more fractionated source terrain, as manifested by high (Th:Sc)N and (La:Sc)N ratios. The fractionated source persists over the stratigraphic interval of Government and Jeppestown Subgroups. It is accompanied by less variation in the Zr:Ti ratio than was present in the Hospital Hill Subgroup (Fig 8.11).
Across the Johannesburg-Jeppestown Sequence boundary, a change to a more mafic source composition with more alkaline rocks is recorded by the lower Th:Sc ratio. This is followed by a sequence of progressively increasing felsic component as indicated by an increase in Zr:Ti ratios (Fig 8.11).
8.4.6 Rare Earth Elements as Provenance Discriminant
The Chondrite-normalised REE patterns (Fig. 8.11) display enrichment in light REE but Eu anomalies could not be established because some REE data is not available. The LREE patterns show similar concentration to average Archean upper crust, and HREE patterns show variable depletion relative to AUC, depending on the sample. HREEs must be contained in the fine grained matrix of the sediment because immature rocks with significant proportions of matrix like the transitional shelf facies rocks contain much higher HREE abundances than the mature orthoquartzites (Fig. 8.13). Sorting processes appear to be the primary factor controlling REE patterns, but the continental signature persists. The only notable difference is relative enrichment of Yb and Lu in Hospital Hill Subgroup, indicating less fractionated source rocks.
Geochemistry
100 AUC*
Mean HH
Mean Gov 10
Mean Jepp Rock : Chondrite Rock
Mean JHB
Mean Turr 1 La Ce Nd Sm Eu Tb Yb Lu
Figure 8-12 : HREE abundance is below average Archean Upper Crust due to fractionation of REE’s into finer grained rocks.
100
AUC* Mean A Mean B 10 Mean C Mean D
Rock : Chondrite Rock Mean E
1 La Ce Nd Sm Eu Tb Yb Lu
Figure 8-13 : Facies controlled distribution of REE. Finer grained siltstone facies display higher REE concentrations than mature shallow marine quartzite facies. Siltstone facies REE patterns approach that of AUC.
111 Geochemistry
8.4.7 Tectonic Discrimination
Bhatia and Crook (1986) used La:Sc and Ti:Zr ratios in greywackes to discriminate the tectonic setting of Palaeozoic sedimentary basins. The specific fields defined Bhatia and Crook (1986) may not be valid for Archean basins due to differences in the Archean tectonics and source rock composition in general. However, the characteristics of the rocks observed in these basins could be are applicable. According to Bhatia and Crook (1986), passive margin basins typically display variable La:Sc ratios and low Ti:Zr ratios which reflects the diverse possibilities of source rocks and the mature nature of the sediments observed in such basins (Fig. 8.14). Volcanic arcs typically display low La:Sc ratios reflecting basaltic and intermediate volcanism, and the Zr:Ti ratio varies depending weather continental crust is involved or not (Fig. 8.14). Active continental margins have somewhat higher La:Sc ratios than arcs and Zr:Ti ratios higher than rifted margins, which reflects continental influence associated with these settings (Fig. 8.14).
On such a La:Sc vs Zr:Ti discrimination diagram, the Government and Jeppestown Subgroups show a large amount of variability and plot across passive margin, active continental margin and volcanic arc fields. The Turffontein Subgroup and Hospital Hill Subgroup on the other hand display low Zr:Ti ratios associated with mature sediments of passive margins (Fig.8.14).
40
Hospital Hill 35 Government Jeppestown 30 Johannesburg Turffontein All 25 Volcanic Arc
20 Ti:Zr
15 Active Continental Margin 10
5 Passive Rifted Margin
0 0 5 10 15 20 25 30 La:Sc
Figure 8-14 : Ti:Zr versus La:Sc ratios for tectonic discrimination, after Bhatia and Crook(1986).
112
Chapter 9 : Summary and Conclusion
9.1 Characteristics of the Witwatersrand Supergroup Source Rocks.
The Hospital Hill Subgroup is a sequence of mature sediments deposited in shallow marine shelf and outer shelf environments. The rocks show physical and geochemical characteristics of supermature sediments, which suggests that they undergone a long duration of transportation and reworking prior to deposition. The source area was composed of mafic and felsic rocks (Table 9.1) as is indicated by the wide range of Th:Sc, La:Sc and linear Nb:Y vs Zr:Ti plots. The abundance of both zircon and chromite minerals is further evidence of mafic and felsic rocks in the source terrain. More fractionated source rocks dominate the Orange Grove Formation, but the Brixton and Bonanza Formations are derived from less fractionated source rocks. Chromite composition is consistent with chromites derived from a primitive igneous source.
Kositcin and Krapez (2004) found zircon populations with ages older than 3100ma for the Orange Grove Formation, with modes centered around 3200ma and also clusters of ages older than 3400ma. Therefore, according to them, the source terrain thus consisted of basement granite-greenstone terrain and of ancient granitoid gneiss complexes sutured by 3.2 Ga TTG granite. The proportion of ultramafic rocks in the source area increases from the Orange Grove Formation to the Brixton Formation (Table 9.1). These findings are similar to those of Wronkiewicz and Condie (1987), who found the most abundant source rock in the Hospital Hill Subgroup is tonalite.
At the base of the Promise Formation a basin wide unconformity is present (Fig. 2.2). The characteristics of the sediments above this unconformity are distinctly different to those observed in the Hospital Hill Subgroup. The rocks of the Government and Jeppestown Subgroups are immature quartzites, deposited in fluvial environments but shallow marine and Summary and Conclusion shelf facies arenaceous and wackestone siltstones are also common. The framework components contain more feldspars, chlorite and lithoclasts, and the rounding and sorting characteristics are more moderate relative to that of the Hospital Hill Subgroup. Lower Zr:Ti ratios in these rocks confirms that the Government Subgroup rocks are only moderately mature and probably contained abundant illmenite. High Th:Sc and La:Sc ratios, and the low abundance of chromite suggests that the source rocks are more fractionated than those of the Hospital Hill Subgroup. However the wide range in Nb:Y ratios in the Government Subgroup, suggests mixing of heterogeneous source rocks (Table 9.1). Nb:Y ratios for the Jeppestown Subgroup indicates a more homogeneous source. Similar conclusions were reached by Wronkiewicz and Condie (1987), who suggest that the abundance of tonalitic source rocks decreases while the granitic, basaltic and komatiitic components increases over this interval. The presence of lithoclasts, chlorite, low Zi:Ti ratios and the presence of epidote and tourmaline suggests that a metamorphic source rock may have been present in the source area.
Kositcin and Krapez (2004), found zircon with ages in the range of 3090-3060 ma for the Government and Jeppestown Subgroups. During this time period, widespread emplacement of potassic granitoid batholiths, granitic plutonism in the Madibe and Murchison greenstone belts and also Dominion Group volcanism took place (Poujol, et al., 2003). These rocks are responsible for the thickening and stabilisation of the Kaapvaal Craton and are associated with the initiation of early subduction tectonics and volcanic arc formation according to Poujol et al. (2003). These rocks could have been important components in the source areas during the deposition of the Government and Jeppestown Subgroups.
Kositcin and Krapez (2006) indicates that a provenance shift occurs at the base of the Bonanza Formation. The geochemical and petrographic data presented in this study indicates the change occurs at the base of the Promise Formation. It appears that the difference in sampling intervals and sampling localities may be responsible for the inconsistent interpretation in timing of the provenance shift.
Another basin wide unconformity is present at the base of the Johannesburg Subgroup, and marks another major provenance change. The Johannesburg Subgroup is composed of mostly coarse grained immature quartzites deposited in a fluvial depositional setting. The rocks are moderately mature, much like the Government and Jeppestown Subgroups. Petrographic analyses reveal that feldspar and chlorite is present but with only minor lithoclasts. Moderate Summary and Conclusion
Zr:Ti ratios confirm the these rocks are chemically more mature than the government and Jeppestown Subgroups. A shift to moderate Th:Sc and La:Sc suggest a less fractionated mix of source rocks (Table 9.1). The disappearance of the lithoclasts indicates that the metamorphic source rocks no longer supplied material to the basin. A small increase in the chromite to zircon ratio also suggests that some unfractionated source rocks were present. The narrow range in Th:Sc, La:Sc, Nb:Y ratios suggests that a homogeneous source area is present, but this is contradicted by the highly variable zircon ages measured by Kositcin and Krapez (2004), so the narrow spread might indicate that the rocks are very well mixed. The zircon population for the Johannesburg contains zircons in the range of 3090-3060ma which were found in the Government and Jeppestown Groups, as well as younger zircons in the range of about 3000-2870ma (Kositcin and Krapez, 2004). The source terrain of the Johannesburg Subgroup probably consisted of a mixture of the granitoid batholiths from which the Government and Jeppestown Subgroups are a derived as well as some intermediate igneous material with ages of 3000-2870 ma.
The Turffontein Subgroup rocks are very coarse and chemically mature, but they display poor to moderate sorting and rounding. The rocks were deposited in a fluvial environment but marine quartzites are not uncommon. It is believed that these rocks were transported in a high energy environment, but the duration of transportation was short. This allows for effective winnowing but insufficient time for physically mature rocks with well-rounded grains to develop, explaining the mature chemical composition but immature physical composition. The source rocks of the Turffontein Subgroup were probably the same as the Johannesburg Subgroup with the higher energy mode of transportation responsible for the observed increase in Zr:Ti ratio. It would also explain the scarcity of feldspars and chlorite in the Turffontein Subgroup. Th:Sc and Nb:Y ratios suggest highly fractionated source rocks, but care must be taken because the mature nature and coarse grainsize of these rocks make trace element analyses unreliable (Table 9.1). The zircon population indicates the presence of 3090-3060ma granite batholiths, as well as 3000-2870 Ma syntectonic granite plutons, as well as Ancient granitoid gneiss in the source area.
115 Summary and Conclusion
Table 9-1 : Summary of the characteristics of Stratigraphic units.
Palaeocurrent Geochemical Zircon Ages Direction Sedimentary Indication of Chromite:Zircon Maturity Framework Minerals (Kositcin and (Beukes, Facies Source Ratio Krapez, 2006) 1995, Nhleko, Composition. 2003) Orange Grove ~3480-3430ma Longshore Marine Supermature Felsic & Mafic Quartz and Feldspar Low Formation ~3200-3100ma Currents Brixton and Longshore Shallow Marine Supermature Mafic > Felsic Quartz and Feldspar High ~3090-3060ma Bonanza FM Currents Government Felsic Quartz, Feldspar, Marine and Fluvial Moderate Metamorphic. Low ~3090-3060ma North West Subgroup Chlorite, Lithoclasts Heterogeneous Jeppestown Felsic Quartz, Feldspar, Marine and Fluvial Moderate Metamorphic Low ~3090-3060ma North West Subgroup Chlorite, Lithoclasts Homogeneous Johannesburg Quartz, Feldspar, ~3090-3060ma Fluvial Moderate Intermediate Low Subgroup Homogeneous Lithoclasts rare ~3000-2870ma Turffontein ~3480-3400ma Subgroup Chemically Mature, ~3200-3100ma Fluvial Felsic Quartz, Chlorite Moderate Physically Immature ~3090-3060ma ~3000-2870ma
116 Summary and Conclusion
9.2 Evolution of the Greater Witwatersrand Basin
Basin reconstruction of the Witwatersrand and Mozaan basins by Beukes (1995), Nhleko (2003) and Nelson and Beukes (1995), reveals that the Hospital Hill Subgroup was deposited in a passive margin setting in a large costal embayment on the south eastern margin of the Kaapvaal Craton (Fig 9.1). A large arcuate shoreline existed to the north and west and the basin was open to the southeast. During Hospital Hill Subgroup times, sediment is believed to be transported by longshore currents within the basin which results in the supermature sediments which were observed. The mature characteristics of the sediment indicates low rates of sediment input and stable tectonic conditions. Erosion of the rocks which constitute the Archean craton, mainly tonalite and greenstone belts, adequately accounts for the provenance indicators that were observed. Progressive unroofing of greenstone belt complexes explains the shift to less fractionated source rocks in the upper part of the Hospital Hill Subgroup.
At the base of the Promise Formation, a major event takes place, which completely changed the characteristics of the Witwatersrand Basin and the sedimentary fill (Fig 9.1). A change to a regime of immature sedimentation associated with evidence of volcanism only in the Jeppestown Subgroup indicates a period of active tectonism. Major changes in basin geometry, palaeocurrent directions, are observed by Beukes (1995), Nhleko (2003), Beukes and Nelson (1995), Burke et al. (1986), Catuneanu (2001). This study has revealed that source rocks rapidly changed from tonalitic basement rocks to mostly 3090-3060ma potassic granitoids, and metamorphic rocks.
It is believed that collision of the Witwatersrand and Kimberley blocks on the western margin of the Kaapvaal Craton occurred, triggered these changes. Many authors have recognised the similarity of the Witwatersrand Supergroup to classic foreland basin sequences (Winter, 1986, Stanistreet and McCarthy, 1991, Coward et al., 1995, Catuneanu, 2001). Lithospheric loading resulting from crustal thickening associated with emplacement of the Kimberley block onto the Western margin of the Witwatersrand block, in a Himalayan style collision, or from thrust stacking of the Witwatersrand block rocks on its western boundary in an Andean type margin. Either of these processes are plausible so peripheral foreland basin or a retroarc foreland basin are possible depositional settings for the Witwatersrand Supergroup (Fig 9.1).
117 Summary and Conclusion
It is suggested that material derived from 3090-3060ma potassic granitoids emplaced along the western margin of the Kaapvaal Craton, was supplied to the basin by an axial river system running in a north south direction along the foot of the orogen. The river entered the basin in the modern day Klerksdorp area, Figure 9.1B. This model explains the change in source composition, the shift in palaeocurrent directions to the northwest (Nhleko, 2003), and the immature sedimentation in the Government and Johannesburg Subgroups, which resulted from rapid upliftment of the source terrain. Burial metamorphism is a possible explanation for the low Zr:Ti ratios and some lithoclasts which are present. Such material has been reported in the Madibe greenstone belt (Hirner et al, 2003), which is likely related to this event. Beukes and Nelson (1995) used the average length of eustatic cycles to suggest an age of around 2950Ma for this collision event.
The shift towards a dominantly braided fluvial depositional environment in the Johannesburg Subgroup was the result of closure of the basin and marks the progression into an overfilled stage of the basin evolution. The composition of the source rocks is still fairly similar to those of the Government and Jeppestown Subgroups, but contains intermediate material with ages of 3000-2870ma. These are the youngest zircons ages and are probably sourced from syntectonic granitoid plutons (Fig 9.1C).
The Turffontein Subgroup formed during the final stage of the Witwatersrand Basin. Tectonic activity had slowed and the orogen was largely eroded away. In addition to the material derived from syntectonic intrusions and the granitoid batholiths, ancient granitoid gneiss became a significant part of the source terrain (Fig 9.1D). This indicates that the orogen was largely removed by erosion and basement material was exposed. It is also possible that this material was sourced from the eastern portion of the Kaapvaal Craton where these rocks are common.
The Amalia, Kraaipan and Madibe greenstone belts and Colesberg Magnetic Anomaly are probably the only remaining remnants of this orogeny today. The current extent of the Witwatersrand Basin is preserved by Platberg age extensional faulting and only a small portion of the original basin remains preserved.
118 Summary and Conclusion
Witwatersrand Block Kimberley Block Witwatersrand/Pongola Basin 3060-3090ma Granitoid Batholiths 3000-2870ma Collision Related Granitoid Plutons
Figure 9-1 :Summary of the Evolution of the Witwatersrand Basin
119 Summary and Conclusion
9.3 Conclusion
The effects of sorting have obscured much of the provenance history of the Witwatersrand Supergroup. However it is possible to establish large scale differences in the source terrain. A major provenance shift occurred at the boundary of the Hospital Hill and Jeppestown Subgroups. This is the record of a major tectonic event during development of the basin. The remainder of the sequence records erosion of syntectonic plutons and finally erosion of the craton basement near the end of the tectonic event. This study has provided new support for a foreland basin origin of the Witwatersrand Supergroup, proposed by Beukes (1995), Beukes and Nelson (1995) and Nhleko (2003), resulting from orogenic collision of the Witwatersrand and Kimberley blocks along the western margin of the Witwatersrand block.
120 References
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127 Appendices
Appendix I : XRF Major Element Geochemistry
Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 S Total (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) TF1 OGL1 88.4 0.26 7.3 1.70 0.02 0.80 0.22 0.63 1.57 0.02 0.57 101.5 TF1 OGM1 71.5 0.49 12.9 6.59 0.06 2.99 0.75 0.22 3.00 0.02 1.22 99.7 TF1 OGM2 98.6 0.06 2.1 0.88 0.01 0.21 0.20 0.16 0.63 0.02 0.02 102.9 TF1 OGU1 89.7 0.16 6.6 0.99 0.02 0.83 0.51 0.03 2.09 0.01 * 100.9 TF1 OGU2 91.5 0.16 5.1 1.49 0.02 1.01 0.39 1.45 1.57 0.03 0.32 103.0 TF1 OGU3 93.8 0.15 4.5 0.80 0.02 0.57 0.34 0.19 1.58 0.02 0.18 102.1 TF1 BUL1 92.0 0.02 2.6 3.86 0.03 1.36 0.43 0.41 0.79 0.02 0.27 101.8 TF1 BUL2 92.7 0.02 4.2 1.43 0.01 0.44 0.20 0.20 3.17 0.02 0.00 102.4 TF1 BUL3 90.1 0.06 6.0 0.96 0.01 0.26 0.20 0.22 3.66 0.02 0.26 101.7 TF1 RIP 1 98.7 0.04 1.6 1.09 0.01 0.42 0.24 0.11 0.46 0.01 0.10 102.8 TF1 RIP 2 68.5 0.60 14.9 5.59 0.04 4.27 0.52 1.26 3.01 0.04 0.33 99.0 TF1 SPEC 1 90.8 0.05 4.4 0.99 0.02 0.73 0.84 0.57 1.34 0.02 0.06 99.8 TF1 SPEC 2 67.8 0.56 13.0 8.87 0.06 4.22 0.62 0.98 2.59 0.04 0.41 99.2 TF1 VER 1 99.0 0.05 2.1 0.60 0.01 0.25 0.19 0.07 0.52 0.02 0.13 102.9 TF1 VER 2 94.7 0.19 4.3 1.32 0.02 0.71 0.35 0.88 0.40 0.02 0.09 103.0 TF1 WIT 1 89.3 0.26 7.4 1.70 0.02 0.79 0.23 0.17 1.54 0.02 0.51 101.9 TF1 WIT 2 95.0 0.16 3.6 2.27 0.07 0.52 0.20 0.62 0.66 0.02 0.07 103.1 TF1 RV 93.1 0.16 4.1 1.92 0.05 1.01 0.55 0.74 1.00 0.06 0.05 102.7 TF1 TOP 91.2 0.25 4.8 2.39 0.03 1.16 0.28 0.12 1.20 0.02 0.06 101.5 BAB1 1 88.6 0.15 8.9 0.67 0.01 0.28 0.33 0.07 2.70 0.02 0.05 101.7
128 Appendices
Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 S Total (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) BAB1 2 90.4 0.34 7.1 1.18 0.01 0.21 0.29 0.47 2.06 0.03 0.89 103.0 BAB1 3 83.3 0.33 11.1 1.56 0.02 0.83 0.45 0.32 3.06 0.02 0.09 101.1 BAB1 4 88.0 0.30 8.6 1.17 0.01 0.42 0.24 0.14 2.52 0.02 0.26 101.7 BAB1 5 92.2 0.12 5.7 0.79 0.01 0.43 0.50 0.29 1.83 0.02 0.06 102.0 BAB1 6 92.6 0.36 4.2 1.89 0.01 0.39 0.21 0.38 1.10 * 1.96 103.2 BAB1 6B 70.9 0.55 13.3 8.58 0.06 2.89 0.35 0.15 2.25 0.04 0.52 99.6 BAB1 7 87.6 0.24 7.0 2.82 0.02 0.87 0.50 0.28 1.54 0.02 1.21 102.1 BAB1 9 89.8 0.19 7.1 1.57 0.02 0.81 0.28 0.17 1.71 0.02 0.33 102.1 JY8 MAR 80.1 0.29 10.7 2.60 0.04 1.21 1.37 1.89 1.92 0.04 0.29 100.4 JY8 1 54.7 0.61 12.2 12.17 0.15 8.24 8.57 1.97 0.65 0.08 0.19 99.6 JY8 2 91.7 0.11 5.9 1.23 0.02 0.57 0.43 0.49 1.43 0.02 0.10 102.0 JY8 3 95.4 0.08 3.8 0.91 0.01 0.39 0.47 0.21 1.00 0.02 0.18 102.4 JY8 4 80.7 0.35 11.1 2.42 0.03 1.33 0.53 0.57 2.60 0.02 0.33 99.9 JY8 5 61.3 0.64 14.9 13.24 0.08 4.55 1.18 0.85 1.63 0.07 0.31 98.7 JY8 6 89.4 0.21 6.8 1.70 0.02 0.71 0.70 1.20 1.28 0.02 0.25 102.3 JY8 7 83.9 0.28 9.6 2.60 0.03 1.03 0.38 1.00 2.05 0.02 0.81 101.7 JY8 8 93.4 0.20 5.3 1.22 0.01 0.43 0.20 0.26 1.23 0.02 0.12 102.4 JY8 9 95.0 0.08 4.3 1.11 0.01 0.39 0.20 0.35 1.02 0.02 0.29 102.7 JY8 10 95.2 0.06 2.3 0.85 0.01 0.30 0.20 0.74 0.53 0.02 0.04 100.3 JY8 11 89.3 0.15 7.0 1.34 0.02 0.85 0.41 1.63 1.18 0.02 0.06 102.0 JY8 12 93.6 0.06 3.0 0.77 0.01 0.34 0.31 1.14 0.69 0.02 0.26 100.3 JY8 13 83.6 0.27 9.1 2.26 0.03 1.02 0.85 1.66 1.87 0.03 0.41 101.0
129 Appendices
Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 S Total (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) JY8 14 85.6 0.23 7.9 1.82 0.02 0.82 0.97 1.93 1.40 0.02 0.28 101.0 JY8 15 79.5 0.28 10.7 2.47 0.03 1.24 1.69 3.31 1.69 0.04 0.18 101.2 JY8 16 86.6 0.16 7.7 1.50 0.02 0.74 1.03 2.01 1.34 0.02 0.14 101.3 JY8 17 88.2 0.15 6.5 1.59 0.02 0.77 1.04 1.52 0.99 0.02 0.24 101.1 JY8 18 66.8 0.65 13.4 6.35 0.06 5.89 1.80 2.53 1.63 0.06 0.22 99.4 DK12 1 93.8 0.09 5.6 0.56 0.01 0.17 0.20 0.03 1.64 0.02 0.32 102.4 DK12 2 94.7 0.08 5.6 0.33 0.01 0.14 0.20 0.04 1.51 0.02 0.03 102.7 DK12 3 97.7 0.06 2.6 0.45 0.01 0.30 0.25 0.51 0.43 0.02 0.02 102.3 DK12 4 93.5 0.14 6.2 0.52 0.01 0.17 0.20 0.24 0.56 0.02 0.19 101.8 DK12 5A 94.3 0.10 5.6 0.50 0.01 0.18 0.21 0.13 0.98 0.02 0.05 102.1 DK12 6 94.9 0.06 5.2 0.43 0.01 0.22 0.21 0.12 1.38 0.02 0.01 102.5 DK12 6A 93.5 0.10 5.3 0.77 0.01 0.39 0.20 0.14 1.32 0.02 0.02 101.8 DK12 7 90.5 0.22 8.4 0.82 0.01 0.24 0.20 0.18 0.87 0.02 0.32 101.8 DK12 8 93.1 0.13 4.9 0.68 0.01 0.31 0.21 0.21 0.15 0.02 0.27 100.0 DK12 9 73.9 0.36 11.9 3.60 0.05 3.53 2.22 2.99 1.36 0.04 0.17 100.2 DK12 10 76.2 0.40 13.0 3.29 0.04 1.43 1.08 2.47 2.14 0.05 0.15 100.3 DK12 11 84.9 0.18 10.6 1.80 0.01 0.58 0.21 0.36 1.90 0.02 0.08 100.6 DK12 12 87.1 0.20 9.4 1.52 0.01 0.52 0.21 0.29 2.15 0.03 0.30 101.8 DK12 13 79.0 0.27 11.3 2.50 0.06 1.62 1.56 1.94 2.28 0.04 0.17 100.7 DK12 13B 82.1 0.33 8.9 2.34 0.03 0.92 0.26 2.69 2.06 0.04 0.77 100.4 DK12 14 83.6 0.28 10.6 2.54 0.03 1.01 0.31 0.47 2.16 0.02 0.23 101.2 DK12 15 82.0 0.30 10.8 3.03 0.05 1.42 0.54 0.77 2.27 0.03 0.15 101.4
130 Appendices
Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 S Total (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) DK12 16 85.7 0.26 8.4 2.60 0.03 1.28 0.30 0.73 1.72 0.03 0.31 101.3 DK12 17 87.1 0.21 7.7 2.31 0.03 1.08 0.27 0.37 1.64 0.03 0.14 100.8 DK12 18 86.3 0.30 8.5 2.81 0.03 0.91 0.31 0.44 1.64 0.03 0.56 101.9 DK12 19 90.8 0.13 3.3 1.40 0.03 0.32 0.24 2.39 1.25 0.03 0.79 100.7 DK12 20 89.2 0.24 7.7 2.15 0.03 0.73 0.23 0.28 1.68 0.02 0.09 102.3 AM1 1 89.5 0.23 6.7 1.18 0.02 0.67 0.70 0.94 1.76 0.03 0.02 101.8 AM1 2 73.1 0.52 14.0 5.14 0.04 2.04 0.83 3.58 2.23 0.03 0.16 101.6 AM1 3 76.4 0.47 12.0 4.49 0.03 1.60 0.84 3.30 1.46 0.02 0.07 100.7 AM1 4 72.0 0.56 14.8 4.81 0.03 1.85 1.09 1.86 2.86 0.03 0.13 100.0 AM1 5 80.2 0.55 10.4 3.37 0.03 1.47 0.67 0.70 2.62 0.02 0.76 100.8 AM1 6 85.9 0.17 6.4 1.05 0.02 0.48 0.81 1.89 1.49 0.02 0.28 98.5 AM1 7 79.7 0.45 9.3 2.34 0.03 1.05 1.12 4.01 2.02 0.03 0.71 100.8 AM1 8 96.3 0.07 3.4 0.75 0.01 0.30 0.28 0.37 0.86 0.02 0.09 102.4 AM1 9 81.8 0.33 10.3 1.72 0.02 1.15 1.16 1.75 2.59 0.02 * 100.8 AM1 10 65.7 0.71 15.1 6.67 0.08 3.73 3.54 2.68 1.69 0.07 0.01 99.9 AM1 11 95.7 0.05 2.8 0.68 0.01 0.29 0.28 0.79 1.39 0.02 * 102.1 AM1 12 92.9 0.08 5.3 0.72 0.01 0.31 0.35 0.78 1.61 0.03 0.02 102.1 AM1 13 82.4 0.24 8.3 1.44 0.02 0.65 1.15 3.76 2.31 0.03 0.12 100.4 AM1 14 88.9 0.17 8.1 1.31 0.01 0.53 0.32 0.21 1.90 0.02 * 101.5 AM1 15 90.4 0.18 6.9 1.31 0.02 0.47 0.30 0.34 1.66 0.02 * 101.6 AM1 16 96.7 0.06 2.7 0.56 0.02 0.20 1.87 0.28 0.27 0.02 * 102.6 AM1 17 95.4 0.05 2.7 0.71 0.01 0.25 0.69 0.41 1.38 0.01 * 101.6
131 Appendices
Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 S Total (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) AM1 18 93.1 0.11 4.7 1.15 0.02 0.35 1.09 0.53 0.80 0.02 * 101.9 AM1 19 75.2 0.44 12.3 3.39 0.04 1.76 1.68 3.20 1.80 0.05 * 99.9 AM1 20 88.4 0.11 6.6 1.11 0.02 0.31 0.72 1.55 3.17 0.02 * 102.0 AM1 21 67.5 0.71 14.8 5.71 0.06 3.31 1.70 2.79 2.12 0.02 0.02 98.8 AM1 22 71.8 0.61 11.8 4.79 0.05 3.84 1.37 4.42 1.78 0.05 0.26 100.8 AM1 23 81.5 0.42 9.0 2.90 0.04 1.54 1.65 2.34 1.38 0.03 0.34 101.1 AM1 25 90.9 0.26 6.1 2.01 0.02 0.71 0.24 1.20 1.46 0.03 * 103.0 AM1 26 86.2 0.21 9.9 1.60 0.01 0.43 0.21 0.97 1.98 0.03 * 101.6 AM1 27 94.3 0.09 4.5 1.21 0.01 0.35 0.22 0.15 0.70 0.02 * 101.6 BAB1 Corr 74.0 0.52 14.6 3.82 0.04 1.84 0.66 0.29 3.01 0.06 0.40 99.3 AM1 Lag 69.1 0.38 10.9 8.52 0.08 4.19 3.40 1.29 1.29 0.05 0.17 99.4 BAB1 Prom 79.8 0.28 8.6 4.50 0.04 1.42 0.85 2.96 1.78 0.03 0.23 100.5
132 Appendices
Appendix II : XRF Trace Element Geochemistry
Sample Ba Nb Pb Rb Sr Th Y Zr Ce Co Cr Cu Ga Hf La S Zn Ni V (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) TF1 OGL1 4.4 2.5 6.1 10.2 8.3 4.5 6.1 44.3 26 1.6 343.2 * * 9 11.8 25.5 6.9 3.4 17.6 TF1 OGM1 481.9 11.3 16.9 100.3 21.6 5 12.6 261.6 48.9 10.9 382.8 * 16.8 14 18.3 765.1 50.2 24.2 59.4 TF1 OGM2 45.4 2.3 11.8 22.6 11.9 5 4.9 74.4 28.3 2.7 525.9 * 1 11 12.2 40 9.1 4.7 23.8 TF1 OGU1 350.9 5.9 17.4 76.5 19.5 2.9 7.5 123.6 42.2 3.4 349.4 * 6.4 9.1 8.2 5.7 13.4 5.9 25.6 TF1 OGU2 245 6.4 15.4 57.8 15.8 1.9 8.3 104.4 34.3 6.5 305.7 2.8 5.1 15.4 10.1 256.2 14 8.4 24.4 TF1 OGU3 244.3 5.7 11.9 63.1 15.1 3.9 6.7 102.6 36.9 4.4 309.2 0.2 4.2 14.3 9.3 202.3 19.7 5.7 21.1 TF1 BUL1 142.4 3.1 11.9 23.1 22.3 2.3 1.8 34.5 26.9 0 267.9 2.5 2 15.2 11.8 205.9 9 11.5 16.8 TF1 BUL2 417.1 3 8.4 74.5 40.8 3.1 49.4 39.9 0.4 277.8 * 2.7 7.2 8.1 11.5 5.5 4.9 14.7 TF1 BUL3 377.8 4.2 11.2 112.2 51.9 0.7 3.5 90.9 43.1 1.1 288.6 * 3.8 7.9 11.1 235.9 5 4.3 16.6 TF1 RIP 1 48.9 2.8 13.5 24.4 8 3.2 3.5 43.5 26.7 1.5 341.3 * 0.9 11.9 5.6 108.9 5.2 5.4 17.6 TF1 RIP 2 364.2 10.7 14.6 123.9 66.4 1.4 13.6 162.2 43.4 8.3 642.1 * 18.1 11.2 14.3 187.1 84.6 29.5 75.7 TF1 SPEC 1 282.3 4 5.7 43.3 60.3 3.8 4.8 70.5 35.6 0.9 228.5 * 4.1 6.3 13.8 61.2 8.3 4.5 17.1 TF1 VER 1 28.9 3.5 15.2 18.6 10.1 3.6 3.9 55.5 27.6 2.3 429.2 * 0.9 5.9 7.7 85.7 4.6 4.7 17.6 TF1 VER 2 81.1 5.4 10.1 17.5 26.4 4.8 6.9 83.6 29.6 2.9 434.4 * 3.9 6.7 7.8 68.8 6.8 9.6 28.9 TF1 WIT 1 249.9 5.8 15.3 56.3 30.1 1.1 6.7 232.2 37.1 5.5 1094.9 * 5.8 7.2 15.6 322.4 14.6 10.4 37.3 TF1 WIT 2 72.9 5 13.2 30 10.6 1.4 6.5 115.1 30.3 2.4 468.5 * 2.8 6.5 12.9 59.1 9.4 10.9 28.3 TF1 RV 135.1 5 11.1 41.4 9.8 1 9.1 85.5 31 3.4 407.9 * 4.1 7.4 7.1 74.2 13.7 9 25.5 TF1 TOP 215.2 6 38.1 50 10.9 5.3 7.5 207.1 37.9 11.5 689.2 * 4.2 9.5 12 86 14.3 14.8 30.9 BAB1 1 443.9 5 11.1 100.9 20.9 2.4 5.5 119 46.5 3.5 337.2 * 9.5 7.3 14.3 48 48.7 4.7 23.7 BAB1 2 289.8 8.2 59.8 70 18.5 22.9 7.4 425 45.8 7.6 508.3 * 5.5 10.9 16.3 630.3 44.2 7.6 33 BAB1 3 512.4 8.4 16.4 102.6 30.3 0.4 8.7 166.2 53 6.6 364.4 * 13.4 10.3 15.6 79.7 54.5 10.7 41.1 BAB1 4 456.3 10.4 20.5 88.6 27.7 8.2 5.4 221.8 51.7 6.3 350 * 9 9.6 18.1 270.6 61.9 7.9 28.1 BAB1 5 356.6 5.6 16.1 70.8 15.9 2.7 4.4 79.4 40.4 8 231.9 * 5.3 7.5 10.2 80 163.1 6.9 20 BAB1 6 180.5 6.3 109.5 41.2 16.5 16.1 0.1 1098.1 44.1 17.5 702.4 * * 16.2 20.5 1927 31.3 14.9 29.9 BAB1 6B 458.9 11.2 24.1 86.2 36.8 4 14.7 155.1 48.1 5.4 392.9 * 17.7 9.9 19.2 329.9 102.6 31.1 64.2 BAB1 7 272.1 5.7 57.2 58.9 34.8 3.8 4.6 128 41.6 8.8 413.6 * 7.2 8.9 16.2 841.2 67 13.8 34.3
133 Appendices
Sample Ba Nb Pb Rb Sr Th Y Zr Ce Co Cr Cu Ga Hf La S Zn Ni V BAB1 9 334.3 4.8 26.4 59.8 24 5.4 6 70.7 44.4 4.8 259 * 8.6 7 21.9 195.1 50 8.5 27.9 JY8 1 195.1 7.8 7.5 36.2 188.3 2.6 12.6 80.8 31.2 18.2 503 0.4 13.7 12.8 10.3 177.7 100.6 50.2 104.4 JY8 2 291.8 3.8 25.6 53.5 51.1 2.4 5 61.5 38 2.1 247.4 * 7.2 6.5 11.1 70.9 78.6 6 21 JY8 3 208.7 3.6 18.1 39.3 29.2 4.1 3.9 55.3 35.7 1.6 269.1 * 3.4 6 16 105.8 76.7 4.5 16.4 JY8 4 584.6 6.5 53.7 95.4 96 5.1 9.7 108.6 56.3 6.4 284 * 14.3 9 20.3 232.5 82.4 11.2 40.6 JY8 5 433.7 11 24 65.2 66.9 6.4 14.5 125.2 47.4 2.8 535 * 19.7 11.1 22.7 180.2 112.7 48.9 84.9 JY8 6 378.1 4.6 23 49.3 91.8 7.7 7.1 121.1 51.5 4 315.7 * 7.1 7.9 25.4 209 129.6 7.9 28.6 JY8 7 557.8 5.9 39.2 73.2 86.8 2.7 8.5 95.8 52.4 5.6 277.1 * 12.3 8.2 20.1 382.9 83.7 11 34 JY8 8 229 5.8 19.2 42.4 37.4 6.7 5 97.5 39.1 3.7 351.7 * 4.7 7.2 14.9 74.5 34.3 6.6 22.8 JY8 9 207.7 5.2 16.2 36.5 27 4.3 3.8 60.3 36.5 2 290.5 * 4.1 6 13.6 195 40.3 5.4 16.7 JY8 10 83.1 4 13.6 19.7 12.9 5.2 3.6 89.1 30.6 2.2 390.1 * 1.2 6.6 12.8 9.8 30.6 4.7 15.5 JY8 11 344 5.1 20.4 41.3 68.4 2 5.5 64.9 41.9 1.6 249.2 * 6.5 6 15.1 63.9 73.5 6.1 21.9 JY8 12 146.6 4.2 14 31 30.1 4.7 3.4 51.8 33.9 2 278.1 * 2.4 5.8 11.5 170.3 61.3 4.3 15.3 JY8 13 441.3 5.2 23.9 72.9 129.1 2.3 8.3 91.4 50.1 5 283.1 * 11.3 8.1 19.1 270.4 106 10.2 38.4 JY8 14 344.7 4.6 22.3 57.9 115.8 5.7 7.1 104.8 46.5 4.8 309.1 * 9.5 7.9 15.5 203.6 116.8 8.7 32.9 JY8 15 440.5 5 26.6 62.9 169.2 6 8.1 111.3 48.4 3.9 308.6 * 11.7 8.6 21.8 164.8 117.2 10 34.8 JY8 16 365.7 3.9 18.1 52.8 121.1 3.6 5.9 73.7 43.8 1.7 271.3 * 8.3 6.6 13.5 129.4 148.1 6.4 26 JY8 17 251.2 4.1 30.5 40.3 116.8 5.4 6.7 75 40.7 3.1 295 * 6.8 6.8 14.4 211.7 107.9 7.1 27.7 JY8 18 379.5 10 35.3 62.5 200.5 2.5 18.8 141.5 44.9 14.7 676.2 * 17.1 11.3 18.2 179.3 216.5 38 82.9 JY8 MAR 411.6 5 29.9 89.4 141.6 3.8 8.7 114.4 48.7 5.3 318.3 * 13.8 8.1 18.9 239.8 144.2 11 35.7 DK12 1 103.2 6.3 15.3 61.5 9.5 6.6 7.2 76.3 32.6 3 265.3 * 5.9 6.8 9.5 233.2 9.3 4.1 18.4 DK12 2 146.5 5.4 8.3 59.7 15.6 5.6 6.2 74.7 33.3 1.6 224.8 * 5.2 6.1 12 18.5 11.1 3.6 16.6 DK12 3 75.1 3.5 6.1 18.3 24.5 4.8 4.4 56.7 30.9 1.8 294.7 * 1.4 6 9.3 16.1 18 3.6 17.6 DK12 4 89.8 5.2 11 23.7 13.7 8.6 6.5 93.7 33.9 4 295.7 * 6.3 6.8 13.1 128.5 16.4 5.2 23.2 DK12 5 64.8 4.1 6.4 19.8 11.5 5.8 5.3 63.3 29.6 1.4 282.7 * 2.1 5.7 5.7 23.7 8.7 4 15.1 DK12 5A 138.8 4.2 13 38.5 19.3 5.5 6 66.6 34.2 2.3 303.8 * 5.3 6.6 17.4 44.6 9.4 4.6 22.1 DK12 6 348.9 4.1 5.4 54.5 15.3 3.2 4 66.1 39.4 1.9 248.4 * 4.8 5.7 6.8 4.5 12.3 3.9 13.5 DK12 6A 258.1 4.8 12.6 52.8 21.3 6.1 9.7 79.2 38.4 3.2 282.1 * 5.9 6.2 12.8 * 18.7 6.3 19.3 DK12 7 125.8 5.4 26.9 34.9 24.4 6.3 7.8 87.1 36.9 6.9 344.7 * 10.7 8.3 19.3 216.4 39.8 7.3 31.6 DK12 8 49 5.8 16.4 10.5 18 7.4 6 91.7 33.1 4.1 328.9 * 5.2 7.2 14.5 178.4 22.8 6.2 17.5
134 Appendices
Sample Ba Nb Pb Rb Sr Th Y Zr Ce Co Cr Cu Ga Hf La S Zn Ni V DK12 9 361.6 7.6 27.4 51.6 212.5 4.2 11.4 121.9 42.4 10.7 473.5 * 14.9 9.3 19.3 159 69.5 23.8 44.1 DK12 10 574.2 6.7 33.1 87.7 173.4 10.8 11.3 174.4 56.7 5.5 275.7 * 16.8 9.8 24.8 112.1 58 13.9 40 DK12 11 272.6 6.4 18 69.6 45.1 4.7 7.7 77.5 40.6 2.7 243.6 * 13.6 6.6 17.3 89.3 41.3 8.5 23.6 DK12 12 302 4.6 48.1 76.8 29 4.5 7.8 97.5 43 3.2 384.1 * 12 7.6 15.7 190.1 21.9 7.2 34.5 DK12 13 680.8 5.8 35.7 96.5 154.5 3 8.4 99.7 57.6 2.8 281.3 * 15.1 7.9 16.8 120.5 50.2 10 35.1 DK12 13B 436.7 6.2 110.9 64.4 55 5.8 8.5 127.7 48.5 14.4 362.3 * 11.3 9 20.9 414.2 41.4 15.6 39.7 DK12 14 504.1 7 35.5 86 74.4 2.9 9.6 97.7 50.7 5.7 282.7 * 14.1 8 21.1 125.8 46 12 33.6 DK12 15 644.6 7.5 49.6 87.9 84.8 3.1 9.9 87.4 55.2 4.2 288.1 * 14.4 8.8 18.2 98.8 49.9 13.4 37.5 DK12 16 436.7 5.9 31.1 53.5 64.8 3.6 7.3 109.5 48.3 4.5 363.9 * 10 8.4 14.3 186 50 12 36.4 DK12 17 421.4 4.7 22.3 52.5 65.2 0.9 5.6 88.4 42.2 3 359.4 * 9.7 8.8 10.4 116.3 50.1 10.5 35.2 DK12 18 391.2 6.1 35.3 50.2 62.8 4.5 8.5 130.3 45.8 5.9 369.1 * 10.2 10.6 15.7 349.2 51.8 13.3 39.7 DK12 19 210.3 3.5 32.5 33.8 36.6 4.3 3.8 144.3 38.8 5.7 346.9 * 5 7.2 16.8 482.6 15.5 7.3 22.9 DK12 20 368.4 5.4 33.7 50.2 48.6 3.3 5.9 146.5 47.4 7.2 319.5 * 8.5 8 20.5 400.6 32.7 10.3 30.7 AM1 1 402.1 6.1 39.1 61.3 93.5 0.9 7.5 116.3 45 4.8 277.6 * 7.2 8.8 10 40.3 29.8 8.1 26.1 AM1 2 417.8 11 22.6 90.3 122.3 3.3 11.3 157 48.4 6.3 335.5 * 16.9 9.9 18.3 133.3 67.4 21 54.1 AM1 3 307.3 10.6 23.7 57.7 87.7 5 11.6 182.3 40.1 7.4 388.8 * 12.9 12.1 21.4 75.3 53.8 19.9 51.6 AM1 4 404.5 11.5 11.8 121.4 119.4 2.1 10.4 173.6 44.8 8.9 402.9 * 18.9 10.6 8.9 136.6 65 21.9 57.9 AM1 5 445.8 9.2 33 101.3 71.6 4.5 9.9 316.8 50.6 12 555.2 1.1 11.8 14.3 17.1 645.6 69.2 18.7 55.7 AM1 6 442.4 4.4 21.9 43.8 142.1 4.8 6.1 103.7 47.7 4.8 262 * 5.9 7.8 11.8 303.1 30.4 6.4 23.4 AM1 7 552.1 7.7 23.8 65.5 150.8 5.7 9.8 285.9 55.5 10.1 381.8 * 9.6 13 19 685 53.5 13.1 38.8 AM1 8 259.9 3.4 15.7 32.7 30.3 5.8 3.7 67.8 41.2 3 285.9 * 2.8 6.1 12.6 101.5 22.7 4.6 17.8 AM1 9 456.2 6.7 26.3 104 207.2 2.4 10.2 197 46.6 9.3 432.5 * 10 11.1 21.1 25.6 42.7 14.5 32.2 AM1 10 303.2 11.2 21.8 74.2 370.2 3.2 19.6 187 44.1 11.7 395.2 * 17.9 11.8 11.9 33.7 79.8 28.1 78.2 AM1 11 356.3 3.4 13.6 42.1 59.4 4.8 4.3 53.7 43.2 2.1 258.2 * 2.1 6.1 10.6 * 20 4.2 15 AM1 12 388.7 3.8 21.1 57.9 90.5 4.6 5.3 66.1 44.8 2.3 231.3 * 4.7 6 12.1 42.2 27.1 4.5 19 AM1 13 613.8 5.4 30.9 76.8 149.9 7 6.6 156.1 61.9 4.5 297 * 8.8 8.4 23.5 116.4 29.7 8 25.7 AM1 14 438 5.3 21 65.4 69.4 5.7 6.4 81.7 50.8 2.7 231.3 * 9.3 6.6 17.4 4.8 31.4 7.1 22.8 AM1 15 423.8 5.1 29.9 54.3 48.8 3.6 6.6 99.4 50.5 4.3 255.2 * 7.7 8.5 17.4 2.2 30.8 7.6 23.6 AM1 16 76.7 3.1 10.6 10.3 36.6 5 3.7 74.2 31.3 0.6 275.9 * 1.5 5.7 15.2 * 15.1 3.5 14.5 AM1 17 221 3.7 12.6 43.6 47.4 4.9 4.5 58 39.6 0.8 261 * 2.7 5.9 13.3 * 11.7 3.8 14.2
135 Appendices
Sample Ba Nb Pb Rb Sr Th Y Zr Ce Co Cr Cu Ga Hf La S Zn Ni V AM1 18 204.3 3.7 48.9 31 87.5 5.8 6.2 95.9 40.1 2.3 297.6 * 3.3 6.3 8.8 * 24.2 5.8 19.7 AM1 19 546.6 6.9 23.1 62.8 256.2 5.2 10.8 178 53.5 8.3 349.7 * 15.6 11.6 14 5.9 63.5 16.9 49.3 AM1 20 465.6 5.6 16.4 91.5 63.6 0.1 22.9 90.9 58.1 1.5 262.4 * 4.6 6.2 21.8 * 42.8 5.1 21.8 AM1 21 486.6 12.2 58.5 86.4 229.6 7.8 25.2 259.9 55.7 12.2 543 * 20 11.7 26.5 44.4 108.2 29.6 73.6 AM1 22 397.2 9.5 43.2 57 169.6 5.7 20.7 191.6 48.4 18.5 713.7 2.2 13.8 14.5 12.1 187.1 127.8 32.1 69 AM1 23 396.3 6.1 44.8 47.3 186.4 9 10.2 223.1 49.5 12.7 526.5 * 9.7 10.8 28.1 358.5 73.6 17.9 43.4 AM1 25 213.9 5.8 29.8 46.3 27.4 5.5 14.3 103.1 41.3 7.6 412.7 * 7.2 8 14.7 * 47.6 13 35.3 AM1 26 342.6 4.8 18.3 74.3 49.6 4.1 7.4 89.1 46 4.7 290.6 * 12.5 6.3 9.3 * 41.1 8.8 32.2 AM1 27 145.2 3.8 28.2 28 35.6 6 7.1 65.5 33.7 3.8 348.8 * 4.5 6.7 13 * 31.6 7.3 23.9 BAB1 Corr 673.9 12.2 82.2 118.3 82.3 8 12.3 194 87.4 22.5 453.9 44.3 18.4 14.3 44.3 667.7 198.7 24.3 67.1 AM1 Lag 375.6 7.5 31.9 47 186.4 5.1 11.8 125.5 55.4 30.5 480.4 36.4 11.3 12 25.2 473.3 91.3 40.2 65.3 BAB1 Prom 323.5 8.3 33.4 72.3 43.6 4.1 7.6 109.5 48 15.8 424.7 35.4 9.7 12 17.9 375.8 66.2 22 37.9
136 Appendices
Appendix III : INNA Trace Element Geochemistry
Sample Sb As Ba Br Ca Ce Cs Cr Co Eu Au Hf Fe La Lu ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppb ppm % ppm ppm TF1 OGM1 0.4 11.0 470 42 4 533 39 0.7 9 9 4.66 21.0 0.29 TF1 OGM2 0.2 4.9 57 18 713 5 0.2 2 0.60 10.0 0.07 TF1 OGU1 0.3 10.0 340 32 2 510 9 0.5 4 0.71 16.0 0.14 TF1 VER1 0.4 8.9 11 658 5 3 2 0.43 5.4 0.06 TF1 VER2 0.3 6.6 54 20 1 628 10 0.4 4 2 1.00 10.0 0.11 TF1 WIT1 0.4 10.0 230 33 3 1870 17 0.6 6 7 1.20 16.0 0.18 TF1 BUL3 0.2 1.5 400 20 430 4 3 0.69 10.0 0.07 TF1 RIP2 0.3 8.1 390 41 5 912 33 0.7 5 3.91 20.0 0.23 TF1 0.2 260 14 1 290 3 0.3 2 0.71 7.3 0.07 SPEC1 TF1 RV1 0.4 5.9 130 20 1 611 10 0.3 2 1.38 10.0 0.12 BAB1 1 0.2 3.7 440 24 3 470 5 0.3 3 0.49 13.0 0.10 BAB1 2 0.5 39.0 310 56 3 751 16 0.3 25 13 0.85 28.0 0.29 BAB1 3 0.2 8.0 460 42 4 480 14 0.4 5 1.11 20.0 0.16 BAB1 6 2.5 74.5 160 70 2 1150 36 0.6 23 32 1.48 36.0 0.43 BAB1 6B 1.3 16.0 400 1.5 40 4 504 28 0.7 4 5.87 20.0 0.22 BAB1 7 1.8 35.0 240 35 3 582 21 0.3 33 4 1.96 18.0 0.10 BAB1 9 0.6 16.0 350 42 3 350 10 0.5 12 2 1.10 23.0 0.09 JY8 1 0.6 1.9 170 6 21 4 817 61 0.7 2 8.78 8.9 0.23 JY8 2 0.2 13.0 280 27 2 350 6 0.4 16 2 0.87 15.0 0.06
137 Appendices
Sample Mo Nd Ni Rb Sm Sc Na Ta Tb Th W U Yb Zn ppm ppm ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppm ppm TF1 OGM1 7 13 150 93 2.8 10.2 0.07 0.7 6.3 2.5 1.7 TF1 OGM2 14 8 1.2 0.9 0.03 1.7 3 0.9 0.4 TF1 OGU1 14 10 66 2.0 2.1 0.06 3.0 1.1 0.9 TF1 VER1 12 5 0.7 0.9 0.03 0.8 1 -0.5 0.4 TF1 VER2 13 7 1.5 4.1 0.08 2.0 3 0.9 0.7 TF1 WIT1 15 14 50 2.4 5.2 0.13 3.5 3 1.5 1.0 TF1 BUL3 16 8 110 1.0 0.7 0.04 2.3 1 0.7 0.4 TF1 RIP2 18 340 120 3.0 15.8 0.87 4.7 2 1.5 1.5 58 TF1 8 6 29 0.9 1.0 0.43 1.5 2 -0.5 0.4 SPEC1 TF1 RV1 13 9 30 1.8 3.5 0.02 1.9 2 -0.5 0.8 BAB1 1 16 9 92 1.5 2.5 0.06 3.2 3 1.7 0.6 BAB1 2 24 16 60 3.3 3.1 0.06 1.8 0.6 19.0 3 9.0 1.6 BAB1 3 12 16 110 86 2.7 6.6 0.22 0.8 4.5 3.5 1.0 BAB1 6 28 24 32 4.1 2.9 0.06 1.2 0.5 11.0 5 11.0 2.3 BAB1 6B 8 15 69 3.0 12.3 0.11 1.1 4.8 2.5 1.5 71 BAB1 7 19 14 120 46 1.8 4.0 0.10 4.1 3 2.1 0.5 52 BAB1 9 13 17 56 2.2 3.9 0.08 3.8 4.3 0.5 JY8 1 8 8 36 2.2 35.0 1.62 1.5 5 1.4 110 JY8 2 18 10 41 1.5 2.3 0.30 2.8 2 2.3 0.4 57
138 Appendices
Sample Sb As Ba Br Ca Ce Cs Cr Co Eu Au Hf Fe La Lu ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppb ppm % ppm ppm JY8 4 0.3 33.0 540 60 4 380 16 0.6 8 3 1.65 31.0 0.12 JY8 5 1.1 14.0 460 1 43 2 680 36 0.8 4 9.06 20.0 0.24 JY8 7 0.3 24.0 450 53 3 340 17 0.6 23 3 1.79 28.0 0.11 JY8 8 0.3 11.0 230 39 1 503 8 0.4 3 0.86 20.0 0.09 JY8 10 0.2 6.7 100 17 -1 532 6 0.3 3 2 0.63 9.1 0.07 JY8 11 0.1 8.5 340 32 1 350 6 0.5 5 2 0.91 16.0 0.07 JY8 12 0.7 7.4 150 27 1 360 5 0.2 66 1 0.57 13.0 -0.05 JY8 14 0.5 11.0 320 49 2 400 12 0.6 8 3 1.26 26.0 0.10 JY8 15 0.8 13.0 390 1 48 2 430 13 0.5 3 1.72 25.0 0.11 JY8 18 0.8 23.0 400 1 37 3 889 43 0.8 4 4.37 17.0 0.27 JY8 MAR 0.4 11.0 340 2 53 4 440 15 0.4 8 3 1.82 27.0 0.11 DK12 1 0.4 9.3 100 23 340 6 0.3 5 2 0.39 11.0 0.11 DK12 5 0.1 1.4 55 17 380 3 0.2 2 0.32 8.7 0.08 DK12 6 0.1 2.0 330 17 340 3 0.3 2 0.33 8.4 0.07 DK12 6A 0.3 5.7 220 31 2 390 7 0.4 2 0.53 15.0 0.12 DK12 8 0.3 7.3 -50 35 470 8 0.5 3 0.48 17.0 0.10 DK12 9 0.7 14.0 350 2 37 2 627 25 0.4 3 2.52 18.0 0.16 DK12 10 0.4 17.0 520 68 4 370 18 1.0 5 2.26 34.0 0.16 DK12 11 0.2 11.0 240 32 3 340 8 -0.2 2 2 1.24 17.0 0.10 DK12 12 0.4 36.0 220 43 2 538 10 0.5 9 3 1.10 22.0 0.10 DK12 13 0.3 17.0 660 47 4 390 11 0.8 3 1.80 24.0 0.11 DK12 13B 0.3 89.9 430 46 3 509 32 0.4 63 4 1.75 24.0 0.15
139 Appendices
Sample Mo Nd Ni Rb Sm Sc Na Ta Tb Th W U Yb Zn ppm ppm ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppm ppm JY8 4 18 19 84 3.2 6.5 0.45 5.5 3.4 0.8 77 JY8 5 3 14 300 58 3.4 18.5 0.57 0.8 0.7 4.9 1.7 99 JY8 7 7 19 63 2.8 5.4 0.58 0.8 5.7 1 3.0 0.6 56 JY8 8 12 13 44 2.0 3.0 0.10 4.4 2 1.4 0.5 JY8 10 12 8 16 1.0 1.1 0.03 1.4 1 0.6 0.3 JY8 11 9 13 33 1.8 2.9 1.01 2.6 2 0.8 0.4 JY8 12 12 9 24 1.3 1.2 0.47 2.1 2 0.7 0.2 57 JY8 14 15 15 44 2.7 4.5 1.49 0.6 4.9 3 2.0 0.6 93 JY8 15 16 20 60 2.8 6.0 2.34 0.6 0.5 4.7 2 1.3 0.7 99 JY8 18 16 400 70 3.3 18.1 1.46 0.5 3.0 2 1.9 160 JY8 MAR 17 20 85 3.1 5.9 1.38 6.3 2 7.1 0.7 78 DK12 1 18 8 53 1.6 1.4 0.04 4.1 2 2.6 0.7 DK12 5 19 6 0.9 0.8 0.03 2.6 2 0.9 0.5 DK12 6 15 5 44 1.1 0.9 0.06 1.8 0.4 DK12 6A 19 10 45 1.7 2.0 0.05 4.1 2.6 0.8 DK12 8 12 12 1.9 2.1 0.04 3.1 2 1.3 0.5 DK12 9 12 200 34 2.6 9.4 2.12 3.4 1.7 1.0 55 DK12 10 7 21 72 4.0 8.2 1.64 0.9 11.0 2 4.3 0.9 DK12 11 8 10 54 1.8 4.2 0.16 0.8 3.2 1.0 0.7 DK12 12 26 12 60 2.2 4.6 0.11 5.6 7.2 0.7 DK12 13 15 18 87 2.8 6.4 1.12 0.5 4.7 1.5 0.6 DK12 13B 21 17 53 2.7 6.4 0.14 6.3 3 9.5 1.0
140 Appendices
Sample Sb As Ba Br Ca Ce Cs Cr Co Eu Au Hf Fe La Lu ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppb ppm % ppm ppm DK12 15 36.0 600 55 4 390 15 0.7 3 2.17 27.0 0.14 DK12 16 0.2 19.0 420 38 2 490 14 0.4 12 3 1.83 20.0 0.12 DK12 17 0.2 13.0 390 22 2 490 11 0.3 3 1.56 10.0 0.08 DK12 19 0.4 23.0 150 36 1 490 12 0.3 19 4 1.00 19.0 0.09 AM1 4 0.6 4.5 340 1.0 35 4 509 23 0.5 5 3.25 18.0 0.18 AM1 5 0.9 18.0 430 53 3 790 31 0.6 140 10 2.28 28.0 0.23 AM1 6 0.7 6.3 410 38 2 330 9 0.7 3 0.79 20.0 0.08 AM1 7 0.8 10.0 510 64 3 500 24 0.8 9 1.70 33.0 0.22 AM1 8 0.7 7.3 230 32 2 400 5 0.3 7 2 0.52 17.0 AM1 9 0.4 15.0 440 37 4 576 19 0.6 13 5 1.17 19.0 0.18 AM1 10 0.7 8.8 270 2 40 6 533 34 1.0 5 4.50 20.0 0.28 AM1 11 0.1 6.6 380 30 1 360 5 0.3 1 0.50 16.0 AM1 12 10.0 380 43 1 310 5 0.5 2 0.49 23.0 0.06 AM1 13 0.4 18.0 640 58 2 430 12 0.6 4 1.04 31.0 0.10 AM1 14 0.2 11.0 370 47 1 300 7 0.7 6 2 0.90 26.0 0.08 AM1 15 0.2 19.0 400 51 2 340 9 0.6 61 3 0.95 27.0 0.09 AM1 16 0.2 3.0 67 1 30 370 3 -0.2 2 0.38 16.0 AM1 17 0.2 5.0 230 0.9 31 350 3 0.3 2 0.51 16.0 0.06 AM1 18 0.4 22.0 160 41 420 7 0.6 3 0.80 22.0 0.09 AM1 19 0.2 11.0 500 1 51 2 460 21 0.9 5 2.42 25.0 0.17 AM1 21 0.6 46.0 440 1 78 3 723 34 1.3 8 3.99 39.0 0.38 AM1 22 0.7 27.0 360 46 2 968 40 1.2 6 3.27 23.0 0.32
141 Appendices
Sample Mo Nd Ni Rb Sm Sc Na Ta Tb Th W U Yb Zn ppm ppm ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppm ppm DK12 15 7 18 80 3.1 7.4 0.16 4.2 2.3 0.8 DK12 16 16 14 49 2.4 5.4 0.11 3.2 2 2.8 0.7 DK12 17 16 8 45 1.3 5.4 0.10 2.4 2 1.3 0.5 DK12 19 14 15 18 1.9 2.5 0.11 3.1 2.8 0.5 AM1 4 6 13 110 2.4 10.2 1.36 1.0 4.2 1.4 1.1 59 AM1 5 12 19 90 3.3 9.0 0.56 0.8 5.9 2 3.1 1.3 53 AM1 6 12 13 35 2.1 2.5 1.26 3.7 2 1.8 0.5 AM1 7 11 24 66 3.8 6.6 1.42 1.2 7.1 1 3.8 1.2 58 AM1 8 20 13 29 1.6 1.1 0.05 3.1 2 0.9 0.3 AM1 9 11 13 96 2.5 5.4 1.34 0.7 3.8 4 1.1 1.1 AM1 10 6 18 72 3.5 17.7 1.65 4.6 1.9 1.9 65 AM1 11 18 12 33 1.5 0.8 0.08 2.3 2 0.9 0.3 AM1 12 14 15 39 2.0 1.3 0.42 0.6 3.2 1 0.7 0.4 AM1 13 13 17 77 3.1 3.5 0.91 0.6 6.2 1 2.2 0.7 AM1 14 9 13 53 2.3 3.4 0.10 3.6 0.8 0.5 AM1 15 10 16 45 2.7 3.2 0.11 4.0 2 1.5 0.5 AM1 16 18 10 1.3 0.6 0.07 2.3 1 0.6 0.3 AM1 17 17 11 37 1.4 1.0 0.23 2.4 2 0.7 0.4 AM1 18 14 14 31 2.3 2.5 0.35 3.0 1 0.6 AM1 19 14 19 76 3.4 10.2 2.44 4.8 1.3 1.0 AM1 21 7 30 76 5.8 15.1 2.19 0.8 0.9 7.5 3.0 2.6 85 AM1 22 10 19 210 65 3.9 13.5 1.84 0.9 4.3 1.3 1.9 110
142 Appendices
Sample Sb As Ba Br Ca Ce Cs Cr Co Eu Au Hf Fe La Lu ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppb ppm % ppm ppm AM1 25 0.2 18.0 170 44 2 623 16 0.7 3 1.44 22.0 0.21 AM1 26 0.2 6.8 350 43 2 410 11 0.4 2 1.08 23.0 0.10 BAB1 PROM 0.5 15.0 240 32 3 470 15 0.8 4 3.47 17.0 0.13 BAB1 CORR 1.9 11.0 690 69 9 502 22 0.9 20 6 2.74 34.0 0.23 AM1 LAG 0.7 5.9 370 3 38 2 569 29 0.6 3 4 6.23 19.0 0.20
Sample Mo Nd Ni Rb Sm Sc Na Ta Tb Th W U Yb Zn ppm ppm ppm ppm ppm ppm % ppm ppm ppm ppm ppm ppm ppm AM1 25 19 15 43 2.9 5.2 0.05 3.0 4 0.9 1.3 AM1 26 15 15 110 65 2.5 4.5 0.17 5.1 2 2.5 0.6 BAB1 PROM 12 63 2.3 6.9 0.37 3.1 1.4 0.9 BAB1 CORR 23 190 120 4.7 12.0 0.26 0.9 8.1 5.5 1.4 71 AM1 LAG 14 56 2.6 17.7 1.00 0.7 4.0 4 1.4 1.2
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