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UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
A Minor Field Study
Kaapvaal Craton basement
exposed in Diamond mines
at Kimberley, South Africa
-Greenstone belt schists and amphibolites-
Susanne Fritiofsson
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B853
Bachelor of Science thesis
Göteborg 2015
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
TableofContents
Abstract .......................................................................................................................................... 4 Sammanfattning.............................................................................................................................. 5 Introduction .................................................................................................................................... 6 Aim ................................................................................................................................................. 6 Geological setting ............................................................................................................................ 7 Social background of Kimberley, South Africa .................................................................................. 9 Petra Diamonds in South Africa ........................................................................................................ 10 Geological background .................................................................................................................. 12 What is a craton? ............................................................................................................................... 12 General geology of Archean cratons and Greenstone belts ............................................................. 12 Archean evolution of the Kaapvaal Craton ....................................................................................... 13 Eastern Kaapvaal craton – Barberton Greenstone Belt (BGB) .......................................................... 15 Western Kaapvaal craton ‐ Kraaipan and Amalia Greenstone Belt ................................................... 16 Komatiites .......................................................................................................................................... 17 Zircons ............................................................................................................................................... 18 Rare earth elements and trace elements .......................................................................................... 19 Method ......................................................................................................................................... 20 Mapping and picking samples for dating underground the Dutoitspan and Bultfontein mine ........ 20 Making thin sections ......................................................................................................................... 21 Mineral identification using thin sections ......................................................................................... 21 Zircon separation and whole rock analysis preparation ................................................................... 21 Zircon separation and puck making .................................................................................................. 22 Zircon identification using Scanning Electron Microscope (SEM) ..................................................... 23 Glass making and whole rock analyses for major elements ............................................................. 23 Rare Earth Elements and trace elements analyses in the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) ........................................................................................................ 24 Zircon dating by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) ........ 26 Results .......................................................................................................................................... 27 Maps from the mine .......................................................................................................................... 27 Thin section result ............................................................................................................................. 30 Major elements from scanning electron microscope (SEM) ............................................................. 35 Rare Earth Element and Trace element using Inductively Coupled Plasma Mass Spectrometry (LA‐
ICPMS) ............................................................................................................................................... 37 Geochronology .................................................................................................................................. 41 2 Discussion ..................................................................................................................................... 44 Maps: ................................................................................................................................................. 45 Thin sections: ..................................................................................................................................... 45 Major elements from Scanning Electron Microscope (SEM): ........................................................... 46 Rare Earth Element and Trace element using Inductively Coupled Plasma Mass Spectrometry (LA‐
ICPMS) ............................................................................................................................................... 48 Conclusion .................................................................................................................................... 51 Acknowledgements ....................................................................................................................... 53 References: ................................................................................................................................... 54 3 Abstract
A minor field study, Kaapvaal Craton basement exposed in Diamond mines at Kimberley, South Africa. Greenstone belt schists and amphibolites. Fritiofsson, Susanne, University of Gothenburg, Department of Earth Science; Geology, Box 460, SE‐
405 30 Gothenburg The Kaapvaal Craton covers an area of approximately 1,200,000 km2 in southern Africa. It was formed and stabilized between 3700 and 2700 Ma ago and it is one of the oldest cratons in the world. The Kaapvaal craton consists of different subdomains that have been welded together probably by processes similar to those of modern day plate tectonics (Brandl et al. 2006). The object of this thesis is to find out what the Kaapvaal Craton basement geology looks like inside two diamond mines in Kimberley, South Africa. It is also to find out the protolith and the age of the greenstone‐schists and amphibolites that were found in the mines. To answer that, five weeks of mapping and sampling in the Bultfontein and Dutoitspan mines owned by Petra Diamonds was performed. Schist and amphibolite samples were taken and brought back to University of Gothenburg, department of Earth Science in Sweden for analyses by Scanning Electron Microscope (SEM), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) and petrography using thin sections and microscope. Never before have any studies been done on these rock types from inside the mines. The western part of the Kaapvaal craton in the Kimberley block has poorly exposed basement rocks due to the Ventersdorp lavas that covered almost the whole area about 2700 Ma ago. To find out the age and the origin of these rocks, I can contribute to the knowledge of how the Kaapvaal craton took its form and how the amalgamation of the different cratonic blocks took place. U/Pb dating of 13 zircons was used to identify the age of one schist sample. The Concordia age result was 2908 ± 18 Ma which coincides with a possible subduction event before the Witwatersrand and Kimberley block collision formed what is now called the Kaapvaal Craton. At that time komatiite lava may have erupted and inherited some zircons from the surrounding rocks and reset those because of their lower closure temperature (900°C) compared to the high‐temperature komatiite (1400 ‐ 1600°C). The age also coincides with the plutonism which happened during the time when the Rehoboth Province and the western Kaapvaal Craton collided, starting from 3,3 Ga to 2,5 Ga. According to rare earth element (REE) diagrams and spider diagrams the schists and the amphibolites have the same origin and have been altered in the same way. Two massive heat periods about 2000 Ma ago may have put the rocks through the hydrothermal alteration which all the samples show. Keywords: Greenstone belt, Archean rocks, South Africa, Kaapvaal craton, Komatiite, U‐Pb zircon dating 4 Sammanfattning
A Minor Field Study, Kaapvaal kratonens djupbergarter exponerade i Diamantgruvor i Kimberley, Sydafrika – Grönstensbälten av skiffer och amfiboliter. Fritiofsson, Susanne, Göteborgs Universitet, Instutitionen för Geovetenskaper; Geologi, Box 460, SE‐
405 30 Göteborg Kaapvaal‐kratonen utgör en area på cirka 1,200,000 km2 i södra Afrika. Den formades och stabiliserades mellan 3700 till 2700 miljoner år sedan och är en av de äldsta kratonerna på jorden. Kaapvaal‐kratonen består av olika subdomäner som troligen blivit sammansvetsade av platt‐
tektoniska processer liknande de som sker idag (Brandl et al. 2006). Syftet med denna uppsats är att ta reda på hur Kaapvaal kratonens djupbergarter ser ut nere i två diamantgruvor i Kimberley, Sydafrika. Syftet är också att ta reda på ursprunget och åldern på de grönskiffrar och amfiboliter tagna från gruvorna. För att kunna svara på det behövdes fem veckor av kartering och stickprovsundersökningar i Bultfontein‐ och Dutoitspan‐gruvorna som ägs av Petra Diamonds. Stuffer av grönskiffer och amfiboliter togs med till Göteborgs Universitet, institutionen för Geovetenskaper i Sverige för att utföra Scanning Electron Microscope (SEM), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) ‐analyser och petrografi med tunnslip och mikroskop. Studier på bergarterna från gruvorna har aldrig tidigare gjorts. I den västra delen av Kaapvaal‐
kratonen i Kimberleyblocket finns det nästan inga synliga exponeringar av de här bergarterna på grund av ett lavatäcke från Ventersdorp‐utbrottet som hände för cirka 2700 miljoner år sedan. Genom att ta reda på åldern och ursprunget på dessa bergarter kan man få mer kunskap om hur Kaapvaal kratonen bildades och hur de olika kratoniska blocken slogs samman. U/Pb‐datering av zirkoner användes för att ta reda på åldern på ett prov från grönskiffern. Åldern resulterade i 2908 ± 18 miljoner år vilket sammanfaller med en trolig subduktionshändelse mellan Witwatersrand‐ och Kimberley‐blockens krock som senare kommer utgöra Kaapvaal‐kratonen som den ser ut idag. Vid den tiden skedde troligtvis ett utbrott med komatitisk lava som fick med sig zirkoner från omkringliggande bergarter och återställde åldern på dessa på grund av den lägre stängningstemperaturen på zirkonerna (900°C) jämfört med den mycket höga temperaturen på komatiten (1400 ‐ 1600°C). Åldern på zirkonerna sammanfaller också med plutonismen som skedde under tiden då Rehoboth‐provinsen och den västra delen av Kaapvaal‐kratonen kolliderade, vilket startade för 3,3 Ga till 2,5 Ga. Enligt diagramen från de sällsynta jordartsmetallerna verkar det som både grönskiffern och amfiboliten har samma ursprung och har blivit omvandlade på samma vis. Det var två stora värmeperioder för cirka 2000 miljoner år sedan som troligtvis ha gett bergarterna dess hydrotermala omvandling som alla stuffer påvisar. Nyckelord: Grönstensbälte, Arkeiska bergarter, Sydafrika, Kaapvaal kratonen, U‐Pb zirkon datering 5 Introduction
The Kaapvaal Craton covers an area of approximately 1,200,000 km2 in southern Africa. It was formed and stabilized between 3700 and 2700 Ma ago and it is one of the oldest cratons in the world. The Kaapvaal Craton consists of different subdomains that have been welded together probably by processes similar to those of modern day plate tectonics (Brandl et al. 2006). The city of Kimberley is located on the Kimberley block 60 km west of the Colesburg magnetic lineament that separates this younger block from the older Witwatersrand block to the east as shown in fig.1 (Schmitz, 2004). The two diamond mines, Dutoitspan mine and Bultfontein mine, owned by Petra Diamonds are located in the city of Kimberley as shown in fig.3. These mines among others have been very important throughout the history of South Africa. My colleague Caroline Lundell is working on the Kaapvaal Craton basement granitoids in the diamond mines in Kimberley, South Africa. We did the field work and collected all samples for both our studies together. I have focused on the greenstone‐schists and amphibolites and Caroline have focused on the granites. The title of Caroline´s project is: “Kaapvaal Craton basement exposed in the Dutoitspan diamond mine and Bultfontein diamond mine, South Africa – Granitoids” and will be finished in 2015. My studies together with her studies are two separate projects but together they are essential for understanding the evolution of the Kaapvaal Craton over the past 3,7 Ga. The financial support for both these projects is contributed by scholarships from the Swedish International Development Cooperation Agency (SIDA). The scholarship is called “A Minor Field Study” and can be applied for by university students that want to do their field studies for their thesis in a developing country. Both I and Caroline applied for Kimberley, South Africa due to its old, interesting and unexposed bedrock geology. Aim
The granitic basement and possible greenstone belt is not exposed at the surface and therefore mapping, dating and other analyses have never been done before on these granites, schists and amphibolites. The aim of this thesis is to work in two diamond mines in Kimberley, South Africa and the questions at issue are:  Where are the boundaries between the schist areas and the other basement rocks?  What are the protolith for the amphibolite and the greenschist?  How old are these rocks? To answer these questions mapping of the side wall rocks in the mines, petrography using thin sections, scanning electron microscope analyses for identifying major elements and laser ablation inductively coupled plasma mass spectrometry analyses for identifying trace elements, rare earth elements and U/Pb zircon dating were done. Ages were calculated and displayed with Isoplot 3.0. Petra Diamonds who own the mines in Kimberley are interested in knowing where the schist boundaries are located because the Bultfontein mine will most likely be sealed off due to lack of diamonds and therefore knowledge of the most strategic and safest locations for placement of plugs in the tunnel is necessary. 6 This study can also be interesting since the Archean basement in the western part of Kaapvaal craton is poorly exposed and further work needs to be done to really explain what have happened with the craton. Thanks to this study this knowledge has increased. Geologicalsetting
The Kaapvaal Craton in South Africa is divided into different blocks. It consists of the Pieterburg block, which has not been in focus during this study, the Witwatersrand block and the Kimberly block as shown in fig.1. In between the Witwatersrand and Kimberley blocks there is a strong N‐S trending magnetic anomaly called Colesburg magnetic anomaly lineament that may represent an epicratonic suture separating these two lithospheric blocks of different age and origin (Schmitz, 2004). The Kalahari line separates the Kaapvaal Craton and the Rehoboth Province to the west, which is shown in fig. 2. Dutoitspan and Bultfontein mines owned by Petra Diamonds are located in the city of Kimberley. The shaft that services these two mines goes down between the kimberlite pipes and is known as Joint Shaft as shown in fig. 3 and 4. The mines are located in the Kimberley Block in the western part of the Kaapvaal craton, about 60 km west of the Colesburg magnetic lineament that separates this cratonic block from the older Witwatersrand Block as shown in fig. 1 after Poujol et al. (2005). The greenstones in the mines comprise N‐S dipping talc‐schist (soap stone) character and might be a southern extension of the Amalia Greenstone Belt (AGB) and the Kraaipan Group which are located further to the north on the craton, fig. 1. The greenstone belts are surrounded by a variety of granitoids ranging in composition from tonalite – trondhjemite gneisses, amphibolites and migmatites. All rocks for this study are taken in the tunnels of Dutoitspan and Bultfontein mine at 760 meters depth below surface and from the decline from 760 – 845 meters depth of Bultfontein, fig. 3. 7 Figure 1. A map representing Kaapvaal Craton and its different geological settings. Picture reference: Poujol et al. 2005, edited by Fritiofsson 2014. Figure 2. A map representing the Kaapvaal Craton and the Rehoboth Province together with the Kalahari Line. Picture reference: Schijndel et al. 2011, edited by Eriksson 2014. 8 SocialbackgroundofKimberley,SouthAfrica
The discovery of diamonds in the Northern Cape province of South Africa paved the way for the rapid industrial development of South Africa in 1880s. First were the alluvial finds in the Orange, Vaal and Riet rivers, followed by the dry diggings in the center of present‐day Kimberley (Lunderstedt, 2002). It is generally accepted that the Eureka diamond was the first diamond discovered in late 1866 or early 1867, shortly followed by the Star of South Africa diamond, which led to the diamond rush to the region. The alluvial diggings attracted the first multinational visitors, but when diamonds were discovered many kilometers inland from the rivers an explosion of diamond diggers came because they had heard stories about that the diamonds were literally lying on the surface, just waiting to be picked up. It did not take long before Kimberley had five big diamond mines; Kimberley (the big hole), De Beers, Dutoitspan, Bultfontein and Wesselton mine (Lunderstedt, 2002). These so called dry diggings led to the discovery and recognition of the famous Kimberley volcanic pipes filled with a new rock type that later became known as kimberlite, named after the settlement that grew up around these mines. (Robey, 2007) Figure 3. Overview of the mines, the Joint shaft and the tunnels. The tunnel down at 760 m depth below surface and the decline from 760 to 845 m, marked in red, have been studied for this thesis. Picture is reproduced with permission of Petra Diamond´s Kimberley Underground Mines. 9 The dry diggings quickly became far more important than the river diggings. Not only did they contain more diamonds per volume of ground being dug up, but the fine‐grained dry kimberlite rock was much easier to process than the alluvial gravels (Janse, 1995). The first of the Kimberley diamond mines to be discovered was the Dutoitspan mine, named so because of the farm Dorstfontein originally belonging to Abraham Paulus du Toit (Lunderstedt, 2002). The diamonds were recognized by J. Robinson in either late 1868 or early 1869 when he acquired some diamonds from the owners of the farm, the Van Wyks. By late 1869 and early 1870 diggings were active next to a pan known as Du Toits pan after which the mine took its name. The discovery of Bultfontein mine, on the neighboring farm of the same name, followed soon after and also by late 1869 diggers were excavating on this site to the southwest of the Dutoitspan diggings. In 1888 Johannes Nicholas de Beer, who also found diamonds on his farm, established a company that would later become one of the world´s leading diamond companies. This company took control of every diamond mine in Kimberley until the Kimberley mine ceased operations in 1914. De Beers mine was shutdown in 1991 and the remaining three mines – Dutoitspan, Bultfontein and Wesselton in 2005. In 2007 De Beers Consolidated Mining Company (DBCM) operations in Kimberley were focused on dump re‐treatment through the relatively new Combined Treatment Plant. Dutoitspan, Bultfontein and Wesselton mines and selected dumps were later put up on sale by De Beers (Robey, 2007). PetraDiamondsinSouthAfrica
Petra Diamonds´ operations are focused in Africa, which produces about 60% of the world´s diamond by value. The company has grown since they acquired mines from De Beers. They operate seven producing diamond mines in South Africa which are divided into two categories: the underground kimberlite pipe mines (Kimberley Underground etc.) and the fissure mines. Kimberley Underground includes three kimberlite pipe mines which are close to each other, Bultfontein, Dutoitspan and Wesselton, fig.3. These mines were essential for the economic development of South Africa. The largest diamond ever recovered at Kimberley Underground was +800 carats. These three mines were closed by De Beers in 2005 and Petra operated Kimberley Underground under care and maintenance from September 2007. Petra was given approval to operate the mines under De Beers´ license and following acquisition in May 2010 (www.petradiamonds.com, 2014). 10 Figure 4. An overview of the city of Kimberley and the Kimberley Underground mines together with The Big Hole. Picture taken from Google Earth and edited by Fritiofsson, 1 August 2014. Figure 5. Personal picture of me (to the right) and my colleague Caroline in front of the Joint Shaft sign at Bultfontein and Dutoitspan mines. 11 Geologicalbackground
Whatisacraton?
A craton is a part of the continental crust which has attained enough stability to sustain accumulation over a long period of time of younger, relatively undeformed cover rocks (Hunter et al.2006). GeneralgeologyofArcheancratonsandGreenstonebelts
Archean cratons are distinguished to have the lowest surface heat flow on Earth and most of the cratons have remained thermally and mechanically stable over the past 2000 – 3000 Ma. This indicates that the roots of the Archean cratons are cool, strong and compositionally distinct from the surrounding mantle. Some geochemical studies imply that the roots are chemically buoyant and highly depleted in incompatible elements, and this would have been achieved by partial melting and melt extraction simultaneously which left behind a residue composed of Mg‐rich harzburgites, lherzolites and peridotites. High‐degree partial melting of mantle peridotite produces magma of komatiitic composition and a solid residue that is very similar to the composition of Archean lithospheric mantle (Kearey et al. 2008). The oldest preserved material on Earth surface are the rocks from the early Archean and they are essential to interpret the evolutionary history of the Earth´s crust. There are only two cratons in the world that have retained relatively large areas of pristine pre‐3100 Ma rocks, the Pilbara craton in northwestern Australia and the Kaapvaal craton in South Africa (Brandl et al. 2006). Because of the vast accessible mineral wealth of the Kaapvaal craton, this region has probably been investigated more in detail than any other Archean craton (de Wit et al. 1992). The early crust comprises mostly high metamorphic grade gneiss terranes with infolded low metamorphic grade greenstone belts or greenstone belt remnants, and both of these are intruded by large volumes of granitoids. The term “greenstone belt” is partly due to the greenish‐grey color present by many of the volcanic rocks occurring in the belts, but is also due to the predominantly greenschist metamorphic overprint affecting these rocks (Anhaeusser, 2014).The greenstone belts are linear irregularly shaped features, usually 10‐50 km wide and 100‐300 km long and are composed mostly of mafic and ultramafic rocks. Their typical dark green color comes from the minerals such as chlorite and tremolite that occur in altered Mg‐rich and Fe‐rich mafic igneous rocks. Together these greenstones and granites form the Archean granite‐greenstone belts (Brandl et al. 2006 and Kearey et al. 2008). Many of these granite‐greenstone belts were formed in the late Archean (2500 – 3000 Ma ago) and those cratonic areas that have an older history have generally been largely deformed and metamorphosed and lost a lot of their original character (de Wit et al. 1992). There are three main stratigraphic units within typical greenstone belts. The lowest usually constitutes of tholeiitic and komatiitic lavas. Komatiites are varieties of Mg‐rich basalt and ultramafic lava that occur only in the Archean crust. The high Mg content (>18wt% MgO) of these rocks is commonly thought to indicate that the magma temperatures were much higher (1400 ‐ 1600°C) than those of modern basaltic magmas. The central unit often consists of intermediate and felsic volcanic 12 rocks and the upper unit contains clastic sediments, such as sandstones and banded iron formations (BIF). The high grade gneiss terranes which often surround greenstone belts typically exhibit a regional metamorphism of the amphibolite and granulite facies (Kearey et al. 2008).In addition to high‐degree partial melting of the mantle, the formation and evolution of the cratonic lithosphere involved a multi‐stage history of many tectonic and magmatic events, probably due to terrane collision and accretion and thickening of the crust. Opinions are divided regarding how the root construction formed, did it involve underthrusting and stacking of subducted slabs, accretion and thickening of arc material or extraction of melt from hot mantle plumes? By applying a lot of criteria from different geologic studies, Kearey et al. (2008) concluded that ancient mantle plumes may have been the most important to the evolution of the Archean lithosphere. Most greenstone belts contain a wide variety of volcanic and sedimentary rocks that reflect the different evolutionary conditions of formation and all have been influenced by geotectonic factors, including the intrusion of ultramafic, mafic and granitic complexes which result in a widespread deformation, metamorphism, metasomatism and mineralization (Anhaeusser, 2014). ArcheanevolutionoftheKaapvaalCraton
The Kaapvaal Craton covers an area of approximately 1,200,000 km2 in southern Africa. It is joined to the Zimbabwe Craton and the Limpopo Belt to the north. To the south and west the Kaapvaal Craton is located next to Proterozoic orogens and to the east is the Lebombo monocline that contains Jurassic igneous rocks associated with the break‐up of Gondwana as shown in fig.1 (Louzda, K.L. 2003). At the west is the Rehoboth Province located and is separated from the Kaapvaal Craton by the Kalahari line, fig.2. Kaapvaal Craton was formed and stabilized between 3700 and 2700 Ma ago and it is one of the oldest cratons in the world. It consists of different subdomains that have been welded together probably by processes similar to those of modern day plate tectonics (Brandl et al. 2006). Due to lack of knowledge about the evolution of the Kaapvaal Craton, there are many different theories about how the cratonic lithosphere evolved. One hypothesis is that the greenstones represent the primordial crust and that the trondhjemitic, tonalitic and granodiaritic (TTG) gneisses were created as a result of partial melting of these belts, but later radiometric data show that the TTG´s predate the greenstones (Hunter et al. 2006). Another assumption is that the Kaapvaal Craton had an intra‐oceanic obduction which resulted in the imbrication and thickening of the oceanic crust and that partial melting gave rise to trondhjemitic, tonalitic and granodioritic (TTG) magmas and the initial separation of the continental lithosphere. Repeated amalgamation of these lithospheric plates finally formed a more stable Kaapvaal continental fragment and this process is assumed to have reached completion at about 3100 Ma ago (Hunter et al. 2006). The Kimberley block is located west of the Colesburg lineament (fig.1), and has an N – S structural trend which is perpendicular to that in the Witwatersrand block. Because the Archean basement of the Kimberley block is poorly exposed through its sedimentary and volcanic strata the data is scarce, 13 however, the zircon dates that exist are from the Amalia and Kraaipan belts of tonalite‐ trondhjemite gneisses dated 2930 to 3180 Ma. The eastern Kaapvaal shield region, or Witwatersrand block shown in fig. 1, is relatively well exposed in eastern South Africa and Swaziland. It consists of continental crust formed between 3700 and 3300 Ma which amalgamated into a coherent unit, by approximately 3200 Ma ago through convergent margin tectonics (Schmitz, 2004). According to Silver (2004) there has been a collision between the western and northern boundaries of the Kaapvaal shield which affected the mantle in the Kimberley and Pietersburg terranes at 2900 Ma ago. This event may explain why the Kraaipan greenstone belt in the north (fig. 1) has an N‐S trend. This collision has been further investigated by Schmitz (2004) which has geochronological data from U/Pb dating of zircons from the western part of the Kaapvaal Craton. The result from his study shows that the Kimberley and Witwatersrand blocks were juxtaposed by subduction and terrane collision between 2930 and 2880 million years ago. The convergence was due to subduction beneath the Kimberley block which culminating in collisional suturing close to the Colesburg magnetic lineament, and the main evidence for this is the different extent of the Witwatersrand and Ventersdorp supergroups, the former only in the Witwatersrand block, the latter covering both. Cornell et al. (2011) describe an event when the western Kaapvaal and an Archean core of the Rehoboth Province joined about 2500 Ma ago as shown in fig 2. The suture zone between these two terranes is represented by the Kalahari Line which has rocks that show hydrothermal alteration due to a subduction zone. The extensive magnetic signature of the Kalahari Line seems to be partly related to the zone of hydrothermally altered granites, and their alteration is probably related to the crustal suture. Before the terranes were joined both had a long history of Archean plutonism, starting with trondhjemites from 3,3 Ga to 2,9 Ga and then joined by granites until 2,5 Ga. Especially two granite intrusion events dated 2854 ± 7 Ma and an older at 2882 ± 7 Ma, are interpreted as granites evolved from trondhjemites by fractionation, and are important events in the Kimberley Terrane. The Vredefort Dome, which occupies the central part of the Kaapvaal Craton, shows an exhumed part of the mid‐crust brought to the surface during the giant meteorite impact event ca. 2023 Ma ago. One proposal is that the Vredefort Dome exposes a near‐continious profile that extends through the entire Archean Kaapvaal crust. There are also regional thermal and magmatic events associated with the 2050 Ma intrusion of the Bushveld Complex, which may be superplume‐related (Cornell et al. 2011). According to Cornell (1978) there are geochemical and isotopic evidence of a metasomatic event that affected rocks older than 2600 Ma about 2000 Ma ago. The Ventersdorp sequence rocks show evidence of metamorphism and some geological features suggesting metasomatic alteration. The author had a hypothesis about a widespread burial metamorphic and metasomatic event which affected the pre‐Transvaal volcano‐sedimentary pile about 2000 Ma ago. The main cause of the metamorphism is considered to be burial to depths exceeding 5 km with a geothermal gradient of more than 200 °C. The general tendency of this metasomatism was towards homogenization within the system, but mineralogical controls caused some chemical diversity. The most common type of metasomatic alteration is silicification. Sodium and potassium metasomatism is also common where fluids from Na‐ or K‐rich granitic bodies can influence the mineralogy and composition of the rock types found close to the intrusions. Serpentinization, 14 carbonate alteration, of ultramafic rocks is generally considered to be a deuteric effect which is hydration without excess water from the surrounding rocks. Steatization is common in layered ultramafic intrusions which have been affected by hydrothermal (or extreme deuteric) alteration to talc or steatite. Ca‐metasomatism generally occurs where Ca‐rich minerals (Ca‐plagioclase or diopside) undergo hydrothermal alteration and resulting in calcium‐enriched fluids (Anhaeusser, 2014). EasternKaapvaalcraton–BarbertonGreenstoneBelt(BGB)
The Barberton Greenstone Belt is one of the best‐studied granite‐greenstone terranes in the world. It consists of a unique sequence of some of the oldest and best preserved lithologies on the planet and it has worked as a general standard for Archean greenstone belts worldwide (Brandl et al. 2006). Barberton greenstone belt is located in the eastern part of the Kaapvaal craton, close to the Mozambique border in the ancient gneiss complex, fig. 1. It covers an area of 6000 km2 and has a strongly folded ENE‐trend (Brandl et al. 2006). It contains well preserved Paleoarchean (3500 – 3100 Ma) volcanic rocks that belong to the Onverwacht Suite and sedimentary rocks belonging to the Fig Tree and Moodies Group. The Onverwacht Group is located in the southern part of Barberton greenstone belt and is divided into seven formations in a 15 km thick imbricated tectonic stack, from the base to the top it is the Sandspruit, Theespruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon complexes, which are bounded by regional shear zones (Furnes et al. 2012a and de Wit el at. 2010). The lavas in the lower part (Sandspruit, Theespruit, and Komati) and upper (Mendon) complexes are composed of komatiite, komatiitic basalt and high‐MgO basalts (MgO = ≥ 24 wt%, olivine dominated), while the middle part (Hooggenoeg and Kromberg) the high – to low‐MgO tholeiitic basalts are dominant. Felsic volcanic rocks and intrusions are common in the Theespruit and Noisy complexes. The ultramafic and basaltic lavas show a rare earth element pattern that are very similar to modern mid ocean ridge basalt (MORB), whereas the felsic rocks are moderately to strongly enriched in light rare earth elements (LREE) which are similar to modern arcs. (Furnes et al. 2012a and Brandl et al. 2006). The dominant rocks in Barberton are the greenstones belts. Away from the margins of the belt the igneous rocks have been altered and metamorphosed to upper greenschist facies. The rocks in the Komati Formation have been influenced by at least three types of metamorphism including sea‐floor metamorphism, burial metamorphism and dynamic (retrograde) metamorphism and each type overprint the other (Anhaeusser, 2014). The dominant mineral assemblages in these rocks are combinations of amphiboles like tremolite‐actinolite and hornblende, serpentine, chlorite, epidote, albite, quartz, illite and sericite formed under metamorphism at temperatures of 120 ‐ 510°C and pressures of 1 – 5 kb, (de Wit et al. 2010). According to Furnes et al (2012b) the tectonic model for the evolution of the Onverwacht suite is proposed as a horizontal lithospheric displacement, which consists of several fault‐bounded and tectonically stacked oceanic complexes that have been successively obducted in a piggy back fashion and is younger upwards. There are also a lot of layered and differentiated ultramafic ‐ mafic 15 intrusions that are associated with rocks in the Onvervacht group. They usually represent units of dunite, harzburgite, peridotite, pyroxenite, gabbro, norite and anorthosite. There are distinctive features of the ultramafic complexes which are different from other igneous intrusions, for example their large proportions of ultramafic components, their subalkaline character, the highly ultramafic nature of the parental magma and the high Mg contents of the olivine and orthopyroxene. However, according to Brandl et al (2006) there are still some questions about the origin of these layered ultramafics. They have been interpreted as subvolcanic, sill‐like feeder chambers, mantle tectonites which formed at the base of the oceanic lithosphere, extrusive cumulate bodies formed in massive komatiite rivers or lakes or Archean ophiolite‐like, ocean crust trapped in suture zones making the position of the collisional convergent plate boundaries. WesternKaapvaalcraton‐KraaipanandAmaliaGreenstoneBelt
The Amalia‐Kraaipan granite‐greenstone terrane is located in the northwestern part of the Kaapvaal Craton, fig 1. It consists of metamorphosed mafic volcanic rocks with interlayered metasediments like banded iron formations intruded by granitoid rocks composed of tonalitic – trondhjemitic – granodioritic (TTG) components. This terrane is poorly exposed due to its cover of the 2710 Ma Ventersdorp Supergroup volcanic rocks and Kalahari sediments (Poujol et al, 2001). The Amalia Greenstone Belt (AGB) is located southwest of the town Amalia and Sweizer‐Reneke and appears as a single, narrow, linear, N – S ‐trending structure, about 4 km wide and about 55 km long as shown in fig. 1. The belt is composed of altered and deformed mafic‐ultramafic rocks and some sedimentary rocks which are represented by amphibolite, quartz‐carbonate‐schists and talc‐
carbonate‐schists and these occur throughout the Amalia greenstone belt. The mafic metavolcanic rocks have komatiitic basalt and Mg‐tholeiitic affinities (Brandl et al. 2006). North of the Amalia greenstone belt is the Kraaipan group located, fig.1. It occurs as three narrow N – S ‐trending belts which are separated by a variety of granitic, gneissic and migmatitic rocks. The volcanic rocks consist largely of metamorphosed and hydrothermally altered, tholeiitic and andesitic basalts and tuffs. Today these rocks are composed mainly of altered massive schistose amphibolites and amphibole‐chlorite‐epidote schists. The whole Amalia‐Kraaipan region is largely deformed. Total lengths of these two belts are approximately 250 km starting from southern Botswana. But there are some indications that the gneisses and migmatites extend down to Kimberley because some are exposed in the mines of Bultfontein and Koffefontein areas in Kimberley (Poujol et al, 2001). According to Anhaeusser (2014) the linear belts show dome‐and‐keel patterns on a local scale, diapirism. Trondhjemitic gneiss plutons have intruded and split the belts into different slivers. That may be the reason why the Archean structural styles in gneissic basement terranes show rocks strongly and repeatedly deformed under amphibolite conditions and also show a wide range of ductile structures like different strained gneisses, migmatites and deformed dykes. According to Brandl et al. (2006) a high‐resolution airborne magnetic and radiometric survey data implies that the Amalia greenstone belt is not a single continuous belt but a number of isolated, N‐S‐
16 trending greenstone belt slivers surrounded by granitoid rocks, mostly unknown owing to the lack of exposure. They discuss that the rocks of Amalia greenstone belt have been affected by three phases of deformation, an initial compressional phase followed by a large scale folding and warping which has given rise to the pervasive cleavage seen throughout the belt. The last stage of ductile deformation was a prominent N –S ‐trending, right‐lateral shear zone. The mineral assemblage in this region suggests that the metamorphic condition has been between the upper greenschist and lower amphibolite facies. An evolutionary model of the crustal development proposed by Poujol et al (2001) is that the N ‐ S‐
trending Amalia‐Kraaipan group of volcano‐sedimentary greenstone succession and the granitoid rock may have accreted to the western edge of the Kaapvaal craton during a time period of 3250 – 2700 Ma ago. However, Brandl et al, (2006) suggest that the settings for the greenstone belt might have been a primitive inter‐arc or back‐arc basin which have accreted together or have been tectonically juxtaposed. Komatiites
Komatiites are rare ultramafic volcanic rocks that erupted mainly in the Archean era. A komatiite contains more than 18 wt% MgO and maximum 25 – 30 wt% MgO, however, this can be difficult to interpret because all the komatiites have been modified chemically by metamorphic processes. Serpentine and chlorite are the main alteration products (Wilson et al, 2003, Grove et al, 2003 and Parman et al, 2004). The MgO content of komatiitic magma is a key value, because it is directly proportional to the liquidus temperature which estimates the melting conditions and records of the komatiites (Parman et al, 2004). Komatiites are also characterized by their low incompatible element content, which is probably due to their high degree of mantle partial melting (30‐50%) (Wilson et al, 2003). Much evidence suggests that the early Earth was much hotter that it is today. The increased temperature from higher concentrations of radioactive isotopes and the heat generated from the segregation of the core were probably large enough to have melted the most of the silicate mantle (Grove et al, 2003). The komatiite magma has higher liquidus temperatures than any other modern magma and their chemical composition characteristics have been used to trace mantle melting, depths, temperatures and processes back into the Archean. They are used as primary evidence that the Earth´s mantle has cooled since the Archean and that the mantle conditions have changed in some way through geologic time (Grove et al, 2003 and Parman et al, 2004). According to Grove et al, (2003), komatiites are typically found interlayered with a variety of igneous lava in the crust attached to depleted cratonic lithospheric mantle. Komatiites represent the oldest ultramafic magmatic rocks preserved on Earth. Early hypotheses about how komatiites were produced envisaged a plume environment which explained why the mantle temperatures were so high (1400 – 1600°C) and the melting process so deep. But now an alternative proposal has been established that the komatiites were produced by hydrous melting at shallow melting depths in a subduction environment. This alternative interpretation predicts that the Archean mantle was only slightly, like 100 °C, hotter than today 17 (Grove et al, 2003). The plume model for the origin of the komatiites is not compatible with the subduction theory (Parman et al, 2004). If some komatiites contained H2O, where would the water originate from? Modern hydrous melts are produced in subduction zones by volatile flow. This can lead to H2O content as high as 10 wt% in some mafic arc magmas, because the subducting slab continuously releases volatiles to the magma. Komatiites have compositional similarities to or are interlayered with lavas similar to boninites. Boninites are produced by high degree of hydrous melting in subduction zones and are characterized by both high MgO and high SiO2 (Grove et al, 2003). Zircons
Zircons form in silica‐rich rocks and over 50 elements have been identified in zircons which include the rare earth elements, uranium and thorium. The radioactive decay of uranium and thorium may cause the zircon structure to become metamict. When zircons become metamict it is due to the alpha decay which disrupts the entire crystal structure in the mineral. Now they do not possess any crystallographic order and have become glass. Most grains are very small and are only visible through a microscope, the crystal structure is tetragonal prismatic and is elongated along the c‐axis with dipyramidal terminations. But detrital zircons that have been through transport may show euhedral to rounded forms (Nesse, 2012). Zircons are widely distributed in many igneous, sedimentary and metamorphic rocks, but are usually only visible through a microscope because of its small grain size. Zircon crystals are resistant to mechanical and chemical weathering and can survive the whole rock cycle. Rims can develop on the outer domain of the grain indicating different ages and environments (Nesse, 2012). Probably the most important element in zircons is U4+ which substitutes for Zr4+, this uranium provides the basis for radiometric dating using U/Pb method. By using this method, relatively precise age‐dating can be done. Of all the isotopic dating methods in use today, the U/Pb method is the oldest and the most reliable when done carefully. The uranium comes in two common isotopes with atomic weight of 235 and 238. Both are unstable and radioactive and become lead in the end. 235U becomes 207Pb and 238U becomes 206Pb and they do that in different rates depending on their half‐lives. The 235U‐207Pb has a half‐life of 704 Ma while the 238U‐206Pb has 4470 Ma, which is why these isotopes are so good for dating most rocks containing zircons. This technique together with the trace elements the zircon contains can often tell a geological story, for example the crustal evolution and development (Nesse, 2012 and Faure, 1998). Natural zircons typically have a closure temperature greater than 900 °C which can explain why they are such a good story tellers and capable of remaining isotopically closed through long periods of metamorphism and partial melting of the host rock (Lee, at al. 1997). According to Deloule et al, (2000), during extraterrestrial impacts zircons can be reset due to the extremely high pressure or temperature. The U‐Pb isotopic system during the impact are totally reset, which means when dating these zircons the age for the impact will be shown instead of the earlier or true age for the zircon. Also according to Cornell, (2014) zircons can be thermal reset during metamorphism due to diffusion above 900 °C. 18 Bobparola et al, (2005) studied the relationships between syn‐ and post‐ magmatic events and the partial resetting of the U‐Pb and trace element chemistry of zircons. They propose that the resetting of the U‐Pb system does not always erase completely the magmatic textures or the typical compositional signatures of the igneous zircons, which is why it can be difficult to see in Scanning electron microscope using backscattered electron and cathodoluminescence detectors if the zircons are reset. The probable mechanism mentioned here to explain the isotopic and elemental resetting is a solid‐state recrystallization facilitated by continuous intergranular fluid circulation in an open system. Resetting is more pronounced in foliated granitoids which were affected by late‐ to post‐
magmatic ductile deformation under subsolidus and wet conditions. Rareearthelementsandtraceelements
When the Earth´s mantle is melted, trace elements strives to be either in the melt phase or in the solid (mineral) phase. Trace elements whose preference is the mineral phase are described as compatible, while the elements whose preference is the melt are described as incompatible, they are incompatible in the mineral structure and will leave at first available opportunity (Rollinson, 1993). The incompatible elements can be subdivided of the basis of the charge/size ratio. Small highly charged cations are known as high field strength (HFS) cations and large cations of small charge are known as low field strength (LFS) cations. Low field strength cations are also known as large ion lithophile elements (LILE). Elements with small ionic radius and a relatively low charge tend to be compatible. High field strength elements – Sc, Y, Th, U, Pb, Zr, Hf, Ti, Nb and Ta. Low field strength elements (Large ion litophile elements) – Cs, Rb, K, Ba, Sr, Eu+2 and Pb+2 (Rollinson, 1993). The rare earth elements (REE) are the most useful of all trace elements. The rare earth elements contain a series of metals with atomic numbers 57 to 71 – Lanthanum to Lutetium. Scandium and Yttrium with similar radius are included. The low‐atomic‐number members are termed the light rare earth (LREE), La – Eu and those with higher atomic number are heavy rare earths (HREE), Gd – Lu (Rollinson, 1993). The rare earth element concentrations in rocks are normalized to a reference standard, which are the values for chondritic meteorites. Normalized multi‐element diagrams, or spider diagrams are based upon a grouping of elements incompatible with respect to typical mantle mineralogy. They are an extension of the chondrite‐
normalized rare earth element diagrams and contain more heterogeneous mix of trace elements. Mid ocean ridge basalt (MORB) ‐normalized spider diagrams are most appropriate for evolved basalts, andesites and crustal rocks – rocks to which MORB rather than primitive mantle could be parental (Rollinson, 1993). A Hughes diagram like that in fig. 24 can tell if a certain rock type is altered or not. There are different fields for Na‐altered, K‐altered and an igneous spectrum where the rocks are considered to be relatively unaltered. For samples plotting in the altered fields all diagrams including large ion lithophile elements should be used with caution. 19 Method
MappingandpickingsamplesfordatingundergroundtheDutoitspanand
Bultfonteinmine
Materials: Maps Bucket Brush Hammer Pencil, papers Camera Spray paint Tape‐measure Head lamps Compass Figure 6. Pictures showing the work procedure underground. A – Bultfontein open pit. B –Measuring strike and dip. C ‐ Drilling machine in a tunnel. D – Hitting the tunnel wall for samples. E – Drawing the geological features. F – Spray‐painting and marking the tunnels. Personal pictures, 2014. Procedure: The maps provided show the tunnels down at 760 meters below surface in the mines. They were used to get an overview where the geological mapping should take place. A tape‐measure was used to measure every five to ten meters in the tunnels and spray paint was used to mark the meter numbers. Geological features in between those were measured and marked as well. Tunnel walls were brushed due to dust and detailed mapping including mineral grain identification, shape and color, strike and dip, drawing of the major features of the tunnel wall and photos for documentation was done. Head lamp was used due to lack of other light sources in the tunnels. Rock samples were picked where there was a possibility to hit the wall with the hammer and to get a fresh surface at all sides of the rock. Six schist samples and three amphibolite samples were taken and put into the bucket and brought back up to the surface and back to Sweden. Work procedure is shown in fig. 6. 20 Makingthinsections
Materials: Big diamond rock saw – DIMAS TS 300E Small diamond rock saw – Logitech CS 10 Thin section cut‐off saw Pen, ruler Thin section glass Plastic padding Thin section machine Polisher Microscope Procedure: Rock samples were cut to a size of 22mm x 25mm x 35mm using two different rock saws. The rock pieces were glued to a thin section glass by plastic padding and had to harden for 20 minutes. A thin section machine was used to grind the samples down to 30‐35 µm and a polisher was used to polish the samples so no lines were showing from the grinder. A microscope was used to confirm that. Mineralidentificationusingthinsections
Materials: Microscope Thin sections Paper, pen Atlas of metamorphic rocks and their textures by B.W.D Yardley, W.S. MacKenzie and C. Guilford, Longman Scientific & Technical, 1990. Atlas of rock‐forming mierals in thin section by W.S. MacKenzie and C. Guilford, Longman, 1980. Procedure: Every thin section was evaluated under these criteria: Pleochroism, extinction, birefringence or color, relief and crystal habit. The name of the rock type, texture of the thin section and if there was any foliation was decided. If there were any minerals that were associated with each other, the modal percentage, which minerals that contribute to the foliation if it was any and if the rock was deformed was evaluated. An age relation was also a criterion, if it was any visible events that could have happened to the rock. Two mineral atlases were used for help in identifying the minerals and textures of the thin sections. Zirconseparationandwholerockanalysispreparation
Materials: Sledgehammer Broom Plastic bags Plastic container with lid A3 paper Rock crusher Jumbo Swing mill ‐ steel container for milling 21 Sieve 400 µm Tissues Compressed air Pan Glass beaker Oven Franz magnetic separator Procedure: Rock samples were crushed into 2 cm big pieces with a sledgehammer on a rock outcrop outdoors. The rock pieces were put into a plastic bag and labeled. One piece of the original rock sample was not crushed into smaller pieces, it was saved as a hand sample and put into the plastic bag. A broom was used to clean the area before next sample was crushed. A few pieces of the crushed samples were put into the Jumbo Swing mill container and crushed in the swing mill for one minute to decontaminate the container. The Jumbo Swing mill was cleaned with tissues and compressed air. The rest of the rock samples were poured into the mill container and crushed for 15 seconds. The coarsely crushed sample was gently poured onto two pieces of A3 paper and halved into a suitable amount for later whole rock analysis and put into the plastic container with lid. A 400 µm sieve was used to sieve the rest of the material for later zircon separation. The whole rock crushed sample was put back into the mill container and crushed one more time for two minutes to make a fine powder for later analysis. A gold pan was used for the 400 micron sized material to separate zircons if available. The zircons were poured into a small glass beaker and put into an oven to dry. The dry zircon sample was passed through a Franz magnetic separator to separate the magnetic grains from the nonmagnetic grains (including zircon and quartz) to make it easier to separate the zircons in the binocular microscope. Zirconseparationandpuckmaking
Materials: Microscope Tweezers Plastic disk Double‐sided tape Paper, pen Zircon sample Epoxy glue Polish‐paper 4000 mesh Procedure: Mineral grains were gently poured onto a plastic disk and zircons were handpicked under the microscope and put onto double‐sided tape in a linear pattern. Epoxy was mixed and poured over the finished disk with all zircons together with some other mineral grains and the resulting 25 mm diameter puck hardened for four days. A 4000 mesh abrasive paper was used to polish the puck. 22 ZirconidentificationusingScanningElectronMicroscope(SEM)
Materials: Puck Scanning Electron Microscope (SEM) – Hitachi S‐3400N Procedure: The puck was inserted in the Scanning Electron Microscope (SEM) and zircon images collected using backscattered electron and cathodoluminescence detectors to determine the internal structure of the zircons prior later Laser Ablation Inductively Coupled Plasma Mass Spectrometry( LA ICPMS) ‐
analyses. Glassmakingandwholerockanalysesformajorelements
Material: Rock powder sample Fritsch (Silicon Nitride) ball mill Scale Plastic spoon Ethanol Deionized (DI) water Oven Paper, pen Tissues Porcelain crucibles Desiccator Mo‐filaments Glassmaker Argon gas Epoxy glue Plastic disk 80 mesh and 1200 mesh SiC paper Scanning Electron Microscope (SEM) – Hitachi S‐3400N CIPW‐norm, written by Kurt Hollocher, Geology Department, Union College, Schenectady, NY Procedure: A scale was used to measure 1 gram of sample powder and 3 grams of deionized water that was put into the ball mill container for decontamination. The container was put into the mill machine for one minute at 800 rpm. The container was cleaned with deionized water and dried with tissues. For grinding, 5 grams of rock powder and 3 grams of ethanol were put into the ball mill container and in the mill machine for four minutes at 800 rpm. The container was put in an oven at 80°C until the sample had dried. The dried sample powder was put on a piece of paper and folded and labeled for later ignition. The ball mill container was cleaned and dried with deionized water and tissues. 23 One gram of the dried sample powder was put into porcelain crucible and weighed. It was dried for 30 minutes at 110°C for removing any alcohol. The sample was ignited at 960°C for 20 minutes and later cooled in a desiccator and weighed accurately again to calculate Loss of Ignition (LOI). The sample powder was put into C‐coated molybdenum boats and transferred into the glassmaker. Argon gas was used to flush the glassmaker chamber for one minute. Glass was made using 65 volts for 5 seconds or until all of the powder was melted. A few pieces of the glass were mounted on a plastic disk and epoxy was poured over to make a puck for later analyses in the Scanning electron microscope (SEM). 1200 mesh SiC paper was used to polish the puck after it had hardened. The polished puck was mounted in the Scanning electron microscope (SEM) and the major elements; Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, and Mo were analyzed using 200 seconds counts for each analysis and three analyses were made for each glass sample. An Excel sheet with the CIPW‐norm was used to calculate the normative minerals from the whole rock sample. RareEarthElementsandtraceelementsanalysesintheLaserAblation
InductivelyCoupledPlasmaMassSpectrometry(LA‐ICPMS)
Material: Puck with whole rock glass samples Glass standards; NIST610 – synthetic glass, BCR‐2G – Columbia River Basalt, BHVO – Basalt and JG‐1 – Japan Granite Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS): Laser Ablation – NWR 213nm Nd:YAG Inductively Coupled Plasma Mass Spectrometry – Agilent 8800 QQQ Glitter program Excel GUPET 2009 – Global Tectonic Diagram Sheet by Prof. David Cornell Procedure: The puck with whole rock glass samples and the standard pucks were mounted into the LA‐ICPMS and the analyses started. The settings for the laser are shown in table 1. Table 1. Table shows the settings for the laser Spotsize (µm) Frequence Warm up (sec) Dwell (sec) Washout (sec) Output (%) (Hz) 45 80 10 30 50 20 24 Table 2. Table shows the elements that were analyzed by the ICP‐MS
Isotope 29 31 39 44 45 47 49 51 53 55 57 59 60 63 65 66 69 72 75 85 88 89 90 93 95 111 Element Si P K Ca Sc Ti Ti V Cr Mn Fe Co Ni Cu Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Cd Isotope 133 137 139 140 141 146 147 151 153 157 159 163 165 166 169 172 175 178 181 182 195 197 208 209 232 238 Element Cs Ba La Ce Pr Nd Sm Eu Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Pt Au Pb Bi Th U First was all the standards analyzed followed by the first four of the whole rock glass samples. After that the NIST610, BCR‐2G and JG‐1 standards were analyzed followed by the next four of the whole rock glass samples, then the same standards again before the last four whole rock glass samples were analyzed and the final step was to analyze all the standards again. All elements analyzed are shown in table 2. Glitter program was used for data reduction of spikes in the diagram. Excel was used for analyses of accuracy of the standards compared to GeoRem preferred values (http://georem.mpch‐mainz.gwdg.de/sample_query_pref.asp, 2014) for the standards and for calculating the average ppm content and the standard deviation of the elements. The ppm calculations from the last excel sheet were placed in the GUPET 2009 ‐ Global Tectonic Diagram Sheet and different diagrams were made to show the results from the analyses. 25 ZircondatingbyLaserAblationInductivelyCoupledPlasmaMass
Spectrometry(LA‐ICPMS)
Materials: Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS): Laser Ablation – NWR 213nm Nd:YAG Inductively Coupled Plasma Mass Spectrometry – Agilent 8800 QQQ Puck with the zircons Zircon standards: GJ = 605 Ma (most common used for LA‐ICPMS analysis), KVT – Kaap Valley Tonalite = 3227 Ma and 91500 = 1064 Ma. Excel – Template for determining age of zircons, by Thomas Zack Isoplot 3.0, by K.R. Ludwig, Berkeley Geochronology Center, 8 Sep 2001, ver.2.49h. Procedure: The puck with the zircons and the standard pucks were mounted into the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) and analyses started. The settings for the laser are shown in table 3. First was the GJ and 91500 standards analyzed, followed by 10 of the zircons from the S5 sample. All the standards were analyzed again. After that the last three zircons were analyzed and finally all the standards again. The elements analyzed are shown in table 4. Data reduction was done for all the data from the analyses using an excel template, and Isoplot 3.0 was used to make Concordia diagram determine the ages of the zircons. Further details on U/Pb zircon dating in Gothenburg can be found in Cornell et al (2013). Table 3. Table shows the settings for the laser Output (%) Spotsize (µm) Frequence Warm up (sec) Dwell (sec) Washout (sec) (Hz) 50 20 5 15 40 5 Table 4. Table shows the elements that have been analyzed.
Isotope 202 204 206 207 208 232 238 Element Hg Pb Pb Pb Pb Th U Time (ms) 5 5 30 60 100 5 5 26 Results
Mapsfromthemine
These maps represent the tunnels of the Bultfontein and Dutoitspan mines at 760 meter depth below surface and the decline from 760 to 845 meters depth in Bultfontein mine, shown in figs. 7, 8 and 9. Every different section of the tunnels going towards the kimberlite pipes have been labeled by a letter (A‐O). Every five to ten meters is marked as numbers. The boundaries between different basement rock types are shown by yellow lines where the names of the rock types are written on either side of the yellow line, with different colors depending on the rock type. A geological feature is shown as a blue thick line across the tunnel at its exact location on the map. The strike and dip of the schist and of other geological features is written beside the schist area or beside the geological feature. A green star shows where a schist sample was taken and a blue star where an amphibolite sample was taken. Red lines show area demarcation. The scale is shown at the bottom of the maps. The most part of the tunnels consist of tonalite and amphibolite. There is often a transition zone of migmatite between the tonalite and amphibolite. In the D‐B tunnel of Bultfontein mine (the east haulage extension) there is a large schist area which covered the whole tunnel right before the kimberlite pipe, as shown in fig. 6. This made the tunnel walls weak and a lot of reinforcement was placed at that part. In the rest of the tunnels of Bultfontein and Dutoitspan the schist appeared as small Figure 7. Map of the tunnels in Bultfontein mine showing the geological bands and slivers, with similar strike, as boundaries and features, strike and dip of the schist and a star symbol where one rock sample was taken. Edited by Thomas Eriksson, 2014. seen in figs. 7 and 8. In tunnel B‐C in Bultfontein (fig. 7) and in tunnel A‐K in Dutoitspan (fig. 8) there are some kimberlite dykes, marked in pink, which cut through the tunnels. These might be fissures from the big kimberlite pipe close by (Robey, 2014). 27 In the decline of Bultfontein (fig. 9) the basement rocks consist of tonalite and amphibolite together with a transition zone of migmatite after 135 meters. There is a fine grained soft tonalite‐dyke after 230 meter. In the bottom of the map there is an arrow pointing towards a green star, which mean that a schist sample was taken there. This schist sample was taken from inside the kimberlite pipe at 845 meters below the surface. Figure 8. Map of the tunnels in Dutoitspan mine showing the geological boundaries and features, strike and dip of the schists and geological features and star symbols where rock samples was taken. Edited by Thomas Eriksson, 2014. 28 Figure 9. Map of the decline tunnel in Bultfontein mine. Edited by Thomas Eriksson, 2014. 29 Thinsectionresult
There are four schist samples and one amphibolite sample evaluated for this study. They were collected from the Bultfontein and Dutoitspan mines in Kimberley, South Africa. The exact location of the samples is shown on the maps of the different tunnels (figs. 7, 8 and 9). Sample A1 is taken from Dutoitspan mine from 760 meters depth in tunnel K‐L after 40 meter (fig. 8). In this tunnel tonalite and amphibolite are the dominate rock types. Located near the K‐L tunnel, sample S5 originates from tunnel M‐N after 25 meter (fig. 8). The schist´s strike and dip was 110/38°. Samples S2 and S3 are collected from tunnel I‐J after 97 meters (fig. 8). This spot was chosen because it was a new development in the Dutoitspan mine, and Petra Diamonds are planning to make a new tunnel there. Both samples are taken from the same location but on opposing sides of the tunnel. Here the schist had a strike and dip of 106/40°. The S1 sample was taken from Bultfontein mine from 845 meters depth inside a kimberlite pipe as shown in fig. 9. It was taken from Drift 6 and opening 22 in the diamond extraction pipe. All samples are related to each other because they are taken from the same mine, except the S1 sample. This sample may have been transported from deeper ground during the kimberlite eruption or it may have fallen down from the side wall rocks above. Mineral 1 – Chlorite Show pleochroism from pale yellow to green. Sheet silicate with undulose extinction. Birefringence 0,00 – 0,01. Moderate relief. Mineral 2 – Talc (or white mica) Show little pleochroism from pale yellow to light green. Sheet silicate with parallel extinction. Second order interference color with moderate relief. Mineral 3 – Brucite (or Serpentine) Colorless in plane polarized light with no pleochroism. Moderate relief. First order interference colors with low birefringence. Tabular crystals form as aggregates of fibers. Mineral 4 – Pyroxene Colorless in plane polarized light with no pleochroism. Moderate to high relief. Second order interference colors with maximum birefringence of 0,023. Twinning and zoning with possible overgrowth. Some grains has rims with different color than the core, the rims has higher birefringence. Parallel extinction. Inclusions of exolution lamellae of another mineral. Mineral 5 – Cordierite Colorless in plane polarized light with no pleochroism. High relief. First order colors – grey to milky white with maximum birefringence of 0,005. Tabular crystals with extinction. No cleavage. Mineral 6 – Quartz Colorless in plane polarized light with no pleochroism. Low relief. 30 First order colors with maximum birefringence of 0,005. Undulose extinction. Mineral 7 – Magnetite Opaque. Tabular crystals, some crystals with cubic structures. Mineral 8 – Zircon Colorless in plane polarized light with no pleochroism, but have a halo which shows pleochroism. High relief with extinction. Very small grains. Mineral 9 – Hornblende Show pleochroism from pale yellow to green. Moderate relief. Second order interference colors. Tabular crystals with extinction. Mineral 10 – Biotite Show pleochroism from pale yellow to brown. Low relief. Sheet silicate with cleavage. Second order interference colors. Elongated crystals with parallel extinction. Mineral 11 – Apatite Colorless in plane polarized light with no pleochroism. High relief. First order color with maximum birefringence of 0,003, grey. It shows extinction. Very small grains. Mineral 12 – Rutile Golden brown color in both plane polarized light and crossed polar light. Very small crystals which have needle‐like appearance. 31 The modal percentage is an approximation of the major minerals in each thin section. A1 – Biotite‐amphibolite with weak foliation and granoblastic polygonal texture with randomly oriented grains of apatite and rutile (fig. 10). The dominating mineral in this sample is green A
B hornblende with big tabular crystals. The biotite has sheet like crystals which are big with almost no signs of deformation due to alteration. The edges of the crystals are sharp and the biotite is randomly oriented throughout the thin section which shows a weak foliation. Some crystals of hornblende show deformation fractures, otherwise both hornblende and biotite crystals are euhedral to subhedral. The accessory minerals are apatite, rutile and brucite (serpentine). Apatite and rutile are randomly oriented throughout the sample. Apatite has stubby crystals with very small grain sizes, while the rutiles Figure 10. A) Plane polarized light and B) crossed polarized light pictures of A1 thin section. have needle‐like crystals and have even smaller grain sizes. The brucite (or serpentine) appear as a matrix which lies in between most of the hornblende and biotite in more or less the whole sample. 90% = Hornblende 8% = Biotite 2% = Apatite, Rutile and Brucite (serpentine) S5 – Pyroxene‐chlorite‐ talc schist with moderate foliation and decussate texture with crosscutting brucite (serpentine) (fig.11). Chlorite and talc are associated with each other and these minerals contribute to the foliation. These minerals dominate the thin section. Brucite (serpentine) crosscuts mostly in the direction of the foliation. The chlorite and talc displays a decussate texture which consists of oriented interlocking sheet like and elongated crystals that form the foliation. The pyroxenes and the cordierite have stubby, prismatic crystals and are spread out throughout the thin section except where the chlorite, talc and brucite (serpentine) are segregated into two parallel layers. The grains do not show much deformation due to alteration and the grains are A Figure 11. A) Plane polarized light and B) crossed polarized light pictures of S5 thin section. 32 B subhedral to euhedral. 65% = Chlorite and Talc (white mica) 25% = Pyroxene 10% = Brucite (Serpentine) and Cordierite S2‐ Pyroxene‐ talc schist with strong foliation and decussate texture (fig. 12). Talc is the dominating mineral and it contributes to the strong foliation in this thin section. The pyroxenes are inclined towards the foliation which also adds to the foliation. There is almost no chlorite, brucite (serpentine) or cordierite in this sample. Parts of the thin section have so small grain sizes that it looks like a matrix instead of separate grains, otherwise the crystals are bigger compared to S5. The edges of the mineral grains show signs of weathering which has not been too extensive but compared to S5, this sample is deformed due to alteration and the grains are subhedral to anhedral. A
B Figure 12. A) Plane polarized light and B) crossed polarized light pictures of S2 thin section. 75‐80% = Talc (white mica) 20% = Pyroxene 5% = Matrix S3 ‐ Zircon‐pyroxene‐talc schist with moderate foliation and decussate texture with crosscutting brucite (serpentine) (fig. 13). Talc is the dominating mineral and it contributes to the foliation. The pyroxenes are spread out throughout the thin section with different grain sizes as anhedral to subhedral crystals. There is almost no chlorite in the sample and the brucite (serpentine) crosscuts in the direction of the foliation. The zircons are very small and are scattered randomly throughout the sample. These show pleochroic halos in plane polarized light which is most distinct in the talc. Strain quartz is present at one edge of the thin section and it show signs of pressure during the crystallization. 33 A
B Figure 13. A) Plane polarized light and B) crossed polarized light pictures of S3 thin section. Cordierite is also present in the sample but at various places and as subhedral grains with different gran sizes. 60% = Talc (white mica) 25% = Pyroxene 10% = Cordierite, Quartz and Zircon 5% = Brucite (Serpentine) S1 – Cordierite‐pyroxene‐chlorite‐talc schist with weak foliation and highly deformed texture together with randomly oriented grains of magnetite (fig. 14). The dominating minerals in this thin section are chlorite, talc and pyroxene. Chlorite and talc crystals are very small compared to the other schist samples, but elongated and fibrous. The pyroxenes and the cordierites are bigger and have tabular crystals. The sample is highly deformed with a weak foliation. The pyroxenes have fragmented into smaller grains which contribute to the weak foliation together with the chlorite and talc. There is almost no brucite (serpentine) in the sample. The magnetite is scattered randomly throughout the sample with various grain sizes, the crystal from varies from almost euhedral to anhedral. B Figure 14. A) Plane polarized light and B) crossed polarized light pictures of S1 thin section. 25% = Chlorite 25% = Talc (white mica) 25% = Pyroxene 20% = Cordierite 5% = Magnetite 34 A
Majorelementsfromscanningelectronmicroscope(SEM)
The major elements were evaluated in weight percent and the values are shown in table 5. The major normative minerals in the amphibolite samples are plagioclase, diopside and olivine. The normative mineral diopside is truly clinopyroxene in the rock. There is normative nepheline in the samples which is probably due to alteration and an enrichment of potassium. In the schist samples the major normative minerals are plagioclase, orthoclase, hypersthene, diopside and olivine. The orthoclase is truly K‐feldspar and hypersthene is orthopyroxene in the rocks. There are also nepheline and magnetite in these samples. Table 5 Major elements from SEM in wt% Sample Na2O MgO A2 A1 S1 S2 S3 S5 1,82 1,59 0,43 0,95 0,59 0,96 13,58 13,41 28,13 22,03 22,22 22,21 Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 11,33 10,68 5,94 10,56 10,09 8,78 45,28
45,96
52,49
45,81
49,10
51,97
0,010 0,128 0,036 0,016 0,045 0,012 0,82 10,63 1,40 0,14 1,10 11,62 1,52 0,13 0,10 4,38 0,08 0,09 4,15 4,27 0,37 0,18 2,79 4,67 0,35 0,14 1,34 6,05 0,17 0,14 2,96 2,77 1,58 2,26 1,92 1,60 FeO FeOT 10,67 13,34
9,97 12,47
5,70 7,12
8,14 10,17
6,91 8,63
5,77 7,21
Escola (1915) suggested that the ACF diagram, fig.15 was a way to illustrate metamorphic mineral assemblages on a simplified three‐component triangular diagram. He focused only on the minerals that appeared or disappeared during metamorphism because those were acting as an indicator of the metamorphic grade. The three pseudo‐components A, C and F are calculated on molecular basis: A = Al2O + Fe2O3 – Na2O – K2O C = CaO – 3.3 P2O5 F = FeO + MgO + MnO (Winter, 2010) The ACF‐diagram in fig. 15 after Cornell (1989) and Escola (1915) shows that the amphibolites A1 and A2 plot in the basalt area while the schists plot very close to the ultrabasite corner but inside the metabasite area. Figure 15. ACF‐diagram from Escola, 1915 indicating that the schist samples plots close to the ultra basite corner while the amphibolites plots in the basalt filed.
35 The TAS‐diagram in fig. 16 after Le Maitre et al. (1989) shows that A1 and A2 are in the basalt area in the subalkaline series, that S1 plots in the basaltic andesite area, S3 in the basalt area and S5 plots in between the basaltic andesite and basalt area and all are in the subalkaline series. S3 plots in the alkaline series at the trachy basalt area. Figure 16. TAS‐diagram from Le Maitre et al., 1989 showing the samples plot in different fields. This result should be looked at carefully due to hydrothermal alteration. The AFM diagram (fig. 17) is most commonly used to distinguish between tholeiitic and calc‐alkaline differentiation trends in the subalkaline series (Rollinson, 1993). Evolution curves can be recognized on the AFM diagram, the parental, or primitive, magmas are closer to the MgO corner and the more evolved ones are closer to the alkalis corner (Winter, 2010). In the AFM‐diagram in fig. 17 after Irvine and Baragar (1971) all samples plot close to the MgO border. Figure 17. AFM‐diagram from Irvine and Baragar, 1971 showing that all samples plot close to MgO corner. According to Rollinson (1993) the Jensens cation plot in fig. 18, is a classification scheme for subalkaline volcanic rocks and is particularly useful for komatiites. The elements were selected for their variability within subalkaline rocks and for their stability under low grades of metamorphism. The diagram clearly shows the komatiites as a separate field apart from the basalts and from calc‐alkaline rocks and that is why it is so useful for Archean metavolcanics. The Fe + Ti‐Al‐Mg‐diagram in fig. 18 after Jensen (1976) shows that the amphibolites A1 and A2 plot in the komatiitic basalt field and the schists in Figure 18. Fe + Ti‐Al‐Mg‐diagram from Jensen, 1976 indicating that all samples are some kind of komatiite.
the komatiite area. 36 RareEarthElementandTraceelementusingInductivelyCoupledPlasma
MassSpectrometry(LA‐ICPMS)
All values from the analyses of the rare earth elements and trace elements are shown in table 6 and presented in ppm. The rare earth elements are presented on concentration vs atomic number diagram on which concentrations are normalized to the chondritic reference value, figs. 19 and 20 from Evensen et al. (1978). In the diagrams from Saunders and Tarney (1984) the elements are arranged into a large ion lithophile‐group (Rb, Ba, K, Th, Sr, La, Ce) followed by an high field strength‐
group (Nb, Ta, Nd, P, Hf, Zr, Eu, Ti, Tb, Y, Yb) shown in figs. 20 and 21. In the rare earth element (REE) diagram for the amphibolites (fig. 19), the diagram shows light rare earth element (LREE) depletion in the solid and heavy rare earth element (HREE) enrichment in the solid. In the rare earth element (REE) diagram for the schists (fig. 20), sample S2, S3 and S5 shows almost the same pattern as the amphibolites with light rare earth depletion and heavy rare earth enrichment in the solid. The S1 sample shows a bird wing‐type profile with an enrichment of both light rare earth and heavy rare earth elements. A bird wing‐type profile is a strong indicator for hydrothermal alteration (Ivanic, 2012). REE-pattern: Amphibolites
10
1
10
1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
A1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
A2
S1
Figure 19. Rare earth element‐pattern for the amphibolites in a diagram from which are chondrite normalized by Evensen et al., 1978. S2
S3
Figure 20. Rare earth element‐pattern for the schists in a diagram from which are chondrite normalized by Evensen et al., 1978. 37 REE-pattern: Schists
100
Sample / Chondrite
Sample / Chondrite
100
S5
In both spider diagrams (fig. 21 and 22) the overall trend of these diagrams is that there is an increase of large ion lithophile elements (LILE) which are the more mobile elements, and decrease of the high field strength elements (HFSE) which are the more immobile elements. This is a result of hydrothermal alteration. There are low values for P, Nb and Sr which can be explained by the minerals found in the thin sections. All values from the rare earth element and the trace element data is shown in table 6. Spider diagram: Schists
Spider diagram: Amphibolites
100
100
Sample/ MORB
1000
Sample/ MORB
1000
10
1
1
0,1
0,1
Ba Th Nb La
A1
Sr
P
Zr
Ti
Y
Ba Th Nb La
Yb
S1
A2
S2
Sr
P
Zr
S3
Ti
Y
S5
Figure 22. Spider diagram for schists with mid ocean ridge basalt (MORB) normalization from Sauders and Tarney, 1984. Figure 21. Spider diagram for amphibolites with mid ocean ridge basalt (MORB) normalization from Sauders and Tarney, 1984. 38 10
Yb
La/Sm 0,79
2,65
0,58
0,71
0,97
0,99
La/Yb 2,35
3,35
1,13
4,39
4,03
2,77
A2 A1 S1 S2 S3 S5 Sample A2 A1 S1 S2 S3 S5 39 0,95
1,14
3,65
2,88
1,93
2,43
29,14 102,94 3,53 319,28 228,04 91,76 Tm 2,40
2,91
0,53
1,44
1,75
1,19
Gd/Yb 0,71
2,26
0,70
0,81
1,06
1,11
1,66 7,56 0,79 3,53 4,25 3,07 Yb 229,41 220,32 2643,92 2040,54 2803,00 2306,66 La Cr 293,24 111,95 247,58 203,20 250,10 107,11 K/Rb 0,09
0,29
0,11
0,12
0,15
0,15
5,92 23,71 1,54 8,65 12,66 8,10 Lu 29,49 81,43 78,18 53,64 63,11 69,41 Ce Co 0,30 0,79 0,87 0,56 0,43 1,18 0,37 1,37 0,34 0,95 0,93 0,44 Rb/Ba Hf 1,03
3,90
0,18
1,13
1,94
1,07
113,79 281,48 1121,04 618,00 777,29 810,01 Pr Ni 0,33 0,98 0,14 12,14 8,61 2,32 0,07 0,15 0,04 0,27 0,09 0,39 Rb/Sr 6,04 21,99 0,76 5,33 9,84 4,94 Ta 57,95 43,26 6,77 3,66 1,62 0,61 Nd Cu 0,90 0,81 6,00 0,05 0,05 0,51 0,65 1,48 1,12 0,57 0,96 2,42 Sr/Ba Pb 1,74
6,66
0,22
1,23
2,21
1,26
45,05 159,71 55,31 113,38 111,64 73,18 Sm Zn 748,67
1626,23
4069,69
3588,45
4347,37
3764,28
0,03 0,21 0,22 0,49 0,30 0,25 REE total Th 0,52
1,81
0,08
0,43
0,76
0,34
8,76 81,84 3,05 178,17 97,50 108,12 Eu Rb 0,10 0,21 0,36 0,87 0,25 0,75 U 1,70
6,57
0,37
1,16
1,85
1,32
26,23 83,44 21,14 14,68 11,33 46,57 Gd Sr 0,26
0,97
0,09
0,19
0,28
0,25
7,63 25,12 5,03 7,20 9,92 10,22 Tb Y 1,54
5,47
0,70
1,21
1,66
1,63
9,00 30,95 11,28 35,53 31,31 10,03 Dy Zr 0,31
1,06
0,18
0,25
0,34
0,35
1,75 3,62 0,34 3,28 1,88 2,52 Ho Nb 0,11
0,35
0,09
0,11
0,15
0,15
1,35 30,51 1,27 43,84 11,16 82,97 Er A2 A1 S1 S2 S3 S5 Sample 149,34 351,39 91,60 110,18 134,66 101,95 Ba 16,13 41,54 18,75 19,06 24,29 21,29 Cs A2 A1 S1 S2 S3 S5 Sample V Sc Sample Table 6 Trace elements and REE in ppm In the diagram shown in fig. 23 from Pearce et al, (1977) the amphibolites plot in the ocean island (OI) area while the schists plot in the mid ocean ridge basalt (MORB) area. In Hughes (1973) diagram shown in fig. 24, every sample plot in the potassium ‐
altered field. Hughes discuss that every normal rock should plot in the field in the middle, the igneous spectrum area, which means that the rocks is relatively unaltered. In this study all samples are altered and the alkalis have drifted away during alteration. Figure 23. This diagram from Pearce et al, 1977 shows a result of the geological setting of the samples which is not trustworthy due to hydrothermal alteration. Figure 24. An igneous spectrum from Hughes, 1973 showing that all samples are K‐altered. 40 Geochronology
The zircons in this study are picked from the S5 sample which was taken from the Dutoitspan mine in tunnel M‐N after 25 meters as shown in fig. 8. The zircons are most likely inherited from the surrounding rocks because zircons can only form in silica‐rich rocks. In low silica rocks Baddeleyite (ZrO2) is more stable. Backscattered electron images are shown in figs. 25 and 27 together with cathodoluminescence images in figs. 26 and 28 which show the internal structure of the zircons. Light colors in the backscattered images means that the zircons have high density, and light colors in the cathodoluminescence images means that the zircons contain less uranium. Rims can be seen in figs. 25 and 26 which have developed during time, but for this study the cores of the zircons were in focus. The red circles seen in figs. 25 and 27 are the locations where the electron beam from the laser ablation inductively coupled plasma spectrometry (LA‐ICPMS) analyzed the zircons. Those places were picked because the density was high, it contained less uranium and it was no deformation cracks there. The zircons with letter “M” are metamict and those were not evaluated, shown in figs. 25 and 27. Zircon grains containing high levels of uranium are dark in the cathodoluminescence images which are due to the alpha decay, and the grains are therefore often metamict. There were also a lot of the zircons found in the S5 sample that were deformed and could not be used for analyzes either. That is why the result comes from 13 zircons in the S5 sample. M Figure 25. Backscattered electron image of zircons from the S5 sample. Figure 26. Cathodoluminescence image of the same zircons from the S5 sample. 41 M M M M Figure 27. Backscattered electron image of other zircons from the S5 sample. Figure 28. Cathodoluminescence image of the same zircons from the S5 sample. In the Concordia age diagram shown in fig. 29 from Ludwig (2001), the data‐point error ellipses are 1 sigma. The blue ellipses are calculated from the age ratios from the 207/235 and 206/238 ages (table 7), while intercept ages are similar but not relevant because of the small spread of data points. The green ellipse shows the Concordia age which is 2908 ± 18 Ma. Figure 29. A concordia diagram from Ludwig, 2001 which shows an age of 2908 ± 18 Ma for the S5 sample. 42 Table 7 Age ratios from U/Pb dating of zircons in the S5 sample Sample Ratio 207/235 1σ Ratio 206/238 1σ rho Age 207/235 1σ Age 206/238 1σ S5
S5
S5
S5
S5
S5
S5
S5
S5
S5
S5
S5
S5
18,2110
16,5126
17,3504
16,6121
16,7903
16,1788
16,7951
15,6566
16,7472
16,6580
16,0715
17,6528
16,1606
0,4824
0,4289
0,4564
0,4375
0,4236
0,4750
0,4243
0,4029
0,4166
0,4289
0,4087
0,4627
0,4082
0,6128
0,5638
0,5910
0,5595
0,5716
0,5492
0,5709
0,5352
0,5695
0,5648
0,5448
0,6034
0,5543
0,0095
0,0083
0,0090
0,0075
0,0068
0,0066
0,0068
0,0069
0,0064
0,0070
0,0062
0,0070
0,0067
0,38
0,32
0,29
0,39
0,30
0,58
0,30
0,34
0,26
0,34
0,30
0,36
0,30
3001
2907
2954
2913
2923
2887
2923
2856
2920
2915
2881
2971
2886
26
25
26
26
24
28
25
25
24
25
25
26
24
3081
2883
2994
2865
2914
2822
2911
2763
2906
2886
2804
3044
2843
38
34
36
31
28
28
28
29
26
29
26
28
28
43 Discussion
The clear absence in the Kimberley block of the crustal components that are older than 3300 Ma is present in the Witwatersrand block. This is taken as a confirmation that the Kimberley block had a distinctive origin and an evolutionary history from the Witwatersrand block. Subduction along the convergent margin became a collisional accretion of the Kimberley and Witwatersrand blocks which formed an N‐S trend in the Kimberley block rocks close to the lineament. The Kraaipan sequences are badly disrupted and deformed with an N –S trending rocks of low grade metamorphic greenstones and higher grade gneisses proximal to the belts (Schmitz, 2004). There are some differences between these two blocks which give clues to a subduction theory, where voluminous magmatism, pervasive deformation and mid‐crustal high‐grade metamorphism at 2930 to 2880 Ma are visible in the Kimberley block but absent in the Witwatersrand block (Schmitz, 2004). Cornell et al. 2011 found cobbles from the west Kaapvaal granite with an age ranging from 3008 ± 4 to 2940 ± 4 Ma and also eleven cobbles with ages between 2500 and 2900 Ma, which have good correlation to an accretion event between the Kimberley terrane and other Kaapvaal terranes. Hydrothermally altered granite cobbles dated 2928 ± 4 Ma at Mabuasehube in central Botswana were found and belongs to the Kimberley Terrane which probably also reflects this accretion event. This event is due to the subduction between the western Kaapvaal and the Archean core of the Rehoboth Province when they joined about 2500 Ma ago. Before the terranes were joined both had a long history of Archean plutonism, starting with trondhjemites from 3,3 Ga to 2,9 Ga and then joined by granites until 2,5 Ga. Especially two granite intrusion events dated 2854 ± 7 Ma and an older at 2882 ± 7 Ma, are interpreted as granites evolved from trondhjemites by fractionation, and are important events in the Kimberley Terrane. Therefore most rocks in the Kaapvaal Craton display different amounts of alteration, deformation, metamorphism, metasomatism and serpentinization. Most of the Barberton intrusions seem to have been derived from Mg‐rich magmas which most likely are komatiite and some of the intrusions has been through alteration involving serpentinization and steatization of the olivine and orthopyroxene in the rocks. The alteration effects can be associated to the metasomatizing fluids, which generally are hydrothermal or magmatic fluids. The fluids can have different compositions and may be sourced from rocks in close proximity to the formation that have been altered, or they may have migrated along faults and fractures or even through permeable volcano‐sedimentary rocks and breccias (Anhaeusser, 2014). Most of the ultramafic rocks in the Barberton group and the greenstone belts in the western part of the Kaapvaal Craton have undergone serpentinization or variable degree of carbonate alteration. The serpentinization or steatization can also be seen in the mineral assemblage in the rocks of this study, as shown in the thin section. 44 Maps:
The maps (figs. 7‐9) show the tunnels at 760 meters depth of Bultfontein mine, Dutoitspan mine and the decline from the 760 meters depth down to 845 meters depth in Bultfontein mine. Every five to ten meters was mapped and also if there was a geological feature in between those five or ten meters. The maps present the boundaries between different rock types, geological features like kimberlite dykes and the strike and dip of the schist. The maps and the detailed mapping of the side wall rocks were done according to the manager and the geologists of the mines preferences. Petra Diamonds who own the mines in Kimberley are interested in knowing where the schist boundaries are located because the Bultfontein mine will most likely be sealed off due to lack of diamonds and therefore knowledge of the most strategic and safest locations for placement of plugs in the tunnel is necessary. The maps also show were all the amphibolite and schist samples for the analyses was taken. The overall picture of the basement rocks in these mines is difficult to distinguish, but the amphibolite and tonalite have a foliation that can be interpreted as a north‐ south direction. Thinsections:
Amphibolite There is only one amphibolite sample evaluated in this study so no comparison is done. The A1 sample (fig. 10) is dominated with green hornblende. The age relations in the amphibolite sample show at least two events of deformation. The first event when fluids entering the host rock forming the brucite (serpentine) which lies mostly in the triple junction between the hornblende and biotite crystals, followed by a deformation event which is shown in the fractures in some of the hornblende crystals. The last deformation event could not have been so extensive due to the weak foliation the sample shows. The apatite and rutile have such a small grain sizes so it is difficult to evaluate them in a correct manner. Apatite is a phosphate mineral and rutile is a titanium dioxide which is a common accessory mineral in metamorphic rocks. Schist It is difficult to distinguish between talc and white mica in thin section but after studying the hand sample and the difficulties grinding the rock to fit for thin sections, talc is the most likely mineral in these samples compared to white mica. The hand sample has a distinctive greasy feel and it is very soft. As seen in figs. 11 and 12 for sample S5 and S2 there are some holes in the thin section due to the grinding of the very soft rock. This confirms that the mineral is most likely talc. The age relations in the schist samples show at least three events of deformation. First, the host rock has been through a thermal alteration when most of the pyroxenes have altered to form mostly chlorite and talc. This first event is followed by fluids entering the rocks and formed the crosscutting 45 brucite (or serpentine) and finally a deformation event which made the foliation of the samples seen in figs. 11‐14. The S1 sample has at least one more event of deformation, from the kimberlite pipe. The rocks inside the pipe got hydrated through the heat and fluids from the eruption and therefore this sample is more altered as seen in fig. 20 in the rare earth element diagram. The grain sizes in the samples varies from S2 and S3 which has the biggest grains, followed by S5 and the smallest overall grain size has sample S1. Zircons were found in sample S3 (fig. 13) as very small grains and in plane polarized light a pleochroic halo was most distinctive in the talc. This halo is due to radioactive decay from the uranium in the zircons. The zircons for dating in this study were taken from the S5 sample (fig. 11) but in the thin section no zircons were found there. That is the reason why so few zircons were evaluated for the dating and why only one rock sample was chosen to represent every schist sample. The S1 sample (fig. 14) has the opaque mineral magnetite. This sample was the one that was taken from inside the kimberlite pipe and may have been through an extensive hydration and metamorphic transformation that could have formed magnetite through a process called serpentinization. The magnetite minerals were scattered randomly throughout the sample with various grain sizes. Some crystals were almost euhedral while some were anhedral and deformed with cracks in the bigger grains. In the rare earth element (REE) diagram (fig. 20), the S1 sample shows a bird wing type pattern which is most likely due to hydration and the metamorphic transformation the rock has been through in the kimberlite pipe. The S2 and S3 samples (figs. 12 and 13) were taken from the same part of the new development in the Dutoitspan mine but from opposing sides of the tunnel as shown in fig. 8. In the thin section they look a bit different and they contain different minerals. This can be a coincidence because a thin section shows a very small part of the host rock. In S3 (fig. 13) zircons, quartz, cordierite and crosscutting brucite (serpentine) were found, compared to S2 (fig. 12) none of these minerals were found. At some places in the S2 thin section, the grain sizes was so small it looked like a matrix, this matrix was not shown in S3. Even if the samples were taken from almost the same spot, the textures and mineral content can differ. This shows the complexity of the schist formations. MajorelementsfromScanningElectronMicroscope(SEM):
In every glass sample it looks like there has been a movement in the glass which may have happened during the heating of the whole rock powder in the glass making chamber. In the S2 sample there are olivine crystals, which probably developed during the cooling of the sample. It was believed that the dark areas in the whole rock glass had lesser amount of molybdenum contamination which is why two of the samples were analyzed in several places, trying to decrease the molybdenum. Molybdenum contamination is due to the Mo‐boats used in the glass making. The normative nepheline in the amphibolite samples is probably due to alteration and an enrichment of potassium. This enrichment could indicate that the amphibolites are alkali basalts which have a rift or plume relation. The magnetite in the schist samples can be produced from peridotites and dunites by serpentinization. 46 This major element composition show that the samples have been altered, and most likely potassium altered, which is also confirmed in the Hughes diagram shown in fig.24. ACF‐diagram, fig. 15. In the analyses, A1 and A2 plot in the basalt field probably because the intermediate content of MgO which is about 13 wt% compared to the schist samples the content of MgO is about 22 wt% (see table 5) and they all plot close to the ultrabasite corner in the metabasite field. According to Winter (2010) the minerals talc, orthopyroxene, tremolite‐actinolite plot in the ultrabasite corner which match the minerals in the schist samples. This indicates that the rocks have been through the greenschist facies metamorphism. Hornblende plots in Winter´s diagram where the amphibolite samples are plotted, which also match the results and shows that these rocks have been through the amphibolite facies. Total alkalis‐silica diagram (TAS), fig. 16. The TAS diagram is one of the most useful classification schemes for volcanic rocks. It divides rocks into ultrabasic, basic, intermediate and acid on the basis of their silica content, (Rollinson, 1993). According to Rollinson (1993) the TAS classification is not appropriate to use when dealing with both K‐rich and Mg‐rich rocks or with weathered, altered or metamorphosed volcanic rocks because the alkalis have likely been mobilized. The classification is intended for the fresh volcanic rocks. That is why, when looking at the diagram (fig. 16), the result of this analysis should be read carefully. However, the result implies that most of the samples are basalts or basaltic andesite which the ACF diagram suggests as well. But all the samples should probably have plotted closer to each other. Their dispersion is probably due to alteration and the diagram is not trustworthy. AFM‐diagram, fig. 17. The result indicates that the schist samples S1, S2, S3 and S5 are more primitive than the A1 and A2 which are more evolved. This may be true according to previous diagrams, but the result should be evaluated with care because of the hydrothermal alteration these rocks have been though. This indicates that all the samples contain quite a lot of MgO and therefore plotted close to that corner. The more mobile alkalis could have disappeared during alteration. Fe + Ti‐Al‐Mg‐diagram, fig. 18. The A1 and A2 plot in the komatiitic basalt field and the schists plot in the komatiite area, which show that these rocks are from the Archean time when the Earth’s interior was much hotter than it is today. It also shows that the rocks must be between 3700 Ma and 2700 Ma when the Kaapvaal craton became stable. Komatiitic basalts usually form during the same time and are chemically alike to komatiites but are generally thicker than the komatiite flows. Because these rocks have such similarities as the komatiites it is suggested that a genetic relationship of the basaltic varieties may represent simple products of fractional crystallization of the komatiitic magma. Komatiitic basalts have melting temperatures as much as 400 °C lower than the komatiites and have similar eruption temperatures to present‐day mid ocean ridge basalts at 1200 °C (Anhaeusser, 2014). This diagram is probably true even if the samples have been through hydrothermal alteration. 47 RareEarthElementandTraceelementusingInductivelyCoupledPlasma
MassSpectrometry(LA‐ICPMS)
Rare Earth Element The rare earth elements have very similar chemical and physical properties. The small differences in size and behavior are exploited by a number of petrological processes causing the REE series to become fractionated relatively to each other. This phenomenon is used to unravel petrological processes and to find the genesis of rock suites. Rare earth element concentrations in rocks are usually normalized to a common reference standard, which most of the time is the values for chondritic meteorites. Chondritic meteorites were chosen because they are thought to be relatively unfractionated samples of the solar system (Rollinson, 1993). In the rare earth element (REE) diagram for the amphibolites (fig. 19), the diagram shows light rare earth element (LREE) depletion in the solid which suggest magmatic or hydrothermal alteration. The diagram also shows heavy rare earth element (HREE) enrichment in the solid which also mean that the sample has been through alteration. In the rare earth element (REE) diagram for the schists (fig. 20), sample S2, S3 and S5 show almost the same pattern as the amphibolites with light rare earth depletion and heavy rare earth enrichment in the solid. The S1 sample shows a bird wing‐type profile. A bird wing‐type profile is a strong indicator for hydrothermal alteration (Ivanic, 2012). This bird wing‐type pattern is due to hydration and metamorphic transformation from the kimberlite eruption and it is most likely to be the reason for the different pattern in the diagram. It was only the S1 sample that was taken from the interior of the kimberlite pipe. When the light rare earth element (LREE) are depleted it acts the same as a mid ocean ridge basalt (MORB) and when there is no depletion in heavy rare earth element (HREE) it implies that it has not been any garnets in the mantle and therefore not a deep melting. This result may confirm the theory about a subduction related event. The amphibolites may not be as affected of the hydrothermal alteration due to its pattern in the REE diagram, because it does show a correct image if the primitive mantle were plotted in the diagram as well. Normalized multi‐element diagrams or incompatible element diagrams (spider diagrams) Elements are often strongly controlled by individual minerals. For example, Zr concentrations may be controlled by zircon, P by apatite, Sr by plagioclase, Ti, Nb and Ta by ilminite, rutile or sphene. Negative Nb anomalies are characteristic of the continental crust and may be an indicator of crustal involvement in magma processes (Rollinson, 1993). In both spider diagrams (fig. 21 and 22) there are low values for P, Nb and Sr which can be explained by the minerals found in thin section. Apatite and rutile were found in the A1 sample (fig. 10) but plagioclase was not found in any of the thin sections. The overall trend of these diagrams is that there is an increase of large ion lithophile elements (LILE) and decrease of the high field strength elements (HFSE) which are a result of hydrothermal alteration. 48 Komatiitic magmas are thought to have had extremely high liquidus temperatures (1400 to 1600 °C) and to have been frequently contaminated with continental crust. Trace elements support this hypothesis. The low Nb value in the spider diagrams (fig. 21 and 22) may confirm that. Some authors proposed that Archean and early Proterzoic basalts erupted on the Kaapvaal craton in Southern Africa were komatiitic in origin but contaminated with continental crust (Rollinson, 1993). It may have been better to normalize the diagrams with primordial (primitive) mantle numbers from Wood et al. (1979), but because of the highly altered rocks it would probably not have made a big difference in the result. In both the igneous spectrum (fig. 24) and the spider diagrams (fig.21 and 22) it shows a clear K‐
alteration. Late hydrothermal alteration of both schist and amphibolite has most likely occurred due to the same pattern in the rare earth element (REE) diagrams and the spider diagrams. The result may also indicate that their protolith is from the same mantle. FeO‐MgO‐Al2O3 diagram, fig. 23. The diagram shown in fig. 23 from Pearce et al. (1977) shows the amphibolites plot in the ocean island (OI) field and the schists in the mid ocean ridge basalt (MORB) area. This diagram is not trustworthy due to the hydrothermal alteration the rocks have been through. Igneous spectrum, fig. 24. A Hughes diagram as shown in fig. 24 shows if a rock is altered or not. An unaltered rock plots in the igneous field in the diagram, and altered rocks in one of the two other fields, depending on whether it is Na‐altered or K‐altered. Only rocks within the igneous spectrum can be correctly interpreted. Rocks which plot in the altered fields have a major element composition that have changed and cannot be correctly interpreted in classification diagrams based on mobile major elements. As seen in fig. 24 all samples from this study plot in the K‐altered field which means that every sample has been altered. The alkalis have drifted away during the alteration and there are now low amount of alkalis left. Geochronology ‐ Concordia diagram, fig.29. Many of the zircons found in the S5 sample were deformed and metamict and that is why only 13 zircons were picked for analyzes in this study. The zircons were picked from one schist sample because all schist samples were thought to have the same origin. Also, as seen in the thin section S3 (fig. 13), the zircons are randomly oriented and very small and no zircons were found in thin section S5 (fig 11) which mean that there is no confirmation that zircons are found at all in the hand sample. Zircons can only form in silica‐rich rocks because in low silica rocks Baddeleyite (ZrO2) is more stable. Baddeleyite can be found in igneous rocks containing potassium feldspar and plagioclase, such as mafic rocks. That is taken as a confirmation that the zircons in this study must have been inherited from the surrounding rocks. 49 The age from the zircons in sample S5 shows a Concordia age of 2908 ± 18 Ma. The intercept age is similar but not relevant because of the small spread of data points. There are also some ellipses that plot above the age line which are probably due to some experimental errors. The age from this Concordia diagram coinciding with the collisional event which happened for 2900 Ma ago, when the Witwatersrand block and the Kimberley block collided through most likely subduction. This can mean that the komatiitic lava erupted at the same time, resetting the zircons which now show the correct age of the event. The age also coincides with the plutonism which happened during the time when the Rehoboth Province and the western Kaapvaal Craton collided. Almost all greenstone belts in the Kaapvaal Craton have been metamorphosed to a greater or lesser extent together with different stages of deformation and intrusions by granitic rocks and other igneous bodies. Factors that affect the metamorphic grade include the size and position of the greenstone belts on the craton, relative to the adjacent, high‐metamorphic grade terranes. Many of the smaller greenstone belts occur as schist belts which are surrounded by gneissic and granitic rocks and are generally metamorphosed to greenschist or amphibolite facies (Anhaeusser, 2014). In Komati Formation, in Barberton greenstone belt, the rocks have been through at least three types of metamorphism and each type overprint the other. There are no systematic metamorphic criteria which fit all greenstone belts, because there are many variables that can affect the nature of metamorphism. Greenstone belts that have been intruded by granitic rocks usually show contact metamorphism with affecting areas ranging from a few meters up to several kilometers wide. Mafic rocks near the contacts are usually transformed into black or dark‐green hornblende or actinolite‐
bearing amphibolites, and ultramafic rocks are altered to talc‐ or talc‐chlorite schists. Depending on the distance from the contact (or the heat source) the rocks display a decreasing metamorphic grade further away from the source (Anhaeusser, 2014). 50 Conclusion
There are different theories about how the Kaapvaal craton became the stable craton as we see it today. Most authors agree that it has been divided in several subdomains that have welded together and become stable at around 2700 Ma ago. In between the Witwatersrand and Kimberley block there is a mid‐crustal suture zone called Colesburg Lineament which is a magnetic anomaly that indicate that these two blocks have been two separate blocks which have collided. On either side of the anomaly the blocks have different rock contents with different ages, which confirm the theory of their origin. The different greenstone belts in Kaapvaal Craton appear to have formed over a time span of approximately 800 Ma, from about 3500 to 2700 Ma. The oldest greenstone unit on the craton is in Barberton Greenstone Belt on the Witwatersrand block with an age of about 3500 Ma. The youngest belts appear to have developed in the western part of the craton in the Kimberley block. Those belts have an age from about 3100 Ma to 2700 Ma before the Ventersdorp volcanic eruption took place and covered the whole area with lava. But the history about the greenstones has to be evaluated together with Archean granitoid rocks which they are closely associated to. It has been suggested that there was a subduction event and a collision between the Kimberley block and the Witwatersrand block along the Colesburg magnetic anomaly lineament, which happened about 2900 Ma ago. The Witwatersrand block subducted under the Kimberley block. Also during that time, starting from 3300 Ma to 2500 Ma, there was massive plutonism in the Kimberley terrane when the Rehoboth Province and the western Kaapvaal Craton collided, and their suture zone are located at the Kalahari line. The greenstone belts in the Witwatersrand block have an ENE‐trend and the belts in the Kimberley block have an N‐S‐trend which can act as an indicator that these two blocks have been separate at one stage. Otherwise the N‐S‐trend may have appeared when the Kimberley block collided with the Witwatersrand block. The linear belts of the Kraaipan group show dome‐and‐keel patterns on a local scale, also called diapirism. Trondhjemitic gneiss plutons have intruded and split the belts into different slivers. That may be the reason why the Archean structural styles in gneissic basement terranes show rocks strongly and repeatedly deformed under amphibolite conditions and also show a wide range of ductile structures like different strained gneisses, migmatites and deformed dykes. The age of the zircons in this study are 2908 ± 18 Ma old. This shows that the zircons are probably inherited from the surrounded rocks when the komatiitic lava erupted. Due to the closure temperature of the zircons, which is about 900°C and the komatiitic lava which have a temperature between 1400 ‐ 1600°C when it erupts, it may reset the zircon age depending on the how long time the zircons were in the hot komatiitic magma before it cooled. This subduction event may have triggered the eruption of komatiitic lava, which could have erupted all the way to the surface or it could have stayed inside the basement rocks as sills or lenses. If the lava stayed in the basement it may be possible that the lava kept its temperature for a longer time and the cooling was slow, which can be a reason why zircons were found reset in one sample in this study. Or the resetting of the zircons could have been a result of the thermal diffusion from the plutonism. 51 If the zircons have been inherited from surrounding rocks they would most likely been older, but now they show the age of the event that made them reset. The main rock types in the mines are tonalite, amphibolite and migmatite. The east haulage extension in Bultfontein mine was dominated by greenstone schist, however, in the rest of the tunnels in both mines the schist appeared as small belts and slivers. At different locations in the mines the boundaries between the different rock types were difficult to distinguish,but the geological features were very distinct. According to the result, it is likely that both the schist and the amphibolites comes from the same mantle and therefore have the same protolith of an ultramafic rock, most likely komatiite. This may be true because of the greenstone belts in the Barberton suite and Amalia‐Kraaipan group also have some komatiitic layers in their basement rocks. The result also show that both the schists and the amphibolites have been through a massive hydrothermal alteration some time during their existence together which probably resulted from two heat periods from the Bushweld Complex and the Vredefort Dome, about 2000 Ma ago. 52 Acknowledgements
I would like to express my very great appreciation to my supervisor here in Sweden, Professor David Cornell for helping me from the start with this project and for his patient guidance, enthusiastic encouragement and useful criticism throughout this research. My grateful thanks are also extended to my supervisor in South Africa, Doctor Jock Robey for his help, local knowledge and social and professional contacts in Kimberley and Cape Town. I would also like to thank the Group Geology and Mineral resource Manager at Petra Diamonds, Andrew Rogers, for letting me and my colleague work down in the mines and also to geologists Charity Mampa and Wiehan Smit for helping and supporting us at the mine. I would also like to thank the staff at Petra Diamonds for the time and help all of you gave us and also special thanks to the staff at the department of Earth Science, Gothenburg University for helping me running the analytical equipment. Special thanks should be given to the Swedish International Development Cooperation Agency (SIDA) which contributed financially to my field work in South Africa through a scholarship. I would also like to thank my colleague Caroline Lundell for our fantastic field work together in South Africa. Finally, I wish to thank my fiancé Thomas for helping me edit some pictures used in this thesis and my family and friends for their support throughout my study. 53 References:
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