A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. / The Sterkspruit Intrusion, in the south-western portion of the Barberton greenstone belt, is a sill-like body containing rocks of gabbroic to dioritic composition, It is hosted by a sequence of komatilitic basalts and komatilites of the Lower Onverwacht Group. The intrusion is considered unique in this area in that it lacks ultramafic components and has no affinities with the surrounding mafic- to- ultramafic lavas, The gabbroic suite also contains an unusual abundance of quartz, and the chill margin shows an evolved quartz-normative, tholelitic parental magma. ( Abbreviation abstract ) / AC2017

Thesis submitted for the degree of Master of Science in the Faculty of Science, University of the Witwaiersrand, Johannesburg. / The following is an account of the stiatigiaphy, structure, metamorphism and mineralization in a complexly deformed area of the northwest part of the Barberton Mountain Land. It is situated at the eastern extremity of the Jamestown Hills and covets a region along the contact zone between the ancient layered rocks of the Archaean Complex and the Nelspruit Granite. In the first section is given a fairly comprehensive account of previous work done in the Barberton region- especially as it applies to the area under discussion. This is followed by a consideration of the petrology and stratigraphy of the area and a description of the various structures encountered. A more detailed statistical treatment of the minor structures is also included and from these results an attempt is made to unravel the tectonic history of the area and to fit it into the regional structural pattern of the Mountain Land as a whole. The area mapped consists of two basically Identical successions separated by a major high angled thrust fault. The northern succession, which has been quite strongly thermally metamorphosed, represents the fairly steeply south dipping northern limb of the Lily Syncline. That to the south has suffered very little metamorphism and forms part of the northern limb of the Eureka Syncline. A well developed and layered basic suite of rocks lying below the Fig-tree Series .and constituting the basal zone of the Lily Syncline, is classed in the Onverwacht Series, It lies in direct contact with the Nelspruit Granite and is considered to represent a metamorphosed succession of impure dolomites with arenaceous and minor shaly horizons, together probably with some basic and acid lavas. The Onverwacht rocks are overlain by metamorphosed Fig-tree shales and "lavas", and these in turn by metamorphosed conglomerates and quartzites of the Moodies System. At the base of the homfelses lies the Consort "Contact" or Consort "Bar", a siUcified, mineralized zone which is the main ore horizon of the Consort Mine. The homfelses grade into rocks which have been termed "lavas" but which are thought to be mote of the nature of crystalline tuffs. The succession to the south of the Main Fault Is on a broad scale Identical to the one just described above, differing mainly in metamorplilc state. Thus, whereas the Onverwacht rocks of the northern succession have been converted to hornblende and tremolite-actinollte schists, similar rocks to the south of the Main Fault have been changed to carbonate-bearing talc and chlorite phylUtes. The basic Intrusive rocks of the Jamestown are considered to be of a much smaller distribution than was previously thought,and are represented mainly by the massive bodies of pure serpentinite. There is a possibility however; that certain of the purer talc- carbonate schists along the Kaap Rivet,represent altered ultrabasic intmsives. The Nelspruit gneiss and migmatlte is considered to represent a completely granitized pre-Swaziland System sequence which at a much later date acted as the basement upon which the layered rocks of the Mountain Land were deposited, A re-moblllzed border phase of this migmatite was largely responsible for the contact metamorphism around the edge of the Mountain Land and late hydrothermal solutions from this same intrusive granite resulted in the mineralization of the area. The isolated patches of black amphibolites situated well within the granite outcrop area, are considered to be isolated downfolded remnants of a once mote extensive sheet of Onverwacht. Their high grade metamorphic state, as with the Onverwacht rocks along the immediate contact zone. Is due to the effects of the Intrusive granites plus the re-heaied migmatite. Three distinct facies of contact metamorphism (related to the Nelspruit Granite) ate recognised in the area. The area can be divided into three fairly distinct structural zones, each one characterized by the good development (as compared to the other zones) of one or mote particular types or styles of deformation. Thus in the Consort Mine area (Zone I), a northwest-trending fold system is the strongest and most apparent structural feature. In the southern part of the area (Zone U), minor ctenulation and conjugate folds are very well developed. In the rocks along the granite contact zone (Zone IH), a metamorphic fabric and well/developed lineations associated with strong shearing, are the most noteworthy structural features. ’ Special attention was devoted to the accurate observation and recording of minor structural features. As a result of this, 4 distinct phases of deformation, corresponding fairly closely to those described by Ramsay (1963), were recognised. The first resulted in strong folding about northeast-trending axes and caused the formation of the major synclines (Including the Eureka and Lily Synclines) and the major faults (Including the Lily, Main Southern and Woodstock Faults). The first period was followed by the widespread development of cleavage, associated with which is the marked flattening and elongation of conglomerate pebbles,and development of various types of lineations including the alignment of metamotphic minerals along the immediate contact belt. Certain of the lineations appear to have formed mainly after crystallization of the granite, and are closely connected with the mechanical deformation (in the form of strong differential shearing) which affected the rocks along the contact. At a late stage during the 2nd period, hydrothermal solutions from the granites entered zones of strong shearing and structural disturbance which had started to develop in the layered rocks. The initial development of the 3rd fold stmctures (especially in the northwest part of the Mountain Land) is considered to have played an important part In the localization of these ore fluids. Both the 1st. and 2nd formed stmctures have been strongly deformed by a northwest-trending fold system. These 3rd phase folds teach their best development in the Consort Mine area where they constitute by far the strongest stmctural feature. This period of folding,which during the very early stages probably played a part in the localisation of the ore fluids, outlasted the mineralizing episode and in the Consort Mine area, strongly deformed the mineralized "Contact”. The marked inflection in the northwest part of the Mountain Land, including the "bending" of the Eureka and Ulundi Synclines, occuned at this stage. Most of the major faults truncate the 3rd folds and are thought to have formed, or to have been rejuvenated,at a late stage during this period of deformation. The 4th and final phase of deformation is represented by the fairly widespread development of minor crenulation and conjugate folds. Their development is largely controlled by the rock types and they only occur in lithologically favourable varieties. The maximum deforming stress which caused these folds must have been nearly vertical. These 4th structures are completely separate and unrelated to the main northwest-trending fold system (3rd structures) with which they were classed by Ramsay (1963), From an economic point of view, three zones of potential mineralization, corresponding in all cases to zones of strong shearing and stmcmral disturbance, occur in the area. The most Important is the silicified and mineralized zone (within the northern limb of the Lily Syncline) between basic schists of the Onverwacht and the overlying Fig-tree homfelses, known as the Consort "Contact" or Consort "Bar". Another potential zone of mineralization is the westward extension of the so-called Lily Fault which occupies exactly the same stratigraphic position as the Consort "Contact", viz. between shales and basic rocks of the Flg-uee and Onverwacht Series respectively, but within the northern limb of the Eureka Syncline. Finally, certain shaly horizons within the "footwall" quartzitic layers of the Consort Mine area also show signs of weak mineralization. / AC 2018

The fluvio-lacustrine rocks of the Beaufort Group, South Africa have long been known for their tetrapod fossil record, which is the richest and most complete Middle Permian to Middle Triassic record for any terrestrial sequence in the world. The abundance of fossil material has enabled the Beaufort Group to be biostratigraphically subdivided into between 8 and 10 tetrapod assemblage zones, of which the lowest three (Eodicynodon, Tapinocephalus and Pristerognathus) are attributed to the Middle Permian. These lower assemblage zones record the earliest therapsiddominated faunas and, because they were recorded during a largely uninterrupted period of deposition, make the Beaufort Group the only place in the world where biodiversity change through the terrestrial Middle Permian can be effectively studied. In the last two decades, much interest has focused on an extinction of marine invertebrates at or close to the end of the Middle Permian (Guadalupian epoch), but the existence of a concurrent extinction in the terrestrial realm is contentious. The Beaufort Group is already well known to record the Permo-Triassic Mass Extinction but it also records an earlier extinction at the top of the Tapinocephalus Assemblage Zone (AZ). This extinction is very poorly understood but recent radiometric dates for many Permian assemblage zones of the Beaufort Group have confirmed a Middle Permian age for Eodicynodon, Tapinocephalus and Pristerognathus assemblage zones and suggest that the end-Tapinocephalus AZ extinction may coincide with the marine extinctions. A recently produced GIS database that accommodates all Beaufort Group fossil material curated in South Africa formed the basis on which the stratigraphic range of individual specimens was calculated. To put the fossil localities in a stratigraphic context, lithostratigraphic information was retrieved from the literature and extensive fieldwork was conducted, which measured stratigraphic sections in key areas and traced the surface outcrop of lithostratigraphic units. In order to compensate for lateral variations in lithostratigraphy, the basin was split into sectors, each represented by a stratigraphic section. The stratigraphic ranges of fossil specimens and, subsequently, of genera and families could then be calculated and a workable biostratigraphic subdivision of the Middle Permian Beaufort Group proposed. The Abrahamskraal Formation, which forms the majority of the Middle Permian Beaufort sequence, can be divided into six lithostratigraphic members based on the occurrence of sandstone ‘packages’. These members were traced laterally across the Basin and their correspondence with fining-upwards cycles was refined and correlated with the newly defined biostratigraphic units. This refined two-pronged stratigraphic subdivision allowed the recognition of a waning period of subsidence in the proximal sector of the Karoo Basin during the Middle Permian. Stratigraphic ranges of individual genera were found to be far more heterogeneous than previously recognised. Dicynodont genera are useful biostratigraphic indicators due to their relative abundance and well-defined stratigraphic ranges, while dinocephalians and pareiasaurs are clustered in the upper part of the Abrahamskraal Formation. The stratigraphic range of Eodicynodon extends further up in the Abrahamskraal Formation than was previously recognised. The Tapinocephalus AZ is restricted to approximately the upper fifth of the Abrahamskraal Formation and is characterised by advanced tapinocephalid dinocephalians and the pareiasaur Bradysaurus. Between these two biozones is a stratigraphic interval dubbed the mid- Abrahamskraal Formation Transition Zone, where both Eodicynodon and advanced tapinocephalids coexisted. A 75 % loss of generic diversity occurred between the upper Tapinocephalus AZ and the base of the Pristerognathus AZ, which corresponds to a stratigraphic interval between the mid-Karelskraal Member of the Abrahamskraal Formation and the mid- Poortjie Member of the Teekloof Formation. Several taxa that survive the end- Tapinocephalus AZ extinction, and are relatively common in the overlying Pristerognathus AZ (scylacosaurid therocephalians, the dicynodont genus Eosimops and the parareptile Eunotosaurus), all became extinct in the upper Poortjie Member at a time when generic originations are increasing. This suggests a second wave of extinctions in a similar fashion to that recorded at the Permo-Triassic boundary.

Thesis submitted for the degree of Master of Science in the Faculty of Science, University of the Witwatersrand, Johannesburg. / The following is an account of the stiatigiaphy, structure, metamorphism and mineralization in a complexly deformed area of the northwest part of the Barberton Mountain Land. It is situated at the eastern extremity of the Jamestown Hills and covets a region along the contact zone between the ancient layered rocks of the Archaean Complex and the Nelspruit Granite. In the first section is given a fairly comprehensive account of previous work done in the Barberton region- especially as it applies to the area under discussion. This is followed by a consideration of the petrology and stratigraphy of the area and a description of the various structures encountered. A more detailed statistical treatment of the minor structures is also included and from these results an attempt is made to unravel the tectonic history of the area and to fit it into the regional structural pattern of the Mountain Land as a whole. The area mapped consists of two basically Identical successions separated by a major high angled thrust fault. The northern succession, which has been quite strongly thermally metamorphosed, represents the fairly steeply south dipping northern limb of the Lily Syncline. That to the south has suffered very little metamorphism and forms part of the northern limb of the Eureka Syncline. A well developed and layered basic suite of rocks lying below the Fig-tree Series .and constituting the basal zone of the Lily Syncline, is classed in the Onverwacht Series, It lies in direct contact with the Nelspruit Granite and is considered to represent a metamorphosed succession of impure dolomites with arenaceous and minor shaly horizons, together probably with some basic and acid lavas. The Onverwacht rocks are overlain by metamorphosed Fig-tree shales and "lavas", and these in turn by metamorphosed conglomerates and quartzites of the Moodies System. At the base of the homfelses lies the Consort "Contact" or Consort "Bar", a siUcified, mineralized zone which is the main ore horizon of the Consort Mine. The homfelses grade into rocks which have been termed "lavas" but which are thought to be mote of the nature of crystalline tuffs. The succession to the south of the Main Fault Is on a broad scale Identical to the one just described above, differing mainly in metamorplilc state. Thus, whereas the Onverwacht rocks of the northern succession have been converted to hornblende and tremolite-actinollte schists, similar rocks to the south of the Main Fault have been changed to carbonate-bearing talc and chlorite phylUtes. The basic Intrusive rocks of the Jamestown are considered to be of a much smaller distribution than was previously thought,and are represented mainly by the massive bodies of pure serpentinite. There is a possibility however; that certain of the purer talccarbonate schists along the Kaap Rivet,represent altered ultrabasic intmsives. The Nelspruit gneiss and migmatlte is considered to represent a completely granitized pre-Swaziland System sequence which at a much later date acted as the basement upon which the layered rocks of the Mountain Land were deposited, A re-moblllzed border phase of this migmatite was largely responsible for the contact metamorphism around the edge of the Mountain Land and late hydrothermal solutions from this same intrusive granite resulted in the mineralization of the area. The isolated patches of black amphibolites situated well within the granite outcrop area, are considered to be isolated downfolded remnants of a once mote extensive sheet of Onverwacht. Their high grade metamorphic state, as with the Onverwacht rocks along the immediate contact zone. Is due to the effects of the Intrusive granites plus the re-heaied migmatite. Three distinct facies of contact metamorphism (related to the Nelspruit Granite) ate recognised in the area. The area can be divided into three fairly distinct structural zones, each one characterized by the good development (as compared to the other zones) of one or mote particular types or styles of deformation. Thus in the Consort Mine area (Zone I), a northwest- trending fold system is the strongest and most apparent structural feature. In the southern part of the area (Zone U), minor ctenulation and conjugate folds are very well developed. In the rocks along the granite contact zone (Zone IH), a metamorphic fabric and well/developed lineations associated with strong shearing, are the most noteworthy structural features. ’ Special attention was devoted to the accurate observation and recording of minor structural features. As a result of this, 4 distinct phases of deformation, corresponding fairly closely to those described by Ramsay (1963), were recognised. The first resulted in strong folding about northeast-trending axes and caused the formation of the major synclines (Including the Eureka and Lily Synclines) and the major faults (Including the Lily, Main Southern and Woodstock Faults). The first period was followed by the widespread development of cleavage, associated with which is the marked flattening and elongation of conglomerate pebbles,and development of various types of lineations including the alignment of metamotphic minerals along the immediate contact belt. Certain of the lineations appear to have formed mainly after crystallization of the granite, and are closely connected with the mechanical deformation (in the form of strong differential shearing) which affected the rocks along the contact. At a late stage during the 2nd period, hydrothermal solutions from the granites entered zones of strong shearing and structural disturbance which had started to develop in the layered rocks. The initial development of the 3rd fold stmctures (especially in the northwest part of the Mountain Land) is considered to have played an important part In the localization of these ore fluids. Both the 1st. and 2nd formed stmctures have been strongly deformed by a northwest-trending fold system. These 3rd phase folds teach their best development in the Consort Mine area where they constitute by far the strongest stmctural feature. This period of folding,which during the very early stages probably played a part in the localisation of the ore fluids, outlasted the mineralizing episode and in the Consort Mine area, strongly deformed the mineralized "Contact”. The marked inflection in the northwest part of the Mountain Land, including the "bending" of the Eureka and Ulundi Synclines, occuned at this stage. Most of the major faults truncate the 3rd folds and are thought to have formed, or to have been rejuvenated,at a late stage during this period of deformation. The 4th and final phase of deformation is represented by the fairly widespread development of minor crenulation and conjugate folds. Their development is largely controlled by the rock types and they only occur in lithologically favourable varieties. The maximum deforming stress which caused these folds must have been nearly vertical. These 4th structures are completely separate and unrelated to the main northwest-trending fold system (3rd structures) with which they were classed by Ramsay (1963), From an economic point of view, three zones of potential mineralization, corresponding in all cases to zones of strong shearing and stuctural disturbance, occur in the area. The most Important is the silicified and mineralized zone (within the northern limb of the Lily Syncline) between basic schists of the Onverwacht and the overlying Fig-tree homfelses, known as the Consort "Contact" or Consort "Bar". Another potential zone of mineralization is the westward extension of the so-called Lily Fault which occupies exactly the same stratigraphic position as the Consort "Contact", viz. between shales and basic rocks of the Flg-uee and Onverwacht Series respectively, but within the northern limb of the Eureka Syncline. Finally, certain shaly horizons within the "footwall" quartzitic layers of the Consort Mine area also show signs of weak mineralization. / AC 2018

A Thesis Submitted to the Faculty of Science, University of the Witwatersrand , Johannesburg for the Degree of Doctor of Philosophy / The Phalaborwa Complex, northeastern Transvaal, South Africa, consists of the main body of clinopyroxenites and subordinate phoscorite, carbonatite and syenite which is surrounded by numerous pipe-like bodies of syenitic compositions and rare clinopyroxenites. Clinopyroxenites of the main complex are characterized by cumulus textures formed by separation and accumulation of coprecipitating clinopyroxene, apatite and phlogopite. Potassium feldspar is an intercumulus phase in feldspathic pyroxenite. "Inch-scale" layering of clinopyroxene, apatite and phlogopite formed as in situ cumulus layering near the outer contact of the complex early in the cooling of the magma. Breccias of monomineralic assemblages such as glimmerite and massive pyroxenite reflect breaking up of early formed rocks by magmatic currents. Clinopyroxenes from clinopyroxenites are characterized by Fe/(Fe+Mg) = 0.07-0.29, low T i 02 (0.00-0.25%), A 120 3 (0.00-1.63%), N a 20 (0.00-1.06%), and Cr, and high Wo component and Sr. Micas from pyroxenites have Fe/(Fe+Mg) = 0.12-0.28, low T i 02 (0.17-1.73%) and have reverse and normal pleochroism and increasing A1 with increasing Fe/(Fe+Mg). Mineral compositions among feldspathic, massive and micaceous pyroxenites overlap; no zonation of the complex from outer contact inward is discernible with respect to the Fe and Mg content. However, phlogopites in "inch-scale" layering have low Fe/(Fe+Mg) of 0.12 and have reverse pleochroism due to F e 3+ entry into the Al-deficient tetrahedral site. Micas from phoscorite and carbonatites have reverse pleochroism, Fe/(Fe+Mg) = 0.05-0.58, low T i 02 (0.00-0.84%) and decreasing A1 with increasing Fe/(Fe+Mg). Olivines range from F o79 to F o91 and have very low Ni content (<0.06% N i O ) . Olivines interpreted as xenocrysts have Fo 84 to F o 8 7 . One of the olivine xenocrysts has an NiO content of 0.29%. Minerals from carbonatites have initial 87S r / 86Sr ratios of 0.70393-0.70623 and 0.71022 and minerals from clinopyroxenites have values of 0.71152-0.71242. Smallscale inhomogeneities exist within samples. Postcrystallization processes can account for variations within samples, but cannot account for variations within a rock type or for raising initial 87S r / 86Sr ratios of pyroxene from 0.7039 to 0.7115. Magmas forming pyroxenites and some carbonatites were generated in a high Rb/Sr mantle from isotopically distinct sources. Hence, liquid immiscibility and differentiation are not viable mechanisms for relating these rocks to one another. Mixing of magmas and assimilation of crust may account for isotopic variations within a rock type. U-Pb dating of uranothorianite and baddeleyite from phoscorite and carbonatite yields an age of 2047+11/-8 m.y. Rb-Sr dating of phlogopites gives widely disparate apparent ages. Nine phlogopites yield an isochron of 2012 + 19 m.y. One sample of phlogopite gave ages of 1661-2360 m.y. in nine different determinations. Older micas may be present in the Phalaborwa complex. Based on the two methods, a best age of the Phalaborwa complex is 2030 + 18 m.y. Two of the syenite pipes, Kgopoeloe and Spitskop, show different levels of emplacement. Kgopoeloe is highly brecciated from a fluid derived from the syenite. Spitskop contains minor breccia and represents multiple injection of syenite. At Spitskop, inward crystallization of the second syenite forms a ring syenite and central syenite; both syenites have cumulus enrichment of minerals. Feldspathic pyroxenite of the Guide Copper Mine is cogenetic with the pyroxenites of the main complex and contains clinopyroxenes with oscillatory zoning. Fluctuations in f02 may be attributed to formation of an immiscible sulphide liquid. Multiple intrusion of the Phalaborwa Complex is proposed. Initial injection of potassic, probably ultrabasic, liquid formed the pyroxenites. A second intrusion of low-silica, carbonate-rich magma formed phoscorite and banded carbonatite. A third intrusion of carbonatite liquid formed the transgressive carbonatite. Magmatic, cumulus processes dominated the formation of the main complex. The syenites of Kgopoeloe and Spitskop are not cogenetic with rocks of the main complex. / AC2017

Thesis (M.Sc.)--University of the Witwatersrand, Science Faculty (Geology), 1965 / The geology of the Johannesburg area is extremely diverse in its engineering aspects. A geological and soil map can therefore be of great assistance in planning detailed site investigations that must be carried out before the foundations of a building can be designed. [No abstract provided. Information taken from introduction] / WS2017

No description available.

Graduation date: 1985

The variations in the composition, specifically the Cr20 S content and the Cr:Fe ratio, and the morphology of the Lower Group (LG) and Middle Group (MG) chromitite seams of the Critical Zone (CZ) across the western Bushveld Complex, including the Ruighoek and Brits sections, is investigated by means of whole-rock chemical data, both major and trace elements analysis, XRD and electron microprobe data. As a result ofthe paucity of exposed or developed LG1 - LG5 chromitite seams in the western Bushveld Complex, this study is confined to the investigation of the compositional variations of the LG6 to MG4 chromitite seams. In only one section, the Ruighoek section, was the entire succession of chromitite seams, from the LG1 - MG4, exposed. The silicate host rocks from the LG6 pyroxenite footwall to the collar of the CC2 drillcore (lower uCZ) in the Rustenburg section were sampled. This study reviews the compositional trends of the silicate host rocks, as the compositional variations of the chromitite seams reflect the chemical evolution of the host cumulate environment and, to a lesser degree, the composition onhe interstitial mineral phases in the chromitite seams. The compositional variations of the LG and MG chromitite seams are attributed to the compositional contrast between the replenishing magma and the resident magma. The chemical trends of the LG and MG chromitite layers and the host cumUlate rOCKS do not support the existence of two compositionalfy dissimilar magmas in the CZ, rather the cyclic layering of the CZ and the chemical variations of the chromitite seams are attributed to the mixing of primitive magma with the resident magma, both of which have essentially similar compositions. The compositional variations of the LG and MG chromitite seams along strike away from the supposed feeder site (Union section) to the distal facies (Brits section) are attributed to the advanced compositional contrast between the resident magma and the replenishing primitive magma pulses. The CZ is characterized by reversals in fractionation trends and this is attributed to the compositional evolution of the parental magma and not to the replenishment of the resident magma by influxes of grossly dissimilar magma compositions. The Cr20 S content and the Cr:Fe ratio of the MG chromitite layers increase from the Ruighoek (near proximal) section to the Brits section (distal facies). This is attributed to the advanced compositional contrasts between the resident magma and the replenishing primitive magma. In contrast, the Cr20 3 content and Cr:Fe ratios ofthe LG6 and LG8a chromitite seams decreases eastwards from the Ruighoek section. The average Cr:Fe ratio for the western Bushveld Complex is between 1.5 and\2.0, nonetheless, a progressively lower Cr:Fe ratio is noted from the LG1 chromitite up through to the MG4 chromitite seam in the Ruighoek section. tn the LG2 - LG4 chromitite interval a deviation to higher.lratios is encountered. A progressive substitution of Cr by AT and Fe in the Cr-spinel crystal lattice characterizes the chromitite succession from the LG1 seam up through the chromitite succession to MG4. The petrogeneSiS of the chromitite seams of the CZ is attributed to magma mixing and fractional crystallization of a single magma type.

The Pletersburg greenstone belt Is located In South Africa, about 300 km northeast of Johannesburg. It hosts a significant amount of gold mineralization and just over 1000 kg of gold have been produced from Its various reefs and secondary deposits. The greenstone belt is interpreted as an Archean ophiolite complex. It comprlses a volcano-sedimentary succession (the Pletersburg Group) which Is subdivided Into a basal greenstone sequence, interpreted as oceanic crust, and an upper sedimentary cover sequence. A number of major shear zones, which are thought to represent thrusts that developed during the subduction of the greenstone sequence, form an integral part of the stratigraphy . Four stages of deformation (D₁-D₄) and four phases of metamorphism (H₁-H₄) (three of which are correlatable with the peak stages of deformation) are recognized. The primary gold deposits are all shear zones related. but they are subdivided into greenstone, sedimentation and granIte-hosted types. Geographically, they occur In three distinct goldfields: Eerstellng, Roodepoort and Marbastad. The greenstone-hosted · Plenaar-Doreen shear complex Is In the Eersteiing goldfield and hosts eight gold occurrences. Within the complex, Girlie North Reef is the 640m-long "pay" section of the Girlie North shear zone. This reef is characterized, macroscopically, by a Quartz-carbonate-chlorite-sulphlde assemblage and, mlcroscoplcally, by the presence of tourmaline, arsenopyrlte and Au. Geochemical evidence Indicates that mineralizing fluids were H₂O and CO₂-bearing and rich In S, K and Al. The wall rock alteratlon was Isochemlcal but Is manifest as a change In mineralogy from a hornblende + plagioclase assemblage to an actlnollte/tremollte + Quartz + clay assemblage. This Is best developed In the hangIng wall of the reef and is thought to have been caused by hydrogen ion metasomatism. The Arsenopyrite Reef was one of the main sediment-hosted shear zone gold producers In the Harabastad goldfield. This reef Is Interpreted as the basal margin of a shear zone whose top contact Is probably represented by the Quartz Vein Reef. The shear zone consists predomonantly of quartz and carbonate, and the two "pay" reefs are characterized by tourmallne. arsenopyrite and Au. No wall rock alteration was identified In this study, Based on the mineralogy and geochemical signature of the Girlie Nortn Reef and the Arsenopyrite Reef, It Is proposed that both were formed at the $The Pletersburg greenstone belt Is located In South Africa, about 300 km northeast of Johannesburg. It hosts a significant amount of gold mineralization and just over 1000 kg of gold have been produced from Its various reefs and secondary deposits. The greenstone belt is interpreted as an Archean ophiolite complex. It comprlses a volcano-sedimentary succession (the Pletersburg Group) which Is subdivided Into a basal greenstone sequence, interpreted as oceanic crust, and an upper sedimentary cover sequence. A number of major shear zones, which are thought to represent thrusts that developed during the subduction of the greenstone sequence, form an integral part of the stratigraphy . Four stages of deformation (D₁-D₄) and four phases of metamorphism (H₁-H₄) (three of which are correlatable with the peak stages of deformation) are recognized. The primary gold deposits are all shear zones related. but they are subdivided into greenstone, sedimentation and granIte-hosted types. Geographically, they occur In three distinct goldfields: Eerstellng, Roodepoort and Marbastad. The greenstone-hosted · Plenaar-Doreen shear complex Is In the Eersteiing goldfield and hosts eight gold occurrences. Within the complex, Girlie North Reef is the 640m-long "pay" section of the Girlie North shear zone. This reef is characterized, macroscopically, by a Quartz-carbonate-chlorite-sulphlde assemblage and, mlcroscoplcally, by the presence of tourmaline, arsenopyrlte and Au. Geochemical evidence Indicates that mineralizing fluids were H₂O and CO₂-bearing and rich In S, K and Al. The wall rock alteratlon was Isochemlcal but Is manifest as a change In mineralogy from a hornblende + plagioclase assemblage to an actlnollte/tremollte + Quartz + clay assemblage. This Is best developed In the hangIng wall of the reef and is thought to have been caused by hydrogen ion metasomatism. The Arsenopyrite Reef was one of the main sediment-hosted shear zone gold producers In the Harabastad goldfield. This reef Is Interpreted as the basal margin of a shear zone whose top contact Is probably represented by the Quartz Vein Reef. The shear zone consists predomonantly of quartz and carbonate, and the two "pay" reefs are characterized by tourmallne. arsenopyrite and Au. No wall rock alteration was identified In this study, Based on the mineralogy and geochemical signature of the Girlie Nortn Reef and the Arsenopyrite Reef, It Is proposed that both were formed at the same time. Textural evidence Indicates that tourmaline, arsenopyrite and Au were all very late In the paragenesis of minerallzatlon. The presence of tourmaline also Indicates a probable granite association. It Is proposed that the maln gold mineralizing event was synchronous with the Intrusion of granitoids (and therefore also with (D₁-D₄) and (H₁-H₄) and that most of the Au was derived from felsic magma. Gold was partitioned Into a magmatic hydrothermal fluid and then transported into the greenstone belt as a chlorIde complex. These magmatiC fluids were channelled up shear zones whIch had already been mineralized with a quartz-carbonate-chlorlte - sulphide assemblage by previous metamorphic fluidS. generated during the dynamic (D₂-related) H₂-phase of metamorphism. The Au was then deposIted as the result of a change In a fluid variable, such as temperature, pH, f0₂, or the activity of Cl (some Au may have been transported In a sulphur complex and so the activity of reduced 5 could also have been Important).