the nature of the witwatersrand hinterland conjectures on the source area problem although the au and u deposits of the witwatersrand basin are generally regarded as modified placer in origin the nature of the source area and the ultimate origin of au and u still remain a major enigma a number of domal outcrops of the archean basement are exposed in the witwatersrand hinterland revealing the presence of a diverse suite of granitoids and a minor 20 greenstone belt component in addition samples from over 130 boreholes drilled adjacent to the basin have revealed the character of subsurface archean basement beneath early proterozoic sedimentary and volcanic cover the granitic subsurface basement is commonly characterized by pervasive hydrothermal alteration evident as propylitization sericitization carbonation and greisenization as well as a variety of vein related alterations comprising quartz or k feldspar or chlorite or co 3 or sulfide or fluorite parageneses these hydrothermally altered granites appear to have been preserved beneath early proterozoic cover but have largely been eroded from surface outcrops the hydrothermally altered granites which are believed to be or 3 050 ma are significantly enriched in au and u with respect to both surface granitoids from the witwatersrand hinterland and the entire range of granites from the barberton mountain land the presence of enriched hydrothermally altered granites in the witwatersrand hinterland lends support to the view that mineralization processes in the basin were dominated in the first instance by placer processes the remarkable concentrations of au and u that are unique to the central rand group of the witwatersrand basin indicate that the sediments in the upper portions of the sequence tapped a fertile source not present in the hinterland during earlier cycles of sedimentation the erosional remnants of this source area are represented by the hydrothermally altered granites which are considered to have been a suite of high level granitoids the upper portions and roof zones of which were mineralized in au and u and easily accessible to erosion the emplacement of these granites and the development of extensive zones of attendant high level hydrothermal alteration was synchronous with witwatersrand sedimentation the hydrothermally altered granites not only contributed to the au and u rich detritus in the witwatersrand but their emplacement may have been linked causally to the deposition of the upper part of the sedimentary sequence

depositional age and sediment source of the auriferous moeda formation quadrilatero ferrifero of minas gerais brazil new constraints from u pb hf isotopes in zircon and xenotime abstract the moeda formation is a metasedimentary clastic sequence of witwatersrand like metaconglomerate and quartzite at the base of the minas supergroup quadrilatero ferrifero of minas gerais brazil which is also known for its world class iron ore deposits the moeda formation can be subdivided into three units units i to iii from bottom to top with each unit consisting of metaconglomerates at their base mostly auriferous grading into quartzite and locally into phyllite this study presents the first combined u pb and lu hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from units ii and iii these data indicate that deposition of sediments occurred 2 68 ga for unit ii and 2 62 ga for unit iii as supported by the youngest magmatic zircon grains the data further provide evidence that the source area of sediments was affected by crust re working during two major magmatic metamorphic events at 2 89 2 87 and 2 72 2 69 ga and minor events between 3 3 and 3 0 and at 2 80 and 2 65 ga this interpretation is supported by mainly subchondritic hf t between 10 and 1 4 and large hf t variations including all magmatic events a few superchondritic hf t obtained for the magmatic events at 3 3 2 9 and 2 7 ga point to limited juvenile crust addition the new dataset indicates that the clastic sediments of the auriferous moeda formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources which are suggested for the mesoarchaean auriferous sediments of the witwatersrand basin of south africa and those of the eastern goldfield province of australia

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

systematics of chlorite alteration at the phelps dodge massive sulfide deposit matagami quebec hydrothermal alteration at the phelps dodge archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages quartz chlorite albite epidote or sericite quartz chlorite or sericite chlorite or sericite and chlorite talc or stilpnomelane alteration at constant al was initiated by hydration addition of fe mg silicification and leaching of ca na which produced al saturated chlorites with a wide range of fe fe mg ratios within the assemblage quartz chlorite albite epidote or sericite with continued addition of fe and less mg combined with extensive leaching of quartz the chlorite became al undersaturated quartz chlorite talc assemblage and ultimately formed a rock composed essentially of chlorite the chlorite talc or stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone chlorites in the si al mg fe system form a rectangular solid solution plane with limited substitution of al for si and extensive substitution of fe for mg in si al space the solid solution plane is outlined by al saturation presence of other al rich mineral and al undersaturation presence of al poor or al free mineral boundaries al iv substitution for si in the chlorites increases from 2 3 to 3 01 formula units based on 8 si al iv with fe fe mg whereas al iv occupancy decreases from 2 88 to 2 65 the valence charge balance is compensated by octahedral site vacancies or by the conversion of fe super 2 to fe super 3 the al iv content of chlorite also increases with temperature chlorites on the al saturated boundary of the solid solution field form a potentially useful geothermometer t degrees c 106 al iv 18 al iv is corrected for changes with the fe fe mg ratio as al iv corrected al iv 0 7 fe fe mg the fe fe mg ratios of bulk rock and contained chlorite correlate well birefringence colors of the chlorite change from green to brown violet and berlin blue as fe fe mg changes from 0 18 to 0 64 the fe fe mg variation apparently depends largely on the proportions of mg rich seawater and fe rich hydrothermal fluid in the altering brine

geochemistry of ikaite formation at mono lake california implications for the origin of tufa mounds the mineral ikaite caco 3 6h 2 o not previously observed in lake environments precipitates seasonally along the shore of mono lake california where ca hco 3 spring water mixes with cold na co 3 lake water during the winter cold water temperatures and high concentrations of po 4 3 and organic carbon inhibit calcite precipitation allowing the metastable ikaite to form during the spring warming however ikaite decomposes to form calcium carbonate and water occasionally leaving pseudomorphs of the primary precipitate the identification of modern ikaite suggests that both pleistocene and holocene tufas in the mono basin originally precipitated as ikaite this mineral may also form in other lake environments but rapid recrystallization after warming destroys the physical chemical and isotopic evidence of formation and alters the geochemical record

volcanic rocks of the witwatersrand triad south africa ii petrogenesis of mafic and felsic rocks of the dominion group abstract a bimodal suite of volcanic rocks builds the bulk of the dominion group which with an age of 2 72 ga is the oldest cover sequence overlying the granite greenstone archaean basement of the kaapvaal craton in the western transvaal south africa the basic lavas are relatively rich in sio 2 50 58 and aphyric and exhibit a large compositional range this variation is typically tholeiitic in that it is characterized by strong enrichment of ti fe and v in differentiated lavas the different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups one group is characterized by evolution to low ti v ratios in evolved lavas and the other by high ti v ratios for nearly all other elements the composition variation of the two groups is extremely similar and overlaps although the high ti v group contains the most evolved compositions and most of the primitive lavas mg numbers 60 belong to the low ti v group although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile theoretical modelling indicates that the variation in both lava groups can be explained by low pressure fractional crystallization of phases in a typical tholeiitic sequence primitive basalts of the low ti v group show a large range in major element compositions and a more restricted range of incompatible trace elements and ree abundances in particular fe 2 o 3 total ranges from 9 to 13 5 and correlates positively with mgo cr and ni and negatively with al 2 o 5 and sio 2 there are no correlations of mg number with incompatible trace elements such as zr and ce these lavas also have lree enriched ree abundance patterns and negative nd these features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements particularly fe al and probably ca and na and uniformly enriched in incompatible trace elements at least 0 3 ga before melting the geochemical style of this enrichment is akin to that associated with modern subduction this enrichment is associated with the development of a thick archaean lithosphere the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies the geochemistry of the dominion basic lavas differs significantly from those of the 3 4 ga barberton komatiitic and tholeiitic suites this suggests that the changing pattern of crustal evolution from metastable greenstone tectonic styles to stable proterozoic styles is reflected in the changing character of the mantle sources tapped by basic magmas the porphyritic dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas nd isotopes and la th and th co ratios suggest that they are partial melts of mafic crustal materials geochemical and petrological features of the dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning this is consistent with some models for the tectonic development of the witwatersrand basin

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

understanding cratonic flood basalts the origin of continental flood basalts has been debated for decades these eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years although flood basalts are found in a variety of settings no locale is more puzzling than cratonic areas such as southern africa or the siberian craton where strong thick lithosphere is breached by these large basaltic outpourings conventionally flood basalts have been interpreted as melting events produced by one of two processes 1 elevated temperatures associated with mantle plumes and or 2 adiabatic decompression melting associated with lithospheric thinning in southern africa however there are severe problems with both of these mechanisms first the rifting circumstances of several wellknown basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep seated plume specifically rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts second the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains as an alternative to these models we propose a new two stage model that interprets cratonic flood basalts not as melting events but as short duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale reservoir creation existence stage i requires long term e g 1 ma supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm appropriate for the archean a slow thermal perturbation e g thermal blanketing or large scale mantle upwelling or a subduction related increase in volatile content the drainage event stage ii occurs in response to an abrupt stress change in the lithosphere which leads to the initiation and propagation of lithospheric dikes such a model accounts for the short eruption time of flood basalts the evidence for lithospheric control of the eruptions and the continued survival of cratonic lithosphere following these magmatic events the most notable consequence of this model is that it implies the existence of large reservoirs of magma comparable to the eruption volume of flood basalts that have been and are still likely present beneath stable continental lithopshere

morphology of witwatersrand gold grains from the basal reef evidence for their detrital origin thousands of rounded particles of fine grained gold released from a crossbedded gold specimen of the basal reef placer in the welkom gold field after acid digestion of the host quartz arenite have revealed morphological features and abrasion textures that are characteristic of detrital gold the size range which is between 0 038 and 0 473 mm has a lognormal distribution with a mean of 2 88 phi units 0 136 mm and a standard deviation of 0 48 this is a characteristic of well sorted sediment the shapes are predominantly disclike with toroidal forms being most common a corey shape factor of 0 08 indicates extreme flattening the preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport a knobby surface texture similar to placer grains recovered from both recent and devonian placers and produced during laboratory controlled tumbling with sand and cobbles in water is considered distinctive of sediment abrasion secondary gold comprising 25 percent of the total gold content is finer grained has an interstitial jagged form and displays unabraded smooth to crystalline surfaces study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive basal reef placer and led to research concerning its metamorphic overprints

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

volcanic rocks of the witwatersrand triad south africa i description classification and geochemical stratigraphy abstract the witwatersrand triad contains thick volcanic sequences confined largely to the dominion group at the base and the ventersdorp supergroup at the top these volcanic sequences are of late archaean to early proterozoic age and are amongst the oldest supracrustal volcanic sequences erupted onto the archaean kaapvaal craton the volcanic rocks have suffered low grade greenschist facies metamorphism but primary textures and in some samples primary mineralogies are well preserved major and trace element analyses of a large number of samples of volcanic rocks from boreholes in the klerksdorp area western transvaal indicate that the chemical compositions of the lavas are largely unmodified by secondary processes and can be used for classification stratigraphic studies and petrogenetic interpretation the volcanics of the dominion group are a bimodal tholeiitic basalt rhyolite association with considerable compositional variation from basalt to tholeiitic andesite within the basaltic suite the ventersdorp supergroup is a weakly developed bimodal tholeiitic basalt dacite suite basic rocks in the klipriviersberg group at the base and in the overlying platberg group are largely basaltic whereas those of the allanridge formation at the top are largely tholeiitic andesites within the klipriviersberg group the more primitive samples occur at the top of the sequence phenocryst rich porphyries of the makwassie formation in the platberg group are largely of dacitic composition and have textural features suggestive of an ash flow origin a distinct geochemical stratigraphy is present within the volcanic sequences and application of discriminant function analysis and an empirical discrimination grid based on ratios of immobile elements p ti and zr allows for unknown samples to be placed within their correct stratigraphic unit

archean foreland basin tectonics in the witwatersrand south africa the witwatersrand basin of south africa is the best known of archean sedimentary basins and contains some of the largest gold reserves in the world sediments in the basin include a lower flysch type sequence and an upper molassic facies both of which contain abundant silicic volcanic detritus the strata are thicker and more proximal on the northwestern side of the basin which is at least locally bound by thrust faults these features indicate that the witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an andean type arc remarkably similar phanerozoic basins may be found in the southern andes we suggest that the continental collision between the kaapvaal and zimbabwe cratons at about 2 7 ga caused further subsidence and deposition in the witwatersrand basin regional uplift during this later phase of development placed the basin on the cratonward edge of a collision related plateau now represented by the limpopo province similarities are seen between this phase of witwatersrand basin evolution and that of active basins north of the tibetan plateau e g the tarim and tsaidam basins the geologic evidence does not appear so compatible with earlier suggestions that the witwatersrand strata were deposited in a rift or half graben

the johannesburg dome south africa new single zircon u pb isotopic evidence for early archaean granite greenstone development within the central kaapvaal craton abstract the johannesburg dome located in the central part of the kaapvaal craton constitutes one of the key areas to better understand the archaean crustal evolution of this part of the craton the dome comprises a variety of archaean granitic rocks intruded into mafic ultramafic greenstone remnants this study presents new precise u pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the johannesburg dome a trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340 3 ma and represents the oldest granitoid phase recognized so far this result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c 3 34 ga this initial magmatic episode involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome was followed by the emplacement of a 3201 5 ma hornblende biotite tonalite gneiss in the south following the trondhjemite tonalite gneiss emplacement a further period of magmatism took place on the dome which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites the age of these dykes has yet to be determined quantitatively but they fall within the time constraints imposed by the age of the trondhjemitic gneisses 3340 3200 ma and later crosscutting potassic granitoids these rocks consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome the southern and southeastern parts of the batholith consist mainly of medium grained homogeneous grey granodiorites dated at 3121 5 ma their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114 2 3 ma an age of 3117 12 ma from zircons extracted from one of the mafic dykes possessing granitic microveins provided confirmation of the timing of this third magmatic event lastly pegmatites that crosscut all these earlier granitoid events are younger than 3114 ma and might be at least 3 0 billion years old these new data provide confirmation of the conclusion that the witwatersrand basin was deposited after c 3074 ma on an archaean basement as young as c 3120 ma the data combined with that from other parts of the kaapvaal craton further supports the view that the evolution of the craton was long lived and episodic and that it grew by accretionary processes becoming generally younger to the north and west of the c 3 5 ga barberton swaziland granite greenstone terrane situated in the southeastern part of the craton

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

high field strength element ratios in archean basalts a window to evolving sources of mantle plumes abstract in terms of high field strength element ratios nb th zr nb nb y and zr y most basalts from non arc type archean greenstones are similar to oceanic plateau basalts suggestive of mantle plume sources a large number of these basalts have ratios similar to primitive mantle composition perhaps the archean mantle was less fractionated than at present and primitive mantle comprised much of the deep mantle and made a significant contribution to mantle plumes the near absence of archean greenstone basalts similar to nmorb in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source dm was volumetrically insignificant the element ratios in basalts also indicate the existence of recycled components himu em1 em2 in the mantle by the late archean this suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the late archean high field strength element ratios also indicate an important contribution of continental crust or and subcontinental lithosphere to some non arc archean greenstone basalts this implies that at least thin continental lithosphere was relatively widespread in the archean

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

an early proterozoic three stage rift system kaapvaal craton south africa abstract a three stage model that constitutes a fully evolved rift system is proposed for a number of superimposed early proterozoic basins on the kaapvaal craton based on interpreted depositional systems and syndepositional tectonics this model consists of three superimposed basins 1 1 a pre graben stage that influenced the dominion group witwatersrand supergroup and klipriviersberg group of the ventersdorp supergroup 2 2 graben development that superimposed the platberg group bothaville and allanridge formations of the ventersdorp supergroup 3 3 post graben thermal subsidence that superimposed the black reef quartzite formation and the chuniespoort ghaap group of the transvaal supergroup oblique collision of the kaapvaal craton and zimbabwe craton is interpreted to have generated stresses that initiated the three stage system at different time intervals compressional deviatoric stress appears to have replaced tensional stress and kept the system in operation tectonically the system is analogous to the baikal rift system and the pannonian basin system while the general configuration of the superimposed basins is comparable to the north sea basin or the north china basin

climate variability in northwest greece during the last interglacial isotopic and palynological data over the period 115 000 to 135 000 years ago from a lacustrine sequence in northwest greece show distinct oscillations during the intervals leading into and out of the full interglacial whereas in between changes were less pronounced but included a brief fluctuation after peak interglacial conditions temperatures decreased and precipitation increased in a series of steps these data suggest that eemian climate variability was similar to that of the holocene both in terms of amplitude and possibly of pacing

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

oxygen isotope sedimentology of gold and uranium bearing witwatersrand and huronian supergroup quartz pebble conglomerates the relation between au and u content and provenance has been evaluated for a number of quartz pebble conglomerates from the archean witwatersrand and early proterozoic huronian supergroups through analysis of the oxygen isotope composition of siliceous detritus within these rocks oxygen isotope compositions of quartz quartzite and chert pebbles as well as of individual sand sized quartz grains in the matrix of the conglomerates were measured using conventional and laser fluorination techniques significant heterogeneity in the delta 18 o values of quartz and quartzite pebbles and quartz sands within individual samples of these conglomerates suggests that they have preserved delta 18 o values of their source rocks preservation of original delta 18 o values of individual sand grains in conglomerates whose matrix displays a typical metamorphic texture implies limited oxygen and silica mobility and recrystallization by pressure solution in the presence of small amounts of intergranular fluids oxygen isotope analyses of quartz pebbles along the main reef conglomerate of the west rand consolidated gold mine witwatersrand indicate that quartz pebble delta 18 o values reflect local variations in provenance yet the overall delta 18 o distributions for quartz pebbles and quartz sands from the au and u bearing main reef and the directly overlying u bearing monarch reef conglomerate in the same mine are identical implying an overall similarity in their source rocks comparison of the quartz pebble delta 18 o distributions between these two reefs and other witwatersrand conglomerates including polymictic conglomerates from the welkom area of the witwatersrand basin moreover suggests that the provenance was similar throughout deposition of much of the central rand group differences in the ore content and au u ratio between conglomerates of the central rand group are compatible with differences in their sedimentology in contrast to conglomerates from the witwatersrand supergroup delta 18 o distributions of quartz pebbles from different conglomerates throughout the huronian supergroup indicate distinct differences in provenance a erude positive correlation between au content and proportion of quartz pebbles with delta 18 o values greater than or equal to 11 5 per mil exists for these conglomerates the u bearing matinenda formation conglomerates in the elliot lake area have a very low proportion of pebbles with delta 18 o values above 11 5 per mil matinenda formation conglomerates sampled closer to present outcrops of archean greenstone terranes as well as conglomerates of the lorrain and livingstone creek formations have a higher proportion of quartz pebbles with delta 18 o values greater than or equal to 11 5 per mil and also higher au contents differences between the delta 18 o distributions for quartz sands and quartz pebbles help to identify sediment sources absence of quartz sands with delta 18 o greater than or equal to 11 5 per mil but presence of quartz pebbles with such values in the same sediment may indicate a source which yields little or no sand sized material in fluvial settings comparison of the delta 18 o values of quartz pebbles and quartz sands in quartz pebble conglomerates to delta 18 o values for quartz from likely source rocks suggests that compositionally homogeneous granites and related pegmatites are the dominant source of uranium whereas hydrothermal lode gold veins are the dominant source of gold

textural and geochemical discrimination between xenotime of different origin in the archaean witwatersrand basin south africa abstract xenotime ypo4 of detrital diagenetic and hydrothermal origin within siliciclastic rocks of the archaean witwatersrand basin south africa has been identified on the basis of petrography and in situ ion microprobe shrimp age data the chemical composition of xenotime determined by in situ electron microprobe analysis can be correlated with its origin this allows the origin of any xenotime grain to be assessed by a non destructive microanalytical method prior to ion microprobe geochronology the main chemical discriminators are mree hree abundance normalised hree slope and eu anomaly and in some cases u and th contents igneous detrital xenotime 2800 ma is distinguished from diagenetic 2780 ma and hydrothermal

stable isotope compositions of quartz pebbles and their fluid inclusions as tracers of sediment provenance implications for gold and uranium bearing quartz pebble conglomerates oxygen isotope compositions of pebbles from late archean to paleo proterozoic gold and or uranium bearing oligomictic quartz pebble conglomerates of the witwatersrand district south africa and huronian supergroup canada were determined in an attempt to define the nature of the source terrain the delta sup 18 o values of quartz pebbles within any one sample typically vary by approximately 4 per thousand or more but occasionally by as much as 8 per thousand even for adjacent pebbles within the same hand specimen in addition adjacent quartz pebbles of widely contrasting delta sup 18 o values also preserve distinct isotopic signatures of their fluid inclusions this overall heterogeneity suggests that the pebbles did not undergo significant oxygen isotope exchange after incorporation in the conglomerates therefore oxygen isotope analyses of such quartz pebbles in combination with a detailed investigation of their mineral and fluid inclusions can provide a useful method for characterizing pebble populations and hence dominant sediment source modes comparison of values found in this study with delta sup 18 o values of quartz from archean granites pegmatites and mesothermal greenstone gold veins i e delta sup 18 o values of sources commonly proposed for the conglomerate ores suggests that uranium is derived from a granitic source whereas gold has a mesothermal greenstone goldmore source low delta sup 18 o values of chert pebbles 9 per thousand to 11 5 per thousand relative to those expected for archean and proterozoic marine cherts commonly ge 17 per thousand effectively exclude marine cherts and therefore auriferous iron formations and exhalatives as likely sources of gold less

a major archean gold and crust forming event in the kaapvaal craton south africa the 2 89 to 2 76 gigayear old conglomerates of the central rand group of south africa host an immense concentration of gold the gold and rounded pyrites from the conglomerates yield a rhenium osmium isochron age of 3 03 0 02 gigayears and an initial 187 os 188 os ratio of 0 1079 0 0001 this age is older than that of the conglomerates thus the gold is detrital and was not deposited by later hydrothermal fluids this middle archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

ages of altered granites adjoining the witwatersrand basin with implications for the origin of gold and uranium one of the main problems in understanding the origin of the witwatersrand basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin another problem is the age and nature of the rocks in the source area recent data1 2 indicate that the source area was largely 3 200 2 900 myr old becoming progressively younger as the basin evolved granites adjoining the basin are commonly overprinted by pervasive and vein related hydrothermal alteration which resulted in an enrichment of both gold and uranium3 4 here we report uranium lead data demonstrating that these hydrothermally altered granites were intruded sporadically both before and during the 360 myr period5 constraining witwatersrand sediment deposition and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling thus fertilization of the crust occurred during basin evolution a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition

hydrothermally altered peraluminous archaean granites as a provenance model for witwatersrand sediments hydrothermal alteration of peraluminous granitic rocks occurring at varkenskraal northwest of the witwatersrand basin consists of two major styles the first style pervasive in nature is represented by two stages i a propylitic alteration and ii a disseminated sericitization both alteration stages were accompanied by a deposition of disseminated sulphides the second style is represented by a vein related alteration in four stages including potassium feldspar minor carbonate and minor sulphide mineralization nodules of uraniferous carbonaceous matter and small amounts of particulate gold could possibly be associated with either one or both styles hydrothermally altered rocks of similar nature have been observed in a number of other localities close to the witwatersrand basin their abundance large volumes and low grade disseminated pyrite gold mineralization indicate that rocks of this type could have been the provenance of the witwatersrand gold bearing sediments

archean age for the granulite facies metamorphism near the center of the vredefort structure south africa granulite facies metamorphic assemblages in rocks exposed near the center of the 2 02 ga vredefort impact structure previously have been interpreted either as early proterozoic genetically related to the 2060 ma bushveld complex or as archean and representative of lower crust that rebounded to upper crustal levels following an impact event zircon and monazite recovered from the granulite facies rocks record high grade metamorphism at 3107 9 ma and a primary age of 3425 ma for detrital zircon a shock deformed but otherwise pristine dolerite dike that intrudes the granulite terrane yields a u pb zircon age of 2560 ma providing a minimum age for the metamorphism these isotopic age data are difficult to reconcile with a regional high grade metamorphic event in the crust beneath vredefort at 2060 ma instead the preimpact high temperature metamorphic history observed in the vredefort lower crustal rocks indicates an enigmatic high temperature event during the stabilization of diamondiferous archean tectosphere

geochemical and geochronological constraints on the nature of the immediate basement next to the mesoarchaean auriferous witwatersrand basin south africa acombined petrological geochemical and geochronological rb srand sm nd whole rock u pb and lu hf zircon and ar arhornblende study on a section of pre witwatersrand basementdrilled at the northwestern margin of thewitwatersrand basin hasrevealed new insights into the nature and tectonic setting of thelikely source area for some of the mesoarchaean auriferouswitwatersrand sediments the protolithsof intersected alteredgran ite and hornblende metagabbro are of indistinguishable age 3062 5 ma and have very similar geochemical signatures trace element characteristics typical of calc alkaline magmatismand evidence of variable contamination with older crust subchon dritic e

geochemistry and origin of late archean volcanic rocks from the rhenosterhoek formation dominion group south africa abstract the rhenosterhoek formation composed chiefly of subaerial volcanic rocks is part of the dominion group 2 8 ga which rests unconformably on older continental crust in south africa chemical compositions of volcanic rocks from three drillcores of this formation do not show systematic differences either between or within cores major and immobile trace element contents indicate that the volcanics are basaltic andesites and andesites with calc alkaline affinities incompatible trace element distributions are similar to those of modern andesites from evolved island arcs or continental margin arcs geochemical modeling indicates that the basaltic andesites and andesites can be produced by open system fractional crystallization with olivine clinopyroxene and plagioclase as principal liquidus phases up to 10 contamination with continental crust is also allowed by the data this succession does not appear to have formed in an oceanic or in a continental margin arc but may have formed adjacent to a continental margin arc system in an incipient foreland basin on the kaapvaal craton