South African Journal of Geology

Print ISSN: 1012-0750
Evidence accumulated over the past two decades is now sufficient to permit an initial quantitative assessment of the patterns of biotic diversity and extinction that occurred during Proterozoic time. Because of limitations in both the quality and quantity of data currently available, however, generalizations thus derived must be regarded as tentative. Nevertheless, read literally, available palaeontological data appear to indicate that the global ecosystem experienced a gradual but massive collapse between 1 000 Ma and the beginning of the Phanerozoic, a supposition consistent with other lines of geological and geochemical evidence. A possible forcing agent for such a collapse appears to have been a decrease in ambient levels of carbon dioxide and a resultant decrease in average global temperature, photosynthetic efficiency, and primary productivity.
Estimated isopaclus of Main Bin! Series (altL^r KeadinH anil Reynolds. IW3),
A tectonosedimentary model is established for the Composite Reef at Far East Vertical Shaft, East Rand Proprietary Mine, Central Rand Goldfield. There the Composite Reef comprises the Main Reef and the Main Reef Leader, with the South Reef occurring sporadically in the hanging wall, redefining the Composite Reef stratigraphy for this area. Tectonic controls on sedimentation persisted throughout the deposition of the Composite Reef, influencing the nature and distribution of the conglomerates. Utilising the Composite Reef model, combined with structural and sedimentological modelling of the Central Rand Goldfield, a tectonosedimentary model for the Main Conglomerate Formation is proposed. Pre- and syn-depositional folding associated with regional basin-wide compression resulted in the formation of the Springs Monocline, the West Rand Syncline and associated DRD Anticline. This was overprinted by northwest to southeast oriented folding associated with left-lateral wrenching on the Rietfontein Fault, forming a corrugated palaeosurface, prior to Main Reef deposition that controlled the palaeoflow direction. Brittle deformation initiated during Witwatersrand times in the form of Riedel and Riedel conjugate shears, normal faults, principal shears and P-shears associated with left-lateral wrenching caused northeast/southwest and east/west cross-cutting channel orientations and northeast/southwest oriented erosion channels. The deposition of the Black Bar, associated with a marine transgression, accumulated in topographically lower lying areas, smoothing the palaeotopography prior to Main Reef Leader deposition. This smoothing effect combined with syn-depositional folding resulted in a single Main Reef Leader channel complex associated with the Robinson Deep Syncline, and restricted Main Reef Leader deposition to an area bounded by the Springs Monocline in the east and the DRD Anticline in the west. Brittle deformation continued during Main Reef Leader deposition resulting in cross-cutting channels. The tectonosedimentary model that has been established increases the confidence of modelling the distribution of conglomerates of the Main Conglomemte Formation, thereby facilitating feasibility modelling of the down-clip, un-mined South Central.
2009 11th SAGA Biennial Conference and Exhibition, Swaziland, 16-18 September 2009 The Waterberg Coalfield is destined to become the major source of energy for South Africa in the future. In 2008, Coaltech Research Organisation funded an airborne magnetic and radiometric survey over the Karoo-age Ellisras Basin in which the coalfield is developed. Interpretation of the magnetic data has provided a novel half-graben model for the structure of this basin. The northern boundary is the block-faulted Melinda Fault Zone, with the southern, less-faulted part of the basin sloping gently to the north. The thickness of the Karoo Supergroup reaches 1 500m in the eastern part of the basin, and decreases to the west. The new geophysical data has contributed much to the understanding of the geological evolution of this important coalfield.
Copyright: 2006 Geological Society of South Africa Borehole radar is an electromagnetic tool that can be applied to assist in the delineation of orebody geometry, ideally using routinely drilled cover and exploration boreholes. Successful trials of borehole radar for delineating reef horizons on South African gold and platinum mines have led to the development of a borehole radar system specifically designed for routine application in those environments. The radar design includes novel elements, including a receiver with instantaneous sampling down the borehole, and it is implemented in probes that can operate in 48 mm boreholes, with development planned for 38 mm boreholes. The radar is known as the Aardwolf BR40. The need for information about dislocations of 3 m to reefs determines the desirable radar resolution while available access geometry determines the range requirement. The electrical properties of typical gold and platinum rocks show that the range/resolution trade-off is feasible for the majority of economically important reef horizons. Boreholes drilled horizontally or upwards are accessed using a borehole crawler. Trials of the radar show that it meets its performance specification. The radar is robust enough for routine work underground and is easy to use. The borehole radar is a useful addition to the toolbox of the mining geoscientist because it can give information about the reef plane along a line, rather than the single point information about the reef given by the borehole.
The results of ion microprobe analyses of cores and rims of grains of zircon from a peraluminous leucogranite intruding and intruded by the Ni-Cu mineralization of the Phoenix deposit in the Tati Granite-Greenstone Terrane of Botswana are interpreted to indicate that: 1) rims formed 1022 ± 16 Ma ago during emplacement of the granite and 2) the cores are xenocrysts, derived from the sources of the granitic magma, which contain zircon of ages between about 1036 Ma and 1786 Ma. These results were unexpected because the Tati Granite-Greenstone Terrane is generally assumed to be part of the Zimbabwe Craton and hence composed of rocks of Archaean age. Emplacement of leucogranite in this area at about 1022 Ma clearly challenges this assumption. Because the leucogranite contains lenses and veins of massive Fe-Ni-Cu sulphides, this mineralization must have occurred more recently than about 1022 Ma. It is possible that the emplacement of mafic to ultramafic magma with its probable immiscible Fe-Ni-Cu-S magma provided the heat to melt rocks of the Penhalonga or Selkirk Formations to produce the leucogranitic magma. If so, all of the Ni-Cu mineralization in the Phoenix deposit took place about 1022 Ma ago.
Low elastic strength of ancient lithosphere based on flexural analyses has been interpreted to reflect elevated regional geothermal gradients in response to higher global heat production in the past. Here we present a flexural analysis of Archean/Palaeoproterozoic sediment cover along the western margin of the Archaean Kaapvaal craton based on seismic stratigraphy. Our results show that between ∼1.93 and ∼1.75 Ga, the Archaean margin of the craton had an effective elastic thickness of 7.5 to 10km compared to its present day value of 60 to 70km. Because the Kaapvaal craton had already stabilized by ∼2.7 Ga and was underlain by 150 to 300km thick strong mantle lithosphere, it is unlikely that the relatively thin elastic thickness along this old margin reflects a change in secular cooling of the Earth. Instead, we interpret the low elastic strength to be a transient marginal tectonic effect similar to that recorded along modern continental margins.
In reply to comments by W.U. Reimold on ”Pseudotachylite in the South Boundary Fault at the Cooke Shaft, Witwatersrand Basin, South Africa by P.W. Mambane et al., S. Afr. J. Geol. 114.2, 109–120. 1. The Introduction statement in Mambane et al. (2011) directs the reader to consider that authors such as Schwarzman et al. (1983), Gibson et al. (1997), Spray (1998), Eiko (2004), Riller et al. (2010) and others, have in the past, considered pseudotachylites as a diagnostic feature of shock tectonics. Mambane et al. make clear in the Introduction that pseudotachylites are formed in a number of settings and under a number of P-T conditions. 2. The perseverant objections to the use by Mambane et al. of the term “pseudotachylite breccia” are not tenable. It is agreed that terminologies used by Shand (1916), Bisschoff (1962), Reimold and Colliston (1994), and Reimold and Gibson (2006), and others, are not well-defined. A better classification scheme is needed to assist in the description of the different types of pseudotachylite, and the meaning of pseudotachylite textures including breccia, “melt breccia”, and “tectonic pseudotachylite”. It seems that Mohr-Westheide and Reimold (2011) and Reimold (2011) are occupied with this task, but that being mostly after review and publication of Mambane et al., which raises the question of how Mambane et al. could consider articles that appeared after the March 2011 acceptation date of their article, or a June 2011 publishing date. For example, Mohr-Westheide and Reimold (2011) was first published on the 17 March 2011, …
Mambane et al. (2011) presented a geological analysis of an occurrence of different fault rock types at the South Boundary Fault on Cooke Shaft, in the West Rand goldfield of the Witwatersrand Basin. They refer to both pseudotachylite and pseudotachylitic breccias (below referred as PTB) being present at that location. They state that based on the conclusion of previous research, namely that “…the Hekpoort Andesite Formation (2241 ± 21 Ma) of the Transvaal Supergroup provides a maximum age for all pseudotachylite in the West Rand goldfield may therefore need revision”, had to be revised, whereas their own findings constrained “the relative age of tectonically–induced pseudotachylite formation…to pre-2.58 Ga…” (their p. 109). Their article does, however, not adequately refer to the previous work on Witwatersrand pseudotachylitic breccias, as borne out of the reference list they provide, and thus I feel obliged to give a plug to previous Witwatersrand pseudotachylitic breccia workers by highlighting some of their findings that seemingly have been overlooked. This may also assist the authors to address their own astonishment about: “It is therefore curious that attempts have been made to correlate pre-2.58 Ga tectonically-induced pseudotachylites from the goldfields with ca. 2.02 Ga impact-induced pseudotachylites in the Vredefort Dome” (p.118), at which instance particular reference to previous work by myself and co-workers is being made. Before I begin to discuss the work by Mambane et al. and their inferences, it is necessary to address a serious problem these authors raise in their Introduction (p.109). They state: “It [ the occurrence of pseudotachylite , the author] is also considered a diagnostic feature of shock tectonics (metamorphism) associated with impact cratering,…”. “Shock tectonics is confusing, as impact-related deformation does not only occur during the shock compression phase, but particularly thereafter – during the modification phase …
Summary alteration map showing alteration throughout the goldfields, in the adjacent granitoid terrane, and probably the interior of the basin (after Phillips, 1988; Klemd et al., 1994; Fox & Winkler, 1997). The cross-section highlights the role of the Jeppestown Shale and Booysens Shale and Ventersdorp lavas as less-permeable barriers, and the pervasive nature of the alteration throughout much of the Central Rand Group.
Log aOz-pH diagram illustrating the dependence of muscovite-pyrophyllite equilibrium on aK and hence pH. Without constraints on aK, this equilibrium cannot be used to uniquely constrain pH and hence gold solubility. Gold solubility reaches a maximum near the H 2 S-Hs-boundary.
Map of the Witwatersrand Basin showing the distribution of metamorphic assemblages. Greenschist facies grade is inferred for all the major gold mines, amphibolite to granulite facies grades were attained in the lower Witwatersrand and underlying basement from the more deeply buried central portions of the basin exposed in the Vredefort Dome. Areas of documented greenschist facies metamorphism are indicated by the diagonal lines and higher grade areas around the _Yredefort Dome by horizontal lines (a= granulite facies; b = amphibolite facies).
An illustration of the peak metamorphic conditions attained in the goldfields (1) and the pressure-temperature evolution of the lower West Rand Group exposed in the Vredefort Dome (2) and the basement to the Witwatersrand Basin exposed in the core of the Vredefort Dome (3); all after Phillips & Law, 1994, and Stevens et al., 1997a; 1997b).
Some inferred alteration reactants and products (Witwatersrand)
The Witwatersrand Basin records evidence of widespread alteration within the Central Rand Group in all goldfields and well into the central part of the basin, and also within mineralized portions of the West Rand Group. This alteration has affected all rock types and is characterized by assemblages of pyrophyllite-chloritoid-chlorite-muscovite-pyrite-rutile-tourmaline-quartz, or combinations of these minerals, plus variable albite, paragonite, margarite, kaolinite, kyanite, sudoite, and pyrrhotite. Alteration assemblages are spatially related to mineralization, with both being best-developed in the Central Rand Group, and within the goldfields. An approximate estimate of the volume of rock in the Witwatersrand Basin affected by this alteration is 5000-50 000 km3, and the temperature of the implicated fluid exceeded 300°C. Based on alteration assemblages, fluid inclusions, and metal ratios, the composition of this fluid had many similarities to the fluids inferred to be responsible for the transport of gold in many slate-belt and Archaean greenstone gold deposits (e.g. similarities in temperature pressure, salinity, redox state, H2O-CO2-H2S composition, and associated elements such as As, Sb, and Hg). The distinctive K-alteration assemblage in the Witwatersrand goldfields of muscovite plus pyrophyllite is more likely to reflect the poor buffering capacity of the Central Rand Group rocks following diagenesis rather than unusual fluid acidity. Acidity is poorly constrained, and K/H was probably not constant throughout the alteration system. Regional metamorphism alteration of greenschist facies grade which stabilized pyrophyllite-chloritoid has affected all the goldfields around the basin margin. This metamorphic pattern in the goldfields has been linked to the amphibolite-granulite facies metamorphism in the Vredefort dome area in the basin centre, and both metamorphic progressions are inferred to be part of one high-temperature-low-pressure metamorphic event. Although the most obvious major thermal event to explain metamorphism around the Witwatersrand Basin is the Bushveld Event at 2050 Ma, the geological evidence better supports an age of metamorphism, generation and migration of substantial fluid and alteration of the Witwatersrand goldfields around 2700-2600 Ma. The latter is also the likely age of gold mineralization. The coincidences of goldfields and alteration spatially, and Witwatersrand alteration fluid and greenstone gold fluid compositionally, give credence to the concept that a major hydrothermal fluid event after burial of the Witwatersrand Basin was a critical element in the formation of this enormous goldfield. This hydrothermal scenario casts serious doubts on the placer model for Witwatersrand gold, but importantly continues, and possibly enhances, the role of sedimentology, stratigraphy, and structure in the localization of gold. In particular, iron and carbon played critical roles in precipitation of gold from solution, and the distribution of both Fe and C was influenced by sedimentary processes.
Rowe and Backeberg (this volume) compares the morphology of cm scale structures formed by fluidization with similar looking structures of the Fold Zone (tens of m scale) for which Blignault and Theron (2010) proposed a model of folding essentially by buckling. The Blignault and Theron model is an integration of structure, glaciological aspects and lithofacies associations as observed over the full outcrop area of ca 300 km. Some of the comments made by Rowe and Backeberg require clarification. They are of the opinion that folding by buckling results in “an essentially unlimited long axis”. However, the extent of fold axes is a function of the extent of the stress field causing the folding. Blignault and Theron (2010) noted that research published during the last ten years on the ice-bed interface, shows how ‘sticky spots” vary spatially and temporally as water pressure at the interface varies. The implication is …
As Dr. Cole correctly deduces, the primary intention of the article was to document the first definitive identification of detrital gold in the heavy mineral deposits of the Vryheid Formation (Ecca Group) near Muden. This discovery was an unexpected facet of a larger on-going study into the source of the economically important modern beach …
The writer wishes to comment on some aspects of the geology, petrology and geochemistry of the paper by Van Tonder and Mouri (2010) that are deemed to be problematic. In their paper the authors have chosen to subdivide the Johannesburg Dome (JD) granitoids into three main suites, namely: 1. a Tonalite Gneiss suite (TG) around the southern boundary of the dome; 2. a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part of the dome; and 3. a Granodiorite/Adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. This subdivision differs from earlier explanations of the geology of the granitic rocks on the dome (Anhaeusser, 1971; 1973). Under normal circumstances, when presenting new interpretations of previously studied geological environments, it is customary to provide new data and argument in support of any suggested changes to the geological status quo. In their study, the authors state that previous geochemical work recorded on the JD is very limited, with only one dataset of major, some trace element, and some rare earth element analyses of isolated areas. The authors then state that in their work an attempt is made, using a complete set of petrographic, mineral chemistry, major-, trace- and REE data, to identify the subcomponents of the Archaean granitoid rocks of the JD, to classify them within the current framework of understanding of the TTG suites and to propose processes involved in the petrogenesis of these granitoids. The writer takes issue with the claim by the authors that they have provided a more complete dataset than the information made available earlier by Anhaeusser (1971; 1973 1999). It is accepted that a copious supply of major-, trace- and REE data was not supplied previously (due to analytical cost considerations), but what was provided were very selective analyses of the best preserved, …
This article is a useful contribution to the study of heavy mineral deposits in the Karoo Supergroup, particularly with regard to the discovery of gold inclusions, which have not been previously reported from the Muden …
The authors of the paper would like to acknowledge the substantial pioneering work done on the Johannesburg Dome (JD) by C.R. Anhaeusser as was referenced in the said paper. Although the subdivision of the JD granitoids proposed in the paper differs from previous work by Anhaeusser (1973), it is based on new and existing geochronological data, field relationships and geochemistry. The proposed suites are broadly similar to those proposed in the new book “The Geology of South Africa” by Robb et al. (2008) of which Anhaeusser was a co-editor: 1. a Granodiorite- to Adamellite Gneiss suite (GAG) with local development of tonalite and trondhjemite gneiss, i.e. probably the ~3.34 Ga rocks of the same composition of Poujol and Anhaeusser (2001) (Lanseria Gneiss, Robb et al, 2008) occurring over much of the northern …
This paper describes and discusses fluid inclusion data from a number of Archaean gold deposits from Western Australia. The deposits discussed are shown, and the nature of mineralization at each of these deposits is given. Analytical techniques are first described to establish the validity of results. Results are then given, and interpretations made of the nature of ore fluid, gold depositional conditions and probable fluid source. Future work is briefly discussed. Refs.
The state of knowledge on the tectonic and geomorphological evolution of southern Africa during the post-Gondwana period is reviewed in the context of Alex du Toit's fundamental contributions to these fields of geology. Basic to an understanding of post-rifting events are, firstly, the high elevation which much of Africa possessed prior to rifting; secondly, the erosion of one to three kilometres from its surface during the Cretaceous; and thirdly, the role of Neogene uplift in re-establishing high elevations, particularly within the eastern half of the subcontinent. This history is traced through the massive denudation of the early Cretaceous, which was followed by the establishment of a dense, integrated drainage net on a well-planed land surface from the Santonian onwards. The configuration of the Upper Cretaceous river system is fundamental to a comprehension of the present distribution of alluvial diamonds and of gems transported into the sea via these conduits. Equally significant for an appreciation of the present macro-geomorphology of southern Africa is the continent-wide planation surface - known as the African Surface - generated by the multi-phase cycle of Cretaceous erosion. This surface forms a readily identifiable datum across the high plains because of the widespread preservation of deep weathering and massive cappings of laterite and silcrete on remnants which have survived later dissection. The African silcretes reflect a world-wide shift to greater aridity at the beginning of the Palaeocene. The evidence for large-scale Neogene uplift, particularly within the eastern half of the subcontinent, is now beyond question and argues for the late development of at least the southern part of the African Superswell. The largest movements post-date the Miocene and have contributed both to the anomalous elevations of the eastern hinterland and to the strong east-west climatic gradient across southern Africa. Controversies surrounding the mechanisms underlying these recent movements appear to have been resolved in favour of buoyancy forces originating from a massive low-density anomaly in the Earth's mantle below East and southern Africa.
Declination observations in Cape Town carried out at the Cape of Good Hope Observatory during the period 1841 until 1903. Also shown are comparisons with the COV-OBS and the gufm1 models.
H-component contour plot as derived from a SCHA model for southern Africa based on the 1903.5 field survey data. Contour intervals are 500 nT. I subsequently used my polynomial secular variation models and calculated annual variation values for D, I and H at 0.5 degree intervals for the area between 25°S and 35°S, and between 17°E and 32°E. Contour plots for D, I and H secular variation are shown in Figures 6, 7 and 8, respectively.
A contour plot showing the secular variation pattern over Southern Africa for D, as determined by a 2nd degree polynomial fit to measurements for 1900 to 1905. The contour interval is 0.5 min/yr.
Inclination secular variation contour plot over Southern Africa for the period 1900 to 1905 as determined by a 2nd degree polynomial model. The contour interval is 0.2 min/yr.
A contour plot for H secular variation over the southern African region as determined from a 2nd degree polynomial model fit to
The requirements of navigation, rather than any scientific interest in geomagnetism, prompted the recording of data of magnetic field components at the Cape of Good Hope (CGH), even before 1600. The importance of knowing the deviation of a compass needle from true north was sufficient to warrant the establishment of a recording station at the Cape in 1841. The first magnetic survey of the Union of South Africa, comparable with that of developed countries in the Northern Hemisphere, was carried out by Beattie (1909), assisted by J.T. Morrison of the University of Stellenbosch, who subsequently reduced the data to 1st July 1903. In this paper we compare the measurements in Cape Town between 1840 until 1903, as well as the field survey of 1903.5, to COV-OBS and gufm1, historical global geomagnetic field models covering the observatory and satellite period of 1840 until 2010 and 1590 until 1990, respectively, while declination observations show clear evidence of a geomagnetic jerk around 1880 to 1883, which correlates with similar observations in Europe a few years earlier (1870). The field survey data of 1903.5 are used to derive a Spherical Cap Harmonic Model for southern Africa using observations of declination (D), inclination (I) and horizontal intensity (H). We also used secular variation measurements for D, I and H obtained during 1900 to 1905 at a few select stations to derive a secular variation model for Southern Africa based on a polynomial approach.
An ammonite identified as Metaplacenticeras subtilstriatum (Jimbo, 1894) was recovered from a borehole in Zululand. It represents the first record of this genus and species outside the Pacific Realm, and permits a precise Late Campanian (Cretaceous) date for that section of the core. -Authors
On March 19th, 2005 a moderate (M~4.6) earthquake struck Monatélé, a small city near Yaoundé, the capital of Cameroon. It was the largest earthquake recorded in the area. No injuries to people or damage to property were reported. We analyse broadband seismograms recorded by the Cameroon regional network to relocate the event and determine the source mechanism. Seismicity is further correlated with the gravity data to investigate the seismotectonic implications. The epicentre lies close to the Northeast segment of the Sanaga Fault, the most continuous tectonic lineament in Central Africa. The focal depth is 11 km, placing the earthquake source in the upper crust. The source mechanism was found to be dextral strike slip, with one nodal plane coinciding with Southwest-northeast strike of the Sanaga Fault. The results from earthquake source mechanism, geological observations, seismicity and gravity analyses all indicate movement between the Northern edge of the Congo Craton and the Southern edge of the Pan-African Belt. The Monatélé earthquake therefore provides evidence that the contact between of the Congo Craton and the Pan-African Belt (the location of Cameroon’s greatest earthquake, M~5.9 in 1945) is still seismically active.
An Archaean greenstone remnant, intermittently developed over a distance of approximately 4 km on the farm Zandspruit 191-IQ, is surrounded and intruded by porphyritic granitic rocks. The greenstones, which consist of cyclically repetitive units of harzburgitic, lherzolitic, and pyroxenitic rocks, have been altered to serpentinites and metapyroxenites, respectively, and the intruding granitic rocks have assimilated xenolithic remnants of the ultramafic rocks. An account is given of the geological relationships in the Zandspruit - North Riding area, including the nature of the later dyke- and sill-like intrusives found in the region. Particular attention is drawn to the manner in which the greenstones have been influenced by the granitic rocks, including descriptions of localities where the ultramafic assemblages are hybridized to dioritic rocks or have been intensely altered as a result of potash metasomatism. -Author
Slack-water sediments as indicators of flood stage can accurately indicate the occurrence and magnitude of modern floods and palaeofloods. No evidence exists for the Buffels River ever experiencing a flood with a discharge exceeding the 1981 Laingsburg flood. Palaeoflood-hydrological modelling for the Wilgehout and Baviaans tributaries, using a prominent knick-line feature and palaeocompetence modelling of in-channel boulders respectively, indicate, that these rivers have experienced palaeoflood discharges of 1.8 and four times greater than that of the 1981 flood. A low probability does exist that the Buffels River at the confluence of the Wilgehout, Buffels, and Baviaans Rivers could in future experience a discharge of 8000 m3/s, which is 25% higher than the 1981 discharge. This prediction highlights the need to incorporate palaeoflood information when calculating realistic design flood figures for potentially high-risk construction such as dams. -from Author
Evidence relating to a possible volcanic eruption in western Lesotho during 1983 is documented and evaluated. It is concluded that a small quantity of highly vesicular, basaltic lava was extruded from a narrow orifice associated with a fracture in basalt of the Drakensberg Group. The eruption was accompanied by a seismic event measuring 4.3 on the Richter scale. The new lava has a chemical composition very similar to that of the 183-Ma-old host rock, but is isotropic and glassy, with no crystalline phases. In contrast, the Drakensberg Group basalt is almost completely crystalline, although many of the primary minerals display chemical alteration, with the mesostasis being devitrified. Excavation of the vent shows it to be fed by a narrow pipe or conduit, the sides of which are thinly coated by glassy material in which flow structures record slumping back of a very small residual portion of the lava after the eruption. The morphology of the conduit, vesicular nature of the lava, the quantity produced (between 0.3 and 1 m3), and the apparent rapidity with which it was extruded seem to preclude the event having been occasioned either by a lightning strike or an electrical discharge from the nearby power line.
Three minute hetermorph ammonites of the genus Worthoceras were recovered from a core of a borehole in Zululand. This is the first record of the genus from South Africa. Their association with Neostlingoceras carcitanense permits accurate dating with the eponymous zone of the Cenomanian stage.
Five Early Archaean (i.e. ~3,0 Ga) metavolcanic and metasedimentary suites, namely Dwalile, Assegaai, de Kraalen, Commondale, and Nondweni, preserved as isolated remnants within enclosing granitoids, are described. Archaean evolution was terminated by the emplacement of post-Pongola granitoids mainly within the core of the Pongola basin. The generation of these granitic melts by partial melting of sialic crust due to depression of the depositional basin is consistent with geochemical data. Northern Natal and the adjacent areas of Swaziland provide a continuous record which will enable models of crustal evolution to be tested. -from Authors
Uranium-lead dating of apatite was undertaken by Laser Ablation-Sector Field-Inductively Coupled Plasma Mass Spectrometry (LA-SF-ICPMS) in situ on apatite from principal rock types of the Loolekop phoscorite-carbonatite intrusion within the Phalaborwa Igneous Complex, South Africa. In situ U-Pb analysis on selected apatite produces U-Pb ages of 2 083.9 ± 41.9 Ma (n = 33; MSWD = 0.87), 2 020.4 ± 116.7 Ma (n = 18; MSWD = 0.91) and 2 034.3 ± 39.0 Ma (n = 17; MSWD = 0.6) for phoscorite, banded carbonatite and transgressive carbonatite, respectively, with a combined age of 2 054.3 ± 21.4 Ma (n = 68; MSWD = 0.86), which we interpret to indicate the timing of emplacement. Apatite U-Pb dates are similar to dates reported in previous studies using zircon and baddeleyite U-Pb systems from the same rock types, showing that apatite can be used as geochronometer in the absence of other commonly used U-Pb-bearing accessory minerals, not only in carbonatite-phoscorite complexes, but in all mafic igneous intrusions. Similar ages for zircon, baddeleyite and apatite indicate little to no re-equilibration of the latter, and suggest that the Loolekop Pipe intrusion cooled below 350°C within ~21 Ma of emplacement. This conclusion is supported by apatite BSE images and trace element systematics, with unimodal igneous trace element characteristics for apatite in each sample. The combination of in situ U-Pb geochronology, trace element geochemistry and BSE imaging makes apatite a useful tool to investigate the emplacement mechanisms of carbonatite-phoscorite complexes, which is particularly advantageous as apatite is one of the main mineral phases in these rock suites.
Single-crystal Pb-evaporation age determinations have been made of zircons from basic lava from a pre-Transvaal volcano-sedimentary outlier in the Derdepoort area, northwestern Transvaal. The 2769.3 ± 2.3 Ma age obtained from these determinations demonstrates that the Derdepoort remnant should be correlated with the Gaborone Granite Complex and Kanye Volcanic Formation in Botswana and with the Kgale Granite Complex in the area north of Mafikeng. Previous alternative correlations with either the Dominion Group or the Ventersdorp Supergroup are no longer tenable. This correlation shows that the Gaborone-Kgale-Kenye event extends over larger areas than previously known. Additionally, the new data point to a decrease in the age of this event from Botswana (2.83 Ga), via the Mafikeng area (2.78 Ga) to Derdepoort area (2.77 Ga), suggesting a regional shift of the center of magmatic activity from west to east during this period. -Authors
Integrated U-Pb geochronology and palaeomagnetic study of mafic to felsic volcanic rocks of the Derdepoort Belt of South Africa are employed to test the hypothesis that the Pilbara and Kaapvaal Cratons were joined as part of a Late Archaean 'Vaalbara' supercontinent. An age of 2782 ± 5 Ma is deduced for eruption of the Derdepoort basalts, bracketed by a concordant SHRIMP zircon age of 2781 ± 5 Ma for overlying felsic volcanics and a concordant isotope dilution zircon age of 2783 ± 2 Ma for underlying granite of the Gabarone Complex. Based on the low (subgreenschist) metamorphic grade of the basalts, the presence of highly stable single domain magnetite, and a positive conglomerate test, the magnetization of the Derdepoort basalts is inferred to date from the time of their emplacement and cooling at 2782 Ma. Results yield a primary palaeopole at 005°E, 40°S (A95 = 18°), and indicate a palaeolatitude of 64.5 ± 17.5°for the Kaapvaal Craton at 2782 Ma. Published palaeomagnetic data for the Mount Roe Basalts of the Pilbara Craton indicate a palaeolatitude of 34.3 ± 6.4°at 2772 ± 2 Ma. The latitudinal separation of 30°implies that the cratons were not contiguous at 2.77 to 2.78 Ga, although the possibilities that the cratons could have been joined during other intervals of time, or that they were non-contiguous parts of a larger continent, are not ruled out.
Metallurgy, and mining metal ores, was first introduced to southern Africa about 2000 yr ago by early farmers. During the first millennium AD iron and copper were exploited, with tin and gold being included by the beginning of the second millennium AD. Indigenous farming communities prospected for iron ore and malachite, and later gold and cassiterite, reduced the ores, fabricated jewellery and implements as well as trade ingots from the metals and participated in intra- and intercontinental trade in these goods. The history of indigenous mining and metals technology is poorly known but the combination of early descriptions by mining engineers and the modern interdisciplinary study of archaeometallurgy is providing better insight into this largely neglected aspect of mining history. -Author
Map of distribution of observatories (red asterisks) and repeat stations (numbered white dots) in the southern African region. Observatories are situated in Hermanus (HER), Tsumeb (TSU); Keetmanshoop (KMH) and Hartebeesthoek (HBK). The exact locations of the repeat stations are given in Table 1.
Secular variation of the Declination for the three epochs 2007.0, 2008.0 and 2009.0 from left to right. The top panel shows the field model derived from the polynomial approach, the bottom panel refers to the spline-based model.
Secular variation of the East component at Hermanus (HER, top) and Keetmanshoop (KMH, bottom) observatories. The black dots show monthly mean secular variation estimates derived as running annual differences. The green curves represent the continuous spline model, while the dashed blue lines show piecewise linear fits. The GRIMM2 model prediction is plotted in yellow for reference. Note the different time scales.  
The southern African region is in close proximity to the South Atlantic Anomaly, a region between Africa and South America where the geomagnetic field is significantly weaker than at other comparable latitudes and is decreasing strongly. Between 2005 and 2009 very dense annual magnetic repeat station surveys were conducted in southern Africa within the framework of COMPASS (Comprehensive Magnetic Processes under the African Southern Sub-continent). This project aims at studying the regional geomagnetic field and particularly its evolutionary behaviour as part of the Inkaba yeAfrica cooperative project between Germany and South-Africa. We have modelled the magnetic field and its secular variation by means of two different techniques, one based on surface polynomials, the other on harmonic splines. Both approaches succeed in describing the characteristic time variations of the magnetic field components in this area. The rapid changes observed in the declination and vertical component during the period of investigation and revealed by the modelling results are of particular interest. Secular variation changes observed in the time-series of the Hermanus and the recently established Keetmanshoop magnetic observatories reveal the occurrence of a geomagnetic jerk in 2007.
Seismic activity recorded by the South African National Seismograph Network in southern Africa during the period January to December 2006 is summarized in this article.The South African National Seismograph Network was expanded to 23 seismological stations and the data communication infrastructure updated to transmit continuous waveform data in near real-time during 2006. Earthquake data, published in the Seismological Bulletins of the Council for Geoscience for 2006, was revisited and all tectonic earthquakes relocated together with mining related earthquakes that were located outside known mine boundaries. The resulting earthquake database comprises a total of 3875 located seismic events, the majority of which (79%) is located in the gold and platinum mines of South Africa. A total of 1460 events were ascribed to chemical explosions in opencast mines and quarries and flagged accordingly in the database. The recorded seismic activity, tectonic as well as mining-related, is discussed in terms of the error in location and the observed frequency-magnitude distribution. A fault plane solution for a M L=4.0 earthquake, located in the gold mines of South Africa, is also presented. The most significant earthquake occurred on the southernmost extension of the East African Rift System in south-western Mozambique and measured 7.0 on the moment magnitude scale. The earthquake was widely felt throughout the region, causing four fatalities, 27 injuries and damage to approximately 160 buildings.
Hermanus X-component secular variation as a function of time between 2005 and 2009.
Y-component secular variation at Hermanus between 2005 and 2009.
A plot showing the Z secular variation pattern between 2005 and 2009.
Quiet-time mean monthly values from the INTERMAGNET observatory at Hermanus (HER) in South Africa were used to study the changes in secular variation during the period between 2005 and 2009. After removing an annual variation resulting from magnetospheric and ionospheric currents by means of a 12-month running means applied to the respective observatory first differences of the X, Y, and Z components, clear evidence was revealed of a strong geomagnetic jerk that occurred during 2007 in this area. The GRIMM-2X model also provided evidence of the occurrence of this jerk in 2007. Of particular interest is that GRIMM-2X predicts the turning points in all the secular variation trends to occur much earlier than revealed by the observatory data. We also observed that the power of this jerk, determined as the difference in slope of the secular variation before and after the jerk, is several times stronger than the global jerk of 1982/3.
The authors would like to thank Hobbs (2015) for his keen scrutiny of some aspects of the paper by Abiye et al. (2015). Even though he used strong phrases including tenuous, irresponsible, negligent, unwitting dissemination of disputable information, we understand that it was due to a lack of understanding of the key aspect of the paper. Abiye et al. (2015) focused on the environmental isotope application in surface water and groundwater interaction, while stream discharge measurement was …
On the 6th of February 2016 at 11:00 hours local time (0900 UTC), KwaZulu-Natal was struck by an earthquake of local magnitude ML=3.8. The epicentre of the earthquake was located offshore in the Durban Basin. The earthquake shaking was widely felt within the province as well as in East London in the Eastern Cape province and was reported by various national media outlets. Minor structural damage was reported. A macroseismic survey using questionnaires was conducted by the Council for Geoscience (CGS) in collaboration with the University of KwaZulu-Natal (UKZN) which yielded 41 intensity data points. Additional intensity data points were obtained from the United States Geological Survey (USGS) Did You Feel It? programme. An attempt was made to define a local intensity attenuation model. Generally, the earthquake was more strongly felt in low-cost housing neighbourhoods than in more affluent suburbs.
Significant lateral variations within certain units in the upper Critical Zone of the western Bushveld Complex have been revealed by exploration and mining activities in the Marikana area since 1965. The UG2 Chromitite Layer in the northwest of the farm is 0.75 m to 0.80m thick with three leader chromitite layers occurring above it within the UG2 Pyroxenite, whilst in the southeast a thicker UG2 Chromitite Layer (1.0 m to 1.2 m) with only one or two leader chromite layers is present. Other lateral variations in thickness and/or lithology in the Merensky Pyroxenite, and the Bastard Pyroxenite/Norite are described. -from Author
Representative cross-sections from underground mapping of the walls of the Merensky Reef drive at Wilgerspruit. Vertical and near-vertical black lines represent faults.
Representative profiles of the Pseudo Reef Unit and the overlying Merensky Footwall Unit in the Pilanesberg, Union and Amandelbult enclaves of the Swartklip Sector.
Postulated direction of intrusion of the Merensky Reef magma into the Pilanesberg, Union and Amandelbult lobes of the Rustenburg Layered Suite in the Swartklip Sector. At the time of intrusion, the sectors are postulated to have been separated by the Southern Gap and Northern Gap domes, which collapsed to form graben structures prior to intrusion of the Upper Zone. Areas shaded red represent Merensky Reef at low levels in the stratigraphy (i.e., in so-called "regional pothole" environments). Areas shaded orange represent Merensky Reef at a relatively high level in the stratigraphy, i.e., the "Normal " reef of Viring and Cowell (1999) or the "Main" reef of Lomberg et al. (1999), where the Merensky footwall succession is most completely developed. The area marked by black vertical lines corresponds to the regional gravity high illustrated by Viljoen (1999), and is interpreted to be the locus of dykes from which the sills that formed the ultramafic layers of the UG1, UG2, Pseudo Reef, Merensky and Bastard Units intruded. Yellow arrows mark the directions of lateral intrusion of the sills.
In the Bushveld Complex, the ultramafic (orthopyroxenite/harzburgite with chromitite) layers that host most of the PGE and chromite mineralization in the Upper Critical Zone display well-documented discordant basal contacts with their anorthositic and noritic host rocks. Whilst not so well documented, there is evidence that the upper contacts of these units are also discordant. We review the nature of the contacts between the ultramafic units and adjacent plagioclase-rich lithologies. These include contact phenomena like pegmatoidal lithologies and thin magmatic reaction chromite stringers. We conclude that most, if not all, ultramafic layers were intruded as sills into pre-existing norite/anorthosite cumulates. The sequence of norites and anorthosites that hosts the ultramafic layers was built up by a prior series of multiple tholeiitic (A-type) magma intrusions. The spectrum of lithologies from melanorite through to (mottled) anorthosite represents differing degrees of partial melting in response to these successive magma influxes. Density and competence contrasts between layers of plagioclase-rich rocks in turn provided pathways for sill propagation of subsequent ultramafic (U-type) magmas. The ultramafic magmas further modified the host norites and anorthosites by processes of partial melting and metasomatism. The ultramafic units themselves accumulated as composite sills in response to multiple magma injections. In the western Bushveld Complex, particularly including the Swartklip Sector in the north-western part of the complex, the Merensky Reef is represented by various facies that occur at different levels in the host stratigraphy. This phenomenon has been referred to by the term “regional potholing”, and has been attributed to the erosion of footwall cumulates by new influxes of magma. We suggest that a series of step-and-stair-type transitions of intruding sills to successive stratigraphic levels might more appropriately explain the various facies of the Merensky Reef.
The Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations. Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.
The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.
The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most non-silica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also – within the resolution of radiometric age data – contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the “just right” degree of heating and very early hydrothermal silicification.
The Schapenburg Schist Belt is one of several large greenstone remnants exposed in the granitoid-dominated terrane to the south of the Barberton greenstone belt and is unique in that it contains a well-developed metasedimetary sequence in addition to the typical mafic-ultramafic volcanic rocks. Detrital zircons within the metasediments have ages as young as ~3.24 Ga and consequently, these rocks correlate with Fig Tree Group sediments exposed in the central portions of the Barberton greenstone belt some 60km to the north, where they are metamorphosed to greenschist facies conditions. The Schapenburg metasediments are relatively K2O-poor, and are commonly characterised by the peak metamorphic assemblage garnet + cordierite + gedrite + biotite + quartz ± plagioclase. Other assemblages recorded are garnet + cummingtonite + biotite + quartz, cordierite + biotite + sillimanite + quartz and cordierite + biotite + anthophyllite. In all cases the peak assemblages are texturally very well equilibrated and the predominantly almandine garnets from all rock types show almost flat zonation patterns for Fe, Mg, Mn and Ca. Analysis using FMASH reaction relations, as well as a variety of geothermometers and barometers, has constrained the peak metamorphic pressure-temperature conditions to 640 ± 40°C and 4.8 ± 1.0 kbar. The maximum age of metamorphism is defined by the ~3.23 Ga age of a syntectonic tonalite intrusion into the central portion of the schist belt. Thus, sedimentation, burial to mid-crustal depths, and amphibolite facies equilibration were achieved in a time span similar to ~15 Ma. The strong bedding-parallel foliation in the metasediments dips to the east at an angle of 75 to 85° In this foliation plane elongated cordierite rods produced during prograde metamorphism define a close to vertical mineral lineation. The metasedimentary succession youngs to the east and in this direction is overlain by older Onverwacht Group rocks. Thus, the cordierite lineation might represent a transposed lineation developed during thrust stacking. Post-kinematic static recrystallization has largely obscured the prograde history of the rocks. However, the pressure-temperature conditions of peak metamorphism, the age of metamorphism, the rapidity of metamorphism following sedimentation, the presence of cordierite on the prograde path, as well as the timing of metamorphism relative to deformation strongly suggest that metamorphism occurred in an arc-style collisional setting. Thus, the rocks of this study most likely represent an exhumed mid-crustal terrane equilibrated during the proposed ~3230 Ma terrane accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient typical for portions of modern orogenic belts of this type.
(a) Generalized geology of the central part of the Barberton Greenstone Belt (e.g. Byerly and Palmer, 1991). The Barb4 borehole (drill site 
Behavior of magnetic moment intensities during stepwise demagnetization of Fig Tree jasper mesobands from the Barb4 drill core. Increase of values were observed from 400°C to 450°C (represented in italic) due to remagnetization of the samples.
Microprobe analyses of siderites in jasper mesobands at different depths of the borehole Barb4.
Responses of the variation in magnetic susceptibility with increasing temperature for jasper mesobands from the Barb4 drill core. Note that continuous heating (red) and cooling (blue) cycles in air (left) and argon (right) are similar. The insets on the right-hand side of diagrams display enlarged susceptibility values along the y-axis above 600°C to show the disappearance of hematite above its Curie temperature ~680°C. 
XRD spectra of the studied jasper mesobands (a) before, and (b) after heating at 700°C in air and then cooling to room temperature. Kut = kutnorite; sid = siderite; hem = hematite; qtz = quartz; mag = magnetite, mgh = maghemite, mgf = magnesioferrite. 
This paper describes the mineral changes associated with the thermal demagnetization of sideritic jasper mesobandsin the ~3.25 Ga Manzimnyame Iron Formation Bed of the Fig Tree Group in the Barberton Greenstone Belt. Thestudy was initiated to determine the timing of hematite formation using paleomagnetic fold and conglomerate tests.If the hematite could be proven to be of early diagenetic sedimentary rather than late post-depositional in origin, itcould hold important clues about redox conditions in Early Archaean depositional basins. However, the stepwisedemagnetization of samples was characterized by an increase of magnetic intensity moment from heating steps above400°C, with erratic directions related to cooling down of samples in a magnetically-shielded room. Follow-up thermalrock-magnetic studies combined with X-ray diffraction identification of mineral phases in unheated samples andsamples that were cooled down after heating to 700°C, indicated that the siderites reacted to form new magneticiron-rich spinels during heating. Some of the siderites are Mg-poor and gave rise to the formation of magnetite witha Curie temperature (TC ) of ~575°C. In contrast, Mg-bearing siderites apparently formed magnesioferrite with a TCof~550°C. Hematite, with a TCof ~680°C, persisted as a stable mineral during the entire heating cycle. Maghemite isanother spinel that formed apparently at the expense of magnetite during cooling of samples from below 575°C. These newly-formed iron spinels acquired strong erratic magnetizations that concealed the pre-existing remanenceof hematite. The timing of formation of the hematite nor its in situmagnetic remanence could thus not be determined.Our results can, however, be used to develop alternative techniques to establish the remanence of hematite not onlyin the Manzimnyame jaspilite but also in other Precambrian iron formations in general.
Greenstone belts are widely distributed within the Archaean Zimbabwe (Rhodesian) and Kaapvaal cratons in southern Africa. At least two ages of greenstone formation have been recognized within the Zimbabwe Craton whereas only a single episode of greenstone development at ca 3.5 Ga has been firmly established within the Kaapvaal Craton. Recently, however, a tentative age of ~3.1 Ga was reported for the formation of the Nondweni greenstone succession which is situated near the south-eastern margin of the Kaapvaal Craton. Rb-Sr, Pb-Pb and limited U-Pb zircon isotopic data obtained for the Mvunyana granodiorite, which is intrusive into the eastern flank of the Nondweni greenstone complex, provide an age of ~3.29 Ga implying that: 1) the Nondweni complex is older than previously reported; and 2) the Mvunyana granodiorite is the oldest granitoid component so far identified in the extensive Archaean granite-greenstone terrane south of Swaziland. -Authors
The Saw Mill Complex (SMC) is a 1 275 m thick layered komatiitic sequence in the 3.3 Ga Weltevreden Formation, uppermost stratigraphic unit of the Onverwacht Group in the northern part of the Barberton Greenstone Belt, South Africa. A series of ultramafic complexes in the Weltevreden Formation have previously been interpreted as layered ultramafic intrusions, consisting of thick, layered ultramafic units of peridotite, pyroxenite, dunite, and gabbro. However, recent work on the Pioneer complex of the Weltevreden Formation has demonstrated an extrusive origin of komatiitic flows and tuffs. The Weltevreden Formation has not been studied in the detail of other Onverwacht Group units, but it is likely composed of a number of individual tectonically juxtaposed terrains brought in by thrust faulting during deposition of Fig Tree Group sediments and felsic volcanics in a magmatic arc setting post-dating the Onverwacht mafic and ultramafic units, which are currently regarded as products of plume-based eruptive centers. This study represents an ongoing effort to elucidate the structure, stratigraphy, and petrogenesis of the Weltevreden Formation. A new 1:1000 scale map and stratigraphic section of the SMC reinterpret the igneous complex as extrusive, with lithologic units interpreted as layered komatiitic flows and interbedded tuffs. Flows accumulated olivine, forming distinct layers from bottom to top of: olivine adcumulate (dunitic lithologies),poikilitic orthopyroxene with abundant olivine inclusions (pyroxenitic lithologies), andolivine orthocumulate (peridotitic lithologies), and sometimes with a cap ofolivine-depleted pyroxene-rich orthocumulates (komatiitic basalts). The chemical variation of flows is almost entirely controlled by olivine accumulation, with dunitic komatiites representing up to 70% accumulated olivine and 30% trapped liquid, and peridotitic komatiites representing only 25% accumulated olivine, while spinifex-textured komatiites are representative of quenched liquid. Minor (up to 1%) chromite accumulation partly affects bulk chemistry, while orthopyroxene accumulation may have an effect on bulk chemistry of SMC pyroxentitic komatiites. Layered flows are up to 120 m thick and some display spinifex-textured chilled upper margins. The original liquid was near primitive mantle composition that likely represents a two-stage melting process. The SMC was rapidly emplaced and was not near a major sedimentary depositional system, as no sediment other than tuff was deposited. Flows preserved in the SMC dominantly formed as open flow pathways, allowing for large quantities of olivine to accumulate in the lower portions of flows. Some flows formed as closed systems, possibly as a result of deforming tuffs below the flows to form lava lakes. Large cm-sized lapilli, including those with aerodynamic shapes, suggest subaerial explosions and possibly near-vent environments (though littoral rootless vents can develop 100’s of km’s from actual volcanic vents). Pillow structures have not been observed in the SMC. Geochemically, the SMC is very similar to Weltevreden Formation komatiites recently studied in the Pioneer complex. They have Al2O3/TiO2 near 30, Gd/Lu normalized values between 0.8 and 1.2, maximum liquid compositions of approximately 33% MgO, and olivines up to Fo94. Oxygen fugacity of the mantle source is most likely to have change in log(fO2) of 0.00 relative to nickel-nickel-oxide buffer. SMC komatiites likely erupted at temperatures in excess of 1615°C based on MELTS modeling, and represent some of the hottest volcanism experienced on the early Earth.
Top-cited authors
M.J. de Wit
  • Nelson Mandela University
Richard Armstrong
  • Australian National University
Bruce S Rubidge
  • University of the Witwatersrand
Andy Edward Moore
  • Rhodes University
Alfred Kröner
  • Johannes Gutenberg-Universität Mainz