A.E. Schoch

Petrochemical evidence on the probable origin of ferriferous metasediments in western Bushmanland

The mineralized Proterozoic metasediments of Bushmanland are characterized by the presence of ferriferous rocks. This includes banded and unbanded iron formations and various types of gossans. These units are not laterally extensive and occur in different stratigraphic levels. The prevalent minerals in the ferriferous rocks are hematite, magnetite, quartz, garnet, muscovite, biotite and sillimanite, but less common occurrences of graphite, alunite, plumbojarosite, gahnite and dufrenite have been noted. The chemical variation (wt%) is extensive: total Fe2O3 (1.3–93.5), SiO2 (4–93), Al2O3 (0.2–14.0), CaO (0.02–20.7), MnO (0.0–14.3), MgO (0.0–5.7), TiO2 (0.0–4.4), Na2O (0.0–2.0), K2O (0.0–1.5) and P2O5 (0.1–7.0). The preliminary nature of the data set precludes, however, firm conclusions regarding stratigraphic control of the chemical composition. The trace-element contents (ppm) extend over several orders of magnitude: Zn (0–7,000), Ba (0–5,200), Cu (0–1,400), Pb (0–1,070) and Ni (6–540). Collectively, the data indicate that most of the ferriferous rocks represent highly metamorphosed sediments.

A progressive ductile shear model for the Proterozoic Aggeneys Terrane, Namaqua mobile belt, South Africa

Abstract Recent structural and stratigraphic study results indicate that the Namaqua orogenesis (ca. 1.2 Ga) involved relative movement of lozenge shaped suspect terranes. Detailed study of one of the terranes, the Aggeneys Terrane, provided important information concerning the deformation process. The principal process was differential shear which produced complex structures such as interference folds and thrust faults. All the observed deformational structures can be explained by a single long-lived compressional event.

The structural development of the Aggeneys Hills, Namaqua Metamorphic Complex

The Aggeneys terrane is composed of gneisses, supracrustal sequences and Broken Hill type base metal sulphide deposits that have been intensely deformed and metamorphosed to upper amphibolite grade. This terrane forms part of the Namaqua metamorphic complex that was amalgamated during the 1.1 to 1.2 Ma Namaqua Orogeny. Preserved stratigraphic sequences can be followed on strike (hundred km scale) in domains of relatively low strain. In high strain (more than 12γ) zones, the sequences are discontinuous. Inversion and duplication owing to thrusting and folding are more prevalent in the latter than in the former. The Aggeneys Hills is situated in a regional high strain zone and comprises a late synformal macro-structure, superimposed on five older structural domains. The exposed dimensions of the macro-structure is 20km by 5km and the strike length of the five recumbent folds is 6 to 17km. Zones of high strain that are localised in relatively ductile quartzite-schist sequences separate the recumbent folds. The geometry and stratigraphic relationships of the macro-folds indicate that the stratigraphic packages in the Aggeneys Hills were tectonically emplaced as fold nappes. Collectively these nappes represent a duplex structure. The five fold nappes have been named Soutkloof, Quarry, Klipbok, Dassie and Dabbiepoort. The Quarry fold nappe was chosen as an example to illustrate the details of the structural development of the Aggeneys Hills. Although the deformation is interpreted to have been a progressive ductile shear process, five successive structural events can be locally recognised (D1–5). The Quarry fold nappe is interpreted to be a large kilometre scale sheath fold. The sheath fold provides information about the nature and orientation of the regional strain ellipsoid; the XY-plane trends easterly and is subhorizontal with the X-direction subparallel to the southwesterly trending tectonic transport direction. The mega sheath fold has an allochtonous structure in its northern limb containing correlates of mineralised stratabound units (the Gams member of the Hotson Formation). Structural duplication of ore bearing units may well be present in the unexposed core of the Quarry structure. Similar structural duplication is to be expected in the rest of the high strain zone of the Aggeneys terrane.

A mid-Proterozoic Volcano-Sedimentary sequence in the Aggeneys Hills Duplex, Namaqua Metamorphic Complex

The western part of the Mid-Proterozoic Namaqua mobile belt consists of a series of tectonostratigraphic terranes, including the Aggeneys terrane. In the centre of the Aggeneys terrane occurs a duplex of five fold nappes, represented by the Aggeneys Hills. One of these, the Quarry fold nappe, contains sections through a distinctive volcano-sedimentary sequence correlated with the Koeris Formation that elsewhere overlies the Hotson Formation with ore-bearing assemblages. A description of the stratigraphic and petrological characteristics of the volcano-sedimentary sequence can supply guides to underlying Broken Hill type deposits in other parts of the Aggeneys terrane. The exposed portion of the Koeris Formation that was mapped in detail in the Quarry fold nappe, is 220m thick and is interpreted to be stratigraphically inverted because of large-scale thrusting and folding. The sequence starts with Mg-rich mafic lavas (now amphibolite with hornblende and cummingtonite), followed by a sedimentary sequence (now quartz-muscovite schist). The succeeding metarhyolite and felsic breccia are overlain by mafic metalava and breccia with intercalated clastic products (now quartz-muscovite schist with mafic lenses). A subsequent major clastic incursion represented by conglomerate (called the main conglomerate), is terminated by massive amygdaloidal lava flows (termed the main amphibolite). The main amphibolite is followed by sedimentary rocks (now schist and quartzite) that are in turn overlain by lava (now homogeneous amphibolite). The sequence ends with volcanoclastic conglomerate/agglomerate succeeded by fine-grained clastic deposits (now quartz-mica schist). Regional metamorphism at amphibolite grade yielded peak temperatures in the range of 400 to 700°C at ≥4 kbar. However epidote-rich parageneses (such as epidote-garnet-quartz) that locally transect the regional fabric, indicates the introduction of Ca, Si and Fe subsequent to the metamorphic peak. Vestiges of earlier alteration are represented by crystallographically zoned cordierite and anthophyllite in rocks of appropriate composition. In most of the exposed sequence, primary features such as vesicles and pillow structures have been retained in spite of the metamorphism, except in zones of high strain. The same can be said of the chemical composition of samples representing typical amphibolite and metarhyolite, matching average tholeiite and rhyolite/rhyodacite. The lavas were generated from an upper mantle source near to 1 600 Ma ago, during extensional crustal conditions subsequent to the infilling of cratonic sedimentary basins in an alluvial-fan/fluvial environment. The volcanism was followed by the Namaqua Orogeny, a long-lived compressional period that included terrane amalgamation and felsic magmatism at ~1 200 Ma ago. The orogeny was terminated during renewed extensional conditions, when easterly orientated shear zones of regional extent were formed near to 1 100 Ma ago.

The stratigraphic interpretation of a highly deformed Proterozoic region in Central Bushmanland, South Africa: first correlation of structurally separated metasediments of the Aggeneys Subgroup

Abstract It is demonstrated that highly deformed and structurally separated Proterozoic metasediments, which have previously defied stratigraphic and sedimentological classification, may be correlated and characterised. This is done by using a multidisciplinary approach, including stratigraphic and structural mapping and sedimentological interpretation, developed during the Geodynamics Programme in Western Namaqualand (1975–1980). The metasediments under consideration belong to the economically important Aggeneys Subgroup, which hosts stratabound massive sulphide deposits at Aggeneys and Gamsberg (Gams). Four isolated areas are considered and it is shown that the Dabenoris, Kabas and Pella Formations, and the lower part of the Achab Formation are equivalent to the lower part of the Wortel Formation, and that the upper part of the stratigraphic column is composed of the upper units of the Achab Formation, the Gams Formation and the Blomhoek Formation. The Zuurwater Formation is, for the most part, equivalent to the lower portion of the Wortel Formation, but the uppermost strata are correlated with the Blomhoek Formation. These results illustrate that it is possible to obtain the information necessary for a palaeobasin analysis from even the most complex Proterozoic regions.

Alteration of the Allanridge Formation of the Ventersdorp Supergroup near Douglas, Northern Cape Province.

The Allanridge Formation of the Ventersdorp Supergroup in the Northern Cape Province consists of andesite to basaltic andesite. The properties of these rocks are described by aid of detail from two localities near Douglas (Kalkdam and Katlani), where seven lava flows can be discerned. Greenschist metamorphism has affected all of the lava flows and sporadic sulphide mineralization has also occurred. The intensity of sulphide mineralization is controlled by porosity so that the amygdaloidal bases and tops of flows are more affected than the massive parts, hence producing a conspicuous bleaching. Thus light amygdaloidal lavas (LA) that are sporadically mineralised by sphalerite, galena and chalcopyrite can be distinguished from less altered dark amygdaloidal (DA) lavas. Sulphides are present in altered amygdales of the LA together with quartz, chlorite and calcite. These minerals were introduced by means of hydraulic fracturing and brecciation features are common. Sphalerite tends to dominate over galena in the amygdaloidal lava flows, while the opposite is true in the breccia zones. Mass transfer calculations reveal that the net mass loss for LA and DA was 7 to 20 % and 3 to 5% respectively. Enrichment/depletion diagrams illustrate that LA is relatively enriched in SiO2, Al2O3, K2O, P2O5, Rb, Ba, Cr and Zn, while the DA is relatively enriched in MnO, Fe2O3, Nb, Cr, V, Co and Zn with respect to unmineralized lava flows. The alteration of DA may be regarded as propylitic, while changes in LA involved potassium metasomatism also. Two generations of mineralization can be discerned. The source of the saline fluids capable of transporting metals is probably in the banded iron formations of the Griquatown and Kuruman Formations. Channelways for fluid migration were provided during extensive deformation.

Picrite from the Katse area, Lesotho: evidence for flow differentiation

Abstract A basalt flow in the Katse area of Lesotho displays composite features with tholeiitic characteristics along its margins and a picritic composition in the centre. Geochemical modelling of the magma and the composition of microscopic melt inclusions indicate that the primary magma was olivine-rich. The chemical variation of the flow is evidence for fractionation of olivine from the magma, causing a trend towards tholeiitic compositions. The derived lava flow was modified by flow differentiation so that olivine concentrated progressively towards the centre, owing to differential flow velocity. Evidence from geochemical modelling corroborates these conclusions.

The geochemistry and petrogenetic relationships of two granites and their inclusions in the Keimoes Suite of the Namaqua mobile belt, South Africa

Abstract The Colston and Straussburg plutons consist of two dissimilar granites belonging to the Proterozoic (1100–1200 Ma) Keimoes Suite along the eastern margin of the Namaqua mobile belt. The intrusives and their enclaves were investigated to establish their genetic relationships. The peraluminous Colston granite exhibits S-type characteristics while the Strausaburg intrusive shows marked similarity with I-type granites. One set of quartz-rich melanocratic inclusions are present in the Colston granite. The corresponding melanocratic enclaves in the Straussburg pluton are comprised of a porphyritic type and a non-porphyritic type, while leucocratic inclusions are also abundant. Field relationships, mineralogical and chemical compositions as well as REE-patterns, point to a strong genetic relationship between the granites and their enclaves. Genetic models involving fractional crystallization, wall-rock assimilation, two-magma mixing, and minimum melt-restite mobilisation, were considered to explain the nature and origin of the inclusions and the host granites, and the observed REE-characteristics make most of the models unlikely. It is concluded that the quartz-rich melanocratic inclusions in both plutons formed as products of partial batch melting from a common source, possibly a basic or intermediate gneiss or granulite, under relatively high XCO 2 conditions. After the intrusion and consolidation of the first melts, water-undersaturated granitic magmas intruded as a secondstage, incorporating the first melt products as melanocratic inclusions. The different crystallization conditions of the melanocratic inclusions and the host granites are highlighted by the difference in chemical composition of the biotites, as for instance illustrated in Ti-Al diagrams. The biotite is accompanied by hornblende in all the rock types with the exception of the Colston granite and the leucocratic inclusions of the Straussburg granite. The leucocratic enclaves in the Straussburg granite are interpreted as depleted restite material which formed at a pressure near 5 kbar, and which was transported to the surface by the granite magma. When the compositions of the granitic rocks are compared with the experimental results for the haplogranitic (Ab-Or-An-Q) system, they prove to be quite near the expected minimum melt compositions for a total pressure of 5 kbar.

The geology and petrochemistry of the Mashikiri Formation along the Olifants River Section, Kruger National Park, South Africa.

The volcaniclastic sediments and nephelinite lavas of the Mashikiri Formation, Lebombo Group, have been well documented in the Pafuri region and in the Sabi region, Zimbabwe. The well-exposed section along the Olifants River in the Kruger National Park provides cogent additional information. The Shishwayini Beds at the base of the succession represents volcaniclastic deposits on the Tshipise Sandstone Member of the Clarens Formation. The rest of the Mashikiri Formation in the Olifants River section is nephelinite in which nepheline is accompanied by olivine, clinopyroxene, devitrified glass, katophorite and opaque oxides. Chemical modelling shows that the nephelinites were derived by partial melting of mantle peridotite enriched in incompatible trace elements and by subsequent fractionation of pyroxene and olivine. Crustal contamination did not play a significant role in derived magma composition. The Olifants River section defines a third source of undersaturated parent magma distinct from the two sources previously identified from outcrops to the north by other authors.

Isotopic relationships of epigenetic Pb-Zn mineralisation in the Ventersdorp Supergroup near Douglas, Northern Cape Province

Hydrothermally altered and mineralised lavas of the ~2.7 Ga Ventersdorp Supergroup were sampled at two localities near Douglas (Kalkdam and Katlani). Propylitic alteration has affected some of the lavas (the dark coloured amygdaloidal type), while others (the light coloured amygdaloidal type), show signs of potassium metasomatism as well, representing at least two mineralisation events. A Rb/Sr age of 2014±38 Ma was obtained on clinochlore from the amygdales and whole rock samples. This is interpreted to be the age of the first hydrothermal alteration and main mineralisation of the lavas. Variable isotopic values for Sr and Pb suggest a complex provenance for the hydrothermal fluids, whilst a narrow range of δ34S in galena (−3.4 to −6.3‰) indicates a specific source region. Four fluid inclusion populations are dominated by a brine population (Tm = −12.9°C) with a model density of 1.07g/cm3 (Th = 117°C). The temperature of the fluid must have been ~270°C according to the chlorite geothermometer of Cathelineau. The temperature of entrapment implies a pressure of roughly 2 kb when the slope of the relevant isochore (1.07g/cm3) is taken into account. Fluids seem to have migrated along channelways that were provided during extensive fracturing such as along the Griquatown fault zone. This migration probably propagated from rocks of the Transvaal Supergroup (Griquatown and Kuruman Formations) into the underlying Ventersdorp lavas.