I.W. Hälbich

Structural history of the southwestern corner of the Kaapvaal Craton and the adjacent Namaqua realm: new observations and a reappraisal

Abstract The rocks along the southwestern margin of the Kaapvaal Craton were deformed up to 7 times during the Early to Middle Proterozoic. The oldest deformation D 1 is recorded in the N-S-trending Uitkoms cataclasite of pre-Makganyene age (>2.24 Ga) on the craton, and interpreted as a bedding-parallel thrust. It is assumed to be a branch rising towards the surface from a blind sole thrust that initiated early N-S-trending F,-folds above it. D 2 is represented by mainly N-S but also NE-SW and NW-SE-trending imbricates and recumbent fold zones ranging in size from small gravity slumps to large tectonic decollements in Asbesheuwel BIF and the Koegas Subgroup, and is younger than D 1 , or equals D 1 in age. These age. These structures pre-date the Westerberg dyke-sheet intrusion. D 3 south-verging folds and thrusts are the oldest post-Matsap deformations, just less than 2.07-1.88 Ga. D 4 are upright to east vergent and N-S-trending folds deforming all previous structures. D 4 post-dates the Westerberg dyke-sheet and probably reactivates N-S folds above the earlier sole thrust during renewed E-W compression. D 5 , producing the main NW-trending Namaqua structures, is only very feebly developed in the Kheis terrain and absent from the cratonic areas overlain by Olifantshoek and older strata, i.e. NE, E and SE of the Marydale High. Very gentle D 6 E-W to ENE-WSW folds produce culminations and depressions in all NW-trending older structures. During D 7 the NW-SE-trending Doornberg Lineament, an oblique left-lateral wrench, and smaller N-trending faults such as the Westerberg Fault developed. These and similar, but right-lateral faults are the last movements along the rim of the craton and occurred around 1.0 Ga. Multiple folding and thrusting with riebeckite mobilization happened prior to Namaqua events and resulted inter alia in discernable duplication and thickening of the Transvaal Supergroup along the southwestern margin of the Kaapvaal Craton and at least some 130 km into the craton interior. This complicates stratigraphic correlation as well as true thickness estimates of BIF units in Griqualand West, and affects the model for the environmental evolution of the Ghaap Group. A structural model of thin-skin decoupling at the base of the Transvaal Supergroup and starting in the Middle-Early Proterozoic is proposed.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

Abstract Much research has been conducted on these banded iron formations (BIF) over the last 15–20 years. This contribution seeks to provide an overview of old and new facts and critical discussion on the latest ideas regarding the origin of these sediments in the early Proterozoic. The recently suggested stratified ocean water model and a new stratified lake water model are compared using new evidence of a stratigraphic, major and trace element, stable isotope and REE nature. It appears that any hypothesis on the genesis of these Transvaal Supergroup rocks will have to satisfactorily account for at least the following: 1. i) A constant supply of enough Fe and Si over at least several hundred thousand years. 2. ii) A macro- and a micro-cyclicity. The latter is the most basic building stone and any hypothesis that can not explain these phenomena must be considered unrealistic. 3. iii) The stratigraphic and isotopic evidence for heterogeneity of the waterbody. 4. iv) The REE and trace element evidence for contributions from different sources. 5. v) The distribution of organic carbon in rock facies and minerals. 6. vi) The difference between Proterozoic and present day atmospheres and surface waters. 7. vii) The fossil record of the early Proterozoic, and coupled to this the role that chelation, complex formation, stable colloids and co-precipitation played in weathering, transportation and deposition of Fe. 8. viii) Factors influencing or controlling cyclicity. 9. ix) The role that atmospheric and crust-mantle evolution plays in producing most large BIF-deposits over a time span of about 500 Ma from the late Archaean into the early Proterozoic. 10. x) The concomitant evidence provided by early Proterozoic paleosols. 11. xi) The fact that several large Proterozoic BIF deposits are immediately preceded by platform carbonates. Finally, the two important ore districts, Sishen in the Northern Cape Province and Thabazimbi in the Central Transvaal, are dealt with. The general geology, mineralogy and genesis of these very high-grade major deposits are presented. New information on ore morphology and new evidence on multiple epigenetic enrichment of BIF-protore are presented and discussed. The modern and specialised ore-mining, -processing and blending techniques at Sishen are explained.

Chemical composition of banded iron-formations of the Griqualand West Sequence, Northern Cape Province, South Africa, in comparison with other Precambrian iron formations

Mesobanded lithotypes (band rhythmites) of banded iron-formation (BIF) from the Griquatown and Kuruman Iron Formations of the Asbestos Hills Subgroup (Transvaal Supergroup, South Africa) have been sampled. 142 major and trace element analyses from diamond drill cores, an underground mine and an open pit mine were carried out. Out of these 19 open pit mine samples were excluded from the interpretation of the analytical results because of a different behaviour of the alkali elements. It is shown that other published data seem to suffer from the same effect. The results show that the chemical composition of the iron-formations is virtually independent of their stratigraphic and geographic localities over hundreds of kilometres across Griqualand West, if averaged element distribution patterns are compared. The major element composition of mesobanded iron-formation samples (magnetite chert and magnetite-carbonate chert, riebeckite-carbonate chert and ironsilicate chert) varies between 30 and 51 wt.% SiO2, 23 and 66 wt.% Fe2O3T, < 0.02 and 0.14 wt.% TiO2, < 0.04 and 1.9 wt.% Al2O3, < 0.02 and 1.05 wt.% MnO, 1.48 and 7.5 wt.% MgO, 0.14 and 12.09 wt.% CaO, 0.11 and 4.26 wt.% Na2O, < 0.007 and 2.39 wt.% K2O and < 0.01 and 1.57 wt.% P2O5. The trace element contents are generally low and range from < 1 to 2 ppm Nb (detected in only one sample), < 2 to 23 ppm Zr, < 3 to 31 ppm Y, < 2 to 152 ppm Sr, < 4 to 5 ppm U (detected in only one sample), < 2 to 240 ppm Rb, < 4 to 4 ppm Th, < 4 to 19 ppm Pb, < 3 to 4 ppm Ga, < 4 to 33 ppm Zn, < 6 to 69 ppm Cu, < 7 to 17 ppm Ni (detected in only one sample), < 5 to 59 ppm Cr, < 4 to 16 ppm V and < 10 to 177 ppm Ba. The intercalated stilpnomelane lutites have a very similar gross composition but regularly display higher concentrations of Al2O3, TiO2, K2O, and Zr. They have a different origin but certainly bear the imprint of the BIF environment and must be considered a clastic contaminant of the otherwise chemically or biochemically precipitated iron-formations. Clastic contamination and subsequent hydrothermal alteration are the most plausible agents which effected the element distribution pattern of the BIF, because they are best to reconcile with a model which assumes hydrothermal fluids as major sources of BIF. It can be concluded, from the general geochemical uniformity of BIF throughout the depository and from the absence of clear-cut relations of elemental distributions and ratios to crustal components (documented by low K/Rb, high Rb/Sr ratios, no relevant correlation between TiO2 and Al2O3), that a basinward hydrothermal system acted as a source for the Fe and Si in the BIF. The geochemical similarity between the microbanded Kuruman Iron Formation and the interbedded granular and microbanded Griquatown Iron Formation suggests deposition of the iron-formations in the same chemical environment over the entire basin from the inception of these conditions until their termination by clastic input. The somewhat lower trace element concentrations in the Griquatown Iron Formation as compared to the Kuruman Iron Formation probably do not mean an environmental change, but may reflect the diagenetic and tectonic evolution of the iron-formations. Regular but short-lived interruptions by distal volcanic ash had no influence on the bulk BIF composition. On the contrary, the volcanics reveal a strong overprint by the regional major element BIF chemistry. Therefore, the relationship between volcanogenic rocks and banded iron-formation seems to be coincidental and related to basin development. The general geochemical similarity of the Griquatown and Kuruman Iron Formations with other Proterozoic and Archaean iron-formations in the world and especially with those of the Hamersley Basin, leads us to the conclusion that the BIF of the Griqualand West Sequence are representatives of typical, large-scale iron-formations of the Precambrian, which all formed in chemically very similar environments.

A carbonate-banded iron formation transition in the Early Protorezoicum of South Africa

Seven new and two resurveyed stratigraphic sections through the important carbonate-BIF transition in Griqualand West are presented and compared with six published sections. Lateral correlation within this zone is attempted but the variability was found to be too great for meaningful subdivision. Substantial lithological irregularity is the only unifying character of this zone, for which the new name Finsch Member (Formation) is proposed. Vertical and lateral lithological variations as well as chemical changes across this zone are discussed with reference to environmental aspects. Local and regional considerations lead to the conclusion that fresh water-sea water mixing occurred in a shallowing basin.

The Gariep belt: stratigraphic-structural evidence for obliquely transformed grabens and back-folded thrust stacks in a combined thick-skin thin-skin structural setting

Abstract This paper deals with the tectonics of the Port Nolloth Zone (PNZ) of the Gariep belt, one of the Pan-African tectono-metamorphic belts along the southwest coast of Africa. Stratigraphical and sedimentological evidence indicates that deposition in the sub-basins was controlled by step and/or graben faults parallel to the craton edge. Some of these structures were inverted during the compressional phase to form oblique wrenches in a subzone proximal to the craton edge, whereas others developed into stacked thrusts, blind ramps and a refolded duplex in a distal subzone. A near vertical ZX plane of deformation strikes north-northwest to south-southeast for the entire PNZ and the shear planes are near horizontal in the distal subzone and near vertical in the proximal area. Because metamorphic grades are very similar everywhere, the difference in style is probably related to the original shape of the basement and the varying lithology of the Gariep rocks. Two-dimensional modelling shows that oblique obduction of an oceanic crustal slab to the southeast, followed by relaxation and renewed compression in three pulses, probably produced a combination of thick-skin and thin-skin elements in a zoned tectonite.

Remagnetizations in Late Permian and Early Triassic rocks from southern Africa and their implications for Pangea reconstructions

-A paleomagnetic study of late Paleozoic and early Mesozoic sedimentary rocks from southern Africa suggests wide-spread remagnetization of these rocks. Samples of the Mofdiahogolo Formation in Botswana and of the Lower Beaufort Group in South Africa were treated using thermal, alternating field and chemical demagnetization. The Mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of

A structural sequence in low strain sedimentary rocks in the Beaufort Group of the south-western Karoo and its bearing on syngenetic and epigenetic mineralization

Abstract Several sets of planar and linear structures are described from the very gently folded Karoo strata of the northern-most tectonic zone of the Permo-Triassic Cape Fold Belt. The superposition of these structures reveals a time sequence. Horizontal compression occurred before compaction was complete. After compaction further horizontal compression developed a sequence of structures that are readily interpreted in terms of visco-elastic strain in multilayers. This analysis reveals that a set of mainly antithetic kinks in the form of monoclines developed out of earlier incipient sinusoidal macrofolds during the final stages of horizontal N-S compression. The model is in agreement with evidence for synsedimentary folding that controlled the sites for erosion channels as well as the direction of sedimentary transport. Exploration efforts for syngenetic and/or fluid controlled epigenetic mineralization in permeable strata of this fluviatile sedimentary sequence should be directed along certain structural lines.

A stratigraphic-structural profile through parts of the Beaufort Group: fold genesis and relationship to U-distribution

Abstract A technique for constructing a composite profile of a very gently folded sedimentary multilayer sequence, some 1200 m thick, is introduced and applied. Accuracy checks and balances are discussed. Real changes in layer thickness are found to be related to the position, attitude and intensity of folds. Upright as well as verging, open sinusoidal megafolds formed during diagenesis and compaction. Monoclines (macrokinks) formed late during the deformation history, and are antithetic with respect to the regional couple force. Most macrokinks eventually became locked. In some cases thrusting of the steep limb seems to have accompanied shortening inside the kinkband immediately before the locking occurred. Other monoclines have developed sets of short listric normal faults on the steep limb that merge with the beds on their downthrow side. The association of all structures is readily explained by a theory of visco-elastic folding of multilayers with varying competency. Some 50% of the stratiform uranium mineralization that is associated with the thicker, more permeable sandstone units occurs in monoclines. The positioning of monoclines relative to the axial planes of the early megafolds seems to be controlled by thickness variations in sequences of macrocycles. These variations can be linked to changes in transport direction across syndepositional early folds. Macrokinks can therefore be considered a valuable additional indicator for exploration targets.