Gariep Belt Research Articles

Controls of host rock mineralogy and H2O content on the nature of pseudotachylyte melts: Evidence from Pan-African faulting in the foreland of the Gariep Belt, South Africa

Abstract Late-orogenic Pan-African tectonics formed an 8 km long fault-fracture zone in the shallow foreland crust of the Gariep Orogen in Mesoproterozoic granitoid basement. Pseudotachylyte was locally formed during a single seismic event that caused brittle failure in cohesive quartz-diorite, granodiorite and granitic gneiss. 40Ar/39Ar laser spot analysis provides an age of 512.5 ± 7.5 Ma for this seismic event. The depth of the hypocentre between 1700 and 5300 m is inferred from the likely thickness of the overlying rock at that time. Abundant quartz melting in conjunction with low host rock temperature indicates a minimum seismic temperature rise exceeding 1030 °C but in some samples dry quartz melting at temperatures exceeding 1700 °C might have been possible. The abundance of hydrous phases in the host rock controlled the volume of melt produced in a given host rock type. Different types of primary melt were formed by the fusion of batches of host rock with different relative proportions of plagioclase, alkali feldspar, quartz, biotite, and, where present, amphibole or garnet. The chemical and physical interaction between these melts depended on contrasting melt mobility and viscosity. In quartz-diorite, glasses of variable composition are well preserved and show the distribution of primary melt species at the time of quenching. These different melt species had limited tendency of mixing, unless mixing was assisted by syn-seismic shearing. In granodiorite, which has a low proportion of hydrous phases, low-viscosity melt was formed at high temperatures, eliminating most host rock fragments and leading to extensive melting of quartz. The low melt viscosity, probably further decreased by the dissolution of free H2O, permitted effective melt homogenisation in such veins. Still in the liquid state these melts segregated, effectively separating secondary melts of sodic–calcic and of K–Mg–Fe-rich composition. After quenching the solid state diffusion of cations in glass was comparatively insignificant.

The P–T–t paths of high-grade gneisses, Kaoko Belt, Namibia: Constraints from mineral data, U–Pb allanite and monazite and Sm–Nd/Lu–Hf garnet ages and garnet ion probe data

The Damara Orogeny is a late Neoproterozoic to Cambrian (ca. 570–480Ma) intracratonic event that affected the Kaoko Belt, the inland branch of the Damara orogen and the Gariep Belt in Namibia and South Africa. This study focuses on the Pan-African evolution of part of the Kaoko Belt between the Puros shear zone and the Village mylonite zone which consists of Mesoproterozoic migmatitic para- and orthogneisses with minor granulite and amphibolite. Pseudosection modeling combined with thermobarometric calculations indicate that the para- and orthogneisses equilibrated at about 670–800°C and ca. 0.6–0.8GPa. Some garnets display a pronounced bell-shaped Ca, HREE, Y and Sr zoning, flat zoning profiles of Mn and Fe and concave upward concentration profiles of Sm and Nd. Pressure–temperature estimates obtained on these garnets reveal similar temperatures of 700–750°C but slightly higher pressures of ca. 0.9GPa. The preservation of distinct major and trace element zoning in garnet and the existence of broadly similar (near prograde) Sm–Nd and Lu–Hf garnet–whole rock ages of ca. 525Ma obtained on the same sample indicate an extremely fast cooling path. Retrograde conditions persisted until ca. 490Ma indicating a slow, late stage near isobaric cooling path. The resulting clockwise P–T–t path is consistent with crustal thickening through continent–continent collision followed by post-collisional extension and suggests that the upper amphibolite to granulite facies terrain of the central Kaoko Belt formed initially in a metamorphic field gradient of ca. 25–35°Ckm−1 at moderately high pressures.

A new lithostratigraphic subdivision and geodynamic model for the Pan-African western Saldania Belt, South Africa

A provenance study involving U–Pb and Lu–Hf isotope analyses of detrital zircon grains as well as whole rock geochemical, Rb–Sr, Sm–Nd and Pb isotope analyses of siliciclastic metasedimentary rocks from the principal tectonostratigraphic units of the Pan-African western branch of the Saldania Belt in South Africa revealed age relationships and source domains that are not compatible with current stratigraphic schemes and geodynamic models for the evolution of this belt. The previous subdivision into three terranes cannot be upheld. The sediments in both the former Tygerberg and Swartland terranes seemingly have the same provenance but differ from those in the former Boland Terrane. The latter is considered a para-autochthonous zone on top of basement rocks of the late Mesoproterozoic Namaqua-Natal Belt. On the basis of our new data it is recommended to unify the former Tygerberg and Swartland terranes into a single terrane for which the name Malmesbury Terrane is proposed. The Colenso Fault, previously regarded as a terrane boundary, is reinterpreted as reactivated old structure inherited from the basement within the Malmesbury Terrane, whereas the major terrane-bounding fault is the Piketberg–Wellington Fault, which can be linked most likely with the Schakalsberge Thrust further north in the Gariep Belt.The age of sedimentation is constrained within a narrow time window between 557 and 552Ma in the Malmesbury Terrane and between 609 and 532Ma in the Boland Zone. No evidence of older, Cryogenian sedimentary rocks could be found, except for the volcanic-dominated Bridgetown Formation, which is presumably a tectonic sliver. The Cambrian Klipheuwel Group is undoubtedly younger than, and unrelated with regard to its provenance to, the other Saldanian sediments.All geochemical evidence points to an active continental margin as source for the sediments in the Malmesbury Terrane and Boland Zone. While the ages of the detritus in the former are strongly dominated by Neoproterozoic, especially Cryogenian, ages, those in the Boland Zone are mainly early Tonian and late Mesoproterozoic. The most likely source of the Cryogenian detritus is the Dionisio Cuchilla-Pelotas magmatic arc in southeastern Brazil and Uruguay, which formed within a splinter of Kalahari Craton, i.e. Arachania. A back-arc position relative to that arc is inferred for the depo-basin in which the redefined Malmesbury Group, making up the Neoproterozoic succession of the Malmesbury Terrane, was deposited. Consequently, our new data support the hypothesis that the Neoproterozoic successions of the western Saldania Belt and their northern equivalents in the western Gariep Belt do not reflect a full Wilson Cycle but only the waning stages of an Ediacaran back-arc basin. The main suture between the Kalahari and Rio de la Plata cratons must be located, therefore, to the west of the Dionisio Cuchilla-Pelotas magmatic arc, probably along the Major Gercino-Sierra Balena Lineament within southeastern South America. This implies that the opening of the modern South Atlantic followed to a large extent the axis of the postulated former Ediacaran back-arc basin.

Chapter 18 The Karoetjes Kop and Bloupoort formations, Gifberg Group, South Africa

Abstract The Vredendal Outlier near the South African west coast lies in an intermediate position between the late Neoproterozoic Gariep Belt further north and the Cambrian Saldania Belt further south. It consists of a sedimentary succession of siliciclastic and carbonate rocks, unified as the Gifberg Group, which contains two diamictite-bearing units, the Karoetjes Kop Formation at the base and the Bloupoort Formation near the top of the group. The diamictite of the Karoetjes Kop Formation represents mainly debris flow deposits in a continental rift setting, with some contribution from retreating glaciers. In contrast, the younger diamticite in the Bloupoort Formation is glaciomarine, is associated with banded iron-formation, is underlain by stromatolitic reef carbonates and is overlain by carbonates. Most of the Gifberg Group is poorly exposed and poorly investigated. In the absence of radiometric age data, stratigraphic interpretation and correlation is based on lithological and chemostratigraphic evidence. The entire Gifberg Group is considered to be equivalent to the much better investigated Port Nolloth Group in the Gariep Belt ( sensu stricto ). Whereas the Karoetjes Kop Formation is correlated with the c. 750 Ma Kaigas Formation of the Gariep Belt, the diamictite and associated banded iron-formation of the Bloupoort Formation are regarded as correlatives of the Numees Formation of the Gariep Belt. The entire Gifberg Group was subjected to transpressional deformation and accompanying low-grade metamorphism during continental collision between the Rio de la Plata and Kalahari plates at the end of the Neoproterozoic and again during the Cambrian accretionary orogeny along the southwestern margin of Gondwana, which led to the development of the Saldania Belt further south.

Chapter 16 The Chameis Gate Member, Chameis Group, Marmora Terrane, Namibia

Abstract The Chameis Gate Member is a poorly exposed and poorly investigated diamictite in the Chameis Subterrane of the Marmora Terrane, which forms the western, completely allochthonous part of the Pan-African Gariep Belt (southwestern Namibia). Its significance lies in its position in an entirely oceanic unit, the Dernburg Formation, which is dominated by mafic volcanic rocks. The diamictite contains exotic dropstones in a mafic volcaniclastic matrix, thus providing evidence for transport by ice away from the continental margin into an oceanic environment. No direct age data are available and stratigraphic relationships are obscured by limited outcrop and intense syn-orogogenic deformation. Preliminary chemostratigraphic data obtained on carbonate rocks below and above the diamictite, imprecise Pb–Pb age data on the largely volcaniclastic silicate fraction within associated stromatolitic reef carbonates, and imprecise Ar–Ar data on early hornblende related to sea-floor metamorphism of the associated volcanic rocks all point to an age loosely constrained between 640 and 580 Ma. Based on a comparison between the tectono-stratigraphic units of the Marmora Terrane with the continental Port Nolloth Group on the one side and the Rocha Group of the Punta del Este Terrane in Uruguay on the other side of the terrane, it is suggested that the diamictite was deposited in a back-arc basin that developed in response to the 640–590 Ma volcanic arc of the Dom Feliciano Belt in southeastern Brazil and eastern Uruguay.

Chapter 17 The Kaigas and Numees formations, Port Nolloth Group, in South Africa and Namibia

Abstract The Port Nolloth Group makes up the eastern, external part of the Pan-African Gariep Belt (Port Nolloth Zone) in southern Namibia and western South Africa. It contains two glaciogenic diamictite units, the older Kaigas Formation and the younger Numees Formation, with intercalated and overlying carbonate-dominated units. Available chemostratigraphic information include O, C and Sr isotope data. Micropalaeontological and geochronological data point to an early Cryogenian age ( c . 750 Ma) of the Kaigas Formation and possibly a middle Ediacaran age ( c . 580 Ma) for the Numees Formation. The former was deposited in an evolving, but eventually failed, continental rift on the western flank of the Kalahari Craton, probably at low latitude. The Numees Formation is a laterally continuous, up to 600-m-thick glaciomarine deposit for which a passive continental margin setting has been suggested. Alternatively, based on more recent data, the depositional setting might have been a back-arc basin. The eroded remnants of the corresponding arc are present in the Dom Feliciano Belt.

The Witputs diamictite in southern Namibia and associated rocks: constraints for a global glaciation?

The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3 Ga, representing the local geology, with the youngest grain at 1030.2 ± 10.9 (2σ) (n = 92 <10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be ‘cap carbonates’, show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their δ13CVPDB isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C–O isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8 ± 10 Ma (2σ errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570 Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03 Ma with the detritus being purely locally derived.

Stratigraphy of the Port Nolloth Group of Namibia and South Africa and implications for the age of Neoproterozoic iron formations

Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neoproterozoic Gariep Belt of Namibia and South Africa. These uncertainties are compounded by major lateral facies changes that complicate correlations locally. Herein, we report the results of integrated geological mapping, chemo- and litho-stratigraphic, and sedimentological studies that shed light on the age and stratigraphic architecture of the Port Nolloth Group. Particularly, we have distinguished an additional glacial deposit, herein referred to as the Namaskluft diamictite, which is succeeded by a ca. 635 Ma basal Ediacaran cap carbonate. This interpretation indicates that the stratigraphically lower, iron-bearing Numees diamictite is not Marinoan or Gaskiers in age, as previously suggested, but is instead a ca. 716.5 Ma Sturtian glacial deposit. A Sturtian age for the Numees Formation is further supported by the discovery of microbial roll-up structures in the dark limestone of the Bloeddrif Member that caps the diamictite. A re-evaluation of the age constraints indicates that all Neoproterozoic iron formations may be of Sturtian age, and thus indicative of secular evolution of the redox state of the ocean.

STRUCTURAL GEOLOGY OF ROBBEN ISLAND: IMPLICATIONS FOR THE TECTONIC ENVIRONMENT OF SALDANIAN DEFORMATION

We present a detailed structural and lithologic map of Robben Island, offshore Cape Town, South Africa. Robben Island is underlain by the Tygerberg Formation, part of the Neoproterozoic to early Cambrian Malmesbury Group of the Saldania Belt. The depositional setting and structural history of the Tygerberg Formation are poorly constrained due to limited outcrop and lack of previous structural studies. Sedimentary structures are indicative of deposition at relatively high rates in a high energy environment and we concur with previous workers that deposition occurred on turbidite fan systems in a tectonically deepening basin. By comparison with active and ancient examples, we suggest that a forearc or trench slope, supra-subduction zone basin is a possible match to the setting of the Tygerberg Formation. However, limits on preservation and insufficient age data prevent comparisons in basin geometry and deposition rates which could be used to test depositional setting with more certainty. Northwest-southeast striking subvertical pressure solution cleavage is pervasive throughout the exposures. Upright folds, with axial planes parallel to the cleavage, plunge 10 to 15° to the northwest or southeast with approximately 20° variation in trend azimuth. The folds are limited in along-axis extent and often occur in asymmetric pairs. Subtle bedding-parallel shear zones divide folds of different plunge directions. This pattern of folds is consistent with experiments and observations of en echelon folding during distributed strain associated with oblique transpression. This finding is consistent with previous studies of parallel, slightly earlier orogenic belts to the north (Gariep and Kaoko Belts) although our observations do not allow us to distinguish whether transpressional strain was sinistral or dextral. Sinistral transpression is considered more likely given the dominantly sinistral strike-slip history on the nearby Colenso Fault and the southward migration of collision along the western margin of Africa during the late Neoproterozoic to early Cambrian.

A Damara orogen perspective on the assembly of southwestern Gondwana

Abstract The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo–São Francisco and Río de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari–Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga–Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Araçuaí Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580–550 Ma in the Kaoko Belt and at 545–530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530–500 Ma, with thrusting onto the Kalahari Craton from 495 Ma through to 480 Ma. Coupling of the Congo and Río de la Plata cratons occurred before final closure of the Mozambique and Khomas (Damara Belt) oceans with the consequence that the Kuunga suture extends into Africa as the Damara Belt, and the Lufilian Arc and Zambezi Belt of Zambia. Palaeomagnetic data indicate that the Gondwana cratonic components were in close proximity by c. 550 Ma, so the last stages of the Damara–Kuunga orogeny were intracratonic, and led to eventual out-stepping of deformation/metamorphism to the Ross–Delamerian orogen ( c. 520–500 Ma) along the leading edge of the Gondwana supercontinental margin.

An evaporitic facies in Neoproterozoic post-glacial carbonates: The Gifberg Group, South Africa

A geological study of the hitherto poorly described Neoproterozoic Gifberg Group, with emphasis on lithogeochemistry and O, C and Sr isotopic composition of the carbonate-dominated Widouw Formation (Vredendal Outlier, westernmost South Africa) revealed that the entire group is an equivalent of the relatively well constrained Port Nolloth Group in the external, paraautochthonous part of the Pan-African Gariep Belt further north. Thus, the Vredendal Outlier can be regarded as the southern extension of the Port Nolloth Zone. Two diamictite units are recognised in the Vredendal Outlier, which can be correlated respectively with the c. 750 Ma Kaigas Formation diamictite and the 583 Ma, syn-Gaskiers Numees Formation diamictite in the Gariep Belt proper. The dominating carbonate unit in the studied area is post-glacial with respect to the older of the two diamictite units. The combined textural, structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds. Sedimentation in a restricted, very shallow and proximal basin led to a wide range in C isotope ratios (δ 13C PDB from − 4.2 to + 4.8‰), very high Sr concentrations (derived from original anhydrite) and initial 87Sr/ 86Sr ratios that are significantly higher (0.70785) than those of coeval seawater. As C and Sr isotopes are commonly used for chemostratigraphic correlation, and high Sr concentrations in Neoproterozic carbonates are often interpreted as evidence of former aragonite, the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition. Thus C and Sr isotope ratios alone in Neoproterozoic carbonates may be less powerful proxies of ancient seawater composition, and high Sr contents are not necessarily indicative of an “aragonite sea”, as previously inferred.

Evolution of the Namaqua-Natal Belt, southern Africa – A geochronological and isotope geochemical review

Juvenile crust formation within the Namaqua-Natal Belt occurred during two principal periods at ∼1.4 and ∼2.2 Ga with little evidence for significant contributions from older crustal sources. Palaeoproterozoic lavas and associated calc-alkaline granitoids are preserved in the Richtersveld sub-province and, to a much lesser extent, in the Bushmanland sub-province. Development of crust within the Aggeneys and Okiep terranes of the Bushmanland sub-province during the Mesoproterozoic involved significant reworking of pre-existing Palaeoproterozoic lithosphere, whereas the Garies terrane (Bushmanland sub-province) and the Gordonia and Natal sub-provinces show little or no reworking of older protoliths. Deformation of Archaean components is evident near the south-western margin of the Kaapvaal Craton but there is no evidence for a significant Eburnean (∼1.8 Ga) orogeny in the Kheis sub-province. Rather, the ‘Kheisian’ fabric is now dated as younger than ∼1.3 Ga, and is thus an early phase of the Mesoproterozoic evolution of the Namaqua-Natal Belt. Graben formation with associated extrusion of lavas and deposition of sediments of the Koras Group occurred relatively early in the history of the Belt, predating most of the intrusive granitoid activity evident in other sub-provinces. Early, juvenile, dominantly mafic to intermediate Mesoproterozoic igneous units formed at ∼1.2–1.3 Ga in the Gordonia and Natal sub-provinces. Two major periods of granitoid intrusion occurred at ∼1.15 and ∼1.03–1.08 Ga and were both of regional extent. The Little Namaqualand Suite intruded at ∼1.15 Ga in the Bushmanland sub-province, as did various individual plutons in the Gordonia and Natal sub-provinces. Spektakel, Keimoes and Oribi Gorge Suite granitoids, often megacrystic in character, were emplaced at ∼1.03–∼1.08 Ga. These latter granitoid suites each span several structural terranes, indicating that accretion of these domains was essentially complete by ∼1.03 Ga. Igneous activity as part of the Namaqua-Natal orogenesis was concluded by ∼1.0 Ga throughout the belt. Subsequent ∼0.85–0.75 Ga magmatism, evident only in the west, reflects the start of a new cycle which ultimately produced the Damara and Gariep belts. The dominant, penetrative regional fabric was produced prior to peak metamorphism at ∼1.02–1.04 Ga. Limited geochronological evidence also records an earlier phase of high-grade metamorphism at ∼1.16 Ga in both the western and eastern sectors of the Belt. Transcurrent shearing is dated at ∼1.06–1.03 Ga in the Natal sub-province. Equivalent shearing, although not directly dated, also occurred in the west as a result of indentor tectonics produced by collision with the Kaapvaal Craton. Subsequent exhumation and cooling of the rocks of the Namaqua-Natal Belt resulted in temperatures as low as ∼350 °C by ∼0.95 Ga. Except in the Gariep Belt and its foreland in the west, there is little evidence for subsequent, Pan-African overprinting or activity within the Namaqua-Natal Belt. The age of the supracrustal gneisses remains contentious. Early, ∼2 Ga metavolcanics and metasediments in the Richtersveld sub-province are presumably also present in parts of the Bushmanland sub-province because granitoids of that age occur in both sub-provinces. The Koeris amphibolites from the Bushmanland Group have provided a ∼1.65 Ga Sm–Nd date, whereas supracrustal gneisses in the Garies terrane (Bushmanland sub-province), Gordonia sub-province and Natal sub-province have provided ∼1.2 to ∼1.3 Ga dates. It therefore seems likely that supracrustal lithologies were deposited during at least three intervals as the Namaqua-Natal Belt developed.

Provenance analysis of selected clastic rocks in Neoproterozoic to lower Paleozoic successions of southern Africa from the Gariep Belt and the Kango Inlier

Conglomerates and associated finer-grained sedimentary rocks, from Neoproterozoic to Lower Paleozoic successions in southern Africa (Kaigas and Numees Formations of the Gariep Belt; Nooitgedagt Member and Vaartwell Formation from the Kango Inlier in the Saldania Belt), were selected for provenance determination. The studied supracrustal rocks are dominated by material derived from the upper continental crust, and display a moderate weathering trend, as derived from the chemical index of alteration, and upper crustal Th/U ratios. Samples characterized by an influence of less fractionated material may be derived from a volcanic arc or from an alkaline source. A volcanic arc component that has been identified in the Kaigas Formation (~770 Ma) detritus is interpreted as eroded from the underlying Meso- to Palaeoproterozoic Orange River Group and/or Vioolsdrift Suite. Alternatively it may represent an undiscovered, unknown arc terrane. Stronger fractionated material identified within samples is reworked and are thought to have formed in a passive margin setting. Detrital material, from a source with a continental arc signature, was observed in both the Nooitgedagt Member and the Vaartwell Formation of the Kango Inlier, but is difficult to substantiate. It has been shown that various local source areas supplied the detrital material to the studied successions. The heterogeneity and the lack of age constraints from the different sources inhibit the modelling of palaeogeographic settings until the entire basin evolution is understood. Correlation of sedimentary successions between isolated outcrops should thus be avoided or treated with greatest caution.

Some observations on diamondiferous bedrock gully trapsites on Late Cainozoic, marine-cut platforms of the Sperrgebiet, Namibia

Namibia's southwestern coast, the Sperrgebiet, hosts one of the world's largest gem diamond placer deposits consisting of fluvial, marine and desert deflation/aeolian placer types. To date, the richest onshore placer discovered in the Sperrgebiet comprises several, Plio-Pleistocene to Holocene, littoral marine packages distributed northwards from the Orange River mouth for some 120 km to Chameis Bay. In this zone, known as Mining Area No. 1 (MA1), these Quaternary marine deposits are floored predominantly by siliciclastic rocks of the Late Proterozoic Gariep Belt that have been bevelled into a number of marine-cut platforms during the different Quaternary high sea level stands (notably, at + 30 m, + 8 m, + 4 m and + 2 m). In many places, these bedrock platforms are extensively gullied and potholed, forming abundant fixed trapsites that promote diamond concentration in the Late Cainozoic littoral sediments. Some observations on the distribution, orientation and incision of bedrock gullies in MA1 are presented here. Three principal bedrock gully types, all of which constitute good diamond trapsites, have been identified in the bedrock footwall between the Orange River mouth northwards to Chameis Bay, a distance of some 120 km: (i) swash-parallel gullies in the southern, proximal sector where the bedrock s 2 dips 45°W and the abrasive gravels are most abundant, thus overriding lithological and structural controls in the footwall; (ii) strike-parallel gullies in the central reach where the bedrock s 2 dips between 80° and 85°W and the abrasive bedload is reduced, resistant lithological units promote the formation of such features along the orientation of the metsediments; and (iii) joint gullies in the northern, distal portion where the bedrock s 2 dips 45–60° to the E and coarse abrasive bedload is virtually absent, the structural pattern in the bedrock is exploited by marine erosion. In these Late Cainozoic placers, diamond concentration is further linked to the depth of the gullies incised into the marine-cut platforms. Maximum incision is noted on the seaward margins of the marine-cut platforms where high-energy littoral processes, acting in a micro-tidal range of ca. 1.8 m under strong prevailing southerly wind and South Atlantic derived southwesterly swell regimes, promote marine erosion and deep gully formation. Therefore, the most favourable diamondiferous trapsites in the Late Cainozoic marine packages of the southwestern Sperrgebiet are those developed in deep bedrock gullies of either swash-parallel, strike-parallel or joint pattern origin on the seaward margins of marine-cut platforms that represent discrete high sea level stands during the Quaternary.

Organic-walled microfossils and biostratigraphy of the upper Port Nolloth Group (Namibia): implications for latest Neoproterozoic glaciations

The occurrence of organic-walled microfossils is reported for the first time from the Neoproterozoic Port Nolloth Group, Gariep Belt (southern Namibia). Acritarchs assigned to Bavlinella faveolata occur in the Hilda Subgroup below the younger of two glaciogenic diamictite units (Numees Formation) within the Port Nolloth Group. The microfossil assemblage in the overlying upper Holgat Formation, above the Numees Formation, is characterized by low diversity (six species), dominance of Soldadophycus bossii, and absence of acanthomorphic or large sphaeromorphic acritarchs. The agglutinated foraminifer Titanotheca also occurs in the Holgat Formation. Combined with available chemostratigraphic data and Pb–Pb ages, this microfossil assemblage indicates an upper Ediacaran age of around 555 Ma for the Holgat Formation. Virtually identical microfossil assemblages, negative-to-positive δ13 C trends,87Sr/86Sr values between 0.7080 and 0.7085, as well as Pb–Pb carbonate ages, make it possible to correlate the Holgat Formation with the Buschmannsklippe Formation (Witvlei Group, central Namibia), the Kombuis Member (Cango Caves Group, southern South Africa) and the uppermost Polanco to lowermost Cerro Espuelitas Formation (Arroyo del Soldado Group, Uruguay). Based on these data, the underlying Numees Formation, the age of which has been only loosely constrained so far and subject to considerable debate, can now be assigned to the c. 580 Ma Gaskiers or the possibly younger (&lt; 570 Ma) Moelv glacial event. The Numees glacial event may be represented in the uppermost Nooitgedagt Member (Cango Caves Group, South Africa) and the lower Barriga Negra formations (Arroyo del Soldado Group, Uruguay), characterized by a negative δ13C excursion and a strong sea-level drop. If this correlation is confirmed, lack of glacial deposits there might have implications for the palaeogeographic extent of upper Ediacaran glaciations. Our preliminary studies show that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphy of Neoproterozoic glacial deposits, especially when combined with C and Sr isotopic data.

A connection between the Neoproterozoic Dom Feliciano (Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts – evidence from a reconnaissance provenance study

A provenance study of Neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the Pan-African Gariep Belt (Stinkfontein Subgroup and Oranjemund Group, respectively) in southwestern Africa, as well as in the Rocha Group of the Punta del Este Terrane (Dom Feliciano Belt) in Uruguay, revealed that the Oranjemund and Rocha Groups can be correlated and most likely formed in the same basin. Thus, the Rocha Group is considered to represent the fill of the westernmost part of a re-activated Ediacaran Gariep Basin. The lower parts of the Oranjemund and Rocha Groups reflect erosion of mafic rocks, whereas the upper parts are derived from a predominantly felsic source area. First major and trace element geochemical data are consistent with oceanic islands of within-plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the Oranjemund Group, whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper Oranjemund Group. Age spectra obtained by SHRIMP U–Pb analyses of detrital zircon grains from the Stinkfontein Subgroup (Port Nolloth Group), the Oranjemund Group, and the Rocha Group are very similar, except for a lack of the youngest age group around 600 Ma in the Stinkfontein Subgroup. In all three units, zircon grains of 1000–1200 Ma dominate, with a further peak in the age distribution between 1700 and 2000 Ma. These ages compare well with the pre-Gariep basement geology in southwestern Africa, where the former age range corresponds to magmatic and high-grade metamorphic activity in the Mesoproterozoic Namaqua-Natal Belt and the latter to an extensive Palaeoproterozoic Andean-type volcanic arc (Richtersveld Terrane). Comparable ages are conspicuously absent in the basement of the Rio de la Plata Craton in South America. Derivation of the Rocha Group sediments from a similar source as the contemporaneous Oranjemund Group sediments is therefore suggested. The most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640–590 Ma magmatic arc of the Dom Feliciano Belt. This interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the Gariep Belt. Two stages of basin evolution are distinguished for the Gariep Basin, an early failed continental rift that subsequently became re-activated as back-arc basin related to the Cryogenian Dom Feliciano arc. Consequently, the Gariep Basin does not represent the main suture between the Rio de la Plata and the Kalahari Cratons. This suture is now suspected to be west of the Dom Feliciano volcanic arc, and might be connected with the Sierra Ballena – Major Gercino Lineament.

Wall rock alteration and lithogeochemical haloes associated with the sediment-hosted Rosh Pinah Zn-Pb-Ag deposit in the Pan African Gariep Belt, southwestern Namibia

Limitations on the life-of-mine of the sediment-hosted Rosh Pinah deposit urged exploration for extensions to existing ore and similar new deposits in the Gariep Belt. Identification and quantification of hydrothermal wall rock alteration and the extent of dispersion haloes of ore indicator elements were considered supportive to this cause. The results from detailed lithogeochemical traverses on the C Mine and Eastern Orefield, coupled with mineralogical studies and the interpretation of a large (n > 400) multi-element data set of the mine area, showed that the extent of wall rock alteration and associated element dispersion is limited. Barium has shown the most extensive dispersion in argillaceous, arkosic and dolomitic-arkosic wall rocks with anomalous values evident up to 100m into the footwall and 50m into the hanging wall of the ore zone. Alteration of detrital K-feldspar clasts by Ba-rich hydrothermal fluids provided a visual, qualitative index, which exceeds the limits of the elemental halo by 10–20m. Zinc and Pb, as well as Fe and Mn, were considerably less effective as indicator elements. Indices devised for the various lithologies of the Rosh Pinah deposit, such as the Barium Alteration Index and Combined Index, as well as modifications to previously proposed indices are based on variations in Fe(t), MnO, MgO, Al2O3, SiO2, BaO, and K2O. The indices are aimed at minimizing the influence of sedimentologically induced variation in chemistry and maximizing those superimposed by hydrothermal alteration. These quantitative parameters allowed the successful distinction between unaffected and hydrothermally altered wall rocks, as well as an indication of proximity to the ore zone. The results confirmed the limited extent of alteration demonstrated by single element distribution, but indicated that a slightly broader zone has been affected. The limited alteration of feldspathic, dolomitic and siliciclastic sediments is a function of the physico-chemical properties and evolution of the mineralizing fluids. It indicates that the fluids were near neutral to weakly acidic, whereas high concentrations of Ba in the ore and wall rocks required the fluids to be reduced (H2S-dominant). These physico-chemical properties of the fluids resulted in rapid precipitation of the metal load in response to a variety of processes such as cooling, increases in pH, dilution or addition of H2S. Consequently, mineralization has proven to be distinctly concentrated within a favourable horizon, with only limited vertical metal dispersion.

Stratigraphic setting of the metalliferous Rosh Pinah Formation and the Spitzkop and Koivib Suites in the Pan-African Gariep Belt, southwestern Namibia

In this paper a stratigraphic scheme is proposed for the Neoproterozoic Rosh Pinah Formation in southwestern Namibia. The formation forms part of the Port Nolloth Zone in the Pan-African Gariep Belt and has become famous for hosting two major economic base metal deposits at Rosh Pinah and Skorpion. It is characterised by the presence of volcanic rocks in an otherwise clastic-dominated sediment succession. Genetically, it is related to progressive opening of a failed intracratonic rift graben in the east (Rosh Pinah Graben) that was separated by a basement horst from a half graben to the west, which developed into the Adamastor Ocean. During early rifting, sedimentation started along the western flank of an actively rising fault scarp as alluvial fans, alluvial plains and fan deltas. The main rock types are arkose, dolomitised limestone, calcarenite, siltstone, mudstone and a variety of volcanic and pyroclastic rocks related to bimodal, predominantly felsic ~750 Ma magmatism. Sedimentation of the Rosh Pinah Formation began during a glacial period that is correlated with the global 720 to 750 Ma Sturtian glaciation. The Rosh Pinah Formation displays repetitive sedimentary cycles that reflect rapid deposition with intervening quiescent periods related to reactivation on basin bounding faults with subsequent thermal and mechanical subsidence. Sedimentary facies play an important role in the localization of sedimentary-exhalative and hydrothermal replacement mineral deposits as fine-grained anoxic sediments. While deposition of the Rosh Pinah Formation volcano-sedimentary successions was sustained in the more active parts of the basin, background sedimentation to the west is reflected by more distal deposits, ranging from planar laminated allodapic limestone to mudstone, followed by platform carbonates. In proximal positions, tilting of the rift shoulders led to partial erosion of the Rosh Pinah Formation and thus to intraformational breccias and olistostromes. The Rosh Pinah Formation is overlain by a regressive, siliciclastic succession, which has been interpreted as reflecting eustatic seawater fall in preparation of a major, probably syn-Marinoan (~580 Ma) glaciation. Finally the original geometry of the Rosh Pinah Formation depository was largely overprinted by intense ductile deformation during the Gariepian orogeny, which started ~580 Ma and reached its peak at ~545 Ma.

Geochemistry of carbonate beds in the Neoproterozoic Rosh Pinah Formation, Namibia: Implications on depositional setting and hydrothermal ore formation

Hydrothermally altered carbonate rocks are an important host to Rosh Pinah-type base metal sulphide mineralisation in the Pan-African Gariep Belt in southern Namibia. In order to elucidate the genesis and environment of formation of these carbonates, as well as the nature of the hydrothermal alteration, a geochemical as well as C and O isotopic study of these rocks was carried out. The results obtained from two localities, one being proximal to an inferred volcanic centre and the other being in a more distal position at the Rosh Pinah mine, revealed a primary sedimentary origin for most of the carbonates. Original limestone beds were altered to variable extent during early diagenesis by a hydrothermal, dolomitising fluid that most likely emanated from local, rift-related felsic magmatism. Enrichment in some base metals, such as Cu, can be related to this magmatism, whereas others, notably Zn and Pb, are not derived from the magma but sourced in the underlying pre-Gariep Mesoproterozoic basement. Sedimentological and isotopic evidence indicates a shallow marine setting in a restricted basin. An anoxic event, required for the formation of massive sulphide bodies, is postulated to have occurred during a cold period and is explained by eustatic drop in sea level, which caused the separation from the open ocean and subsequent starvation of the depositional basin for the Rosh Pinah Formation.

Late Vendian Closure of the Adamastor Ocean: Timing of Tectonic Inversion and Syn-orogenic Sedimentation in the Gariep Basin

New Pb-Pb age data obtained on stromatolitic dolomite from the oceanic Marmora Terrane in the Pan-African Gariep Belt support previous Ar-Ar age data on early amphibole, that has been related to sea floor metamorphism, and indicate formation of oceanic crust in the Neoproterozoic Adamastor Ocean (Gariep Basin) around 600 Ma. A maximum age for oceanic crust in the Gariep Basin is given by 741±6 Ma, which is the best available minimum constraint on continental rifting in the Gariep Belt. Tectonic inversion to basin closure overlaps in time with the end of a younger glacial period that is recorded both in the oceanic and continental realms and is correlated with the global c. 580 Ma Marinoan glaciation. The rock record up to that time in the Marmora Terrane is completely different to that in the para-autochthonous, continental Port Nolloth Zone in the east of the belt, but the post-glacial sediment successions bear many lithological and geochemical similarities. These younger successions are therefore interpreted as reflecting syn-orogenic deposits in a foredeep, the age of which is constrained between c. 580 and 550 Ma. Comparison of three chemostratigraphic profiles through the post-glacial carbonate succession shows that enrichment in carbonate 13C is, in the first instance, a function of proximity to the basin margin and thus of the extent of ocean water stratification.