Pan-African Gariep Belt Research Articles

Pan-African accretionary metamorphism in the Sperrgebiet Domain, Gariep Belt, SW Namibia

The newly described Sperrgebiet Domain is a part of the Palaeo- to Mesoproterozoic Namaqua–Natal Province that is exposed as a basement inlier within the Pan-African Gariep Belt in SW Namibia. Clastic metasediments of the c. 1700Ma Aurus Schist of the Sperrgebiet Domain exhibit Pan-African fabrics and metamorphic mineral assemblages that are not present in the low-grade metamorphic Gariep Supergroup against which it is tectonically juxtaposed. Metapelitic and semi-pelitic schists consist of phengitic white mica, chlorite, minor garnet and quartz ± biotite, and contain pseudomorphs after a prismatic aluminous mineral, likely chloritoid or staurolite. Calculated pseudosections for four mica schist samples constrain P–T conditions of around 10kbar and 500°C, and the late paragenesis of garnet indicates that these conditions were attained along a prograde, clockwise trajectory. These conditions and the low apparent geotherm (∼14°C.km−1) are typical of subduction zone metamorphism, and are similar to those documented for peri-contemporaneous accretionary metamorphism in the adjacent Damara Belt to the NE. A subduction origin has also been proposed for exotic ‘blueschist’ blocks within the Gariep Belt; however, the absence of an accretionary metamorphic imprint in the Gariep Supergroup indicates that the Sperrgebiet Domain was decoupled from these lithologies for much of the Pan-African Orogeny. Our results are the first indication that the continental margin of the western Kalahari Craton may have been subducted during the latter stages of Pan-African convergence and Gondwana assembly.

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.

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.

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.

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.

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.

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.

An Eburnean base metal source for sediment-hosted zinc-lead deposits in Neoproterozoic units of Namibia: Lead isotopic and geochemical evidence

The lead isotopic composition of galena from the Neoproterozoic sediment-hosted Zn-Pb sulphide deposit at Rosh Pinah and the oxidised Zn deposit at Skorpion in the Pan-African Gariep Belt, southwestern Namibia, as well as that of galena from minor occurrences in the wider Rosh Pinah ore province was investigated and is compared with that of other major sediment-hosted base metal ore deposits hosted by Neoproterozoic strata in southwestern Africa and Brazil. The isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the Rosh Pinah deposit and its stratigraphic equivalents. The pre-orogenic Rosh Pinah deposit and the inferred progenitor for the secondary Skorpion deposits have very similar Pb isotopic compositions. In contrast, syn-orogenic deposits (e.g. Tsumeb-type) show a larger proportion of radiogenic Pb from the country rocks that were infiltrated by orogenic ore fluids.

Fluid evolution in and around the Rosh Pinah massive sulphide deposit in the external Pan-African Gariep Belt, Namibia

A regional fluid inclusion and stable isotope (H, O, C) study of the external part of the Pan-African Gariep Belt, with emphasis on the Rosh Pinah Zn-Pb sulphide deposit, demonstrated that the bulk of those fluids present at the time of syn-tectonic peak metamorphism and subsequent uplift and exhumation are of aqueous composition with low salinity. Exceptions are few moderately saline inclusions that are limited to the tectonic contact with the pre-Gariep basement near the volcano-sedimentary Rosh Pinah Formation. These are explained by derivation from a nearby, locally developed lagoonal, possibly evaporitic, facies. The infiltrated host rocks largely controlled the composition of the syn-orogenic fluids and no evidence was found for a regional pervasive flux of orogenic brines through the external parts of the belt. Very rare pre-orogenic fluids discovered in Rosh Pinah mine samples display markedly different composition and density that are in line with a derivation from a volcanic source. These fluid inclusions are interpreted as remnants of the original ore fluid that infiltrated the host sediments and pyroclastic rocks at a temperature of around 400°C and a hydrostatic pressure of a few hundred bars. This infiltration led not only to intense brecciation and silicification of arenitic footwall and sulphide precipitation but also to the dolomitisation of original limestone host rock. The good preservation of the syn-rift Rosh Pinah ore bodies during the Gariep orogeny is explained by the lack of regional orogenic brine flow.

New U-Pb zircon ages for the Kuboos pluton in the Pan-African Gariep belt, South Africa: Cambrian mantle plume or far field collision effect?

The age of the Kuboos pluton, which forms part of a series of alkaline, apparently post-orogenic intrusive bodies along the Kuboos-Bremen line, was re-assessed using single zircon U-Pb isotope analyses. A new age of 507 ± 6 Ma obtained for the youngest intrusive phase, a monzogranite from the Kuboos composite pluton is clearly younger than the main phase of deformation in the Gariep Belt (545 ± 2 Ma). This age corresponds to a major thermal event, as reflected by Rb-Sr and Ar-Ar mineral ages and by coeval post-orogenic granite in the Damara Belt, and also with Ar-Ar ages of syn-tectonic muscovite, related to a late tectonic pulse in the Pan-African Gariep and Saldania Belts, their basement and corresponding foreland basins. While the earlier 545 Ma tectonothermal event is explained by the closure of the Adamastor Ocean between the Kalahari and the Rio de la Plata cratons, the younger 500 Ma tectonic pulse can be linked to subduction beneath the western margin of Gondwana. The alkaline magmatism, which is represented by the Kuboos pluton and which is contemporaneous with the latter tectonic event, can be explained either by a mantle plume underneath southern Africa or as a distant effect of collision along the Gondwana margin. The occurrence of a 1.1 - 1.2 Ga xenocrystic zircon in the Kuboos pluton hints at the external Gariep Belt being floored by Kibaran crust.

Marine evaporites from an oceanic island in the Neoproterozoic Adamastor ocean

We report a hitherto unknown occurrence of ancient Neoproterozoic evaporite deposits from an allochthonous terrane in the Pan-African Gariep belt in Namibia. Low contents of Rb, Cs, Ba, Zr, Hf, Th, and U, flat chondrite-normalised rare earth element (REE) patterns, 87Sr/ 86Sr ratios as low as 0.7075, Na–Cl–Br systematics of fluid inclusion leachates, and high δ 11B values for stratiform tourmalinites, together with geologic evidence, such as association with oceanic basalt, gabbro, and stromatolitic dolomite, point to a marine evaporitic origin. An atoll environment on an oceanic island is envisaged as a likely depositional setting. In the associated mafic sequence we found a diamictite with metre-sized ice rafted detritus, suggesting the presence of sea ice cover at relatively low latitude around the time of evaporite deposition. Based on chemostratigraphic ( 87Sr/ 86Sr, δ 13C) comparison with passive continental margin sediments in the para-autochthonous external part of the Gariep belt, a correlation of the mafic diamicitite with the global Varangian (590–560 Ma) glaciation is proposed.

Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia and South Africa

Within the Pan-African Gariep Belt in southwestern Africa a para-autochthonous, predominantly sedimentary sequence, deposited in a rift graben and subsequently on a passive continental margin with a Kibaran basement, is distinguished from an allochthonous, predominantly oceanic terrane that was thrust on top of the former during collision of the Rio de la Plata and the Kalahari plates. 40Ar/ 39Ar mineral cooling ages presented here set the following constraints: after Kibaran high-grade metamorphism, the consolidated basement cooled through ∼300°C at 1006 Ma. Rifting between 781 and 741 Ma was followed by the formation of oceanic crust, the lower parts of which cooled through 500°C sometime between 630 and 600 Ma. After inversion from extension to compression, some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge—a process that is indirectly dated between 573–576 Ma by hornblende plateau ages. Continent–continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547–543 Ma as indicated by further hornblende plateau ages. Muscovite cooling ages for the internal zone are between 526 and 529 Ma, those for the tectonically deeper, external zone are 483 Ma in the north and between 495–506 Ma in the south of the belt. In conjunction with published data, these results suggest successive closure of first the northern Adamastor ocean (Kaoko Belt), followed by the Khomas sea (intracontinental Damara Belt), and finally the southern Adamastor ocean (Gariep Belt).

Geochemistry and tectonic setting of magmatic units in the Pan-African Gariep Belt, Namibia

Major-, trace-, rare-earth-element, and RbSr and SmNd isotope data of the various low-grade metamorphosed magmatic units in the Pan-African Gariep Belt, southwestern Namibia, presented in this paper, provide the basis for the reconstruction of the evolution of the late Proterozoic Adamastor ocean between the South American and Kalahari Cratons. The Gariep Belt is subdivided into two major zones, a para-autochthonous, predominantly sedimentary rift and passive continental margin succession (Port Nolloth Zone, PNZ) and the allochthonous, predominantly mafic Marmora Terrane. The latter has been thrust in southeasterly direction over the former. All of the magmatic activity in the PNZ can be related to lithospheric stretching. The earliest magmatic activity is expressed as bimodal volcanism with continental within-plate affinity along an embryonic rift graben (Rosh Pinah Formation) and is found within the older sediment package of the para-autochthonous external zone of the orogen. After ∼ 24 Ma, this was followed by the intrusion of mafic, tholeftic dykes into the basement and the lower parts of the PNZ, heralding the opening of the Gariepian basin at 717 Ma. There is evidence of a series of seamounts, or an aseismic ridge, with oceanic within-plate characteristics in the Marmora Terrane. Such volcanic piles make up most of the Schakalsberge Complex and parts of the Chameis Complex. Both complexes are separated by turbidites (Oranjemund Complex), which are interpreted as continental slope deposits. In the Chameis Complex, metagabbros predominate. Most of them are part of the oceanic seamounts, but some have a markedly different chemistry, indicative of a mid-ocean ridge setting. During Pan-African collision, the Marmora Terrane formed an accretionary wedge that was thrust over the passive continental margin succession. Peak metamorphism in the thickened crust was reached at ∼ 545 Ma, with erosion of the orogen having commenced as early as 540 Ma.