Controls of gold-quartz vein formation during regional folding in amphibolite-facies, marble-dominated metasediments of the Navachab gold mine in the Pan-African Damara Belt, Namibia

The Navachab gold mine in central Namibia is situated in amphibolite-facies, marble-dominated metasediments of the Neoproterozoic Damara Sequence in the Pan-African Damara Belt. Gold mineralization is hosted by two main, and at least three minor, sets of auriferous quartz-veins that are developed on the subvertical northwestern limb of the regional-scale, shallow doublyplunging anticline of the Karibib dome. The orientation, relative timing and progressive deformation of quartz veins indicate that veining occurred during folding and fold amplification of the Karibib dome. This deformation forms part of the main phase of northwest-directed collisional tectonics in the Damara belt at ∼540 to 550 Ma. The two main vein sets include (1) bedding-pa rallel, shallowly-plunging ore lenses and shoots situated at the base of a prominent marble unit, and (2) a laterally extensive swarm of shallowly-dipping quartz veins that truncate the host rocks at high angles. The bulk of the bedding-parallel lenses represent dilational jogs that opened during flexural flow along bedding-parallel slip planes during the amplification of the Karibib dome. Flexural flow and associated bedding-parallel fluid infiltration were concentrated close to the contact between marbles and the underlying siliciclastic formations. This lithological contact, in particular, represented a pronounced rheological contrast resulting in increased slip rates during fold amplification. The highly discordant sets of shallowly-dipping, sheeted quartz veins were emplaced during the fold lock-up stage of the Karibib dome. Quartz veining occurred when the northwestern limb of the dome was rotated to subvertical attitudes, so that bedding-parallel flexural slip ceased to be active. Extensional fracturing and veining was facilitated by the presence of transiently supralithostatic fluid pressures. The high-amplitude fluid pressure cycling associated with extensional fracturing is likely to have triggered gold precipitation in the shallowly-dipping veins. Although bedding-parallel veins and discordant veins are broadly contemporaneous, related to the regional-scale D2 phase of fold-and-thrust tectonics, cross-cutting relationships between the two main vein systems indicate that the two vein sets have represented separate entities and succeeded each other rather than forming an interconnected fracture mesh. © 2005 Geological Society of South Africa.Articl

Lithostratigraphic correlations in the western branch of the Pan-African Saldania belt, South Africa: The Malmesbury Group revisited

A new lithostratigraphic subdivision is proposed for the low-grade metamorphic, supracrustal rocks of the Pan-African Saldania Belt of the Western Cape Province South Africa, previously referred to as the Malmesbury Group. Two tectonostratigraphic groups can be distinguished that appear to be separated by an unconformity. The lower of these groups is termed the Swartland Group. There are no absolute age constraints for the Swartland Group, but a penetrative D1 deformation in rocks of the Swartland Group is correlated with the onset of oblique crustal convergence along the Pan-African West Coast Belts at ca. 580 to 570 Ma, which also provides an upper age limit of the Swartland Group. The structural evolution and style of deformation suggest a correlation of the Swartland Group with part of both the Gilberg Group of the Vanrhynsdorp Basin and the Port Nolloth Group of the Gariep Belt to the north of the Salclania Belt. The Swartland Group is unconformably overlain by mainly clastic sediments that lack evidence of the early D1 deformation and that are here referred to as the Malmesbury Group. A lower age bracket of the Malmesbury Group is provided by the presumed age of deformation of the underlying Swartland Group (ca. 575 Ma), while a minimum age of ca. 550 Ma is indicated by the intrusion of the oldest phases of the Cape Granite Suite that cross-cut the lithologies and fabrics of the Malmesbury Group. The lithological evolution and age of the Malmesbury Group suggests it correlation with the late orogenic clastic deposits of the Late-Neoproterozoic to Early-Cambrian Nama Group that formed in the eastern foreland basins of the emerging Pan-African orogenic belts. The new subdivision no longer assumes the presence of three allochthounous terranes in the Saldania Belt and allows for a better correlation of formations with those in Pan-African belts to the north. © 2003 Geological Society of South Africa.Articl

Thrust-related dome structures in the Karibib district and the origin of orthogonal fabric domains in the south Central Zone of the Pan-African Damara belt, Namibia

The high-grade metamorphic south Central Zone (sCZ) exposes an oblique crustal section through the internal parts of the Pan-African Damara belt in Namibia. The structural pattern of the south Central Zone is characterized by kilometer-scale, northeast-trending dome structures for which a number of different origins have been proposed. Detailed structural mapping of the Karibib and Usakos domes in the Karibib district indicates an origin of the two domes as large tip-line folds located above blind thrusts. The two domes are overridden from the southeast by a crystalline thrust sheet along the Mon Repos thrust zone and the thrusts and associated folds form part of a deeply-eroded, foreland- (northwest-) vergent fold-and-thrust belt. Intrusive relationships of syn- and post-tectonic granitoids constrain the timing of thrusting to between 550 and 540 Ma, corresponding to the main phase of collisional tectonics in the Damara belt. On a regional scale, the south Central Zone is underlain by two orthogonal fabric domains. Northwest-vergent tectonic transport, documented in this study, dominates the lower amphibolite-facies northeastern parts of the south Central Zone. In contrast, a southwest-directed orogen-parallel extrusion is widely documented for the lower structural levels of the upper-amphibolite- to granulite-facies southwestern parts of the south Central Zone that contain voluminous synkinematic granites and widespread migmatization. Fabric development in the two orthogonal domains can be shown to be contemporaneous and related to the main collision between the Congo and Kalahari cratons at ca. 540-550 Ma. We suggest that the formation of the two orthogonal fabric domains represents the different crustal response of the rheologically layered crust to the late-Proterozoic crustal shortening. At shallower crustal levels, shortening of sufficiently strong crust in the Karibib district resulted in vertical thickening and orogen-normal folding and thrust transport. Shortening of the deeper levels of the sCZ resulted in pure shear deformation of the rheologically weakened and partially molten crust and the lateral, orogen-parallel displacement of rocks. © 2004 Elsevier B.V. All rights reserved.Articl

Constraining the timing and migration of collisional tectonics in the Damara Belt, Namibia: U-Pb zircon ages for the syntectonic Salem-type Stinkbank granite

A U-Pb zircon LAM-ICP-MS age of 549 ± 11 Ma from the Stinkbank granite in the south Central Zone of the Damara belt represents the first robust age for the crystallization of a member of the regionally widespread suite of syntectonic Salem-type granites. This age confirms previous assertions that suggest that the main phase of crustal shortening related to the collision between the Congo and Kalahari Cratrons occurred between 550 and ∼540 Ma in the south Central Zone, the leading edge of the Congo Craton. Ages from syntectonic granites reported in the literature from the north Central and the Northern Zone record progressively younger ages of ∼530 to ∼515 Ma and late-stage folding in the Northern Zone is suggested to have occurred between ∼510 to ∼500 Ma. The younging of ages that date the main collisional phase towards the north is interpreted to track the northward propagation of the deformation and metamorphic front from the main suture zone to the northern foreland of the Damara belt. Geochronological data are, sparse, and the lack of detailed structural data precludes the retrodeformation of the fold-and-thrust belt along this 250 km long section. However, the age brackets suggest a migration of the orogenic front at a rate of at least 5 to 7 mm/a, but more likely in the order of several centimetre per year, if the internal strain is removed. © 2006 December Geological Society of South Africa.Articl

Continental growth and convergence-related arc plutonism in the Mesoarchaean: Evidence from the Barberton granitoid-greenstone terrain, South Africa

U-Pb zircon ages from trondhjemitic gneisses of the Badplaas domain, located in the southwestern parts of the Mesoarchaean Barberton granitoid-greenstone terrain of South Africa, document a hitherto largely unrecognized plutonic event. The geochronological results indicate the addition of juvenile, felsic crust over a period of 60 Myr between ca. 3290 and 3230 Ma and prior to the main collisional event in the granitoid-greenstone terrain at 3230 Ma. The timing and duration of plutonism, together with the structural and compositional heterogeneity of the Badplaas domain suggest that the Badplaas domain represents part of a convergence-related magmatic arc. On a regional scale, the spatial and temporal relationships between plutonism, metamorphism and deformation are interpreted to preserve an almost complete inventory of a Mesoarchean arc-trench system. This includes (1) the 3290-3230 Ma convergence-related magmatic arc, (2) a largely coeval deposition of the back-arc type volcano-sedimentary 3260-3225 Ma Fig Tree Group of the Barberton greenstone belt, (3) relics of the underplated crust preserved in high-P, low-T rocks to the immediate east of the magmatic arc, juxtaposed against (4) high-T medium-P rocks of the overlying plate along the major crustal structure of the Inyoni shear zone. These findings corroborate the notion that thermal and rheological conditions on the early Earth supported, at least locally, crustal growth and the extraction of buoyant felsic crust along convergence-related magmatic arcs. © 2010 Elsevier B.V. All rights reserved.Articl

Metamorphism and exhumation of the basal parts of the Barberton greenstone belt, South Africa: Constraining the rates of Mesoarchaean tectonism

The Paleo- to Mesoarchaean Barberton granitoid-greenstone terrain of South Africa consists of two main components: the low-grade metamorphic supracrustal greenstone sequence of the Barberton greenstone belt in the north and a high-grade metamorphic granitoid-gneiss terrain to the south. The boundary between the two different domains corresponds to the highly tectonized, amphibolite-facies rocks of the Theespruit Formation that occur along the margins of the greenstone belt. These rocks record high-P, low-T peak metamorphic conditions of 7.4 ± 1.0 kbar and 560 ± 20 °C that are very similar to estimates from other areas of the high-grade terrain and were attained during the main phase of terrain accretion in the greenstone belt at 3230 Ma. In contrast, the greenstone sequence ca. 4 km to the north only records low greenschist-facies metamorphism, indicating that a metamorphic break of ca. 18 km exists between the high-grade terrain and the greenstone belt. The main phase of deformation in the Theespruit Formation was initiated under peak metamorphic conditions and continued during retrogression. Retrograde P-T estimates and mineral reactions indicate that retrogression involved near-isothermal decompression of ca. 4 kbar prior to cooling into the greenschist-facies, suggesting that the fabric in these rocks is an exhumation fabric that accommodated the juxtaposition of the high-grade terrain against the greenstone belt. Geochronological constraints, combined with the depths of burial indicate that exhumation of the high-grade rocks occurred at rates of 2-5 mm/a and are comparable to the exhumation rates of crustal rocks in younger orogenic environments. The extremely low apparent geothermal gradients of ca. 20 °C/km that are recorded in the high-grade terrain are inconsistent with models of a hotter and weaker crustal environment in the Archaean. Rather, the depths of burial and structural integrity of this terrain suggest that the Mesoarchaean crust was cold and rigid enough to allow tectonic stacking and crustal overthickening and had a rheology similar to modern continental crust. © 2005 Elsevier B.V. All rights reserved.Articl

Granulite facies anatexis in the Ancient Gneiss Complex, Swaziland, at 2.73 Ga: Mid-crustal metamorphic evidence for mantle heating of the Kaapvaal craton during Ventersdorp magmatism

The 3.7-3.2 Ga Ancient Gneiss Complex (AGC) of Swaziland, located S and SE of the Barberton Greenstone Belt (BGB), is a complex geological terrane where few studies have focussed on the timing and conditions of deformation and metamorphism, despite the fact that minor metasedimentary units within the AGC provide excellent opportunities for constraining metamorphic evolution. One of these, the Mkhondo Valley Metamorphic Suite consists of migmatitic metapelitic granulites, within which the peak metamorphic assemblage garnet + cordierite + biotite + plagioclase + quartz ± K-feldspar + melt is dominant. NCKFMASHTO pseudosection modelling of this anatectic assemblage constrains peak metamorphic conditions to 850-830 °C and 4.4 kbars, followed by near isobaric cooling of the terrane to 680 °C and 3.9 kbars. These conditions indicate a period of anomalously high heat flow within the crust due to the direct addition of mantle heat. U-Pb SHRIMP dating of zircons from an in situ leucosome indicates an age of ca. 2.73 Ga for peak granulite facies metamorphism, approximating the age of Ventersdorp Supergroup flood basalt volcanism on the craton and widespread intracratonic granitic magmatism. The current study suggests that substantial heat input to the Kaapvaal cratonic crust during underplating by Ventersdorp related magmas, possibly related to the presence of a ∼2.7 Ga mantle plume marginal to the craton, was responsible for high-grade metamorphism in the AGC and a period associated with regional crustal anatexis. The resultant mid-crustal ductility and weakening may have initiated mobilisation of the mid-crust as gneissic domes, producing basement uplift and sedimentary recycling of auriferous reefs of the Central Rand Group of the Witwatersrand Supergroup. © 2009 Elsevier B.V. All rights reserved.Articl

Extensional detachment faulting and core-complex formation in the southern Barberton granite-greenstone terrain, South Africa: Evidence for a 3.2 Ga orogenic collapse

The Barberton greenstone belt in South Africa is an Early- to Mid-Archaean, very low-grade metamorphic supracrustal belt that is bordered in the south by a mid- to lower crustal gneiss terrain. Detailed mapping of the contacts between the supracrustal and gneiss domains along the southern margin of the greenstone belt shows that the supracrustal rocks are separated from the high-grade metamorphic gneiss terrain by an extensional detachment that is situated at and close to the base of the belt. The extensional detachment is approximately 1-km wide and its location corresponds with the heterogeneous, mélange-like rocks of the Theespruit Formation. Within the detachment zone, two main strain regimes can be distinguished. Amphibolite-facies rocks at and below the granite-greenstone contacts are characterized by rodded gneisses and strongly lineated amphibolite-facies mylonites. The bulk constrictional deformation at these lower structural levels records, in a subhorizontal orientation, the vertical shortening and horizontal, NE-SW directed stretching of the mid-crustal rocks. The prolate, coaxial fabrics are overprinted by greenschist-facies mylonites at higher structural levels that cut progressively deeper into the underlying high-grade basement rocks. These mylonites have developed during non-coaxial strain and kinematic indicators consistently point to a top-to-the-NE sense of movement of the greenstone sequence with respect to the lower structural levels. This relationship between bulk coaxial NE-SW stretching of mid-crustal basement rocks and non-coaxial, top-to-the-NE shearing along retrograde mylonites at upper crustal levels is consistent with an extensional orogenic collapse of the belt and the concomittant exhumation of deeper crustal levels. The exhumation was initiated under amphibolite-facies conditions at depths of approximately 18 km. The extensional collapse is coeval with or shortly follows the main D2 collisional event in the Barberton greenstone belt at ca. 3230-3220 Ma. Voluminous plutonism at ca. 3225 Ma along the northern margin of the belt is possibly related to the orogenic collapse and associated decompression melting of lower crustal rocks. The extensional collapse coincides with the onset of the coarse-clastic Moodies Group sedimentation which suggests that the small, isolated Moodies basins formed as supradetachment basins in the collapsing hanging wall of the detachment. The steepening of lithologies and fabrics to their present-day vertical attitudes is ascribed to a late-stage solid-state diapiric component of the exhumed hot and buoyant basement gneisses that underlie the relatively cool and dense mafic and ultramafic supracrustal succession. © 2003 Elsevier B.V. All rights reserved.Revie

The assembly, metamorphic and deformation history of the Stolzburg pluton and surrounding rocks, Kaapvaal craton, South Africa

International audienc

Table 1. The Donkerhuk batholith, Namibia: A giant S-type granite emplaced in the mid crust, in a fore-arc setting

Partial chemical analyses for rocks of the main Donkerhuk batholith