Vredefort Impact Event Research Articles

Impact‐Generated Permeability and Hydrothermal Circulation at the Vredefort Impact Structure, South Africa

AbstractThe 2.02 billion year old Vredefort impact structure in South Africa offers a unique opportunity to study large‐scale impact processes on Earth. Vredefort's large size (∼250 km in diameter) and eroded topography provides the opportunity to study the effects of shock physics at depth and post‐formation hydrothermal alteration. In this work, we simulate the formation of the Vredefort structure building upon recent shock physics (iSALE) simulations. We expand those simulations to cover a wider range of input conditions, and compute impact‐driven porosity and permeability. The latter quantities are used to perform fluid mobility simulations (HYDROTHERM). We find that the Vredefort event produced significant impact‐generated porosity (up to 30%) in an annulus from about 50 to 100 km from the center and up to several kilometers in depth. The corresponding estimated permeability (up to 10−12 m2) would have allowed for large scale subsurface fluid flows. Our hydrothermal calculations show that the Vredefort impact event could have generated a complex crustal fluid pattern within the crater rim that lasted for hundreds of thousand years, with localized flow concentration regions, opening a new interpretation for the mobilization and location of the ore deposits in the Witwatersrand basin. The combined approach utilizing impact and hydrothermal simulations constitute a powerful tool to understand geochemical processes at Vredefort, as well as to assess the ability of large impacts to drive crustal chemistry with far‐reaching consequences for the prebiotic evolution of the early Earth.

Geochemistry of a confirmed Precambrian impact ejecta deposit: The Grænsesø spherule layer, South Greenland

AbstractSamples from a single outcrop of the Graenseso spherule layer, Midternaes, South Greenland, consist of a spherule‐bearing dolomixtite with matrix‐supported intraclasts up to 1 m in size. In addition to field observations, we performed mineralogical and whole rock geochemical analysis, including electron microprobe, neutron activation analysis, X‐ray fluorescence, and mass spectrometry of the horizon and the overlying and underlying strata. We show that the spherules are petrographically similar to those in the Zaonega spherule layer, Karelia, Russia. Our petrographic and chemical results are consistent with the previous suggestion that the Grænsesø layer correlates with the Zaonega layer, and it is possible that both layers are related to the Vredefort impact event. The samples containing spherules, as well as the overlying rocks, have elevated REEs compared to the underlying pre‐impact layer, suggestive of a new continental source of sediment that may be coincident with the impact event. Zircons separated from the lower part of the Grænsesø spherule layer display complex age patterns suggesting that they have genetically different origins based on distinctly different Th/U ratios. Crystallization ages of all groups are ≥ 2.8 Ga, with ~2.8 Ga representing a time of major crustal growth globally. Therefore, we cannot conclusively determine in this study if the zircons are locally derived or if they are transported with the ejecta. The spherule layer was deposited by a high‐energy, subaqueous debris flow, an origin that is consistent with the hypothesis that the layer was deposited by impact‐induced waves and/or currents.

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was <2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic.

Identification and provenance determination of distally transported, Vredefort-derived shocked minerals in the Vaal River, South Africa using SEM and SHRIMP-RG techniques

The record of meteorite impacts on Earth is incomplete due to the destruction of impact craters by erosion and burial. Shocked minerals residing in sediments may help further document the impact record. To evaluate the survival of shocked minerals in fluvial systems we investigated ∼10,000 sand grains from seven sites along the Vaal River in South Africa, ranging from 103 to 759km downriver from the 2.020Ga Vredefort impact structure, for the presence of detrital shocked minerals. Detailed observations were made of >7000 quartz grains by petrographic analysis, and >3300 zircon and monazite grains by scanning electron microscopy, using BSE, CL, and EBSD methods. Shock microstructures were identified in detrital quartz, monazite and zircon grains in all of the Vaal River sediment samples analyzed, from the Vredefort Dome to the confluence with the Orange River, 759km downstream. Analysis of U–Th–Pb and REE compositions of detrital shocked zircon and monazite by SHRIMP-RG allows correlation of the grains to Archean basement exposed in the Vredefort impact structure.Three distinct detrital shocked zircon age populations at 3104Ma, 3065Ma and 2867Ma were identified. Apparent ages of detrital shocked monazite range from 3044Ma to 2157Ma, including two groupings at 2640Ma and 2454Ma. The Vredefort impact event does not appear to have caused significant Pb-loss for the majority of the grains; Neoproterozoic lower concordia intercepts (1093–751Ma) in the zircon populations indicate most Pb-loss occurred during the Kibaran orogeny in southern Africa.This study demonstrates that shocked minerals survive long distances (at least 750km) of transport within fluvial environments, and can be correlated to known bedrock localities using U–Th–Pb dating. The implications of these results are: (1) detrital shocked minerals within modern sediments can be used to identify the existence of an impact structure; geochronology of shocked zircon and monazite can provide target bedrock ages and metamorphic history that can constrain the location of the structure. (2) The persistence of ancient shocked zircon in modern fluvial sedimentary systems suggests that a detrital shocked mineral record of early impact events on Earth may be preserved in siliciclastic sedimentary rocks.

Comment on 40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex (South African Journal of Geology, 109, 393 410), by Alexandre et al. (2006)

Alexandre et al. (2006) recently presented the results of a detailed structural study of brittle-ductile deformation, which could, as they state, be seemingly coincident with regional low-grade metamorphism of the pre-2.15 Ga Transvaal Supergroup strata between the Vredefort impact structure of ~2.02 Ga age (Kamo et al. , 1996; Gibson et al. , 1997) and the Bushveld Complex that was intruded at ~2.06 Ga (Walraven, 1997; Buick et al. , 2001). Alexandre et al. (2006) also performed six 40Ar/39Ar step-heating dating experiments with synkinematic white mica specimens. The results comprise ages of ~2.15 Ga and 2042.1 ± 2.9 Ma, the latter derived from “plateaus and pseudo-plateaus, as well as cumulative probability statistics” ( ibid ). These authors suggest that “at ~2.04 Ga the rocks of the Transvaal Supergroup in the central [Kaapvaal] craton were a part of a more extensive fold-and-thrust belt, named here [ibid] the Transvaalide fold-and-thrust belt ”. Alexandre et al. (2006) admit the difficulties to explain the ~2.15 Ga phase (although they gave a try), but assign the time at ~2.04 Ga to “orogenic activity at the margins of the craton”. Alexandre et al. (2006) conclude (p.402) that “The age of 2042.1 ± 2.9 Ma is based on unambiguous plateau and pseudo-plateaus ([their] Figure 6), and therefore can be considered a robust and reliable one…”. They proceed to discuss that this age is clearly distinguishable from the ages of the Bushveld magmatic event and the Vredefort impact event, and that a definite event at that time was also supported by other chronological results for rocks from the entire region between the Limpopo Mobile Belt and the Vredefort impact structure. In the light of these authors’ conclusion based on the age of 2042.1 ± 2.9 Ma, it appears reasonable to scrutinize the argon chronological results …

40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex

Rocks of the Palaeoproterozoic Transvaal Supergroup, situated between the Bushveld Igneous Complex and the Vredefort Impact Structure in the central parts of the Kaapvaal craton (South Africa), experienced a regional low-grade metamorphism synchronous with brittle–ductile deformation that formed small- to medium-scale folds, cleavages, monoclines and thrusts in the dolomites, slates, and phyllites. Single-grain 40Ar/39Ar step-heating laser probe dating of synkinematic white mica from the phyllites indicates ages of ~2.15 Ga and 2042.1 ± 2.9 Ma (plateaus and pseudo-plateaus, as well as cumulative probability statistics). These ages are clearly distinct from the emplacement age of the Bushveld Igneous Complex (~2.06 Ga) and from the Vredefort impact event (~2.02 Ga), both of which have previously been suggested as possible causes of the deformation. The new data suggest that at ~2.04 Ga, the rocks of the Transvaal Supergroup in the central craton were part of a more extensive fold-and-thrust belt, named here the Transvaalide fold-and-thrust belt. While it is difficult to explain the significance of the ~2.15 Ga ages, the ~2.04 Ga event may be related to orogenic activity along the margins of the Kaapvaal craton at this time.

Mineral Deposits and Earth Evolution, Geological Society Special Publication 248.: I. MCDONALD, A. J.BOYCE, I. B. BUTLER, R. J. HERRINGTON, AND D. A. POLYA, EDITORS. 268 Pp. London. 2005. ISBN 1-86239-182-3. Price 75.00.

This book contains 13 papers based on presentations at the Geological Society’s Fermor Flagship Meeting in 2003 on World Class Mineral Deposits and Earth Evolution. The volume begins with two papers on impact features and ore deposits. The first, by Grieve, is a review and classification of impact-hosted deposits that formed before, during, or after the impact. Among other things, it reviews evidence that the Sudbury magmas are of crustal origin and stops just short of telling us whether this crust was already enriched in Cu and Ni. The other paper, by Hayward et al., presents trace element evidence against significant mobility of Witwatersrand gold during the Vredefort impact event, and drives yet another nail in the coffin of the hydrothermal model. A second set of papers deals with ore deposits and tectonic processes. De Wit and Thiart show that Archean …

New U–Pb SHRIMP zircon age for the Schurwedraai alkali granite: Implications for pre-impact development of the Vredefort Dome and extent of Bushveld magmatism, South Africa

The Schurwedraai alkali granite is one of a number of prominent ultramafic-mafic and felsic intrusions in the Neoarchaean to Palaeoproterozoic sub-vertical supracrustal collar rocks of the Vredefort Dome, South Africa. The alkali granite intruded the Neoarchaean Witwatersrand Supergroup and has a peralkaline to peraluminous composition. A new zircon SHRIMP crystallization age of 2052 ± 14 Ma for the Schurwedraai alkali granite places it statistically before the Vredefort impact event at 2023 ± 4 Ma and within the accepted emplacement interval of 2050–2060 Ma of the Bushveld magmatic event. The presence of the alkali granite and associated small ultramafic-mafic intrusions in the Vredefort collar rocks extends the southern extremity of Bushveld-related intrusions to some 120 km south of Johannesburg and about 150 km south of the current outcrop area of the Bushveld Complex. The combined effect of these ultramafic-mafic and felsic bodies may have contributed to a pronouncedly steep pre-impact geothermal gradient in the Vredefort area, and to the amphibolite-grade metamorphism observed in the supracrustal collar rocks of the Vredefort Dome.

Structural analysis of the collar of the Vredefort Dome, South Africa—Significance for impact‐related deformation and central uplift formation

Abstract— Landsat TM, aerial photograph image analysis, and field mapping of Witwatersrand supergroup meta‐sedimentary strata in the collar of the Vredefort Dome reveals a highly heterogeneous internal structure involving folds, faults, fractures, and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort impact event. Broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike‐slip displacements of, typically, tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from ˜17% at a radius from the dome center of 21 km to &lt;5% at a radius of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults or earlier, during the shock pulse. In addition to shatter cones, quartzite units show two fracture types—a cmspaced rhomboidal to orthogonal type that may be the product of shock‐induced deformation and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints, and the presence of radial and tangential dikes of impact melt rock, confirm the impact timing of these features and are suggestive of late‐stage collapse of the central uplift.

Gold mineralization within the Witwatersrand Basin, South Africa: evidence for a modified placer origin, and the role of the Vredefort impact event

Abstract The chemical composition of gold within the Archaean metasedimentary rocks of the Witwatersrand Supergroup displays significant heterogeneity at the micro-, meso-and regional scales. A detailed electron microbeam analytical and petrological study of the main auriferous horizons in the Central Rand Group throughout the Witwatersrand Basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres. Contemporaneous chlorite formation was strongly rock-buffered. Gold mobilization occurred under fluid-poor conditions at temperatures that did not exceed 350 °C. Widespread circulation of mineralizing fluids within the Central Rand Group is not supported by the gold and chlorite chemical data. Brittle deformation that affects most of the paragenetic sequence of the Central Rand Group late in its post-depositional history is followed by sequences of mineral growth and dissolution that appear throughout the Central Rand Group and have consistent textural relationships with gold. The consistent location within the paragenetic sequence, the wide regional and stratigraphic extent of the brittle deformation, together with mineral chemical and petrological data suggest that the Vredefort Impact Event (2.02 Ga) was the cause of this late deformation, and that post-impact fluid-poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event.

40Ar-39Ar Dating of and Structural Information on Tectonite-Bearing Faults in the Witwatersrand Basin: Evidence for Multi-Stage, Tectono-Thermal Activity in the Central Kaapvaal Craton

Tectonites from ~2.7-Ga-old Platberg (mid-Ventersdorp) rift-related fault zones in H.J. Joel and Harmony gold mines (southern Free State Goldfield) were dated by the 40Ar-39Ar step-heating technique. A mylonite from a bedding-parallel thrust zone at H.J. Joel Gold Mine yielded an age plateau with ages between ~2.08 Ga and ~2.07 Ga (interpreted age 2076 ± 5 Ma). A cataclasite from a north-striking shear zone yielded a partial (60%) age plateau with ~2.095 to ~2.065 Ga ages, resulting in an interpreted age of 2088 ± 7 Ma. Both partially recrystallised tectonites exhibited 39Ar-loss at, or more recent than, ~1.83 to ~1.75 Ga. A completely recrystallised ultracataclasite from a major northeast-striking fault zone yielded only ages <~1.75 Ga. Seemingly two phases of low-temperature resetting at ~1.83 to ~1.75 Ga and ~1.35 to ~1.20 Ga affected this sample. The argon age systematics of mylonitic fault rocks from Harmony Gold Mine that are crosscut by younger ultracataclastic-pseudotachylitic breccias and that are developed along individual shear planes of the Merriespruit Thrust and De Bron Fault zones are disturbed. Only one mylonite yielded a plateau age of 2019 ± 8 Ma, which is within errors of the age of breccia development due to the Vredefort impact event. Most of these mylonites record ~1.83 to ~1.75 Ga ages. Several other ultracataclastic-pseudotachylitic breccias could not be precisely dated, as the samples do not contain sufficient potassium. The spectra of these samples are saddle-shaped with strong indications of significant excess argon. Saddle ages of 2841 ± 15 Ma and 2020 ± 20 Ma were obtained for one sample each from the De Bron and Merriespruit fault zones. Ages in the interval ~2.095 to ~2.065 Ga document a thermal overprinting event of pre-Bushveld-age, which led to complete resetting of the isotope system of older cataclasites and mylonites. This thermo-metamorphic event is believed to represent the initial phase of the Palaeoproterozoic Magondi (Ubendian) Orogeny at ~2.1 to 2.06 Ga along the western margin of the Zimbabwe Craton, which resulted in intense magmatic activity throughout the northern and central Kaapvaal Craton. Pre- to early orogenic magmatism in the Kaapvaal Craton culminated in the structurally controlled emplacement of the Bushveld Igneous Complex, which caused regionally persistent greenschist facies metamorphism (~2 to 3 kbar; 350°C) and the formation of peak-metamorphic mineral assemblages in West and Central Rand Group lithologies in the northern and central Witwatersrand Basin at ~2.06 to ~2.05 Ga, but appears not to have thermo-metamorphically effected the southern portion of the basin. Besides geological evidence for Vredefort-related generation of ultracataclastic-pseudotachylitic breccias along some of the major fault zones in the Free State Goldfield, ages that could be related to the Vredefort impact event at ~2.02 Ga were obtained from ultracataclastic-pseudotachylitic breccias cutting across earlier developed cataclasites and mylonites along the De Bron Fault and Merriespruit Thrust Zone. Ages <~2 Ga represent post-Vredefort tectono-thermal or hydrothermal events that have previously also been documented from breccias in the north-central Witwatersrand Basin. Expected Platberg formation ages of ~2.71 Ga were not obtained from any of the analysed cataclastic and mylonitic fault rock specimens, due to the intense and almost complete resetting of their isotopic systems during later (pre-Bushveld, Bushveld, Vredefort, post-Vredefort) tectono-thermal events. These results clearly indicate that post-2 Ga tectono-hydrothermal activity affected the central part of the Kaapvaal Craton. This activity is interpreted as a manifestation of orogenic activity in the Proterozoic mobile belts around the craton margins during the Eburnean (Kheis) and Kibaran/Grenville (Namaqua-Natal) orogenies.

Review of Pseudotachylites in the Witwatersrand Basin, South Africa

The Vredefort Dome, located in the central part of the Witwatersrand Basin in South Africa, is the type locality for pseudotachylite. Pseudotachylite at the Vredefort Dome is generally regarded to be of impact origin. Pseudotachylites which are closely associated with faults are, however, also known to be common along the northern and northwestern edges of the Witwatersrand Basin. In order to compare pseudotachylites from the Vredefort Dome and from the surrounding Witwatersrand Basin, different studies were undertaken in the past. Mode of occurrence, microscopic textures, geochemical analyses and chronological measurements of pseudotachylites are briefly reviewed in this paper.In the Vredefrot Dome, pseudotachylites are commonly observed except in the central part of its core. In the surrounding Witwatersrand Basin, they are reported from drill core sections and in underground workings. The matrix in pseudotachylite from the Vredefort Dome is mostly a recrystallized melt phase, while those from the surrounding Witwatersrand Basin seem to be composed of clastic material. Pseudotachylites both from the Vredefort Dome and the surrounding Witwatersrand Basin are geochemically closely related to their host rocks. Although evidence for more than one generation of pseudotachylite has been presented, both in the Vredefort Dome and the surrounding Witwatersrand Basin, it is widely believed that most of them were formed as a result of the Vredefort impact event (ca. 2.0 Ga). Other fault rocks reported from the surrounding Witwatersrand Basin are older than the pseudotachylites and therefore not related to their formation.

40Ar/39Ar constraints on the age of metamorphism in the Witwatersrand Supergroup, Vredefort dome (South Africa)

Muscovite, biotite and amphibole bulk step-heating and laser-probe 40Ar/39Ar results from metasediments and meta-lavas of the Witwatersrand Supergroup and Dominion Group exposed in the Vredefort dome provide constraints on the age of greenschist to amphibolite facies metamorphism of these rocks. The majority of apparent ages from biotite and sericite grains cluster between ~2100 and ~2000 Ma and are interpreted as reflecting an initial metamorphism (M1) accompanying the ~2060 Ma Bushveld magmatic event and a later lower-grade overprint (M2) accompanying the 2023 ± 4 Ma Vredefort impact event. The spectra also show variable degrees of post-2000 Ma resetting that has also been observed in samples from elsewhere in the Witwatersrand Basin. Despite temperatures in excess of 500 °C during M2, disturbance of the M1 40Ar/39Ar spectra is generally limited, reflecting the short-lived and largely anhydrous nature of the M2 event and the absence of accompanying strain. Single-grain biotite laser-probe data confirm that the extent of within-sample resetting varies, and probably reflects more intense deformation and alteration effects towards ubiquitous pseudotachylite-bearing microfaults that formed during the impact event. The complex spectra in M1 muscovite and amphibole are difficult to interpret but may at least partly reflect contamination by excess argon during M1.

The Katdoornbosch--Witpoortjie Fault: a ring thrust of Vredefort Event age

A fault, designated the Katdoornbosch—Witpoortjie Thrust, was traced in the field continuously from the area north of Klerksdorp to the West Rand. It is shown that the Katdoornbosch Thrust and the Witpoortjie Fault form elements of the same structure. Evidence is presented why the Witpoortjie Fault should be designated athrust. Thrust faults related to the Vredefort Event are recognized mainly by their 2 Ga, post-Transvaal age, their concentric arrangement around the centre of the structure, and their centrifugal vergence, away from the centre. Where chert beds of the Malmani Subgroup were intersected, thrust faults of Vredefort Event age can be recognized by a unique type of brecciation, displaying chocolate-tablet-type boudinage, with fracturing having taken place in three-dimensions. The brecciation is confined uniquely to the ramp zones and was probably formed during the release of three-dimensional, hydrostatic stress, when fault detachment took place. The recognition of this type of brecciation along the planes of the thrust faults associated with the stage of centrifugally directed movement, during the formation of the Vredefort Structure, provides an important criterion in their recognition. The Katdoornbosch—Witpoortjie Thrust is one of a set of similarly orientated, time-related thrusts, formed during the Vredefort impact event.

Complex metasomatism of an Archean placer in the Witwatersrand Basin, South Africa; the Ventersdorp contact reef; a hydrothermal aquifer?

The Ventersdorp contact reef is an auriferous conglomerate horizon and economic orebody on top of the Late Archean Witwatersrand Supergroup in South Africa. It differs from the other conglomerate beds (reefs) in the Witwatersrand basin in having a massive metabasalt sequence as its hanging wall and by its intense post-depositional alteration. A geochemical, petrologic, and fluid inclusion study of the Ventersdorp contact reef reveals a polyphase hydrothermal alteration history. Regional metamorphism under acidic conditions was followed by potassic alteration along the reef. The potassic alteration, which also extended into the hanging-wall and footwall rocks, was subsequently followed by a chloritization episode that was restricted to the reef and its immediate contacts. Mass-balance calculations suggest that the latter alteration required only minor fluid influx from an external source, whereas the potassic alteration reflects the progressive dehydration of the siliciclastic footwall rocks and associated hydration of the basaltic hanging wall. Based on microtextural observations on quartz using SEM-cathodoluminescence imaging, the chloritization event is ascribed to the 2020-Ma Vredefort impact event that created the necessary secondary permeability in the form of microfractures particularly within the conglomerate bed. Considering that the Ventersdorp contact reef is generally regarded as the most intensely altered reef in the Witwatersrand basin, the relatively minor fluid infiltration established in this study suggests that, contrary to prevailing opinion, the other auriferous conglomerate beds might have experienced even less influx of externally derived hydrothermal fluids.

Detrital origin of hydrothermal Witwatersrand gold—a review

The Witwatersrand ‘basin’ is the largest known gold province in the world. The gold deposits have been worked for moren than 100 years but there is still controversy about the ore forming process. Detailed petrographic studies often reveal that the gold is late in the paragenetic sequence, which has led many researchers to propose a hydrothermal origin for the gold. However, observations, such as the occurrence of rounded, disc‐like gold particles next to irregularly shaped or idiomorphic secondary gold particles in the same sample, suggest an initial detrital gold source within the Witwatersrand strata. Mineral chemical and isotopic data, together with SEM cathodoluminescence imaging and fluid inclusion studies, provide evidence for small‐scale variations in the fluid chemistry – a requirement for the short‐range mobilization of the gold. The existing data and observations on the Witwatersrand rocks support a model of hydrothermally altered, metamorphosed placer deposits, with at least two subsequent gold mobilization events: hydrothermal infiltration in early Transvaal time (2.6–2.5 Ga) and during the 2.020 Ga Vredefort impact event.

Chlorite Thermometry in the Witwatersrand Basin: Constraints on the Paleoproterozoic Geotherm in the Kaapvaal Craton, South Africa

A series of post‐depositional alteration events can be distinguished in the late Archean Witwatersrand Basin. They range from thrust‐controlled metamorphism during pre‐Transvaal crustal thickening, burial metamorphism and hydrothermal infiltration during crustal thinning and associated deposition of the 2.6‐2.1 Ga Transvaal Supergroup sediments, thermal metamorphism related to the 2.06 Ga Bushveld Igneous Complex, and hydrothermal alteration as a consequence of the 2.023 Ga Vredefort impact event. The relative importance of these events varies locally, with different events responsible for the maximum temperatures achieved in different parts of the basin: the pre‐Transvaal metamorphism affected the northern margin of the basin, whereas the syn‐Bushveld thermal metamorphism is particularly pronounced in the lower parts of the Witwatersrand Supergroup. Chlorite geothermometry, in conjunction with fluid inclusion microthermometry and the thermodynamic modeling of equilibrium mineral assemblages, was applied ...

The age and thermal evolution of the Vredefort impact structure: A single-grain UPb zircon study

Single-grain UPb SHRIMP analyses of a heterogeneous zircon population from a small granite body in the Archean basement core of the Vredefort Dome indicate an age of 2017 ± 5 Ma for the granite, which is identical, within errors, to the previously reported age determinations for the Vredefort impact event. Petrographic evidence from the granite indicates that it crystallized after the impact shock event, but some doubt remains whether it is a true impact melt, a decompression melt generated during rapid uplift of the crater basement following the impact event, or a melt generated during an earlier, 2.05–2.06 Ga, granulite facies anatectic event that was still at least partially molten at the time of the Vredefort impact event. Rare zircon xenocrysts displaying planar microdeformation features and giving discordant Archean ages are overgrown by polycrystalline zircon rims with UTh compositions similar to that of the main ∼2.02 Ga zircon population, but with discordant 207Pb/ 206Pb ages, indicating more recent Pb-loss.

Original size of the Vredefort Structure: Implications for the geological evolution of the Witwatersrand Basin

Abstract— Historically, there have been a range of diameter estimates for the large, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as ∼124–140 km, based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted Transvaal outliers. Taking erosion into account (&lt;6 km) and scaling to original final rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based on an empirical relation of central uplift diameter to rim diameter, spatial distribution of pseudotachylites, and concentric large scale structural patterns, give a similar estimate of close to 300 km for the original final rim diameter. An impact structure of this size is expected to have had an original multi‐ring form. At this size, the Vredefort impact structure encompasses the bulk of the Witwatersrand Basin, which appears to owe its preservation to the Vredefort impact. In addition, the Vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area, which, in recent interpretations, is believed to be a component in the creation of the world‐class gold deposits of the Witwatersrand Basin.

A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre

UPb isotope systematics of shock metamorphosed zircon grains from pseudotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an impact origin for this structure. Zircon grains from these rocks exhibit planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks from the Sudbury Structure. A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. UPb isotopic results for individual, pre-impact zircon grains indicate crystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 2023 Ma and indicate both impact- and post-impact related Pb loss. The data and granular morphology of a zircon grain from the Granophyre indicates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar microstructures in zircon are ubiquitous, there are also some unshocked, low-U grains, and these record a ca. 3.1 Ga primary age. The older ca. 3.1–3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma.The resilience of zircon shock features to post-impact alteration and annealing, in combination with precise UPb dating of individual shocked grains provide a valuable method for indentifying ancient, metamorphosed and tectonically modified impact structures.