Stable isotope study of the Archaean rocks of the Vredefort impact structure,central Kaapvaal Craton,South Africa

The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in whole-rock δ ¹⁸O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (n = 35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures, suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies greenstone remnants from the ILG have δ ¹⁸O values that are on average 1.5‰ higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower δ ¹⁸O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ ¹⁸O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ ¹⁸O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

南非主要金矿集区研究现状及存在问题

南非是世界上金矿石资源量最为丰富的国家,其金矿床主要产于东北部地区,主要有三种类型:1赋存于兰德盆地砾岩型建造中的"兰德"砾岩型金矿床,2产在绿岩带中的绿岩带型金矿床,和3主要与白云岩有关的白云岩型金矿床,以前两种类型为主。"兰德"砾岩型金矿床通常品位高,开采规模最大,目前采矿的深度最大,是南非最重要的矿床类型,其矿床成因长期以来存有争议,主要有两种观点,分别是砂矿模式和热液模式,就目前资料来看笔者等认为其代表了两个不同的矿化阶段;绿岩带型金矿床则以巴伯顿地区为代表,矿床具有规模大、易开采等特征,因此在南非金矿床开采中占有重要位置,其矿床主要赋存于石英脉或剪切带中,矿床成因主要有三种,分别是火山成矿模式、转换滑脱构造模式和构造交叉模式。     

New constraints on the auriferous Witwatersrand sediment provenance from combined detrital zircon U–Pb and Lu–Hf isotope data for the Eldorado Reef (Central Rand Group,South Africa)

Combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains from the auriferous Eldorado Reef conglomerate, upper Central Rand Group, reveal new insights into the provenance of the sediments and thus, by implication, possibly also into that of the gold. Most of the detrital zircon grains, which are of magmatic origin, yielded Mesoarchaean ages clustering around 2.94 and 3.06 Ga. A subordinate zircon population gave ages with maxima at 3.28 and 3.44 Ga. The Mesoarchaean zircon grains mostly show super-chondritic ?Hf_t of up to +5.2, whereas the Palaeoarchaean zircon grains have nearly chondritic composition with ?Hf_t between −1.3 and +2.0. The new dataset of the Mesoarchaean zircon populations provides the first unambiguous evidence of the formation of juvenile crust not only at 3.06 but also at 2.94 Ga. As the analysed zircon grains are from the ruditic fraction, they must be derived from a comparatively proximal source in close vicinity to the Central Rand Basin. Based on currently available data, this source was most likely a magmatic arc that existed at the northern edge of the Witwatersrand Block at 3.06 Ga. An additional source might be the 2.94 Ga magmatic rocks of the Kraaipan Greenstone Belt that occurs to the west of the Witwatersrand Block. The minor fraction of Palaeoarchaean zircon grains in the Eldorado Reef perhaps stem from sources that are isotopically similar to the Barberton Greenstone Belt and the Limpopo Belt but were more proximal to the Central Rand Basin.     

Widespread fluid infiltration during metamorphism of the Witwatersrand goldfields: generation of chloritoid and pyrophyllite

Abstract Chloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages, but they are also developed locally in the regionally persistent metapelites that have basin-wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates. The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite-rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid. The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn-metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration. Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi-pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapelites is inferred.     

南非维特沃特斯兰德盆地绍斯迪普金矿床地质特征、成矿模式和找矿模型

对南非兰德砾岩型金矿床的研究成果进行总结,概括了绍斯迪普金矿床的区域地质背景、岩石地层划分,较为系统地分析了矿床的地质特征和矿床成因。得出如下结论:1金的主要来源可能与太古宙含金花岗绿岩带有密切关系;2金矿体主要受太古宙文特斯多普砾岩地层和中兰德群中的Turffontein亚群埃尔斯伯格组砾岩地层控制;3金矿床可能经历了早期沉积成岩成矿阶段—后期改造富集多个(Klipriviersberg火山活动作用时期和与2050Ma布什维尔德杂岩体有关的变质作用高峰时期)阶段,为复合成因的层控改造型矿床。研究结果对该类矿床的深入研究和走出去的中国企业有重要的参考价值。     

A Kaapvaal craton debate: Nucleus of an early small supercontinent or affected by an enhanced accretion event?

Incorporation of the Kaapvaal craton within a speculative Neoarchaean–Palaeoproterozoic supercontinent has long been debated, and this idea provides a potential solution to solving the apparently enigmatic provenance of the huge quantities of gold within the famous Witwatersrand auriferous deposits of Kaapvaal. Within a framework of a postulated Neoarchaean “Kenorland” (“northern”; present-day reference) supercontinent, we examine possible “southern” cratons that may have been contiguous with Kaapvaal: Pilbara, Zimbabwe, Dharwar, São Francisco, Amazon, Congo. Brief reviews of their basic geology and inferred evolution in syn-Witwatersrand basin times (c. 3.1–2.8 Ga) show no obvious support for any such supercontinental amalgamations. An alternative idea to explain a measure of gross similarity amongst several Neoarchaean cratons is through global events, such as a c. 3125–3000 Ma cratonic-scale erosive event interpreted for both Pilbara and Kaapvaal, and a much more widespread magmatic event at c. 2760–2680 Ma. We postulate that a global superplume event at c. 3.0 Ga included a plume beneath the Kaapvaal cratonic nucleus, thus halting any subduction around that terrane due to the thermal anomaly. Such a speculative global magmatic event is assumed to have enhanced production of juvenile oceanic crust at mid-ocean ridges, including those “offshore” of the thermally elevated Kaapvaal nucleus. Intra-oceanic obduction complexes may have built up fairly rapidly under such conditions, globally, and once the plume event had abated, “normal” plate tectonics would have resulted in composite (greenstone-tonalite, possibly also including granite) terranes accreting with nuclei such as Kaapvaal. This enhanced plume-related cratonic growth can be seen as a rapid accretion event. Formation of the envisaged ophiolite complexes possibly encompassed deformation-related first-order concentration of gold, and once accretion occurred around Kaapvaal's nucleus, from north and west (present-day frame of reference), a second-order (deformation-related) gold concentration may have resulted. The third order of gold concentration would logically have occurred once placer systems reworked detritus derived from the orogens along the N and W margins of Kaapvaal. Such conditions and placer gold deposits are known from many Neoarchaean cratons. The initial source of gold was presumably from the much hotter Mesoarchaean mantle and may have been related to major changes in Earth's tectonic regime at c. 3.0 Ga. The unique nature of Kaapvaal is probably its early stabilization, enabling formation of a complex flexural foreland basin system, in which vast quantities of placer sediments and heavy minerals could be deposited, and preserved from younger denudation through a unique post-Witwatersrand history.     

Chemographic analysis of assemblages involving pyrophyllite, chloritoid, chlorite, kaolinite, kyanite, quartz: application to metapelites in the Witwatersrand goldfields, South Africa

The Witwatersrand goldfields contain abundant assemblages that include pyrophyllite, chloritoid, chlorite, kaolinite and/or kyanite, with quartz. A chemographic analysis of the system Fe(Mg)-Al-Si-O-H involving these minerals yields 22 potential phase diagrams. Using orientation criteria and thermodynamic calculations as further constraints, this list has been reduced to three possible diagrams. New thermodynamic data favour one of these in particular.
This chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the F(M)ASH system, as stated by previous studies, but could be from pyrophyllite + chlorite → chloritoid + quartz + H₂O.
The metamorphic temperature variation between Witwatersrand goldfields exceeded 65 C, based on chlorite and chloritoid compositions. The lower and upper pressure limits are constrained by the andalusite to kyanite, and the sudoite/chlorite to carpholite boundaries, i.e. 1.5–2.8, and 7 kbar, respectively. The widespread pyrophyllite, chlorite and Fe-chloritoid in all the Witwatersrand goldfields, and the local occurrence of sudoite indicate a consistent low-pressure environment in which Mg-chloritoid would not be stable.     

The indigenous origin of Witwatersrand “carbon”

In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes.     

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.     

The Witwatersrand deposit and the problem of the ore formation

The problem of the origin of the Witwatersrand gold-uranium deposit is considered. The fluvial and explosion hydrothermal origin of pyrite spheroids and Au-bearing pseudoconglomerate is denied. Evidence for the primary character of spheroids and the autochthonous crystallization of pyrite under endogenic conditions is furnished. The association of gold with carbonaceous rocks is interpreted in terms of the nuclear magnetic physicochemical phenomena. It is concluded that the Witwatersrand deposit is a hydrothermal metasomatic deposit.