Academic literature on the topic 'Geochemistry – South Africa – Rooiberg'

Thesis (MSc.)--Stellenbosch University, 1994.<br>ENGLISH ABSTRACT: The NAD deposit forms part of the A-Mine Complex in the Rooiberg tin field in the north-western Transvaal. Mining started in 1986 and the mine was closed recently following a decline of tin prices and metallurgical problems with ore recovery. The deposit is hosted by the Boschoffsberg Quartzite Member of the Pretoria Group sediments and is structurally controlled within the so-called Tin Zone. Extensive alteration of the original host rocks occurred during the mineralisation event. Potassic remobilisation and redistribution appear to have taken place pervasively. The nature of the original host rock was altered to such an extent that it cannot be classified correctly. Previous workers refer to the host rock as an arkose because of its feldspathic nature. Wall rock alteration in the hanging- and foot wall of the different ore lodes (fractures) is generally similar. Very little change is observed in whole rock chemistry away from the lodes. The mineral chemistry of the ore lode minerals is indicative of fluid composition and mineralising conditions in the NAD deposit. The FeO/(FeO + MgO) ratio of tourmalines indicates deposition at a distance from the source of the mineralising fluids. Pyrite trace element chemistry differs from that of the Leeuwpoort deposit, indicating changing fluid chemistry. Contrary to previous assumptions that the carbonate present is ankerite, abundant siderite occurrences were found. Pyrite may co-precipitate with siderite under specific physico-chemical conditions. Cassiterite is generally trace element poor and relatively enriched in Fe. Hydraulic brecciation of the fractures as well as the tourmaline chemistry are strong indicators of a hydrothermal origin for the NAD deposit. However, the origin of the hydrothermal fluids is still uncertain.<br>AFRIKAANSE OPSOMMING: Die NAD- tinafsetting vorm dee/ van die A-Myn-kompleks in die Rooiberg tinveld in die noord-wes Transvaal. Die myn is in 1986 geopen en is onlangs gesluit weens swak tinpryse en metallurgiese ertsherwinningsprobleme. Die tinafsetting kom voor in die Boschoffsberg Kwartsiet lid van die Pretoria Groep sedimente. Dit is strukturee/ gekontroleer binne die sogenaamde Tin Sone. Vorige navorsers het na die waardgesteente as 'n arkose verwys op grand van die hoe veldspaat-inhoud. lntensiewe en uitgebreide verandering van die oorspronklike waardgesteentes het plaasgevind tydens mineralisasie. Deurdringende kalium-hermobilisering en -herverspreiding het plaasgevind. Die aard van die oorspronklike waardgesteente is egter tot so 'n mate verander, dat dit chemies nie korrek gek/assifiseer kan word nie. Wandrotsverandering van die verskil/ende ertsskeute in beide die voet- en dakgesteentes is baie soortge/yk. Die heelrotsgeochemie van die wandgesteentes verskil baie min weg van die ertsskeute af. Die mineraalchemie van die ertsskeut-minerale is aanduidend van die v/oeistofsamestelling en mineralisasietoestande van die NAD-tinafsetting. Die FeO/(FeO + MgO) verhouding van toermalyne dui daarop dat afsetting op 'n afstand vanaf die bron van die mineralisasie-vloeistowwe plaasgevind het. Die spoor-elementchemie van die NAD-piriete verskil van die piriete van die Leeuwpoort tinafsetting, wat verandering in vloeistofsamestelling aandui. In teenstelling met vorige aannames dat die karbonate in die Rooiberg tinafsettings as ankeriet bekend staan, is daar volop sideriet gevind. Piriet kristalliseer dikwels saam met sideriet onder spesifieke fisiese en chemiese toestande. Kassiteriet is meestal arm aan spoor-elemente, maar is relatief verryk in Fe. Hidroliese breksiering van die skeute en die toermalyn-chemie dui sterk op 'n hidrotermale oorsprong vir die NAD afsetting. Die bron van die vloeistowwe is nog nie definitief vasgestel nie.<br>Digitized at 300 dpi Colour PDF format (OCR), using ,KODAK i 1220 PLUS scanner. Digitised, Ricardo Davids on request from IBL 10 September 2013

The Cu/(Cu+Ni) ratios associated with the Basal Gabbro also display the vertical reverse fractionation trend, supporting the supercooled margin model. The disseminated sulphides in the lowermost units, are regarded as being the result of sulphur saturation induced by contamination from the dolomitic and quartzitic xenoliths. This is supported by isotope data which indicate the high degree of contamination in the lowermost units of the Complex. The results of this study are used to propose a model for the petrogenesis and metallogenesis of the Uitkomst Complex, whereby the Complex is closely related to the Bushveld Complex. The Basal Gabbro, as supported by its chemistry and style of mineralisation (Cu-rich), represents a supercooled margin to the lowermost units of the Uitkomst Complex, which stoped upwards into the surrounding sediments, assimilating the country rock xenoliths, and precipitating sulphides. Following this was a period in which large quantities of magma moved laterally through the system before the magma flow waned, and closed system crystallisation ensued. As the body cooled, the primarily magmatic water was superceded by the hydrothermal magmatic water released from the xenoliths, and later by geothermally driven circulating meteoric water, producing the extensive alteration. This alteration was accompanied by considerable stress and the development of fractures and shears. Finally the Complex was itself intruded by diabase sills and later dolerite dykes.

Bibliography: pages 100-109.<br>The Kaap Valley Tonalite (KVT) was a homogenous hornblende+ biotite tonalite that had been subjected to overail propylitic and potassic alteration. Petrographic studies reveal that carbonate alteration and increased proportions of hydrous minerals occur along the KVT and Barberton greenstone belt contact zone, and along major shear zones within the KVT.

Bibliography: leaves 84-91.<br>A geochemical investigation of salt distribution in soils of the semi-arid of Western Cape, South Africa, was carried out following a pilot study which revealed widespread soil salinity in the Berg River catchment. The present study looks at the distribution of the salts along a toposequence in a subcatchment of the Berg River underlain by Malmesbury Group shale. The objectives were: to investigate salt distribution in relation to landscape topography; to identify potentially harmful trace elements associated with the salinity; and to determine the processes responsible for salt distribution. Ten soil profiles were dug to a depth of 200 cm at points along a slope of angle 10.20 along and 367 m long. The profiles at the crest of the toposequence are underlain by silcrete and alluvium while Malmesbury shale parent material underlies the rest of the profiles. Contour drains disrupt the natural shape of the slope. The soil texture is loamy sand to clay loam at the crest and loamy to sandy-clay loam in the midslope. Mineralogy at the crest is quartz dominated and kaolinite occurs in the mid and lower slope.

Banded iron-formations (BIFs) are chemically precipitated sedimentary rocks in which Fe-rich bands or laminae alternate with Fe-poor ones. They formed within a specific time-span of the geological record. Their occurrence is restricted between 2.3 and 1.9 Ga, and characterises virtually all the major Precambrian-aged sedimentary basins of the world. The Precambrian Transvaal Basin in Griqualand West, South Africa, is noted for its well-developed BIF units. The Kuruman and Griquatown BIFs comprising the Asbesheuwels Subgroup (up to 1000m thick) are the best known and thickest of these. As far as metallogenesis is concerned, the Kuruman BIF is of major importance, for it carries the world's largest crocidolite (blue asbestos) deposits. The uppermost, youngest member of iron-formation deposition in the Griqualand West Sequence is represented by the Voëlwater BIF. The direct association between the latter and the giant Mn-deposits of the Kalahari Field, renders the Voëlwater association unusual, if not unique, in the geological record. The Voëlwater BIF represents a typical example of the so-called "Superior-type", and in the area of study it has undergone late-diagennetic to low-grade metamorphic processes. This is evident from the mineralogical composition and textural signature of the various BIF lithologies. Specifically, the minerals that make up the Voëlwater BIF are mainly chert(quartz), Fe-oxides (magnetite and hematite), Fe-silicates (greenalite, stilpnomelane, minnesotaite, riebeckite, Fe-mica), Fe-carbonates (members of the dolomite-ankerite series and siderite), calcite and pyrite. Soft-sediment deformation structures and shear-stress indicators are abundant in carbonate-rich and granular, silicate-rich BIF lithologies respectively. The bulk chemical composition of the study rocks is relatively simple and is characterised by the abundance of essentially three elements, namely Si, Fe, and Ca, which make up more than 90% of the total chemical composition of the Voëlwater BIFs. The detrital component of the study rocks is negligible. Mn-enrichments characterise all the transitional lithologies towards the interbedded Mn-orebodies, as well as the well-developed, hematitic BIF-unit between the Ongeluk lavas and the lower Mn-horizon. In terms of trace element composition, no significant enrichments or depletions, were encountered, except for some unusually high values of Sr and Ba and Co in carbonate-rich and Mn-rich lithologies respectively. Geochemical comparisons on the basis of major, trace and light rare-earth element composition verified the similarity between the Voëlwater BIF and other major Superior-type BIFs of the world (e.g. Kuruman, Griquatown, Sokoman, Biwabik, Gunflint, Mara-Mamba, Brockman, etc.). The processes that led to the formation of the Voëlwater BIFs may have been very similar to the ones described in various genetic models proposed in recent years. They would have involved a combination of: i. hydrothermal processes related to mid-ocean ridge (MOR) or hot-spot activity that acted as major iron suppliers; ii. storm-mixing in stratified oceans (bottom, anoxic, Fe⁺² reservoir-thermo- pycnocline zone-upper, mixed, SiO₂-saturated layer), largely dictated by seasonal changes and contemporaneous volcanism; iii. periodic, convection-driven upwelling mechanisms acting as major Fe-precipitators; and, iv. organic carbon productivity that was responsible for the anoxic diagenesis of the initial sediment. However, the origin of Fe and Mn for the genesis of the Voëlwater sediments was difficult to explain with typical convection-cell models in active mid-ocean ridges, in contrast to previous hypotheses. Instead, large-scale endogenous processes in the form of magma convection, underplating, differentiation and associated degassing, may have played a critical role in the supply of metals for the formation of large amounts of BIFs in the Precambrian. The present study of the Voëlwater BIF also bears strong implications regarding the metallogenesis of Mn in the Precambrian. The common association of Mn with carbonate-bearing sediments, the transitional character of the Voëlwater BIF towards carbonate lithologies (Mooidraai dolomites) and the critical timing of the deposition of the former in terms of the Precambrian atmospheric-lithospheric- hydrospheric evolution, may be important indicators for the exploration of large Mn-deposits in Precambrian sedimentary basins of the world.

The 2.9 Ga old Usushwana Complex in the Piet Retief-Amsterdam area (south-eastern Transvaal) represents an exposed segment of a layered intrusion. It has the form of a dyke-like body elongated in a northwesterly direction, and extends to an estimated depth of 3000 -5500 m. Lithologically, the Complex consists of a cumulate succession of mafic rocks capped by granitoids and has intruded along the contact between the basement and the supracrustal sequences of the Kaapvaal Craton. Differentiation of an already contaminated gabbroic magma resulted in an ordered stratigraphic sequence comprising progressively more evolved lithotypes, with at least two imperfect cyclic units developed over a stratigraphic thickness of about 700 metres (Hlelo River Section). Meso- to orthocumulate textured gabbros and quartz gabbros grade upwards into magnetite- and apatite-bearing quartz gabbros, interlayered with discontinuous magnetitite horizons. The gabbros in turn grade into hornblende-rich, granophyric granodiorites. The differentiation process is regarded as having been considerably enhanced by the assimilation of acidic material, derived by partial melting of the felsic country rocks at the roof of the magma chamber. Recrystallisation of these rocks gave rise to the microgranites that locally overlie the granodiorites. Mineralogical, textural and geochemical features indicate a relatively advanced fractionation stage, suggesting that the exposed sequence of the Usushwana Complex in the study area represents the upper portion of the intrusion. No significant mineralised occurrences were identified. However, on the basis of similarities between the Usushwana Complex and other mafic layered intrusions which host significant ore deposits, it is suggested that economic concentrations of base metal(Cu-Ni) sulphides, PGE and chromitites are likely to be developed at lower stratigraphic levels.

Bibliography: pages 123-137.<br>Arseniferous sulphide-hosted gold deposits associated with the Barberton greenstone belt, located in the Mpumalanga province of South Africa, have been mined on various scales since late last century. The Barbrook mine is located close to the Swaziland border in an area of the Barberton mountainlands which forms part of the Kaap subcatchment of the Crocodile River. Scattered small communities depend on local streams and boreholes in the area for drinking and irrigation water. Hence, the prime motivation for this study was the potential risk to human health associated with enhanced As mobility as a result of the sulphide mineral weathering which accompanies gold mining and processing. Sampling of local waters, soils and sediments, as well as the ore and waste rock at Barbrook was undertaken during a week of fieldwork in August 1996. A range of chemical analyses were performed on the various samples, including As analysis by hydride generation-atomic absorption spectroscopy and inductively coupled plasma - mass spectrometry. The Barbrook ore bodies are structurally controlled and hosted by banded iron formation, carbonaceous shales and carbonate-bearing schists. Within these ore bodies, As is mainly associated with auriferous arsenopyrite (FeAsS), although minor amounts also occur within pyrite (FeS₂) and ullmannite (NiSbS). Of the streams which flow through the mining lease, those at most risk of As contamination are Barbrook Creek, Low's Creek and Crystal Stream. Low's Creek flows through the main mine area, adjacent to the level 1O mine adit entrance, and past the processing plant before converging with Barbrook Creek. The latter receives water from the Shiyalongubo Dam which is the main water source for the area. Immediately downstream of the level 10 mine adit entrance, the steep eastern bank of Low's Creek is lined for several hundred metres with waste rock containing low grade ore. Tailings material from the processing plant is pumped to two tailings dams located about 5 km to the northwest of the plant and close to Crystal Stream.

With an estimated erupted volume of 300,000 km3 and an areal extent of more than 200,000 km2, the Paleoproterozoic (2.06 Ga) silicic volcanic rocks of the Rooiberg Group (Kaapvaal Craton) in northern South Africa forms one of the largest and to the same time oldest silicic large igneous provinces (SLIPs) known. This large volume of rocks can be sub-divided into four formations: the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formations. The results of this study show that a clear chemostratigraphy (by using major elements such as TiO2, SiO2, Na2O, K2O, P2O2, MgO, and Fe2O3) can be established in the area north of Loskop Dam, dividing the rocks of the study area into the Damwal, Kwaggasnek and Schrikkloof formations. The studied rocks are characterized by aphanitic lavas bearing amygdales, spherulitic textures and flow-bands with some sedimentary and pyroclastic interbeds. The dacites could mainly be described as high-Mg felsites (HMF), whereas the rhyolites could be described as low-Mg felsites (LMF). The negative Eu anomaly, Nb and Ta values of the upper part of the Rooiberg Group range between 5.38-24.2 and 0.45-1.86 ppm, respectively, similar to crustal compositions. Furthermore, Nb/Ta values range from 10.91-14.83 (also similar to typical crustal compositions) while few samples from the Damwal Formation exhibit higher values of 15.13-16.02, similar to mantle-derived compositions. Tectonic discriminant diagrams show that the rocks used in this study evolved from fractional crystallization of a mafic liquid although all samples plot in fields with crustal signatures. Plot of ƐNd and 87Sr/86Sr show a mantle-derived origin for the upper part of the Rooiberg Group. However, ƐNd values of the upper part of the Rooiberg Group range between ~-10 to ~-6, typical of crustal composition or continental basalts formed in the crust. From the results, the Rooiberg Group exhibit both mantle (as observed in the Dullstroom and lower Damwal formations) and crustal signatures as exhibited by the Kwaggasnek and Schrikkloof formations. This is interprested as a result of the interaction of the thick crust and a shallow mantle source within the Bushveld Province during magmatism. Furthermore, similarities in geochemical signatures between the Rooiberg Group and selected SLIPs around the world suggest a similar origin for SLIPs by fractional crystallization of a mafic melt and melted (or assimilated) crustal material.<br>Thesis (PhD)--University of Pretoria, 2017.<br>Geology<br>PhD<br>Unrestricted