A geological model of shear zone gold deposits in the Pietersburg Greenstone Belt, South Africa
- Authors: Franey, N J
- Date: 1987 , 2013-04-17
- Subjects: Greenstone belts -- South Africa , Gold ores -- Geology -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5032 , http://hdl.handle.net/10962/d1007190 , Greenstone belts -- South Africa , Gold ores -- Geology -- South Africa
- Description: The Pletersburg greenstone belt Is located In South Africa, about 300 km northeast of Johannesburg. It hosts a significant amount of gold mineralization and just over 1000 kg of gold have been produced from Its various reefs and secondary deposits. The greenstone belt is interpreted as an Archean ophiolite complex. It comprlses a volcano-sedimentary succession (the Pletersburg Group) which Is subdivided Into a basal greenstone sequence, interpreted as oceanic crust, and an upper sedimentary cover sequence. A number of major shear zones, which are thought to represent thrusts that developed during the subduction of the greenstone sequence, form an integral part of the stratigraphy . Four stages of deformation (D₁-D₄) and four phases of metamorphism (H₁-H₄) (three of which are correlatable with the peak stages of deformation) are recognized. The primary gold deposits are all shear zones related. but they are subdivided into greenstone, sedimentation and granIte-hosted types. Geographically, they occur In three distinct goldfields: Eerstellng, Roodepoort and Marbastad. The greenstone-hosted · Plenaar-Doreen shear complex Is In the Eersteiing goldfield and hosts eight gold occurrences. Within the complex, Girlie North Reef is the 640m-long "pay" section of the Girlie North shear zone. This reef is characterized, macroscopically, by a Quartz-carbonate-chlorite-sulphlde assemblage and, mlcroscoplcally, by the presence of tourmaline, arsenopyrlte and Au. Geochemical evidence Indicates that mineralizing fluids were H₂O and CO₂-bearing and rich In S, K and Al. The wall rock alteratlon was Isochemlcal but Is manifest as a change In mineralogy from a hornblende + plagioclase assemblage to an actlnollte/tremollte + Quartz + clay assemblage. This Is best developed In the hangIng wall of the reef and is thought to have been caused by hydrogen ion metasomatism. The Arsenopyrite Reef was one of the main sediment-hosted shear zone gold producers In the Harabastad goldfield. This reef Is Interpreted as the basal margin of a shear zone whose top contact Is probably represented by the Quartz Vein Reef. The shear zone consists predomonantly of quartz and carbonate, and the two "pay" reefs are characterized by tourmallne. arsenopyrite and Au. No wall rock alteration was identified In this study, Based on the mineralogy and geochemical signature of the Girlie Nortn Reef and the Arsenopyrite Reef, It Is proposed that both were formed at the $The Pletersburg greenstone belt Is located In South Africa, about 300 km northeast of Johannesburg. It hosts a significant amount of gold mineralization and just over 1000 kg of gold have been produced from Its various reefs and secondary deposits. The greenstone belt is interpreted as an Archean ophiolite complex. It comprlses a volcano-sedimentary succession (the Pletersburg Group) which Is subdivided Into a basal greenstone sequence, interpreted as oceanic crust, and an upper sedimentary cover sequence. A number of major shear zones, which are thought to represent thrusts that developed during the subduction of the greenstone sequence, form an integral part of the stratigraphy . Four stages of deformation (D₁-D₄) and four phases of metamorphism (H₁-H₄) (three of which are correlatable with the peak stages of deformation) are recognized. The primary gold deposits are all shear zones related. but they are subdivided into greenstone, sedimentation and granIte-hosted types. Geographically, they occur In three distinct goldfields: Eerstellng, Roodepoort and Marbastad. The greenstone-hosted · Plenaar-Doreen shear complex Is In the Eersteiing goldfield and hosts eight gold occurrences. Within the complex, Girlie North Reef is the 640m-long "pay" section of the Girlie North shear zone. This reef is characterized, macroscopically, by a Quartz-carbonate-chlorite-sulphlde assemblage and, mlcroscoplcally, by the presence of tourmaline, arsenopyrlte and Au. Geochemical evidence Indicates that mineralizing fluids were H₂O and CO₂-bearing and rich In S, K and Al. The wall rock alteratlon was Isochemlcal but Is manifest as a change In mineralogy from a hornblende + plagioclase assemblage to an actlnollte/tremollte + Quartz + clay assemblage. This Is best developed In the hangIng wall of the reef and is thought to have been caused by hydrogen ion metasomatism. The Arsenopyrite Reef was one of the main sediment-hosted shear zone gold producers In the Harabastad goldfield. This reef Is Interpreted as the basal margin of a shear zone whose top contact Is probably represented by the Quartz Vein Reef. The shear zone consists predomonantly of quartz and carbonate, and the two "pay" reefs are characterized by tourmallne. arsenopyrite and Au. No wall rock alteration was identified In this study, Based on the mineralogy and geochemical signature of the Girlie Nortn Reef and the Arsenopyrite Reef, It Is proposed that both were formed at the same time. Textural evidence Indicates that tourmaline, arsenopyrite and Au were all very late In the paragenesis of minerallzatlon. The presence of tourmaline also Indicates a probable granite association. It Is proposed that the maln gold mineralizing event was synchronous with the Intrusion of granitoids (and therefore also with (D₁-D₄) and (H₁-H₄) and that most of the Au was derived from felsic magma. Gold was partitioned Into a magmatic hydrothermal fluid and then transported into the greenstone belt as a chlorIde complex. These magmatiC fluids were channelled up shear zones whIch had already been mineralized with a quartz-carbonate-chlorlte - sulphide assemblage by previous metamorphic fluidS. generated during the dynamic (D₂-related) H₂-phase of metamorphism. The Au was then deposIted as the result of a change In a fluid variable, such as temperature, pH, f0₂, or the activity of Cl (some Au may have been transported In a sulphur complex and so the activity of reduced 5 could also have been Important).
- Full Text:
- Date Issued: 1987
Anorogenic alkaline ring-type complexes of the Damaraland Province, Namibia, and their economic potential
- Authors: Potgieter, J E
- Date: 1987
- Subjects: Alkalic igneous rocks -- Namibia
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4907 , http://hdl.handle.net/10962/d1001567
- Description: Anorogenic alkaline ring-type complexes form within continental plate settings. Alkaline magmatism is derived from the upper mantle, in which mantle metasomatism plays an important part, as well as from partial melting of the lower crust. Radial and concentric fractures develop during the ascent of alkaline magma. Extrusion of basic and felsic magma takes place along these fractures with felsic volcanics building-up central volcanoes. As a result of emptying of the magma chamber, the superstructure of the volcano collapses and a caldera is formed. During the caldera stage syenitic and granitic material are intruded into ring fractures. Alkaline ring-type complexes may be classified as (i) alkaline qranite and syenite-type and (ii) carbonatite and undersaturated-type. These ring-type complexes occur as distinct igneous provinces. Some major provinces occur in Brazil, Corsica, Namibia, Nigeria, Norway, Saudi-Arabia and Sudan. In Namibia the Damaraland igneous province is of Mesozoic aqe and it contains 15 alkaline ring-type complexes . These complexes are situated along north-eastern trends which correspond to transform directions of the South Atlantic. During the opening of the South Atlantic (Gondwana breakup) Pan-African age lineaments were reactivated which allowed emplacement of anorogenic alkaline magmatism. A zonation of alkaline granite and syenitetype in the west and carbonatite and undersaturated-type ring-complexes in the east correlates with down- and upwarp axes parallel to the line of Gondwana fragmentation. Alkali- and H⁺-metasomatism is related to the alkaline and syenite-type whereas alkali metasomatism (fenitization) is associated with carbonatite and undersaturated-type ring-complexes. Sn, W and Ta mineralization is associated with alkaline granites of some of the alkaline granite and syenite-type ring-complexes. Fe, F, PO₄ , Nb, Th, REE, Sr, Zn and Pb mineralization is associated with carbonatite complexes. Potential exists for: (i) porphyry Cu-Mo and epithermal-type (Au, Ag, Pt-metals, base metals) mineralization in the alkaline granite and syenite-type ring-complexes and (ii) disseminated Cu, Au, Aq and Pt-metals in carbonatite and undersaturated-type ring-complexes
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- Date Issued: 1987
Gold metallogeny of Australia
- Authors: Rankine, Graham M
- Date: 1987
- Subjects: Gold ores -- Geology -- Australia , Gold mines and mining -- Australia , Gold -- Metallurgy -- Australia
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4928 , http://hdl.handle.net/10962/d1004676
- Description: The gold metallogeny of Australia is predominantly confined to the Archaean and Palaeozoic Provinces. The Archaean gold occurrences are predominantly hosted in ultramafic-mafic dominated greenstone belts, with less associated tofelsic-volcanic and sedimentary sequences. Most gold occurrences are confined to shear zones or faults, and adjacent discoveries of economic laterite-hosted deposits, host rocks. Recent are presently under investigation and will supply a significant proportion of production in the future. The Proterozoic gold deposits of Australia , are confined to geosyncinal sequences, commonly turbidites (eg: Telfer), with other hydrothermal deposits associated directly to granites. An important feature of the North Australian Craton deposits, is the spatial association of most deposits to granite bodies, although a genetic link has not been established conclusively. The Roxby Downs deposit in South Australia is a unique occurrence of gold in association to copper, uranium and R.E.E. This deposit is tentatively related to intraplate alkaline-magmatism, with further work necessary. The most significant recent discovery of gold mineralization in Australia is in the Drummond Basin in Queensland. This epithermal is tentatively related to mineralization within the Georgetown Inlier. The latter mineralization is Permo-Carboniferous, in a Proterozoic (and possibly Archaean) sequence of schists. It is tentatively suggested that all the gold mineralization in northern Queensland may be related to single tectonic event, a feature which requires further study . Other mineralization in the Phanerozoic includes the turbidite-hosted metamorphogenic deposits of Victoria, the rift related deposits in New South Wales and magmatic related deposits in Queensland. The gold deposits in Australia may in the future be classified in a tectonogeological framework, similiar to the layout of this dissertation, particularly once further data becomes available on recent discoveries.
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- Date Issued: 1987
Risk analysis assessment of the influence of geological factors on exploration and mining investment alternatives : development of a microcomputer simulation model
- Authors: Mallinson, Clyde A
- Date: 1987
- Subjects: Geology, Economic , Risk assessment -- Computer simulation , Mineral industries -- Economic aspects -- Computer simulation , Prospecting -- Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4904 , http://hdl.handle.net/10962/d1001564
- Description: A microcomputer risk analysis model is developed and used in an exploration - delineation simulation to analyse in particular the effects of geological factors on exploration mining investment alternatives. Analysis of results indicate that geological parameters can have profound effects on such investment alternatives and that the role of the geologist in determining and evaluating the significance of the various geological factors is critical. Simulation examples highlight some of the key geological parameters and show how changes in these parameters influence both the expected mean results and the standard deviations of such means. The risk analysis model provides an ideal means of conveying the importance of the different geoiogical factors on exploration - delineation - mining investment alternatives and may be used as a geological education aid
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- Date Issued: 1987
The geology and alteration/mineralization of the Van Rooi's vley W/Sn deposit, Namaqua metamorphic complex, South Africa
- Authors: Smithies, Robert Hugh
- Date: 1987 , 2013-03-01
- Subjects: Mineralogy -- South Africa , Geology -- South Africa -- Namaqualand
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4924 , http://hdl.handle.net/10962/d1004526 , Mineralogy -- South Africa , Geology -- South Africa -- Namaqualand
- Description: Scheelite, wolfram Ite and cassIterIte mIneralizat ion is hosted wIthin numerous quartz-tourmaline-feldspar-fluorite veins at Van Rooi's Vley, N.W. Cape Province . MineralizatIon and hydrothermal alteration within, and around, these veins is hIghly complex and reflects the intricate interaction of hydrotherma l activity upon a structurally deformed sequence of ProterozoIc med ium to high-grade gneisses. Four distinct stages of alteration and mineralization occurred, including a l ate 'epithermal stage'. Although the location of mineralization was strongly controlled by st ructure, the concentration of mineralizati on was controlled by physicochemical variables, of which host-rock geochemistry was particularly important . Further W/Sn mineralization occurs on a local scale, some of which is spatia lly related to minor leucogranite dykes. Leucogranite bodies are not uncommon within the region and some are enriched in Wand Sn. By comparing FIB ra tio s,W/Sn ratios, the alteration mineralogy, the ore mineralogy and the Fe-content of tourmaline, the deposits within the Van Rooi's Vley area can be placed into a 'proximal' to 'distal' classification, with respect to a common source of mineralizing hydrothermal fluids. The Van Rooi's Vley deposit, whilst affiliated to greisen-style deposits, represents a ' distal' quartz-vein lode deposit. , KMBT_363 , Adobe Acrobat 9.53 Paper Capture Plug-in
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- Date Issued: 1987
The petrology and geochemistry of the upper critical zone of the Bushveld complex at the Amandelbult section of Rustenberg Platinum Mines Limited, Northwestern Transvaal, South Africa
- Authors: Field, Matthew
- Date: 1987 , 2013-03-06
- Subjects: Geology -- South Africa -- North-Western Transvaal , Petrology -- South Africa -- North-Western Transvaal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5037 , http://hdl.handle.net/10962/d1007499 , Geology -- South Africa -- North-Western Transvaal , Petrology -- South Africa -- North-Western Transvaal
- Description: A study of petrological and geochemical variations through the upper Critical Zone of the Bushveld Complex at Amandelbult section of R.P,M. was undertaken. The sequence at this locality may be divided into seven "units" two of which appear to be complete, possessing the sequence harzburgite-pyroxenite-norite-anorthosite. The other five Units lack basal, intermediate or upper members. Considerable lateral variations are apparent in this sequence, but these are restricted to the Lower Pseudo Reef-Merensky Reef interval, tne same portion of the succession which is affected by pothole structures. The single most important petrographic feature of genetic significance is the occurrence of annealed, recrystallized anorthosite immediately underlying ulstramafic layers. This, together with the undulatory nature of the contact between the two rock layers, suggests that the ultramafic layer was emplaced as a hot liquid over a pre-existing, crystalline anorthosite floor, and that some remelting of this layer occurred. Variations in the chemical make-up of constituent silicate minerals reveal a number of significant processes which may have been operative in the magma chamber prior to crystallization, Olivine grains, for instance, exhibit extremely wide chemical variations both within single layers and from one layer to the next. These variations are best explained by re-equilibration processes with spinel and base metal sulphides, rather than by wide variations in original liquidus compositions. It appears that the compositions of the initial liquids from which each basal olivine-bearing layer crystallized, were approximately similar. Variations in the iron-magnesium ratio of ortho-pyroxenes indicate well defined continuous fractionation trends in units which are considered to be complete. Magnesian compositions are recorded in ultramfic members, while increasingly iron-enriched values are recorded upwards through the sequence pyroxenite-norite-antorthosite. Plagioclase grains exhibit less well defined fractionation trends, but it is clear that an upward increase in An is encountered through indivitual Units. This is in direct contrast to the trend exhibited by orthopyroxene. A further feature of plagioclase grains is the considerable degree of chemical zonation exhibited by them. In cumulus grains this is commonly manifested as strongly reversed rims, while in intercululus grains normal zoning is ubiquitous. Whole-rock chemical variations through the succession indicate that cyclical variations occur through successive Units, but that these merely reflect changes in modal mineralogy and not liquid fractionation trends. Such trends can be shown for selected element ratios, where these elements are known to partition into a single mineral phase. Rations of pyroxene components such as the nickel/scandium ratio, exhibit a saw tooth pattern through successive Units, while ratios of plagioclase components such as the strontium/alumina ratio have unique, fairly constant values for each individual Unit but different values for successive Units. The latter type of cyclicity is not always strictly confined to lithologically recognized boundaries between Units, and a slight overlap into overlying ultramafic layers is apparent. An investigation of variations in trace element levels in a single layer in five widely separated boreholes revealed that there is some evidence for a lateral fractionation trend from the southwest (more primitive) to the northeast (more evolved), although the small number of data points available preclude definite conclusions. There exists in the data some evidence that the Giant Mottled Anorthosite differs chemically from the other anorthosites in the study section, and that it more closely resembles rocks of the Main Zone. This evidence is particularly apparent in variations of the chromium/aluminium ratio of orthoyroxene grains, and in the An content of plagioclase grains, both of whose trends exhibit distinct inflections at the base of this member. The features of the succession at Amandelbult are best explained by the model of Eales et al. (in press, a), which visualizes the input of a number of pulses of new, hot liquid into a magma chamber containing the fractionated residua of previous influxes. At a critical point in time, just prior to the mafic Merensky Reef input, a large input of gabboic liquid was intruded at high levels in the chamber. The lower portions of this liquid mixed with the residua of earlier mafic inputs, which in turn mixed with new inputs of mafic, typical Critical Zone liquids. Thus the lower portions of the study section represent mixtures of new Critical Zone liquids with the residua of previous such influxes, while the upper portions have the added complication of mixture with a Main Zone-type liquid. The unique chemical character of the Giant Mottled Anorthosite appears to be a direct manifestation of the influence of the Main Zone liquid. , KMBT_363 , Adobe Acrobat 9.54 Paper Capture Plug-in
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- Date Issued: 1987
The tectono-metallogenesis during the irumide and pan-African events in South West Africa/Namibia
- Authors: Tregoning, Trevor Denzil
- Date: 1987 , 2013-03-05
- Subjects: Orogeny -- Namibia , Ore deposits -- Namibia , Mines and mineral resources -- Namibia , Geodynamics , Rifts (Geology) -- Namibia
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4922 , http://hdl.handle.net/10962/d1004490 , Orogeny -- Namibia , Ore deposits -- Namibia , Mines and mineral resources -- Namibia , Geodynamics , Rifts (Geology) -- Namibia
- Description: A large portion of South west Africa/Namibia is underlain by 2 great orogens. They are the Irumide (Sinclair/Rehoboth) and Damara Orogenies. The L-shaped Irumide Province forms part of a belt which extends over the subcontinent from Namaqualand to as far as Zambia. The volcano-sedimentary sequences of the Irumide are believed to have formed in intracratonic rifts and pull-apart basins during the period 1400 to 900 Ma. The evolution of the NW trending Sinclair Group proceeded by means of 3 major cycles each beginning with the emplacement of basic to intermediate magmas followed by felsic ones. The cycle ended off with subsidence, deposition of immature clastic debris and final tilting of the volcano-clastic sequence. It was suggested that the extensive calc-alkaline lavas present, developed within a magmatic arc above a subduction zone, but this proposal has not been generally accepted. The NE trending Klein Aub-Witvlei Basins consist essentially of red bed alluvial fans and lacustrine sediments with minor volcanics near the base. The red beds and aeolian sediments were deposited in an arid climatic condition. The regional greenschist facies metamorphism and deformation is attributed to a major tectono-thermal event at 1100 Ma. The Damara Orogen (900 - 550 Ma) forms part of the Pan-African mobile belt system of global proportions. The NE trending intracontinental branch (aulacogen) and 2 coastal branches constitute a triple junction with its focal point near Swakopmund. The NE extension of the intracontinental belt has been linked with the Lufilian Arc hosting the renown Zambian Copper Belt deposits. In South West Africa/Namibia this belt hosts many different mineral occurrences which can be grouped into rift and collision related deposits. The tectonic history of the Damara Orogen supports a geodynamic-evolution-with-time hypothesis and represents a transitional phase in which limited Wilson Cycle Tectonics was active. The Theory of Mantle Advection is invoked to explain rifting, thinning and subsidence. Extensive ensialic rifting resulted in a relatively stable Northern Carbonate Platform and several deep troughs hosting turbiditic sequences. Crustal rupture in the Khomas Trough allowed for the emplacement of ocean floor tholeiites known as the Matchless Amphibolite Belt. Subsequent ocean closure and collision resulted in deformation, metamorphism and generation of predominantly S-type granites. The southern continental plate was partially overridden by the northern plate during final collision at 550 Ma. These low angle thrust faults allowed for the emplacement of the Naukluft Nappe Complex on top of younger Nama sediments. The break up of Gondwanaland during the Mesozoic with the splitting of the Atlantic Ocean was responsible for the intrusion of anorogenic alkaline ring complexes along the extension of the NE trending transform faults within the intracontinental branch of the Damara Orogen. A close relationship between the tectonic setting and mineral deposits has been recognized in both the Irumide and Damara Orogenies. In the Irumide, stratiform syngenetic copper deposits are hosted by alluvial fan, playa and lacustrine sediments. The uninterrupted sedimentation from the Irumide to Damara Orogen resulted in similar stratiform copper deposits during the early stages of rifting. In the Damara Orogen the rifting (extensional) phase is characterized by 4 main mineralizing systems: diagenetic/syngenetic (Kupferschiefer-type), epigenetic/hydrothermal Cu-Pb-Zn (Mississippi Valley-type), volcanogenic cupriferous pyrite (Besshi-type) and volcano-exhalative Pb-Zn (Red Sea-type). The collision (compressional) phase was accompanied by 4 main mineralizing processes: epigenetic/hydrothermal Cu-Pb-Zn, hydrotheral/metasomatic Sn-W-rare earth, metamorphogenic Au and U-bearing anatectic melts. The key to the selection of viable exploration targets lies in the understanding of the field evidence and the geodynamics modelling to explain the evolution of the orogen and its associated mineral deposits.
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- Date Issued: 1987
Petrology and geochemistry of the lower group chromitites and host rocks on the farm Zandspruit 168 JP, Western Bushveld Complex
- Authors: Botha, Michael James
- Date: 1988
- Subjects: Petrology -- Africa, Southern , Geochemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4905 , http://hdl.handle.net/10962/d1001565
- Description: The eight Lower Group chromitite layers of the Ruighoek Pyroxenite in the area west of the Pilanesberg (LGl - LG7), on the farm Zandspruit 168 JP, were sampled in borehole cores drilled through the succession. The stratigraphic succession faIls within Cameron's (980) units B to E of the Critical Zone. The lowermost layer of the Lower Group, the LG 1 chromitite, is located some 440 metres below the MG 2 (Middle Group) chromitite layer, above which the first appearance of cumulus plagioclase in the Complex is seen. By convention, this horizon is designated the top of the Ruighoek Pyroxenite in the Western Bushveld Complex. Homogeneous units of chromite-bearing orthopyroxenite (bronzitite), exhibiting inconspicuous layering defined in terms of variations in orthopyroxene grain-size host all but one of the Lower Group layers; the LG 4 chromitite layer is exposed within an olivine-rich subunit 23 metres in thickness (C₃ subunit). The cumulative thickness of chromitite is 2,92 metres or 0,8 per cent of the studied section, which is 381 metres in thickness. Minimum and maximum thicknesses of the LGl - LG7 layers exposed in drill core are 17 and 81 centimetres, respectively, with minor chromitite layers ranging between 2 and 5 centimetres in thickness. Weighted mean Cr₂0₃ contents of units B to E vary between 1,17 and 3,22 per cent, with the latter estimate representative of the D₂ subunit which hosts the LG 6 chromitite layer. The LG 6 is correlated with the Steelpoort layer of the Eastern Bushveld Complex, and varies between 76 and 81 centimetres in thickness under a large portion of the farm Zandspruit. An undisturbed succession striking N15°E and dipping 12 - 15°E is depicted within the studied area, which is bounded on the eastern side by the north-striking Frank fault. Major folding of the layered succession is evident to the north of the area, where the layering adjacent to the trace of the fault dips 35° to the southwest. Particular attention is paid in the present study to (a) the nature of chromitite layers and their host rocks, (b) the contrast between the mineral chemistry of weakly disseminated chromite and grains within massive ore layers, (c) concentrations of Cr, V, Ni, Co, Sc and Ti in orthopyroxene in relation to stratigraphic height, and levels of Sr, Ba and Zr associated with hypothetically pure, intercumulus plagioclase feldspar, and (d) possible mechanisms which induce crystallization of chromitite layers containing 50 per cent Cr₂0₃ from magma with a Cr content of less than 1 000 ppm. Electron microprobe studies of chromite in relation to mineralogical and textural environment clearly reveal that (a) the proportions of Cr and Al cations are linked to paragenesis: higher Al/Cr ratios characterize olivine-bearing domains, whereas grains intergrown with plagioclase feldspar exhibit low AI/Cr ratios, and (b) Al contents rise with a decline in Mg/(Mg + Fe²⁺) from high values to a value of 0,450, then decrease with a further decline in Mg/(Mg + Fe²⁺). The paragenetically later trend is emphasized in a large population of chromite grains which escaped early encapsulation in orthopyroxene crystals and continued to grow in the environment of intercumulus plagioclase. Within-and between-sample compositional variation of grains in silicate-rich domains is modelled in terms of in situ growth increments, diffusive homogenization of zonal structures, and residence time within interstitial melt. Fractionation trends, as measured by Mg/(Mg + Fe²⁺) ratios in whole-rock and/or microprobe studies of orthopyroxene, are reversed in relation to stratigraphic height towards the top of the B unit and in the overlying C unit. These data are supported, for example, by lower vanadium contents and higher Ni/Sc ratios in hypothetically pure orthopyroxene. Small olivine crystals in chromite-rich domains are enriched in Ni relative to coarse-grained olivine in adjacent dunite: a feature attributed to early isolation of primocrysts from magma in the former case, and in situ equilibration between olivine crystals and Ni-depleted residual melt in the latter case . Similarly, rising Ni contents and Mg/(Mg + Fe²⁺) ratios of orthopyroxene with increasing stratigraphic height in the footwall of the LG 6 chromitite layer, linked to a progressive decline in orthopyroxene grain-size, are effects which may arise out of early separation of interstitial melt from orthopyroxene cumulates. A model is thus proposed which (a) links the thickness of chromitite layers to the vertical separation between successive layers or the thickness of fine-grained orthopyroxenite in the footwall, (b) ascribes copious nucleation of chromite to liquid mixing of this footwall derived, Cr - depleted contaminant with influxes of hot, primitive magma, and (c) tenders the notion that the present modal proportion of mesostasis in the footwall of a chromitite layer serves as a reciprocal measure of the volume of fractionated exudate
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- Date Issued: 1988
The geology and metallogeny of the Otavi mountain land, Damara orogen, SWA/Namibia, with particular reference to the Berg Aukas Zn-Pb-V deposit a model of ore genesis
- Authors: Misiewicz, Julian Edward
- Date: 1988
- Subjects: Geology -- Namibia -- Damaraland , Ore deposits -- Namibia , Orogeny , Metallogeny , Geodynamics
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4959 , http://hdl.handle.net/10962/d1005571 , Geology -- Namibia -- Damaraland , Ore deposits -- Namibia , Orogeny , Metallogeny , Geodynamics
- Description: The Olavi Mountain Land is a 10 000 km2 mineral province located at the eastern extremity of the exposed Northern Platform of the Damara Pan African orogenic belt. The Olavi Mountain Land is tbe most important mineral province on the Northern Platform. Exploitation of tbe Cu-Pb-Zn-V province has been on-going since the possession of the territory by the German colonial authority in 1890. Production has been mostly from four mines which in order of importance are Tsumeb, Kombat, Berg Aukas and Abeoab. A second mineral province on the Northern Platform located in the west is centred on Sesfontein where as yet only insignificant mineralization has been noted. Besides these localities, the Northern Platform is conspicuously devoid of notable mineralization. The aim of this thesis has been to document tbe Berg Aukas deposit, an important end-member type of mineralization in the Otavi Mountain Land. The basic premise bas been to show tbat the derivation and localization of the mineralization is a consequence of two broad controls which can be simply summarised as features of the basement and of the carbonate sequences. The geodynamic evolution of the Damara Belt commenced with intra-continental rifting approximately 900 Ma ago. Rift grabens trending north-east were filled by the Nosib Group which comprises mostly clastic lithologies but also some volcanics. The earliest and largest rift is referred to as the Northern Rift. Separation of the Congo, Kalahari, and proto-South American cratons resulted in rifting and rapid downwarping so that an encroaching sea and an Olavi Group carbonate shelf developed along the northern margin of the Northern Rift. Significantly, the carbonates only covered the Northern Rift in the area of the Otavi Mountain Land where a basinal dome, referred to as the Grootfontein Basement High, marked the basin edge. In the west, the carbonates covered the less important Sesfonfein Rift, and it is only in these two areas where Nosib sequences underlie the carbonate platform. Carbonate sedimentation was interrupted by a major period of crustal readjustment and the deposition of an extensive mixtite throughout the geosynclinal Swakop Trough and Northern Platform. This is referred to as the Chuos Formation and subdivides the Olavi Group into a lower Abenab and an upper Tsumeb Subgroup. Reversal of spreading led to plate collision and subduction of tbe Kalahari craton beneath the Congo craton. It was accompanied by orogenesis which resulted in F1 folding of the Northern Platform into a series of north-easterly trending intermontane basins into which a molasse sequence known as the Mulden Group was unconformably deposited. Following this major north-south deformation mild east-west compression initiated F2 folding and the formation of doubly plunging synclines. The Berg Aukas Syncline represents a primary depositional basin which was subsequently folded. The original basin was formed by late Nosib rifting wben spreading caused the Swakop geosynclinal Trough to form. Carbonates of the basal Berg Aukas Formation were deposited in a lagoonal setting typified by reef and fore-reef facies witb peri-platform conditions. Rapid subsidence caused these sediments to be overlain by deep water carbonates of the Gauss Formation. Two styles of mineralization known as the Tsumeb-type and Berg Aukas-type are stratigraphically, isotopically, and mineralogically distinct. The Tsumeb-type is a cupriferous variety of discordant bodies confined to the upper sequences beneath the Mulden unconformity. The Berg Aukas-type is a Zn-Pb variety confined to tbe basal unconformity. The Berg Aukas deposit comprises three ore bodies known as the Northern Ore Horizon, the Central Ore Body, and the Hanging Wall Ore Body. Sphalerite and galena constitute the bypogene ore. Willemite, smithsonite, cerussite, and descloizite are important supergene ores. A review of genetic models concludes that a magmatic origin initially proposed for tbe Tsumeb deposit is entirely rejected and a basin dewatering model in line with Mississippi Valley-type deposits is proposed. The syntectonic nature of mineralization at Berg Aukas and elsewhere in the Otavi Mountain Land indicates that orogenesis encouraged dewatering and leaching of metals from a broad mineralizing front along the margin of the Swakop Trough. These were transported by acidic saline brines which migrated along the clastic aquifers and structural conduits provided by the Northern Rift. Fluid inclusion studies indicate that the hydrothermal fluids at Berg Aukas were very saline (23% TDS) and were transported at temperatures ranging between 92° to 210°C. Hydrothermal fluids which mineralized Berg AukaS-type deposits migrated along the basal unconformity towards the basement high and were responsible for hydrothermally altering the basement granites and gabbros and the Nosib clastic rocks. Tsumeb-type deposits resulted by migration of fluids through the carbonate pile and along north-easterly trending basement geofractures. As a consequence of variation in transport, the Berg Aukas-type and Tsumeb-type fluids leached different sources and therefore derived mineralogically and isotopically seperable characteristics. The localization of the Berg Aukas ores was controlled by the carbonate stratigraphy and structure. Hydrothermal karsting and ore deposition took place on the contact between Massive Grey and Light Grey Dolostones which represents a permeability contrast. The movement of the hydrothermal fluids was controlled by north-south trending vertical fractures caused by F2 folding which resulted in a peric1inal structure. Hydrothermal karsting was accompanied by ca1citic, dolomitic and silicic alteration. The heated acidic fluids initiated solution collapse and a variety of breccia types. Supergene processes resulted in oxidation and upgrading of the ore. Vanadium derived indirectly from gabbros in the basement complex were transported as calcium metavanadate complexes and deposited on contact with the oxidizing base metal sulphides.
- Full Text:
- Date Issued: 1988
The petrology, mineralogy and geochemistry of the main zone of the Bushveld Complex at Rustenburg Platinum Mines, Union Section
- Authors: Mitchell, Andrew Alexander
- Date: 1988
- Subjects: Mineralogy -- South Africa Petrology -- South Africa Rustenburg Platinum Mines Geochemistry -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4903 , http://hdl.handle.net/10962/d1001563
- Description: Union Section of Rustenburg Patinum Mines is situated in the northwestern part of the Bushveld Complex, some twenty kilometres north of the Pilanesberg Alkaline Complex. The mining lease area covers a roughly triangular segment of Lower, Critical and Main Zone rocks, transgressed to the north and south by magnetite-bearing ferrogabbro of the Upper Zone. The Main Zone at Union Section is the focus of this study. The prime source of sample material for the study is the deep exploration borehole SK2, but additional, supplementary samples were collected on surface and underground, as well as from a second surface exploration borehole, SK4. In line with the recommendations of SACS (1980), the top of the Critical Zone, and therefore the base of the Main Zone, is taken to be the top of the Bastard Cyclic Unit. Sharpe (1985) suggested that the succession from the base of the Main is an isotopically separate entity Zone up to the Pyroxenite Marker from the rest of the Bushveld layered succession. This is not strictly true, as there is evidence that more than one parental magma was involved in the formation of this interval. It is, however, true that there are fundamental differences, particularly in isotopic makeup, between the Main Zone rocks below the pyroxenite Marker and those above (the latter having been assigned by Molyneux (1970) to subzone C of the Main Zone). Kruger et al. (1986, in press) suggested that the Pyroxenite Marker marks the base of the Upper Zone, and this convention is adhered to here. The implication of this is that the rocks which formerly constituted subzone C of the Main Zone are now considered part of the Upper Zone. The Main Zone rocks below the pyroxenite Marker were originally subdivided by Molyneux (1970) into two subzones, A and B. The results of the present study indicate that this subdivision is not justified. Instead, eight units have been distinguished in the Main Zone on geochemical, petrological and mineralogical bases. Each of these units is characterized by a coherent set, or progression, of chemical and petrological characteristics. The specific assignment of genetic connotations to these units has been deliberately avoided , at least until further studies of the Main Zone prove this to be justified. The demarcation of the eight units is illustrated in the composite diagram (Fig. 34) in the back pocket of this work, and the reasons for the subdivisions are listed in Table 6 (at the end of chapter 7 of this thesis). Until the late 1970's, it was thought that most layered cumulates formed by crystal settling (Wager and Brown, 1968). More recently, there has been a fundamental conceptual change, and many workers now believe that most cumulate rocks formed by in situ crystallization at the floor and walls of the magma chamber (McBirney and Noyes, 1979, Irvine, 1980a; Campbell, 1987). There is, however, some evidence for the physical separation of phases undergoing cotectic crystallization, particularly in the Upper Critical Zone and lower part of the Main Zone (Eales et al., 1986). This process, which has been alluded to in the past by various authors (Ferguson and Botha, 1963; Vermaak, 1976) involves the flotation of early-formed plagioclase crystals due to their positive bouyancy in tholeiitic liquids. The result is an apparent decoupling of the chemistry of pyroxene and plagioclase, as in unit IV of the Main Zone, where plagioclase becomes more anorthitic upwards, whilst pyroxene becomes more iron-rich. There is some substantial evidence, particularly in reversals in the strontium isotope initial ratio and the orthopyroxene Mg/(Mg+Fe) ratio , for multiple intrusion in the Main Zone. Although the largest and most important magma influx in the Main Zone was a high-R₀ aluminous tholeiite, as suggested by Sharpe (1985), the intrusive history of the Main Zone is believed to be far more complex than Sharpe (op. cit.) suggested. Significantly, there is strong evidence for small influxes of Upper Zone-type (Fe-rich tholeiite) magma in the upper reaches of the Main Zone. These are believed to be precursors to the major influx of Upper Zone-type magma at the pyroxenite Marker (Kruger et al, 1986, in press). The fate of intercumulus liquids in cumulate rocks has recently recieved substantial attention (Sparks et al., 1985; Morse, 1986; Barnes, 1986: Campbell, 1987). It is believed that the migration, or at least redistribution, of intercumulus liquids has played a vital role in modifying fractionation trends in the Main Zone. More importantly, the accumulation of late-stage intercumulus liquids is believed to be responsible for the formation of the Fe-rich ultramafic pegmatite bodies that interrupt the layered cumulates in borehole SK2 , Adobe Acrobat 9.53 Paper Capture Plug-in
- Full Text:
- Date Issued: 1988
A preliminary appraisal of the mineral potential of Venda based on a reconnaissance geochemical soil sampling survey and literature review
- Authors: Wilson, Michael George Carey
- Date: 1990
- Subjects: Mines and mineral resources -- South Africa -- Venda
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4994 , http://hdl.handle.net/10962/d1005606 , Mines and mineral resources -- South Africa -- Venda
- Description: A reconnaissance soil sampling survey was carried out over Venda by Cycad (Pty) Ltd and the samples were analysed for 36 elements using XRF techniques, by Anglo American Research Laboratories in Johanuesburg. The data resulting from this survey forms the basis for the present interpretive study. Initially the sample positions were co-ordinated, then the geological, soil and sample types were allocated to each point. Twelve lithological groupings were chosen which incorporated most of the available data, retained significant geological characteristics and consisted of statistically significant sample populations. Statistical manipulation was undertaken for each of the lithological groups. Using a final population of 5768 samples, means were determined and anomalous values were identified using a threshold of mean plus two standard deviations. Due to time and budgetary constraints, 24 of the 36 elements were chosen for statisical manipulation and fourteen of these, with particular economic significance, were chosen for plotting, wherever significant numbers of anomalies were present. The element overlays were plotted so as to coincide with 18 of the 25, 1:50 000 topographic sheets covering Venda , the remaining 7 having inadequate sample coverage to yield meaningful contours. In this way a total of 175 element overlay sheets were plotted, each showing contoured element levels, with selected anomalous values. The treatment of the vast body of information made available by the Cycad sampling programme has thus been selective and has continually been aimed at highlighting and concentrating attention on the areas of greatest indicated mineralization potential, rather than on specific anomalies. In this regard it is felt that the present study has been successful, in spite of limited sample coverage in some areas. Combining the results of this study with a modern tectonically-based appraisal of mineralization potential and a knowledge of the local geology and previously known mineralization gleaned from an extensive literature review, the following types of mineralization are considered to have the highest potential in Venda: i) Nickel-copper-platinum mineralization as well as magnesite, in the olivine dolerite sills which intrude the base of the Karoo Sequence in Northern Venda. ii) Coal in the basal Karoo Sequence sediments in a broad zone from Jazz 715 MS in the west, and along the Klein Tshipise fault from Amonda 159 MT to the Mutale Copper Fields then east of these to the Kruger National Park. Where intrusives invade these lower Karoo sed iments the potential exists for amorphous graphite. iii) Hydrothermal copper and possibly gold and silver concentrations, in Nzhelele and Sibasa Formation rocks, particularly those associated with faults known to have been active in post-Soutpansberg times. iv) Sediment-hosted massive sulphide deposits (Cu-Pb -Zn) close to basin margin faults, near intersections with cross cutting faults that have resulted in localized basin formation. These are most likely in the Soutpansberg sediments. v) Martle, flake-graphite and late stage skarn mineralization (including lead, zinc, gold and tungsten), in calcareous rocks of the Gumbu Formation.
- Full Text:
- Date Issued: 1990
The geology and alteration-mineralisation of the Gamigab Tin Prospect, Damaraland, Namibia
- Authors: Walraven, Felix Caspar
- Date: 1990
- Subjects: Mineralogy -- Namibia -- Damaraland , Geology -- Namibia -- Damaraland , Tin mines and mining -- Namibia -- Damaraland
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4956 , http://hdl.handle.net/10962/d1005568 , Mineralogy -- Namibia -- Damaraland , Geology -- Namibia -- Damaraland , Tin mines and mining -- Namibia -- Damaraland
- Description: The stratigraphy at the Gamigab Sn prospect consists of two mainly schistose units separated by a thick marble unit which have been assigned to the Orusewa, the Karibib and the Kuiseb Formations respectively. Four phases of folding affected the lithologies with the south-south-west trending F2 folds defining the main structures in the region. The area underwent low grades of metamorphism. Temperatures were in the range 420° to 500°C and pressures less than 2 kbars. The effects of contact metamorphism are seen in the south-east and south-west. Regional metamorphism outlasted the deformation and contact metamorphism started late during deformation. Two Karoo-age intrusions penetrated the metasediments north of the mineralisation. One is an altered porphyry plug and the other is a weathered dolerite plug, the latter containing xenoliths of undeformed Karoo sediments. Cassiterite is hosted within east-west trending quartz veins that cross-cut previously altered schistose country rocks. The alteration types include sericitisation, tourmalinisation, carbonatisation and ferruginisation. Preliminary Rb/Sr dating on muscovite from the alteration zone suggests an age of 509 ± 11 Ma. Breccias of probable hydrothermal origin are spatially associated with the mineralisation. These hydraulic breccias occur in antiformal structures within the marble and developed in response to a sudden pressure release due to a build up of fluids at the contact between the schistose Orusewa and carbonate Karibib Formations.
- Full Text:
- Date Issued: 1990
Gold mineralization in an archaean granite-greenstone remnant west of Melmoth, Natal ore genesis and implications for exploration
- Authors: Bullen, Warwick David
- Date: 1991
- Subjects: Gold ores -- Geology -- South Africa , Greenstone belts -- South Africa -- KwaZulu-Natal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4967 , http://hdl.handle.net/10962/d1005579 , Gold ores -- Geology -- South Africa , Greenstone belts -- South Africa -- KwaZulu-Natal
- Description: The previously undifferentiated, "Melmoth Granite-Greenstone Remnant" (MGGR¹) crops out over an area of about 360 km² in northern Natal, South Africa. The greenstone sequence is comprised mainly of mafic metalavas with lesser serpentinite, talc schist, dacitic tuff, quartz-muscovite schist, quartzite and calc-silicate rocks. The greenstones are intruded by syntectonic trondhjemitic gneisses, late-tectonic granodioritic gneisses and post-tectonic granite dykes. Four phases of deformation and metamorphism are recognized. Epigenetic, disseminated and quartz vein-hosted gold mineralization is associated with D₂ shearing - a positive correlation existing between the intensity of the shearing, the thickness of the shear zone and the grade of ore it contains. Auriferous quartz veins are distinguished from an earlier generation of barren vein quartz on the basis of mineralogy, texture and relationship to the s-fabric. The mineralization occurs in zones of dilation associated with shear zone refraction. Associated wall rock alteration includes sericitization, argillization and chloritization. An ore genesis model based on the aforementioned parameters, is proposed. Finally, an exploration programme has been devised in order to locate undiscovered gold deposits in the MGGR. The programme could probably be applied, with minor modifications, to shear zone-hosted gold deposits in other granite-greenstone remnants in northern Natal. ¹- Name suggested by writer.
- Full Text:
- Date Issued: 1991
Mantle xenoliths from the Abrahamskraal kimberlite : a craton-margin geotherm
- Authors: Nowicki, Thomas Edward
- Date: 1991
- Subjects: Kimberlite , Kimberlite -- Inclusions
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4898 , http://hdl.handle.net/10962/d1001557
- Description: The Abrahamskraal kimberlite pipe (group I) occurs approximately 5 km to the south-west of the geophysically defined margin of the Kaapvaal craton in the central Cape Province, and contains a variety of crustal and mantle xenoliths. This study focusses on xenoliths of deep-seated origin (mantle and lower-crustal), and in particular on garnet-orthopyroxene bearing assemblages which are amenable to thermobarometric techniques. Four major types of deep-seated xenolith have been identified, i.e. peridotites, dunites , eclogites, and garnet pyroxenites. The petrographic features and mineral compositions of these xenoliths are described . Pressures and temperatures of equilibration have been calculated primarily using the garnet-orthopyroxene thermometer of Harley (1984), and the Al-in-enstatite barometer of Nickel and Green (1985). The peridotites are coarse-textured (Harte, 1977), magnesium -rich rocks, and are typical examples of the common type I peridotites which generally dominate mantle xenolith suites in kimberlites. Garnet peridotite xenoliths define a geotherm which lies along a typical theoretical conductive geothermal gradient for shield areas (Pollack and Chapman, 1977), and which extends to a maximum pressure of 41 kb (~130 km). Comparison of the Abrahamskraal geotherm with that constructed for the northern Lesotho xenolith suite (calculated using the same thermobarometer couple), suggests that the lithosphere at the Namaqua /Kaapvaal boundary is not significantly thinner or hotter than that underlying the craton. Modelling of the craton boundary under the constraints provided by the Abrahamskraal geotherm, and by the distribution of diamond-bearing kimberlites in southern Africa, indicates that the Abrahamskraal kimberlite has sampled relatively thick, cool , Namaqua lithosphere. It is suggested that, in terms of diamond distribution, the age and magmatic history of the Namaqua lithosphere is of greater significance than its thickness. Two varieties of dunite occur at Abrahamskraal. Coarse-textured dunites with Mg-rich olivine compositions similar to those of the peridotitic olivines, probably originated by similar (but perhaps more extreme) processes to those which formed the peridotites. A finer-grained and relatively Fe-rich variety of dunite may represent ultramafic cumulates formed by fractionation of basic or ultrabasic magmas within the mantle. Two varieties of eclogite have been distinguished. Coarse-grained eclogites which yield relatively high temperature estimates, are believed to have originated from depths similar to those determined for the garnet peridotites, i.e. from the lower lithosphere. A distinctly finer grained variety of eclogite, yields significantly lower temperatures which may be based on frozen-in equilibria. A maximum depth of approximately 87 km (~ 27 kb) has been estimated for these xenoliths, but they may have originated from significantly shallower (possibly lower-crustal) levels. The garnet pyroxenite xenoliths are generally orthopyroxene-rich rocks which contain varying amounts of garnet (8 to 33 %) and clinopyroxene (0 to 64 %). Textural features indicate that the garnet and possibly some of the clinopyroxene has exsolved from an originally A l -rich orthopyroxene. The rocks are significantly more Fe-rich than the peridotite xenoliths, and their constituent minerals show a wide range of Mg/Mg+Fe ratios. The pressure-temperature array defined by the garnet pyroxenites is approximately isothermal, and spans a depth range from approximately 30 to 95 km. It deviates strongly (to higher temperatures) from the ambient geothermal gradient at the time of kimberlite emplacement, as inferred from the garnet peridotite xenoliths. The pressures and temperatures calculated for the garnet pyroxenites are based on mineral equilibria which are believed to have been frozen-in during cooling from an intial hightemperature (probably molten) state. Qualitative modelling of possible cooling paths in pressure-temperature-composition space, indicates that the apparent depth range displayed by the garnet pyroxenites, approximates the true depth range over which these rocks were emplaced. However, the apparent pressures calculated from core compositions are significantly lower than the true pressures at which the original rocks formed . The garnet pyroxenite xenoliths appear to represent a major, possibly Namaqua age (~1000-1400 Ma), magmatic event involving the emplacement of large amounts of mafic magma over a significant depth range in the shallow upper mantle
- Full Text:
- Date Issued: 1991
Mineralogy, petrology and geochemistry of the lower and lower critical zones, Northwestern Bushveld Complex
- Authors: Teigler, Bernd
- Date: 1991
- Subjects: Mineralogy -- South Africa -- North-Western Transvaal Petrology -- South Africa -- North-Western Transvaal Geochemistry -- South Africa -- North-Western Transvaal
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4978 , http://hdl.handle.net/10962/d1005590
- Description: This study of the lower part of the Rustenburg Layered Suite in the Western Bushveld Complex is based mainly on drill core samples from three localities, which are approximately 130 km apart. The NG-sequence, situated in the northwestern sector of the complex (Union Section, R.P.M.) extends from the floor of the complex to the base of the upper Critical Zone. The sequence is ca. 1800 m thick and it comprises mainly ultramafic cumulates, namely pyroxenites, olivine pyroxenites, harzburgites and dunites. Norites and anorthos ites are present only in minor proportion. Within the upper half of the NG-sequence ten prominent chromitite layers are correlated with the LGI MG4-interval. Correlation is also established between published sequences and the two other sequences studied, located 8 km and 55 km, respectively, east of Rustenburg. Whole-rock chemical data (major and trace elements), microprobe and Sr isotope data are presented. Petrographic studies provide modal analyses and measurements of grain size. All petrographic, mineralogical and other geochemical data point to an origin of the cumulates of the NG-sequence by crystallization from liquids of the U-type lineage and derivatives thereof. No evidence is found for the involvement of parental liquids with a distinctly different composition or crystallization order (A-liquids). However, subtle compositional variations of the parental liquids are evident in slight changes of the Cr content in orthopyroxene or in variations of Sr isotope ratio. The NG-sequence is characterized by intervals with reversed fractionation trends caused by repeated influxes of pristine magma (during periods of high magmatic activity) resulting in a high degree of rejuvenation. These intervals are overlain by others with a normal fractionation trend, interpreted as cumulates formed in periods with low or no magmatic activity, in which fractional crystallization controlled bulk composition of the evolving liquid. The Lower Zone in the NG-sequence is dominated by a progressive shift towards more primitive compositions, while in the Critical Zone fractionation was the major operating process in the magma chamber. However, during deposition of the pyroxenitic lower Critical Zone several replenishment events occurred, during which fresh Cr-rich magma was emplaced. Massive chromitite layers were deposited after mixing between the newly emplaced magma and the resident residual liquid shifted bulk compositions into the primary field of chrome-spinel. Cumulus plagioclase crystallized after bulk composition of the residual liquid was driven to the orthopyroxene plagioclase cotectic by continued fractional crystallization; this occurred once in the Lower Zone, yielding a single, thin norite layer, and again in the upper Critical Zone of the NG-sequence. A facies model is proposed based on the stratigraphic and compositional variations along strike in the Western Bushveld Complex. This model explains the variations by means of the position of the sequence with regard to a feeder system. The olivine- and orthopyroxene-rich, but plagioclase-poor NG-sequence represents the proximal facies, while the SF-sequence (poor in ferromagnesian phases, but plagioclase-rich) is developed as a distal facies, close to the Brits graben.
- Full Text:
- Date Issued: 1991
The alluvial diamond deposits of the lower Vaal river between Barkly West and the Vaal-Harts confluence in the Northern Cape province, South Africa
- Authors: Matheys, Fabrice Gilbert
- Date: 1991
- Subjects: Diamond deposits -- South Africa , Geology, Economic
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4933 , http://hdl.handle.net/10962/d1005545 , Diamond deposits -- South Africa , Geology, Economic
- Description: The alluvial diamond deposits along the Vaal River, between Barkly West and the Vaal-Harts confluence, have been worked for more than one century by thousands of private diggers. The diamonds are recovered from two sedimentary units of Cenozoic age, the Older Gravels and the Younger Gravels. These rest on a basement of Ventersdorp Supergroup andesites and Karoo Sequence sediments, which have been intruded by Cretaceous kimberlites. The gravels are, in turn, overlain by the Riverton Formation and the Hutton Sand. On a large scale, tectonic setting, geomorphology and palaeoclimate have played a major role in the formation of diamondiferous placers in the area under investigation. A study of the sedimentology of the Younger Gravels was carried out with the aim of acquiring an understanding of the processes responsible for the economic concentration of high quality diamonds. An investigation of facies assemblages, clast composition, clast size, palaeocurrent directions external geometry, particle morphology and led to the conclusion that the Younger Gravels were deposited in a proximal braided stream environment during high discharge. A small-scale experiment was carried out to test the efficiency of different sedimentological trap sites in concentrating kimberlite indicator minerals. The results show that the concentration of indicator minerals is dependent on the size fraction chosen, bed roughness and gravel calibre. The examination of surface features on pyrope, picroilmenite and chrome diopside from kimberlite led to the conclusion that most of these minerals are locally derived. Diamond grade variations within the Younger Gravels are influenced by a combination of factors, including bed roughness, channel width and sorting process from the source. Alluvial diamond exploration programmes must take into account the tectonic setting, the palaeoclimate, the level of erosion, the stability of the drainage system in the area as well as the presence of local trap sites in the river profile. It is concluded that the diamonds are the product of a long and complex process of erosion, reworking and concentration and are derived from Cretaceous kimberlites in the area.
- Full Text:
- Date Issued: 1991
The genesis and controls of gold mineralization south of Rehoboth, Namibia
- Authors: Whitfield, Derek
- Date: 1991
- Subjects: Gold ores -- Geology -- Namibia , Gold ores -- Geology -- Africa, Southern
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4948 , http://hdl.handle.net/10962/d1005560 , Gold ores -- Geology -- Namibia , Gold ores -- Geology -- Africa, Southern
- Description: Gold mineralization is hosted within gossanous quartz-haematite veins in volcano-sedimentary lithologies of the Klein Aub - Rehoboth basin of the Irumide Belt, Namibia. Mineralization and hydrothermal alteration are restricted to deformed lithologies particularly the metasediments. Lithological relationships, geochemistry and metallogenic characteristics of the Irumide Belt suggest an intra-continental rift setting. Copper mineralization is well known along the length of the belt, from Klein Aub in the southwest to Ghanzi in the northeast, whereas gold mineralization appears restricted to the Klein Aub Rehoboth basin. The gold is envisaged as having being leached initially from graben fill sequences during rift closure and basin dewatering. Location of the mineralization is strongly controlled by structure and lithological contact zones. Such zones are percieved as having acted as conduit zones for escaping mineralized fluids during basin closure and deformation. Apart from the lack of an effective mineralizing trap, all features consistent with the development of an ore deposit are present. The largest mineralization traps within the area studied are shear zones followed by lithological contact zones. The Mebi and Blanks gold mines are developed over large shear zones while the Swartmodder and Neuras gold mines are situated over mineralized lithological contacts. The Swartmodder copper mine yielded ore from a mineralized schist enclave within granite. Copper and gold occurrences are attributed to two contrasting styles of mineralization. Copper mineralization is suggested to have developed during initial rifting of the belt (ie. stratabound sedimentary exhalative type), while the gold and minor copper resulted from rift closure and basin dewatering. Although no economical orebody was realized during the course of this study a model is proposed for the development of mineralization within the Irumide basement lithologies as a working hypothesis for future exploration.
- Full Text:
- Date Issued: 1991
The genesis of ilmenite-rich heavy mineral deposits in the Bothaville/Delmas area, and an economic analysis of titanium, with particular reference to the Dwarsfontein deposit, Delmas district
- Authors: Brennan, Michael Brendan
- Date: 1991
- Subjects: Heavy minerals -- South Africa -- Transvaal , Titanium , Geology -- South Africa -- Transvaal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4949 , http://hdl.handle.net/10962/d1005561 , Heavy minerals -- South Africa -- Transvaal , Titanium , Geology -- South Africa -- Transvaal
- Description: A number of ilmenite-rich heavy mineral deposits occur along the northern margin of the intracratonic Karoo basin, and are hosted by the fluvio-deltaic Middle Ecca Group. Coastal reworking of delta front sands within a regressive, microtidal shoreline may be considered as a depositional model for the development of the heavy mineral deposits. An economic analysis of titanium suggests long term positive demand, and sustained high prices for this commodity. An evaluation of the Dwarsfontein ilmenite-rich heavy mineral deposit, using available data, indicates how important it is for deposits of this type to be situated close to an upgrading plant or export harbour.
- Full Text:
- Date Issued: 1991
The geology, petrology and geochemistry of the mineralization and hydrothermal alteration at Ongeama, Ongombo and Matchless West Extension, Namibia
- Authors: Moroni, Marilena
- Date: 1991
- Subjects: Geology -- Namibia , Petrology -- Namibia , Geochemistry -- Namibia
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5045 , http://hdl.handle.net/10962/d1007693 , Geology -- Namibia , Petrology -- Namibia , Geochemistry -- Namibia
- Description: The Matchless Amphibolite Belt (Damara Orogen, Namibia) hosts several volcanogenic-exhalative, sediment-hosted stratiform cupriferous pyrite deposits. These are thought to be related to submarine volcanism during the early evolutionary stages of a narrow Damaran ocean, the Matchless Trough. The mineralized bodies examined (Ongeama, Ongombo and Matchless West Extension) are deformed and metamorphosed to low-medium grade (greenschist-amphibolite facies). They are associated with metapelite and amphibolite country rocks, and crop out as prominent limonite-rich gossans. The elongated shape of the sulphide bodies suggests a structural control. The mineralization normally consists of a variably developed massive sulphide portion, either quartz-, talc- or amphibole-bearing, and a stratigraphically overlying, extensive horizon of sulphide- and baryte-bearing exhalite (magnetite quartzite and less common talc- and actinolite-bearing schists). Lateral and vertical mineralogical changes within the mineralization match with significant variations in the element distribution. A metamorphosed and deformed alteration pipe, indicating the position of the fluid conduit, can be recognized in association with some ore bodies. The formation of quartz-muscovite and chlorite alteration envelopes (Ongeama, Matchless West Extension) and the presence of subtle mineralogical changes (Ongombo) in the immediate wallrocks, accompanied by extensive redistribution, leaching and introduction of elements from outside, suggest the hydrothermal metasomatic origin of the alteration zones. Element zoning within the mineralized bodies can be related to the original position of the vent, possibly coinciding with the intersection of the axis of the alteration pipe with the sulphide body. Cu, Zn, Au (pro parte) and Mo are enriched proximal to the vent, whereas Pb, Ba, Mn, Ag, Au, Sn, Bi and W enrichment characterizes the distal facies of the mineralization. In spite of the obliterating and disrupting effects of the regional dynamo-metamorphism, the element distribution within the mineralization and alteration zones examined is comparable with the geochemical trends observed in present-day mineralizing systems in early- stage oceanic environments (e.g. Guaymas Basin). During exploration for blind volcanogenic mineralization, the detection of hydrothermally altered rocks is fundamental in indicating the proximity to the mineralization. The localization of the alteration zone is also important in the interpretation of the regional geology of the explored area: in deformed terrains the assessment of the stratigraphic position of the alteration zone, relative to the mineralization, helps in establishing the polarity of the sequence.
- Full Text:
- Date Issued: 1991
The nature and origin of gold mineralization in the Tugela valley, Natal Structural and Metamorphic Province
- Authors: De Klerk, Ian Duncan
- Date: 1991
- Subjects: Gold ores -- Geology -- South Africa , Gold mines and mining -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4979 , http://hdl.handle.net/10962/d1005591 , Gold ores -- Geology -- South Africa , Gold mines and mining -- South Africa
- Description: The project area is situated within the Tugela Valley, located in the Northern Marginal Zone of the Natal Structural and Metamorphic Province, and this work outlines the different styles of gold mineralization found in the Tugela Valley. Two different styles have been recognized and both have economic significance:- 1) Epigenetic shear zone-hosted gold occurs in late-stage relatively undeformed thin quartz veins confined to shear zones, and is present in both the greenschist facies Natal Thrust Belt and the amphibolite facies Natal Nappe Complex. However the vast majority of these occurrences are concentrated within the thrust front (i.e. the Natal Thrust Belt). The gold grades (up to 7 g/t) and the hydrothermal alteration assemblages associated with the epigenetic deposits have been documented. 2) An as yet unrecognized occurrence of syngenetic gold mineralization is found associated with the sediment-hosted exhalative massive, to semi-massive, sulphides of the iThuma prospect, located within the amphibolite facies Natal Nappe Complex. Here gold (up to 3 g/t) is concentrated together with the main sulphide are, as well as some gold enrichment (230ppb) in the hydrothermally altered footwall feeder pipe. It is proposed that the epigenetic mineralization was formed as a consequence of the northward directed abduction of the major thrust slices of the Natal Nappe Complex. This increased the permeability of the rocks and provided channelways for the focussing of fluids. Deposition took place at the thrust front where metamorphic hydrothermal fluids interacted with meteoric water.
- Full Text:
- Date Issued: 1991