Vein and replacement type Sn and Sn-W mineralization in the Southern Kaoko Zone, Damara Province, South West Africa/Namibia
- Authors: Petzel, V
- Date: 1986
- Subjects: Geology, Structural -- Namibia -- Damara Mobile Belt , Geology, Structural -- Namibia -- Kaoko Belt , Veins (Geology) -- Namibia -- Damara Mobile Belt , Veins (Geology) -- Namibia -- Kaoko Belt , Mineralogy -- Namibia -- Damara Mobile Belt , Mineralogy -- Namibia -- Kaoko Belt , Geodynamics -- Namibia -- Damara Mobile Belt , Geodynamics -- Namibia -- Kaoko Belt
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5041 , http://hdl.handle.net/10962/d1007633
- Description: The ENE trending Brandberg West - Goantagab Sn-W belt is located in the Southern Kaoko Zone of the northern coastal branch of the Damara Orogen. The lithologies in this area are turbiditic and consist of three schist units separated by two marble horizons, all of which are correlated with the Swakop Group. The formations are intensely folded by at least three episodes of which the first two are coaxial and resulted in prominent, approximately N-S trending, structures. Sn and Sn-W mineralization predominantly occurs as vein and replacement type mineralization. Vein type mineralization occurs as Brandberg West, Frans Prospect, Gamigab Prospect and the Goantagab Mining Area. The vein type mineralization is accompanied by intense alteration, consisting of greisenization, sericitization, hematitization and carbonatization. Replacement-type, hematite-cassiterite mineralization, occurs in the Goantagab Mining area in the marble close to, or at the schist marble contact. Intense ferruginous alteration of the marbles in this area, is associated with veins, which terminate against, or cross cut the marble. A regional metal zonation, ranging from Sn-W mineralization with minor sulphides at Brandberg West to Sn-sulphide mineralization at Goantagab can be detected. This metal zonation is attributed to the distance of the mineral locality from the source area, with Goantagab representing a distal and Brandberg West a proximal position relative to the source area. Structural, mineralogical and geological features of the mineralization in this area suggest that processes of ore genesis may be related to anorogenic magmatism of Karoo age.
- Full Text:
- Date Issued: 1986
Turbidite-hosted gold deposits
- Authors: Leeming, Prudence Mary
- Date: 1985
- Subjects: Gold ores , Turbidites
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5010 , http://hdl.handle.net/10962/d1005939 , Gold ores , Turbidites
- Description: Turbidite-hosted gold deposits contribute a significant proportion to world lode gold production and have also provided substantial gold to alluvial resources. Turbidity current deposits occur throughout geological time within Archaean greenstone belts, Proterozoic orogenic belts and rifted passive continental margins, and Palaeozoic geosynclines. Representing the end member of the sedimentary cycle, turbidites have the attribute of preservation not only on an individual bed basis but also due to below wave base accumulation in submarine deeps. Cyclic deposition according to the Bouma sequence punctuates turbidite deposition by a series of diastems. Accumulation of organic, pelagic and chemical sediments may concentrate gold to protore enrichment levels i n a primary sedimentary environment. Dewatering during diagenesis and low-grade metamorphism under reducing conditions may redistribute gold with transport as low energy organo- and thio-complexes. Gold may precipitate with diagenetic pyrite and silica near black shale and/or partially replace fine carbonate detritus. Gold solubility increases with high grade amphibolite facies metamorphism (T 400ºC) when efficient leaching of gold and transport by simple chloro- and hydroxychloro - complexes to lower greenschist regions takes place. Reduced permeability of turbidite strata induces hydrofracturing which focuses dewatering solutions. Gold is deposited due to pressure and temperature decrease or local changes in physico - chemico conditions caused by the reaction of fluids with wall rocks (reactive beds in turbidites are predominantly carbonaceous strata). The largest of turbidite - hosted goldfields are confined to back -arc or marginal sea basins with restricted oceanic circulation. The richest concentrations of gold occur proximal to the original source within the greenschist facies formations lowermost in a thick turbidite sequence and exhibit strong combined structural and lithological association. Turbidites represent important strata for the concentration and preservation of gold not only during sedimentation and diagenesis but also during later deformation and metamorphism.
- Full Text:
- Date Issued: 1985
Trace element and sulphur isotope variations of sulphides in the Koperberg Suite, O’okiep Copper District, Namaqualand, South Africa: implications for formation of sulphides and the role of crustal sulphur assimilation
- Authors: Marima, Edmore
- Date: 2022-04-06
- Subjects: Sulfur Isotopes , Magmatism South Africa Namaqualand , Sulfides , Koperberg Suite (South Africa) , Copper sulfide , Sulfur Absorption and adsorption
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/291117 , vital:56820
- Description: The major economic copper sulphide deposits hosted in the late Mesoproterozoic intrusions of the Koperberg Suite in the O’okiep Copper District immediately overlie sulphur-bearing paragneisses of the Khurisberg Subgroup in an otherwise low-sulphur granitic basement. The dominant sulphide assemblage (chalcopyrite and bornite) hosted in the Koperberg Suite is also atypical of the intermediate solid solution (iss) assemblage (chalcopyrite and pyrrhotite) observed in most Cu-Ni magmatic sulphide deposits. This study presents sulphur isotope and in-situ trace element analysis of sulphides from the Koperberg Suite and the Khurisberg Subgroup with the view of placing constraints on the role of sulphide-bearing supracrustal metasedimentary of the Khurisberg Subgroup as a source of additional sulphur in the genesis of these deposits, and ore-forming (sulphide formation) processes which result in trace element variations registered by sulphides hosted in the Koperberg Suite. The high concentrations (up to 2100 ppm) of monosulphide solid solution (mss)-incompatible trace elements (e.g., Te, Se, Bi, Ag, Pb), and the depletion in Ni and Co (<40 ppm) of sulphides hosted in the Koperberg Suite are instead consistent with the derivation of such sulphides from a Cu-rich sulphide melt which segregated from a Ni-rich sulphide melt prior to magma emplacement in the middle crust, in agreement with one of the petrogenetic models for the Koperberg Suite proposed in the existing literature. The low S/Se ratios ( ̴650-10300) of sulphides hosted in the Koperberg Suite and the high S/Se ratios ( ̴18800-56000) registered by the main sulphide phase (pyrite) in the Khurisberg Subgroup argues against crustal contamination of the Koperberg Suite magmas by the Khurisberg Subgroup. The S/Se and Cu/S ratios of coexisting bornite and chalcopyrite hosted in the Koperberg Suite are positively correlated with the bornite modal abundance in the Koperberg Suite. Such trends are interpreted to be consistent with progressive oxidation of sulphide melt, a process which results in the crystallisation of iss-bornite assemblage and/or replacement of iss with bornite due to the enrichment of Cu and depletion in S of the sulphide melt. The oxidation of sulphide melt is likely to have been effectuated by the fractional crystallisation of mss in a prior sulphide melt segregation event and/or the fractional crystallisation of Fe2+-dominated silicate phases. Fractionation of the Cu-rich melt sulphide melt (segregated from mss) also tends to enrich the residual sulphide melts in Se. Thus, the chalcopyrite-dominated assemblage with S/Se ratios of ̴1300-10200 observed in the less basic rocks in the Koperberg Suite (leucodiorites and leuconorites) is interpreted to have formed from the least evolved sulphide melt, whereas the bornite-dominated assemblage with S/Se ratios of ̴650-5500 observed in the more mafic members of the Koperberg Suite (orthopyroxenites and norites) is interpreted to have formed from the most evolved sulphide melt. The ẟ34S isotopic signatures in sulphides of the Koperberg Suite (-1.4 to +1.91‰) and the proposed contaminant, the Khurisberg Subgroup (-1.2 to +3.5‰), overlap with the those of the Koperberg Suite below the Khurisberg Subgroup (+0.74‰) and typical mantle-derived magmatic rocks (0 ± iv 2‰). Hence, the sulphur isotope variations are inconclusive as an indicator of possible crustal sulphur assimilation into the intruding mantle magma. However, considering the trace element systematics and the sulphur isotope data, the Koperberg magmas likely attained sulphur saturation at deeper crustal levels. , Thesis (MSc) -- Faculty of Science, Geology, 2022
- Full Text:
- Date Issued: 2022-04-06
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.
- Full Text:
- Date Issued: 1987
The tectonic framework of magmatism and mineralization in the western United States
- Authors: Empsall, J S
- Date: 1982 , 2013-04-03
- Subjects: Plate tectonics -- United States , Magmatism -- United States , Mineralogy -- United States
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5019 , http://hdl.handle.net/10962/d1006328 , Plate tectonics -- United States , Magmatism -- United States , Mineralogy -- United States
- Description: KMBT_363 , Adobe Acrobat 9.53 Paper Capture Plug-in
- Full Text:
- Date Issued: 1982
The tectonic evolution of the Cape Fold Belt: constraints from fluid inclusion characteristics in syntectonic quartz veins
- Authors: Proctor, Briony
- Date: 2017
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/8019 , vital:21335
- Description: Syn-tectonic quartz veins formed along faults, folds and tension gashes in rocks of the Cape Supergroup (CSG) of the central Cape Fold Belt (CFB) comprise mainly hydrous saline fluids. These veins may also contain CO₂ Syn-tectonic quartz veins formed along faults, folds and tension gashes in rocks of the Cape Supergroup CO₂ , or CH4 and CO₂, or neither CO₂ nor CH4. The majority of inclusions are two-phase and fluid rich, and the most common fluid composition is H2O-NaCl. The final melting temperature, and therefore salinity, differs very little across different structures (fluids in all structures show maximum salinities between 2.5 and 6 wt% NaCl equivalent). Thrusts, reverse faults, strike- and oblique-slip faults, and folds all have similar homogenization temperatures (Th). Primary H2O-NaCl inclusions show Th between ~130 and 200 °C, and H2O-NaCl-CO₂ inclusions have slightly higher Th, between ~140 and 240 °C. Secondary inclusions in all structures have a similar Th range to primary inclusions, but have a lower maximum Th (~130-180 °C). Inclusions containing CH4 have the highest Th (~210 - 300 °C). Microthermobarometry indicates that fluids associated with contractional structures, such as thrust faults or folds, from the Ordovician lower Table Mountain Group (TMG) show lower greenschist facies trapping conditions (~170-175 MPa and ~240-300 °C). These veins also show a plastic deformation overprint (recrystallization of quartz and foam textures), at temperatures higher than the trapping conditions (~ ≥300 °C), indicating that temperatures increased subsequent to hydraulic fracturing, quartz precipitation and thrust slip. These structures formed on a prograde path, at approximately 335 Ma, at a time when the overlying CSG rock column was approximately 6800 m thick. This event pre-dated the thermal peak of the Cape Orogeny at ~276-261 Ma by ~60 million years. Further up in the stratigraphy of the CFB, in the Devonian upper Bokkeveld Group, fluid inclusions in quartz veins associated with a thrust fault show similar trapping pressure (~200 MPa) to the structures in the lower CFB. At 335 Ma, the stratigraphic overburden on this sample locality would not have been sufficiently thick to exert the calculated pressure. This fault may have formed at a later time. The observed pressure of ~200 MPa may have been created either by the higher Bokkeveld Group, the entire Witteberg Group, and further CSG rocks that were eroded prior to the deposition of the Permo-Triassic Karoo Supergroup, or by tectonic thickening of the CSG by prograde thrusting. Still further up in the CSG, fluids from a fold sample from the Witteberg Group record quartz precipitation at lower greenschist facies conditions and subsequent plastic deformation during folding. The formation of this fold postdates the thrusting in the lower TMG, and may correlate in time with deformation during the thermal peak in Middle Permian time (~276-261 Ma).
- Full Text:
- Date Issued: 2017
The surface geology of the Lavino Chrome Mine of the farm Grootboom 336KT, eastern Transvaal
- Authors: Tinney, Christopher Bruce
- Date: 1992
- Subjects: Chromium ores -- Geology -- South Africa -- Mpumalanga
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5067 , http://hdl.handle.net/10962/d1013404
- Description: A mapping project of the surface geology of the Lavino chrome mine and its surroundings was initiated in order to establish the surface geological relationships in the area. In so doing the chromitite layer presently being mined has been identified and potential exploration targets in the area have been outlined. The Lavino Chrome mine field area is situated within the eastern lobe of the Bushveld Igneous Complex. The area is bounded by in the north by the Steelpoort Lineament, in the west by the Dwars River fault and in the east by the contact with the Transvaal Sequence floor rocks. Layered igneous rocks (pyroxenites, norites and anorthosites) of the Rustenburg Layered Suite dominate the geological landscape at the Lavino mine. The fact that outcropping igneous rocks of the Critical Zone abut directly against the quartzite floor rocks on the mine property makes this area unique in the Bushveld Complex. The hills in the field area are capped by mafic/ultramafic iron-rich sheet - like bodies. Extensive strike-slip faulting is seen in outcrop in the area to the north/northwest of present mining operations. On the basis of field relationships, the main chromitite layer presently being mined at Lavino is identified as the Middle Group chromitite layer MG 1. Three other prominent chromitite layers stratigraphically associated with MG 1 are identified as the Middle Group chromitites MG 2, MG 3 and MG 4. Several other less prominent outcropping chromitite layers are tentatively identified as those belonging to the Lower and Upper group of chromitites. The disconformable nature of the contact between the layered igneous rocks and the Transvaal Sequence floor rocks has resulted in the development of a wedge of undifferentiated pyroxenites in the north of the field area. The economically important LG 6 chromitite layer may be developed in subcrop within this wedge.
- Full Text:
- Date Issued: 1992
The stratigraphy and structure of the Kommadagga subgroup and contiguous rocks|
- Authors: Swart, Roger
- Date: 1982
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4919 , http://hdl.handle.net/10962/d1004378
- Description: The Lake Mentz and Kommadagga Subgroups were deposited in a marine environment and are characterised by a heterogeneous sequence of sediments, which range in grain size from clays to grits. During the first phase of deposition the Kweekvlei Shale and Floriskraal Formations were deposited in a prograding shoreline environment, whereas the succeeding Waaipoort Shale Formation is interpreted as represnting a reworked shoreline . The final phase of deposition of the Cape Supergroup was a regressive one in which the Kommadagga Subgroup was formed. The coarsening upward cycle of this subgroup represents a deltaic deposit. A significant time gap appears to exist before the deposition of the glacial-marine Dwyka Tillite Formation. Structurally, the area was subjected to deformation by buckle folding at about 250 Ma into a series of folds with southward dipping axial planes. Only one phase of deformation is recognised in the study area . A decrease in pore space, mineral overgrowths,formation of silica and calcite cements and development of autigenic minerals such as opal, stilpnomelane, analcite, prehnite, muscovite and various clay minerals are the characteristic diagenetic features of the sediments. The mineralogical evidence suggests that the maximum temperature and pressure of burial was 150 C and 4 to 5 Kbar respectively.
- Full Text:
- Date Issued: 1982
The Springfontein prospect : a case study of a tertiary age epithermal hot spring deposit in the Eastern Cape
- Authors: Nichols, Richard Lionel
- Date: 1991
- Subjects: Hot springs -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5058 , http://hdl.handle.net/10962/d1012963
- Description: The Springfontein Prospect is a Tertiary aged epithermal Hot Spring deposit that was investigated for precious metal mineralisation. It is located some 14 kilometres due west of East London (Cape Province), within Beaufort Group sediments and Karoo dolerite. Prior to the work described in this case study, the location was known for it's abundant plant fossils and barite mineralisation. A gossan sample collected near the main barite vein returned 1,07 g/t Au and 26,6 g/t Ag. Remote sensing studies of the site revealed a set of north-south lineaments paralleling the barite vein system. Geophysical surveys confirmed this orientation and revealed an extensive alteration system underlying the prominent sinter terrace. A detailed soil geochemical survey returned elevated values in the classical epithermal gold mineralisation element suite (Rg, Tl, As, Bi, Sb, Te, Mo, Ba and Pb - Bonham, 1986). Trenching of geochemical anomalies revealed zones of intense argillic alteration and vein stockworks). Four percussion and three diamond drill holes intersected a 'feeder-fissure' system of veins, alteration and brecciation, but failed to repeat gold levels seen at surface. Mineralogical and petrographic studies of the cores determined temperatures of formation of important indicator minerals (e.g. adularia and zeolites). Litho-geochemical work revealed mercury (and thallium) to be most elevated in the feeder systems. Stratigraphic and paleontological observations determined that the Springfontein Tertiary deposit was clearly different to the other siliceous (silcrete) units that crop out in the Eastern Cape region. A number of distinguished visitors to the prospect, with epithermal deposit experience, confirmed that the characteristics and dimensions of the system is within those of mineralised deposits elsewhere. A brief review of current epithermal models are presented. The conceptual geological model for the Springfontein prospect evolved through the exploration programme. The final consensus is that it best fits Bonham's (1986) alkalic model and the Tertiary epithermal event was sustained by rifting associated with the break-up of Gondwanaland.
- Full Text:
- Date Issued: 1991
The significance of unconformities in the development of Witwatersrand gold and uranium placers
- Authors: Beater, Christian Douglas
- Date: 1983 , 2013-04-03
- Subjects: Placer deposits , Gold ores -- Geology -- South Africa -- Witwatersrand , Uranium ores -- Geology -- South Africa -- Witwatersrand
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4925 , http://hdl.handle.net/10962/d1004535 , Placer deposits , Gold ores -- Geology -- South Africa -- Witwatersrand , Uranium ores -- Geology -- South Africa -- Witwatersrand
- Description: Most of the economic gold and uranium placers are developed on low angle disconformities in the Central Rand Group and concentrations of gold and uranium are usually at their optimum on unconformity surfaces. Examples include the Kimberley Reef and South Reef of the East Rand, the Main Reef Leader of the Central Rand, the Carbon Leader of the Carletonville goldfield, the Vaal Reef of the Klerksdorp goldfield and the Basal/Steyn placers of the Welkom goldfield. The individual goldfields represent fluvial fans which are composed of a large number of tectonogenetic sedimentary packages separated by unconformities. The tectonic responses between cycles of sedimentation produced unconformities and tectonically controlled cyclic sedimentation is one of the key factors culminating in the preparation and deposition of auriferous placers within the Witwatersrand succession. Unconformities, which represent breaks in sedimentation, result in the preconditioning of palaeosurfaces and redistribution of sediments and heavy minerals on them. Winnowing of sands produced heavy mineral residual accumulations on erosion surfaces which were generally preserved by small-pebble lags or algal mats. Reworking of units truncated by the unconformities provided additional gold, uranium and heavy minerals to unconformity surfaces.
- Full Text:
- Date Issued: 1983
The sedimentology of the Zerrissene turbidite system, Damara Orogen, Namibia
- Authors: Swart, Roger
- Date: 1991
- Subjects: Sedimentology Sediments (Geology) -- Namibia Sedimentation and deposition Turbidites
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5000 , http://hdl.handle.net/10962/d1005613
- Description: The Zerrissene turbidite system of central-western Namibia is a late Proterozoic sequence which consists of dominantly siliciclastic turbidites interbedded with minor turbiditic and hemipelagic marbles. The basin in which these sediments were deposited is located at the junction of the coastal and intra-cratonic arms of the Pan-African Damara Orogen, and an understanding of the sedimentary evolution of this basin is therefore important to the understanding of the development of the orogen as a whole. One major and two minor phases of folding have deformed the sediments, but the grade of metamorphism is low and sedimentary structures are often well preserved. Further, the area lies entirely within the Namib Desert and the lack of vegetation cover results in good outcrops providing an unusual opportunity for examining a large Precambrian turbidite system. The system consists of five formations: three siliciclastic and two mixed carbonatesiliciclastic units. The floor of the system is not exposed, and the oldest sedimentary rocks which outcrop are siliciclastics of the Zebrapiits Formation. This is overlain successively by the Brandberg West Formation (dominantly calcareous), the Brak River Formation (siliciclastic), the Gemsbok River Formation (calcareous) and the Amis River Formation (siliciclastic). Nine silicilastic turbidite facies have been recognised in the basin. These are facies A₂ (disorganised onglomerates), B₁ (horizontally laminated to massive greyackes), C₂ ("classical" turbidites), Dl (sandstone-shale couplets with base cut-out Bouma sequences), D₂ (sandstone-shale couplets with less sand than shale and base cut-out Bouma sequences), E (coarse, discontinuous sandstone-shale couplets), F (slumped units), G (shale) and H (glacial dropstones). Four facies are associated with the carbonate horizons, and these carbonate facies are given the suffix c to distinguish them from similar siliciclastic facies. These are facies Ac (disorganised and graded marble breccias), facies Cc (graded carbonates), facies Gc (hemi-pelagic marbles) and facies G (pelagic shales). The basal Zebrapiits Formation is made up of relatively thin packages of thin- to thickbedded, laterally continuous facies D₁, D₂ and B₁ beds encased in thick envelopes of shale. This type of sequence is typical of a distal lobe-fringe, and requires an unconfined basin-floor on which it can develop. The overlying Brandberg West Formation consists of a basal portion of interbedded facies Cc and G, followed by a sequence dominanted by facies Gc. This sequence is interpreted as representing outer-apron carbonate turbidites, derived from multiple point sources (facies Cc), with background pelagic settling (facies G) overlain by hemi-pelagic deposits (facies Gc). A reversal back to siliciclastic turbidites followed with deposition of the Brak River Formation. This sequence comprises relatively thick packages of laterally continuous facies B₁, D₁, and D₂ beds sandwiched between facies G shales, a succession characteristic of a lobe to lobe-fringe environment with intermittent abandonment of lobes. An unconfined basin floor adjacent to a passive margin is required for the development of this type of sequence. Glacial dropstones (facies H) are found in the upper portions of this formation, and slumped beds are also present (facies F), but are uncommon. The facies F beds are only found in association with facies H and are therefore considered to be genetically related. Slumping of beds was possibly caused by an oversupply of sediment from ice-rafting which caused instability. The overlying Gemsbok River Formation has a sequence similar to the Brandberg West Formation in that the basal portion consists of interbedded facies Cc and G, which is overlain by a thick sequence of largely facies Gc beds. Minor facies Ac beds occur near the top of the overall sequence. This formation is interpreted as an outer-apron succession with the facies Ac beds representing distal inner-apron deposits, indicating progradation of the system. The youngest unit in the basin, the Amis River Formation, shows strong lateral variation from west to east. In the west the sequence comprises laterally continuous facies B₁, C₂, D₁ and D₂ with rare, discontinuous facies E beds. Facies G is relatively minor in the sequence. In the east the succession is dominated by facies D₁, D₂ and G, and this succession is interpreted as a sequence of distal turbidites which were deposited on a basin-plain. The system developed by aggradation rather than progradation as only minor cycles are developed. Geochemical and petrological features indicate that the entire siliciclastic system was derived from a granite-recycled orogen terrane. Palaeocurrent data are unreliable because of the deformation, but transport was initially from the south-west, moving later to the west and north-west. The provenance of the carbonates is uncertain as reliable palaeocurrent indicators are rare, but they could have been derived either from South America or from the extensive carbonate deposits developed on the north-western margins of the basin. The Zenissene siliciclastic turbidite system represents the distal portion of a major submarine turbidite system, the more proximal parts of which now lie west of the exposed basin, either under the Atlantic Ocean or in eastern South America. The calcareous deposits developed as an apron adjacent to a multiple point source, the position of which is at present unknown.
- Full Text:
- Date Issued: 1991
The sedimentology and depositional model of VS5 reef at Beatrix mine and surrounding areas of the Freestate Goldfield, South Africa
- Authors: Shivambu, Steven
- Date: 2017
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4942 , vital:20745
- Description: Historically, placers of economic importance at Sibanye Gold's Beatrix No. 3 Shaft were Beatrix Reef, Aandenk Reef and Composite of the two reefs. Recently, the VS5 placer has emerged to be a significant contributor of ore tonnages mined on the Shaft as mining advances into deeper levels towards the northern portion of the mine lease area. Gold concentration in the VS5 Reef is highly variable from uneconomic, through marginal to economic gold values. A good knowledge of the sedimentological framework of the VS5 Reef was considered necessary in order to interpret the variable distribution of the gold within the VS5 placer. The aim of this study was to determine the depositional environments that played a major role during the formation of the VS5 placer by means of investigating the macroscopic sedimentological parameters of this particular reef. These sedimentological parameters were used as the basis for the subdivision of VS5 Reef into different facies recognized in the mine and surrounding areas. It was recognized that the VS5 placer was formed in a distal, braided fluvial environment by the reworking of the pre-existing Aandenk Reef by new materials containing significant amount of nondurable materials (see definition on page xii) such as shale detritus. Where there is no evidence of reworking of the older Aandenk Reef, the VS5 Reef occurs as poorly sorted, polymictic conglomerate with abundance of non-durable detritus. This is referred to as the Immature (IV) VS5 facies and occurs in the northern portion of the study area. The degree of reworking of the gravel bars by waves and current action resulted in the formation of well sorted, oligomictic conglomerates of the Beatrix facies in the southern portion of the mine. At the boundaries between the Immature VS5 and Beatrix facies occurs the Transitional (Sub-mature) VS5 facies, characterized by reef comprising a polymictic upper portion and a basal more mature oligomictic unit. The Transitional VS5 facies extends across the current central northern mining faces of Beatrix No. 3 Shaft with a northwest-southeast trend. There is a strong correlation between the VS5 lithofacies and distribution of gold mineralization. The Immature VS5 facies is poorly mineralized, with gold values averaging 200 c.m.g/t and lower. The Transitional (Sub-mature) VS5 facies has elevated gold values, ranging from 300 c.m.g/t to 1500 cm.g/t. Mineralization in this unit tend to be bottom loaded as well as at the base of each cyclic unit. The Beatrix facies records the highest grades averaging >1500 cmg/t. The improved understanding of the VS5 lithofacies made it possible to predict gold mineralization and aid planning to mine into viable VS5 areas.
- Full Text:
- Date Issued: 2017
The role of northwest striking structures in controlling highgrade ore shoots at the Syama Gold Mine, Mali, West Africa
- Authors: Soro, Ali
- Date: 2020
- Subjects: Syama Gold Mine , Gold ores -- Geology -- Mali , Veins (Geology) -- Mali
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/145209 , vital:38418
- Description: This study intended to investigate the relationship between the NW striking structures and the high-grade ore shoots at the Syama gold mine in Mali, West Africa. All structural data collected since 1987 from drill core have been integrated to allow the interpretation and modelling of these NW-SE structures. The structures collected were grouped into three main groups; foliations/shears/faults, veins and joints/contacts/fractures. Micromine software was used to plot the structures, printed out on A3 paper and interpreted manually using tracing paper. Analysis and interpretation of stereographic plots has shown that the majority of the high-grade zones are generally located at the intersection of the NNE structures and the NW structures. The observed cross-cutting relationship between the NNE and the NW structures suggests two different generation of faults. It is suggested that the NW structures were active during the D4 deformation event (Standing, 2007) and have played a role in reactivating earlier (D3) NNE structures, allowing greater fluid flow and enhancing the gold grade. These zones are mainly defined by brecciation and stockwork veining. The E-W structures are believed to be the latest and are attributed to the D5 event. Although gold mineralisation is grossly controlled by the NNE structures, the NW structures need to be considered as major gold enrichment upgrading factors at Syama. It is therefore strongly recommended that ongoing exploration at Syama specifically target the intersection of the NW and NNE structures as favourable zones for high-grade mineralisation.
- Full Text:
- Date Issued: 2020
The Precambrian metallogeny of Kwazulu-Natal
- Authors: Hira, Hethendra Gangaram
- Date: 1998
- Subjects: Metallogeny -- South Africa , Metallogeny -- South Africa -- KwaZulu-Natal , Geology, Stratigraphic -- Precambrian
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4993 , http://hdl.handle.net/10962/d1005605 , Metallogeny -- South Africa , Metallogeny -- South Africa -- KwaZulu-Natal , Geology, Stratigraphic -- Precambrian
- Description: The Precambrian rocks of KwaZulu-Natal comprise the Archaean granite-greenstone remnants of . the Kaapvaal craton and Late Archaean volcanics and sediments of the supracratonic Pongola Supergroup. These Archaean rocks have been intruded by numerous mafic/ultramafic complexes and voluminous granitoid intrusives of various ages. To the south, the basement rocks are represented by the Mid- to Late-Proterozoic Natal Metamorphic Province (NMP). The NMP comprises three discontinuity-bound tectonostratigraphic terranes. These are, from north to south, the Tugela, Mzumbe and Margate Terranes. The Tugela Terrane has been interpreted as an ophiolite suite that was thrust northwards onto the stable Archaean craton as four nappe structures. Continued thrusting resulted in the two southern terranes being thrust northwards over each other, resulting in numerous sinistral transcurrent shear zones and mylonite belts. The greenschist facies Tugela terrane has been intruded by mafic-ultramafic complexes, alpine serpentinites, plagiogranites and a number of alkaline to peralkaline granitoids. The Mzumbe and Margate Terranes comprise arc-related, felsic to mafic supracrustal gneisses and metasediments that were intruded by syn-, late- and post-tectonic granitoids. Mineralisation in the granite-greenstones consists of structurally-hosted lode-gold deposits. These deposits have many characteristics in common with lode-gold deposits found in other granitegreenstone terranes throughout the world. The Nondweni greenstones also contain volcanogenicrelated massive sulphide deposits. The Pongola Supergroup is host to lode-gold mineralisation and placer gold mineralisation. These placer deposits have been correlated with deposits found in the similarly-aged Witwatersrand Basin in an adjacent part of the craton. The metallogeny of the NMP can be described in relation to the various stages in the tectonic evolution of the belt. The initial, rifting and extension-related stage was characterised by arcrelated magmatism and volcanic arc activity. Alkali basalt magmatism due to hot-spot activity in the oceanic basin in which the Tugela Terrane initially accumulated, produced magmatic segregation deposits, while volcanic-arc activity is responsible for the submarine-exhalative massive sulphide mineralisation. All the mineralisation within the NMP is structurally-related. These thrusts and shear zones were developed during obduction and thrusting during the NMP event, and created the paths necessary for the migration of mineralising fluids. Alpine-type ophiolite deposits were also emplaced along these zones. Epigenetic, shear zone-hosted gold mineralisation occurs in the Tugela and Mzumbe Terranes. Mineralisation occurs within quartz veins and is also disseminated within the sheared host-rocks. The Mzumbe Terrane also contains small showings of massive sulphide deposits that were related to volcanogenic exhalative processes during the formation of this terrane. Potential for finding further mineralisation of this type appears to be good. The massive sulphide deposits formed early in the evolution of the belt, and were deformed and metamorphosed during the later accretionary processes. The southernmost Margate Terrane is characterised by a lack of metalliferous mineralisation, but hosts the extensive, and economically important, limestone deposits of the Marble Delta. The recently discovered spodumene-rich pegmatite deposits of this terrane may also be considered for exploitation. Post-collisional magmatism and metamorphism resulted in extensive rapakivi-type granite/charnockite plutons
- Full Text:
- Date Issued: 1998
The precambrian iron-formations in the Limpopo belt as represented by the magnetite quartzite deposits at Moonlight, Koedoesrand area, Northern Transvaal
- Authors: Badenhorst, Jaco Cornelis
- Date: 1991 , 2013-02-20
- Subjects: Iron ores -- Geology -- South Africa , Quartzite -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5064 , http://hdl.handle.net/10962/d1013309
- Description: This dissertation is based largely on data that was accumulated during the execution of an exploration program by Iscor Ltd in the Northern Transvaal. The program included geological mapping, geophysical surveys and drilling, on Precambrian iron-formations in the Central Zone of the Limpopo Belt. The structure, stratigraphy, metamorphism, and economic importance of the magnetite quartzites and associated lithologies of the Moonlight prospect are discussed. The lithologies underlying the Moonlight prospect area consist of various pink- and grey-banded gneisses and pink granulite, together with a variety of metasedimentary supracrustal rock-types and concordant serpentinite bodies. The gneissic rock-types consist of chlorite-quartz-feldspar gneiss, chlorite-quartz-feldspar augen gneiss, hornblende-quartz-feldspar gneiss, biotite-quartz-feldspar gneiss, felsic and mafic granulite, and foliated amphibolite. The metasedimentary lithologies are represented by calc-silicates and marble, white quartz-feldspar granulite, magnetite quartzite, metaquartzite and garnet-bearing granulite and gneiss (metapelites). The concordant ultramafic bodies consist of serpentinite with lesser amphibolite, dunite, and chromitite. Intrusive pegmatites and diabase dykes are also present in the prospect area. Metamorphism reached granulite-facies, and more than one retrqgrade metamorphic event is recognized . Amphibolite-facies assemblages are present, but it is uncertain whether they represent another retrograde event . Polyphase deformation has produced intense and complex folding , resulting in irregular magnetite quartzite orebodies. The high metamorphic grades have resulted in medium- grained recrystallization of the magnetite-quartzites with a loss of prominent banding often associated with these rock-types . The magnetite quartzite occurs as three seperate but related ore zones, consisting of one or more ore-bands seperated by other lithologies. All three zones form poor outcrops and suboutcrops in a generally flat lying and sand covered area. · Although representing a low-grade iron ore (32% total Fe), the magnetite quartzite deposits at Moonlight are regarded as potentially viable due to the large opencast tonnages available at low stripping ratios, and the relatively cheap and easy beneficiation process needed to produce a magnetite concentrate with 69-70% total Fe.
- Full Text:
- Date Issued: 1991
The porphyry copper system and the precious metal-gold potential
- Authors: Gendall, Ian Richard
- Date: 1994
- Subjects: Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4992 , http://hdl.handle.net/10962/d1005604 , Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Description: It has been established that porphyry copper/copper-gold deposits have formed from I Ma to 2 Ga ago. Generally, they are related to the Mesozoic-Cenozoic interval with few reported occurrences from the Palaeozoic or Precambrian. A reason cited is the erosion of these deposits which are often related to convergent plate margins and orogenic belts. Observations of the alteration and mineralisation within and around porphyry copper/copper-gold systems have been included in numerous idealised models. These alteration and mineralisation patterns are dependent on the phases of intrusion, the tectonic setting and rock type, depth of emplacement and relationship to coeval volcanics, physiochemical conditions operative within and surrounding the intrusive and many other mechanical and geochemical conditions. Island arc and cratonic arc/margin deposits are generally considered to be richer in gold than their molybdenum-rich, intra-cratonic counterparts. Metal zonation may occur around these copper/copper-gold deposits, e.g. copper in the core moving out to silver, lead, zinc and gold. This zonation is not always present and gold may occur in the core, intermediate or distal zones. Examples of gold-rich porphyry deposits from British Columbia, Chile and the SW Pacific Island regions suggest gold is closely associated with the potassic-rich zones. Generally these gold-rich zones have greater than 2% magnetite and a high oxygen fugacity is considered to be an important control for gold deposition. High Cl contents within the magma are necessary for gold mobility within the host intrusive centres. Beyond this zone HS₂ becomes an important transporting ligand. Exploration for porphyry copper-gold deposits includes an integrated geological, geophysical and geochemical approach. Petrographic work through to Landsat imagery may be used to determine the chemical conditions of the system, ore association, favourable structural zones and alteration patterns, in order to focus exploration activities.
- Full Text:
- Date Issued: 1994
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
The petrology of the Merensky cyclic unit and associated rocks and their significance in the evolution of the Western Bushveld Complex
- Authors: Kruger, Floris Johan
- Date: 1984
- Subjects: Petrology -- Africa, Southern Petrofabric analysis
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5006 , http://hdl.handle.net/10962/d1005636
- Description: A brief review of the various models proposed to account for the Bushveld Complex shows that there are two main hypotheses. These are the Multiple Intrusion hypothesis and the In Situ Crystallization hypothesis. The latter also allows for multiple additions to the crystallizing magma, and several variants involving the number of these inputs , their composition, volume and timing have been proposed. To facilitate description and investigation of the study section, the stratigraphic nomenclature of this part of the Rustenberg Layered Suite is revised and clarified. It is proposed that the boundary between the Critical Zone and Main Zone be placed at the base of the Merensky cyclic unit, and thus the whole of the Merensky and Bastard cyclic units are included in the Main Zone. Furthermore, the extremely confused terminology for smaller units within the Merensky and Bastard cyclic units is resolved by discarding the term Reef as a formal term and substituting lithological terms such as Merensky pegmatoid, Merensky pyroxenite, Bastard pyroxenite and Merensky mottled anorthosite etc. It is recommended that the term Reef be retained as an informal term to designate the mineralized horizon which may be mined, regardless of lithology. The term "pegmatoid" is restricted to stratiform or lensoid masses of coarse grained feldspathic pyroxenite or harzburgite which are part of the layered sequence. The transgressive vertical pipe-like, coarse-grained ultramafic "iron-rich bodies are termed "ultramafic pegmatites ". The main features of the Merensky and Bastard cyclic units are the regular chemical and mineralogical changes that occur with respect to stratigraphic height in these units. In the Merensky cyclic unit there is a smooth iron enrichment in the orthopyroxenes upward in the succession and a transition from pyroxenite at the base to mottled anorthosite at the top of the unit. The Bastard cyclic unit is broadly similar to the Merensky cyclic unit. A variety of textures and chemical features are in disequilibrium in some samples but not in others, and great complexity is evident wh en individual samples are studied in detail. The initial ⁸⁷Sr/⁸⁶Sr ratios of plagioclase separates and whole rocks from the study section show a distinct step-like increase in the Merensky cyclic unit to .70806 at the base of the, Merensky cyclic unit to .70806 at the base of the Bastard cyclic unit. In contrast , samples from below the Merensky cyclic unit have a constant initial Sr-isotopic ratio, as do the samples from the Bastard cyclic unit. These isotopic and chemical data, and available published geologic relationships suggest that a major new influx of basic magma occurred after the Footwall unit was deposited and that this mixed with the residual magma in the chamber and then precipitated the Merensky and Bastard cyclic units. The crystal settling theory as outlined by Wager and Brown (1968) fails to account for the chemical and stratigraphic variations observed in the study section. The theory of bottom crystallization, initially proposed by Jackson (1961), more adequately explains the features observed. Applying a model outlined by Irvine (1980a & b), it has been established from chemical data, that the Merensky cyclic unit crystallized from a magma layer with a thickness roughly equivalent to the average thickness of the cyclic unit itself (±10m). A similar exercise on the Bastard unit was not possible. The formation of the Footwall unit is still enigmatic. Infiltration metasomatism and sintering can modify the petrographic and chemical characteristics of rocks and minerals after deposition at the liquidus stage. During the solidification of the crystal mush a separate vapour phase may form in the crystal mush, which could move up through the crystal pile. This process may ultimately be responsible for the generation of potholes and pegmatoidal horizons, such as the Merensky pegmatoid. The upward increase in the initial ⁸⁷Sr/⁸⁶Sr ratio within the Merensky cyclic unit is strong evidence that infiltration metasomatism has played an important part in the generation of the Merensky cyclic unit. This process, coupled with fluid enrichment, may also result in the formation of pegmatoid layers. Sintering appears to have been a common process in the mottled anorthosites of the study section and may have severely reduced the amount of trapped interstitial liquid in these rocks.
- Full Text:
- Date Issued: 1984
The petrology of the Khale dolerite sheet
- Authors: Eales, Hugh V
- Date: 1953
- Subjects: Petrology -- Botswana , Diabase -- Botswana
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5076 , http://hdl.handle.net/10962/d1014853
- Description: In the hills and in the plain of the Khale area are exposed two sheets of quartz-dolerite, the upper sheet being locally mushroom-shaped. The rock corresponds loosely with the Downes mountain type of dolorite, and contains numerous horizons of coarse-grained dolorite-pegmatite. The plagioclase felspars are described with special reference to the variation of grain-size, the features of zoned crystals, and the possible causes of a peculiar brown clouding in the basic zones of the crystals. Phenocrysts of augite and pigeonite, as well as plagioclase are recorded in the chilled contact rock. The micrometric data indicate that the mineral composition of the specimens is largely governed by the oxidation state of the iron, and that the Soret effect has caused the marginal rock to be enriched in basic elements. Hydrothermal veinlets composed largely of chlorite occur in both the dolorite and the country rock, and evidence suggests that veins of both residual material and mobilised granite occur in the upper finer-grained dolorite.
- Full Text:
- Date Issued: 1953
The petrology of the basalts of the Dordabis Formation in the vicinity of Dordabis in central S.W.A./Namibia|
- Authors: Williams-Jones, Ian Eric
- Date: 1985 , 2013-03-14
- Subjects: Basalt -- Namibia , Petrology -- Namibia , Geochemistry -- Namibia
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5062 , http://hdl.handle.net/10962/d1013277
- Description: The late Proterozoic volcanic and sedimentary sequence in the Dordabis area SWA/Namibia has been named the Dordabis Formation and subdivided, on the basis of field, petrological and petrographic evidence, into the Opdam and Bitterwater Members. Relict phases including augite and minor plagioclase only occur in the Bitterwater metalavas, as recrystallisation is complete in the Opdam metal avas. The composition of the relict feldspars ranges from labradorite in the ophitic basalts to oligoclase in the blastoporphyritic metalavas. The feldspars in the Opdam member are albitic in composition (An content 0,0 to 1,7) . Epidote compositions are typical of those occurring in metabasic rocks. Samples with high-iron whole-rock compositions are accompanied by high concentrations of Fe3+ in concomitant epidotes. Sixty three samples were analysed using X-ray fluorescence spectrometry to determine concentrations of major and 16 trace elements. Although greenschist facies metamorphism, metasomatism and shearing have produced scatter in the more mobile element concentrations, variation trends in other elements closely resemble modelled low-pressure fractional crystallisation trends. The Dordabis metalavas are petrologically classified as sub-alkaline, tholeiitic continental basalts. Low K/Rb ratios and low ratios of less incompatible to more incompatible elements probably reflect a source that has either been metasomati ca lly enriched or that has undergone little previous partial melting. Overlapping whole-rock variation trends indicate that the generally more evolved Opdam and primitive tholeiitic Bittenwater lavas are cogenetic. A comparison of the Dordabis Formation with the Sinclair Sequence and the Koras Group shows that their ages, petrology, petrography, associ a ted sedimentary suites and depositional environments are similar. It is concluded that they may possibly be coeval equivalents.
- Full Text:
- Date Issued: 1985