A review of sediment-hosted gold deposits of the world with special emphasis on recent discoveries outside the U.S.A
- Authors: Daglioglu, Yasar Mehmet
- Date: 1996
- Subjects: Gold ores , Sedimentation and deposition
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4997 , http://hdl.handle.net/10962/d1005609 , Gold ores , Sedimentation and deposition
- Description: Most of the Great Basin sediment-hosted gold deposits are located along well defined, northwest-striking trends. Trends coincide with faults, intrusive rocks and magnetic anomalies. Sedimentary host rocks are siltstone, sandstone, conglomerate, argillic, interbedded chert and shales. Silty bedded silty dolomites, limestone and carbonaceous shales are the most favourable hosts. High, and locally, low-angle faults are very important structural features related to the formation of the ore bodies. High-angle faults are conduits of hydrothermal fluids which react, shatter and prepare the favourable host rock. Decalcification, silicification, and argillization are the most common hydrothermal alteration types. Jasperoid (intense silica replacement) is a significant characteristic; not all of these deposits are gold-bearing. Most deposits contain both oxidized and unoxidized ore. Fine grained disseminated pyrite, arsenian pyrite, and carbonaceous material are the most common hosts for gold in many deposits. These deposits are also characterized by high Au/Ag ratios, notable absence of base metal and geochemical associations of Au, As, Sb, Hg, Ba and TI. Recently numerous sediment-hosted gold deposits have been recognized in different regions of the world. They vary in their size, grades, textwe, host rock lithology, degrees of structural control and chemical characteristics. However, they have many common features which are very similar to the general characteristics of sediment-hosted gold deposits in the Great Basin, U.S.A. Besides these similarities, several unusual features are recorded in some newly discovered deposits elsewhere, such as predominant fault controlled paleokarst related mineralization and the lack of two very common trace elements (Hg, TI) in Lobongan/Alason, Indonesia; and Early Proterozoic age metamorphosed host rocks and lack of Sb in Maoling, China. The discovery of the deep ores in the Post-Betze and Rabbit Canyon, Nevada, proposed sediment-hosted Au emplacement at deeper level (4 ± 2 km; Kuehn & Rose, 1995) combined with a lack of field evidence for paleowater table and paleosurface features has ruled out a shallow epithermal origin. Recent discoveries in other parts of the world throw important new light on the ongoing genetic problems. Intrusive rocks are present in nearly all sediment-hosted gold deposits. Numerous intrusion-centred districts worldwide are characterized by tWo or more different mineralization types and consequently by metal zoning. Sediment-hosted gold deposits are proposed as a distal part of intrusion-centred magmatic hydrothermal systems (Sillitoe &Bonham, 1990).
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- Date Issued: 1996
Basin analysis and sequence stratigraphy a review, with a short account of its applicability and utility for the exploration of auriferous placers in the Witwatersrand Basin
- Authors: Van Eeden, Johan
- Date: 1996
- Subjects: Geology, Stratigraphic , Sedimentary basins -- South Africa , Placer deposits -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4934 , http://hdl.handle.net/10962/d1005546 , Geology, Stratigraphic , Sedimentary basins -- South Africa , Placer deposits -- South Africa
- Description: The Witwatersrand basin is unique in terms of its mineral wealth. The gold in the Witwatersrand basin is mainly concentrated in the placers and two types of unconformities are associated with the placer formation. This paper attempts to quantitatively describe the origin and depositional process of placers within the context of basin analysis, geohistory and sequences stratigraphic framework. Several tectonic models have been proposed for the evolution of the Witwater~rand basin and it seems as if a cratonic foreland basin accounts for many of the observed features observed the Central Rand Group basin. The tectonic subsidence curve generated for the Witwatersrand Basin clearly implies foreland basin response which was superimposed an older, deep seated extensional basin. These compressive tectonics can be superimposed on extensional basins, where the shift from extensional to compressional tectonics lead to inversion processes. The critical issues about the Witwatersrand basin which were addresed in this review, is the validity of basin wide correlation of placer unconformuties and whether sequence stratigraphy is applicable to fluvial systems of the Witwatersrand sequence. It is believed that the Central Rand Group was deposited as alluvial - fan deltas by fluvially dominated, braidplain systems with minor marine interaction which had a considerable impact on the preservation of economically viable placers. Most important to the exploration geologist is the recognition of stacking patterns of the fluvial strata to determine change in the rate at which accommodation was created. Identifying sequence boundaries and other relevant surfaces important for identifying these stacking patterns of the sequences, depends entirely on the recognition of a hierarchy of stratal units including beds, bedsets, parasequences, parasequence sets and the surfaces bounding sequences. Placers are closely associated with the development of disconformities and therefore become important to recognise in fluvial strata. If these placers are to become economic, the duration of subaerial exposure of the unconformities that allowed the placers to become reworked and concentrated must be determined. In order to preserve the placer, a sudden marine transgression is necessary to allow for minimal shoreline reworking and to cap the placer to prevent it from being dispersed. The placers in the Witwatersrand basin occur in four major gold-bearing placer zones in the Central Rand Group. Accordingly they can be assigned to four supercycles, which are cyclical and therefore predictive. It is the predictive nature of these rocks and the ability of sequence stratigraphy to enhance this aspect, which is a pre-requisite for an effective exploration tool in the search for new ore bodies or their extension in the Witwatersrand basin.
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- Date Issued: 1996
Early Cretaceous alluvial palaeosols (Kirkwood formation, Algoa Basin, South Africa) and their palaeoenvironmental and palaeoclimatological significance
- Authors: Frost, Susan
- Date: 1996
- Subjects: Paleopedology -- South Africa -- Eastern Cape , Paleoclimatology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4939 , http://hdl.handle.net/10962/d1005551 , Paleopedology -- South Africa -- Eastern Cape , Paleoclimatology -- South Africa -- Eastern Cape
- Description: The Kirkwood Formation in the Bushman's River area of the Algoa Basin is characterised by a number of fining-upward cycles. These have been interpreted as indicating deposition in a dynamic aggrading meandering river system with the channel deposits (conglomerates grading upwards into sandstones) fining upward into the overbank deposits (mudrocks). Channel, channel-margin and overbank deposits were recognised. The three mudrock sequences logged comprise compound pedofacies sequences of multistorey, simple and cumulative palaeosols. Distinctive palaeopedological features, such as root traces and pedotubules, soil horizons and structures, mottles, and iron-rich and calcareous glaebules and calcareous hardpan lenses and layers were used to identify a number of palaeosols within the mudrock sequences. Each mudrock sequence comprises multistorey entisol, inceptisol, alfisol, ultisol, aridisol and vertisol profiles at different stags of pedogenic maturity. The entisols and inceptisols are relatively immature profiles formed close to the meandering river channel and are classified as channel-margin palaeosols. The ultisols, alfisols, aridisols and vertisols are more mature and formed at some distance from the channel. They are classified as proximal floodbasin or distal floodbasin palaeosols depending on their maturity, distance from the channel and grain-size. Slickensides, desiccation cracks, and iron-rich concretions occur, indicating multiple cycles of wetting and drying. A low water-table beneath the floodplain is indicated by both the prominent maroon-brown colouration of the mudstones, caused by oxidation during deposition, and the general lack of evaporites in the sequence. Calcretes comprising calcic and petrocalcic horizons are very common in the lower mudrock sequence, rare in the middle mudrock sequence and relatively common in the upper mudrock sequence. The calcretes generally consist a nodular zone which may, in some cases, be capped by a thin hardpan layer. The calcic palaeosols commonly show stages of carbonate accumulation which indicate at least 10 000 years of formation. The lack of calcrete formation in some of the profiles may indicate frequent flooding and high sediment accretion rates or a decrease in the influx of Ca²⁺-rich aeolian dust into the depositional basin. Clay alluviation is common in many of the profiles and soil structures are commonly well developed. The palaeosols are interpreted as having formed on an aggrading floodplain in a warm to hot (25-30°C), semi-arid climate with a low but seasonal rainfall (100-500mm per annum).
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- Date Issued: 1996
Exotic deposits derived from porphyry copper systems in Chile
- Authors: Diaz Acevedo, Nelson Simon
- Date: 1996
- Subjects: Porphyry -- Chile , Copper mines and mining -- Chile , Copper ores -- Chile
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5050 , http://hdl.handle.net/10962/d1011149 , Porphyry -- Chile , Copper mines and mining -- Chile , Copper ores -- Chile
- Description: The exotic orebodies related to cal-alkaline porphyry copper deposits. are sub-horizontal lenticular bodies of secondary copper minerals that impregnate Tertiary gravels and bedrock of different ages. They lie immediately downslope of the porphyry copper deposits, that is to say. they are related to the propylitic halo of the main deposits, and are considered to have originated with the deposition of copper minerals from solutions that overflowed during the secondary enrichment process. Supergene alteration took place between the late Oligocene and Miocene, by which time both orehodies (exotic and porphyry copper) were established. The paucity of tile denudation since the Miocene in the Andean segment from 21º to 26º latitude S. due to the dominance of a hyperarid climate explains the remarkable preservation of the shallow porphyry copper systems, supergene enriched blankets and associated deposits. This is reflected in the limonites, where the typical boxworks have been partially or totally destroyed on surface by the superleaching. As a result of the lateral migration of the copper-bearing solutions, the exotic deposits show a zonation. alteration and mineralization whose characteristics depend among other factors. on the reactivity of the bedrocks and the Cu/S ratio of the mother deposit. In these deposits three zones can be recognized: Proximal (0 to 2 km Intermediate (2 to 3) and Distal (3 to 8-14 km) with palaeodrainage control. The associations and mineralogical abundance are related to the climate (rain. temperature). In some deposits two important units are detected and they are the Cu-phosphates and Cu-lixiviable (to sulphuric acid) units. The size of tile Chilean exotic deposits varies between 100 and 3.500.000 tons of copper, with a total known resource of 8 M tons of copper. The large exotic deposits are comparable to the resources of a medium-sized porphyry copper- type deposit. The discovery of the exotic deposits is related to the exploration of porphyry copper deposits, where a mass balance of the leached column must be done. Moreover the lithology and alteration of the propylitic halo. permeability, structures, geophysics and geochemistry should be considered. The diorite model is not compatible with a supergene enrichment process, expressed by the absence of colour anomalies, exotic deposits and in the presence of secondary minerals like jarosite, which is coherent with the pyrite deficiency of the system. The projects are for exploitation of reserves by open pit methods. The projects plan to extract and to crush copper oxide ore which will he pre-treated with concentrated sulphuric acid prior to heap leaching, solvent extraction and electrowinning. The copper output varies between 10,000 and 50,000 ton per year of catilode copper. The total investment varies between 20 and 100 millions dollars. For project calculations, estimation of 65 % - 82 % copper recovery and 37 - 40 kg/ton net acid consumption can be used due to the nature of ore. The leaching time is estimated as 30 to 180 days for heaps of 4,5 to 30 metres high. As a result of this, the plant capacity is determined by an annual equivalent of 10,000 to 25,000 ton Cu per year. Pit planning has heen carried out using diverse software on the basis of 5 x 5 x 5 m and 10 x 10 x 10 m block models, evaluated using a kriging package built into the program, giving an overall waste/ore ratio (induding pre-stripping) of 0: I (in an outcrop) to 3: I. The location of the mine and plant sites are associated with the porphyry copper in production, for this the already installed facilities can be used. So there is no need to build a new mine camp or access. The environmental impact is summarized relating to the characterization of the physical, biological and socio-cuitural effects, using the framework of the Base Line study and the Impact evaluation derived from the construction and project operation phases. The conclusions and recommendations will diminish, mitigate and/or eliminate impact derived from the specific activities.
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- Date Issued: 1996
Geochemical exploration in tropical terrains with special reference to base metals
- Authors: Chiconela, Domingos Rubão
- Date: 1996
- Subjects: Geochemical prospecting -- Research -- Tropics , Geological mapping -- Research -- Tropics , Weathering -- Research -- Tropics , Geochemistry -- Research
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4953 , http://hdl.handle.net/10962/d1005565 , Geochemical prospecting -- Research -- Tropics , Geological mapping -- Research -- Tropics , Weathering -- Research -- Tropics , Geochemistry -- Research
- Description: In tropical areas, the high rainfall induces severe-and pervasive weathering, producing a thick soil cover. The lithologies underneath may b~ recognised using geochemical mapping, which is based on certain elements that have the ability to differentiate between various lithologic units. Elements that are independent of the weathering process are normally selected for this purpose. The chemistry of mobility of base metals is an important factor to take into account when evaluating the mobility and distribution of these elements in a soil profile. Factors such as pH, Eh, organic material, clay minerals, Fe and Mn oxides are normally key aspects to be considered. When iron-rich rocks undergo deep weathering, lateritic profiles are developed. These are widespread in a belt bordering the equatorial zone, including the Brazilian shield, West and East Africa, parts of India and Northern Australia. In these profiles, the high rainfall promotes intense leaching of the different horizons. Where the pre-existing profiles are mostly preserved, the base metals are distributed throughout the profile: in the upper ferruginous horizon, goethite and hematite can adsorb large amounts of Mo, resulting in large dispersion halo. Other base metals such as Cu and Zn are less resistant in these freely-drained profiles and, therefore, they may be partly leached from the profile. In the lower horizons, Cu, ,zn, Ni and Co are retained, hosted in kaolinite and smectite, and thus, a high geochemical contrast will be identified in this horizon at the expense of a decline in the size of the dispersion haloes. The pre-existing profiles can be truncated, with a thin stone line developing at the contact between the lateritic profile and the recent soil. The conditions in these environments favour the retention of most of the pathfinder and target elements in all soil horizons, with the B horizon showing the highest contrast. If the primary rock is rich in AI, a bauxitic profile will be developed. The world distribution of bauxites closely resembles that of laterites. The behaviour of Co and Ni is very similar to that of iron during the bauxitization. Furthermore, the factors that induce residual enrichment of Al with removal of Fe in the soil profile will cause significant depletion of Co and Ni in these profiles. These metals are then concentrated at the base of the profile because of precipitation from downward percolating solutions. Many karst bauxite deposits in Southern Europe are enriched with Ni and Co in the basal horizon. Such horizon is mined as nickel ore in the bauxites of the Lokris region in Greece. Copper and molybdenum are strongly enriched.in bauxitic profiles. Concentration ratios are 8 and 3.2 for Cu and Mo respectively. Molybdenum is closely related to goethite and hematite, and therefore, the high concentration of Mo in a bauxitic profile will be consistent with the horizon where iron is concentrated. Copper concentrates at the base of the iron rich-horizon but also appears enriched in the saprolite together with Co. When sulphide bodies occur, in this environment, deep and penetrative weathering has resulted in considerable near-surface mobilization of iron and silica. The supergene alteration commonly obscures the identity of the primary sulphides at the surface. In this case, geochemical assessment of the resulting gossan has proved to be crucial in mineral exploration. A search in the secondary mineral assemblage, volatile and precious metals may lead to the information on the composition of the primary sulphide assemblage. The conclusion that will be reached is that if the geochemical properties (mobility, affinities with Fe or Mn oxides and/or clay minerals) of each of the base metals are understood, an appropriate sampling (optimum size-depth combination) will then be done. In such cases, a subdued, weak, but significant, geochemical response will be identified in the surface horizon.
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- Date Issued: 1996
Gold exploration in tropical and sub-tropical terrains with special emphasis on Central and Western Africa
- Authors: Breedt, Machiel Christoffel
- Date: 1996
- Subjects: Geochemical prospecting -- Tropics , Geochemical prospecting -- Africa , Gold ores -- Geology -- Tropics , Gold ores -- Geology -- Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4966 , http://hdl.handle.net/10962/d1005578 , Geochemical prospecting -- Tropics , Geochemical prospecting -- Africa , Gold ores -- Geology -- Tropics , Gold ores -- Geology -- Africa
- Description: The aim of this dissertation is an attempt to' provide a general guide for future gold exploration in tropical and sub-tropical terrains. The dissertation includes a brief discussion of the various exploration techniques used in regional and local exploration. This provide the necessary background knowledge to discriminate between the constraints and applications and to be able to select the techniques which are more suitable for gold exploration in tropical and sub-tropical terrains. Weathering, gold geochemistry and soil formation, fields often neglected, are emphasized to illustrate the importance of the mobility and dispersion of gold in the weathering of the lateritic soil profile. A sound knowledge and experience in regolith mapping is to the advantage of the explorationist. Case studies with special emphasis on Central- and Western Africa are included to illustrate the effectiveness of some of the gold exploration techniques in tropical and sub-tropical terrains. Gold exploration is a highly complex and demanding science and to be successfull involves the full intergration of all geological, geochemical and geophysical information available. An intergrated exploration method and strategy would enhance the possibility of making viable discoveries in this highly competative environment where our mineral resources become more depleted every day. Where applicable, the reader is refered to various recommended literature sources to provide the necessary background knowledge which form an integral part of gold exploration.
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- Date Issued: 1996
Overview and comparison of Besshi-type deposits ancient and recent
- Authors: Schoeman, Philo
- Date: 1996
- Subjects: Ore deposits -- Japan , Ore deposits -- Canada , Ore deposits -- Namibia , Ore deposits -- South Africa , Ore deposits
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4983 , http://hdl.handle.net/10962/d1005595 , Ore deposits -- Japan , Ore deposits -- Canada , Ore deposits -- Namibia , Ore deposits -- South Africa , Ore deposits
- Description: Besshi-type deposits range in age from early Proterozoic to early Tertiary, of which the largest number are late Proterozoic, early Palaeozoic or Mesozoic in age. No Archaean examples of Besshi-type deposits are known, probably due to insufficient availability of sialic crust for erosion and clastic marine sedimentation before the start of the Proterozoic. All Besshi-type deposits are contained within sequences of clastic sedimentary rock and intercalated basalts in a marine environment. The basalts and amphibolites are principally tholeiitic in composition. Besshi-type deposits characteristically form stratiform 1enses and sheet-like accumulations of semi-massive to massive sulphide. The main ore assemblage consists dominantly of pyrite and/or pyrrhotite with variable amounts of chalcopyrite, sphalerite and trace galena, arsenopyrite, gold and e1ectrum, barite being absent in general. The median Besshi-type deposit (n=75) contains 1.3 million tonnes (Mt) of massive sulphide with a Cu grade running at 1.43%. It is suggested that Besshi-type deposits form by both exhalative and synsedimentary replacement processes when considering geological features and comparisons with modern analogues in the Guaymas Basin, Middle Valley and Escanaba Trough. The currently forming metalliferous sediments in the Red Sea provide for a brine pool model explaining the lack of footwall feeder zones below sheet-like deposits. Where thick sulphide lenses are contained in some Besshi-type deposits, combinations of exhalative precipitation and sub-sea-floor replacement of permeable sediments and/or volcanic rocks, take place in the upper parts of submarine hydrothermal systems.
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- Date Issued: 1996
Petrology and geochemistry of the basal gabbro unit, Uitkomst complex
- Authors: Strauss, Toby Anthony Lavery
- Date: 1996
- Subjects: Petrology -- South Africa -- Mpumalanga -- badplaas , Geochemistry -- South Africa -- Mpumalanga , Gabbro -- South Africa -- Mpumalanga
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4989 , http://hdl.handle.net/10962/d1005601 , Petrology -- South Africa -- Mpumalanga -- badplaas , Geochemistry -- South Africa -- Mpumalanga , Gabbro -- South Africa -- Mpumalanga
- Description: The Cu/(Cu+Ni) ratios associated with the Basal Gabbro also display the vertical reverse fractionation trend, supporting the supercooled margin model. The disseminated sulphides in the lowermost units, are regarded as being the result of sulphur saturation induced by contamination from the dolomitic and quartzitic xenoliths. This is supported by isotope data which indicate the high degree of contamination in the lowermost units of the Complex. The results of this study are used to propose a model for the petrogenesis and metallogenesis of the Uitkomst Complex, whereby the Complex is closely related to the Bushveld Complex. The Basal Gabbro, as supported by its chemistry and style of mineralisation (Cu-rich), represents a supercooled margin to the lowermost units of the Uitkomst Complex, which stoped upwards into the surrounding sediments, assimilating the country rock xenoliths, and precipitating sulphides. Following this was a period in which large quantities of magma moved laterally through the system before the magma flow waned, and closed system crystallisation ensued. As the body cooled, the primarily magmatic water was superceded by the hydrothermal magmatic water released from the xenoliths, and later by geothermally driven circulating meteoric water, producing the extensive alteration. This alteration was accompanied by considerable stress and the development of fractures and shears. Finally the Complex was itself intruded by diabase sills and later dolerite dykes.
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- Date Issued: 1996
Semi-automated extraction of structural orientation data from aerospace imagery combined with digital elevation models
- Authors: Slabber, Frans Bresler
- Date: 1996
- Subjects: Image processing , Geographic information systems , Information storage and retrieval systems -- Geology , Geology -- Data processing
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5001 , http://hdl.handle.net/10962/d1005614 , Image processing , Geographic information systems , Information storage and retrieval systems -- Geology , Geology -- Data processing
- Description: A computer-based method for determining the orientation of planar geological structures from remotely sensed images, utilizing digital geological images and digital elevation models (DEMs), is developed and assessed. The method relies on operator skill and experience to recognize geological structure traces on images, and then employs software routines (GEOSTRUC©) to calculate the orientation of selected structures. The operator selects three points on the trace of a planar geological feature as seen on a digital geological image that is co registered with a DEM of the same area. The orientation of the plane that contains the three points is determined using vector algebra equations. The program generates an ASCII data file which contains the orientation data as well as the geographical location of the measurements. This ASCII file can then be utilized in further analysis of the orientation data. The software development kit (SDK) for TNTmips v5.00, from MicroImages Inc. and operating in the X Windows environment, was employed to construct the software. The Watcom C\C++ Development Environment was used to generate the executable program, GEOSTRUC© . GEOSTRUC© was tested in two case studies. The case studies utilized digital data derived from the use of different techniques and from different sources which varied in scale and resolution. This was done to illustrate the versatility of the program and its application to a wide range of data types. On the whole, the results obtained using the GEOSTRUC© analyses compare favourably to field data from each test area. Use of the method to determine the orientation of axial planes in the case study revealed the usefulness of the method as a powerful analytic tool for use on a macroscopic scale. The method should not he applied in area with low variation in relief as the method proved to be less accurate in these areas. Advancements in imaging technology will serve to create images with better resolution, which will, in turn, improve the overall accuracy of the method.
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- Date Issued: 1996
The geology of the Unki platinum-base metal deposit, Selukwe subchamber, great dyke, Zimbabwe
- Authors: Murahwi, Charley Zvinaiye
- Date: 1996
- Subjects: Platinum ores -- Zimbabwe
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4962 , http://hdl.handle.net/10962/d1005574 , Platinum ores -- Zimbabwe
- Description: This thesis focuses on platinu'm group element (PGE) mineralization in the Unki Section of the Selukwe Subchamber of the Great Dyke (Zimbabwe), and is based on drill hole intersections and underground and surface exposures of the Main Sulphide Zone (MSZ) which hosts significant concentrations of PGE. The petrological and geochemical data presented are part of a broader study currently underway and the present are restricted to the 2m section of the PGE-rich MSZ encountered in drill hole MR126. The PGE-rich MSZ at Unki is unique in having a shear, locally referred to as the Footwall Shear, developed at or close to its base . It is however, similar to the other PGE occurrences on the Great Dyke (MSZ) in having its hanging-wall restricted to within 1m of the websterite/bronzitite contact. Slight axial tilting to t he west is indicated by steeper dips on the eastern flank. The sulphide concentration wit hin the MSZ can be used as a rough guide to the PGE-rich zone, but is not sufficiently precise to be used in stope control. The visual identification of the potentially mineable zone remains a problem that is unlikely to be solved. Based on petrological evidence, the bulk of the sulphides with which the PGE are associated, are cumulus in status. This provides unequivocal evidence for an orthomagmatic origin of the MSZ. The dominant platinum group mineral (PGM) phase is the Arsenide/Sperrylite group which is most commonly found at the contact zones between base metal sulphides (BMS) and gangue. The PGM range up to 90 ~m in length. Geochemical evidence from the analyses of cumulate orthopyroxenes through the 2m PGE-rich MSZ interval at Unki reveals a trend of arked Fe enrichment upwards which corresponds to an enrichment in sulphide. This indicates that precipitation of sulphide was caused by fractionation with lowering of temperature in the magma. The Fe enrichment is followed by a reversal in Mg# of orthopyroxene which corresponds to the decrease in sulphide content, suggest i ng that the termination of the PGE-rich MSZ was due to an increase in temperature associated with an influx of new magma. Coupled with these magmatic events are a complex interplay of chemical and physical processes occurring at a critical stage in the overall fractionation of the Great Dyke magma chamber. The overall persistence and continuity of t he PGE zone as observed in the Unki area is consistent with the inferred orthomagmatic origin of the mineralization
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- Date Issued: 1996
The geology, geochemistry and stratigraphic correlations of the farm Rietfontein 70 JS on the south -eastern flank of the Dennilton Dome, Transvaal, South Africa
- Authors: Crous, Stephanus Philippus
- Date: 1996
- Subjects: Stratigraphic correlation -- South Africa -- Transvaal , Geology, Stratigraphic , Geochemistry -- South Africa -- Transvaal , Geology -- South Africa -- Transvaal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4960 , http://hdl.handle.net/10962/d1005572 , Stratigraphic correlation -- South Africa -- Transvaal , Geology, Stratigraphic , Geochemistry -- South Africa -- Transvaal , Geology -- South Africa -- Transvaal
- Description: The study area is located between Loskop Dam and the town of Groblersdal, on the southeastern flank of the Dennilton dome, and is underlain by lithologies of the Pretoria Group, Bushveld Complex mafics and ultramafics and acid lavas that resort under the Rooiberg felsites. Field work comprised of geological mapping, soil-, hard-rock- and stream sediment geochemistry, various geophysical techniques and diamond drilling. The rocktypes that resembles the Rustenburg Layered Suite on the farm Rietfontein 70JS is subdivided into a Mixed Zone, Critical Zone and Main Zone, on grounds of geochemical and certain geophysical attributes. The Mixed Zone that overlies the Bushveld Complex floor-rocks, is furthermore separated into an i) Lower-, ii) Middle- and, iii) Upper Unit. The Lower Unit of the Mixed Zone consists primarily of magnetite-gabbros, iron-rich pegmatites, harzburgites and feldspathic pyroxenites. The Fe-rich constituents of this stratigraphic horizon generates a pronounced magnetic anomaly within the study area. On the basis of; amongst other parameters, Zr/Rb and Sr/Al₂0₃ ratios, the magnetite-gabbros are postulated to conform to lithotypes in the vicinity of magnetite layers 8 to 14 of Upper Zone Subzone B in a normal Bushveld Complex stratigraphical scenario. Similarly, it is argued that the feldspathic pyroxenites and norites that display elevated chromium values are analogues to normal Critical Zone rocktypes of the Rustenburg Layered Snite. A more elaborate and precise stratigraphic correlation for the Critical zone was, however, not possible. It is advocated that a volume imbalance was created by the hot, ascending mafic magmas of the intruding Bushveld Complex, resulting in the updoming of certain prevailing basement features such as the Dennilton Dome. In addition to this ideology, it is proposed that the Mineral Range Fragment is in fact a large xenolith underlain by mafics, after being detached from the Dennilton Dome during the intrusion event. Evidence generated by this study unequivocally indicate that the potential for viable PGE's, Ni, Cu and Au within a Merensky Reef- type configuration or a Plat Reef-type scenario under a relatively thin veneer of acid Bushveld Complex roof-rocks on the eastern flank of the Dennilton Dome, appears feasible.
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- Date Issued: 1996