A remarkable correlation between hydrothermal gold mineralisation and geological structure is discussed. The mineralisation occurs in vein quartz which occupies a fissure in Archaean greenstones of the Basement Complex in Rhodesia, It has been determined that gold which is silver-rich is typical of low-grade ore, and is associated in space with sulphides that crystallised early in paragenesis. Silver-poor gold is characteristic of highgrade ore which is concentrated along the crestal zone of anticlinal warps in the fissure. It is suggested that the local pattern of fracturing and folding is related to fundamental wrench faulting.

The gold metallogeny of Australia is predominantly confined to the Archaean and Palaeozoic Provinces. The Archaean gold occurrences are predominantly hosted in ultramafic-mafic dominated greenstone belts, with less associated tofelsic-volcanic and sedimentary sequences. Most gold occurrences are confined to shear zones or faults, and adjacent discoveries of economic laterite-hosted deposits, host rocks. Recent are presently under investigation and will supply a significant proportion of production in the future. The Proterozoic gold deposits of Australia , are confined to geosyncinal sequences, commonly turbidites (eg: Telfer), with other hydrothermal deposits associated directly to granites. An important feature of the North Australian Craton deposits, is the spatial association of most deposits to granite bodies, although a genetic link has not been established conclusively. The Roxby Downs deposit in South Australia is a unique occurrence of gold in association to copper, uranium and R.E.E. This deposit is tentatively related to intraplate alkaline-magmatism, with further work necessary. The most significant recent discovery of gold mineralization in Australia is in the Drummond Basin in Queensland. This epithermal is tentatively related to mineralization within the Georgetown Inlier. The latter mineralization is Permo-Carboniferous, in a Proterozoic (and possibly Archaean) sequence of schists. It is tentatively suggested that all the gold mineralization in northern Queensland may be related to single tectonic event, a feature which requires further study . Other mineralization in the Phanerozoic includes the turbidite-hosted metamorphogenic deposits of Victoria, the rift related deposits in New South Wales and magmatic related deposits in Queensland. The gold deposits in Australia may in the future be classified in a tectonogeological framework, similiar to the layout of this dissertation, particularly once further data becomes available on recent discoveries.

M.Sc. (Geology) / Please refer to full text to view abstract

Please read the abstract in the front matter of this document. / Dissertation (M Eng (Electrical Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

The "A" -Reef occurs within the Aandenk Formation of the Central Rand Group, Witwatersrand Supergroup, in the Welkom Goldfield. It comprises the Witpan and Uitsig Reefs which are both oligomictic conglomerates, and are exploited for their gold content by Anglo American Corporation. The main Witpan channel complex is orientated in a NW-SE direction and occurs close to No.3 Shaft of Western Holdings Mine where it is currently being mined. The Witpan Reef varies in thickness from 7-220cm, with lateral facies changes controlling the thickness. Within the reef, gold is associated with degradation surfaces, carbonaceous material, increase in pebble sphericity, and channel edges. Two channel edges have yielded the best gold values on No.3 Shaft although the potential for further payable gold lies in the recognition of sieve conditions. The potential also exists for extensions of the "carbon"-bearing Uitsig channel currently being mined on President Steyn Mine. The palaeo-environment proposed for formation of the Witpan Reef is a braidplain that was partly reworked by a brief transgression. Ore evaluation using geostatistics was considered a valid technique as the dataset is sufficiently well structured. Semi-variograms in the channel and across-channel directions differ markedly. It was found that variograms of gold in cmg/t lacked sufficient structure for modelling, however, log semi-variogram modelling followed by simple log-kriging and back-transformation, proved to be the most successful method. Owing to the morphology and distribution of gold within the reef, a geologically based geostatistical valuation method is proposed. The potential for further exploration of "A" -Reef depends on a substantially higher gold price. In this event, exploration of Uitsig Reef to the southwest of the current mining area is recommended as well as a new exploration strategy for Witpan Reef.

The distribution of gold in igneous rocks and minerals is described and discussed. Not all the gold in igneous rocks is contained within early formed crystal lattices. Evidence that gold can be associated with late stage crystallizing phases is described. It is concluded that some of the gold in hydrothermal and volcanogenic deposits may have come from a primary magmatic source rather than having been leached from solid country rock. Gold is probably transported as chloride complexes at temperatures greater than about 300°C. At lower temperatures it is probably transported with other metals as sulphide and thio-sulphide complexes. The precipitation of gold from the transporting medium is brought about by changes in the physico-chemical conditions within that medium. Decrease in pressure is probably not a major cause of precipitation in volcanogenic environments. The geology of volcanogenic iron-formations is described and discussed, relative to the development of greenstone belts. Oxide facies iron-formations were formed in shallow oxidizing environments. They are associated with volcanogenic and clastic sediments. Sulphide facies iron-formations were precipitated in the deeper parts of geosynclinal structures. They are associated with mafic and ultramafic rocks similar to modern oceanic volcanic assemblages. Carbonate facies iron-formations were deposited in the regions between oxide facies and sulphide facies. Other banded iron-formations are found associated with base metal massive sulphide deposits related to arctype volcanic centres. These deposits are found in the regions where carbonate facies iron-formations were formed. Exploration for and exploitation of gold deposits in Archaean iron-formations are discussed. Geochemical exploration programmes are aided by the association of gold with trace amounts of base metals. Geophysical exploration methods that can be employed include magnetometer, I.P. and E.M. surveys. The metallurgical treatment of the ores should include "roasting" because a large proportion of the gold occurs as submicroscopic grains within sulphide mineral crystals.

The general characteristics of gold deposits are reviewed, and a classification of gold deposits based on mineability is proposed. Evaluation, mlnlng and beneficiation methods are briefly discussed. It is concluded that the most viable targets for small-scale companies comprise deposits that require the least pre-production time and expense. Great potential exists for the small-scale reclamation of gold from tailings dumps and abandoned mines in Southern Africa. There is also potential for developing new smallscale gold mines in the Archaean greenstone terranes of the Zimbabwean and Kaapvaal cratons.

The first Witwatersrand gold deposits in the Orange Free State were discovered under younger cover rocks in the 1930's with the aid of drilling and geophysics. The Welkom gold deposits are found in the sedimentary rock sequences of the Central Rand Group, which represent unconformity bounded genetic packages. The structural configuration of the goldfield is one of a north to south trending synform that is split near it's axis by the De Bron and Homestead faults. The "B" Reef is a highly variable, erratically mineralised reef that lies on an unconformity at the base of the Spes Bona Formation. The "A" Reef Zone consists of several placers 1 including the "Reworked BPM" 1 the Witpan, the Uitsig, the Hanging Wall Grits and the Upper "A" Reef. The Beatrix Reef lies at the base of the Eldorado Formation on an unconformity surface overlying the Virginia Formation in the southern part of the Welkom Goldfield. The origin of gold in the Witwatersrand basin can be classified into the modified placer theory, the syngenetic theory and the epigenetic theory. From the distribution of basin edge unconformities it can be deduced that the Welkom fan depository was tectonically active on the western, southern and eastern margins during sedimentation. Palaeocurrent studies indicate that sediment was transported predominantly from the south and west. It is thought that the "B", "A" and Beatrix Reefs were all deposited in a braided stream environment. A multidisciplinary approach to ore evaluation of Witwatersrand deposits is considered to be the best method, where sedimentology, geostatistics and structural geology are used.

A vast amount and variety of mine tailings are produced around the world each day. In the gold¬mining industry in South Africa the residue of crushed are is disposed of in large tailings or hydraulic fill dams. The outer walls of these dams are built up of layers of material, each of which is allowed to dry before the next layer is placed. In order to study the stability of these walls, the appropriate engineering properties of the tailings must be ascertained. Due to the construction technique used in tailings dam construction, the outer walls are in an unsaturated state, which also means that suctions are generated within the tailings. Various techniques exist to measure suction, most of which are indirect methods. The recent development of the mid-¬plane suction probe at the University of Pretoria created the opportunity of measuring suctions directly on desiccating samples of gold mine tailings. A test method has been developed from which soil mechanics parameters can be derived from suction measurements. The experimental programme consisted of a series of these newly developed tests on fine and coarse samples of gold mine tailings, as well as on different particle size ranges. The experimental results were used in the development of a new method of predicting the air-entry value, with only the grading of the tailings known. A new method of predicting the soil-water characteristic curve up to the air-entry value was also proposed. The results of the research showed that the tailings remain saturated up to the air-entry value. The clay, fine silt and medium silt sized tailings was found to be the controlling particle size ranges in the development of suctions. The vast amount of parameters and information gained through the use of the proposed test method clearly indicates its effectiveness in studying the performance and characteristics of a material drying from saturation. The results also indicated the effectiveness of the mid-plane suction probe for the direct measurement of suction. / Dissertation (M Eng (Geotechnical Engineering))--University of Pretoria, 2006. / Civil Engineering / unrestricted

Gullies have been regarded as the "artery" in mining because they provide the only access route for material, people and ore in stopes. It thus becomes vital to provide the industry with suitable guidelines for gully layouts, geometry and the support required at all depths to keep the gully safe at all times. The research has indicated that best practices for gully layouts have been well recognised, but often poorly applied for many years. To address the issue of best gully practices, research was based on a review of past practices, underground visits, mine standards, codes of practice and the use of numerical modelling as a tool to back- analyse the underground conditions observed. The recommendations provided do not attempt to develop any new techniques for gully protection. They try to provide a guide for best practice under various geotechnical conditions. Based on depth, or stress environment, a broad based recommendation for gully geometry is provided. Using numerical modelling calibrated to underground observations, optimum widths and spans for each mining layout used at different depths are provided as a prescriptive guideline. • Sidings can only be omitted where stress damage does not occur. • A minimum siding width of 2m is recommended wherever sidings are cut. • Lagging sidings should be avoided and used only if absolutely essential. • In high stress areas gullies should be footwall lifted behind the stope faces or within wide headings. • All sidings must be cut on reef. Off reef sidings are not acceptable. • Correct blasting practice is essential to ensure stability of gully shoulders. • Gully width and span between support over gullies should be minimised. • Gullies must be kept straight. Excavation and cleaning of downdip sidings remains problematical from a practical mining point of view and future research is recommended in this area. / Dissertation (MSc (Mine Strata Control))--University of Pretoria, 2006. / Mining Engineering / unrestricted