Five mines located in contrasting geological and structural domains have been selected for in situ stress determinations. In the Namaqualand Metamorphic Complex results at Carolusberg and Broken Hill mines indicated the maximum principal stress (sigma 1) to be near horizontal and orientated approximately normal to the general structural grain. In direct contrast, the mines situated within the stable platform of the Kaapvaal craton, Deelkraal (Au), Venterspost (Au) and Northam (Pt) display a near vertical sigma 1.
The orientation of the stress field in both domains, although distinctly different, is unaffected by depth below surface, lithology or structural attitude. In both domains the magnitudes of the principal stress components increase linearly with depth producing approximately constant K-ratios that are characterstic for each domain.
The residual or remnant stress field appears to be the dominant factor influencing the measured field in metamorphic terraines.
In situ stress determinations have been made in five operating mines over the Past ten years. The purpose of these investigations was to obtain a better understanding of the virgin stress field in the various areas. This allowed improved mine planning for the individual operations.
The studies were undertaken in deep level mines at Carolusberg near Okiep, Deelkraal and Venterspost gold mines on the far West Rand and Northam Platinum mine in the north-western Transvaal. The only shallow operating mine investigated was Broken Hill at Aggeneys in the north-western Cape Province (Fig 1). All the above operations are managed by Gold Fields south Africa but function as separate companies.
Methodology and equipment used at all sites was similar. Strain measurements were made with standard CSIR triaxial strain measuring equipment. This technique is fully described by Van Heerden (1976). Elastic constants of the various rock types were determined by COMRO triaxial loading tests. Deviations from the above methods or practical problems encountered at the individual mines will be discussed separately.
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It is the purpose of this paper to collate and compare the results obtained from the various investigations and to interpret these in terms of their contrasting tectonostrati-graphic setting, variable host rock lithology and geological age. Although the various mines studied form part of three distinct mining districts, tectonically they are situated in two well defined structural domains.
The base metal deposits of Carolusberg and Broken Hill are hosted by the Namaqualand Metamorphic Complex situated in the 1,9 Ga Namaqua-Natal Mobile Belt (Fig. 1). Host rocks of the Witwatersrand gold placers are slightly metamorphosed clastic sediments deposited on the tectonically stable Kaapvaal Craton some 2,7 billion years ago. Platinum deposits are also confined to this Craton and are associated with the Bushveld Igeneous Complex which intruded the Transvaal Sequence (2,1 Ga).
Rocks grouped as the Namaqualand Metamorphic Complex are exposed in the north-western Cape Province and southern fringes of Namibia, covering an area of approximately 80 000 km2. This complex comprises highly metamorphosed and polyphase deformed rocks of sedimentary, volcanic and intrusive origin.