The presence of a wide package of economically minable conglomerate bands at Western Areas Gold Mine, associated with the earlier selective mining of certain of these reefs at narrow stope widths, has led to the design of several mining methods aimed at a greater percentage extraction of the ore body. These mining methods include and combine: room and pillar mining, caving, open stoping and destressing. When used at the depths involved i.e. between 800 and 3000m these methods entail unusual challenges in Rock Mechanics Design. This paper describes the mining standards derived in conjunction with the relevant Rock Mechanics criteria.
Western Areas Gold Mine is situated approximately 50 km South West of Johannesburg, near the town of Westonaria. The mine chiefly exploits the conglomerate strata of the Upper Elsburg and Ventersdorp contact reefs of the Witwatersrand Super Group while conventional mining of Middle Elsburg reefs also occurs.
The Witwatersrand rocks are overlain by the volcanic Ventersdorp supergroup and the dolomites and other rocks of the Transvaal sequence. The thickness of the lavas of the Ventersdorp sequence diminish toward the North East, until in the North Eastern portion of the mine the Upper Elsburg reefs are directly overlain by dolomite or Black Reef shales. The dolomites are repositories of enormous quantities of ground water and present a flooding hazard when intersected. Although a dewatering programme is underway this has not yet been completed. The Upper Elsburg reefs and the VCR form an near continuous package of conglomerates with quartzite bands between. The gold content of the conglomerates varies and differing horizons and cuts are payable in different areas of the mine.
(Figure in full paper)
The reef package thins to the West where the various bands reach their paleo-shorelines. Thus the reef package varies from 2 m to 60 m and more in thickness; however as a general rule the package is of a higher payability in the West. All reefs dip at approximately 15 degrees to the South.
From 1961 when the Western Areas Gold Mining Company Limited, North Division was sunk until the mid 1980's the Upper Elsburg reefs were mined by conventional stoping along various high grade bands; stoping widths varied from 1 m to 3 m. Mining was supported on timber and later grout based packs. Due to the faulting and variability of the gold content this mining was often discontinuous, several bands were often mined above or below each other with middlings as small as 3m.
(Figure in full paper)
Subsequent mechanized mining in these areas has been in an environment which is more or less destressed with thin middlings to over or under lying stopes. The presence of pillars and mining not shown or poorly indicated on mine plans due to the multiple horizons mined adds to the difficulties of achieving a further extraction of reef from these areas. Other areas, in particular the deeper portions of the mine, had not been previously mined but are now being exploited by mechanized mining methods.
