- Copyright © 2000 Society of Exploration Geophysicists
Major South African gold-mining companies have recently turned to 3-D seismic surveys for mine planning in Witwatersrand Basin (e.g., Pretorius et al., 1997). We present examples showing that 3-D seismic data provide the gross structure and depth of the prospective stratiform ore horizons (the original objective of such surveys), and that careful analysis of 3-D seismic attributes—backed up with borehole control, direct geologic observations, and synthetic seismograms—can provide important information on small-scale faulting (throws of 10–20 m) and lithologic variations of the ore-zone conglomerates.
The Target gold project (Gray et al., 1994) is immediately north of the Welkom gold field, Orange Free State, South Africa (Figure 1). Avgold estimates mineral resources of 50.5 million tonnes (at 7.65 grams/tonne Au) and plans to produce 330 000 ounces of gold per annum. More than 50 surface boreholes have been drilled to the ore zones (depths = 1800–2800 m) to test ore grade. These provide stratigraphic control, particularly along the central spine of the prospect. The ore zone comprises up to 22 auriferous conglomerate “reefs” of the topmost Central Rand Group's Dreyerskuil and Eldorado formations and is immediately below the Klipriviersberg lavas (Figure 1).
The Precambrian 3-1-2-7Ga Witwatersrand Basin had an Andean-style collisional setting, dominated by southeast-verging thrust tectonics (Coward et al., 1995). Basin fill comprises shales and quartzites of the West Rand Group (Figure 1) overlain by the quartzites and auriferous conglomerates of the Central Rand Group. The succeeding ∼2.0-km thick Klipriviersberg lavas unconformably overlay the Central Rand. In the Target area, sediments and bimodal volcanics were deposited in graben systems during upper Klipriviersberg to Platberg times. These reactivated and dissected the earlier thrust system. The unconformably overlying Pniel Group can be looked upon as postrift lavas and sediments. These Precambrian rocks …