Summary:
Target is located on the western margin of the Achaean Witwatersrand Gold Basin, which is on the Kaapvaal Craton. The sediments of the Central Rand Group occur within an oval-shaped basin, which has a 160 kilometrelong axis through the Welkom area and Johannesburg, and a short axis of about 80 kilometres. The Central Rand Basin is superimposed on the West Rand Group or Lower Witwatersrand Basin, which has a much larger aerial extent at the centre of the Kaapvaal Craton.
A thrust fault system has resulted in the post-depositional folding of the strata into a synclinal shape. This “border feature” is the western limit of the graben structure, which is some 10 kilometres wide and hosts most of the Welkom gold mines. The eastern limit of this graben is the well-defined De Bron Fault. The Target 1 gold prospect is a northward continuation of the Free State goldfield.
The full potential of the Basal Reef, which produces 85% of the gold from this area, has yet to be established in the Target area because, given time constraints, initial drilling focused on the shallower Elsburg and Kimberley reefs. The reefs in the Aandenk (Kimberley) Formation include the B Reef at the base, the Big Pebble Reef and the A Reef.
The Eldorado (Elsburg) Formation is developed as a sequence of oligomictic auriferous conglomerates referred to as the EA Reefs, which have been mined extensively at the Loraine gold mine. The Elsburg reefs are overlain by a remnant of the diamictite facies of the south, termed the boulder beds at Lorraine. The reefs and associated quartzites represent alluvial sediment influx from a source area to the west. The distribution of gold mineralisation is clearly related to the sedimentology and this primary sedimentological control of gold distribution is understood. However, research has shown that some remobilisation of gold has taken place over small distances. This is not extensive enough to mask the sedimentary controls.
Mineralisation
Exploitable mineralisation within the EA reefs, as developed in the Target Mine area, is associated with the presence of medium.
Although the lithostratigraphic location of the reef zone can generally be recognized throughout the mine, reef development and hence payability has no definable lateral contiguity, with favourable reef development being restricted to channelized zones of reworking. In this regard the laterally most persistent (dip and strike extent) reefs are the EA1, EA3, EA7 and EA12.
A further important point to note is that the minable extents of most reefs are generally restricted to the western limb of the asymmetric syncline and, in addition, to within 40m of the sub-outcrop areas. Payable extensions of reefs east of the trough axis have been encountered for selected reefs.
In the northern portions of Target, the exploitable mineralisation within the EA reefs is associated with medium to coarse clast to matrix supported oligomictic pebble horizons. However, allogenic (buckshot) pyrite along with free milling gold can be associated with oligomictic grit horizons especially east of the trough axis, representing distal facies of the above assemblage.
In limited exposures towards the south of the lease area, allogenic (buckshot) pyrite is associated with medium to coarse grained, clast supported polymictic pebble horizons, with clasts of yellow, black and green shale, black chert and quartzite and white vein quartz. This may represent a localized influx of unsorted material into an area of reworking.
It is proposed that the Heeversrust (Eslburg) Member comprises several, locally overlapping alluvial fans with entry, in general, from the west. Beyond the present position of the trough axis (80m to 100m east of sub-outcrop against the EA12 reef), it is envisaged that a lower energy braided fluvial environment predominated.
Deposit Type
The Target deposit is an alluvial fan-type deposit. The bulk of the mineralisation is specifically within the Eldorado Fan, a structure with dimensions of approximately 135m vertically, 450m down-dip and 500m along strike. This fan is connected to the subsidiary Zuurbron Fan, located between Target and Loraine, by a thinner and lower grade sequence of Elsburg (EA) reefs termed the Interfan area. To the north of the Eldorado Fan, several fans are known, including the Siberia and Mariasdal fans.