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Location: 2 km E from Welkom, South Africa
2VHF+3M, Free StateWelkomSouth Africa
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Mining takes place in a structurally complex zone between two major north-south trending faults: the De Bron/Homestead Fault in the west and the Saaiplaas Fault in the east. The orebody has been subjected to severe deformation and contains numerous folds (anticlines and synclines) as well as an abundance of smaller faults. The dip of the reef bands is very variable – from 45 degrees to the east, adjacent to the western side of the lease, to less than two degrees in parts of the southern area.Production is hosted within two quartz pebble conglomerate bodies, developed above unconformity surfaces, the Basal and the B reefs. Approximately 80% of the centares (1 centare = 1 square metre) are from the Basal Reef horizon and 20% from the Basal B Reef horizon.The primary facies of Basal Reef intersected at Masimong 5 Mine is the Black Chert facies. It comprises two upward fining cycles. The lower one has a small to medium pebble conglomerate at its base, overlain by a grey ortho-quartzite. The upper unit consists of a pyrite-rich grit. The upper cycle tends to erode large segments of the Lower higher grade cycle in the south-west, with resultant lower values. Towards the east of the shaft, it appears that only the upper unit is present. Carbon is almost always present on the bottom contact of the lower cycle of the black chert facies.The B3 facies of the B-Reef comprises polymictic medium to large pebble-supported conglomerates, with porphyry clasts, and yellow and green shale pebbles (fragments). The shale clasts are angular and tabular with the most common being yellow to mustard in colour. The non-durables account for 5-10% of the total clast composition. The durable pebbles are mainly black, smoky and vein quartz, sub rounded to rounded. Pebble sizes range from 0.7-4.5cm, and are poorly sorted. The matrix has a yellow-green argillaceous composition with noticeable yellow and black speckling. The B3 can occur on the shale or above the B2 quartzite. It can be a solid band or inter-bedded. The B3 facies is the primary facies being mined at Masimong and range from a pebbly quartzite to matrix or clast supported conglomerate with carbon present in the latter.
