By Dr Nicolaas C Steenkamp
Nkomati mine was South Africa’s only primary nickel mine and has now been placed on care and maintenance. Dr Nicolaas C Steenkamp took a drive past to see how the landscape has changed and to revisit its potential.
Hidden between the pine tree plantation on the Eastern Escarpment between the towns of Badplaas and Barberton, is the Nkomati Nickel Mine. The winding road that leads down to the mine has long been a favoured route for motorcycle enthusiasts. It provides scenic bends down the escarpment through the grasslands of the Highveld to the plantations established in recent years, offering a panoramic view of three open pits and surface mine infrastructure.
The Nkomati Nickel Mine was developed in the Uitkomst Complex, indicated to represent a conduit ultra-mafic deposit associated with the Bushveld Complex. The Uitkomst Complex intruded between the chemical dolomite and clastic shale sediments of the Transvaal Supergroup and basement granite. Weathering resulted in the partial exposure of the conduit along the escarpment. The Uitkomst Complex consists of a layered succession of up to 8km in length, with a thickness of up to 800m.
Original mining started with gold mining on the farm Little Mamre, which also gave its name to the mining operation. The gold mining operation was, however, short lived and it took several years before the layered mafic intrusion gathered enough interest to start a deep exploration drilling program. The presence of a massive sulfide orebody on contact between the intrusion and the basement rock initiated the underground mining operations with the sinking of a vertical shaft to access the orebody.
The Uitkomst Complex was unique in terms of the range of mineralisation that it presented. The sulphide mineralisation hosted nickel, platinum group minerals (PGMs) and copper mineralisation. The massive sulphide orebodies were developed and this was the first target of the underground mining. The lower portion of the Uitkomst Complex also hosted the Main Mineralised Zone (MMZ) of disseminated and semi-massive suphides. This was overlain by the chromatiferous-peridodite mineralised zone (PCMZ) that contains both disseminated sulphides and chromite. This was in turn overlain by a massive chromite layer. Each one of these mineralised zones were separately developed at various stages of mining.
One of the open-pit operations.Image supplied by Dr Nicolaas C Steenkamp
The Nkomati mine used trackless mining equipment to drill, blast, load and haul the ore to an underground crusher. The trackless fleet consisted of drill rigs, LHDs, dump trucks, a utility vehicle, a roof bolter and a long-hole drill rig. Stoping involved primary and secondary extraction. A backfill plant was used to ensure that the orebody was extracted at a rate in excess of 98%. After the ore had been crushed at the underground primary crusher, it was hoisted and transported by an overland conveyor belt over a distance of approximately 300m to a concentrator processing plant. The ore was then fed into the secondary crusher at this processing plant. The crushed ore was fed into the primary and secondary ball mills, where it was milled. The milled ore was then passed through a flotation circuit from which two types of concentrates were produced. Following this process, the moisture content of the concentrates was reduced by passing it through two filter presses. The high-grade concentrate was then sent to Botswana for smelting. The matte was sent to Xstrata for refining.
The underground mining operation was followed by the development of three open pit operations that exploited the MMZ and PCMZ, including the more weathered material. One of the main challenges in the open pit operation in the MMZ was the frequent occurrence of massive calc-silicate xenoliths. These calcsilicate xenoliths formed as a result of thermal alteration of the host dolomite country rock by the intruding mafic conduit.
Total proven and probable reserves in the MMZ and PCMZ as reported on 30 June 2019, were estimated at 6.79million tonnes grading 0.3% nickel. MMZ stockpiles were estimated at 200 000t grading 0.27% nickel, while PCMZ stockpiles were estimated at 600 000t at 0.19%. Total mineral resources as at the same date were estimated at 46.35million tonnes grading 0.40% nickel, 0.13% copper, 0.02% cobalt and 0.97g/t platinum, palladium, rhodium and gold.
Underground: calc-silicate xenolith in ultramafic host rock containing the disseminated sulphide mineralisation of the nickle ore. Image supplied by Dr Nicolaas C Steenkamp
Due to the high variability of the MMZ and the PCMZ, the stockfeed produced was processed at two concentrators, during both underground and open pit mining operations. The Nkomati underground mine was placed on care and maintenance in December 2015 and the last of the three open pits reached the end of its economic life in March 2021.
There is however potential to access the remaining underground mineable pillars and walls. The plan is to access the underground workings from the open pit through the high wall on the north-eastern side of the pit where it has stopped and access the underground with multi-portals. The expected ramp-up period would be in the order of 36 months at a very low layout cost. At a volume of around 200 000t a month, its good grade and by-product resplendent underground orebody would underpin a life-of-mine of about 15 to 20 years for the Nkomati Nickel Mine operations. The demand generated by the EV battery market and increase in the nickel commodity market price over the last couple of months1 may be the catalyst to seriously consider restarting underground mining operations.
The potential re-establishment of the Nkomati Nickel Mine remains one of the probable projects to keep an eye on as an indicator of the ability of the South African mining sector to capitalise on the EV market.