Modikwa Platinum Mine, South Africa

The Modikwa platinum mine lies along the border between the Mpumalanga and Limpopo provinces, around 15km north-west of Burgersfort. The mine is an underground operation extending over an area of 14,278ha.

Owned jointly by African Rainbow Minerals (ARM) and Anglo Platinum, the mine began operations in the mid-1920s. During 2008-2009, the mine produced nearly 2.45 million tons of run of mine (ROM).

Modikwa mine reserves

The mine contains up to 56 million tons of mineral reserves. Measured and indicated reserves are estimated at between 115.2 and 145.7 million tons.

Modikwa platinum mine geology

The Modikwa mine is 450m deep with an average density of 3.72t/m³. It hosts a north to north-west trending igneous layer that dips 10° towards the west.

"The Modikwa platinum mine lies along the border between the Mpumalanga and Limpopo provinces."

Both the upper group 2 reef (UG2) and Merensky reef are found within the mine area. The UG2 reef is present as a chromitite layer characterised by an average thickness of nearly 60cm. The three thickest chromitite layers overlie on top of the main seam.

The mine area is rampant with gentle wave-like movements of the UG2 with less than 2m amplitude. The north shaft area is characterised by randomly distributed potholes of different sizes. In contrast, the south shaft area contains fewer potholes. The area is more disrupted by faulting.

The Onverwacht hill area is marked by an abundant presence of large ultramafic pegmatoid intrusions. The intrusions disturb and replace the UG2 locally.

Platinum mine exploration

To estimate the mineral reserves, over 700 surface diamond drill holes and more than 1,000 underground channel samples are carried out. Reserves are classified as measured mineral resources if a block falls within 66% of the semivariogram range (from the closest sample) and between six to 30 samples are considered for estimation.

Similarly, if ten to 30 samples are used for estimation and the block lies within the semivariogram range, the resources are categorised as indicated mineral resources. If a block is found beyond the range of the semivariogram and between 30 and 100 samples are used, it is categorised as inferred mineral resources.

"During 2008-2009, the mine produced nearly 2.45 million tons of run of mine."

Logs and values are maintained in independent electronic databases after the rigorous validation of statistics. For estimation purposes, the values are combined. Based on the sample analysis, the 4E grades are limited at 13g/t. Two fire-assay techniques are used for estimating individual assay grades for platinum (Pt), palladium (Pd), rhodium (Rh) and gold (Au). Nickel (Ni) and copper (Cu) grades are determined using wet chemical techniques.

Estimation is performed separately for the three subunits that divide the mining cut of UG2. This includes the UG2 chromitite layer, the hanging wall and the footwall. Depending on the space between the drill holes, two-dimensional mining block models, including 250m x 250m and 500m x 500m-sized blocks, are established. Using ordinary kriging Pt, Pd, Rh, Au, Ni and Cu grades are interpolated for the UG2. For hanging and footwall units, the interpolation method used includes inverse distance squared.

"A fleet of automated equipment is used for mining operations at the Modikwa platinum mine."

The chromitite's width and density are also estimated into the block models. Ore lost because of pegmatoidal intrusions, faults, dykes and potholes is accounted for by applying discount factors to tonnages ranging between 10% and 30%.

To measure the quantity of footwall waste included in the mining cut, a minimum mining cut of 102cm is used. In case the main seam thickness and hanging wall exceeds 102cm, an additional 5cm of footwall waste is considered. As the basal contact of the UG2 layer is high-grade, it is not left in the footwall while mining.

Modikwa mining operations

A fleet of automated equipment is used for mining operations at Modikwa. Accessing UG2 is achieved via two primary declines from the ground. ROM processing is carried out at the Modikwa concentrator. The platinum-group element (PGE)-rich concentrate is sent to the Polokwane smelter and the refining facilities of Anglo Platinum.