The Great Dyke of Zimbabwe developed as a series of initially discrete magma chamber compartments, which coalesced as the chambers filled. On the basis of structure, style of layering and continuity of layers, the Great Dyke has been sub-divided into five sub-chambers, namely the Wedza, Selukwe (Shurugwi), Sebakwe, Darwendale and Musengezi subchambers. The stratigraphic units in each sub- chamber are classified into the ultramafic (lower) and the mafic (upper) sequence. The ultramafic rocks are dominated from the base upwards by dunite, harzburgite and pyroxenite, while the mafic rocks consist mainly of gabbro and gabbronorite. Narrow layers of chromitite occur at the base of cyclic units throughout the ultramafic sequence. The PGM-bearing horizon is known as the Main Sulphide Zone (MSZ), which is part of the lower sequence and is located below the contact with the mafic sequence. The MSZ is located in the P1 pyroxenite some 5m to 50m below the ultramafic/mafic contact. The MSZ is a continuous layer, 2m to 10m thick, and forms an elongated basin. The zone strikes in a north-northeasterly trend and dips between 5° and 20° on the margins, flattening towards the axis (centre) of the basin. The areas where the dip is less than 9° is referred to as the ‘Flats’; these have historically been the target for mining due to the ease of operating. The areas with dips between 9° and 14° are referred to as the ‘Upper Ore Resources I’ and those with dips above 14° are referred to as the ‘Upper Ore Resources II’. Peak base metal and PGM values are offset vertically with palladium peaking at the base, platinum in the centre and nickel towards the top. Visual identification of the MSZ is difficult, therefore systematic monitoring of the reef using various sampling methods is needed to guide mining.

The current mine infrastructure consists of five portals (decline shafts). The deepest operating depth is some 310m at Bimha Mine (Portal 4). Boundaries between individual portals are based on a maximum strike length of 3km to 6km or are terminated on known geological discontinuities such as major faults. Minor faults and other geological discontinuities are present at the operations and are accounted for as geological losses during the Mineral Resources and Mineral Reserves estimation process. At all the mines, Zimplats employs a mechanised room and pillar mining method on a narrow reef to extract ore from stopes whose nominal width is 2.5m.

The trackless mechanised machinery consist of low profile single boom face rigs for drilling, low profile roof bolters for support drilling, 10t load and dump (LHDs) and 30t dump trucks which are referred to as a fleet. A single fleet is allocated about 20 rooms and its total face length is dependent on the sizes (widths) of the pillars and rooms at the operation. The mining cycle consists of face drilling and blasting, support installation, face preparation and sampling and loading and hauling. The production target for each fleet varies from 16 000t in the Upper Ores I and more than 21 500t of ore per month, in the flats depending on the particular mine, ground conditions and the existing pillar layout. The typical layout comprises 7m panels with different sizes of in-stope pillars, which are determined by the depth below surface and geotechnical considerations. At Mupfuti and Bimha mines, a series of barrier pillars are set out on a ‘paddock’ around the smaller stope pillars. This pillar layout is meant to contain any likelihood of cascading pillar failure should in-stope pillars fail. Ngwarati and Rukodzi mines do not have barrier pillars nor paddocks owing to their shallow depth below surface. At all the mines, the spans of rooms may decrease and pillar dimensions may increase in bad ground. A combination of roof bolts and tendons is integral to the support design.

The new mine, Mupani mine, is now on reef and is on course to replace the mature Rukodzi and Ngwarati mines which will be depleted in FY2021 and FY2025, respectively.

At Rukodzi and Ngwarati mines, the broken rock is loaded onto trucks by LHD and trucked to a surface crusher while Mupfuti and Bimha mines have underground crushing plants and ore is tipped into the crusher and conveyed to surface.

Ore from the mines is processed by two concentrators (one at Selous Metallurgical Complex and the other at Ngezi).

Approximately one-third of the mined ore (2.2Mt) is transported by road trains to the concentrator at SMC, which operates a single semi-autogenous grinding mill (SAG), while the rest is transported by overland conveyor system to the crusher and ball mill concentrator modules at Ngezi.

Zimplats has in place two operating flotation plants at Ngezi Mine and the Selous Metallurgical Complex (SMC), and a smelter at the SMC.

Ore from the mines is processed by two concentrators (one at SMC and the other at Ngezi). The concentrator at Ngezi has two similar modules, which were commissioned in 2009 and 2013, respectively. Each module has a capacity of 2.0Mtpa, which makes up a total of 4Mtpa against total production of 6.4Mtpa. The SMC concentrator has a capacity of 2.2Mtpa. Concentrate from both Ngezi plants and SMC is then smelted in an arc furnace and converted to matte at SMC. The resulting matte is despatched to Impala’s refinery in Springs under the terms of a LoM agreement with Impala.

The mined ores are treated separately in the concentrators, reflecting the differences in their respective mineralogy. The ore is milled together with water to produce a fine pulp, which is pumped to the flotation section. Here chemicals are added to the pulp enabli ........