The Great Dyke developed as a series of initially discrete magma chambers, 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 and the stratigraphic units in each sub-chamber are classified into the ultramafic (lower) and the mafic (upper) sequence. The ultramafics are dominated from the base upwards by dunite, harzburgite and pyroxenite while the mafics consist mainly of gabbro and gabbronorite. Narrow layers of chromitite occur at the base of cyclic units throughout the ultramafic sequence. The most important segment of the Great Dyke is the Hartley Complex which is the 100 kilometre long portion that straddles the boundary between the Sebakwe and Darwendale sub-chambers. It contains approximately 80 per cent of the total PGM resources in Zimbabwe. PGM distribution within the Hartley Complex is asymmetric with higher grade, narrower profiles along the western margin.

The platinum-bearing horizon is known as the Main Sulphide Zone (MSZ) which is part of the lower sequence and is located from about 5m up to 50m below the contact with the mafic sequence. Much of the MSZ and overlying mafic sequence have been eroded away and part of the Mafic sequences remain as the hills of the Great Dyke and outcrops of the MSZ have been mapped adjacent to the hills. The MSZ is a thin, continuous, stratiform zone whose upper layers have a high sulphide enrichment of 2 to 8 % of iron - nickel - copper sulphides and is remarkably persistent in all subchambers. It occurs all along the Great Dyke where it overlaps the bronzitite/ websterite contact and ranges from 2 to 10 m thick in pyroxenite. The base of this nickel - copper - rich layer is straddled by a one to five metre thick zone of elevated precious metal values (Pt, Pd, Au, and Rh) and contains up to 5% disseminated sulphides while the sulphide content of the rest of the PGM zone is less than 0.5%. This drop in sulphide content with depth in the MSZ is related to the distribution of metal grades in the ore body so it is generally used as a grade marker to identify the ore zone during mining and exploration activities. It can normally be located visually in drill core and with careful observation it can also be identified underground.

The mineralisation is vertically gradational and distributed around this high-grade pivot zone which is the marker horizon. The selected cut on which Mineral Resources are based is determined by economic cut off within the MSZ and not based on a geological boundary. Given the difficulty of visually locating the MSZ, the smaller faults and related jointing give rise to inherent dilution of ore during mining. The effect of dilution is minimised because of the gentle transition from economic to sub economic grades especially in the footwall. Other disruptions to the mineralisation include post mineralisation intrusions into the MSZ such as large areas of disrupted metal profiles and washouts have been located elsewhere in the MSZ though no Bushveld style potholes have been mapped at Ngezi.

Production at Zimplats is from trackless mechanised underground operations and one contractor operated open-pit mine. The four underground mines use a room and pillar mining method which is designed to extract ore from a 2.5 metre (m) cut in the production stoping sections and up to 3.5m in the main development roadways. The underground mines are set out on dips of less than 9° while the open pit is operating on dips less than 6°. Work is underway to establish an equally efficient means of mining the resources which exceed 9° up to an initial target of 14° using appropriate low profile trackless mining machinery (TMM).

The mining envelope for the underground operations has been increased from 3km on strike to 6km while using the same equipment and maintaining the same efficiencies as on the shorter strike. The access declines (incline shafts) are centralised in each of the mines’ resource boundaries, which splits each of the mines into a North and South Section. The position of the declines is governed by the local geology and ground conditions therefore there is invariably some asymmetry between the 2 sections of the mine and this is regularised during the mine production sequencing and scheduling. The TMM consist of a low profile (LP) single boom face rig, for production drilling, and an LP roof bolter ,for support drilling, a 10t load and haul (LHD) and a 30t dump truck.

At the various portals, the broken rock is either trucked out to a surface crusher or tipped into an underground ore pass and then conveyed to surface using the footwall decline conveyor.

Some portals employ the use of an underground crushing plant and the crushed ore is conveyed to surface.

A low angle shear in the deeper sections of the Bimha Mine has impacted ground conditions over a large area. The accelerated deterioration in conditions resulted in the temporary closure in July 2014. The affected production teams from this mine were redeployed to the other three underground mines. Redevelopment is on target and the mine will achieve design production volumes of 1.8Mtpa in April 2018. The Ngezi South Open Pit, which was reopened during 2015, will then be shut down.

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

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 enabling the valuable material to be separated from the waste. The concentrate containing the minerals is sent to the smelter for further processing and the waste material (or tails) is pumped to tailings dams.

The concentrate is dried before being charged in submerged electric arc furnaces. Here the PGMs and other metal sulphides are separated into furnace matte from the unwanted material (or slag oxides). Iron sulphide is removed from the furnace matte by oxidation to an iron oxide slag in the converter section of the smelter, by blowing air through the molten furnace matte. The converter matte is granulated and sent to the Refineries, while the furnace and converter slags are granulated and reprocessed in the concentrator to recover the remaining valuable metals.

The Zimplats Smelter operations consist of a concentrate filtration plant, a Drytech flash dryer, a modified Elkem / Hatch 13.5 MVA circular furnace, two 10’ x 15’ Peirce Smith converters, and a matte granulation plant. The Zimplats smelter uses a three-field electrostatic precipitator, supplied by ELB-Brandt, for removal of particulates in the gas streams from both the converter and furnace.

Thickened concentrate (45% solids) is pumped from the thickener into one of the two receiving tanks equipped with agitators. The tanks are mounted on load cells, for metallurgical accounting purposes, and each full tank receipt is treated as a batch. Concentrate from receiving tanks is then pumped to an agitated storage tank.

A single Drytech flash dryer is in use, and is designed for a throughput of 10 600 tons of concentrate per month (dry), or to evaporate 2 248 tons of water per month at 17.5% moisture in cake.

Wet concentrate cake from the filter press is dropped into a reclaim hopper through the double screw feeders onto a dryer feed conveyor.

The concentrate cake is then discharged into a heavy-duty back-mixer. A dried concentrate recycle stream is mixed with wet filter cake in the back-mixer (this practice was stopped since the Larox was commissioned). The wet concentrate is discharged from the back-mixer onto a cake disintegrator. The disintegrator serves the purpose of distributing feed evenly, and breaks concentrate further to feed into the incoming hot-gas air stream.

The concentrate is conveyed up the drying column with a hot air stream, and concentrate is dried on the way. Collection of dry concentrate is done through a cyclone underflow. The cyclone overflow goes through a settling chamber or dryer heat-sink box, then into the dryer bag-house. The bag-house is a fabric filter, jet-pulse type, with fabric material resistant to acids and moisture, and operates at a temperature of 140ºC. The dryer product enters the bag-house through the inlet at the bottom section of the bag-house, and rises upwards through the bags. Ultra-fine concentrate is trapped on the bags. Each row of bags in the module is pulsed down in rapid sequence to release the concentrate fines; this sequence is fully automated. An emergency air dilution valve has been installed upstream of the bag-house to protect the fabric filter bags from over-temperature conditions (opened on signal from a temperature switch). The released concentrates are dropped onto a screw conveyor to the concentrate holding bus. Outlet moisture content of the dried concentrate has been consistently averaging 0.2 – 0.3% (cf. <0.5% design).

The concentrates from the dryer holding bins are pneumatically conveyed to the furnace holding bins. The furnace is the ‘hub’ of the smelter operation. The circular (12 m shell diameter) three-electrode submerged-arc Elkem furnace is rated at 13.5 MVA with a power density of 134 kW/m2 at 12 MW.

Zimplats has two 10’ x 15’ Peirce-Smith converters with 22 tuyeres (18 in use), which are utilised to produce converter matte of 0.6% Fe from furnace matte containing 42% Fe and 31% S.