Hard rock mining
All of Petra’s operations are mining ‘hard rock’ kimberlite pipe diamond orebodies, as opposed to alluvial deposits (i.e. deposits of diamonds which have been removed from the primary kimberlite source by natural erosive action over millions of years, and eventually deposited in a new environment such as a river bed, an ocean floor or a shoreline).
Open Pit mining
Petra is employing open pit mining at the Williamson mine in Tanzania and the Ebenhaezer satellite pipe at Koffiefontein in South Africa. Mining of a diamond-bearing kimberlite starts with the excavation of a pit into the kimberlite pipe. In this process, called “open-pit” or “open-cast” mining, the initially weathered ore material is removed with large hydraulic shovels and ore trucks. Hard rock is drilled and blasted with explosives so the broken material can be removed.
Block caving is currently used as the mining method at Finsch, Cullinan, and Kimberley Underground mines in South Africa. Block caving was first introduced at Kimberley, South Africa in 1951 and is now used extensively as an underground mining method. It has proved to be a safe, low cost, high volume mining method. In recent years advances in block caving technology have allowed the mining method to be used for ore extraction in many large-scale orebodies world-wide.
Utilising this method, the full orebody or an approximately equi-dimensional block of ore is fully undercut to initiate caving. The undercut zone is drilled and blasted progressively and some broken ore is drawn off to create a void into which initial caving of the overlying ore can take place. As more broken ore is drawn progressively following cave initiation, the cave propagates upwards through the orebody or block until the overlying rock also caves and surface subsidence occurs. The mechanisms by which caving takes place under the influence of redistributed stresses and/or gravity are extensively studied and well known.
The broken ore is removed through the production or extraction level developed below the undercut level and connected to it by drawbells through which the ore gravitates to drawpoints on the extraction level. In most current block caving operations, the broken ore is removed from the drawpoints by LHD vehicles, although the Kimberley Underground mines still use the more traditional gravity-based grizzly or slusher systems. From the extraction level, the ore is transported to the haulage level and out of the mine, sometimes following underground crushing. Block caving may be used in massive orebodies which have large, regular “footprints” and either dip steeply or are of large vertical extent. It is initially a capital intensive process, requiring considerable investment in infrastructure and development before production can commence. However, once the caves have been propagated, it is a low cost mining method which is capable of automation to produce an underground “rock factory”. Historically, block caving was used for large-scale, low strength and usually low grade orebodies which produced fine fragmentation.
This method owes the first part of its name to the fact that work is carried out on intermediate levels (that is, sub-levels) between the main production levels. It follows the same basic principles as the Block Caving mining method, but the caves are smaller in size and not as long lasting. This method of mining is quicker to bring into production than block caving, as the related infrastructure does not require the level of permanence needed for a long-term block cave. However, the operating costs are slightly higher, as more drilling and blasting is required.
Petra utilises this mining method to supplement Block Caving, in order to provide production flexibility. For example, at the Finsch mine sub-level caving will be used as part of the mine’s expansion programme to maintain underground production levels during the transition from the Block 4 cave to the Block 5 cave.
The front cave mining method is used at Petra’s Koffiefontein mine in South Africa. Front caving was developed from the overdraw system used on the two lower levels of the sub-level caving operations at the Shabanie Mine, Zimbabwe. In essence, the method involves retreating on one or more levels from an initiating slot which can be in the centre of the orebody as at Koffiefontein, or against the orebody boundary. The lower level is the production level on which so-called semi-permanent drawpoints are fully developed ahead of undercutting on the upper level. This upper level also provides initial temporary drawpoints from which the swell from each blasted ring is drawn. The undercut is retreated in stages to points above the semi-permanent drawpoints in a manner similar to that used in sub-level caving. Ideally, the method should work best with two production levels rather than one.
Petra is mining ‘fissure’ orebodies at the Helam, Sedibeng and Star mines in South Africa. Erosion will eventually wear down a kimberlite ore body until only the root zone remains, leaving only this narrow ‘fissure’ zone of magmatic kimberlite to be mined. These fissures are vertical orebodies that are generally very narrow, with an average width of less than 70cm. Fissure mining, otherwise known as ‘dyke mining’, yields much lower tonnages and is generally complex because the ore body is narrow. More than one type of kimberlite can be present within dyke systems due to multiple intrusive phases varying in both space and time. This, too, can further complicate the mining process. Few companies have successfully tackled fissures for long periods, and Petra is regarded as a leader in the fissure mining field.