Rosh Pinah
Rosh Pinah is a mature underground operation with production sourced from a number of discrete zinc-lead sulphide orebodies. The mine has been in operation since 1969 and has continually replenished ore resources from extensions to the known Mineral Reserves through exploration drilling and development.
The mine has previously been able to generate Mineral Reserves based on the exploration activities sufficient only to sustain a five-year plan. The lack of Mineral Reserves and short mine life has made it difficult for the mine to attract and retain people to work on the mine, especially after the opening of the nearby Scorpion Mine, which has a significantly longer projected mine life. The short mine life also makes the raising of finance to address issues such as housing all that more difficult.
The mine has identified three options for the future:
- Managed closure over a five-year period;
- Maintain status quo and continue to replenish Mineral Reserves; and
- Adopt the “Modified Focus” plan that increases development and exploration drilling for three years and generates sufficient reserves for at least a 10-year LoM.
The latter option would enable the mine to attract financing for renovating and upgrading the processing plant and providing housing for mine employees.
The Modified Focus plan has been approved by the Rosh Pinah Zinc Corporation Board subject to review by the Board depending on the viability of the exploration project. For the purpose of the valuation of the mine SRK have based their model on maintaining the status quo, that is a five-year plan (supplied by the mine) that replenishes Mineral Reserves on an ongoing basis with necessary capital for further exploration.
Mining has been conducted from eight principal orebodies (“orefields”) and future Mineral Reserves are contained in five orefields. The main focus is currently the Eastern Orefield that has a significantly higher grade (10.8% zinc). Production is currently limited by the capacity of the process plant which is ~780ktpa. Total tonnes mined are in the order of 1Mtpa. Access to the underground workings is through three single declines with portals at 400m above the mean sea level (“AMSL”). The orefields are accessed via drives and ramps with the main levels developed at 30m vertical intervals. The deepest production level is currently 20 Level (20m AMSL). Ventilation is effected through surface exhaust fans. Several auxillary fans are used to direct air to the working faces.
Inflow of water has increased slightly with the increase in depth. The mine has recently installed a cascade dam system in order to provide a more consistent supply of water to the production areas.
The mining method employed is sub-level open stoping which is locally referred to as “silling” and “benching”. The footwall and hangingwall contacts of the orebody are exposed at 30m intervals through development. The benches are then drilled with up-rings with 74mm holes to the footwall and hangingwall contacts on the level above.
The broken ore is loaded by LHDs into 20 – 40t trucks and hauled to the main tip on 370m level. The ore is then crushed and transported by conveyor to surface. Average haul distances are in the order of 3.5km. The mine has recently installed a service bay on 370 Level close to the tip where daily maintenance will be carried out. All maintenance has previously been carried out on surface which has reduced the availability of the machines. The daily servicing particularly of drill rigs on 370 Level is still not conducive to high levels of availability.
The mine has increased production by 65% over the last three years. Development in the current year is planned to increase by 4,000m and exploration drilling to 43,000m in order to increase confidence levels in Mineral Resources that will sustain a longer mine life. All development is being undertaken by the mine. Mineral Resource management is undertaken on the mine, mine design and scheduling is undertaken by an outside contractor who spends two weeks of a month at the mine. Monthly scheduling of the rings is done by utilising Minesite a 3D program to Xpac, an autoscheduler.
Only those mineral resources in the Measured and Indicated categories and contained within Mining Area No. 39 of the Republic of Namibia are converted into Mineral Reserves.
Mineral Reserves are estimated with reference to a 3D mining model drawn on the geological model. The design process utilises knowledge of the mining method, equipment constraints and geotechnical limitations in order to achieve optimum resource recovery. The model shows the stoping layout based on the chosen mining method, the development layout and the main infrastructure components required for access and ventilation. In-situ tonnage and grades are calculated for the main recoverable minerals, i.e. zinc, lead, silver and copper and reported as RoM quantities.
The mining model is designed with prior knowledge of the rock mass and ore body characteristics which influence the mining method. The rock mass ratings at Rosh Pinah enable ore extraction by open stoping methods without backfilling. Localised geotechnical problems are encountered and countered by installation of the relevant roof support. The planned reserve extraction ratio (including recovery from temporary support pillars) and waste dilution levels are audited by outside consultants.
The local geohydrological system has been modelled for environmental reasons but does not impose constraints on mining. There are no aquifers other than localised fault and joint systems which are de-watered as mining progresses. The Rosh Pinah ore bodies are generally suited to extraction by sub-level open stoping method. Where the dip of the orebody does not allow gravity flow the room and pillar method is applied. Reserves are blocked out with reference to the resource model without further reference to cut-off grades. The economic reserve is determined through cost/revenue estimates. Reserves are estimated on 3D models of detailed stope and development designs.
Grade control is in the form of accurate drillhole ring design and the accurate drilling of drill holes to the required length. Underground observations confirmed that this was taking place.
Planned dilution is estimated from the reserve models. No further dilution factors are applied. Mining recovery is assumed to be 100% on the basis of tonnage reconciliations. It is known that ore densities are undervalued and this may be accounting for the real mining losses (the reconcialiation indicates that the ore density is slightly greater than what is currently being used in tonnage estimates and is balancing out real mining losses). Investigations into ore densities are currently being undertaken. All planned waste is included in the total reserve tonnage and grades.
New infrastructure is limited to underground tunnel extensions which are reported under the headings “Decline Development” and “Infrastructure Development” in the LoM schedule.
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