B2Gold digs down at Wolfshag

Augetto Graig - Construction of a four-metre-diameter, 200-metre-deep ventilation shaft commenced during July 2021 and is progressing on schedule, with the raise bore platform completed. The next phase of ground consolidation entails the drilling and pressure grouting of pylons 40 metres deep to ensure geotechnical stability of the raise bore platform.

According to Eric Mouton, project manager for Wolfshag, “Fruition of the feasibility plans for the underground operation at Otjikoto has created much excitement within the B2Gold Namibia team. Despite shaping by the Covid-19 epidemic and impact of nuisance groundwater, plans continue full steam ahead to reach the high-grade ore by early 2022. Recent exploration drilling confirms the continuity of high-grade mineralisation along the eastern periphery of the underground mine, the so-called OTG-shoot, which adds further potential to extend the underground mining life beyond 2025.”

B2Gold’s public relations superintendent for Namibia, Namasiku Nalisa, recently elaborated that the first blast of the Wolfshag Underground Project decline was on 30 October 2020, after stabilising the Otjikoto Phase Two pit highwall. The portal position is located at its base, 140 metres deep.

A fall arrest structure was then constructed at the entrance to the decline to ensure safe access for vehicles and personnel entering the underground operation, according to Nalisa.

Concurrently with decline development, in-pit infrastructure was erected, including a cement batch plant, control room, workshops, stockpiling facilities and installation of services to provide water, compressed air, electricity and communication via a leaky feeder system to underground.

“Currently the decline has advanced to a distance of 677 m from the portal, with its deepest end located 223 m below surface. The decline is 5 m wide and 5.5 m high and slopes at -7 degrees. Systematic ground support with resin bolts and wire mesh is conducted in all excavations and no personnel is allowed to work under unsupported roof,” she added.

Active groundwater extraction to the order of 280 m3/h is done from two deep dewatering wells drilled from surface. The second installation, PB02, completed during April this year, consists of a 20” borehole drilled to 430 m deep, cased with 16” stainless steel casing and equipment with a pump-motor assembly installed at 390 m below surface. Its purpose is to dewater the underground infrastructure ahead of production activities.

“A third installation is considered, depending on the effectiveness of the first two wells,” she said.

Groundwater enters the decline at a rate of 20-30 m3/h from unconnected Karst cavities, which cannot be actively dewatered from surface, and is a nuisance factor that has to be dealt with by the development contractor.

“The Murray & Roberts Cementation experts have been called in to perform grouting of these water-carrying fissures ahead of the production face,” said Nalisa.

Murray & Roberts Cementation and Lewcor (MRCL), as a joint venture, employ 106 people on site, of whom 18 are expatriates and 87 Namibians.

The core group of expatriates, consisting of experienced underground development specialists and operators, provide on-the-job training to their Namibian counterparts.

A number of Namibian service providers are subcontracted to provide services to the operation, such as catering, accommodation, cleaning, transport and civil construction, providing numerous other employment opportunities. At the peak of initial development activities, during raise boring, MRCL will directly employ 127 Namibians.

During September 2021 the raise bore will be mobilised to site to drill a pilot hole 200 m down to intersect the decline. A reamer head will be fitted to the drill string and will cut the four-metre-diameter shaft into the rock as the drill string is retrieved back up. Upon completion of the ventilation shaft, two 400 kW primary ventilation fans will be installed at the collar of the ventilation shaft to force fresh air directly to the underground operations, without mobile equipment or residence time heat addition, she elaborates.

Dynamic modelling of the ventilation circuit will determine air speed and volume of air available in all underground workings. An enclosed escape way will be installed in the ventilation shaft to provide a secondary exit way in fresh air in the event of a mine fire. The ventilation exhaust will be out via the decline.

A partial redesign of the underground infrastructure was completed during the first half of 2021, reducing unproductive waste development with more than 3 000 m and optimising the ore grade. As part of this process, the decline was repositioned and reduced in length, allowing access to the closest ore stopes. A 600-metre long exploration drive to define the southward, down-plunge extension of the existing reserve was added to the 2022 production plan. Diamond drilling from the exploration drive will also investigate known mineralised occurrences at depth below the envisaged mine workings.

A total of 184 blasts were done to develop the main decline and associated underground infrastructure, such as sumps, re-muck bays and substation bays. An advance of 4.05 m per blast is targeted in the main decline.

Over the life of mine, 124 000 m of long holes will be drilled and charged with 912 tonnes of emulsion explosives. A total of 116 stopes will be mined and backfilled with 860 000 tonnes of cemented rock fill and 210 000 tonnes of uncemented rock fill. A total of 1,56 tonnes of ore will be extracted grading 5.36 g/t at a rate of 1000 tonnes per day, over a period of four years (from year 2022 to 2025) to produce 270 000 ounces of Au, incorporating 90% recovery and 10% dilution factors.