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There’s life left in old Martha yet as Newmont starts extracting the eastern wall of the open pit and pitches for a new underground mine. By ALAN TITCHALL
Newmont uses a three-year rolling gold price average to assess the viability of its projects and with gold over US$1200 an ounce, the company sees plenty of reason to hang onto its Waihi hold. Preparatory work for the east layback began in late April as work on the south wall layback finished. The new project will increase the pit size by a marginal three percent by extending the pit’s length by 100 metres (to 960 metres), yet produce 1.7 million tonnes of ore. Trio design
With the existing underground Favona mine due to close next year, Newmont Waihi says a second underground mining opportunity exists made up of the Trio, Union and Amaranth veins that sit 500 metres to the northeast of the Favona underground portal near Union Hill.
Trio will reach the same depth as Favona at 330 metres and the design includes upgrading underground facilities, such as the wash pad, and expanding amenities to cope with extra personnel, and the addition of an underground service bay. If consented (and it requires a full resource consent application process) the Trio project would extend underground mining at Waihi and has the potential to produce about 1.7 million tonnes of ore (and 3.5 million cubic metres of waste), for about 200,000 ounces of gold at six to seven grams per tonne. One in three trucks will cart up ore, as opposed to waste rock, and each 80-tonne load will contain about five ounces of gold. If approved, mining could start around the middle of 2012. Newmont is anticipating resource consents conditions for Trio to be the same as those for Favona, which operates in close proximity to a number of residential properties and complies with stringent limits on noise, vibration and dust.
Martha’s last ore
The east layback is not big – about half the size of the work carried out over the past three years mining the south wall of the Martha pit. About $10 million has been budgeted for the works involved, which includes moving infrastructure, particularly the crusher, 11kV lines, and workshop. The east layback entails a triangular shaped cutback from the edge of the existing eastern pit perimeter – about 100 metres in an east-west direction by 160 metres in a north-south direction. The layback area is contained by the Grey Street noise bund, which will be extended to the east and then grassed and planted. As work is within the noise bund, the project did not need notification, however getting consents and approvals for features of design did delay progress on the project until April this year. The east wall of the open pit partly overlies the historic ‘Milking Cow’ collapse zone. This wall was formed in 2003 as an interim slope but subsided and failed. The east layback will mine the failed slope and produce an overall pit shape that is more stable than the current pit, says Newmont, and effectively places the majority of the historic underground cracking and collapses, associated with and around the Milking Cow collapse zone, below the future lake level.
Also relocated was an historic refinery building, one of three known to still remain standing in the country. The building, dated around 1913, was moved to a site near the pit rim walkway where it can be appreciated by the public.
Q&M Vol.7 No.4 August-September 2010 |
For the past three years Newmont Waihi Gold has been mining the south wall of the Martha Mine open pit to complete its stability cutback. Now the focus is on mining the eastern layback, which will operate under existing consents until at least April 2014. At the same time, Newmont is pitching consents for a new underground mine accessed from the present Favona portal.
The new mine would use the existing Favona portal and two access drives, 590 metres and 790 metres long, which would be tunnelled to the top and bottom of the new prospect.
The principal source of noise at the new project is expected to be the ventilation fan, likely to be positioned on Union Hill. Under the District Plan the nighttime noise level is 40dBA.
Contracted preparation work involved clearing pine trees from the top of the north-eastern wall, and dismantling and relocating the crushers started over the winter. Only minor modifications were necessary to the ore conveyor to accommodate the new crusher location.