In the era of underground mining at Martha between the 1880s and 1952 the primary objective was just to get the gold out of the ground. There were no rehabilitation plans in place, and mining was not planned with rehabilitation of the site in mind. For more information see How Have Environmental Considerations Changed Over Time?
Pit design and pit wall stability
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| Above: The truck on the haul road gives an indication of the size of the open pit. To view a fact file detailing the mining vehicles used at Martha Mine click here. |
At the modern Martha Mine site, rehabilitation of the open pit was an important part of the planning of the project. The pit slope design recommended for the Extended Project took into account the following factors:
These factors are somewhat special compared to many post-mining situations in other countries, and they are influenced by the location of the open pit within the township of Waihi. As such, a degree of conservatism has been built into the design process.
The upper slopes for the Extended Project are 10 degrees flatter than for the previously licensed project; 30 degrees over 70% of the pit walls above the proposed lake level. This provides a flatter slope on which to establish vegetation.
The geometry of the pit slope depends to some extent on the geology. In hard 'fresh' rock the slopes can be steeper than with lower strength rock. The pit slopes consist of a series of batters and berms. Above lake level the batter slopes are approximately 15 metres high, and the berms approximately six metres wide. The berms allow for control of rainfall runoff, and provide access at various points around the pit for maintenance and monitoring. A single haul road spirals from the north east corner to the pit floor.
The potential for land outside the pit being affected by pit instability has been assessed in detail. The stability was checked for historic factors that included pit flooding, earthquake loading and the effects of old stopes. The filling of the pit to create the lake will improve the long term stability of the pit walls. The slopes have been designed conservatively and in accordance with best engineering practice. In addition to this, worst case what if scenarios have been assessed. Based on these analyses a 35 metre wide buffer zone has been proposed surrounding the pit so that residences and business premises will not be affected.
Geological sampling
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| Above: Geological mapping of the pit floor. |
To separate ore from waste rock, samples (nominally 5.0kg) are taken from the pit floor at set intervals (nominally 1.0m) along survey lines, or channels (nominally 7.5m apart). The samples are crushed, dried and subsampled, then sent to an assay laboratory for analysis. The assay results are used to mark out zones of ore and waste rock, which are mined separately. The definition of ore and waste rock depends on the cut-off grade. If the cut-off grade is one gram per tonne (1.0g/t) all rock with more than 1.0g/t of gold will be marked out as ore while all rock which contains less than 1.0g/t of gold will be marked out as waste rock. The cutoff grade at Martha Mine is as low as 0.75 gm/tonne.
Mining
After sampling and mapping, some areas require blasting to loosen the rock prior to excavation. Ore and waste rock are mined by excavators loading onto trucks. Mining machinery is large, and it can be difficult for the operator to delineate the areas of ore and waste. For this reason a person stands close to the excavator to ensure that material is correctly separated.
Dewatering
In order to develop the open pit it is necessary to dewater the mine to ensure that the water level remains below the pit floor. A maximum of 15,000 cubic metres of water per day with a yearly average of 10,000 cubic metres per day can be pumped from the open pit. Some of this water is used on site; for example in dust control. The remaining water is pumped by pipeline along the conveyor route. Excess water is treated at the Water Treatment Plant prior to discharge to the Ohinemuri River. For further information on how Martha Mine deals with water see Water Treatment.
Monitoring
A significant amount of monitoring takes place in and around the open pit. This includes:
The Peer Review Panel
An independent peer review panel comprising experts in the fields of geotechnical engineering, hydrogeology and rehabilitation carries out inspections, reviews data and reports to the regulatory agencies.
