In the era of underground mining at Martha 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
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:
- the pit will be a normal mining operation during its life, but post construction the mine will be allowed to fill to RL 1104 (mine datum) and a lake formed. For further information see Rehabilitation and Closure.
- areas of the slopes above RL 1104 will be rehabilitated both during and after mining
- the slopes need to be designed for long term stability
- the Cornish Pumphouse must be protected.
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 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.
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.
The south wall of the open pit, adjacent to the Cornish Pumphouse, received special attention in design and analysis. In 2006 the pumphouse was relocated to a stable site (for more information on the Cornish Pumphouse relocation project click here) and work began on the southern stability cutback. This will be completed in 2010.
To separate ore from waste rock, drill samples are 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. Other areas can be ripped using a bulldozer. Ore and waste rock are mined by excavators loading onto trucks.
