With a population of about 4,700 people, the town of Waihi is situated on the east coast of the North Island of New Zealand.The surrounding area supports horticulture and dairy farming. Waihi is an important service centre for these activities, as well as playing an increasing role in tourism.
Waihi township is within ninety minutes drive of half the population of New Zealand, including the major cities of Hamilton, Auckland, Rotorua and Tauranga. Local areas of natural beauty include the Karangahake Gorge, Waihi Beach, and the coastline north and south.
Waihi township slopes gently down from the foot of Martha Hill to the Ohinemuri River and has an altitude averaging about 90m above sea level.
The climate of Waihi is affected by its local topography. The rainfall in Waihi is relatively high, averaging in excess of 2,100 mm/year. The township lies at the northern end of an elevated basin, which is the source of the Ohinemuri River and its tributaries. The Ohinemuri River flows westwards just south of the town and then through the Karangahake Gorge to the Firth of Thames.
The mining licence was granted for the open pit Martha Mine in 1987. The Favona underground operation, began in 2004, has been producing ore from stopes since 2006. Martha and Favona produce around 100,000 ounces of gold and around 750,000 ounces of silver annually.
Together the Martha and Favona mines support a direct workforce of around 350 people. This comprises Newmont Waihi Gold employees and the two major contractors Macmahon Contractors (NZ) Ltd and HWE.
The Martha Project was the first major hard rock mining operation to be commissioned following the resurgence of the gold mining industry in New Zealand in the late 1970s. The Martha Mine is situated in the Waihi township, and the processing plant, waste rock embankments, water treatment plant and Favona Mine are located two kilometres away in a rural area. Because of its location, the Martha mining operation has received a high level of public scrutiny. This is reflected by the stringent conditions set for the project.
Mining at Martha is by open pit methods. Ore and waste rock are crushed at the surface facilities area prior to transportation along a 2.7 kilometre conveyor. On arrival at the processing plant gold and silver are extracted from the ore using conventional carbon-in-pulp treatment methods.
Waste rock is used to construct an engineered earth embankment to contain the tailings. A water treatment plant treats excess water prior to discharge
The Extended Project.
As described in the Timeline, the modern Martha Mine operation commenced in 1987. In January 1999, following the granting of the relevant consents, the Extended Project commenced. For more information see Permitting the Martha Mine
The main features of the Extended Project are described below:
- Information gained from exploration indicated that additional ore lay within the boundaries of the Martha Mine Extended Project. This ore resource was estimated to be approximately 7.5 million tonnes at 3.3 grams per tonne gold and 26 grams per tonne silver; equivalent to approximately 800,000 ounces of gold and 6.4 million ounces of silver.
- The Extended Project increased the Martha Mine's life by an additional seven years.
- The original pit was deepened and widened. Pit dimensions: length at the surface approximately 850m, width 600m and depth around 250m.
- Adjacent to the open pit is a surface facilities area. In this area, workshops, offices, fuel bays, crushers and explosives magazines are located.
- In order to reduce noise effects on the town, noise bundswere built in selected areas. The largest and most obvious is the Grey Street noise bund. Its function is to reduce noise, particularly from the surface facilities area. The noise bunds are grassed and planted with trees to improve visual effects.
- After excavation of the rock, the material is hauled in trucks to the crushers. Following crushing, the ore is transported to the processing plant, and the waste rock is carried to the waste disposal area for selective placement within the waste rock embankment.
- Ore and waste are transported by conveyor at different times. As part of the Extended Project, the conveyor was upgraded and widened from 1.0m to 1.35m to increase its capacity.
- Ore is delivered to the processing plant. As part of the Extended Project the processing plant was upgraded to allow an increase in ore processing, to a maximum of 1.3 million tonnes per annum.
- The Extended Project produced additional waste rock and tailings. The waste rock to be disposed of increased from approximately 13 million BCM (bank cubic metres) to approximately 26 million BCM. Similarly, the amount of tailings increased from 10 million cubic metres to 17 million cubic metres.
- A second tailings storage facility (Storage 1) had already been permitted under the original 1987 consents. Investigations for the Extended Project confirmed this to be the most suitable site for the modified storage facility (Storage 1A).
- The water treatment plant was upgraded to allow up to 20,000 cubic metres of water a day to be treated and discharged (subject to a number of conditions including water quality and river flow). A pipeline was constructed to a second discharge point.
- On completion of mining, the open pit area will be rehabilitated to form a lake. Surrounding areas will be developed to form parklands.
- At the waste disposal area, rehabilitation of the waste rock embankments will be progressive. At closure the surfaces of the tailings storage facilities will be rehabilitated to form a combination of wetlands, ponds and grassed areas.
The Southern Stability Cutback
In 2006, nearing the end of Martha Mine open pit operations, the southern stability cutback (SSC) began. This involved mining a wedge of material amounting to approximately nine million tonnes from the southern pit perimeter to the base of the open pit. The southern pit wall was altered from a 45 degree angle to a 40 degree angle, providing a gentler slope and effectively removing weight from the wall to help to stabilise it. This work extended pit operations to mid 2010.
The Favona Underground Mine
Late in 2004, construction began on the decline for the Favona underground operation. While some ore was mined during construction, full ore production from stopes began in 2006. Currently Favona ore and Martha ore are both fed to the processing plant and treated alternately.
In September 2008 the Favona Mine had almost 12,000 metres of drives, including the decline, and was approximately 335 metres below the surface.
Exploration
Newmont Waihi Gold’s exploration and geological investigations continue. The company invests around $9M a year on exploration near-mine and in the region.
above: The town of Waihi. The Martha Mine can be seen in the middle of the picture, with the tailings impoundment behind it. To the left (north) lies the Coromandel Peninsula, to the right (southeast) the Bay of Plenty. This photograph was taken in May 2008.
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