Green from Gold "The Rehabilitation of Golden Cross"

Introduction

Tucked away in a Coromandel valley down a dead end road is a modern New Zealand gold mining success story. Faced with rugged terrain, some of the highest rainfall in the country, and complex geology, the Golden Cross mine has endured cyclones, a major land movement, and antimining misinformation to become a productive operation, and the first modern mine in New Zealand to successfully move into planned closure and final rehabilitation.

above from left: Golden Cross prior to mining in 1990, during active mining in March 1994, and during rehabilitation, March 2001.

The Site

The Golden Cross mine site is situated eight kilometres north-west of Waihi in the Waitekauri Valley at the base of the Coromandel Peninsula. The mine site ranges from 270 metres to 460 metres above sea level on steep to rugged terrain, which receives in excess of three metres of rain per year. The site is bordered on three sides by land controlled by the Department of Conservation, including the ecologically sensitive Coromandel State Forest Park.

A Brief History

Between 1895 and 1920 the Golden Cross underground mine produced just over two and a half tonnes of gold. When the mine closed the site became farmland.

In 1977 two exploration licences were taken out. Initial investigation showed potential. A prospecting licence was obtained in 1980 and a drilling programme commenced in 1981. By mid 1985 exploration had revealed a potentially mineable deposit. Project feasibility followed. An Environmental Impact Report, a mining licence application and applications for water rights were lodge in late 1987. Water rights were granted in 1988. The mining licence was granted in April 1990, having been the subject of an appeal to the Planning Tribunal. Bonds of $12.1 million were required to be lodged to allow the project to proceed.

The new Golden Cross mine began production in December 1991. Mining was by underground and open pit methods. The Golden Cross underground mine was the first underground gold mine to operate in New Zealand since the nearby Martha Mine at Waihi closed in 1952. A skilled workforce was developed mainly by training workers hired locally.

above: Geologists underground at Golden Cross during the active mining phase. Over 11 kilometres of tunnels reaching a depth of 300 metres below the surface were driven through the rock.

Coeur d'Alene Mines Corporation purchased the Golden Cross mine in March 1993 in an 80%/20% joint venture with New Zealand company Viking Mining.

Between December 1991 and April 1998 the mine produced a total of 20.5 tonnes of gold and 52 tonnes of silver with a present day value of approximately $430 million, most of which was spent in New Zealand. At its peak Golden Cross employed 243 staff, as well as indirectly employing approximately 750 people in service and support industries. The company's annual staff payroll exceeded $10 million.

The mine officially closed on 17 December 1998 and became the first modern mine in New Zealand to move into planned closure and final rehabilitation. The area is now used for grazing, wetland and native habitat.

The Mining Operation

Ore hauled from the underground and open pit mines was processed at the mill located on the site. Over 11 kilometres of tunnels reaching a depth of 300 metres below the surface were driven through the rock. Waste rock was used to backfill the tunnels and build the tailings impoundment. Ore was crushed then put through the mill to remove the gold and silver. Once the precious metals had been removed, the remaining fine rock and water mix, now called tailings, was pumped to the tailings dam. Cyanide used in the gold extraction process was recycled using a new, patented process. This was a world first for its use on a commercial scale.

above left: The Cyanisorb plant recycled cyanide used in the gold extraction process to the leach circuit for reuse.

above right: The Golden Cross process plant with the Coromandel State Forest in the background, 1997. The plant has since been removed and the area rehabilitated.

The waste rock embankment that impounded the tailings was constructed with suitable rock from the open pit. On the surface the area may just look like a hill and a lake, but it is actually a very carefully engineered structure. It consists of a series of layers and zones, each of a different material, and each carefully placed. In addition the structure features an underground drainage system. The construction of the dam took place over a number of years in a series of lifts as material became available and storage capacity was required. Fault zones run through the area, so the dam was designed and built to withstand a Maximum Credible Earthquake much larger than the 1931 'quake that devastated Napier which measured 7.9 on the Richter scale.

above: The waste rock embankment is a carefully engineered structure consisting of a series of layers and zones.

Tailings from the processing plant were pumped through pipelines to the tailings dam. Excess water was decanted and piped to the water treatment plant before discharge.

The dam contains over five million tonnes of tailings. As it was filled, the tailings consolidated. At closure a cap of fill and topsoil was placed around the perimeter of the dam extending out over the tailings. The area was contoured and revegetated. The central area was left as a body of water that drains clean water through the outlet channel into the Waitekauri River.

above: As part of the rehabilitation process, staff lay geotextile cloth around the perimeter of the tailings pond. Clay and topsoil is then layered on top and the area revegetated.

Environment

New Zealand's permitting and operating requirements are amongst the most stringent in the world. Numerous Resource Consents require monitoring and reporting.

Environment Waikato took the lead role under the RMA for environmentally related issues with Hauraki District Council occupying the role of second tier regulator behind the regional council.

From the original planning stage it was obvious that effective water management would be the key to successful environmental management of the project.

Steep terrain and high rainfall on site required the mine to use an advanced water management system. During operation a series of diversion drains collected water from surrounding hills and diverted it away from the site, into the local stream system. Underground drains collected any natural or tailings seepage from the tailings impoundment area and directed this water to the water treatment facility. Water pumped from underground was directed to settling ponds or the water treatment plant prior to discharge into the Waitekauri River.

above left: The water treatment plant. above right: Monitoring water on site.

All water on site was monitored according to a rigorous quality and quantity schedule. At the water treatment plant, in addition to a comprehensive water quality monitoring system, juvenile rainbow trout were used to establish a real-world real-time biomonitoring system to monitor the potential effects of discharge water on the receiving stream ecology. Three test aquaria were set up; one contained 100% river water as a control, another 20% water treatment plant discharge and 80% river water, and another 100% water treatment plant discharge. Water was continuously pumped through the aquaria. Treated water was taken from the retention pond 6–12 hours before planned discharge. If fish appeared distressed the discharge to the river could be stopped prior to that water entering the discharge pipe and the water treatment plant put into recycle so that potential problems could be identified and rectified.

This was the first use of continuous effluent biological monitoring in New Zealand.

above: Juvenile rainbow trout became the modern equivalent of the miners' canaries when they were used to establish a biomonitoring system to monitor the potential effects of discharge water on the receiving stream ecology.

During operation any potential long-term effects of the mine discharge into the Waitekauri River were assessed by monitoring the plants and animals that live in the aquatic environment. Changes in the abundance and distribution of aquatic life are regarded as reliable indicators of an effect. Independent studies of algae, macro invertebrates (e.g. snails, caddis fly) and fish were conducted. Annual analyses of fish tissue for bioaccumulation were also performed. Regulatory agencies were also involved, cross-checking results and providing independent data.

above left: Regular monitoring of the Waitekauri River. above right: Over 100,000 thousand native trees and shrubs have been planted in and around the mine site.

The on site and in steam biomonitoring programmes have shown no measurable impacts to fish or other aquatic organisms during the operation of the mine.

In addition to its regulatory requirements the company undertook an extensive programme of rehabilitation and enhancement in line with its philosophy of 'Producing and Protecting'. This was a voluntary initiative by the mine and has lasting benefits:

1. Recognised stands of native trees have been extended to establish local corridors with similar stands and enhance wildlife habitat.

2. Riparian habitat enhancement programmes were instituted to protect stream banks from erosion and increase habitat quality both in-stream and on riparian boundaries. This has involved co-operating with local farmers to exclude stock from riparian areas.

3. Over 100,000 native trees and shrubs have been planted on and around the mine site.

4. Swampy areas located away from the riverbed were fenced and planted with native species to encourage wetlands establishment and enhancement.

Plant material was generated from seed eco-sourced from the local area and from seedlings raised at a local nursery to preserve the local vegetation's distinct genealogy.

Consultation

The Joint Venture has benefited from regular ongoing consultation with a Peer Review Panel and a local Community Consultative Group. Initially developed during the operational phase of the mine, both groups have continued through into the closure phase. The Peer Review panel members were firstly approved by Environment Waikato and then jointly appointed by Environment Waikato and Coeur Gold.

above: Local residents and members of the Community Consultative Group meet on site to observe rehabilitation progress in 1998.

Specific conditions of the Water Rights required the establishment of a Peer Review Panel to assist Regional Council staff assess the technical compliance of the operations. The panel members were appointed and funded by the company, but were independent of it, and reported to council staff. The Peer Review Panel provides assurance to the regulating authorities that the ongoing onsite works conformed to good industry practice. The perspective provided is that of an objective industry professional. The system worked extremely well in practice with panel members providing balanced insight for council staff on industry good practice in specialised fields of engineering, hydrology, geochemistry, revegetation, and later landslide mitigation and mine closure. The panel removed the problems that industry sometimes experiences when trying to present complex technical material to council staff, while at the same time providing council staff with a degree of comfort that their decisions were based on expert recommendation.

The Community Consultative Group has met on a regular basis for four years. The group is comprised of Regional Council staff members, District Councillors, environmental groups, iwi, and local residents. Regular site visits have been well attended by members of the CCG and the local community.

Land Movement Remediation

In mid 1995 routine monitoring for a future raise of the tailings impoundment identified a re-activation of historic land movement beneath the impoundment. This deeply seated movement was ultimately determined to be occurring over an area of one square kilometre, where a slab of ground up to 100 metres thick was moving slowly down-slope on an old slip surface.

above: A slab of ground up to 100 metres thick on which the tailings dam was built was moving slowly downslope on an old slip surface,

The company notified regulatory agencies and held public meetings to discuss the situation and address public questions and concerns. A formal Community Consultative Group was established to keep local people informed of site developments.

The media struggled to come to terms with the situation. Used to dealing with disaster scenarios, they had neither the technical knowledge nor the vocabulary to adequately explain the situation. The company produced an eighteen minute video explaining in layperson's language the situation and likely remedial action. This was widely circulated. At the same time a world class investigation and remedial measures programme was implemented at a cost of $27 million.

above left: GPS monitoring of land movement. above right: Monitoring of water quality at horizontal bores.

Throughout 1996 the Golden Cross mine site became perhaps the most measured and studied piece of ground in the world as mine staff and experts from New Zealand and overseas worked to first identify the problem and then work out how to deal with it.

The solution took several forms. To reduce groundwater pressure and lubrication of the slip surface where movement was occurring, a series of horizontal drainage holes was drilled at numerous sites. A drainage tunnel between four and five metres in diameter was driven through the rock some 15 metres below the slip surface. Holes bored upwards into the slip surface drained more water. Water volumes in the tailings impoundment were reduced. A saddle filter buttress was constructed of rock quarried nearby to add strength to the tailings impoundment structure.

Throughout all of these operations groundwater and surface water were constantly monitored. Water of a suitable quality was released off site. If necessary, water was diverted to holding or settling ponds for further treatment.

By late 1996 movement rates had been reduced significantly. By late 1997 the problem had been generally controlled, and by 2000 the annual rate of movement detected was less than the annual uprising of the Southern Alps.

Throughout all of this time the structural integrity of the tailings impoundment was maintained.

Closure

All mines eventually close. It is how they close that is vitally important. In the past the traditional definition of mine closure was to surrender the mining licence and walk away.
This definition no longer applies. Changes in the industry have evolved in line with changes in public expectations of stewardship for the future. Modern mines are planned with closure in mind.

above left: Fertiliser application is an integral part of the rehabilitation process. above right: Cattle graze on rehabilitated pasture in 1999. In the background the removal of the process plant is almost complete.

Today, mine closure is an integral part of the mining cycle, and is investigated and planned for before a mine begins to operate. Mine sites are rehabilitated and stabilised so they are suitable for a sustainable land use that is compatible with the surroundings. Former mine sites in New Zealand are now being used for farming, forestry, recreation and conservation.

Effective closure involves a range of issues. Closure must meet all regulatory requirements as laid down in the conditions on the Mining Licence and Resource Consents. In addition, human resource management and community involvement and consultation add a social facet to the procedure.

Rehabilitation activities at a modern gold mine include: decommissioning the mine, providing surface drainage and erosion protection across the entire site, establishing self sustaining vegetative cover, meeting water quality standards, and minimising post-closure maintenance requirements.

The Peer Review Panel has described the Golden Cross operation as a world class mine closure.
Closure at Golden Cross focussed on the issues of handling site stormwater, compacting and sealing potentially acid generating waste rock, and creating long term stable structures. These objectives had been planned for both during initial planning and mine operation.

Buildings and equipment have been dismantled and removed. The mine's operational footprint has been recontoured to more closely resemble the surrounding landform.

Some administrative buildings remain. These will be used for recreational or educational or business use of the site. A range of partnership options is currently being investigated.

Major drainage channels have been constructed to handle runoff from a 1000-year rainfall event (715mm in two days).

The water treatment plant will continue to operate until all water on site meets discharge criteria and can be released into the Waitekauri River without any mechanical treatment. From this time the wetland areas will act as passive water treatment systems.

above left: The open pit has been capped, recontoured, and drainage channels added. above right: The rehabilitation process almost complete, 2001. The recontoured open pit can be seen in the foreground with the rehabilitated tailings dam in the background. The bush at bottom left is part of the Coromandel State Forest Park.

Potentially acid forming waste rock has been sealed and capped. The open pit has been contoured with low permeability mine overburden and revegetated. Drainage channels have been formed. The tailings impoundment has been partially capped and the perimeter revegetated. Drainage channels maintain the level of the freshwater pond.

Areas surrounding the minesite that had previously been cleared for farmland have been revegetated with native species. Bush areas adjacent to the Coromandel Forest Park have been fenced to exclude stock and provide a buffer zone.

A comprehensive monitoring programme keeps track of landform stability, vegetation rehabilitation, surface water quality, groundwater quality, and flora and fauna.
Final rehabilitation will have been completed when all consent conditions have been satisfied and the area has been returned to a self sustaining, stable landform.

Bonding provisions ensure that no ratepayer funds are required to successfully rehabilitate the site or cope with any future maintenance.

above: The tailings dam, summer 2001. Walking tracks, footbridges, and picnic facilities have been provided. Information panels along the tracks display the history of Golden Cross.

Conclusion

After another productive life the Golden Cross mine has closed for a second time. Within a short period of time little evidence will remain of the mine's successful second life. This very lack of evidence is part of the successful closure operation nearing completion as Coeur Gold redevelops a self-sustaining environment at the head of the Waitekauri Valley.

There will be other evidence, however: the sort modern gold mines like to leave behind.

Evidence of a productive partnership between the mine and the community.

Evidence of the stewardship role a modern gold mining company actively accepts.

And evidence of the commitment of the Coeur d'Alene Mines Corporation to Producing and Protecting.

It's what gold mining in New Zealand is all about.

And it's evidence that you really can get Green from Gold.