TAILINGS STORAGE OVERVIEW
Tailings is the finely ground rock left over after the gold and silver have been extracted. A tailings storage facility (TSF) is a structure built for the purposes of storing these tailings and other by-products from the gold and silver extraction process. The tailings from the Waihi mines are currently stored in tailings impoundments. Waihi’s tailings impoundments are carefully engineered rock structures that are designed to withstand significant seismic events. They are made up of several separate layers and are designed to the same specifications as a water-retaining structure.
The current TSF1A and TSF2 have insufficient capacity for the tailings volume from processing ore from the proposed Martha Open Pit Phase 5 and Gladstone pit.
As part of Project Quattro, OceanaGold Waihi is proposing to construct a new tailings storage facility (TSF3) immediately east of the current facilities, and raise the crest on TSF1A. Additional storage options are also being investigated that may include disposal using Gladstone pit.
Once these investigations have concluded, we will consult and seek feedback on all the options to inform the final project design.
TAILINGS STORAGE FACILITY 3
TAILINGS STORAGE FACILITY 3 - OVERVIEW
If approved, Tailings Storage Facility 3 (TSF3), will be constructed immediately east of the current facilities.
Geotechnical investigations have identiﬁed that TSF3 construction will ﬁrstly require material unsuitable for use in its foundation to be stripped from its footprint. This area is about 60 Ha, and the material will be stockpiled on OceanaGold Waihi land close to Trig North Road. It is likely that this material will be used for future rehabilitation work or mixed with other materials for use later in the project.
To provide for a structure of similar geotechnical integrity as the existing embankments, backﬁlling and contouring with a structural ﬁll mined from the Gladstone and Martha pits will be required.
During the initial foundation works, silt control will be provided to avoid dirty water discharging to waterways. There will also be an option to pump this water to the existing water treatment plant. At a minimum, erosion and sediment control will follow Waikato Regional Council guidelines.
TSF3 construction will also involve building an uphill diversion drain, perimeter drains to capture stormwater runoff, and a perimeter road to provide access for operation and maintenance.
For operations, a fully lined silt collection pond will be constructed in the lowest area of the TSF3 site. All runoff collected in the pond will be pumped to the water treatment plant. The new collection pond will also include an overﬂow spillway to the Ruahorehore Stream.
TSF3 will be fully lined to reduce the potential for any seepage to enter the groundwater system. At the lower levels, TSF3 will be lined with a 1.5 mm thick liner similar to that used in landﬁlls, while at higher levels, a layer of low-permeability, compacted clay is proposed.
All designs and plans are put through a rigorous review process and will need to adhere to all regulatory requirements.
If the required consents are granted, the structure will be extensively monitored throughout and beyond its construction phase, and all monitoring data will be annually collated and reported. The data and reports will be independently reviewed to ensure the ongoing structural integrity and safety of the structure. The independent review findings will be reported to the Council after each review.
The outer walls of the TSF3 embankment will be progressively rehabilitated to pasture during construction. This involves applying a layer of subsoil material, followed by a layer of topsoil and then agricultural seed mix. The land will then be grazed by young dry-cattle.
Native trees and shrubs have been planted on various areas of TSF1A and TSF2. These plantings provide a food source and nesting sites for birds. Similar planting is planned for TSF3.
Once the deposit of tailings ceases, it will continue to consolidate, and after a relatively short period of time, water quality in the tailings storage pond will reach a level suitable for direct discharge to the river.
When the tailings impoundments are finally closed, they will be partially capped, leaving a wetland and small pond on the top. The pond outlet structures will allow ﬁsh passage from nearby waterways. The ponds will be able to support the range of aquatic organisms typically found in such pond-like environments and wetlands. The riparian planting adjacent to the pond edge will trap sediment and nutrients in the runoff waters and assist in maintaining water quality.
TAILINGS STORAGE FACILITY 1A CREST RAISE
A lift can be achieved by steepening the slopes at the crest and reducing the TSF1A dam crest width. It is likely that embankment fill material required for the lift could be obtained from existing stockpiles and from Martha Open Pit Phase 5.
Raising the crest of TSF1A will increase its storage capacity. This will allow tailings discharge into the current storage facilities for longer, including the construction period of TSF3.
GLADSTONE PIT TAILINGS STORAGE
The option to backfill the pit with tailings is currently being investigated. In-pit tailings storage, as the name suggests, is the process of backfilling an open pit mine with tailings. OceanaGold Waihi is currently undertaking technical studies into the viability of in-pit tailings storage using Gladstone open pit. Once these investigations have concluded, we will consult and seek feedback on all the options; this will contribute to the final project design.
After mining of Gladstone is complete, the pit could be backfilled with tailings, capped with layers of rock and soil, and planted.
OceanaGold Waihi has used the in-pit storage and capping method at the now-closed Souvenir open pit at the Globe Progress mine in Reefton.
The management of tailings and the structural integrity of our TSFs are critical to community safety and environmental protection. Investigations into using Gladstone pit as a tailings storage facility are early and ongoing.
Regardless of the outcome of the studies, our approach to tailings storage management will remain the same:
1. Robust design and site management including consenting, operational management, monitoring and reporting.
2. Regular auditing of conformance with standards and consent requirements at a site and corporate level.
3. Independent reviews by third-party experts.