The Wafi-Golpu mine in Morobe prefers to release its tailings into the waters of the Huon Gulf, according to an updated feasibility study report released last week.
Three types of tailings management options have been considered during the various studies undertaken since 2012, those being various terrestrial tailings storage facilities, dry-stacking and DSTP (deep sea tailings placement).
The study of 45 sites for terrestrial tailings storage options for the Wafi-Golpu Project has highlighted the following:
- The required storage volumes would result in a large disturbance footprint over an area which can have high traditional heritage and economic value, high biodiversity, and/or displacement of communities and their livelihoods;
- The project area has high seismicity and complex geology, including active faulting, which could at some sites result in liquefiable soils. Complex design would be required to partly mitigate such factors, and that would carry high risk and high cost in both construction and ongoing operation;
- The project area has high rainfall and large water catchment, which would require significant and costly water management treatment solutions. Any structure would contain very large amounts of water with commensurate risks;
- Due to terrain and geotechnical complexity, multiple storage sites and types of tailings management would be required for a life of mine solution; and
- The mining operation would be exposed to complex tailings operations, closure and rehabilitation risk and the residual risks for terrestrial tailing storage facilities would remain high in perpetuity.
DSTP studies have been conducted as part of the 2017-18 work programme. Oceanographic and environmental studies in the Western Huon Gulf to date have confirmed that area to be a highly suitable environment for DSTP.
It hosts a deep canyon leading to a very deep oceanic basin with no evidence of upwelling of deeper waters to the surface.
The tailings are expected to mix and co-deposit with a significant, naturally occurring loading of riverine sediments from the Markham, Busu and other rivers that also are conveyed via the Markham Canyon to the deep sea.
Around 60 million tonnes per annum (mpta) sediment has been estimated.
The pelagic, deep-slope and sea floor receiving environment has a very low biodiversity as a result of the riverine sediment transport, deposition
and regular mass movements (underwater landslides).
These same riverine sediments are expected to also bury the co-deposited tailings at closure and promote benthic recovery to pre-mine conditions.
Oceanographic studies have confirmed that a 200m deep outfall for the tailings disposal will meet the draft PNG Guidelines for Deep Sea Tailings Placement, prepared by the Scottish Association for Marine Sciences on behalf of the State of Papua New Guinea.
In the light of the factors considered in relation to terrestrial tailings storage, the outcomes from the study of 45 terrestrial sites and the outcomes of the DSTP study work undertaken to date, the updated Feasibility Study identifies the use of DSTP as the preferred tailings management solution.
Papua New Guinea has three existing active DSTP operations (Lihir, Simberi, Ramu Nickel), one permitted (Woodlark) and one closed (Misima).