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15/9/2017 — Conference Special
AusIMM2017: Martha wall slips still under remediation
By Simon Hartley

A decision on the viability of OceanaGold Corporation (TSX & ASX: OGC) to resume mining its closed Martha pit mine at Waihi appears to be at least a year away; plus a further three years of preparation.

The historic Martha pit suffered two slips in 2015 and 2016, the latter and largest bringing down more than 2 million tonnes of the North Wall, blocking access and partially filling the bottom of the pit, costing about $US3 M in remediation work so far.

At the AusIMM New Zealand Mining Conference in Christchurch, OceanaGold geotech engineer at Waihi, Liam Ireland, updated delegates on progress.

The North Wall was now “stabilised,” but only after much remediation work, including dewatering and bringing down other threatening slips, with the pit area now under care and maintenance.

Ireland reconfirmed what had been speculated at the time of big slip in April 2016, that historic underground works and weakness in the actual rock structure had contributed to the failure; at times further undermined by periods of heavy rain.

The 2016 slip had been closely monitored and was predicted, with the public in Waihi warned of the likelihood of an imminent slip. The top of the North Wall slip was just 40 metres from a public road.

Ireland had time-lapse photographs and video of numerous slips and also the subsequent remediation works, including heavy machinery working atop the slip; showing little room for mistakes.

“This was a live beast and we couldn't be complacent about what we were doing,” he said.

The most recent remediation had been work on an east block and north-west block, of which the latter required about 10%-15% removed.

To date OceanaGold has removed about 500,000 tonnes of rock.

Following the presentation, Ireland said OceanaGold still wanted to resume mining, given there was an estimated 80,000 ounces of gold still below the pit's bottom.

In calendar 2016 at Waihi OceanaGold extracted 300,000 oz of gold, from its underground Correnso and Empire tunnels. Waihi has an established further 570,000 oz of gold to be mined and mine life beyond 2019.

He estimated it would take a further year for the company to look at all options, then consenting could take a further year, then site preparation could take another two to three years.

*Simon Hartley is senior business reporter and assistant chief reporter for the Otago Daily Times.

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Liam Ireland.
The North Wall slip at the Martha mine.