A 'sinkhole' has opened up near 'old underground workings' at the moth-balled Golden Cross mine.
It is about 25m across, with slumped material falling into the quite large hole and is situated on the eastern wall of the previously backfilled Golden Cross open pit.
Hauraki District Council strategic planning project manager Mark Buttimore said the hole was first noticed in early June when a helicopter crew flew over the area doing unrelated work. They contacted Coeur Gold about the sinkhole.
Mr Buttimore said council had a meeting at the site of the slump with Environment Waikato and New Zealand engineering firm Tonkin & Taylor (working for Coeur Gold).
"Tonkin & Taylor are monitoring the area, they have monitoring points near the slump which appears to be above old underground stopes which might not have been completely filled when Coeur finished its mining at Golden Cross," Mr Buttimore said.
Coeur Gold senior vice president for environment and community, Luke Russell told Waihi Leader the site area has been secured with construction of a deer fence around the fracture to ensure public safety.
"With regard to potential remediation efforts, the hole appears to have "choked itself" off and collapse of upslope material in to the hole appears to have effectively stopped. We will continue to monitor the situation to confirm site stabilisation. When weather conditions permit, we will evaluate reshaping the sides of the hole by pushing the loose open pit fill in to the hole and regrading and seeding the area."
He said the hole is located within the former open cast pit at Golden Cross.
"The pit was partially backfilled as part of the site rehabilitation conducted in 1999-2000. The hole appears to be related to a weaker zone of materials that follows an inactive fault zone within the former open pit mine.
"This weaker zone of material appears to have intersected with a vertically drilled non-backfilled underground mine stope (or working). It appears this weaker material within the faulted zone collapsed in a conical or wedge like shape into the unfilled stope. It is now about 5-10 meters in depth."
He said it is quite localised and not expected to develop further.
Tonkin & Taylor geotechnical engineer, Grant Loney said: "The company was monitoring the extent of the feature and carrying out an assessment of possible remedial backfilling options."
Hauraki District Council instructed the Coeur Gold caretaker to close the road gates to ensure members of the public did not get near the sinkhole.
- The Waihi Leader