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Three staff exposed to unsafe levels of naturally occurring mercury vapour at Oceana Gold's processing plant at Macraes in East Otago prompted a $500,000 upgrade of its gold smelting room.
The incident, in January 2010, found no adverse effects to the three staff who were treated orally for removal of mercury.
Voluntary tests on up to 30 other staff found no signs of raised mercury levels, Oceana's chief operating officer, Mark Cadzow, said when contacted yesterday.
The incident prompted a critical report from the Department of Labour, citing breaches of health and safety regulations, but no prosecution was sought as Oceana had dealt with the issues.
Mr Cadzow said the mercury vapour was not used as part of the gold-processing system but occurred naturally in the ore being processed at the time. He was unable to say why some ore would have higher mercury content levels than others.
"Routine testing during the past 20 years has not shown it [raised mercury levels] to be an issue," Mr Cadzow said yesterday.
The incident in January 2010 was not reported to the Department of Labour until June, and was made public following an Official Information Request to the department by Fairfax media.
Mr Cadzow said the department was not told until June about the incident because once Oceana picked up the raised mercury levels in its routine audit process, it instigated more tests to confirm the higher levels, then informed the Department of Labour of its findings.
The three staff were reported to have been exposed to 20 times the safe mercury-vapour levels.
"There were varying degrees of elevated levels recorded, but at all times the necessary protective equipment was worn by our Oceana Gold employees, thereby negating potential harm," Mr Cadzow said.
He said the gold room had been upgraded, at a cost of up to $500,000, including new ventilation systems and enhanced personal safety equipment.
It was coincidental that a new smelting room would be built soon, he said.