Ocean floors hide mineral treasure trove

By Kent Atkinson

Seafloor deposits of gold, zinc, copper, silver and lead may only need to be 15m thick to make mining them underwater profitable, says a company prospecting off the coast of New Zealand.

Seacore, a specialist in marine drilling, is taking core samples of the seafloor massive sulphide (SMS) deposits, using a drill rig mounted on a work vessel.

It has been hired by a London-listed mining explorer and developer, Neptune Minerals, to prospect for billions of dollars of minerals northeast from White Island, along the Kermadec Arc, which is riddled with seafloor volcanoes and hydrothermal vents.

The company is headed by Robert Goodden, who told New Scientist magazine he was a little apprehensive.

"Seldom have I been in a place where the consequences of a 'yes' are so significant, and of a 'no' so dreary," he said. "The whole world will be watching."

In effect, a decade's research by New Zealand scientists and the commercial prospecting now under way have potentially put the mineral wealth of the world's oceans up for grabs, and the Neptune venture is being closely watched by mining companies.

There are 20,000km of volcanic arcs in the western Pacific region alone, of which only 2 per cent have been surveyed. Recent estimates indicate that these arcs contain at least 200 submarine volcanoes.

In addition to sampling the New Zealand deposits for Neptune, Seacore will take its vessel to waters around Papua New Guinea to do similar work for Nautilus Minerals, of Sydney.

If the deposits are shown to be economic, the work will create a new global industry - and debate over whether mining the seafloor is less intrusive and environmentally destructive than mining on land, or whether it will destroy potentially unique, uncatalogued reservoirs of biodiversity.

Where the vents occur, valuable minerals dissolved in the superheated water of up to 320C from under the earth's crust are left as deposits on the seafloor when they hit the cooler waters of the sea.

David Heydon, of Nautilus Minerals, told New Scientist that even though some of the sulphide deposits still growing on the seafloor might cover only 4000 sq m each, all the necessary mining equipment could easily be moved from one deposit to another.

"Instead of needing a minimum deposit size of, say, 20 million tonnes for your investment to be economic, as on land, you could mine 10 sites of two million tonnes."

A study his company commissioned from consultants Worley Parsons Engineering showed that even if a deposit contained only copper, mining it would cost only about half of the price of developing a new land-based mine.

Giant goldminer Placer Dome has spent US$2.7 million ($3.86 million) exploring the Nautilus prospects, and will pay Seacore US$4 million for the core drilling off PNG. Placer will receive up to 75 per cent of any gold mined, and Nautilus will get the non-gold minerals, particularly copper.

On the Kermadec Arc, mineral samples dredged up by New Zealand scientists in 1996 were found to contain 18 per cent zinc (by weight), 15 per cent copper, and six parts per million of gold - a higher concentration than some on-shore gold deposits.

The initial finds were at the Brothers volcano, which is three times the size of White Island and lies in 1850m of water.

Since then, researchers have explored many of the 75 seafloor volcanoes along the arc and found many have hydrothermal vents belching super-hot water with so much dissolved mineral content that they are called "black smokers".

Seafloor mining
* Undersea vents are natural smelters of precious metals with "chimneys" of metal-rich deposits building up around them.
* Eventually the chimney formations collapse under their own weight and the process starts again, creating big, metal-rich deposits.
* Over millions of years, they can become the enormously valuable mineral deposits mined on dry land, such as at Broken Hill and Mt Isa in Australia.


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