Ironsand prospecting on the West Coast

Ironsands laced with gold down the West Coast of the South Island have become a target of exploration companies both on and offshore, writes LINDSAY CLARK.

Beach sands have been mined for gold since diggers first hit the beaches during the West Coast gold rush of the 1860s, and exploration companies in recent decades have focused on the heavy mineral sands as a source of titanium-rich ilmenite and rutile, but much of the heavy sands down the West Coast also consist of iron oxides sourced from the metamorphic rocks of the western Southern Alps. This includes magnetite, commonly used as an ore by the world steel industry.

The rocketing price of iron ore in the past few years, as feedstock for China’s rapidly growing steel industry, has turned mineral explorers attention to new sources of ore around the world.

Iron ore prices were little changed at about US$30 a ton for the 25 years until 2005, when price climbed to US$55 a ton. Two years ago the price was up to US$80 a ton, then in 2008 iron ore soared to US$140 a ton.

Although the sharp world economic downturn is expected to cause a sharp drop in iron ore prices drop in 2009, so far explorers are still showing keen interest in New Zealand ironsands – on the west coasts of both the North and South islands.

Offshore, down a 500 kilometre strip of the West Coast, marine diamond experts De Beers Marine is exploring for ironsand as well as placer gold for Seafield Resources, owned by E Oppenheimer & Sons.

Australia’s Fortescue Metals - the self-proclaimed “new force in iron ore” - is also interested in the West Coast’s iron sands onshore and has applied for a 2600 square kilometre prospecting permit mostly inland from Greymouth up the Grey Valley.

Fortescue, which has rapidly expanded to be Australia’s No 3 iron ore exporter just behind BHP Billiton and Rio Tinto, has applied for approx 25,600 square kilometres of exploration and prospecting permits around the country, “principally for iron sands exploration,” the company says in its September 2008 quarter project update.

Two of Fortesque’s largest prospecting applications are in the onshore and offshore west coast Northland beaches all the way from Auckland to Cape Reinga.

Interestingly, Fortesque is also seeking to explore for offshore “detrital iron mineralisation” off the Pilbara coast of West Australia.

Further south down the south Westland coast between Ross and Haast, Alexandra-based Goldmines New Zealand and Auckland-based GSL are understood to be also exploring for ironsand as well as gold.

Consultant geologist John Youngson of Dunedin who is a specialist in heavy mineral sands including alluvial gold, says geologists have overlooked the South Island West Coast as a source of iron ore.

Dr Youngson says that much of the ironsands found down the West Coast have less impurities from a steelmaking point of view than the titanomagnetite ironsand used by New Zealand Steel for its Waiuku plant.

Though the iron oxides constitute only a small proportion of the rocks in the Alps, natural processes serve to sort and concentrate the heavy mineral sands along the coast.

The rocks in the Southern Alps are being constantly uplifted. High rainfall and glaciation has moved massive volumes of rock and sand down to the sea. Wave action grinds rock into sand and northward flowing sea currents have swept away the lower density sand grains leaving the heavier sands behind.

Dr Youngson says the sea has the effect of upgrading the deposits. “The sea is the ultimate concentrator”, he says.

Heavy mineral sands and the even heavier gold is often left together in certain locations such as near the gold mining town of Ross.

“Some of the Ross gold deposits are contained in high concentrations of ironsand,” Youngson says.

Ironsands are often carried quite a long way offshore. Airborne magnetic surveys carried out offshore off Westland by Seafield Resources are expected to be a guide to potential gold and ironsands deposits.