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Scientist on the trail of seafloor minerals - 02/01/1998

Scientists preparing to explore the ocean floor to the ocean floor to the north east of New Zealand believe it could represent the most exciting mineral prospect since gold was first discovered in New Zealand.

They are on the trail of a discovery made by chance in 1996 when the government research ship, Tangaroa, dredged up mineral samples. They contained 18 percent zinc (by weight), 15 percent copper, and six parts per million gold -- a higher concentration than in some on-shore gold mining areas.

The scientists believe the gold strike almost certainly came from a ''black smoker'' -- an undersea vent where hot fluid from inside the Earth's crust gushes out of the ocean floor. Scores of black smokers are believed to be in a belt running several hundred kilometres northeast of White Island in Bay of Plenty.

Known as the Southern Kermadec Arc, it is an undersea equivalent of Rotorua and Taupo -- an area of intense volcanism and geothermal activity.

Black smokers get their name because they belch hot water and dark clouds rich in particles containing manganese, iron, zinc, copper and occasionally a little gold. They were discovered in the late 1970s and are still regarded as a relatively new phenomenon in earth science.

Fluid coming out of the vents is commonly between 250 and 380deg. Under high pressure, it reacts with chemicals in the surrounding rock. Minerals dissolve in this superheated fluid ''like sugar in tea''. But that all changes when the hydrothermal fluid enters the ocean, which is usually at 4deg.

The sudden cooling causes dissolved minerals to precipitate out to form hydrothermal plumes, and associated chimneys, which grow like stalagmites. When the chimneys become too heavy or unstable, they often fall over and new ones form.

The plumes are rich in methane and hydrogen sulfide. Although a factory spewing out these poisons would be shut down, scientists say that despite their apparently toxic environment, black smokers are hosts to lush ecosystems and thriving communities of shrimp, mussels, tubeworms and myriads of smaller organisms.

Scientists have believed for about a decade that the region to the north of Bay of Plenty -- the Southern Kermadec Arc and the Harve Trough -- might contain scores of hydrothermal vents and mineral mounds on the ocean floor.

When the Tangaroa dredged up samples from one of these mounds, Cornel de Ronde, a mineral geologist with the Institute of Geological & Nuclear Sciences Limited in Wellington, put together a team of scientists to return to the area and find out more.

In early 1999 he and colleagues will take a New Zealand research ship to the area and map it for hydrothermal vents -- the first time this has been done in New Zealand's territorial waters.

Dr de Ronde says there are about 25 places worldwide where hydrothermal mineral deposits have been recorded on the ocean floor. But what makes New Zealand unusual is the presence of gold and the shallow depth at which the vents occur.

Most other black smokers are in mid-ocean ridges at around 2.5km deep. In the Southern Kermadec Arc they are mostly less than 1.5km deep. And some places -- at the summits of undersea volcanoes for instance -- they are as shallow as a few hundred metres.

The presence of high-grade gold traces on the seabed in the Kermadec Arc does not surprise Dr de Ronde. Most of the other known black smokers around the world are in areas dominated by basalt rock. Its magma is much less likely to carry gold. But New Zealand's offshore volcanoes are andesite and dactite in character -- far better candidates for bearing gold.

Dr de Ronde rates the discovery of black smokers as one of the ''sexier discoveries'' in earth science in the past 20 years. Until then, no-one was sure how to account for the iron and manganese dissolved in the world's oceans. It seems much of it comes from black smokers.

Most of the world's lead, zinc and copper massive sulphide deposits which are being mined today began, millions of years ago, at similar ocean vents and over time were tectonically uplifted to the Earth's surface.

Dr de Ronde has managed to secure $1.3 million in funding over the next three years from the New Zealand government's Public Good Science Fund to map hydrothermal vents. A large chunk of the money will be spent on the 15-day scientific cruise in early 1999.

Joining him will be Dr Ian Wright of NIWA and specialists from the Seattle-based National Oceanic & Atmospheric Association. Dr de Ronde says he is extremely fortunate to have secured the services of two American scientists -- Dr Ed Baker and Gary Massoth -- considered world leaders in mapping hydrothermal vents.

Using NIWA's research ship, Tangaroa, they intend towing a long cable packed with sophisticated instruments over the areas where hydrothermal vents are likely to occur. Travelling at just one knot, the heavy cable will drape almost vertically off the ship's stern. They plan to ''tow-yow sample'' about 200 nautical miles of sea floor. The ''tow-yow'' method involves winching the instrument package up and down to roughly match bottom contours.

Keeping the business end of the cable between 50m and 400m from the bottom is important. It's in this zone where the plumes maintain their form. Beyond this, they dissipate and it is difficult to detect them. The scientists will be looking for subtle changes in temperature and electrical conductivity. The instruments will also continuously measure the concentrations of manganese, iron, and hydrogen sulphide.

The area they will focus on has been ''quite well'' mapped for topography and geology. There are at least three active undersea volcanoes within the zone. Finding hydrothermal plume ''bullseyes'' will provide scientists with focal points for future study.

Dr de Ronde hopes that one day he will be able to persuade a deep-sea research ship equipped with a submersible to come to New Zealand. Perhaps early next century we may get a first-hand look at the gold-gushing black smokers off the New Zealand coast, he says.

Whether any undersea mineral deposits will be recovered, and the costs of doing so, are not part of Dr de Ronde's immediate investigation, although he points out that deep-sea technology is advancing rapidy. The main concern is knowledge. He says what science has yet to learn about the ocean floor far surpasses what is already known.

'' We know more about the surface of the moon than about the bottom of the ocean,'' he says.