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The first round of pioneering offshore test core drilling for placer gold, and other heavy minerals off the West Coast of the South Island, funded by the two big South African-based gold and diamond companies AngloGoldAshanti and De Beers, has just been completed by Seafield Resources. LINDSAY CLARK looks at some of the technical aspects of the task.
Neil Fraser, the manager of the Seafield project for the marine mining technical services company AuruMar, says from Nelson that the drilling of up to 10 metre long cores of sea bottom deposits had been successful. Fraser, a Cape Town-based geologist formerly with offshore diamond mining company De Beers Marine in South Africa, says the West Coast cores are currently being prepared to be sent off to assay laboratories for analysis. Drilling undisturbed geological drill cores from a small vessel which is moving up and down in waves close to shore is a challenge onshore drillers might shudder at. Fraser says that De Beers Marine mainly used airlift tools to suck diamond-rich seafloor material up from the seafloor but the New Zealand project wanted to collect undisturbed seafloor profiles of up to 10 metres deep. This required core drilling. To do this, Seafield decided to build its own drilling equipment in New Zealand specifically for local conditions. The custom-built sampling tool is designed to be winched over the stern of the operating vessel and to stand upright on the sea floor while drilling. The 13 metre-tall rig contains a sonic vibro-corer which sits on top of the 10 metre-long core drill pipe. The sonic vibro-corer uses high-frequency vibrations to vibrate the core pipe as it penetrates down into the sand or gravel to give a relatively undisturbed core. The sonic vibro-corer unit is driven from a hydraulic powerpack on board the ship through flexible hydraulic hoses. The drill can use either a 180mm diameter drill pipe or larger 330mm pipe to gather heavy mineral samples or to drill in sand containing gravel. To stop the core falling out of the drill pipe when it is withdrawn, ‘tulip flaps’ on the inside of the bit automatically fall down to prevent the core escaping. At the end of each hole, the sampling tool is winched back to the surface using the Launch and Recovery system (or LARS). This consists of:
Once the drilling tool is recovered on board after drilling, the core is extruded out of the core barrel into catcher trays, before packaging into thick plastic bags for logging ashore and later lab analysis and assaying. The drilling tool was built by Wellington-based company New Zealand Diving and Salvage with design input from De Beers Marine.
The construction and commissioning of the equipment has been supported by numerous other contractors from both Wellington and Nelson. Fraser says the Marsol Pride vessel was ideal for the drilling task as it had four-point anchor mooring to keep the ship firmly fixed in one position whilst drilling. The vessel has a large low aft deck, a shallow draft of four metres making it easy to work close to shore plus plentiful on-board accommodation for up to 60 persons. On the recent drilling for Seafield there were 10-15 support crew on board in addition to the MarSol Pride’s crew of between 12 and 15. The prospect drilling was carried out between Greymouth and Ross, and north and south of Franz Joseph and Fox glaciers. The original concept for the West Coast project was developed by international placer gold consultant Dr John Youngson of Placer Solutions (2004) who remains a geological consultant to Seafield. Dr Youngson told Q&M a year ago that much of the heavy sands down the West Coast consist mainly of iron oxides sourced from the Southern Alps, including magnetite a common ore used by the world steel industry. Exploration for heavy mineral sands on the West Coast have focussed on titanium-rich ilmenite and rutile. Youngson says that natural processes serve to sort and concentrate the heavy mineral sands along the coast. 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. Effectively this has upgraded the deposits. “The sea is the ultimate concentrator,” Youngson says. Heavy mineral sands and the even heavier gold are often left together in certain locations containing former glacial moraines such as near the gold mining town of Ross. Most of the nine million ounces of alluvial (or placer) gold mined on the West Coast was found on or near these glacial moraines. The former glacial moraines formed in ice ages but now lying undersea are likely to be key targets for Seafield.
Q&M Vol.7 No.3 June-July 2010 |
Seafield is a Kiwi-based company owned by another South African company E Oppenheimer & Son, which holds substantial interests in AngloGoldAshanti and De Beers.
Nelson marine support company Unimar led construction of the LARS and provided the vessel – the Nelson-based 60 metre multi-purpose vessel Marsol Pride – along with a Kiwi crew, for the West Coast drilling.