Geophysics Leads the Way in an Aggressive Exploration Program

The Sudbury Basin has been very, very good to Inco. In the last 98 years, the company has pulled 750 million tonnes of ore out of the ground, containing 9.8 million tonnes of nickel and 9.0 million tonnes of copper. The ore has come mainly from large deposits, called “contact deposits”, near the base of the Sudbury Igneous Complex, such as Creighton, Frood-Stobie, Garson and Levack. Beneath them, in older rocks, are smaller bodies often with high grade copper and platinum group elements, called “footwall deposits”. Nickel is also found in structures called “offsets” that protrude from the igneous complex several kilometres into the older rocks.

The target of the exploration program today is new ore near surface, or near existing underground workings, that may be mined at low cost compared to existing operations. The Ontario Division has had particular success in the past decade, finding the 153 orebody at McCreedy East, an additional contact and footwall ore zone at Victor, the Kelly Lake deposit, and a new ore zone at the Totten mine.

Working with Laurentian (MERC) and Carleton universities, the University of Toronto and the Geological Survey of Canada, Inco has developed new conceptual models and targets for exploration. While contact deposits have already been well explored, the less well-known footwall deposits (such as McCreedy West) and offset deposits (such as Copper Cliff and Totten) show promise.

The Division is using new exploration tools, particularly geophysics, to find and define mineralization. For reconnaissance work, Inco uses audio magneto-tellurics (ATM), a system that can locate large, conductive, mineralized systems to depths of more than one kilometre, albeit with poor spatial resolution. Another system called UTEM, developed originally at the University of Toronto, can define ATM discoveries more precisely.

UTEM probes used in diamond drill holes may detect the presence of conductive sulphide mineralization within 300 metres of the probe. This means that a drillhole that is only a few centimetres across can be used to search a volume of rock with a diameter of 600 metres. The team is therefore getting far more exploration value from each hole.

Downhole UTEM is also being used in Inco’s “Old Hole” program. At a cost of $3 per metre, the UTEM can test if there is nickel mineralization within 300 metres of previously drilled holes. This is far cheaper than drilling a new hole for $100 per metre. It is also fast: one or two holes can be probed in a day.

On the production side, smaller EM probes, which measure the conductivity in the immediate vicinity of the hole, are used to survey blast holes. The amount of nickel in the ore is known to be proportional to its conductivity, so the geologists can estimate the grade of nickel in areas that are about to be mined. This allows Inco to mine the ore very precisely, taking all the ore and avoiding mining waste rock. It takes about a day to probe the 20 holes in a stope.

MIRARCO (at Laurentian University) is working with the exploration team to examine the ways of measuring how rocks will break when they are blasted, which should aid in designing the exploration program and mining methods.

The year 2000 exploration budget for the Sudbury Basin is $11 million, including 52,000 metres of surface and 40,000 metres of underground diamond drilling.