MINERAL EXPLORATION — CASE STUDIES — Exploration strategies evolve at Voisey’s Bay — First-phase results hinge on pulse-EM system

In the poisoned atmosphere of political controversy, court-ordered delays and rock-bottom metal prices that has left Inco (N-T) gasping for air, it is easy to forget that Voisey’s Bay represents one of the truly great discoveries of the 20th century. It is among the largest, highest-grade and best-positioned nickel-sulphide deposits in the world — and it will, one day, make its owners rich.

This long-term promise has inspired several dozen mining companies, including Inco’s Voisey’s Bay Nickel, to conduct major exploration programs in the vicinity of the Labrador deposit during the four years since its discovery. After all, most nickel sulphide deposits, including those of the famous Noril’sk (Russia), Kambalda (Australia) and Sudbury camps, occur not in isolation but as part of a group.

But finding local analogues of what is being described increasingly as a one-of-a-kind orebody is proving to be a challenge — such a challenge, in fact, that most of the initial participants have thrown in the towel. The main player, Voisey’s Bay Nickel, declined to be interviewed for this story, though the company is conducting exploration as far as 65 km north of the main deposit.

“The Voisey’s Bay sulphide deposit is rather unique in both its physical setting and in its association with relatively early troctolitic rocks that are part of the Nain Plutonic Suite (NPS),” says Bruce Ryan of Newfoundland’s geological survey. “We don’t know of any other old troctolitic rocks that are part of the NPS at this point.”

In the early days of the Labrador staking rush, press releases overflowed with references to “troctolite,” the olivine-rich mafic intrusive rock that hosts the Voisey’s Bay deposit. Companies without troctolite dykes on their property were often shunned by investors and geologists alike.

But some geologists now believe troctolite is merely a sub-group of a more widespread olivine gabbro and that some of the older gabbros are valid exploration targets in themselves.

“A lot of the work done recently suggests that the rocks mapped as troctolites are actually olivine gabbros and that troctolite is only a special type of olivine gabbro,” says Harvey Keats, president of Donner Minerals (DML-V), one of the companies still active in the Labrador exploration drive.

Donner, along with its major partner, Teck (TEK-T), and a host of junior partners, is banking on the potential for these gabbros to host significant quantities of sulphide mineralization. This season, the joint venture will spend up to $14.8 million to find new targets and follow-up the massive sulphide teasers encountered last year on the South Voisey’s Bay property, 90 km from the Voisey’s Bay deposit.

Gaining an understanding of the geology of the region and the processes that led to the emplacement of Voisey’s Bay has been a crucial component of Donner’s exploration strategy. Consequently, the company has come up with a geological model similar to the Voisey’s Bay model to guide exploration on its South Voisey’s Bay claims.

This claim group contains the largest concentration of gabbros outside the Voisey’s Bay discovery area. Most lie near the boundary of the Nain and Churchill structural provinces, which was the likely conduit for the magma.

The gabbros are similar in composition and age to the troctolite of the Voisey’s Bay deposit. They are also contaminated with gneiss, the local country rock, suggesting that the interaction between magma and country rock required to segregate large amounts of sulphide did take place.

Last year, Donner proved that massive sulphide mineralization is associated with the olivine gabbros, and it did so by intersecting a 1.1-metre zone grading 11.75% nickel, 9.7% copper and 0.43% cobalt at 177 metres down-hole.

The company is also beginning to identify the type of feeder zones that are thought to have played a vital role in concentrating sulphides at Noril’sk, Jinchaun in China, and Voisey’s Bay.

But the difference between the Voisey’s Bay deposit and other local prospects — and the main challenge to exploration — is the degree of preservation in the magmatic system.

“What we are looking at in Voisey’s Bay seems to be the dyke feeder and the lowest part of a larger magma chamber,” says Ryan. “It’s only in the lower levels that you find mineralization. If you take something such as Kiglapait [a nearby trotolitic pluton], estimates suggest you may have to drill down as far as 9 km through the centre before you strike the feeder zone.” Donner is facing a similar obstacle at its South Voisey’s Bay prospect, though the depths are not nearly as extreme.

“Here, the whole sill is preserved, and in some cases the sill is actually under the gneiss itself,” says Keats. “So you also have the geophysical challenge of seeing through graphite and pyhrrotite in the gneiss.” To overcome this challenge, Donner turned to the most powerful geophysical tools for nickel sulphide exploration: gravity and magnetics. The company also conducted horizontal loop electromagnetic (EM) surveys to detect pockets of mineralization.

But although the gravity surveys outlined potential troughs in the gabbros where sulphides may have ponded, the anomalies identified by the horizontal loop technique turned out to be either graphite zones or graphite and pyrrhotite disseminations within the gneiss.

This year, Donner chose instead to use a low-frequency, multi-channel pulse-EM system to see through the graphites to the mineralization at the base of the gabbros. The system has identified at least one strong anomaly near the 1997 massive sulphide intersections.

“It took quite a while to develop a technique to pick up mineralization at the base of the gabbro,” says Keats.

The performance of the pulse-EM technique may well make or break this season’s exploration program, because the detailed drilling planned for the second phase will only take place if a significant discovery is made in the first phase. In August, Donner discontinued drilling in order to catch up on geophysical work. Only time will tell if the resumption of drilling leads to the discovery of a Voisey’s Bay analogue.