MINERAL EXPLORATION — Downhole geophysics helps keep Voisey’s Bay II dream alive

Although they have yet to find economic base metal concentrations, juniors Columbia Yukon Resources (CYR-A) and International CanAlaska Resources (ICA-V) refuse to abandon the theory that the Eastern Deeps zone at the Voisey’s Bay nickel-copper-cobalt project in Labrador does not end where Inco’s property ends.

Falconbridge (FL-T) joined the team working on the VBE-1 project earlier this year, and the three firms now share the costs equally and rotate the operatorship (Columbia Yukon currently holds the reins). Beginning in 1998, Falconbridge can elect to earn a 25% interest in the property by spending $22 million over three years. It can raise its interest to earn 51% by delivering a bankable feasibility study and to 60% by advancing the property to commercial production.

Exploration at VBE-1 has centred on the search for nickel-copper-cobalt mineralization associated with troctolite. Since the project began in 1995, airborne surveys and trenching have given way to diamond drilling and downhole geophysical surveying.

The partners are now working through a 13,500-metre diamond drilling program; geophysical work is finished for the year. In the area being drilled, geophysically derived targets are in the order of 800 to 1,500 metres down.

“The equipment we’ve got is able to go deeper than that, if we start to get into some interesting rocks,” notes Benjamin Ainsworth, vice-president of exploration for Columbia Yukon. “We’re on our third suite of holes now. There are several different structural elements in the claim block, and we’re trying to make sure we get a hole in each one.”

Large-loop electromagnetic (EM) surveys at surface were completed on the northern part of VBE-1 last year, but only a few weak features were revealed, Ainsworth says. In the same area, a 30-metre piece of drill stem was lost at a depth of 871 metres, and the hole was capped for the season. Although no massive sulphide mineralization was intersected in the 1996 drill hole, petrographic analysis of a core sample taken from near the bottom of the hole indicated that traces of pentlandite and chalcopyrite are present.

The unfinished hole was completed this year down to 1,500 metres. Such deep holes are expensive to complete, so in order to keep the number of holes drilled to a minimum and to aid in spotting the holes, downhole (sometimes called borehole) geophysics has been employed at VBE-1.

As JVX President and geophysicist Blaine Webster explains: “It’s always been a good guiding method. Deep drilling is expensive, and by using borehole geophysics, your radius is greater. You just get more exploration value out of your drill hole using these methods.”

With downhole geophysics, probes that measure different properties are lowered into a drillhole to collect continuous or point data that are graphically displayed as a geophysical log. The geophysical logging system consists of probes, cable and drawworks, power and processing modules, and a data recording unit. Logging systems are controlled by a computer and can collect multiple logs with one pass of the probe.

Types of downhole surveys include gravity, resistivity, electromagnetic and magnetotelluric. Downhole geophysics was an important factor in the discovery of the Louvicourt copper-zinc-gold deposit in Quebec, and in several of the deep mines in the Sudbury basin.

Downhole gravity surveying is used in oil exploration, but diamond drill holes used in hard-rock mining are too narrow for gravimeters to fit.

Using downhole resistivity surveying, an electrode is lowered into a hole, and an ohmmeter is used to measure any drop in resistivity, which can be the mark of a metallic body of some sort. The resistivity will then shoot up again after passing the lower edge of the body, thus defining its size in that hole.

The shape of the body can be determined by matching the results with those from nearby holes.

The pulse EM downhole surveying method, in which a coil transmits an alternating magnetic field which induces a current in any nearby electrical conductors, was used in the aforementioned deep hole in 1996, and again when the hole was re-entered and completed this year.

“The pulse EM downhole data give us, depending on contrast, a couple of hundred metres around the hole,” says Ainsworth. “It extends our search pattern quite a bit, and if we’re just getting into something interesting at the end of a hole, it allows us to have a look several hundred feet beyond the hole.

That’s good economic sense.”

Downhole surveying costs the VBE-1 partners about $10,000 per hole. “The interpretive software is expensive, and to hire a fella to interpret it all is expensive,” says Webster, but the overall cost is still low compared with the cost of deep diamond drilling.

Audio-magnetotelluric (AMT) surveying has also been used at VBE-1. AMT is a method of examining resistivity and conductivity contrasts at depth. A large loop is laid on the ground, and magnetic field readings at various frequencies are taken. Telluric (non-induced) currents passing through the earth’s crust at a given location are measured.

Ainsworth says an orebody can be quite elusive when it is down as deep as the targets the VBE-1 partners are now investigating.

“There’s always a concern that when you’re down at 1,500 ft., you might not see it, even with AMT.

“We’ve got to be really careful that we don’t put all of our drilling into a graphite horizon [graphite, like metal sulphides, is a good conductor]. A conductor is a conductor.”

When results from several holes are combined and compiled, the primary result of these downhole surveys is a map of the relative conductivity of the rocks in a given area. In some cases, if the content of adjacent rock types is great enough, it has also aided in geological mapping of the rock types.

Oddly, proximity to seawater can render AMT surveys confusing and inaccurate, as Ainsworth explains: “You can be 500 metres or more from the ocean, and because of the conductivity of seawater, you get weird things happening that really obscure your results. You can’t use audio-magnetotellurics around water too easily.”

While VBE-1 is quite close to the Labrador Sea, AMT has nonetheless proven useful and accurate there, especially as an aid in the mapping of underground structural features.

The VBE-1 partners’ combined budget for 1997 is $3.3 million, much of which is being used to finance the completion of nine deep diamond drill holes. All of the geophysical work is finished for the year.

As for next year, Ainsworth says it is too early to say exactly what the VBE-1 partners’ exploration plans and budget will be. Naturally, much depends on the results of this year’s work. But after almost three fairly fruitless seasons, the future of the entire Voisey’s Bay area play (excluding Inco’s claims) in terms of nickel-copper-cobalt potential is very much in doubt. The company with the claim block just east of VBE-1, Absolut Resources (alr-a), has more or less given up on finding an extension of the Eastern Deeps after its deep drilling yielded little.

Nevertheless, after much deep drilling, ground and downhole surveying and other work, no one can say the VBE-1 partners did not give it a shot.