MINERAL EXPLORATION — Perseverance pays for Cominco in Yukon

In 1994, Cominco (CLT-T) discovered the ABM polymetallic deposit at its Kudz Ze Kayah property in the southeastern Yukon, confirming the area’s potential for hosting volcanogenic massive sulphide deposits.

The discovery triggered a staking rush, and, in 1995, Westmin Resources (WMI-T)and partner Atna Resources (atn-t) followed Cominco’s lead by discovering the Wolverine deposit, 20 km east of Kudz Ze Kayah.

Subsequent work by Columbia Gold Mines (cob-v) at its Fyre Lake project, in the southern region of the Finlayson-Wolverine district, has resulted in the expansion of the Kona copper-gold-cobalt Besshi-type deposit.

Kudz Ze Kayah, which derives its name from the Kaska Dena language and translates roughly as “caribou country,” lies in the Yukon-Tanana Terrane near Finlayson Lake, 190 km northwest of Watson Lake and 110 km southeast of Ross River. The project site is above the tree line at an elevation of 1,400 metres in the northeastern range of the Pelly Mountains.

The ABM deposit is hosted by a thick sequence of Devonian-Mississippian-age felsic volcanic pyroclastics comprising quartz and feldspar crystal tuffs, fine lapilli ash tuffs, and ash tuffs with lesser rhyolite flows or sills.

Immediately above the deposit are felsic pyroclastics, which are intensely deformed and altered to quartz-muscovite-carbonate schists containing fine pyrite and quartz veinlets.

.SBlind deposit

The ABM, like the Wolverine, is a blind deposit in that it is not exposed at surface. The shallow-dipping ABM straddles the centre of a broad, U-shaped valley and is entirely covered by up to 12 metres of glacial till and alluvial sediments.

By the end of last year, 139 drill holes had outlined a minable open-pit reserve of 11.3 million tonnes grading 5.9% zinc, 1.5% lead and 0.9% copper, plus 1.3 grams gold and 133 grams silver per tonne.

The Finlayson Lake district had been previously explored by several companies, including Cominco. The company’s first pass through the area, in 1977, involved a regional geochemical survey. The program was widely spaced and uncovered no evidence of the ABM deposit.

As part of an ongoing zinc exploration program in eastern Yukon, Cominco was drawn back to the area in 1992 to follow-up on anomalous base metal stream silt sampling, which had been carried out by the Geological Survey of Canada in 1988. One sample, which turned out to be 800 metres down-stream from the subcrop of the deposit, returned 1,820 parts per million (ppm) zinc, 102 ppm lead and 75 ppm copper. Cominco’s initial soil sampling in the ABM drainage basin uncovered highly erratic anomalous values — up to 878 ppm zinc and 116 ppm lead.

In 1993, the company spent $20,000 to send in geologist Chris Schultze and Bruce Mawer, a retired geologist and prospector, to investigate further.Rhyolite rocks were found exposed along the ridge on either side of the valley.

William Wolfe, general manager of Canadian exploration for Cominco, says that although the rocks displayed quartz-sericite alteration, no significant mineralization was found.

What caught Cominco’s attention was Mawer’s discovery of a lone cobble-size (15-cm) piece of banded massive sulphide-magnetite float on the side of the valley. The rock assayed 9% zinc, 8% lead and 0.8% copper, plus 4.8 grams gold and 583 grams silver.

“It was the kind of thing we were looking for,” says Wolfe. “Once we found the first one, we scoured the hillside and pounded every rock there was, trying to find another one, but that was the only rock we found. Without that boulder, we might not have moved on to the geophysical stage.”

Geophysical crews were mobilized to the property in September, and a few reconnaissance transient electromagnetic (EM) lines were run in the area.

Preliminary geophysical work showed a “booming anomaly” under the valley cover The anomaly was detailed with horizontal-loop EM and magnetic geophysical surveys, which identified a strong 700-metre-long conductor with a high magnetic signature. Both magnetite and pyrrhotite were found to be present.

On the basis of the geophysical interpretation, the initial diamond drill holes were spotted. In April 1994, the first hole was drilled, intersecting two intervals of massive sulphides — an upper 7.5-metre interval and a lower 15-metre interval — separated by 10.8 metres of waste rock. The total 22.5 metres of massive sulphides averaged 10% zinc, 2.8% lead and 0.53% copper, plus 2.9 grams gold and 278 grams silver.

A proper grid was established, and exploration crews carried out a combination of magnetics, transient EM and gravity geophysical surveys. The results, says Wolfe, “outlined the deposit beautifully.”

The transient EM survey was particularly successful in that it clearly showed the deposit’s limits, defining not only the shallow updip edge, but the downdip limit as well.

.SPossible resource

In 1994, after 50 holes were drilled on 100-metre centres (40 inside the deposit and 10 outside its periphery), a possible resource was estimated at 13 million tonnes averaging 5.5% zinc, 1.3% lead and 1% copper, plus 1.2 grams gold and 125 grams silver.

A further 90 holes of infill drilling were completed in 1995 in an attempt to achieve 50-metre spacings (and, in some cases, in the top part of the deposit, 25-metre spacings).

Cominco spent a total of $11 million in its effort to find and outline the ABM deposit and take it to the feasibility stage. Project permitting began in 1996 with the submission of environmental assessment documentation, and regulatory approval is anticipated by year-end.

Cominco once held more than 10,000 claims covering 2,000 sq. km of ground in the Finlayson-Wolverine Lake region. The company has reduced some of its holdings and has optioned to Pacific Bay Minerals (pbm-v) 19 separate blocks comprising 518 claims.

For the past two years, Cominco has been drill-testing targets outlined by airborne geophysics. Exploration work, comprising geological mapping, geochemistry and geophysics, is ongoing. The company’s 1997 exploration budget for the area is about $2 million, compared with $4.2 million in 1996.

Wolfe, who believes the Finlayson-Wolverine Lake area is still prospective for new massive sulphide discoveries, adds a footnote: “As far as technology goes, the Canadian discoveries of the 1990s, including this one, were found by guys walking in the bush and collecting samples — they were not found by satellites, or any of this other wild technology.”