Cominco doubles reserve estimate at Pebble Copper

A drill program carried out by a unit of Cominco (TSE) during 1991 more than doubled preliminary reserves at the 100% owned Pebble Copper deposit in the Bristol Bay region of Alaska.

The deposit is now estimated to contain a preliminary resource of 500 million tons grading 0.35% copper and 0.012 oz. gold per ton. This estimate, based on 40,000 ft. of drilling in 82 holes, represents a substantial increase from previous estimates of 200 million tons grading 0.4% copper and 0.012 oz. gold based on a widely spaced drill program carried out in 1990. Cominco geologists were attracted to the region in 1985, because of modelling and interpretation of published maps and reports, even though the area of interest did not have any previously known mineral occurrences. The initial target was gold deposits similar to Snip in northwestern British Columbia, and by 1987, gold-bearing epithermal veins were discovered (about five miles from the porphyry centre). This mineralization was not found to be continuous, but mapping and sampling outwards from the veins revealed some evidence of porphyry-style sulphide mineralization. There was little outcrop, and soil geochemistry in 1988 outlined the combined copper and gold anomaly where the discovery hole was ultimately drilled.

Major companies have at times been criticized for showing little return for the vast sums they spend on exploration. But Pebble Copper reflects Cominco’s stated philosophy to develop many of its projects internally from grassroots or early stage exploration prospects.

Owen Owens, Cominco’s vice-president of exploration, views the discovery as “significant, and a credit to our people in the field.”

The deposit is covered by a thin oxidized leached cap followed by a zone of supergene enrichment where copper mineralization is associated with chalcocite. Copper mineralization within the main body of the deposit is associated with chalcopyrite and other copper sulphide minerals. Open pit mining would be feasible at Pebble Copper, and the deposit contains a higher-grade core previously estimated to contain 55 million tons of 0.5% copper and 0.015 oz. gold per ton with a very low stripping ratio. More drilling is planned to confirm tonnage and grade in this core area, and metallurgical work is also planned. But the main project focus now will be on elements relating to permitting and engineering.

This will include conceptual work on dock sites, access corridors, tailings and waste rock disposal sites and water quality. The first phase of geotechnical studies has already been completed, and base-line environmental studies are now in progress.

All exploration work to date has been serviced by helicopter as the region has no roads, no power grid and no ports. Environmental and permitting requirements are expected to be challenging, but Cominco’s preliminary discussions with government are reported as “positive.”

The deposit is in moderate terrain north of Cook Inlet, about 60 miles from coastal waters and about 200 miles southwest of Anchorage. The Bristol Bay region is world famous for its commercial sockeye salmon fishery as well as its sports fishing.