The latest round of drilling by
The first nine holes of this year’s program all returned significant intervals of high-grade copper, with the best of these intersecting 147.8 metres grading 1.46% copper, 0.31 gram gold and 8.92 parts per million silver.
Since its discovery last summer, the Northeast zone has returned grades roughly three times that of existing reserves, which stood at 31.9 million tonnes grading 0.36% copper and 0.34 gram gold per tonne when operations were suspended in 2001, owing to low metal prices.
President Brian Kynoch describes the Northeast zone as “one of the highest-grade copper-gold discoveries in British Columbia.” He also notes that it outcrops on surface, thus eliminating the need for prestripping.
The new discovery and improved metal prices are spurring Imperial’s efforts to revive its wholly owned, fully permitted mine project. But Kynoch and Chairman Pierre Lebel envision a much different mine than the one placed into production in 1997. They’re proposing a higher-grade, lower-cost, more technologically advanced operation — one that’s better able to survive the cyclical nature of the mining business.
Along with the exploration effort to outline higher-grade reserves at the Northeast zone, Imperial is carrying out a research program for leaching of copper oxide ores, which typically had low recoveries in the past. The low recoveries were a reflection of the fact that the mill at Mount Polley was designed to produce concentrates from sulphide deposits. However, most of the remaining reserves are found in the as-yet-unmined Springer pit, where a major portion of the copper occurs as oxide minerals. In contrast, the partially mined Bell pit was predominantly sulphides.
Assuming leaching of oxide copper minerals is feasible, Imperial would be able to strip the oxide layer down to the underlying sulphides, which are amenable to the flotation circuit. On a property-wide basis, the company would be able to re-assess exploration targets that had been abandoned because of their high oxide copper content.
Leaching plant
Some preliminary tests on oxidized material have shown that about 80% of the acid-soluble copper can be recovered, compared with recoveries of 11% when oxide ores were treated in the existing flotation plant. If ongoing results confirm these recoveries, Imperial would forge ahead with plans to permit and develop a leaching plant that would operate alongside the existing flotation plant.
The addition of the leaching plant would then boost overall recoveries, which had averaged about 75% for copper and 66% for copper during the initial production run. This, in turn, would significantly lower costs and improve the mine’s overall economics.
The $115-million Mount Polley mine produced more than 133 million lbs. copper and 370,000 oz. gold during its operating period (1997-2001). Based on preliminary estimates, mining could resume for an initial investment of about $15 million. As an added bonus, smelting and refining costs for concentrates are significantly lower than when the mine operated.
The leaching method for the oxides is also expected to be low-cost (though leach times are long) as the material is suitable for direct electrowinning (no solvent extraction).
While the research effort continues, Imperial will continue to explore the high-grade Northeast zone, which lies about 1.5 km northeast of the previously mined Bell Pit.
The boot-and-hammer discovery was made last summer, after construction of a logging road exposed a showing in an under-explored portion of the property. It was a fortunate and unexpected event, as the known deposits at Mount Polley have magnetic signatures, whereas the Northeast zone has no associated magnetic response.
Mount Polley hosts alkaline porphyry copper-gold deposits, each of which has distinctive mineralization, alteration and breccia characteristics. The Northeast zone discovery is distinguished from other known deposits by a higher copper-to-gold ratio (and higher copper grades), higher silver and bornite content, and lower magnetite.
Subsequent trenching and drilling revealed a hydrothermal breccia over a 325-metre strike length, which remains open along strike to the southeast and to depth.
The first hole drilled into the zone returned 56.9 metres grading 2.54% copper, 1.15 grams gold and 17.4 grams silver. Hole 7 returned 204 metres of 1.02% copper, 0.4 gram gold and 7.31 grams silver.
The latest round of drilling also returned some long intersections, including 132.5 metres grading 1.22% copper, 0.53 gram gold per tonne, and 8.48 ppm silver. Some 45 holes have tested the zone to date.
Imperial notes that related breccias continue in other directions. These will be tested in ongoing exploration programs. In light of the unexpected discovery, which was not revealed by previous geophysical surveys, Imperial is examining various geophysical methods to establish a signature of the zone. An induced-polarization survey is already under way to identify extensions of the Northeast zone, and new targets.
Imperial has assays in hand from the first dozen holes drilled to explore beneath the Bell pit. One of the best holes intersected 68.7 metres grading 1.15% copper and 0.86 gram gold. Drilling is also under way at the Springer zone.
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