Eighteen months ago, while scouting around Mexico’s Sonora state,
Corner Bay now views the Alamo Dorado project as a potential heap-leach silver mine with low operating costs and a quick payback period. A prefeasibility study has been completed, along with some preliminary metallurgical testwork, and infill drilling will be carried out in an attempt to upgrade reserves and resources contained in the shear-hosted deposit.
“It’s a brand-new discovery, yet, because of the high grade, we think it has the earmarks of a robust mine even in this depressed market,” says Corner Bay President Peter Mordaunt. “And it’s unusual in that there is only one other silver heap-leach mine, Coeur d’Alene’s Rochester [in Nevada].”
Based on calculations by Mintec, a consulting firm from Arizona, the Alamo Dorado project hosts a geological resource of 47.8 million tonnes grading 42.06 grams silver and 0.15 gram gold per tonne, or about 77.6 million oz. silver-equivalent.
Reserves minable by open-pit methods stand at 24.7 million tons of 71 grams silver and 0.22 gram gold, or 66.5 million oz. silver-equivalent. This calculation is based on a 20-gram cutoff grade for silver, a silver price of US$5.28 per oz., and a gold price of US$300 per oz. Corner Bay sees potential to expand reserves in several directions, and at depth, where several holes stopped in mineralization.
Metallurgical tests performed by Metcon Research on four composite samples derived from drill-hole material yielded average recoveries of 89.52% for gold and 67.23% for silver. Corner Bay views these recoveries as “very good for testwork at this level.” The silver and gold are predominantly free/exposed and surrounded by oxidized rocks — and therefore, it is believed, easily recoverable by heap leaching.
The open-pit reserve has a stripping ratio of 1.09-to-1, which includes a prestrip of 700,000 tonnes of low-to-medium-grade ore. A 12-year mine life is envisioned, with the low-to-medium stockpiles going to the heaps in year 12.
The Mintec study is based on a mining rate of 2.4 million tonnes per year. A series of three pit phases is planned, beginning with a higher-grade starter pit where silver could be produced at cash cost of US$1.38 per equivalent ounce. This would increase to US$1.58 per oz. in years 3 to 5, and US$2.59 per oz. in years 6 to 12.
“These numbers are still preliminary,” says Mardaunt. “But because of the supergene mineralization in the starter pit, we see potential for payback of capital in less than a year. We think we should be able to finance this project with up to 80% debt.”
Once the infill drilling is complete (on 50-metre spacings), results will be incorporated into a final feasibility study. In the meantime, Corner Bay plans to take a bulk sample for advanced metallurgical testing, which may include a 5,000-tonne test heap. Results from this work will then be incorporated into the final study. The low-sulphidation epithermal deposit does not contain any deleterious elements, such as arsenic or antimony.
Mordaunt sees potential beyond existing reserves and resources, and, toward that end, a regional exploration program on the surrounding land package is planned for this year.
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