Heap leach ore at El Indio is a byproduct of mining in the Tambo open pits. Low-grade ore running on average 1.2 g per tonne — waste rock were it not for heap leaching — is piled in 6-m lifts up to 30 m high. The current recovery rate is 40%. That should improve by 10 percentage points once a new crusher is installed. It will crush to — 34 inches. The current size is — 2 inches. The leach piles sit on a sequence of dry tails (the bottom layer), a high-density polypropylene liner, a geotextile, and another layer of dry tails.
Several modifications, many of them resulting from a technical visit made by Tambo staff to LAC’s Richmond Hill heap leach mine in South Dakota, will be introduced, said Jorge Aoun, Tambo’s superintendent. At Richmond Hill, for example, drip lines are buried 40 cm beneath the surface. This reduces evaporation and conserves cyanide temperatures. A specially-equipped bulldozer digs the trenches to bury the lines. Aoun is also considering adding a fourth carbon column.
The leachable ore is trucked from the Tambo pit area. Tambo has two main operating pits — Wendy and Kimberly/Lauren. The annual mill ore tonnage is 235,000 tonnes. Reserves stand at 2,694,614 tonnes grading an average 6.4 g gold per tonne. (About 28,000 tonnes represent underground reserves.) There is another 2.4 million tonnes of leachable material.
A strip ratio of 10:1 (waste to ore) this year will decline next year to 8:1. “That will be the norm afterwards,” said Aoun. Benches are generally 10 m high and 6 m across. The crawler drills operate on a 3×3-m pattern in ore (4-inch-diameter holes) and 3.5×3.5 in waste.
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