New metallurgical results confirm the feasibility of heap-leaching material mined from the Alamo Dorado silver-gold deposit, reports
Situated in Mexico’s Sonora state, Alamo Dorado hosts 79.5 million tonnes grading 46 grams silver and 0.18 gram gold per tonne. The estimate is based on a cutoff grade of 10 equivalent-grams silver, which assumes metal prices of US$5.28 per oz. for silver and US$300 per oz. for gold.
According to an independent prefeasibility study, an open pit would be developed in three stages, with progressively lower grades and higher stripping ratios. Material grading 10-25 equivalent-grams (that is, material considered “low-grade”) would be treated as waste but stockpiled until metal prices improve (T.N.M., Oct. 2/00).
Recent column tests suggest that at least 75% of the silver from material grading more than 90 equivalent-grams would be liberated after 206 days under leach. The addition of mid-grade material would reduce this to 67%. Both samples include low-grade material, and each yielded recovery rates higher than those used in the prefeasibility study.
Results for gold fared worse: recovery rates from both samples were 10% lower than expected. However, as the yellow metal is a byproduct only, Corner Bay does not expect the loss to affect project economics.
Metcon Research of Tuscon, Ariz., which carried out the tests, notes that recoveries of both metals are sensitive to crush size. The samples reported above were ground to minus 3/8-inch.
Corner Bay says the new results bode well for the project, which stands to become one of the world’s richest and largest heap-leach silver operations.
Geologically, Alamo Dorado sits on the western flank of the Sierra Madre Occidental and is hosted by quartzites and schists that are structurally controlled in a setting proximal to granodiorites that intrude them.
Weathering has completely oxidized the deposit, freeing up the disseminated silver and gold from its previous host minerals. Where the former metal is tied to secondary oxide minerals, it is usually chlorargyrite — a soluble chloride mineral found in secondary enrichment zones above silver veins subjected to intense weathering.
Metcon is now testing samples from each of the different grade categories to determine if higher recoveries are possible. The tests will also ascertain if column height influences leaching, though preliminary results suggest otherwise.
Corner Bay is seeking financing for a bankable feasibility study, as well as personnel to manage it. Deutsche Bank Securities has been engaged as a financial advisor.
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