The infinitely small particle size of Barrick’s gold, and of other producers on the Carlin trend, is directly attributable to its hot-spring origin. Less well known, the accompanying pyrite (plus up to 50% marcasite) is also extremely finely divided. Typical Barrick sulphide is in the 10-35-micron range (37 microns=400 mesh). Marcasite, though, can form rosettes up to 200 microns in diameter. The iron sulphides are finely disseminated and make up no more than 3%-4% of the ore. Thin-section studies show that the individual grains of pyrite/marcasite are rimmed with an arsenic-enriched phase and it is in this phase that the gold is concentrated.
Because of the fineness of the mineralization and its low sulphide tenor, the ore cannot be concentrated economically by flotation. Whole-ore processing is necessary, and autoclaving as a pretreatment was selected by Barrick. Newmont plans to use its patented, bacteria-assisted oxidation process in the heap-leach mode. The third alternative is roasting and Newmont will also use this method on their carbonaceous ores.
Feed to the autoclaves assays 1.7%-1.8% sulphur. After oxidation it approximates 0.03%. If the feed runs 1.4% sulphur or less, sulphur is added in the form of purchased flotation concentrate to satisfy the kinetics of the oxidizing reaction. Superheated steam can be injected into the autoclave as an alternative.
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