Geology

The ore deposits are hosted by the platform carbonate and lesser siliclastic sedimentary rocks of the Triassic Star Peak Group. The McCoy gold deposit is a proximal skarn developed principally in limestone of the upper Smelser Pass Member of the Augusta Mountain Formation. The Cove gold-silver deposit is located 1.5 km (1 mi) northeast of the McCoy deposit and is comprised of two separate ore bodies.

Cove Sulfide Mineralization

The most abundant sulfides at Cove are pyrite, sphalerite, marcasite and galena. Arsenopyrite, pyrrhotite, and chalcopyrite are present in minor amounts and a variety of tin-sulfide minerals occur in trace amounts. The mineralization of the Cove sulfide ore is displayed in Table 1.

Metallurgy of Cove Sulfide Deposit

With the discovery of refractory sulfide material in the Cove pit, numerous metallurgical tests were conducted on the ore to determine the most economical treatment. Roasting, pressure oxidation, bioleaching and pyrite concentration (flotation) were all evaluated on the basis of cost and recovery.

Milling Circuit

A flexible milling flow sheet was developed for a grinding circuit capable of processing both oxide and sulfide ores. Initially, the design utilized the existing heap leach ore crusher to provide 4,536 tonne/day (5,000 ton/day) of feed stock for a single-stage grinding circuit. The broad range of work indices of the different ore types and the ability to expand the grinding capacity were considerations in the selection of the ball mills.

Selection of the SAG mill was based on the above tests, the capability of feeding 4,536 tonne/day (5,000 ton/day) to the ball mill circuit and a delivery lead time that would not adversely affect the construction schedule. The 6.4 m diameter by 3.4 m length (21 ft by 11 ft) mill met this criteria. Again, flexibility and expandability were the main focus in the design of the primary circuit.

Cyclone overflow advances feed to the rougher flotation circuit, which consists of six Outokumpu 42 m³ (1,500 ft³) flotation cells.

Lead Flotation

The four flash floatation cells and first two rougher flotation cells have been isolated for a bulk lead, pyrite concentrate from which a marketable lead concentrate is made, see Figure 2. The bulk concentrate pulls only 4% of the feed, but carries a high grade of gold and silver. The pH of the bulk concentrate is adjusted up to 9.0 with lime and 50 g/tonne (0.1 lb/ton) sodium cyanide is added for pyrite suppression.

Pyrite Flotation

Rougher flotation cells 3 through 6, cleaner flotation cells 1 through 4 and the lead column tail combine to make the pyrite concentrate, see Figure 3. This concentrate is then classified through four 38 cm (15 in) cyclones in closed circuit with the pyrite regrind tower mill to 80% minus 44 µg (325 mesh, U.S.) .

Reagents

The recovery of gold and silver is the bottom line. When this mill was built, the idea of expansion and renovation was built in. The recovery of gold and silver have been established over the course of 27 different flotation campaigns. To date 83% gold recovery has been achieved on a consistent basis, but silver recovery has varied, from 59% to 76%. Considering the average ratio of silver to gold is around 50 to 1, any increase in the silver recovery is cost beneficial and essential to this operation.