The Wharf Operation deposits are considered Tertiary epithermal replacement mineralization, both intrusive-hosted and sediment-hosted (Coeur internal report, 2014). Depth of mineralization at the Wharf Operation (~1.5 km; Paterson and Giebink, 1989) is considered deeper than normal for epithermal deposits, but low temperature of mineralization are consistent with epithermal-type deposits (Paterson, 1990), although these temperatures may represent late stage of fluorite mineralization.

The main trends of mineralization at Wharf parallel the strike of a major joint sets that Shapiro and Gries (1970) measured in upper and lower Deadwood Fm. outcrops. At Golden Reward, the mineralized trends strike north-south. Mineralized trends at Golden Reward may reflect more of an influence from basement structure, as the later faults in the Golden Reward graben area may have been reactivated along faults or moved along existing foliation planes, and mineralization occurred along the same trend as the faulting.

Gold or gold-bearing minerals may be disseminated in porphyry, or may be confined to fractures (Paterson and Giebink, 1989; Paterson, 1990). Highly fractured porphyry is known to be the most mineralized, and preliminary work shows a correlation of fractured porphyry with gold, in contrast to unfractured and unmineralized rocks within mineralized zones. This, however, may be due again to gold-bearing sulfides occurring as the primary economic mineral. Porphyry near fractures is usually the most oxidized, grading outwards into gray, blocky porphyry with very small (< 0.25 mm) grains of disseminated sulfides. Atomic absorption (AA) assaying is limited in determining gold encapsulated in sulfides, but assays of fractured samples that are oxidized will readily measure gold formerly in sulfides. Additional fire assays may help to better understand this association.

Visible native gold observed at Wharf has been only seen twice during current mining activities; one occurrence was found within a fluorite vein. Because fluorite is considered a late-stage mineral (e.g., Paterson et al., 1989), the fluorite-gold occurrence may indicate a later, minor mineralizing event, distinct from earlier, goldbearing sulfide mineralization. Thin sections of very high-grade (1.173 and 0.615 oz/ton Au) samples from the Annie Creek mine analyzed by Schurer and Fuchs (1991) show native gold associated with hematite, goethite, jarosite which forms from oxidation of iron sulfides, and arseniosiderite which forms from oxidation of arsenopyrite; quartz was also associated with gold. A thin section from the lowestgrade sample in this study (0.195 oz/ton Au) showed no native gold, but abundant sulfides, which was assumed to indicate lattice or submicron gold. The discussion suggests that oxidation of arsenic-bearing sulfides may have remobilized gold in an arsenic complex and deposited it as native gold in higher-grade samples (Schurer and Fuchs, 1991).

Wharf Operation is currently a conventional truck and loader heap leach gold mine. The mine has been in continuous operation since 1983. Wharf leases nearly all the earth moving equipment used at the mine.

In-situ ore and waste must be blasted prior to removal. Several historic pits that were partially backfilled are being mined again and the backfilled material is considered rehandle and does not require blasting. Waste material removed for access to the ore is taken to one of the rock disposal sites. Rock disposal sites are all designed to fill existing pits and are reclaimed as soon as possible after placement.

Crushing
A single crushing circuit is used to process ore before being transferred to the leach pads. This crushing circuit has undergone numerous modifications over its history to accommodate operational conditions and optimize performance.

Ore is hauled from the pit with CAT 777 and 785 haul trucks that dump directly into a hopper or onto a stockpile adjacent to the hopper. Stockpiled ore is fed into the hopper by a loader at times when a direct ore haul is not available.

Ore is transferred by an apron chain feeder to a vibrating grizzly where oversize rock is diverted in to a Nordberg C140 jaw crusher. The jaw crusher reduces the rock down to a 6-inch nominal size before dropping onto a conveyor belt along with the fines from the grizzly. Powdered lime is added to the ore from a silo as it is conveyed up to the secondary crushing stage. The lime application rate can be adjusted as needed to control solution pH during leaching.

Before being fed into the secondary crusher, the ore passes over a screen deck to remove final product sized material. These fines are conveyed to the final product pile and oversize rock drops into a Nordberg HP 500 cone crusher where it is reduced to 2 ½ inch nominal size. Crushed ore is then conveyed to an adjacent screen plant to remove product size material before being conveyed to the tertiary crushing stage.

The tertiary stage of the crushing system consists of four Nordberg Omnicone 1560 cone crushers. Ore enters the tertiary stage through a diverter that distributes the rock to the four cone crushers. Each cone has its own screen deck to remove product size material before dropping into the crusher. This product size material is conveyed to the final product pile. Rock that is larger than product size falls in the tertiary cones and is crushed down to ¾-inch nominal size. The total crushing system throughput of 750 to 1,000 tons per hour can be achieved with only three of the tertiary cones operating. A fourth cone can be left in standby in the event of a failure or planned maintenance on one of the other cones.

After leaving the tertiary crushers the ore is conveyed back to the screen plant and any material not meeting the final product size is recirculated back into the tertiary cones until it meets specifications. The final product size target is 90 percent passing ¾-inch and 80 percent passing ½-inch.

Ore is trucked to the crusher located at the east end of the plant/pad area to be crushed to a nominal size of 80 percent minus ¾-inch passing. The crushing plant can process between 4.2 and 4.6M tons of ore per year, depending on ore hardness. Lime is added to the crushed ore. Once crushed, the ore is trucked to leach pads to be stacked in 20-foot-high lifts.

Stacked ore is then leached with dilute sodium cyanide solution. Gold and silver in the pregnant (metal-bearing) leach solution (PLS) are recovered by adsorption on activated carbon and the barren (non-metal bearing) leach solution is recycled to the heap leach pad. Spent ore is rinsed, neutralized and denitrified and then removed from the leach pad to be placed on a designated spent ore storage area.

Gold and silver is recovered from loaded carbon utilizing a modified pressure Zadra method. The rich electrolyte (RE) from the elution process is then processed by electrowinning, depositing the metals into an ........