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REVETT MINERALS the Rock Creek Evaluation Adit will begin the process of more clearly defining the characteristics of the ore body. Reviving the Troy Mine The Troy mine sits below Mt. Vernon just off Montana state Hwy 56. The Revett Formation outcrops at about 4,500 ft and dips at about 10% back under Mt. Vernon. Stratigraphically, the St. Regis Formation overlies the Revett Formation, which overlies the Burke Formation. The Revett has three members: Upper, Middle and Lower Revett. There are three quartzite units in the Upper Revett (the Upper, Middle and Lower Quartzites). Each is mineralized and Troy mines from all three. The largest amount of mineral- ization occurs in the Upper Revett Middle Quartzite and it's the most exten- sively mined member. The Troy mine has active headings in the A, C and plans to develop the I Beds of the Lower Revett. "We are mining in the A and C beds, and we are driving a decline down to the I Bed," said Doug Miller, vice president of operations, Revett Minerals. "We hope to begin development to the I Bed by July." At the time E&MJvisited; the mine in April, crews were driving a drift through a northern extension of the C Bed before starting the decline to the I Bed. Miller, who worked the Troy mine dur- ing the Asarco days, describes the mine as a typical room-and-pillar mine using a top slice and benching operation. "We take a top cut and then bench the re- maining thickness of the ore," Miller said. "When we were originally mining the Middle Quartzite, the bench was typ- ically 45-ft high. The remaining ore today has a total thickness of 30 ft. So we are talking a 20-ft top slice and benching the remaining 10 ft. The bench can be a lit- tle thicker in the C Bed, 10- to 45-ft Located in northwest Montana near the Idaho border, the Troy mine extracts copper and silver from the Revett Formation. thick, mostly averaging 20 ft. The A Bed bench averages 10 ft." The mine uses a fleet of Cat and Atlas Copco underground haulers and Cat 980 and 988 front-end loaders to load and haul muck. The mine also oper- ates three 2-boom jumbos, three bolters and three bench drills. "We're constant- ly upgrading our equipment, and we recently ordered a new bench drill so we will probably retire two of the others," said Bruce Clark, mine manager, Troy mine. "We have two AC jumbos and one Sandvik jumbo and another on order. We're also running three 60-ton Cat haul trucks, along with two 50-ton AC haul trucks, and one smaller Cat truck used primarily in the development areas—so six trucks total." The miners drill a 5-hole burn and load it with emulsion. "The ideal blast keeps the muck pile in one general vicin- ity, which works better for mucking, con- trolling dust and ventilation," Clark said. "We start with a 24-ft wide pilot cut and then it slabs out to 40-ft wide on final finish. We maintain 40-ft wide spans on the drifts and a 30-ft width on the cross cuts." The emulsion works well with wet holes and also helps with water quality. "We have top slice and benching operations in the C and A Beds," Clark said. "We are developing this C bed oper- ation off to the north of the fault. Then we have another crew mining in the Lower Quartzite. There are some mining activities in the Middle Quartzite in the south ore body and a development pro- ject in the east ore body." "We have about 25 faces available to drill and blast, just for the top slice drilling," Miller said. "With five trucks and three primary loaders, we have three to four active mining areas at one time." Crews can be drilling and loading holes in one area while others are mucking in another. The equipment simply rotates between the faces. The Troy mine is using the new Cat 980K front-end loader with a 450-hp Tier 4 diesel engine. "Because of the federal- ly mandated underground diesel particu- late matter (DPM) rules, we have tried to repower equipment with as much Tier 3 www.e-mj.com JUNE 2012 • E&MJ; 65