Meikle mine opens.
The state of Nevada has a well-deserved reputation as a gambler's paradise. Mining companies have gambled in the state, and many have come out winners. Toronto-based Barrick Gold Corp. took its chances in January 1987, when it anted up $62M to purchase the Goldstrike deposit as an exploration target. Shortly thereafter it hit the jackpot. Today the property hosts gold reserves of almost 30M oz. It has an established open-pit mining operation and is entering a new phase with the opening of the Meikle underground gold mine.
The Meikle gold orebody is one mile north of the Betze-Post open-pit mine on Barrick's Goldstrike property in the northern portion of the Carlin Trend of Northeast Nevada. About one hour's drive from the bustling town of Elko, the 7,000-acre property is easily accessed by a road that passes several large open-pit gold operations that have made the Carlin Trend famous.
Barrick's Goldstrike property was purchased as an exploration target even though it hosted a known deposit. Shortly after the purchase, the Betze deposit was discovered to add to the reserves of the Post oxide deposit discovered in 1982. Since then, the property's reserves have increased dramatically and by year-end 1995 totalled 29.9M oz gold. The Betze and Post deposits were developed as one large open-pit mine: the Betze-Post Mine contains 23.3M oz with an average grade of 0.203 oz/st gold. The Meikle mine has 6.6M oz with an average grade of 0.68 oz/st gold.
The Meikle deposit was discovered in September 1989, when drill hole EX89-4 intersected a length of 540 ft grading 0.41 oz/st gold from a depth of 1,305-1,845 ft. The area had originally come to the attention of geologists when they noticed surface silicification. Subsequent geochemical and geophysical surveys had outlined a number of anomalies that were drill-tested. The tenth deep exploration hole penetrated the heart of the deposit.
Preliminary evaluation by consulting engineers Steffen, Robertson, and Kitsten, in late 1991 led to a decision to proceed with development of the mine. Detailed engineering design of the shaft system was undertaken by Dynatec Mining Ltd., while Kilborn Engineering designed the surface facilities. Drawing on its internal expertise Barrick, formed a team to design the underground infrastructure and mining system. The design work formed the basis for the issue of tender documents, the selection of contractors, and the procurement of key equipment items. According to Louis Dionne, vice president-Canadian operations and the person responsible for getting the project into production, "the mining operation was designed for the orebody." The equipment was purchased to suit the orebody and people were hired for their suitability to work the deposit. Dionne was brought on board to get the mine up and running because of his experience in Canada, where most gold mining operations are underground and accessed through vertical shafts, unlike most gold mines in the United States that are open-pit or adit operations.
Meikle is a large, shaft-accessed underground operation. With his experience in underground operations, Dionne was responsible for guiding the project to the production stage. Reporting directly to Dionne is Rod Pye, mine manager. Pye has been with the project from its earliest days, first as a consultant and then as project manager. His background includes work in Zambia and South Africa followed by nine years as a consultant from which he was recruited to his present position. Dionne and Pye built the team at Meikle, a task that Pye relished. He enjoys putting a team together and believes that, "if you can do that, then most of the problems will go away."
Another key player was Herman Pieterse, vice president-capital development, who managed the surface construction of the project and put his experience with refrigeration in South Africa to work.
To John Breen, chief mine engineer and early member of the development team, one of the biggest challenges of bringing the project to completion was the scheduling. The different phases of the project had to be coordinated with the various contractors on site. Due to careful planning, facility development began at the same time as mine development.
Ore reserves were determined in-house and audited by Pincock, Allen & Holt, Inc. Extensive metallurgical testing was conducted by Lakefield Research, and hydrogeological studies were prepared by Legette, Brashears & Graham, Inc.
Dionne is proud of the development team on this project and its smooth operation. It was their efforts that enabled the mine to officially open on Sept. 21, 1996, on schedule and within its $180M budget. Commercial production began in the last quarter of the year.
The capital costs for developing Meikle have been $180M, or approximately $30/oz gold, financed through cash flow from the Betze-Post Mine. Mining, processing, and administrative costs are expected to be about $75/st, or about $125/oz.
The deposit is hosted in a complex series of collapse, tectonic, and hydrothermal breccias, which are stratigraphically located within the Middle to Upper Devonian Popovich Formation, which is composed of several facies of limestone. The formation ranges from 400-800 ft thick. Approximately 85% of the reserves are hosted by the Popovich. Overlying the Popovich Formation is the Middle to Upper Devonian Rodeo Creek Member, composed of a series of argillite and calcareous siltstone units.
Underlying the Popovich Formation is the fossil-bearing facies of the Silurian-Devonian Roberts Mountains Formation. This forms a prominent paleo-topographic high (RMH) along the length of the Meikle orebody.
The Paleozoic rocks are intruded by a series of Late Jurassic diorite to monzonite dike-and-sill complexes that are commonly strongly mineralized along their footwall and hanging wall contacts.
The rocks in the mine area have been subjected to a number of regional deformations; reactivation of earlier structures is observed at all scales.
The Meikle orebody has been divided into three ore zones based on geometry and relative depth. The Upper Main Zone is relatively shallow dipping, and occurs immediately below the Rodeo Creek-Popovich contact. The Lower Main Zone dips steeply (65 [degrees] - 75 [degrees]) to the northeast and is located below the Post fault system along the eastern boundary of the RMH. This portion of the orebody forms the high-grade core of the deposit, and has an average thickness of about 200 ft. The zone has a strike length of approximately 1,200 ft and is oriented N20 [degrees] - 30 [degrees] W. The Lower Main Zone appears to be the main feeder zone for mineralization.
The South Meikle Ore Zone occurs in the same stratigraphic position as the Upper Main Zone, but lies west of the RMH. Ore is hosted within breccias and mudstones underlying the lampro-phyric sill complexes along the Rodeo Creek-Popovich contact, and in many cases, the sills themselves host high-grade mineralization along their footwalls. The average thickness of ore at South Meikle is 40-50 ft.
The dominant control on mineralization at Meikle appears to be the north-northwest striking Post fault-system that provided the main conduits for both intrusive rocks and gold-bearing fluids.
Gold is found in arsenic-rich fine-grained pyrite and is usually invisible because of its minute size. Drill core that assays 13.35 oz/st gold even under close inspection has no visible indication of the gold it contains. This type of mineralization presents a challenge underground, as the only way to distinguish ore from waste is by sampling and assaying. According to chief geologist Jeff Volk, rush samples from the stopes can be analyzed in [less than]6 hours that allows for timely routing of broken ore.
Main Development Challenges
The proximity of the Betze-Post Mine was a major factor in making the Meikle deposit economically feasible. Because the Betze-Post open-pit mine was operational, much of the necessary infrastructure was present on the Goldstrike property; the only thing to be done was to build the underground mine. The Meikle Mine Development Plan, formulated in 1992, called for the development of the top of the orebody between a depth of 800-1,225 ft. Louis Dionne saw four main challenges to the project: heat, water, mine access, and ground conditions.
The Meikle mine is hot. Touching the rock on the 1225 level is like touching a water kettle a few minutes the water had boiled. Ground water temperatures average 140 [degrees] F, creating an environment that would be unsuitable for working. To overcome this, Barrick had to lower the water, which was accomplished in tandem with Betze-Post. Most of the ore in the Betze-Post pit lies below the original water table. This required pumping water from the ore zone that drew the water table down to 600 ft below the pit floor. This draw-down also lowered the water table in the Meikle deposit to a point below the planned level of operations for the first phase of mine development.
As a next step, Barrick borrowed technology used in the deep mines of South Africa. Olla van der Walt of T.W.P. Ltd. designed a cooling system composed of refrigeration plants, main fans, cooling towers, and a 22.5M Btu/hr bulk-air-cooler that is capable of pumping 518K [ft.sup.3] air cooled to 45 [degrees] F into the mine to produce an average temperature in the mine workings of 81.5 [degrees] F. This system began operating in June.
Mine access was a concern because the rock above the orebody was considered to be weak ground. It was determined that sinking a shaft would be more cost effective than driving a ramp to access the orebody. Today there are two shafts: an 18-ft-dia circular concrete production shaft sunk to a depth of 1,480 ft, with stations at 925, 1075, and 1225 levels, and a 16-ft-dia concrete ventilation-shaft sunk to its ultimate depth of 1,320 ft, with stations at 860, 925, 1075, and 1225 levels. The production shaft has been designed for future deepening to open up the lower part of the orebody.
Ground conditions were an early concern, based on drill core examination and testing. After dewatering and underground access was attained, it was discovered that rock conditions were better than expected. Remi Proulx, senior engineer, explains that with dewatering, the rock "tightened" and became more competent. To secure the back, one Secoma Mercury 14-1B rock-bolting jumbo was purchased. Split Set stabilizers and rebar with resin are used to reinforce the back, and wire mesh is used to keep any loose rock from dropping.
Extensive Rock Mass Rating (RMR) work on surface drill core helped to determine the best place for shaft and facility location. These structures and facilities were placed as well in the small confining space of higher RMR, but away from the bulk of the orebody, resulting in the two shafts being placed only about 350 ft apart.
The overall orebody dimensions, including the South Meikle orebody, are 2,300 ft in a north-south direction, with a 950-ft elevation difference from 875-1,825 ft below surface.
As of Dec. 31, 1995, reserves at the Meikle Mine were as follows:
Reserve Volume Gold Grade Contained Classification (K st) (oz/st) (K oz) Proven and Probable 8,373 0.683 5,719 Possible 1,350 0.650 877
The first mining phase calls for a production rate of 2,000 st/d or 730K st/yr to produce about 400K oz/yr gold. Mine life is approximately 12 years, using current reserves and mining rates.
Ore will be extracted using two different mining methods: about 75% of the ore will be bulk mined using long-hole open stoping with cemented rock fill, and the remainder will be taken using an underhand drift-and-fill method. Both methods require cemented rock fill using high-quality, crushed-and-sized aggregate. The long-hole open-stoping operation that is being used to start production has stope dimensions typically 25 x 100 ft, between sublevels spaced 50 ft apart vertically, and containing about 10K st/stope.
Underhand drift-and-fill will be used starting early in 1997 in areas where the orebody is not amenable to open stoping: primarily areas of flatter-lying or narrow mineralization in less competent rock. In these areas the ore is extracted by a series of drifts the size of which depends on the rock strength in the area being mined. When individual drifts are completed, they are completely filled with cemented back-fill to permit the extraction of ore adjacent to them without opening up large expanses of unsupported rock.
The back-fill system can provide fill at a rate of 4,000 st/d. Crushed waste rock from the Betze-Post pit is transferred underground along with cement and fly ash. An automated underground Besser-Appco mixing plant prepares back-fill that is distributed to the mined out stopes by 22-st Dux trucks.
The main horizontal access in the upper mining area is on the 925, 1075, and 1225 levels. These levels are connected by a ramp system to the sublevels.
Breaking and Moving Rock
The drilling equipment consists of three Tamrock Monomatic H105D, single-boom jumbos and two Minimatic H205D twin double-boom jumbos. These drills are both electro-hydraulic with HL500S rockdrills. Muck is moved by three 6-[yd.sup.3] Tamrock EJC LHDs, four 3.5-[yd.sup.3] Wagner ST-3.5 scooptrams (both models equipped for ejector buckets and remote-control operation), and one 2-[yd.sup.3] Wagner ST-2D scooptram. The haul truck fleet consists of five, Dux DT22 underground trucks and one Dux-DT20 underground teledump-truck. The truck shop is located on the 1225 level and measures 190 x 30 ft by 20-ft high. The location allows easy access from the ramp system and the production shaft with entry from both ends of the shop. Exhaust from the shop goes directly to the ventilation shaft.
Oversize rock from development is reduced by two Teledyne TM-20H pedestal-mounted hydraulic-breaker systems with TB-825XS hydraulic breakers. All production ore is crushed prior to hoisting by means of a Kue-Ken 36 x 48 in. jaw crusher located on the 1225 level.
Loading pockets are located on the 1180 and 1330 levels. The upper pocket has a capacity of 173 [ft.sup.3] and is semi-automated. The lower pocket with a capacity of 195 [ft.sup.3] is fully automated.
Ore is hoisted to surface by two 10-st, 195-[ft.sup.3] bottom dump skips using an Ingersoll-Rand 144-in., 1,250 hp double-drum production-hoist. Beneath one of the skips is a trailer cage with a maximum load of 17 people, or a maximum combination load of 39.7K lb. The production shaft is also equipped with a personnel hoist with a capacity of 12 people and a service hoist that is also available for future shaft-deepening duty.
Once on surface, ore is dumped into a 500-st coarse-ore bin. The bin is emptied into 190-st trucks for transport to the Goldstrike mill where it is supplanting some lower grade Betze-Post ore in the recovery process.
When in full production, approximately 200 people will be employed at the mine that will operate 24 hr/d, 7d/wk. According to Steve Long, mine superintendent and a fourth generation miner himself, miners work 11.5-hr shifts in a "super-long change rotation" that sees the miners work 14 d/mo but gives them at least one 7-d/mo period off. The mine is non-union, and the miners are eligible for a production bonus based on meeting or exceeding monthly targets.
Meikle has been fortunate in acquiring experienced operating personnel from the Cannon mine, which is a similar operation. The former closed in late 1994. As production at Meikle increases and development winds down, some of the Dynatec personnel will be hired and some miners with underground experience will be moved from the Betze-Post Mine. By the end of 1996, when the mine is in full production, the contractor's role will be completed and the mine will be run with its own staff.
With the beginning of commercial production, and the tough job of development behind it, management's thoughts will focus on increasing production and adding to reserves. The orebody is open to the north and at depth. The design of the mine and the capabilities of the equipment can allow for increased production without straining the system. To supplement the reserves, exploration is ongoing on the Rodeo and East and West Griffin prospects, which are south of the Meikle orebody.
Barrick's good fortune continues with the beginning of production in one of North America's richest underground gold mines.
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|Title Annotation:||Nevada gold mine|
|Publication:||E&MJ - Engineering & Mining Journal|
|Article Type:||Cover Story|
|Date:||Oct 1, 1996|
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