Chile: La Coipa Mine

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La Coipa Mine

Overview

La Coipa is an open pit gold and silver mine located in Region III (Atacama) northern Chile 140 km NE of Copiapó. When it opened in 1991, La Coipa was one of the first large-scale precious metals mining operations in Chile. La Coipa is owned and operated by Compañia Minera Mantos de Oro (MDO), a Chilean company which is 100% owned by Kinross as of 2007.


Location

La Coipa Mine is located in Region III (Atacama) northern Chile 140 km NE of Copiapó.The road distance between Copiapó, the nearest city, and La Coipa is approximately 140 km. The first 30 km are paved and the remaining part is a third-class dirt road; the road is in good condition but can be icy during the winter. The mine is connected to the Chilean national power grid system. The mine lies in the Domeyko Cordillera between 3,800 m and 4,400 m, with the staff accommodations and facilities situated at approximately 3,300 m elevation. The plant site elevation is 3,815 m, and current and future mining operations are at elevations ranging from 4,040 m to 4,390 m.

The climate at site is considered pre-arid Mediterranean, subject to low temperatures, strong winds and some snow during the winter. Despite the adverse climate, mining operations are performed year-round without interruption. Temperatures range from a high of 25°C in the summer (January) to a low of approximately -10°C in the winter (August). Water is scarce in the area, but the Maricunga Salar provides sufficient water to fulfill industrial needs through a 40 km pipeline. Vegetation is sparse. Wildlife observed in the region around the mine includes foxes, vicunas and guanacos. Pink flamingos are also frequently seen around the salars except during their migration period in the winter. [3]


History

A small underground copper-silver mine was operated a century ago 2 km SE of present day operations. Attention was not received from any exploration geologists until the late 1970s, when after the El Indio gold deposit, international companies began to explore the Maricunga district in the high Andes for colour anomalies. Pick and shovel miners discovered gold-bearing structures between today’s Farellon and Coipa Norte orebodies. Systematic exploration by Exploraciones y Minerales Sierra Morena S.A. began in 1981 and a channel sample, 60 m long, gave an average silver content of 21.5 oz/tonne.[1]

A feasibility study was commenced in 1988 and showed a mine life to 2002. In 1989, engineering began for a 15 ktpd plant, which was then commissioned in 1991. On August 1, unionized workers at La Coipa voted to accept a new three-year collective agreement, ending a strike that began on July 8. The strike reduced La Coipa’s forecast production for July by approximately 9,400 gold equivalent ounces. [1, 2008]


Mineralogy

The property is located in the northern Chilean Tertiary volcanic belt referred to as the Maricunga Belt. The mineral deposits were formed by the precipitation of gold and silver from circulating mineral bearing hydrothermal fluids in fracture and breccias zones. These permeable zones are controlled by major faults. [4] The mineralogy of the precious metals mineralization within the oxidized and enrichment zone consists mainly of cerargyrite, native Au, native Ag and electrum with minimal embolite, iodargyrite and argentojarosite. Within the sulphide zone, enargite and native Au are the dominant minerals, but variable amounts of chalcopyrite, bornite, covellite, mineralization tenantite-tetrahedrite, sphalerite and galena are also present. [3] High gold values in the eastern portion of Coipa Norte are in direct association with advanced argillic alteration. Higher silver grades accompanied by little or no gold, especially those in western Coipa Norte, are associated mainly with silicification and less commonly with advanced argillic assemblages. [3]

 Operations

The La Coipa mine currently operates three open pits: Ladera Ferellon, Coipa Norte and Puren. The Can Can deposit is scheduled to be mined later in the mine life. Production for 2008 was 226,293 gold equivalent ounces. [1, 2008] La Coipa uses conventional truck-shovel mining.

The mine employs 460 people and it is scheduled to cease production in 2012 if additional reserves are not found; however, Kinross believes that there is potential for additional reserves and resources to be discovered near the present mine site (Dec/07). Seven holes (2,063 metres) were drilled at Coipa Norte targeting mineralization extending beyond the east wall of the pit and beneath the pit floor. Nineteen holes (3,289 metres) were drilled targeting mineralized extensions of ore exposed at the bottom of the pit. [1, 2008]

Mine Cycle


Drilling and Blasting

  • 4 drills - 5-6m * 6-7m drilling pattern, ore and waste use different diameter drill holes
  • 600t per month of explosive at a powder factor of 200-320g/t
  • ANFO + heavy ANFO used for blasts, emulsions used wet holes
  • Pre splitting on bench edges – 20m 75o towards free face, 4-6” diameter with a 1” uncoupled charge

Loading and Hauling

  • One 25yd3 shovel
  • Two 28 yd3 Front End Loaders
  • One 23 yd3 FEL
  • One 13 yd3 FEL
  • 13 CAT 785B 154 tonne trucks

Three to four trucks are assigned to each shovel/FEL and the trucks travel 6 km out of the pit and then a further 9 km to the dump location.

Crushing
(thumbnail)
La Coipa Norte Pit Wall Failure along South Fault
  • Crusher 6’5’’*42”
  • Currently 16ktpd ore at plant, 4:1 stripping ratio
  • Total recovery 80ktpd (16ktpd ore)

Pit planning

  • 10m high benches, doubled or tripled when possible
  • Ramp 8-10% grade
  • 70o faces, interramp 45-53o
  • Equipment is specifically created for the mine
  • 85% availability for trucks, 88% FEL, 84% drills
  • Cutoff grade is 0.6 g/t Au or 20 g/t Ag depending on the mineralisation present

A pit wall failure occurred in the Coipa Norte pit during January 2008. Replacement ore to feed the mill in 2008 was sourced from stockpile ore which contained lower gold grades. [1, 2008] This area (the South Fault) has experienced 3 failures in the last 4 years. The wall slope has been adjusted, and due to the failures, 40 000 oz became unrecoverable and a cleanup cost of USD 2M was incurred. The Coipa Norte pit has rock strengths ranging from 20-80 MPa and contains lots of geotechnical structures.

Milling Cycle

The ore is processed in a mill that consists of 3 stages of crushing, single ball milling, leaching, and belt filtration. A feed of 1.5 g/t Eq Au is received at the mill. Pregnant solution is treated using the Merrill-Crowe process of zinc precipitation to produce gold and silver. [5, 1991].

  • Run of Mine is approximately 1m in size and the primary crusher reduces to 3 ½ “
    (thumbnail)
    Stacked Dry Tailings at La Coipa [2]
  • 28’ SAG mill, 15ktpd plant, can increase to 18ktpd
  • 55% solids for leaching, 8 tanks in series of cyanide (500g/tonne)
  • 1,000ppm free cyanide in leaching tanks
  • 24 hrs leach time
  • Counter current decantation used, precipitation of Au, Ag and Cu
  • Waste product of Hg produced 3,000kg/mo
  • Dore bars of 90-95% Ag and the rest is gold
  • Recovery 80% Au, 50-75% Ag

Belt filters dewater 18,000t of tailings a day to <20% water content. The tailings are conveyed to the stack area and deposited by a mobile radial stacker. One pass takes 6 months to complete. Vacuum filters were used to dewater the tailings in the hope of retaining dissolved gold from the process solution. The high rate of dewatering helps to lower the saturation of the tailings stack which helps in water conservation and stability in the high seismic area. The tailings are then spread out and compacted to increase the density of the stack. The deposit is much more stable than a paste disposal site. [2, 2004] The current height of the dry tailings is 100 m and it contains 110M tonnes of material. 



Gold equivalent = [Au grade] + [Ag grade x {Ag price/ Au price} x {Au recovery/Ag recovery}]

$4/t total cost (mine+mill)


References

[1] Kinross Gold Corp. (2008). Annual Report. Available online: [1]

[2] Engels, J. & D.Dixon-Hardy.(2004) Disposal Techniques: Dry Stacking La Coipa. Available online: [2]

[3] Keystone Mining Post. La Coipa Mine. Available online: [3]

[4] 24hrgold. (2008). La Coipa. Available Online: [4]

[5] Oviedo, L.; N. Fuster; E. Grez and A. Aguilar. “General Geology of La Coipa Precious Metal Deposit, Atacama, Chile.” Economic Geology. Vol 86 (1991): 1287-1300

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