Peñasquito Polymetallic Mine, Zacatecas, Mexico
Canadian mining company Goldcorp is expanding its Peñasquito open-pit polymetallic mine project in the north-eastern corner of the state of Zacatecas, Mexico. Pre-stripping began in August 2007, the oxide plant is scheduled to start up in July 2008 and the first metal is due to be produced in late 2008 with mill start-up in early 2009.
Pre-stripping began in August 2007 and the oxide plant commenced operations in the second quarter of 2008.The first gold was poured in May 2008 and in October 2009 the first lead and zinc concentrates were produced. By July 2009, the first sulphide process line was constructed. Line 2 is scheduled for start-up by the third quarter of 2010. Both the lines have a design capacity of 50,000tpd.
The high-pressure grinding rolls that have a design capacity of 130,000t will be constructed by the last quarter of 2010. The mine was officially inaugurated on 22 March 2010. Ramp-up to full production is expected in 2011.
The operating company is wholly owned Goldcorp subsidiary, Minera Peñasquito, SA de CV. Goldcorp owns 100% of the mineral rights to the area, which covers about 39,000ha.
The initial project capital cost was estimated at $1,494m, and the sustaining capital was estimated at $562m. By March 2010, approximately $1.7bn was invested in the project. A 2% NSR royalty is owed to Royal Gold on production from Peñasquito. Life of the mine is put at a minimum of 19 years, and the payback period is 6.1 years. No specific capital was raised for this project; the company says it was financed internally.
The project lies within the Mexico Geosyncline, a 2.5km-thick series of marine sediments deposited during the Jurassic and Cretaceous periods and consisting of a 2,000m-thick sequence of carbonaceous, calcareous turbidic siltstones and interbedded sandstones underlain by a 1,200m-thick limestone sequence.
The two sierras in the area are separated in the western half of the district by the Mazapil Valley, a synclinal valley underlain by the Upper Cretaceous Caracol Formation. The Caracol siltstone-sandstone section is generally flat-lying in the valley with occasional small parasitic anticlines and drag folds along faults. A large granodiorite stock is believed to underlie the entire area and the sediments are cut by numerous intrusive dykes, sills and stocks of intermediate to felsic composition.
The site has two dominant styles of mineralisation – sulphide and recent oxidised – at the surface, and the economically important styles of mineralisation are sedimentary-rock-hosted sulphide veinlet/stockwork deposits, breccia-hosted disseminated and veinlet sulphide deposits and secondary oxide deposits.
Total proven and probable reserves at Peñasquito are 17.82Moz of gold, 1,070.1Moz of silver, 3,214t of lead and 7,098Mt of zinc. Average ore grades are 0.13g/t for the gold oxide and 13.37g/t for the silver oxide grade; the average sulphide ore grades for the gold and silver are 0.32g/t and 18.62g/t respectively. Average lead and zinc sulphide ore grades are 0.19% and 0.44% respectively.
Total measured and indicated resources include 4.5Moz Au, 81.6Moz Ag, 1,205t Pb and 4,062t Zn. Inferred resources are 1.2Moz Au, 81.6Moz Ag, 357.14t Pb and 714.28t Zn.
The project is being built to process initially a nominal 25,000t/day of oxide ore and 65,000t/day of sulphide ore. Over the first three years of operation, further construction and expansion of the sulphide plant will provide for a rate increase to a nominal 130,000t/day.
The expanded operation is expected to produce an average of 1.7Moz gold-equivalent ounces a year – about 400,000oz of gold plus silver, zinc and lead.
Run-of-mine (RoM) oxide ore will be discharged from haul trucks onto a heap leach pile. Lime will be added to the ore before being placed on the pad, and the ore will then be leached with cyanide solution. This pregnant leach solution will then be clarified, filtered and de-aerated, then treated with zinc dust to precipitate the precious metals.
The precipitated metals will then be pressure filtered and the resulting filter cake smelted to doré.
RoM sulphide ore will be processed through a conventional crushing, milling and flotation facility. Haul trucks will deliver ore to a primary, gyratory type crusher, then the crushed ore will be conveyed to a SAG mill, dual-ball mill set-up, and pebble crusher facility. Ground and classified sulphide ore will be piped to the flotation circuit where the ore will be processed to produce lead and zinc concentrates.
The concentrate slurries will first be thickened and then dewatered using pressure filters. The dewatered concentrates will be stockpiled before loading onto road vehicles for transport to in-country smelters or to the ports for export to foreign smelters.
Although there is already an adequate road network in the region, access roads to the site will be improved and bypass routes around affected municipalities built.
Power will be brought in from the 400kV national grid via a tapped-in point reasonably close to the site. It will then be transmitted at 400kV from the 400kV/400kV switchyard tap-in via a new overhead power line to the mine site's main 400kV/69kV substation.
Process water will initially be obtained from wells to be developed on site and supplemented during later development from pit dewatering.
The tailings impoundment is designed as a zero-discharge facility with the capacity to store excess water temporarily from mill operations and expected climatic events including the design storm. Water will be reclaimed as needed from the tailings facility for use in the mill.
M3 Engineering and Technology, of Tucson in the US, is the EPCM contractor.