High altitude, higher hopes for Pascua-Lama
Project director Ron Kettles needs more than one word to sum up the Pascua-Lama project. “It’s a world-class deposit at high altitude in a bi-national setting. And the mineralogy is unique.”
The reward, when Barrick Gold Corp. begins production in 2009, will be an annual output of approximately 750,000 oz of gold and 30 million oz of silver on average for the first 10 years of the mine’s life. The deposit is estimated to contain 17.6 million oz of gold and 643.2 million oz of silver in reserves, supporting a planned expansion in the fourth year of production. Pascua-Lama is going to be a big mine with two decades of production ahead of it.
When Barrick merged with Lac Minerals Ltd. in 1994, it acquired the Pascua-Lama property. The deposits are located in the Frontera region, which straddles the border between Chile and Argentina. The region was recognized immediately as a prime target to expand Barrick’s gold production, and exploration became a priority.
In 1994, the deposit was estimated to contain 1.8 million oz of gold. Through exploration drilling over the ensuing years, Barrick has proved up the reserve almost ten-fold. Barrick started the development project in 2000-01, but this was postponed as the price of gold stubbornly refused to go above US$300/oz. In July 2004, with the Mining Treaty facilitating mineral development along the border between the two countries ratified, and with higher gold prices, Barrick proceeded with the development.
For the bi-national Pascua-Lama project, an Additional Specific Protocol–ratified by the ministries of Foreign Affairs of both countries in 2004–was required to clarify access to the site. The Pascua portion of the deposit, which will sustain the majority of the open pit, lies largely in Chile while the Lama portion of the deposit and the processing plant lie on the Argentine side of the border. In Argentina, the project is reached by following the road that passes Barrick’s Veladero mine. Finalization is pending for certain specific provisions related to cross-border services and the application of Protocol Rules related to customs and immigration.
The capital cost of building the mine, processing plant and infrastructure now stands at between US$1.4-$1.5 billion. That amount will put the project into production at a rate of 33,000 tonnes/day. Cash costs are estimated to be in the US$130-140/oz Au range. Another US$250 million will be spent in the first four years of operation to expand the plant, adding a flotation circuit and boosting throughput from 33,000 to 44,000 tonnes/day.
The extended exploration period had a big pay-off: estimates at the end of 2004 put proven and probable reserves at 327.27 million tonnes grading 1.7 g/t Au and 61.0 g/t Ag. Another estimated 39.43 million tonnes of material at 2.2 g/t Au and 51.6 g/t Ag fall in the resources.
Work this year is progressing on the various permitting processes. Once permits are obtained in both countries, construction will proceed in 2006.
Mining the mineralization
The gold, silver and copper mineralization at Pascua-Lama is part of a mineralized acid sulphate system that was structurally controlled within Upper Paleozoic and Middle Tertiary intrusive and volcanic rock sequences.
Basement rocks in the Pascua-Lama area are dominated by a multiphase granite pluton that may be a phase of the Permian Guanaco Sonso sequence of intrusives and volcanics. In the deposit area, the granite intrudes older diorites and volcanic pyroclastic units and is, in turn, intruded by diorite stocks and dikes of mid-Tertiary Bocatoma age. During Tertiary time, all of the previously described rocks were cut by subvertical fault zones and hydrothermal breccias located at complex fault intersections. Typically the breccias show a strong correlation to zones of intersection of two or more major faults.
The mineralogy at Pascua-Lama is unique. The deposit has both oxide and sulphide ores, with very finely disseminated mineralization. The sulphide ore has the potential to generate acid, and it must remain dry during mining, crushing, transport and grinding to avoid corrosion of the equipment.
Barrick is taking every precaution necessary. The company will build a canal to route most of the non-contact meltwater away from the waste rock and the pit. Any water that comes in contact with the waste dump, will be captured and channelled to a storage pond and from there it will be sent through a water treatment plant before being discharged into the natural water course. This process complies with the established laws regarding effluent discharges.
Mining will be centred around 4,600 m elevation. The plan calls for an open pit that lies 70% in Chile and about 30% in Argentina. This pit will be exploited at a rate of 45,000 tonnes/day starting in 2009. A smaller, 4-million-t pit that lies entirely in Argentina, is slated for mining in the last two years of mine life.
The mine plan calls for conventional truck-and-shovel methods, with Anfo as the blasting agent. Barrick designers plan to purchase three electric- and two diesel-powered blasthole drills. The diesel units have the advantage of mobility in parts of the pit without electricity. Plans also call for two hydraulic and two electric rope shovels. The size and number of haulage trucks was still under discussion when CMJ spoke to Barrick about the project in April.
Processing the ore
The primary crusher will be located at 4,750 m elevation on the Chilean side of the border. Ore will be conveyed 3.5 km down to the coarse ore stockpile. The 1.2-m-wide conveyor will have a 15 slope, and its first 2.7 km will pass through a 4.2-m by 4.5-m tunnel. Three feed lines will be installed to move coarse ore from the stockpile to the milling circuit in the processing plant, at 4,000 m elevation.
The plant will start up at a rate of 33,000 tonnes of refractory ore per day in 2009. Milling will be followed by washing through three thickener cells to remove soluble salts and iron that would impede gold recovery. This will be followed by a neutralizing step involving the addition of lime to raise the pH to 7.0-8.5. The solution will then flow to a Merrill-Crowe plant with twin leaching lines and five counter-flow clarifiers. The clarified solution will then go to the smelter where it will be reduced to dor that will be refined elsewhere.
The plant capacity will be increased to 44,000 tonnes/day three years later when processing of non-refractory ore begins. The plant expansion will involve adding a three-stage conventional flotation circuit to make 4,800 tonnes/year of copper concentrate. The copper concentrate, containing high amounts of gold and silver, will be custom processed elsewhere.
Barrick has chosen a tailings impoundment site that will cover 4 million m2. Design of the facility was underway in April when CMJ spoke with Kettles, who explained that the tailings circuit will be entirely self-contained. There will be a process of cyanide destruction and tailings thickening ahead of the tailings pond. The pond bottom and the upstream side of the dam will be lined with low-density polyethylene, plus there will be both a subdrain system and an overdrain system. This will be a contained closed circuit from which all water is recycled. Containment will be provided for each of the facilities as well as emergency holding ponds.
The Lama plant site lies in a valley surrounded by the Andes Mountains. The east side of the site is bordered by the Rio Turbio, which has a natural pH of 3.5. The southeast side is bordered by the pristine Rio Canito. These rivers will be diverted during the operating life of the mine. The closure plan includes the construction of a runoff capture area and lined canal to direct runoff to the tailings pond instead of its natural destination, the Rio Taguas. This is an example of the type of detail Barrick has addressed and will also explain to the Argentine authorities.
Barrick’s commitment to the development of Pascua-Lama in a responsible manner is important to help carry the company forward well into the 21st century.