Pascua Lama mine |
Location: Andes Mountains, on the Chilean-Argentine
border.
Products: Gold,
Silver, Copper.
Owner: Barrick Gold.
The Mine life: The mine life is expected to be 25 years.
Deposite Type:
The gold, silver and copper
mineralisation and alteration assemblages at Pascua-Lama are associated with a
structurally controlled acid sulphate hydrothermal system hosted by intrusive and
volcanic rock sequences of the Upper Palaeozoic and Middle Tertiary age.
Alteration and mineralisation are of the high-sulphidation, epithermal type.
Throughout the Pascua-Lama district, the alteration and mineralisation appear
to have been strongly controlled by structure. This control is most evident
along the Esperanza, Pedro and Quebrada de Pascua fault systems. As is typical
with high-sulphidation epithermal deposits, the principal metal commodities at Pascua-Lama
are gold and silver, the copper content is sub-economic.
The presence of hypabyssal intrusive host
rocks that are not related to mineralisation is unusual for high sulphidation
deposits, making Pascua-Lama (along with Barrick’s Alto Chicama deposit in
Peru, which is hosted by meta-sedimentary rocks) somewhat unique among deposits
of this type.
Overview
Pascua-Lama is located
at an altitude of 3,800m to 5,200m. The Chilean part of the mine constitutes
75%, while 25% is located in Argentina. The development activities of
Pascua-Lama were stopped in April 2013, following a Chilean court's orders on
issues of sanitation and violation of the Glacier monitoring plan. The Diaguita
indigenous community filed a petition for the closure of the project. Barrick
Gold's plea to reopen the project was rejected by a local appellate court in
Copiapó, Chile, on 24 April 2013.
The Chile's Supreme
Court, however, issued a ruling in September 2013 overturning the Copiapó court
order. Following the ruling, Barrick Gold will construct a water management
system at the Chilean section of the mine in order to receive environmental
approval for the project. The water management system is expected to be
completed by the end of 2014.
Barrick Gold announced
its decision to temporarily suspend the Pascua-Lama project, in October 2013,
in order to reduce its debt burden. Construction of facilities required for
obtaining the environmental approval will, however, be completed. The company
plans to resume the mine's development in future.
The Argentinean segment
was to include critical infrastructure such as the processing plant and
tailings storage facility.
Barrick Gold had
estimated the development capital cost of Pascua-Lama to be $3bn at 2009
prices, but the construction delay increased the estimated capital costs of the
project to approximately $8bn to $8.5bn at 2012 prices. The development of the
mine would have created more than 5,500 jobs during construction and more than
1,600 jobs during production phase.
Geological Setting
The Pascua-Lama deposit is situated at the crest of the high
cordillera of Region III, along the international border between Chile and
Argentina and on the northern edge of a major mineralised trend known as the El
Indio belt. This trend, along which a number of major precious metal deposits
are located (including the nearby Veladero mine), stretches 47km south of
Pascua-Lama to the world-renowned El Indio deposit and adjacent Tambo deposit
(both closed).
The geology in the region is dominated by extrusive volcanic rocks
that are locally intruded by hypabyssal stocks of varying size and numerous
dikes and sills (Figure 6-1). Volcanic activity began with deposition of the
Permian Guanaco/Zonso felsic ash flows from a caldera 15km east of Pascua-Lama
and subsequent intrusion of the Permian-Triassic Chollay crystalline felsic
rocks along the extent of the El Indio belt. These events were followed by
intrusion of the Triassic Pascua-Lama granite complex in the immediate vicinity
of the Project. Deposition of extrusive volcanic rocks and continued intrusive
activity resumed in the Oligocene with the Bocatoma diorite stocks (33-36Ma),
the Tilito dacite ash flows (27.2-17.5Ma) the Escabroso mafic andesite and
andesitic flows (21.0-17.5Ma), and the Cerro de Las Tortolas I andesites (16.0
±0.2 -14.9 ±0.7Ma), after which volcanic activity decreased markedly in the
vicinity of the El Indio belt. Subsequent activity was confined to the Vacas
Heladas intermediate dacitic domes, lava flows and felsic tuffs (12.8-11.0Ma),
and the Late Miocene rhyodacite dikes at Pascua. The most recent activity in
the region included deposition of the post mineralisation silicic Vallecito rhyolites
south of Pascua-Lama in the vicinity of Cerro de Las Tortolas, and the Upper
Pliocene Cerro de Vidrio rhyolite. All ages are from Bissig et al., (2000a
& 2001) and Martin et al.,(1995).
Regional
structure in and around the gold deposits and prospects in the El Indio belt is
dominated by northerly-trending high angle reverse faults, normal faults and fold
belts oriented parallel to the major structural grain of this portion of the
Andean Cordillera. Pascua-Lama is positioned near the center of a northerly
trending graben that contains nearly the entire Tertiary volcanic sequence that
is distributed along the spine of the cordillera in Chile and Argentina. This
graben is bounded by two high angle reverse fault zones, the Baños del Toro/Chollay
located 10km west of the deposit and the El Indio zone situated 16km to the
east. The graben is cut at Pascua and El Indio by strong, west-northwest
fracture zones, which form loci for mineralisation. Large elliptical fracture
zones are also present immediately to the east and/or northeast of both El Indio/Tambo
and the Pascua-Lama/Veladero deposit areas, and these zones may have
contributed to host rock permeability.
Metallurgy and Mineral Processing
The
Pascua-Lama (and Esperanza) ore is extremely complex and highly variable,
ranging from relatively straight forward oxide zones which are amenable to cyanide
leaching, to highly altered sulphide zones containing soluble sulphate minerals
with some cyanide-amenable gold/silver and some refractory gold/silver hosted
in sulphides. The majority of silver occurs in an enriched blanket of secondary
mineralisation in the upper zones of the deposit with silver grades typically four
to five times those of the underlying primary zones.
The
deposit is hosted in a high-sulphidation hydrothermal system consisting of
acidic material that requires a washing stage to remove soluble iron and copper
sulphate salts that are detrimental to subsequent processing. Ore material in
the deposit is classified as two main types:
Non-Refractory
and Refractory, both ore types are crushed, wet ground and washed in similar circuits.
The washed Non-Refractory ore is subject to direct cyanide leaching only with
pregnant solution which is recovered from the counter current decantation
(“CCD”) circuit, treated in a conventional Merrill Crowe (zinc precipitation)
circuit to produce gold/silver doré. The washed
Refractory
ore is subject to flotation with cyanide leaching of the flotation tails.
Solution recovery and precious metal production from the leached tails is via
the CCD and Merrill Crowe circuits to produce gold/silver doré. The flotation
circuit produces a final gold/silver rich concentrate of nominally 12% copper
for export to smelters.
The proposed nominal plant capacity is 45,000t/d of ore, 30,000t/d
for Non-Refractory ore and 15,000t/d for Refractory ore, according the
following schedule:
• Year 1, Q1: Two lines,
30,000t/d Non-Refractory ore;
• Year 2, Q4: Three lines,
45,000t/d Non-Refractory ore; and
• Year 3, Q3: Two lines,
30,000t/d Non-Refractory ore and one line, 15,000t/d Refractory ore.
Mine Production and Mineral Reserve Estimate
SRK audited the Mineral Reserve estimate that was prepared by Barrick
(Table 2). SRK is of the opinion that the estimation strategy and methods
employed meet or exceed current industry standards and the reserves have been
classified according to CIM guidelines. The LoM plan was based on calculations
prepared in mid-2008 for the Feasibility Study and not the end of year Mineral
Reserve estimate disclosed in this report.
The difference between the LoM plan and the Mineral Reserve
estimate is not considered material to Silver Wheaton.
Mining commences in 2011 with pre-stripping. The amount of pre-stripping
required is 66.4Mt and this is scheduled to be mined in an 18-month period using
the owner’s equipment. The first ore is produced in late 2012. The production phase
commences in 2013. The LoM production schedule is shown in Table 3.
The average ore plus waste mining rate is 66.0 Mt/y, comprising
18.3Mt/y of ore and 48.8Mt/y of waste. The average overall strip ratio is
2.71:1, exclusive of the pre-production period. The average overall strip ratio
inclusive of the pre-production period is 2.88:1.
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