Location: Elqui, Coquimbo, Chile.
Products: Copper & Gold.
Owner: Royal Gold,Inc.
Ore Type: Porphyry copper-gold deposit, hosted by altered andesitic and dacitic volcanic rocks, and small stocks and irregular dykes of potassium-rich tonalitic porphyry.
The Andacollo mining district is located in the Coquimbo region of Chile at 30°14’ south, 71°06’ west, some 55 km southeast of La Serena, at a mean elevation of 1030 m within a semi-arid hilly landscape. Current mining activity in the district is concentrated on copper and gold. These metals are mined, respectively, from a porphyry copper deposit and epithermal, manto and vein gold deposits of adularia–sericite type.11,13 Other types of mineralization include mercury veins hosted by carbonate rocks. The gold veins are controlled by a northwest-trending set of normal faults, whereas the manto-type mineralization is strata-bound and largely confined to andesite breccias, dacites and sites of strong fracturing. The lateral and vertical continuity of the mantos is strongly controlled by rock type, faulting and intensity of fracturing. The gold deposits have been the focus of a recent study,11 but comparable information on the Andacollo porphyry has not become available.
Andacollo’s operating profit from August 22 to December 31, 2007 was $27 million before the effects of the revaluation of copper inventory to fair value on acquisition and negative pricing adjustments. The revaluation established a higher value for copper inventories, based on market prices at the date of acquisition. This increased our cost of sales by $24 million and the subsequent decline in metal prices resulted in a loss on the sale of these inventories. In addition, the mine recorded negative pricing adjustments of $2 million since they acquired it in August 2007. After these adjustments, Andacollo’s operating profit was $1 million. Copper cathode production in 2008 is expected to be approximately 20,000 tonnes and capital expenditures are planned at US$190 million, including US$185 million on the hypogene development.
Geological setting and Mineralization
The Andacollo deposits are the products of a complex hydrothermal system and consist of a porphyry copper-gold deposit and peripheral strata-bound manto gold deposits and veins with minor associated base metals. The hydrothermal system was part of the Pacific porphyry copper belt which was generated during development of an Early Cretaceous magmatic arc displaying shoshonitic petrochemical affiliations. Rocks that crop out in the area include a volcanic sequence, the Arqueros and Quebrada Marquesa Formations, consisting of andesitc and dacite flows, volcanic breccias, and pyroclastic rocks of Early Cretaceous age. Intrusive rocks range from diorite to granodiorite in composition and date between 87 and 130 Ma. The porphyry copper-gold deposit is zoned vertically downward from a leached capping through a supergene enrichment blanket to a hypogene sulfide zone. Alteration is characterized by central potassic (K feldspar-biotite), phyllic, and peripheral propylitic zones. Abundant northwest-trending tensional fractures were superimposed on the porphyry copper-gold deposit and surrounding areas during the later stages of the evolving mineralized system. The fractures channeled mineralizing fluids from the central parts of the porphyry copper deposit outward for up to 5 km. Replacement by adularia and sericite took place together with deposition of gold-bearing pyrite and minor amounts of zinc and copper where these fluids encountered permeable dacite flows and andesite flow breccias. The alteration process caused remobilization of aluminum and alkalies and addition of K 2 O, which attains values of 12 to 13 wt percent. The Andacollo system is interpreted to be a porphyry copper-gold deposit that is transitional outward to distal epithermal, adularia-sericite-type contact metasomatic gold orebodies.
Chuquicamata Copper Mine
250km north-east of Antofagasta, 1,200km north of Santiago. (22°17'S 68°54'W)
Classification: open pit copper mine.
OreType : Copper porphyry.
Dimension: Size of pit: L=4,500m, W=3,540m, D=800m.
Production: Gold-Copper 650,000 metric
Tons annually. A=2,800m a.s.l.
Chuquicamata, in northern Chile, is the world’s greatest orebody. It was mainly controlled by initial intrusions (probably at 36 to 33 Ma) through mineralization (last major hydrothermal event at 31 Ma) to postmineral brecciation and offset by the West Fault system. The Chuquicamata Porphyry Complex consists of the East Porphyry, West Porphyry, Banco Porphyry and Fine Texture Porphyry. Potassic alteration, the early stage of alteration, affects all porphyries. Veins of quartz molybdenite, up to 5 m wide and cutting all porphyries, were emplaced between the early and the main stages. Main-stage veins occupy many of the same structures of the early stage and may involve massive remobilization of earlier mineralization. The late stage formed digenite with relatively coarse grained covellite from deep in the sericitic zone. A leached capping and oxide copper ore, replacing an upper chalcocite blanket, overlie a high-grade supergene chalcocite body that extends up to 800 m in depth.
Summary of Geological Setting
Chuquicamata is closely related to Eocene, early Oligocene porphyritic intrusions that occur within the middle to late Cenozoic, north-south striking Domeyko Fault system.
The oldest rocks in the Chuquicamata district occur in a north-northeast trending belt of Paleozoic metasedimentary and metaplutonic rocks. These rocks include gneissic granite, metadiorite, quartz diorite, and minor tonalite recrystallized in varying degrees to amphibolite.
The porphyritic rocks in the Chuquicamata pit, with the dominantly barren Fortuna Complex to the west and the intensely mineralized Chupui Porphyry Complex to the east, are separated by the major postmineral West Fault. Rocks with textures essentially identical to those of the Chuqui Porphyry Complex extend northward at least 9 km through the Radomiro Tomic mine (Cuadra et al., 1997; Cuadra and Rojas, 2001).
Fortuna Intrusive Complex
The Fortuna Intrusive Complex borders to the open pit and contains only low-grade mineralization. It has been structurally juxtaposed against the intensely mineralized Chuquicamata Porphyry Complex by large-scale, postmineral movement on the Wets Fault, which is documented by Dilles et al. (1997), Tomlinson and Blanco (1997), and previous workers. The Fiesta Granodiorite phase of the Complex is volumetrically dominant and is intruded by small irregular bodies of San Lorenzo Granodioritic Porphyry and minor Tetera Aplite Porphyry. Fiesta Granodiorite is weakly mineralized with copper oxides in the uppermost northwestern benches of the pit. Sulfides occur only near contacts of the San Lorenzo porphyries.
Pre-Chuqui porphyry intrusions
The Elena and East Granodiorites are exposed on the eastern margin of the pit. They intrude metasedimentary rocks that were originally shale and sandstone with minor limestone. Whereas Elena Granodiorite is mineralogically and texturally similar to the East Porphyry, the East Granodiorite is texturally distinctive and clearly older. A radiometric dating of the Elena Granodiorite indicates a Jurassic (dating of zircon) to Early Cretaceous age (dating of biotite), published by Ambrus (1979). All of these rocks at the east edge of the pit are essentially poor of mineralization.
Chuqui Porphyry Complex
Practically the entire Chuquicamata orebody is hosted by the Chuqui Porphyry Complex, made up of East, West, Fine Texture, and Banco porphyries. Their textures vary widely, and most exposures are affected by some degree of hydrothermal alteration and pervasive cataclastic deformation. The probably oldest and largest intrusion is the East Porphyry with hypidiomorphic-granular texture. The West Porphyry is finer grained and with quartz eyes in an aplitic groundmass. Locally both porphyries are weakly foliated. Banco Porphyry is more porphyritic and finer grained than East Porphyry, which it intrudes. From West Porphyry it differs in having an abundance of small plagioclase crystals in the aplitic mass. The Fine Texture Porphyry is distinctly finer grained than normal East Porphyry but has also a hypidiomorphic-granular texture. Contacts with East Porphyry may be abrupt but usually faulted. Because of the overprinting of most dikes by quartzsericite alteration, their identification is very difficult. Furthermore is seems, that Banco and Fine Texture porphyries have been affected by all of the same stage of alteration and mineralization as the East Porphyry.
A large part of the copper at Chuquicamata occurs in veins and veinlets filling faults and faultrelated shatter zones. In the main orebody practically all of these fractures have been opened and mineralized more than once. Early-stage veinlets of quartz and quartz-K feldspar contain no or only very minor sulfide. They are cut by more continuous quartz veins, to 5 cm wide, containing minor molybdenite and traces of chalcopyrite. Large banded quartz veins, known as blue veins, are typically 1 m or more in width. They contain abundant molybdenite and truncate the previous veins. Furthermore, they are commonly surrounded by sericitic alteration, but this is due to superposition of younger pyritic veins following the same structures. Veins and veinlets of the main stage contain pyrite, chalcopyrite, bornite, and digenite, decreasing amounts of quartz and increasingly well-developed sericitic alteration halos. Locally, the earliest of these veins appear to contain pyrite without Cu sulfide (Lindsay et al., 1995). Relatively late main stage veins contain enargite ± pyrite and minor sphalerite. Later on, veinlets and fractures are filled with relatively coarse grained covellite (to 1 mm) and digenite with and without pyrite.
Hypogene Alteration and Mineralization
Just like El Salvador and many other porphyry copper deposits, vein relationships lead to the definition of an early stage defined by K feldspar stable alteration and early quartz veinlets, a transitional stage defined by quartz-molybdenite veining, and a main-stage defined by pyrite-bearing veins with sericitic halos. A more unusual and controversial late stage is defined by coarse-grainedcovellite-digenite veinlets without pyrite and possibly hypogene sphalerite rims on other sulfides (Fréraut, et al., 1997).
Supergene Mineralization and Alteration
After Taylor (1935) and Jarrell (1944) the rich oxide copper orebody has been largely mined out, but considerable resources of lower grade material remain in the north end of the pit and beyond (North zone, Fig. 1; Cuadra et al., 1997; Ossandón and Zentilli, 1997). Oxide ores contain a large variety of minerals but in chief antlerite, brochantite, atacamite, chrysocolla, and copper pitch. Also residuals of chalcocite are implied. The ore was overlain by leached capping and was an eastward and upward extension of the chalcocite zone, indicating it was a supergene chalcocite enrichment blanket oxidized in situ. It is the upper of two chalcocite blankets with a leached horizon in between. A lower enrichment zone has more reactive alteration assemblages and contains decreasing chalcocite and/or covellite proportions downward. In the central zone of intense brecciation, the two enrichment blankets (copper leaching and chalcocite enrichment) merge and reach their maximum depth.
Mining and processing
Coldec uses conventional open pit mining methods at Chuquicamata. A conventional truck-and-shovel operation constitutes the mining activity. Large quantities of the ore are crushed within the pit. Underground conveyors transport the crushed ore to the mill bins.
An Outokumpu flash smelter is installed for smelting the concentrate. The concentrate is then passed through a converter with electric furnace. After the slag is cleaned, the concentrate passes through four Pierce Smith converters. Blister copper is then sent to six anode furnaces. The electrolytic refinery has a capacity of 855,000t per annum. Three anode casting wheels were installed and are fed by the furnaces.
Underground mining at Chuquicamata
The new underground mine, scheduled begin operations in 2019, will comprise of four production levels, a 7.5km main access tunnel, five clean air injection ramps, and two air-extraction shafts.
The tunnels will deepen the mine by nearly 787m by the end of production in 2060. The underground mine will be developed at an estimated cost of $4.2bn and will produce an estimated 140,000 tonnes of ore per day. It mine is expected to produce 366,000t of copper and 18,000t of fine molybdenum per year.
Sinclair Knight Merz undertook the conceptual engineering of the mine, including identification, option studying and analysis for ore excavation and handling. For ore extraction, panel caving and macro blocks were studied. Three extraction panels were identified at different depths for both options. Excavation panels identified for panel cavings were at 1,841m, 1,697m and 1,409m above sea level and 1,841m, 1,625m and 1,409m for macro blocks.
The copper ore reserves of the Chuquicamata underground mine are estimated to be 1,700mt grading 0.7% copper and with an average molybdenum content of 502ppm.
|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.
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.
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.
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.