MODULE 1 - THE ANDES, PART A - GOLD DEPOSITS
Major gold in the Andes is distributed within a number of belts from Peru in the north to the El Indio belt in the south. This tour was planned to visit key deposits in each of the main belts over this interval. In addition it is planned that the tour should take in prime examples of each of the different styles of gold ore in this interval, from high to low sulphidation to porphyry gold and high grade underground vein systems.
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Yanacocha is a major mine in the high Andes of northern Peru at an elevation of 4250 m. It is some 625 km north of Lima and comprises a number of heap leach gold mines that together account for a reserve at Dec 2000 of the order of 1135 t (36.6 Moz) of contained gold at grades of 0.9 to 1.6 g/t Au, currently extractable at a cash cost of $US 88/oz. Production in 2000 amounted to 55 t (1.8 Moz) Au. The ore is porous, is ideal for heap leaching and does not require crushing. The deposits are hosted by the mid to late Miocene Yanacocha Volcanic Complex. This complex is a flow dome field with associated pyroclastics and minor lacustrine sediments. It unconformably overlies both deformed and peneplained Cretaceous sediments, predominantly limestone, shale and quartzite and local Tertiary volcanics. Hornblende andesite tuffs were erupted during the waning stages of hydrothermal activity, while post-mineral ash flow tuffs and minor coeval rhyodacite intrusives are found nearby. The dome field covers an area of 19x7 km and comprises andesitic to dacitic flows emplaced within precursor tuffs. The sequence was intensely altered within 1 Ma of dome emplacement. Pervasive acid-sulphate alteration occurred in multiple coalescing centres. The alteration is zoned with central massive to vuggy quartz, progressing outward into quartz-alunite, quartz-kaolinite (alunite) and mixed clay layers with kaolinite and pyrite. The disseminated and fracture controlled gold mineralisation succeeded the main stage alteration. The mineralisation is hosted by siliceous alteration, fringed by a lower grade zone of quartz-alunite. In the main sulphide zone early quartz-pyrite is over-printed by a high-sulphidation assemblage of enargite-pyrite-covellite-digenite with minor sphalerite and galena in leached vugs, irregular fractures and as breccia matrix. The gold is associated with silver bearing enargite and minor arsenian pyrite. The mineralisation is controlled by a combination of structure and lithology, but is also locally linked with diatreme brecciation and dacitic stocks and dykes. The deposit is operated by Minera Yanacocha which is 51.35% owned by Newmont Gold Company.
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The Pierina Au-Ag ore deposit in the Cordillera Negra of north-central Peru is a high sulphidation (acid sulphate) epithermal deposit hosted by Tertiary pyroclastics. As of December 2000 proven+probale reserves totalled 92.5 mt @ 1.93 g/t Au for a total of 175 t (5.65 Moz) of contained Au, with around 1750 t Ag. Resources amounted to 17.7 mt @ 1.04 g/t Au for a further 18 t Au. The ore will be treated by heap leach processing (80% recovery) to produce 25.5 t (0.821 Moz) Au in 2000 at a cash cost of $US40 per oz. The life of mine cash cost is predicted at less than $US 90/oz. The waste:ore stripping ratio is 1.4:1. The Cordillera Negra is made up of Jurassic to Cretaceous sediments overlain by Eocene to lower Miocene andesites, dacites and rhyodacites. These are cut by a late Tertiary (16.0 to 2.7 Ma) granodioritic batholith. A 70 km belt of alteration in the Cordillera Negra corresponds to a series of eruptive centres and Pb, Zn, Cu, Ag and Au deposits (including Pierina) localised by cross structures. This alteration zone directly correlates with a NNW trending Pliocene-Pleistocene graben filled with pumice and tuffs over an andesite basement. Pierina is the first high sulphidation deposit discovered in the district. The hosts are rhyodacitic pumice and lithic tuffs overlying andesitic lavas, while a quartz-feldspar porphyry intrudes on the southern margin and is believed to be either late stage of post mineralisation. The southern section of the graben contained the dome/vent from which the pyroclastics were erupted. There is a strong lithological control to the mineralisation and alteration at Pierina. Residual vuggy silica containing alunite is the main host to ore. This core zone is surrounded by alunite with minor pyrophyllite and dickite, which also locally hosts ore, in turn surrounded by an outer kaolinitic to illitic clay zone. The pumiceous tuffs are more strongly mineralised and altered, containing the bulk of the ore, although lesser mineralised pervasive quartz-alunite is also found in the overlying lithic tuffs. The basal andesites have undergone clay-pyrite alteration with veinlet mineralisation. A sulphide feeder zone has been intersected to the south. Gold is disseminated in the vuggy-silica altered rhyodacite pumice, present as micron sized native gold grains associated with Fe-oxides, quartz and pyrite. More than 95% of the ore at Pierina is oxidised, which is regarded as both supergene and hypogene, with overall higher grades of up to 90 g/t Au. Remnant, overprinted kernels of pyrite-enargite-covellite-native sulphur are found locally. The pre-oxidation phases of quartz-alunite acid leaching created a porosity for pervasive pyrite-enargite-covellite, and quartz-pyrite-Au veinlet stages with associated lower 0.5 to 2 g/t grades. The final barite-Au veinlet phase contains up to 1.5 g/t Au. Hypogene mineralisation is dated at 14.9 Ma. The ore zone covers an area of 900x300 m. Pierina is operated by Barrick Gold Corporation═s wholly owned subsidiary Minera Barrick Misquichilca SA. The operation is operated and controlled by Barrick Gold Corp. .
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Pascua - El Indio
The El Indio Belt of Central Chile, some 500 km north of Santiago, and 180 km to the east of the coastal town of La Serena contains the three significant deposits, namely El Indio Au-Cu, and the Tambo and Pascua Au orebodies. The El Indio-Tambo group of mines are nearing exhaustion at current gold prices, while proving continues at Pascua to take advantage of future higher gold prices. The host sequence comprises Oligocene to Miocene andesitic to rhyolitic tuffs and volcanics preserved in a fault bounded depression. The rhyolitic and dacitic host rocks have been strongly altered and bleached over considerable areas. Mineralisation in the belt is related to both an early porphyry type event at 12 to 10 Ma, and a later high sulphidation overprint at 7 to 6.5 Ma.
El Indio is a high sulphidation Au-Ag-Cu deposit hosted by welded ash flow tuffs that underlie andesitic volcanics in a north-south striking fault bounded depression. Mineralisation appears to be related to the emplacement of small felsic domes and is of late Miocene age. The ore is present as fault controlled veins covering an area of 500 x 150 m. Mineralisation was emplaced in two well defined phases, namely initial massive enargite-pyrite veins, and second stage quartz-gold veins. The massive sulphide veins are accompanied by kaolinite and lesser alunite alteration and are rich in Cu, but gold poor. The second stage has associated sericite and lesser pyrophyllite, with local bonanza gold grades. Total production plus reserves are quoted as 23.2 mt @ 6.6 g/t Au, 50 g/t Ag, 4% Cu. Past production of bonanza grade ore totalled 0.5 mt @ 121 g/t Au.
The Tambo mine is around 5 km to the south of El Indio, separated by Cerro Canto, an 800 m thick pile of altered dacitic tuffs with minor dacite and quartz porphyry intrusives, interpreted to be a volcanic feeder zone. A similar, but subsidiary centre, Cerro Elephante, is immediately to the east of Tambo. The three separate pits that constitute the Tambo ´mine═ are characterised by ´higher level═ barite-alunite vein and breccia ores. Grades in the three pits average around 7 g/t Au.
Pascua is some 50 km to the north of El Indio, on the Chile-Argentina border. At the end of 2000 it had a proven+probable reserve of 545 t (17.5 Moz) of contained gold comprising 325 mt @ 1.71 g/t Au. In addition however the mineral resource includes a further 320 mt @ 0.8 g/t Au, accounting for another 245 t (7.9 Moz) Au. The operations are controlled by Barrick Gold Corp..
In addition to a detailed visit to Pascua-Lama, there was also a one day workshop, hosted by Barrick Exploration staff, on the El Indio Belt, including drill core from the region and specifically from the El Indio gold deposit.
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La Coipa - Maricunga Belt
The La Coipa high sulphidation gold deposit lies within the Maricunga Belt of gold deposit. The Maricunga Belt is separated from the El Indio Belt to the south by a barren gap without significant known gold deposits, and is some 150 km to the east and north-east of the Chilean coastal town of Copiapo. It comprises a NNE trending chain of andesitic to dacitic volcanoes that are part of a late Oligocene-Miocene continental margin plutonic volcanic arc. This arc embraces a series of predominantly epithermal acid-sulphate gold-silver deposits (including La Coipa) and porphyry gold orebodies (including Refugio, Aldeberan, Marte & Lobo). To the north, another "barren gap" with few significant gold deposits, separates the Maricunga Belt from El Penon, the next significant gold mine in the Andean chain (see below).
In addition to the visit to La Coipa, there was a one day field workshop on the Maricunga Belt, including a visit to the Marte porphyry gold deposit.
The La Coipa operation is based on six orebodies, namely Ladera-Farellon, Farellon Bajo, Coipa Norte, Brecha Norte, Can-Can and Chimberos. Ladera and Coipa Norte originally contained 52 mt @ 1.58 g/t Au, 60 g/t Ag, while Ladera comprised 9 mt @ 0.19 g/t Au, 170 g/t Ag. As of Dec 31 2000 the remaining proven and probable reserves were 46 mt @ 1.1 g/t Au, 63 g/t Ag. All but Chimberos are hosted by Triassic black shales and sandstones of the "basement" and by early Miocene tuffs and tuffaceous breccias. Silver is found mainly in the volcanics while gold occurs preferentially in the basement sediments. Chimberos lies within Jurassic sediments with silicified shales and sandstones. Intrusives dykes and sills of latite composition, and a dacite dome complex are spatially related to the pyroclastic sequence. Mineralisation accompanied the volcanic event. While there is a strong structural control to the mineralisation, in the form of north-south to NE-SW faults, there is also an important lithological influence - both representing permeability controls. The orebodies has a tabular to mushroom shaped form. Alteration varies in form in the different host lithologies, with high gold grades and alunite-kaolinite▒dickite-quartz argillic assemblage in the Triassic sediments at Farellon and eastern Coipa Norte, while the western Coipa Norte and Ladera have high silver/gold ratios in the Tertiary pyroclastics, with associated vuggy residual silica. Ladera is largely supergene altered with a suite of jarosite-goethite▒gypsum▒barite. Western Coipa Norte is silicified and supergene altered in its upper sections, while the depth it is been subjected to an argillic domain. Gold occurs in quartz with secondary oxides and in jarosite. The precious metals are present as cerargyrite, silver halides, argentite, native silver, electrum and native gold. Copper, lead and zinc sulphides, including enargite are present at depth.
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El Penon is located some 160 km to the south-east of the Chilean coastal city of Antofagasta at an elevation of 1400 m. It defines a new gold district and is in the same vicinity as the Escondida copper mine. After the discovery hole was drilled in early 1994, the deposit was proved, and following a 10 month construction period, the first gold was poured in September 1999. The deposit is associated with an approximately 60 Ma rhyolite dome complex which intrudes a sequence of volcanics (dacite flows, tuffs and volcaniclastics). The rhyolite complex represents multi-phase intrusive and extrusive activity. The El Penon deposit comprises a low sulphidation adularia-sericite epithermal vein system that is related to the rhyolitic dome complex, both temporally and spatially. The mineralisation, which is strongly fault controlled, has been dated at 59.4 ± 1.4 Ma by K/Ar dating of the Adularia. The main NNW trending mineralised faults dip at 80 degrees west, while the NNE trending faults dip at 65 to 75 degrees east or west. These faults are predominantly dip-slip and reflect both extensional and compressional regimes. Alteration is dominated by quartz ± adularia replacement or flooding within or near the veins systems, grading outwards into quartz-sericite/illite ± adularia. Argillisation is locally developed with in rhyolite, although argillic and sericitic alteration is best developed in the ash flow tuffs and dacite flows of the dome complex. Gold and silver are associated with a variety of quartz vein textures and grain sizes. Chalcedonic to coarse-grained quartz is found in banded, saccharoidal, comb and bladed carbonate replacement vein textures. Hydrothermal brecciation is common, having prepared the brittle rhyolites for vein emplacement, resulting in zones that are locally up to 22m in true width. The mineralised veins are believed to have formed from periodically boiling low salinity (< 2 wt% NaCl) fluids at between <200 to 255 degrees C. The oxide zone which extends 250 to 280 m below the current surface comprises largely silver halides, native gold and silver, and electrum with varying fineness. Base metals are rare in both the oxide and primary ore, although trace sphalerite, galena and chalcopyrite are found in the early banded veins in the unoxidised ore. During 2000 production totalled 9 t (0.289 Moz) Au and 124 t (4 Moz) Ag at an average gold recovery of 94% and cash cost of $US 48 (total production cost of $US 96) per oz. Reserves at December 2000 were 4.7 mt @ 10.1 g/t Au, 186 g/t Ag for 47 t (1.532 Moz) Au . Mineral resources total 4.5 mt @ 8.4 g/t Au. 160 g/t Ag for another 38 t (1.217 Moz) of Au. The mine is operated and controlled by Meridian Gold Inc.
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Antofagasta Seminar -
Common to both Module 1A and 1B
The Setting, Geology & Metallogeny of the Andes.
This day long seminar was common to both Module 1 Part A and Part B. It addressed the following topics:
- The tectonic setting & geology of the Andean Cordillera in Peru and Chile.
- The distribution of copper and gold within the Andes, both geographically, lithologically and temporally.
- An overview of gold in the Andes, including details of deposits not to be visited.
- An overview of the iron-oxide copper-gold deposits of the Andes, including details of deposits not to be visited.