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.
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The Mantos Blancos copper-silver deposit is located some 45 km NE of the Chilean coastal city of Antofagasta. It lies within the Atacama Fault Zone and is hosted by a Triassic sequence of acid volcanics, mainly rhyolites and dacite which dip at 10 to 45 degrees SE and, cut upper Palaeozoic sediments and metasediments. These are in turn overlain by Jurassic clastics and limestones, Jurassic andesites and Cretaceous andesites and dacites. In the mine area the host volcanics are intruded by a sill like sub-volcanic andesite body, by sheets of dacite and abundant andesite dykes. The mineralisation occurs as irregular bodies of oxide and sulphide copper with economically significant associated silver. The oxide minerals atacamite and chrysocolla are common in the upper levels of the sulphide body associated with faulting and intense brecciation. The primary copper sulphides comprise chalcocite, bornite, chalcopyrite and covellite and are found in irregular, lenticular bodies roughly parallel to stratification. These bodies usually have a central-upper high grade zone of chalcocite and bornite, a lateral-lower low grade variable halo with bornite-chalcopyrite, fringed on the outer margins by a pyrite zone with occasional associated chalcopyrite. Silver is found in the lattices of both the oxide and sulphide minerals and correlated with the Cu grade. Alteration is dominated by albitisation and silicification that is distributed homogeneously through the volcanic sequence. Specular hematite is found in the barren upper levels and red hematite in the mineralised zones. These represent four principal alteration types, namely: 1). Na metasomatism manifested by albitisation of feldspars as well as albite veining and pore filling; 2). incipient to intense addition (or locally removal) of Mg and Fe, reflected by chloritisation or bleaching; 3). intense hematisation in the form of disseminated and stringer specularite and by intense pervasive red hematisation to many of the rocks within the deposit; 4). silicification, represented by quartz phenocrysts, microcrystalline aggregates in the groundmass and as occasional veinlets and amygdule fillings. Economic grade ore is found over an interval of 3x1.5 km and to a depth of 450 m. In 1995 the pre-mining resource was calculated at 170 mt, of which 91 mt were oxide ore @ 1.4% Cu and 89 mt @ 1.6% Cu and 17 g/t Ag. The mine is operated by Minera Mantos Blancos S.A., and Anglo American Group company.
After the visit the tour group concluded, that while this deposit has been included within the Iron oxide Cu-Au class in past literature, it is not typical of this class of deposits.
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The Mantoverde mine is located some 400 km south of Antofagasta and 100 km north of the Chilean coastal town of Copiapo. It falls within the Atacama Fault Zone (AFZ) and is hosted by cataclased andesitic volcanics and coeval (?) early Cretaceous diorite porphyry stocks that have been subjected to low-grade regional metamorphism. The deposit is associated with specularite rich breccias in brittle faults belonging to the AFZ, which were emplaced during an extensional phase of sinistral strike-slip and dip-slip tectonism of that complex Jurassic to early Cretaceous major regional fault system. The deposit consists of three breccia units paralleling the 12 km long Mantoverde Fault for at least 1500 m. The Mantoverde Fault which strikes NNW and dips at 40 to 50 degrees east, joins two major branches of the AFZ. The hangingwall Manto-Atacama breccia is a ±100 m wide, matrix supported, specularite rich, hydrothermal breccia that follows the Mantoverde Fault along strike and down dip although it thins with depth. The degree of brecciation and mineralisation decreases gradually outwards from the fault into the andesite wall rocks. In contrast the footwall Manto Verde Breccia is a ±20 m wide cataclasite with relatively sharp mineralisation boundaries. Oxidation extends to 200 m producing copper carbonates, sulphates and silicates with minor Cu chlorides. Supergene enrichment is only thin and poorly developed. Little is know of the primary sulphides which comprise chalcopyrite and pyrite in a specularite matrix. District wide chloritisation and K-silicate alteration is characterised by microcline replacement and veining in the andesite country rock. Sericitisation is superimposed and localised in the Mantoverde Fault and neighbouring fracture zones. The latter accompanied the emplacement of the specularite and Cu-Au mineralisation and has been K/Ar dated at 121 and 117 ± 3 Ma. Fluid inclusion studies suggest boiling and homogenisation temperatures of 180 to 320 degrees C. The geologic features of the deposit are interpreted to indicate a common magmatic-hydrothermal origin to the apatite bearing magnetite deposits of the Chilean Iron Belt, but may represent a younger phase to the copper deficient systems. The copper resource at Mantoverde to the 800m level was estimated prior to mining at 120 mt @ 0.72% Cu (0.2% Cu cutoff) including a mineable reserve of 85 mt @ 0.82% Cu. The mine is operated and controlled by Minera Mantos Blancos S.A., and Anglo American Group company.
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Punta del Cobre District
A series of iron oxide Cu-Au(-Zn-Ag) deposits occur within the Punta del Cobre belt which hosts the Punta del Cobre district, sensu strictu and La Candelaria the largest of these deposits. The Punta del Cobre belt lies within an Early Cretaceous continental volcanic arc and marine carbonate back-arc basin terrane whose sequences are intruded by Early Cretaceous granitoid plutons that form part of the Chilean Coastal Batholith. The Punta del Cobre belt deposits are found fringing the eastern margin of the batholith within (eg., La Candelaria) or just outside the contact metamorphic aureole (eg., the Punta del Cobre district). Andesitic volcanic and volcaniclastic host rocks are intensely altered by biotite-quartz-magnetite. This style of alteration extends much further to the east of the intrusive contact than the metamorphic mineral associations in the overlying rocks that are clearly zoned outboard. Local areas of intense calcic amphibole veining that overprints all rock types occur within the contact metamorphic aureole. Chalcopyrite mineralization which crosscuts and thus post-dates all of the major metamorphic and metasomatic assemblages is paragenetically late. Deposits found close to the contact of the batholith and the deeper parts of the sequence in the Punta del Cobre district are characterised by abundant magnetite with associated biotite-quartz alteration, which is overprinted by fracture-controlled calcic amphibole, and chalcopyrite-pyrite mineralization. Potassium feldspar-chlorite and/or biotite ▒ quartz plus magnetite ▒ hematite occur in the intermediate parts of the hydrothermal system. Up-section and, in places, laterally, these assemblages grade into pervasive albite-chlorite-calcite-hematite that are spatially associated with Cu-Au mineralization in the more distal portions of the system. Mineralization is controlled by tectonic structures and their intersection with massive volcanic rocks and overlying volcaniclastic rocks. Isotopic ages of alteration minerals associated with the metallic mineralisation indicates that the bulk of the iron oxide mineralization formed between 116 and 114 Ma, and the main copper-gold mineralization between 112 and 110 Ma, and that hydrothermal activity was coeval with both the emplacement of the Copiap¸ Batholith and regional uplift. They also imply burial during the mineralisation was no greater than 2-3 Km. Deposits other than La Candelaria account for another 120 mt @ 1.5% Cu, 0.2-0.6 g/t Au, 2-8 g/t Ag, with the larger deposits producing up to 1.5 mt of ore per annum.
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The La Candelaria deposit is located some 20 km south of Copiapo. Like Mantoverde and Mantos Blancos (see above) it is localised near the Atacama Fault Zone within the Coastal Cordillera of northern Chile, and the Chilean Iron Belt. It is hosted by early Cretaceous andesitic rocks, predominantly flows and volcanic breccias that are masked by a thick sequence of limestones with minor evaporites. This sequence has been intruded and strongly metamorphosed and metasomatised within the broad 2.5 km wide contact aureole of a multiphase lower Cretaceous (119 to 97 Ma) batholith which is exposed <1 km to the west of Candelaria. The accepted mine stratigraphy of the sub-horizontal host rocks is based on alteration assemblages. This comprises an intensely biotitised Lower Andesite, overlain by the banded and brecciated Tuff, which in turn is succeeded by an albitised Upper Andesite and the fine grained and biotitised Metasediments. The mineable reserve comprises 470 mt @ 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag within a geological resource of 600 mt at a similar grade. The ore deposit has a grossly stratabound form, with a thickness of >350 m in its central section. It lies within the Lower Andesite and the Tuff units. The ore is present as veins, veinlets, breccia fillings and coarse to fine-grained disseminations of chalcopyrite in association with 10 to 15% magnetite, lesser pyrite and subordinate pyrrhotite. The alteration is not obviously zoned, comprising dominant pervasive biotite in the lower 70% of the orebody, accompanied by calc-silicates, mainly actinolite and scapolite. The overlying limestones have been skarn altered, accompanied by restricted gold and copper, although small gold rich copper skarns in this lithology cropped out at two localities above the main Candelaria orebody. While the altered limestones are stratabound garnet bearing skarns, the mineralisation within the andesite has the characteristics of a wall rock porphyry deposit, although no porphyry has been identified. The main alteration assemblage within the andesites are albite-quartz-epidote with pyroxene and scapolite, while the underlying tuffs and conglomerates have albite, quartz, epidote, magnetite, amphibole and scapolite. This alteration is interpreted to have resulted from a six stage process, namely: 1). An early texture preserving albitisation which decreases in intensity and pervasiveness downwards; 2). Biotite-quartz-magnetite to produce widespread brown Fe rich biotite, disseminated quartz-magnetite and large magnetite-quartz bodies, but has no accompanying significant Cu or Au; 3). Main Ore Stage - the bulk of the Cu and Au accompanied this stage, superimposed on the previous barren alteration, and is characterised by iron rich calcic amphibole, chalcopyrite, magnetite and hematite after magnetite with pyrite albite and quartz, plus possibly cogenetic barren scapolite-pyroxene-amphibole and calcic garnet skarns in the overlying Upper Andesites; 4). Epidote-chlorite with minor associated chalcopyrite; 5). Hematite-calcite-chalcopyrite stage represented by hydrothermal breccia cements, and locally important mineralisation; 6). Anhydrite and calcite-chlorite stage. The mine is operated by the Phelps Dodge subsidiary Compania Contractual Minera Candelaria.
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El Romeral is one of the largest and best known of the magnetite-apatite deposits of the Chilean Iron Belt which lies within the Coastal Cordillera of Chile and is associated with the Atacama Fault Zone. It is 22 km north of the Chilean coastal town of La Serena. The Chilean Iron Belt is around 30 km wide and extends over a north-south interval of approximately 600 km. It embraces a large number of magnetite-apatite accumulations, some of which are mined for iron on a large scale. El Romeral has two main orebodies composed of magnetite microscopically intergrown with actinolite. The Main orebody is a steeply dipping, lenticular magnetite rich mass in a re-entrant between two lobes of the Romeral diorite pluton. The North orebody comprises conformable pods of magnetite within steeply foliated actinolite rich biotite schists. Both have a northerly trend within a north trending network of anastomosing strike-slip faults, and are sub-parallel and adjacent to the western lobe of the diorite pluton. Both orebodies have gradational boundaries with the enclosing rocks which also contain magnetite, actinolite, plagioclase, diopside, clinozoisite, titanite, chlorapatite, mariolitic scapolite, tourmaline, chlorite, pyrite, calcite, micas and clay. The host rocks include late Palaeozoic schists, phyllites and quartzite, cut by Cretaceous andesite porphyry and diorite. The diorite predates the magnetite ores which were accompanied by pervasive actinolitisation at temperatures in the range 475 to 550 degrees C. Magnetite emplacement was also accompanied by sinistral movement on the Romeral Fault, and by intra-mineral diorite-aplite dykes in fault movement induced gash fractures, and then by post mineral chloritisation. The Min orebody had dimensions of 850x250 m maximum and extended to a depth of 400 m, with 20 to 70% Fe grades. The North orebody outcropped over a distance of 300 m, was 20 to 50 m thick and extended down dip for 100 m. The South-east extension is some 500 m to the south of the Main Orebody and is 800 m long and 100 m wide, although its top is 175 m below the surface. Prior to mining the orebodies were estimated to comprise around 200 mt of ore. Reserves in 2000 were 44 mt @ 49% Fe. The mine capacity was 4 mt of ore per annum. The mine is controlled and operated by Compania Minera del Pacifico S.A..
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The Andacollo District is located in the early Cretaceous shoshonitic volcanic arc of the Coastal Belt of Chile, some 480 km to the north of Santiago and 56 km south-east of La Serena. It embraces a large mineralised system comprising a substantial disseminated and stockwork copper gold body that had a geological resource of 250 mt @ 0.62% Cu and 0.25 g/t Au, (operated by Compa÷Ôa Minera Carmen de Andacollo) partially ringed by an outer zone, mainly to the north and west, of stratabound, low sulphidation, manto style gold ores (mined by Compa÷Ôa Minera Dayton Limitada). Gold has been worked from gravels in the district since Inca times, with an estimated past production of over 100 t (3Moz). At the end of 1997, the declared gold resource totalled 2.9 Moz (90 t) of contained Au at grades of 1 to 1.2 g/t Au in more than four deposits. The copper-gold body is central to the district, with the mantos having been worked up to 5 km radially outwards. Both styles of mineralisation have up to several percent of associated hematite and/or magnetite, in places being present as fine dustings of hematite giving the host rocks a pink to red tinge.
The host to the mineralisation is a 2000-3000 m thick sequence of early Cretaceous volcanics and sediments, principally andesite and dacite flows, volcanic breccias and pyroclastic rocks with limestone lenses. These are intruded by 130 to 87 Ma diorite to granodiorite, and dislocated by a series of north-south and north-west trending faults that have influenced the geometry of the mineralised system.
The central Cu-Au body comprises an upper 30 m thick leached cap, a 40 m supergene blanket with 70 mt of 0.6 to 1.5% Cu ore, overlying a 100-200m thick hypogene zone that has an inner core of pyrite-chalcopyrite and specular-hematite/magnetite grading 0.3 to 0.8% Cu. The best grades are in zones of increased fracturing. This inner zone has a very low pyrite:chalcopyrite ratio. It grades out into a pyrite-chalcopyrite intermediate zone and a high pyrite:chalcopyrite halo. Some 80% of the ore is contained within early Cretaceous andesite and dacites, while 20% is hosted by small apophyses and stocks of Cretaceous porphyry. Alteration comprises a potassic core, surrounded by phyllic alteration and propylitisation.
The main host to gold mineralisation in the Andacollo mantos is a suite of shallowly dipping dacitic and andesitic flows, flow breccias and pyroclastic units. The location of the mantos is influenced by the north-west trending fault set dominant in the district. While transgressive in detail however, the mantos are grossly concordant, being stratabound, confined to specific andesite breccias and dacites within this sequence of alternating flows and breccias. They are characterised by potassic (adularia) alteration with associated pyrite and magnetite. The overlying andesite is rarely mineralised in manto form, but contains mineralisation in vertical structural breccias zones. Although the gold is associated with pyrite as disseminations and as veinlets, it is not held within the pyrite, making heap leaching an effective processing method.
While the Andacollo district lies within the same tectonic setting, the same host sequence and is the same general age as the iron-oxide copper-gold family deposits of the Coast Range, it has quite different alteration and trace element associations that are more characteristic of porphyry style mineralisation. As such the visit was very instructive, highlighting the contrast in ore styles and that both styles may occur in the same belt and setting.