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Carmen de Andacollo |
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Chile |
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Cu Au
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Super Porphyry Cu and Au
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The Carmen de Andacollo porphyry copper gold deposit lies within the Andacollo district, which is hosted by 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 (#Location: 30° 15' 2"S, 71° 5' 40"W).
It embraces a large mineralised system comprising a substantial disseminated and stockwork copper gold body separated by a major fault from a zone of stratabound, low sulphidation, manto style gold ores to the NW and west to SE. Gold has been worked from gravels in the district since Inca times, with an estimated past production of over 100 t (3 Moz). 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 structural setting of the deposit area consists of the following main faults (after Araya et al., 2012), by relative age from the oldest to the youngest of, syn-mineralisation structures, comprising the:
i). Carmen Fault, a NW trending sub-vertical fault that roughly coincides with the southern limit of higher-grade Cu mineralisation. It is considered the oldest of the faults although there is little physical evidence of its location, other than changes in Cu and Hg grade, which is diffuse and not a hard boundary.
ii). Hermosa-Twila-Andacollo Set (plus a subsidiary NW trending faults). The Hermosa and Andacollo faults strike at 290 to 315° and dip 70°W. They have undergone multiple movements, and are considered to form part of a duplex with the Twila Fault. There has been significant offset of marker units across the Hermosa Fault in its central sections. There are a large number of NW trending faults that correspond to the NW oriented porphyries. These structures are considered to control the conduit for the main Cu mineralisation and to be coeval with the main mineralising event. They do not significantly offset other faults.
These are followed by post-mineralisation structures, which include:
iii). a set of numerous, small scale, north-south faults, considered to be synchronous with the north-south oriented porphyries. The consist of a series of extensional faults that displace both the early intrusions and the ore deposits. There may have been re-activation along the Hermosa and Andacollo faults during this event, which provided channels for ascending Hg solutions, although there is strong evidence of subsequent reactivation of north-south systems and the Andacollo-Hermosa faults, causing significant normal displacement, in some places of up to 200 m, post-secondary enrichment.
iv). a major NE set of faults that cut all previous strutures and were a site of supergene Hg deposition. Three major faults with this orientation offset the lithologies of the mine area by 5 to 10 m. The southern two of these faults dip to the NW and the northern dips to the SE. They are considered the youngest of the major faults and crosscut earlier events including the Cu mineralisation. The Hg mineralisation shows continuity relative to the NE and north-south fault, so it is possible that the Hg mineralising fluids were plumbed through the system via both fault sets.
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. The sequence has been subdivided into (from Araya et al., 2012):
• Lower Volcanic Unit, or Arqueros Formation, a Barremian age volcanic sequence, mainly composed of andesitic flows and auto-breccias with smaller interspersed tuffs, volcanoclastic rocks. Other lithologies include coarse porphyritic andesite, microporphyritic andesite, medium porphyritic andesite, fine porphyritic andesite, andesitic breccia, aphanitic andesite and limestone.
• Upper Volcanic Unit, or Quebrada Marquesa Formation, which is Aptian to Albian in age, and is mostly pyroclastic rocks that are similar to the Lower Volcanic Unit, other than they have sections with a greater permeability, which strongly favoured the invasion of hypogene mineralising fluids. Lithologies defined include crystalline tuff, vitreous tuff, trachytic lava, lithic tuff and volcaniclastic rocks. For details of the stratigraphic subdivision of this formation see the separate Andacollo Gold record.
• Intrusive Rocks, that cut all previous stratified units, the earliest of which are 130 to 87 Ma tonalite, diorite and granodiorite, with later stage stocks and dykes of rhyolitic, dacitic and andesitic composition. The earliest of these later stage intrusions, the medium and coarse dacitic porphyries, are predominantly oriented NW, and are directly related to alteration and mineralisation that gave rise to the ore. The second group is represented by rhyolitic porphyry (known as the 'Culebron Porphyry') and andesitic porphyry, which are both largely oriented north-south and are considered to be post-mineralisation, and as such are usually considered to be waste. Intrusions from the porphyry deposit have returned ages of 98±2 and 104±3 Ma (K-Ar; SERNAGEOMIN, reported in Reyes, 1991).
• Breccia Unit composed of a number of variants, which include the Hermosa Breccia, Hydrothermal Breccia, Magmatic Breccia and Fault Breccias.
• Superficial Gravels mainly recent sediments of natural origin, composed of semi-consolidated polymictic fragments which overlay all of the units described above.
Both the porphyry Cu-Au and epithermal manto Au mineralisation is located in the downthrown block west of the north-south Andacollo fault and may represent the shallow, apical parts of the more deeply eroded felsic intrusions that crop out east of the fault. Most of the porphyry copper mineralisation is concentrated in a single central body and in contiguous satellites to the north (La Coipa and Perlita) and South (Hermosa). The mineralisation covers an area of 1500 x 1300 m and is generally circular in plan, truncated to the east by the Andacollo fault (Reyes, 1991).
The main central Cu-Au body comprises an upper 30 m thick leached cap, averaging 0.07% Cu, characterised by goethite, jarosite, and hematite. Oxide copper minerals are only present locally defining a and Oxide Zone' which cannot be consistently differentiated visually (Araya et al., 2012.
This leached capping overlies an ~40 m thick supergene blanket that occupies and area of 1.5 km2 with ~70 Mt of 0.6 to 1.5% Cu ore. It comprises an upper zone of strong supergene enrichment, defined by the absence of chalcopyrite and dominant chalcocite. This is underlain by a zone of weak secondary enrichment, characterised by the presence of both chalcocite and chalcopyrite (Araya et al., 2012.
The underlying hypogene zone is 100 to 200 m thick, and has an inner core of pyrite-chalcopyrite and specular-hematite/magnetite with subordinate molybdenite, and minor gold, which carries 0.3 to 0.8% Cu. The best grades of both supergene and hypogene mineralisation 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 (Araya et al., 2012.
The upper hypogene zone is characterised by primary minerals, i.e., chalcopyrite, pyrite, bornite and tennantite, but no secondary Cu, with cavities. This passes down into an interval where rock fractures and cavities may be partially filled with gypsum, then to a zone where the rock fractures and
cavities are completely filled with gypsum ±anhydrite. The top of this zone marks the end of supergene activity This is underlain, in turn, by the main sulphide body where fractures and veinlets are filled with calcite (Araya et al., 2012).
Three principal, progressively shallowing, hypogene alteration and mineralisation domains have been established (after Araya et al., 2012):
• Late magmatic domain which includes potassic alteration, principally secondary biotite or K feldspar, propylitic and albitic assemblages. Within this domain, chalcopyrite-bornite (traces), magnetite-chalcopyrite and chalcopyrite-molybdenite assemblages accompany the potassic alteration where secondary biotite dominating, whilst where K feldspar is the main potassic mineral, chalcopyrite-bornite is the predominant sulphide assemblage. Where deeper core albite alteration occurs, it is associated with an assemblage of chalcopyrite-magnetite-specularite-(bornite)-pyrite(?). Pyrite, and traces of pyrite-chalcopyrite, specularite-chalcopyrite and magnetite-chalcopyrite are found within peripheral propylitic zones.
• Phyllic domain subdivided into the main and late phyllic events, accompanied by sulphide assemblages of i). pyrite and pyrite-chalcopyrite, and ii). pyrite-chalcopyrite-tennantite-enargite-tetrahedrite respectively.
• Argillic domain which includes late hydrothermal intermediate argillic, accompanied by pyrite-cinnabar, pyrite-tennantite-enargite, cinnabar and pyrite-tennantite-chalcopyrite, and supergene argillic alteration with chalcocite, chalcocite-(covellite) and supergene cinnabar.
The main host to gold mineralisation in the Andacollo epithermal veins and mantos to the west is the suite of shallowly dipping dacitic and andesitic flows, flow breccias and pyroclastic units of the Lower and Upper Volcanic units described above. The epithermal mineralisation is 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. See the separate Andacollo Gold record for details.
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 deposits of the Chilean Coast Range, it has quite different alteration and trace element associations that are more characteristic of porphyry style mineralisation.
In ca 2000, the Cu-Au porphyry mineralisation was exploited by Compañía Minera Carmen de Andacollo and had a geological resource of 250 Mt @ 0.62% Cu, 0.25 g/t Au. At the same time the epithermal gold mineralisation was mined by Compañía Minera Dayton Limitada.
The mine has been operated by Compañía Minera Teck Carmen de Andacollo since 2007. NI 43-101 compliant ore reserves and mineral resources at the end of 2015 were (Teck website, visited June 2016):
Ore reserves
Mill ore
Proved reserve - 140.4 Mt @ 0.36% Cu, 0.12 g/t Au + Probable reserve - 276.6 Mt @ 0.33% Cu, 0.12 g/t Au;
TOTAL reserve - 417.0 Mt @ 0.34% Cu, 0.12 g/t Au.
Heap leach ore
Proved reserve - 300 Mt @ 0.37% Cu Soluble + Probable reserve - 1000 Mt @ 0.27% Cu Soluble;
TOTAL reserve - 1300 Mt @ 0.29% Cu Soluble.
Mineral resources which are exclusive of reserves
Mill ore
Measured resources - 12.2 Mt @ 0.30% Cu, 0.08 g/t Au + Indicated resource - 153.6 Mt @ 0.28% Cu, 0.09 g/t Au +
Inferred resource - 47.5 Mt @ 0.28% Cu. 0.09 g/t Au;
Heap and dump leach ore
Measured resources - 8.2 Mt @ 0.34% Cu Soluble + Indicated resource - 30.6 Mt @ 0.15% Cu Soluble +
Inferred resource - 40.0 Mt @ 0.37% Cu Soluble.
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 28/6/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Carmen Andacollo
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Selected References:
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Araya, V., Maldonado, A. and Astudillo, J., 2012 - Geology of Carmen de Andacollo Deposit: in Congreso Geológico Chileno, 13, Antofagasta, Chile, 05-09 Agosto, Articulos de congresos, pp. 25-27.
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Lincoln J B, Tellez C 1995 - The Andacollo gold project, IV Region, Chile: in Green S M, Struhsacker E, (Eds), 1995 Geology & Ore Deposits of the American Cordillera, Field Trip Guidebook compendium Geol. Soc., Nevada pp 492-495
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Oyarzun R, Ortega L, Sierra J, Lunar R 1996 - The manto-type gold deposits of Andacollo (Chile) revisited: a model based on fluid inclusion and geologic evidence: in Econ. Geol. v91 pp 1298-1309
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Reyes M 1991 - The Andacollo strata-bound gold deposit, Chile, and its position in a porphyry copper-gold system: in Econ. Geol. v86 pp 1301-1316
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Richards, J.P., Lopez, G.P., Zhu, J-.J., Creaser, R.A., Locock, A.J. and Mumin, A.H., 2017 - Contrasting Tectonic Settings and Sulfur Contents of Magmas Associated with Cretaceous Porphyry Cu ± Mo ± Au and Intrusion-Related Iron Oxide Cu-Au Deposits in Northern Chile: in Econ. Geol. v.112, pp. 295-318.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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