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Mantos Blancos
Chile
Main commodities: Cu Ag


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The Mantos Blancos copper-silver deposit is located in the Coastal Range of northern Chile, some 45 km NE of the Pacific coastal city of Antofagasta in northern Chile (#Location: 23° 25' 52"S, 70° 3' 22"W).

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° SE and, cut upper Palaeozoic sediments and metasediments. Rock units within the Mantos Blancos ore deposit comprise a rhyolitic dome and its magmatic–hydrothermal breccias, intruded by dioritic and granodioritic stocks and sills. The dioritic and granodioritic stocks locally grade upwards into magmatic–hydrothermal breccias. These rocks are informally grouped as the Mantos Blancos Igneous Complex, and are all mineralised to variable degrees. Late mafic dykes crosscut all previously mentioned rock units and are essentially barren (Ramírez et al. 2006).

These rocks are 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 rhyolitic dome forms the central part of the deposit. It is partially preserved in the open-pit walls. Pervasive alteration has made the contacts between different internal flows difficult to differeentiate, although, near-horizontal and vertical flow laminations are evident, varying from between 1 to 4 cm in thickness. West of the pit, the felsic dome is intercalated with felsic tuffs and andesitic lava flows, and is intruded by dioritic and granodioritic sills. The dome is composed of a rhyolite porphyry with fragments of corroded 1 to 5 mm quartz and feldspar phenocrysts set in an intensively altered felsic groundmass (Ramírez et al. 2006).

The rhyolitic magmatic–hydrothermal breccia system comprises several sub-vertical monomictic and matrix-supported rhyolitic magmatic and hydrothermal breccia bodies within the felsic dome intrusion. These are composed of irregular bodies with a vertical extent of 100 to 250 m, and semi-ovoid to circular sections that are 50 to 100 m in diameter. The matrix comprises intensely altered rhyolitic rock flour with disseminated sulphide minerals The clasts are altered, with irregular shapes, poorly sorted, and vary in size between 1 cm and several metres. Late dioritic to granodioritic magmatic–hydrothermal breccias intrude the rhyolitic magmatic and hydrothermal breccias in the centre of the ore deposit (Ramírez et al. 2006).

A bimodal subvolcanic complex of porphyritic dioritic and granodioritic stocks and sills intrude the rhyolite dome. At least five gently dipping sills of both rock types occur in the mine, varying in thickness between 10 and 50 m. The granodiorite porphyry comprises 10 to 30% phenocrysts of hornblende, plagioclase, quartz and biotite, set in a groundmass of quartz, feldspars, biotite and hematite microlites. The diorite porphyry contains 5 to 10% pyroxene and minor amphibole phenocrysts in a groundmass of fine-grained pyroxene, plagioclase and magnetite. In both lithologies, the porphyritic texture grades to being aphanitic near the intrusive margins. The diorite porphyry has millimetre-size amygdules filled with quartz and quartz-sulphide. Mutual intrusive relationships between both granodioritic and dioritic rocks are common, and enclaves of one in the other have been frequently evident. Dioritic enclaves have convolute to flame-like contacts with the host granodiorite, whilst, the granodioritic enclaves exhibit sharp or brecciated contacts with the surrounding diorite (Ramírez et al. 2006). 40Ar/39Ar data on amphibole yielded ages of 142.18 ±1.01 Ma for the granodiorite, and 141.36 ±0.52 Ma for the diorite (Oliveros 2005).

Polymictic and matrix-supported pipe-like magmatic–hydrothermal breccias occur at the top of some dioritic and granodioritic stocks, hosted within the rhyolitic dome and spatially related to north-south trending faults. The central and largest breccia body is crosscut by at least three metre-sized sills, two of which are dioritic and the other granodioritic in composition. The breccias occurs as near vertical bodies, with a vertical extent of ~700 m, and diameters between 100 and 500 m. These bodies are regarded as not reached the upper levels of the ore deposit, as they have only been recognised at deeper levels of the pit. The upper parts of these breccia pipes have hydrothermal characteristics, as evidenced by the presence of a matrix mainly composed of hydrothermal gangue and ore minerals. They are composed of altered angular and subrounded clasts of the rhyolitic dome and granodioritic and dioritic porphyries, and are poorly sorted, ranging from 1 cm to 15 m ibn size. Downwards in the breccia bodies, magmatic features are progressively evident, with granodioritic fragments in an altered and mineralised dioritic matrix, as well as dioritic fragments in a granodioritic matrix (Ramírez et al. 2006).

Mafic dykes include partially altered, vertical, late-mineral, dioritic dykes that are preferentially oriented NNE, and subordinately north-south to NNW. They are 1 to 12 m thick and represent ~15% of the total rock volume in the deposit, with a porphyritic texture, composed of 10 to 25% phenocrysts of altered plagioclase, amphibole and minor pyroxene. The phenocrysts are set in a very fine-grained groundmass of feldspar and amphibole with minor biotite and magnetite (Ramírez et al. 2006). An
40Ar/39Ar date of amphibole from one of these dykes in the mine returned an age of 142.69 ±2.08 Ma of age (Oliveros 2005).

The Mantos Blanco mineralisation displays two superimposed hydrothermal events, namely:

i). an older phyllic alteration probably related to felsic magmatic-hydrothermal brecciation at ~155 Ma. The event is characterised by an assemblage chalcopyrite, bornite, pyrite, quartz and sericite. It occurs: a). as disseminations in the matrix of irregular and sub-vertical bodies of rhyolitic magmatic–hydrothermal breccias; b). as planar veinlets; c). disseminated within the rhyolitic dome and in fragments of the hydrothermal breccias; and d). as isolated crystals or rim assemblages within and on quartz phenocrysts of the rhyolitic dome. Chalcopyrite and bornite are the most abundant sulphides within the rhyolitic magmatic–hydrothermal breccias, which are surrounded by chalcopyrite and pyrite. The sulphide minerals occur as open space filling within fractures in the phyllic veinlets, and often exhibit weak alteration halos of sericite and quartz. Due to the intense and widespread superimposition of the main (second) hydrothermal event, it was not possible to establish the extent and intensity of this first event. It probably extended to all rocks of the rhyolitic dome (Ramírez et al. 2006).
40Ar/39Ar dating of sericite from this first event returned an age of 155.11 ±0.786 Ma (Oliveros 2005).
ii). a younger (142 to 141 Ma) potassic, propylitic and sodic alteration phase, coeval with dioritic and granodioritic stocks and sills, and dioritic dykes. The principal ore formation is genetically related to this second hydrothermal event, and comprises hydrothermal breccias, disseminations and stockwork-style mineralisation, associated with sodic alteration. The ore stage 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:

i). Na metasomatism manifested by albitisation of feldspars as well as albite veining and pore filling;
ii). incipient to intense addition (or locally removal) of Mg and Fe, reflected by chloritisation or bleaching;
iii). intense hematisation in the form of disseminated and stringer specularite and by intense pervasive red hematisation to many of the rocks within the deposit; and
iv). silicification, represented by quartz phenocrysts, microcrystalline aggregates in the groundmass and as occasional veinlets and amygdule fillings.

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 hypogene sulphide assemblages have distinctive vertical and lateral zoning, centred on magmatic and hydrothermal breccia bodies, which are interpreted to constitute feeders to the main mineralisation which is largely distributed in irregular, lenticular bodies roughly parallel to stratification. A barren pyrite root zone is overlain by pyrite-chalcopyrite, and followed upwards and laterally by chalcopyrite-digenite or chalcopyrite-bornite. A digenite-supergene chalcocite assemblage characterises the central portions of high-grade mineralisation in the breccia bodies. Silver is found in the lattices of both the oxide and sulphide minerals and correlated with the Cu grade.

Economic grade ore is found over an interval of 3 x 1.5 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 of sulphide ore @ 1.6% Cu and 17 g/t Ag.

Remaining mineral resources at 31 December, 2014 at a 0.2% Cu
Total cutoff (Anglo American Annual Report, 2015) were:
  Sulphide ore (flotation)
      Measured + indicated resource - 90.8 Mt @ 0.64% Cu at a 0.2% Cu
Total,
      Inferred resource - 22 Mt @ 0.56% Cu
Total,
  Oxide ore (Vat and Heap Leach)
      Measured + indicated resource - 18.4 Mt @ 0.43% Cu
Acid Sol.,
      Inferred resource - 16.3 Mt @ 0.29% Cu
Acid Sol.,
  Oxide ore (Dump Leach)
      Measured + indicated resource - 11.0 Mt @ 0.17% Cu
Acid Sol.,
      Inferred resource - 70.7 Mt @ 0.18% Cu
Acid Sol..

Remaining Ore Reserves and Mineral Resources at 31 December, 2022 (Capstone Copper Reserve and Resources online, viewed March 2024) were:
  Sulphide + Mixed ore (Flotation)
      Proved + Probable Reserve - 115.682 Mt @ 0.66% Cu, 5 g/t Ag,
      Measured + indicated Resource - 206.373 Mt @ 0.65% Cu, 5 g/t Ag, (Inclusive of Ore Reserves)
      Inferred Resource - 22.490 Mt @ 0.47% Cu, 3 g/t Ag,
  Oxide + Mixed ore (Dump + Heap Leach)
      Proved + Probable Reserve - 4.646 Mt @ 0.29% Cu
Acid Sol.,
      Measured + Indicated Resource - 119.981 Mt @ 0.21% Cu
Acid Sol., (Inclusive of Ore Reserves),
      Inferred Resource - 23.565 Mt @ 0.19% Cu
Acid Sol..

The mine is operated by Minera Mantos Blancos S.A., which was initially an Anglo American Group company, but was sold to be part of Mantos Copper S.A. in 2015. In 2021, Mantos Copper merged with Capstone Mining to become Capstone Copper which took control of Mantos Blancos.

The most recent source geological information used to prepare this decription was dated: 2001.    
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.


Mantos Blancos

  References & Additional Information
   Selected References:
Chen H,  2010 - Mesozoic IOCG Mineralisation in the Central Andes: an Updated Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 259-272
Espinoza S, Veliz H, Esquival J, Arias J, Moraga A  1996 - The cupriferous province of the Coastal Range, Northern Chile: in Camus E, Sillitoe R H, Peterson R (Eds), 1996 Andean Copper Deposits: New Discoveries, Mineralisation, Styles and Metallogeny Soc. Econ. Geol.   Spec Pub no. 5 pp 19-32
Haynes D W,  2000 - Iron Oxide Copper (-Gold) Deposits: Their Position in the Ore Deposit Spectrum and Modes of Origin: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.1 pp. 71-90
Maksaev V and Zentilli M,  2002 - Chilean Strata-bound Cu- (Ag) Deposits: An Overview: in Porter T M (Ed.), 2002 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.2 pp. 185-205
Oliveros V, Feraud G, Aguirre L, Ramirez L, Fornari M, Palacios C and Parada M,  2008 - Detailed 40Ar/39Ar dating of geologic events associated with the Mantos Blancos copper deposit, northern Chile: in    Mineralium Deposita   v43 pp 281-293
Palacios, C., Ramirez, L.E., Townley, B., Solari, M. and Guerra, N.,   2007 - The role of the Antofagasta-Calama Lineament in ore deposit deformation in the Andes of northern Chile : in    Mineralium Deposita   v.42, pp. 301-308.
Ramirez Rodriguez R D  1995 - Geology of Mantos Blancos Mine: in Green S M, Struhsacker E, (Eds), 1995 Geology & Ore Deposits of the American Cordillera, Field Trip Guidebook compendium, 1995 Geol. Soc., Nevada    pp 466-481
Ramirez, L.E., Palacios, C., Townley, B., Parada, M.A., Sial, A.N., Fernandez-Turiel, J.L., Gimeno, D., Garcia-Valles, M. and Lehmann, B.,  2006 - The Mantos Blancos copper deposit: an upper Jurassic breccia-style hydrothermal system in the Coastal Range of Northern Chile: in    Mineralium Deposita   v.41, pp. 246-258.


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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