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Potrerillos District - Cobre, El Hueso |
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Chile |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Potrerillos Mining District includes the Potrerillos (Cobre) porphyry copper and El Hueso high sulphidation gold-silver deposits which are 3 km apart. The district is located some 30 km south-east of the El Salvador mine, 800 km north of Santiago and 120 km east of the Pacific coast. It is also immediately north of the Maricunga gold district.
Pre-Jurassic basement units in the Potrerillos district include Carboniferous to Triassic granite, diorite, tonalite, and quartz syenite, metavolcanic rocks, and minor quartz-muscovite schist. These basement lithologies are overlain by the Jurassic, Tarapaca back-arc basin sequence of more than 400 m of calcareous mudstone and limestone of the Montandon Formation, grading upward into the 290 m of mixed carbonate and clastic sedimentary rocks with interbedded andesite and basaltic andesite flows of the Asientos Formation, which are in turn is conformably overlain by siltstone and limestone with inbterstratified basaltic andesite flows of the Late jurassic to Early Cretaceous Pedemales Formation. These sequences were overlain by Palaeocene bimodal andesitic and rhyolitic volcanics which are up to 350 m thick.
Eocene to Oligocene transpression, linked to deformation along the major regional, north-south trending Domeyko fault system, produced a belt of asymmetric folding, and thrust to reverse faulting in the Potrerillos district, including the Potrerillos Mine fault, an east-southeast-vergent thrust fault with approximately 1.5 km of displacement. The Potrerillos porphyry deposit and the El Hueso Au deposit are both located in the upper plate of the Potrerillos Mine fault. Between 40.8 and 32.6 Ma, during the late Eocene to early Oligocene, a series of granodioritic, quartz monzodioritic, and tonalitic plagioclase-porphyritic intrusions were emplaced in the Potrerillos district.
The north-south elongated, 36 and 35 Ma, 1.6 x 0.5 km monzodiorite to granodiorite Cobre porphyry, which hosts the Potrerillos porphyry Cu-Mo-(Au) deposit, was emplaced syntectonically with movement along the flat lying Potrerillos Mine thrust fault. The Potrerillos deposit is largely confined to the Cobre porphyry, although skarns and related contact metamorphism were formed in the surrounding Montandon, Asientos, and Pedernales Formations, with manto-like bodies of Cu mineralisation in limestone horizons of the Asientos Formation. Distal alteration extending outward from the Cobre porphyry include disseminated pyrite and secondary actinolite in sandy lithologies and pyroxene-garnet-quartz-anhydrite-calcite skarn and hornfels grading outward to pyrite-sphalerite in thermally metamorphosed marls and impure carbonate rocks. The latter extend up to 1500 m from the Cobre porphyry, while pure limestone horizons are silicified within 900 m of the contact.
The first stage of hypogene copper mineralisation occured as disseminated and vein hosted chalcopyrite and bornite with subordinate pyrite and magnetite, accompanied by potassic alteration. Late stage enargite-pyrite was related to a quartz-sericite alteration assemblage interpreted to represent the roots of an advanced argillic-altered lithocap that crops out at the nearby El Hueso gold deposit. NNW oriented, late, postmineral pebble dykes follow minor faults. The first stage bornite-chalcopyrite-magnetite hypogene mineralisation of the Cobre porphyry typically contained from 0.1 to 0.3 g/t Au, and locally up to 6 g/t, which extended outward into the calcareous wall rocks.
Published production and reserve/resource figures include:
87 Mt @ 1.65% Cu (Sulphide reserves, Anaconda Copper Co., 1916);
56 Mt @ 1.63% Cu (Oxide reserves, Anaconda Copper Co., 1916);
210 Mt @ at an average grade of 1.1% Cu, 0.25 g/t Au (Production from large scale block caving at Cobre mine, 1926 to 1958);
278 Mt @ 1.1 to 1.2% Cu and 0.3 to 1.25 g/t Au (Resources, Cobre mine, Colley et al., 1989);
215 Mt @0.74% Cu (Reserves, Camus, 2003);
100 Mt @0.76% Cu (Leachable reserves, Camus, 2003);
The El Hueso high sulphidation gold-silver deposit consists of replacement-style mineralisation focused in steep, east-west trending faults and adjacent porous, gently dipping bioclastic, limestone beds, particularly within the Asientos Formation. The faulting which controls the mineralisation, is interpreted to be pre-minerall, being related to early Tertiary compression. The deposit is truncated at depth by the Potrerillos Mine fault, which would appear to be post mineralisation. The dominant ateration is silicification developed in porous lithologies accompanied by spotted or banded quartz-illite (40.25 ±0.05 Ma) assemblages in less porous rocks. A post-mineralisation advanced argillic alteration assemblage of alunite (dated at 36.23 ±0.07 Ma), pyrophyllite, zunyite, dickite, woodhousite, rutile, diaspore and quartz occurs in crosscutting veins related to steep faults. This younger alteration is similar in age to the Cobre porphyry Cu-Mo mineralisation, but younger than the alteration associated with the gold ores.
The El Hueso had initial mineable reserves of 16 Mt @ 1.68 g/t Au, 7.8 g/t Ag.
For more detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1997.
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.
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Selected References:
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Marsh T M, Einaudi M T, McWilliams M 1997 - 40Ar/39Ar geochronology of Cu-Au and Au-Ag mineralization in the Potrerillos district, Chile: in Econ. Geol. v92 pp 784-806
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Morales-Leal, J.E., Campos, E., Kouzmanov, K. and Riquelme, R., 2023 - Alunite supergroup minerals from advanced argillic alteration assemblage in the southern Atacama Desert as indicators of paleo-hydrothermal and supergene environments: in Mineralium Deposita v.58, pp. 593-615. doi.org/10.1007/s00126-022-01149-5.
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Rivera S L, Vila T and Osario J, 2004 - Geologic characterisitcs and exploration significance of gold-rich porphyry copper deposits in the El Salvador region, northern Chile: in Sillitoe R H, Perello J and Vidal C E, 2004 Andean Metallogeny: New Discoveries, Concepts and Updates, Society of Economic Geologists, Denver, SEG Special Publication 11 pp 97-111
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Thompson J F H, Gale V G, Tosdal R M and Wright W A, 2004 - Characterisitics and formation of the Jeronimo carbonate replacement gold deposit, Potrerillos District, Chile: in Sillitoe R H, Perello J and Vidal C E, 2004 Andean Metallogeny: New Discoveries, Concepts and Updates, Society of Economic Geologists, Denver, SEG Special Publication 11 pp 75-95
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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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