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El Salvador - Indio Muerto district |
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Chile |
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Cu Mo
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Super Porphyry Cu and Au
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The El Salvador supergene enriched and hypogene porphyry deposits and the related Damiana exotic copper accumulations are located in the Indio Muerto district of Northern Chile, some 140 km east of the Pacific coast port of Chañaral, 1100 km north of Santiago and 200 km south of the Escondida deposit. They contain in excess of 12 Mt of copper metal.
The hypogene mineralisation of the main El Salvador deposit was formed during the younger Eocene (42 to 41 Ma) episode of two distinct superposed magmatic events evident in the Indio Muerto district, the older being of Paleocene (63-58 Ma) age. A 200x50 km Tertiary volcanic field surrounding the deposit (and the Potrerillos porphyry copper 25 km to the north) embraces a variety of high K Paleocene volcano-plutonic activity, including collapse calderas associated with explosive rhyolitic magmatism, a post collapse rhyolite dome field, and andesitic-trachyandesitic strato-volcanoes. The structures which controlled this volcanism also influenced the emplacement of porphyry related Eocene intrusives some 14 Ma after the cessation of Paleocene activity. The porphyries that formed copper deposits in the Indio Muerto district include both an early 44 Ma phase of multiple rhyolitic sub-volcanic intrusions as well as the main Cu-Mo porphyry related Eocene granodioritic-dacitic stocks which were mineralised between 42 and 41 Ma. Together these form a 6 km long NNE trend.
The intruded rocks are a mainly shallowly south dipping 3 km thick section of Cretaceous andesitic volcano-sediments in the lower levels, unconformably overlain by Tertiary andesites and rhyolites, including abundant ignimbrites. These are cut by a generally vertically to north plunging composite granodioritic porphyry plug with a diameter of around 2 km representing a multiple intrusive and hydrothermal event responsible for the mineralisation at El Salvador.
Four main porphyries make up this granodioritic complex, as follows:
i). X Porphyry - the earliest, is fine grained, weakly porphyritic and equigranular, and predated the bulk of the mineralisation.
ii). K Porphyry - is a feldspar porphyry which occupies the south-eastern lobe of the main mass at El Salvador/Turquoise Gulch and accompanied the main phase of mineralisation and alteration.
iii). L Porphyry - is a feldspar porphyry with an aplitic matrix which post-dates K Porphyry and is the largest mass in the complex, being about one km across. It is relatively barren and un-altered and has obliterated some of the mineralisation associated with K Porphyry.
iv). A Porphyry - is the name applied to a group of minor, discontinuous dykes and sills cutting all of the preceding rocks, ranging from a few centimetres to 10 m in thickness but not generally being traceable for more than a few metres to tens of metres, to a maximum of 100 m.
v). Igneous breccias and Latite Dykes - a series of igneous breccias with heterolithic clasts are found within the complex, the largest with a diameter of up to 300 m. These are followed by post mineralisation latite dykes.
Early mineralisation, which was mostly emplaced with the K Porphyry (before the last major feldspar porphyry, the L Porphyry) and accounted for 75% of the 5 Mt of copper in the main Turquoise Gulch deposit, is characterised by distinctive quartz veins - A veins - with a disseminated K-silicate assemblages of alkali feldspar-biotite-anhydrite-chalcopyrite-bornite or chalcopyrite-pyrite. The bornite content of the A veins decreases outwards from the central zone of K-silicate alteration as chalcopyrite increases until pyrite appears and increases at the expense of chalcopyrite. Pyrite increases then declines in an outer epidote-chlorite-calcite propylitic zone where minor chalcopyrite-magnetite veining is found, giving way further out to specular hematite.
The A Veins which contain the bulk of the mineralisation are characterised by an assemblage of 50 to 95% fine equigranular quartz with K-feldspar, anhydrite, rare biotite and sulphides disseminated within the veins which have a thin halo of K-feldspar alteration. They are generally, discontinuous, 1 to 25 mm thick and carry chalcopyrite and bornite proportional to their location within the mineral zonation described above, and traces of molybdenite. Repeated A veining started prior to the introduction of X Porphyry and continued until after the emplacement of L Porphyry.
The main bornite-chalcopyrite core of the deposit contains 0.5 to 2% total sulphide, with copper grades varying from 0.3 to 1%. In the surrounding pyrite-chalcopyrite zone the sulphides range from 0.75 to 2.5%, but again vary from 0.3 to 1% Cu, while in the outer pyrite fringe the total sulphide varies considerably, ranging from 0.5 to 6%, with copper generally averaging less than 0.2%.
A transitional phase of quartz veining post-dated the emplacement of L Porphyry and removed part of the early stage alteration/mineralisation. These are the B veins which lack K feldspar, are generally flat and are characterised by an assemblage of coarse, elongated, cockscomb textured quartz with anhydrite and sulphide, often in bands within the vein which lack defined halos. These veins also carry tourmaline, which increases towards the surface. They always cut A veins, are usually regular and continuous and 5 to 50 mm thick. They carry molybdenite and chalcopyrite which tend to be coarse grained and banded.
Late mineralisation is associated with the D veins and is characterised by abundant pyrite and K-feldspar destructive alteration which is more fracture controlled and with more disseminated mineralisation. These late sulphide D veins cut all earlier veins and rock types except the latite. They are 1 to 75 mm thick, irregular and 'lacing' and contain pyrite with lesser, but upward increasing quantities of bornite, chalcopyrite, enargite, tennantite, sphalerite or galena, accompanied by an anhydrite gangue, but only minor quartz and are surrounded by feldspar destructive sericite and sericite-chlorite alteration.
At depth below the primary mineralisation the sulphide system persists but with the total sulphide content diminishing, molybdenite more common in quartz veins and magnetite becoming an important component, accompanied by a corresponding reduction in K-silicate alteration and an increase in the abundance of albite. Cu values also decrease to form a barren core below the chalcopyrite-bornite zone in the upper levels.
Supergene enrichment was responsible for the original commercial orebody at Turquoise Gulch comprising an approximate pre-mining (1957) reserve of 300 Mt @ 1.6% Cu as secondary Cu sulphides replacing chalcopyrite and bornite but only coating pyrite. Supergene alteration is characterised by kaolinite and alunite. This reserve was present in the form of a chalcocite blanket approximately 1.5 km in diameter and 200 m thick. Surface expressions of alteration and mineralisation were observable throughout an elongated zone extending over some 5 by 10 km.
El Salvador commenced as an underground mine operated by the Anaconda Company in 1959, with an associated open pit, smelter and electrolytic refinery. The mine is now part of the Codelco Chile Salvador Division. In 1995 a new 25 000 tonne per year capacity hydrometallurgical line also started up. Currently both sulphides and oxides are being extracted from underground and open pit mines, each with its own plant for copper recovery to produce electro-refined and electro-won cathodes, refinery slimes and molybdenum concentrate.
The total resource of the Indio Muerto district is estimated to be 866 Mt @ 1.41% Cu, 0.01% Mo, 0.115 g/t Au.
Production in 1999 totalled 91 700 tonnes of fine copper from 11.76 Mt of ore (both oxide and sulphide) averaging 0.7% Cu.
For more detail consult the reference(s) listed below which were the principal source of the information on which this summary was based.
The most recent source geological information used to prepare this decription was dated: 2002.
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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Gustafson L B and Hunt J P, 1975 - The porphyry copper deposit at El Salvador, Chile: in Econ. Geol. v.70 pp. 857-912
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Gustafson L B, Orquera W, McWilliams M, Castro M, Olivares O, Rojas G, Maluenda J, Mendez M 2001 - Multiple centers of mineralization in the Indio Muerto District, El Salvador, Chile: in Econ. Geol. v96 pp 325-350
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Gustafson, L.B. and Quiroga, G.J., 1995 - Patterns of mineralization and alteration below the Porphyry Copper orebody at El Salvador, Chile: in Econ. Geol. v.90, pp. 2-16.
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Hedenquist, J.W., Watanabe, Y. and Arribas, A., 2020 - Hypogene alunite from the El Salvador District, Chile, indicates potential for a blind porphyry copper center: in Econ. Geol. v.115, pp. 231-239.
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Lee, R.G., Dilles, J.H., Tosdal, R.M., Wooden, J.L. and Mazdab, F.K., 2017 - Magmatic Evolution of Granodiorite Intrusions at the El Salvador Porphyry Copper Deposit, Chile, Based on Trace Element Composition and U/Pb Age of Zircons: in Econ. Geol. v.112, pp. 245-273.
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Mote T I, Becker T A, Renne P, Brimhall G H 2001 - Chronology of exotic mineralization at El Salvador, Chile, by 40Ar/39Ar dating of Copper Wad and supergene Alunite: in Econ. Geol. v96 pp 351-366
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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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Sillitoe R H, McKee E H 1996 - Age of supergene oxidation and enrichment in the Chilean Porphyry Copper Province: in Econ. Geol. v91 pp 164-179
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Watanabe Y, Hedenquist J W 2001 - Mineralogic and stable isotope zonation at the surface over the El Salvador Porphyry Copper Deposit, Chile: in Econ. Geol. v96 pp 1775-1797
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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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