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La Coipa

Chile

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The La Coipa cluster of high sulphidation gold deposits lies within the Maricunga Belt of northern Chile, located some 140 km to the north-east of the Chilean coastal town of Copiapó, and ~1000 km north of Santiago. The mine lies in the Domeyko Cordillera between 3800 and 4,400 masl (#Location: 26° 48' 34"S, 69° 16' 06"W).

The Maricunga Belt of gold deposits is separated from the similarly aligned El Indio Belt to the south by a barren gap without significant known gold deposits. It comprises a NNE trending chain of andesitic to dacitic volcanoes that are part of a late Oligocene-Miocene continental margin plutonic volcanic arc. This arc embraces a series of predominantly epithermal acid-sulphate gold-silver deposits (including La Coipa) and porphyry gold orebodies (including Refugio, Aldeberan, Marte & Lobo). To the north, another "barren gap" with few significant gold deposits, separates the Maricunga Belt from El Penon, the next significant gold mine in the Andean chain.

Regional Setting

The regional sequence in the La Coipa district is as follows (after Belanger, 2003), from the oldest outcropping rocks:
Chinches Formation, comprising Devonian to Carboniferous sandstones, shales and mudstones;
Pantanoso Formation or El Leoncito sequence - Permo-Triassic dacite to andesite flows;
The succeeding Mesozoic cover is largely composed of sedimentary rocks, including the;
La Ternera Formation, a late Triassic sequence that includes arkosic sandstones, but is predominantly carbonaceous black shales (Brüggen, 1950), which are a significant host to mineralisation at La Coipa where they form the basement;
Lautaro Formation, Jurassic marine limestone and volcanic rocks;
Quebrada Monardes Formation, late Jurassic to early Cretaceous sandstone;
Quebrada Seca Formation, late Cretaceous volcaniclastic rocks.
The Mesozoic cover is capped by andesitic breccias and agglomerates of late Cretaceous to early Tertiary age. These include the Tertiary West Volcanic Ridge, a NNE-SSW-trending volcanic cordillera. It comprises a stratigraphically lower section of 25 to 20 Ma (late Oligocene to early Miocene) rhyolitic to dacitic tuffs, breccias and domes, and an upper suite of 20 to 14 Ma (early to mid-Miocene) dacite to andesite flows, feeders and related strato-volcanoes of the Cerros Bravos volcanic complex. The Au-Ag orebodies at La Coipa are partially hosted by the lower section of this complex.
Late Miocene gravels and interbedded ignimbrites of the San Andŕes Formation represent the top of the stratigraphic column in the district.

The oldest intrusive rocks are late Palaeozoic granitoids that occur mainly in the northern and southern parts of the district, while north-south trending Paleocene to Eocene tonalite porphyries outcrop as a discontinuous belt in the central part. Small dacite to andesite domes, dykes and sills related to the Tertiary volcanic rocks are locally exposed (Belanger, 2003).

The La Coipa area is characterised by NNE-SSW trending horsts and grabens, produced by reverse faulting, but are concealed by early to mid- Miocene volcanic rocks. A subsequent set of NW-SE trending normal faults and a secondary set of NE-SW striking faults were superimposed on the earlier structural framework. Intersections of the reverse and normal fault systems controlled the location of Tertiary volcanic rocks at La Coipa (Belanger, 2003).

Deposit Geology

The principal lithostratigraphic units hosting mineralisation at La Coipa are the Triassic La Ternera Formation and the overlying Tertiary volcanic rocks. Although the complete Mesozoic to Cenozoic succession is represented in neighbouring areas, at La Coipa, the Tertiary volcanic sequence lies directly on sedimentary rocks of the La Ternera Formation (Belanger, 2003).
  The Triassic La Ternera Formation is mostly composed of black shale interbedded with feldspathic sandstone, including thin (~2 mm) carbonaceous and bituminous layers. To the east of Ladera-Farellón, red sandstone of unknown age crop out, characterised by gypsum layers and fracture fillings (Belanger, 2003).
  The Tertiary volcanic sequence consists of pyroclastic rocks, volcanogenic sediments, tuffs and locally latite breccias, dacite flows and quartz latites intruded by dykes, sills and domes of similar composition. This volcanic event culminated in the accumulation of a felsic tuff sequence. Locally, particularly in the Ladera area, a shale fragment breccia occurs at the contact between the Triassic and Tertiary sequences, believed to represent a talus breccia formed during horst and graben development. This horizon is a favourable host for high-grade ore, probably because of its permeability (Belanger, 2003).
  Two types of intrusive rocks are recognised at La Coipa, namely: i). a few small 21 Ma latite dykes and sills in the west, and ii). 22-23 Ma dacite plugs in the central and surrounding areas of the mineralised zones. Dacite is the dominant intrusive rock in the La Coipa district, occurring as plugs that outcrop in NW-SE trending belts east of the main mineralised zones at Ladera- Farellón and Coipa Norte, as well as in a similar-trending belt east of Quebrada Los Terneros. Zones of weakness following the NNE-SSW trending reverse faults and associated northwest-southeast-trending normal faults controlled the emplacement of these intrusive bodies. These dacite plugs have no observable intrusive contacts with the volcanic sequence that hosts mineralisation, and their ages are similar and their spatial relationships close. It is therefore probable they are closely related to the alteration-mineralisation episode at La Coipa (Belanger, 2003). A dacite dome southeast of Ladera-Farellón hosts copper- and silver-bearing veins that have been mined sporadically since the end of the 19th century (Belanger, 2003).
  The structural control on mineralisation is interpreted to be defined by north-south strike-slip faulting, with dextral movement along these structures generating a subordinate set characterised by NE oriented dilational jogs. These faults and jogs provided efficient conduits for magmatic fluids to ascend to shallower crustal levels from a likely porphyry source at depth. At least two plumbing systems appear to have been developed. Post-mineralisation faulting along sinistral NW structures produced step-like displacements that led to the current configuration of the main orebodies in the La Coipa mine area (Belanger, 2003).

Alteration

The principal alteration types observed at La Coipa, are a s follows, after Belanger (2003):
Silicification occurs in brecciated zones adjacent to structures that carry high silver and gold contents, although most often, it occurs as lenses within pervasively replaced tuffs or tuffaceous breccias that are not always associated with high precious metal values. Silicified, breccia zones are typically associated with a very high content of limonite, including jarosite. It is the dominant alteration type in the upper parts of Coipa Norte and to a lesser degree at Ladera.
Advanced argillic (alunite-kaolinite + dickite-quartz) alteration - This assemblage is mainly associated with high gold grades, predominantly in the eastern part of Coipa Norte and Farellón. Alunite occurs as stockwork veinlets, generally associated with jarosite and locally scorodite. At Farellón, gold is closely associated with alunite, with the highest grades restricted to the alunite stockwork, whilst the surrounding rocks only carry low-grade, disseminated values. Advanced argillic assemblages are also found in the lower parts of the high-grade silver orebody. At Ladera, advanced argillic alteration is restricted to the deepest zones.
Intermediate argillic (illite-smectite + sericite) alteration - Precious metal mineralisation is somewhat uncommon in zones of this alteration assemblage. Generally, clays and white mica are either found peripheral to the ore-bearing zones, or within strongly fractured fault zones. Nevertheless, small, silver-rich lenses are associated with this clay assemblage, both at Ladera and in the western silver-rich orebody at Coipa Norte.
Vuggy silica - This alteration style is dominant in and above the Ladera orebody, and is interpreted to be a product of hydrothermal leaching under highly acid conditions that occurred in the upper parts of the ore zone.
Supergene alteration (jarosite-goethite + gypsum + barite) - This mineral assemblage is dominant in much of the Ladera orebody and in the upper and deep, highly fractured zones of Farellón and Coipa Norte. It mainly occurs in vugs within previously leached rocks and, less commonly, in veinlets that crosscut them. Jarosite and goethite are the most common minerals, whilst gypsum and barite remnants are found in the upper parts of the supergene interval, although well formed crystals may also be locally encountered at depth.

Mineralisation

The mineralisation at La Coipa is very well zoned, characterised by:
i). an upper zone of secondary Ag enrichment,
ii). intermediate zone of Au-Ag in the oxidised zone, where mineralisation occurs as disseminations and structurally controlled deposits. Disseminated ore occurs within both the volcanic and sedimentary basement rocks, and is closely associated with silicic and advanced argillic alteration styles. The structurally controlled deposits are typically formed at lower elevations, but still occur within the precious metal zone. They are often hosted by basement metamorphic rocks (lutites and sandstones) where they are of higher grade. Within the oxidised and overlying enrichment zones, the mineralogy of the precious metals is mainly cerargyrite, native Au, native Ag and electrum with minimal embolite, iodargyrite and argentojarosite.
iii). primary Au-Cu mineralisation in the lower reduced zone, at deeper levels, where different controls on Cu-Au mineralisation are observed, as at Ladera- Farellón and Can-Can, with grades of up to 4% Cu being common in veins and manto-like tabular bodies. Within the sulphide zone, enargite and native Au are the dominant minerals, although variable chalcopyrite, bornite, covellite, with tennantite-tetrahedrite, sphalerite and galena are also present. Generally, these sulphides appear as fine veinlets in weakly altered black shales. Gold mineralisation generally occurs in association with powdery alunite, distributed along stockwork fractures, varying from 50 µm to fractions of a micron. Jarosite and scorodite are common minerals in gold-bearing alunitic stockworks. Gold is normally in the native state, although electrum is seen locally. High gold values in the eastern portion of Coipa Norte are directly associated with advanced argillic alteration, whilst higher silver grades are accompanied by little or no gold, especially in western Coipa Norte, where they are mainly with associated with silicification, and less commonly with advanced argillic assemblages.

The La Coipa operation is based on at least six orebodies, namely Ladera-Farellon, Farellon Bajo, Coipa Norte, Brecha Norte, Can-Can and Chimberos. These deposits had the following original, pre-mining reserves, based on a cutoff grade of 1.0 g/t Au equivalent:
  Ladera-Farellón - 52.1 Mt @ 1.58 g/t Au, 60.36 g/t Ag,   Coipa Norte - 9.15 Mt @ 0.19 g/t Au, 171.7 g/t Ag,
Initial construction commenced in 1987, with mining soon after.
Progressive ore reserves and mineral resource estimates for the operation are as follows:
  Dec 31 2000 - proven+probable reserves - 46 Mt @ 1.1 g/t Au, 63 g/t Ag.

  Dec 31 2002 - proven+probable reserves - 35.6 Mt @ 1.13 g/t Au, 56.0 g/t Ag, plus
                  measured+indicated resources - 0.82 Mt @ 0.61 g/t Au, 33.1 g/t Ag.

  Dec 31 2006 - proven+probable reserves - 10.14 Mt @ 1.33 g/t Au, 85.3 g/t Ag, plus
                  measured+indicated resources - 11.47 Mt @ 0.99 g/t Au, 29.3 g/t Ag.

  Dec 31 2010 - proven+probable reserves - 21.73 Mt @ 1.34 g/t Au, 47.2 g/t Ag, plus
                  measured+indicated resources - 14.66 Mt @ 1.03 g/t Au, 43.8 g/t Ag.

  Dec 31 2013 - proven+probable reserves - 5 Mt @ 0.5 g/t Au, 32.8 g/t Ag, plus
                  measured+indicated resources - 13.5 Mt @ 1.48 g/t Au, 44.9 g/t Ag.

  Dec 31 2015 - proven+probable reserves - None declared,
                  measured+indicated resources - 13.816 Mt @ 1.8 g/t Au, 64.8 g/t Ag, plus
                                      inferred resources - 2.121 Mt @ 1.5 g/t Au, 45.2 g/t Ag.

All of these deposits, with the exception of Chimberos are predominantly hosted by Triassic black shales and sandstones of the "basement" and by early Miocene tuffs and tuffaceous breccias. Silver is found mainly in the volcanics while gold occurs preferentially in the basement sediments.

Chimberos lies within Jurassic sediments with silicified shales and sandstones. Intrusives dykes and sills of latite composition, and a dacite dome complex are spatially related to the pyroclastic sequence. Mineralisation accompanied the volcanic event. While there is a strong structural control to the mineralisation, in the form of north-south to NE-SW faults, there is also an important lithological influence - both representing permeability controls. The orebodies has a tabular to mushroom shaped form.

The form of alteration described above varies with the different host lithologies. High gold grades and an alunite-kaolinite±dickite-quartz argillic assemblage are characteristic in the Triassic sediments at Farellón and eastern Coipa Norte, while the western Coipa Norte and Ladera have high silver/gold ratios in the Tertiary pyroclastics, with associated vuggy residual silica. Ladera is largely supergene altered with a suite of jarosite-goethite±gypsum±barite. Western Coipa Norte is silicified and supergene altered in its upper sections, whilst at depth it is been subjected to an argillic domain.

This summary is largely based on sections of the report: Belanger, M., 2003 - Technical Report La Coipa Mine, Chile, prepared for Kinross Gold Corporation La Coipa Mine, Chile.

The most recent source geological information used to prepare this summary was dated: 2003.     Record last updated: 2/11/2014
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.


Ladera-Farellon

Coipa Norte

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Anonymous  2001 - La Coipa: in   Extract from Placer Dome web site http://www.placerdome.com/properties/content/sites/lacoipa/lacoipa.html    4p
Davidson J, Mpodozis C  1991 - Regional geological setting of epithermal gold deposits, Chile: in    Econ. Geol.   v86 pp 1174-1186
Oviedo L, Fuster N, Tchischow N, Ribba L, Zuccone A, Grez E, Aguilar A  1991 - General geology of La Coipa precious metal deposit, Atacama, Chile: in    Econ. Geol.   v86 pp 1287-1300
Sillitoe R H  1995 - La Coipa, Chile: in Sillitoe R H,  Exploration and Discovery of Base- and Precious-Metal Deposits in the Circum-Pacific Region During the Last 25 Years Metal Mining Agency of Japan    pp 66-68
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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