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The Quimsacocha high-sulphidation epithermal Au-Ag-Cu deposit is located ~480 km south of Quito, 110 km SE of Guayaquil and ~30 km southwest of the city of Cuenca, in Azuay Province, southwestern Ecuador (#Location: 3° 2' 38"S, 79° 12' 54"W).
It is one of a number of Miocene porphyry and epithermal copper-gold deposits that occur in two main clusters, which define two mineral districts in western Ecuador, including Chaucha, Gaby-Papa Grande, Quimsacocha and Portovelo-Zaruma (Azuay-El Oro District), and Junín and Alpala (Imbaoeste District). These districts are ~440 km apart and located within 100 km of the Pacific coast, 200 km SSE of Guayaquil and 75 km north of Quito, in southwestern and northwestern Ecuador respectively. Other deposits are distributed along the Miocene porphyry belt between these two districts, and to the north and south into Colombia and Peru.
Tectonic and Regional Setting
For detail of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador and the broader North Andes and Panama copper-gold province.
The Quimsacocha deposit lies in the southern part of the Chaucha continental terrane, in the Western Cordillera (Cordillera Occidental) of Ecuador. The Chaucha terrane is bounded to the NW by the NNE trending Bulubulu fault system, and by the the Girón fault system to the SE, interpreted to represent long-lived structures, that during each reactivation phase, influenced the location of intrusive and subvolcanic bodies, while also acting as conduits for mineralising hydrothermal fluids.
The Chaucha terrane is predominantly composed of a variety of schists and semi-pelitic gneisses of the Palaeozoic (Devonian) La Victoria sequence, and thick volcanic/volcaniclastic rock packages that together form the basement to much of the area. The latter includes the Oligocene to early Miocene Saraguro Group andesitic and rhyolitic lavas and tuffs. Towards the south of the deposit area, a succession composed of rhyolitic and dacitic tuffs containing biotite, plagioclase and quartz are subdivided into the Plancharrumi, Jubones, La Fortuna and La Paz formations, which are considered to be the upper part of the Saraguro Group. The post-Saraguro early to middle Miocene volcanosedimentary units were deposited in the Santa Isabel basin. Towards the north, the late Miocene Turi, Quimsacocha and Tarqui formations, as described below, are mapped. Quaternary alluvial deposits along the Girón valleys cover wide areas, masking sections of the Saraguro Group sequence. Gabbroic bodies of the Pallatanga Unit cross-cut the above formations. Quartz diorites and granodiorites/tonalites intrude the Cretaceous metamorphic rocks and the Sacapalca Unit, a lower member of the Saraguro Group. These granitoids are usually medium- to coarse-grained, while some are typical of subvolcanic (high level) intrusions, and form complexes that include micro-granodiorites and micro-diorites.
The geology surrounding the deposit is characterised by Upper Miocene volcanic and volcaniclastic rocks. The deposit is situated between the major Gañarin and Girón faults, while a 4 km diameter caldera, which is cut by a multi-phase diatreme, lies along the Gañarin fault, 400 m west of the main Quimsacocha mineralisation.
The Upper Miocene succession comprises (Valliant et al., 2012), from the base:
Ayancay Group, sandstones, conglomerates which overlie the Oligocene to early Miocene Saraguro Group
Turi Formation, a sequence of tuffaceous breccias, conglomerates and sandstones containing abundant andesitic clasts and occasional fragments of tuffaceous breccia. The lower contact is well exposed with basal layers of massive conglomerate and boulders. The outcrops have a radial pattern with shallow dips to the south and east, away from the caldera. Turi Formation has been confirmed to be late Miocene (9 to 8 Ma).
Turupamba Formation, is composed of rhyolitic to dacitic tuffs with a lesser lapilli tuffs, and appears to be composed of numerous minor ash falls with intercalations of fluvial and lacustrine sedimentary rocks.
Quimsacocha Formation, which hosts the Quimsacocha deposit and consists of banded andesitic lava flows and flow breccias with phenocrysts of fresh plagioclase as well as andesitic tuffs and breccias. This formation is exposed over an area with a ~20 km diameter, within and around the caldera, on which it is centred.
Tarqui Formation, which has a maximum thickness of ~400 m and outcrops mainly on the eastern flank of the caldera where it overlies the Turi Formation, and unconformably succeeds all of the older formations. In the vicinity of the Quimsacocha deposit, exposures of the Tarqui Formation are mostly strongly weathered tuffs. In contrast, further north the unit is mostly composed of thinly banded tuffs, tuffaceous sandstones and conglomerates, with common plant remnants and coal.
The deposit area is predominantly underlain by the Quimsacocha Formation, where it comprises a series of andesite flows and monolithic volcanic breccias, distributed radially from the Quimsacocha caldera. Three distinct lithologies are exposed at surface: i). A porphyritic, banded, dacite flow, located on the east, NE and SE of the caldera; ii). An andesite flow breccia that occupies nearly every part of the caldera except the north and NE sectors; and iii). Porphyritic, banded, andesite flows, that are either hornblende- or plagioclase-rich, and occur on the south, east, and NE flanks of the caldera (Valliant et al., 2012).
The Quimsacocha Formation is essentially flat lying and usually does not outcrop in the deposit area. Typically the exposed resistant flows described above overlie a succession of lapilli and breccia tuffs, lenses of lava, and ash and crystal tuffs, which are seldom seen in outcrop but are intersected in drill holes, which also encounter dacite porphyry dykes intruding the tuffs. In addition to these dacite dykes, a number of post-mineral dacitic plugs intrude the volcanic sequence (Beate et al., 2001). Porphyry intrusions directly associated with mineralisation have not been described, although diatreme breccias outcropping on the east side of the caldera contain clasts of porphyry-style Cu-Au mineralisation, possibly suggesting a buried porphyry system.
The Quimsacocha high sulphidation epithermal Au-Cu-Ag mineralised system occurs at lithological contacts between the andesitic lavas and underlying tuffs, and is thickest in the tuffs. The alteration system surrounding the deposit covers a north-south elongated area of ~12 x 6 km, and was formed by multi-phase fluid injections, controlled by lithological and structural features. Alteration is typical of high sulphidation systems with a nucleus of vuggy and massive silica, fringed both laterally and vertically, by alunite, kaolinite, dickite and pyrophyllite, and yet further out by argillic alteration represented by illite and smectite clays.
The deposit is a flat lying, north-south striking, cigar shaped body, comprising a high grade core surrounded by a lower grade shell. The north-south mineralised zone hosting the resource has a strike length of ~1600 m, is 120 to 400 m wide and up to 60 m thick beginning ~120 m below surface.
Two generations of pyrite are recognised: i). Pyrite 1 (Py1), which is generally <50 to 60 µm in size, and is disseminated throughout the silicified rock; and ii). Pyrite 2 (Py2), usually >400 µm in size, found in fractures and cavities. Pyrite is found either in its massive form or, in some cases, is partially replaced by enargite. Enargite occupies cavities and fractures, occasionally together with barite, and is younger than both pyrite generations. Galena and sphalerite are typically found on the margins of the mineralisation, mainly in fractures and cavities, sometimes coexisting with kaolinite. Corbett (2006) suggested a paragenetic sequence as follows:
Pre-ore stage 1 - alunite and kaolinite and minor early vuggy to massive silica;
Pre-ore stage 2 - a major episode of vuggy to massive silica and early pyrite, plus some chalcopyrite;
Ore stage 1 - minor continued vuggy to massive silica and major pyrite, early gold plus some continued chalcopyrite, enargite, early covellite and barite;
Ore stage 2 - further strong vuggy to massive silica, continued strong pyrite, gold, covellite tennantite-tetrahedrite, argentite-galena and continued barite;
Post ore - renewed kaolinite, sphalerite, quartz veinlets and waning pyrite and gold.
The main mineralising event was characterised by an early phase caused by a strong inflow of volatile, acidic fluids which cooled progressively and were neutralised by their reaction with country rock, leading to the formation of sub-horizontal massive and vuggy silica bodies surrounded by zoned advanced argillic and argillic alteration haloes (kaolinite, illite, dickite, pyrophyllite, alunite, etc.). The sulphides and gangue minerals associated with the mineralisation (mainly enargite and second-generation pyrite) were deposited by late fluids inside the silicified bodies. Gold occurs as inclusions in enargite associated with py2 (Corbett and Leach, 1998; and Corbett, 2006). Hypogene alunite associated with advanced argillic alteration gave an age of ~7.1 Ma (40Ar/39Ar; Schütte et al., 2010).
Mineralisation encountered in outcrop in the northern part of the caldera area is mainly composed of pyrite, traces of enargite, barite and gold. To the south, in the Rio Falso area, it is common to find traces of cinnabar, stibnite, barite and pyrite, while only traces of cinnabar and pyrite are apparent to the north, in the Cerro Casco and Costillas areas. Some outcrops show typical steam-heated alteration (e.g., at Loma Larga in the south), and are usually unmineralised. In other areas, there are high mercury anomalies along with occurrences of granular silica, alunite crystals with kaolinite, and traces of pyrite. It is also common to find locally massive silicification with weak development of vuggy texture (e.g., at D1). Towards the south (Jordanita area), low temperature opaline or chalcedonic silica is observed.
Published Canadian NI 43-101 compliant resource estimates for the Quimsacocha deposit (INV Metals Inc., 2012; Valliant et al., 2012) at an ~0.4 g/t Au cutoff, include:
Indicated resource - 32.6 Mt @ 3.2 g/t Au, 22 g/t Ag, 0.20% Cu; and
Inferred resource - 2.3 Mt @ 2.2 g/t Au, 27 g/t Ag, 0.22% Cu.
This summary is sourced from the "Geological Setting and Mineralisation" section of Valliant et al. (2012), prepared for INV Metals Inc.
The most recent source geological information used to prepare this summary was dated: 2012.
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.
References to this deposit in the PGC Literature Collection:
Bissig, T., Clark, A.H., Rainbow, A. and Montgomery, A., 2015 - Physiographic and tectonic settings of high-sulfidation epithermal gold-silver deposits of the Andes and their controls on mineralizing processes: in Ore Geology Reviews v.65, pp. 327-364.|
Schutte P, Chiaradia M, Barra F, Villagomez D and Beate B, 2012 - Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology: in Mineralium Deposita v.47 pp. 383-410|
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