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Cerro Bayo - Laguna Verde, Brillantes, Bahia Jara, Mallines

Chile

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The Cerro Bayo epithermal silver-gold district in southern Chile, just south of Lago Gral Carrera, 545 km south of Puerto Montt, 130 km south of Coyhaique, the capital of Region XI, and 8 to 20 km west of Chile Chico and the Argentine border. Mineralisation is distributed over an area of ~12 x 10 km at Laguna Verde in the west, to Brilliantes to the NE, Bahia Jara to the east and Mallines to the SE (#Location: Laguna Verde - 46° 32' 47"S, 71° 59' 53"W).

Epithermal gold and silver mineralisation was recognised in the Cerro Bayo district in 1984 by Freeport Chilean Exploration Company, who continued exploration until 1989, when it sold its Chilean interests to Coeur d'Alene Mines Corporation. After further exploration and proving, Coeur d'Alene Mines commissioned a treatment plant in 1995, and undertook mining in two campaigns, initially from 1995 to 2000 as open pit exploitation mainly in the Laguna Verde area and again from 2002 to 2008, predominantly from underground operations. Exploration after the 2008 shutdown, discovered the large Delia vein in the Laguna Verde area late that year. The project was purchased by Mandalay Resources in 2010, with mining restarting based on the Delia vein in September of the same year.

The regional setting and geology of Patagonia is described in the Southern Andes and Patagonia record.

The Cerro Bayo district is located towards the western margin of the Jurassic 188 to 152 Ma Chon Aike large igneous province, hosted by the 158 to 140 Ma Upper Jurassic felsic rocks of the largely rhyolitic Ibáñez Formation. This formation contains an increased subduction component compared to the in-plate character of the older part of the province further to the east in Argentina.

The geology of the Cerro Bayo district is dominated by the andesitic to rhyolitic rocks of the Ibáñez Formation, which is subhorizontal to shallowly (<20°) east dipping and hosts all epithermal veins in the district. It is intruded by dacitic domes near Laguna Verde, rhyolitic domes and dykes at Bahía Jara and Mallines. The felsic pyroclastic rocks of the Ibáñez Formation were followed by a gradual transition to the Lower Cretaceous (Vanlanginian to Hauterivian) shallow marine limestones and quartzites of the Toqui Formation, remnants of which are preserved as an ~30 m thick unit, representing a marine transgression following cessation of volcanic activity. These are, in turn, covered by subaerial felsic pyroclastic rocks of the Aptian Divisadero Group, which outcrops 20 km south of Cerro Bayo and reflect basin inversion. Eocene to Pliocene basaltic plugs and lavas intrude and overlie the Mesozoic sequence (e.g., Poblete et al., 2014).

The Ibáñez Formation has been subdivided into four stratigraphic units:
Unit 1 - ~112 m of andesitic to dacitic coherent lavas and volcaniclastic successions. The veins in the Brillantes area are hosted by this unit.
Unit 2 - ~150 m thick, composed of a lower, variably welded rhyolitic to rhyodacitic pyroclastic fragmental succession, containing 5 to 10% quartz and plagioclase, locally including K feldspar crystals in an argillised ash matrix. This unit is an important host at Laguna Verde.
Unit 3 - ~20 to 60 m of volcanosedimentary dominated rocks containing laminated volcaniclastic crystal-rich sandstones to conglomerates, probably representing detritus from unit 2. It is widely distributed in the district and separates the felsic pyroclastic rocks of units 2 and 4.
Unit 4 - up to 300 m of variably welded rhyolitic to rhyodacitic pyroclastic deposits, similar to Unit 2, although less quartz rich. In contrast to the underlying units, only granitic lithic clasts have only been observed in this unit which hosts epithermal veins in the Mallines and Bahía Jara areas.

Intrusive rocks include a suite of north-south aligned aphyric to fluidal quartz-K feldspar porphyritic subvolcanic domes that extend from Mallines to the Bahía Jara area in the east. In the southern part of this belt of domes, similar textured north-south rhyolite dykes outcrop. A volumetrically less significant series of ~NE-SW aligned domes outcrop from the Laguna Verde to the Cañadón Verde area in the west. These have a porphyritic fluidal texture with 20% phenocrysts which include ~45% quartz, ~45% K feldspar (partly replaced by clays ±sericite), and ~10% biotite. Domes near Laguna Verde contain light and dark (opaque minerals) bands. These rock are dominantly composed of a fluidal fine-grained potassium-rich matrix with ~1% largely unaltered phenocrysts of 90% K-feldspar, 9% quartz and <1% biotite.

Over 90 major veins have been identified in the district to 2017. These veins, which vary from 0.5 to 5 m in thickness, infill steeply dipping, pre-existing faults and fractures. They were formed by multiple mineralising events distributed over an interval of ~34 m.y. that produced open-space filling, banding and brecciation typical of low-sulphidation style vein deposits. Economic mineralisation typically occurs as sub-horizontal ore-shoots within veins, extending up to a kilometre or more along strike and 200 m vertically. Ore minerals comprise a complex sulphide suite, dominated by pyrite, electrum and various silver sulphosalts, in a gangue of quartz with minor adularia and barite that exhibit assemblages suggesting at least three stages of precious metals deposition. Base metal sulphides are common in the veins, though not abundant.

Laguna Verde

The Laguna Verde area, in the northwestern part of the district, contains numerous veins which have a dominantly northwesterly strike, although some mineralised structures have a northerly or a west-northwesterly strike. A significant proportion of the mineralisation is breccia hosted after hydrothermal brecciation. Vein textures are dominantly composed of white saccharoidal quartz and grey banded quartz as well as calcite, adularia and locally, fluorite (Townley, 1997; Pizarro, 2000). Bladed textures are only locally evident. Gold and silver are typically found as electrum or in their native form. The sulphide content generally increases with vein depth and includes pyrite, sphalerite, chalcopyrite and galena, as well as proustite/pyrargyrite (Townley, 1997; Pizarro, 2000). Dating of samples from a vein at Laguna Verde yielded a plateau age of 111.9±2 Ma, whilst intensely illite altered wall rock adjacent to a vein from another sample gave a K-Ar age of 114±3 Ma (Tippet et al., 1991, in De la Cruz and Suárez, 2008). Outcrops of the Dagny, Fabiola, Yasna, Delia NW and Delia SE veins consist of narrow, chalcedonic quartz fracture fillings just a few tens of metres above ore whereas other veins, such as Bianca, have no surface expression at all.

Brillantes

This area is located in the northeastern part of the district and contains, from east to west, the Constanza, Roberta, Francisca and Brillantes veins hosted by units 1 and 2 of the Ibáñez Formation. To 2014 no precious metal production had come from these veins. The Roberta vein strikes at 190° and dips 75°, with an average width of 20 cm at surface. Observed surface mineralization (at 500 masl) mainly consists of fine-grained disseminated subhedral pyrite crystals associated with chalcedonic quartz and adularia. Galena, malachite and azurite are present in the Constanza vein surface outcrops. Vein adularia from the Roberta vein yielded a plateau age of 133.0±1.5 Ma.

Bahía Jara

Several veins have been exploited in this area in the eastern section of the district, and have some of the highest-grades encountered (e.g., Lucero vein). These veins, which are located immediately west of the Cerro Bayo Dome and the north-trending Cerro Bayo fault, strike at 315 to 330°, dipping steeply to the NE, some displaying slickensides with sinistral strike-slip displacement. Farther west, the Guanaco I-IV veins have a similar strike, but dip steeply to the SW. The veins near the Cerro Bayo Dome contain chalcedonic, drusy and minor grey quartz. Locally, quartz is intergrown with barite, while quartz also replaces tabular calcite (lattice texture). Veins in the Guanaco area have grey, saccharoidal and crustiform quartz textures. The dominant vein fill in ore shoots is saccharoidal quartz and locally hydrothermal breccia, with chalcedonic quartz clasts and cement of grey quartz with pyrite and proustite. The sulphide assemblage of the Guanaco I vein (Townley, 1997) is composed of disseminated pyrite, overgrown by and intergrown with sphalerite, the latter containing chalcopyrite inclusions. Disseminated anhedral bornite occurs in fractures, partly replaced by covellite of possible supergene origin along cleavage planes. Arsenopyrite is rare, only occurring as subhedral disseminated grains, while native silver and gold are present as small (<20 µm) anhedral and subhedral grains. Late coarse-bladed barite is common throughout the Bahía Jara area, either as late vein infill or as filling of NE- to east-striking fractures. Vein samples have been dated at 124.9±1.1 Ma. Additional samples from the Guanaco area were dated at 145±5 Ma (adularia K-Ar) and 137.0±1.4 Ma (40Ar/39Ar). Approximately 1 km north of the Cerro Bayo Dome, a K-Ar age of 128±9 Ma and a 40Ar/39Ar date of 128.4±2.6 Ma were obtained for the same adularia sample.

Mallines

This area is located in the southeastern part of the district and is the highest topographic exposure of mineralisation on the district. With the exception of the Cascada vein, no resources large enough to warrant mining operations had been delineated to 2014. Four main vein sets are recognised with orientations that vary considerably:
• The oldest veins, which are subvertical and strike at 150 to 160° with widths from 0.5 to 1 m. They are mainly filled by saccharoidal, drusy, and lesser grey quartz with minor lattice textures of quartz replacing tabular calcite. Hydrothermal alteration adjacent to the veins comprises silicification with illite, minor kaolinite, and locally, chlorite. Drusy quartz, regular boxwork (with hematite filling), and fine-grained sulphides are evident.
• The second generation crosscuts the first, with a predominant 110° strike and subvertical to 60° dips that have metre-scale sinistral dilatational jogs. These veins have widths of up to ~1 m and comprise chalcedonic, saccharoidal, minor drusy and grey quartz, with local crustiform textures, and include quartz and adularia. The fringing alteration minerals are mainly illite, minor kaolinite, and silicification (Poblete, 2011).
• The third vein system crosscuts the previous two with strike trends of 165 to 180° and dips from vertical to 60° and dextral or sinistral offsets of up to 60 cm. These are between 0.2 and 10 m wide, with Veta Madre being the widest. They contain saccharoidal, chalcedonic, drusy, bladed, crustiform (quartz + adularia) and grey quartz, and have been subjected to minor brecciation. Sulphides include highly corroded euhedral and subhedral disseminated pyrite and lesser arsenopyrite, overgrown by proustite-tetrahedrite (Townley, 1997). Fine-grained disseminated pyrite is associated with the grey and bladed quartz. Adjacent hydrothermal alteration mostly consists of silicification accompanied by illite, smectite and minor kaolinite (Poblete, 2011).
• The fourth set is cut by the third, with no conclusive temporal relationship to the previous two, and has subvertical east-west to NE strikes.
Dating has returned ages of 156±5 Ma (K-Ar) and 142.1±1 Ma (
40Ar/39Ar) from the same adularia sample. A separate determination gave a 40Ar/39Ar age of 144.4±1.6 Ma.

Resources and Reserves

Remaining Mineral Resources and included Ore Reserves at the end of 2016 (Mandalay Resources Reserve and Resource Report 2017) were:
  Mineral Resources
      Measured resource - 0.105 Mt @ 2.47 g/t Au, 352 g/t Ag;
      Indicated resource - 0.915 Mt @ 3.05 g/t Au, 349 g/t Ag;
      Measured + Indicated resources - 1.020 Mt @ 2.99 g/t Au, 349 g/t Ag; for 30.5 t Au and 356 t Ag.
      Inferred resource - 0.543 Mt @ 2.49 g/t Au, 206 g/t Ag; for 1.34 t Au and 112 t Ag.
  Ore Reserves
      Proved reserve - 0.103 Mt @ 1.91 g/t Au, 282 g/t Ag;
      Probable reserve - 0.876 Mt @ 2.33 g/t Au, 282 g/t Ag;
      Proved + Probable reserves - 0.979 Mt @ 2.29 g/t Au, 282 g/t Ag; for 2.24 t Au and 276 t Ag.

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


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Poblete, J.A., Bissig, T., Mortensen, J.K., Gabites, J.,Friedman, R. and Rodriguez M.,  2014 - The Cerro Bayo District, Chilean Patagonia: Late Jurassic to Cretaceous Magmatism and Protracted History of Epithermal Ag-Au Mineralization: in    Econ. Geol.   v.109, ppp. 487-502.


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