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El Penon

Chile

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The El Peñón low sulphidation gold deposit is located some 160 km to the south-east of the Chilean coastal city of Antofagasta at an elevation of 1400 m (#Location: 24° 24' 29"S, 69° 29' 42"W).

After the discovery hole was drilled in early 1994, the deposit was proved, and following a 10 month construction period, the first gold was poured in September 1999.

The El Peñón mine is located in the Central Depression of the Atacama Desert, which is underlain by Late Cretaceous to Early Eocene magmatic arc rocks, of the 'Paleocene belt'. The geology of the region comprises basaltic to rhyolitic lavas and tuffs, subvolcanic porphyritic intrusions, and granitoid stocks, which extend from southern Peru to central Chile, where it is host to epithermal gold deposits (e.g., El Guanaco, San Cristóbal, El Huesco, Manto de la Falda and La Coipa) and subvolcanic porphyry systems (e.g., Andacollo).

Late Cretaceous, 84 to 65 Ma calc-alkaline magmatic arc volcanic rocks were deposited in a series of narrow fault-bound extensional basins, the margins of which are intruded by dioritic to monzonitic plutons. Compressive tectonism from 65 to 62 Ma resulted in the inversion of the Late Cretaceous basins, accompanied by uplift and erosion of Late Cretaceous plutonic rocks to the west of the basin, and syntectonic magmatism along the basin-bounding faults. Volcanic rocks continued to be deposited through the rest of the Cretaceous in new northeast-trending trans-tensional basins partially controlled by reactivation of basin-bounding faults.

Volcanism continued into the middle Eocene, when subvolcanic domes and sills were emplaced and the mineralisation at El Peñón was emplaced. Deformation took place in the mid- to late Eocene with uplift of the Precordillera to the east, accompanied by low angle offset of the El Peñón vein system.

The deposit is associated with an ~60 Ma Eocene rhyolite dome complex which intrudes a sequence of volcanics (dacite flows, tuffs and volcaniclastic rocks). The rhyolite complex represents multi-phase intrusive and extrusive activity.

The host sequence within the district is as follows:
Aeropuerto Formation, Lower Cretaceous fluvial sandstones, breccias with coarse sediments, conglomerates and volcaniclastic breccias;
Quebrada Mala Formation, Upper Cretaceous fluvial and lacustrine sands and silts, andesitic to basaltic lavas and breccias, rhyolitic to dacitic ignimbrites Augusta Victoria Formation Upper Cretaceous, sanidine-biotite rhyolite ignimbrites, fluvial sandstones and trachytic lavas;
Paleocene to Eocene volcanic and subvolcanic units, dacite lavas or domes, andesite flows rhyodacitic ignimbrites, volcaniclastic breccias, intercalated fluvial sediments, late rhyolite, dacite domes, feeders and hypabyssal intrusions;
Cretaceous to Eocene intrusives;
Diorite and monzonite stocks.

The deposit is mostly hosted by Late Cretaceous to Early Eocene basaltic to rhyolitic pyroclastic flows, lavas, volcaniclastic breccias and tuffs, whilst several thin Early Cretaceous rhyolite tuff and dacite to andesite flow layers occur in the northern part of the deposit area. These rocks are intruded by Late Cretaceous diorite and monzodiorite stocks and dacite domes.

The mineralisation is hosted by near-horizontal to gently dipping Eocene to Paleocene basaltic to rhyolitic volcanic rocks, comprising a lower sequence of volcanic breccia and andesitic to basaltic flows, overlain by rhyolitic to dacitic pyroclastic rocks, dacitic to andesitic flows, and volcanic breccia. Rhyolitic intrusives, domes, and associated flows are intercalated with earlier volcanic units.

The El Peñón deposit comprises a low sulphidation adularia-sericite epithermal vein system that is related to the rhyolitic dome complex, both temporally and spatially. The mineralisation, which is strongly fault controlled, has been dated at 59.4 ± 1.4 Ma by K/Ar dating of the Adularia. There are 13 main and many subsidiary veins in nine vein systems. The main NNW trending mineralised faults dip at 80° west, while the NNE trending faults dip at 65 to 75° east or west. These faults are predominantly dip-slip and reflect both extensional and compressional regimes. NNE to NE-striking fault zones are a relatively proportion of the overall deposit. Vein widths range from a few tens of cm to >20 m. Individual mineralised shoots vary from <1 to 4 km in strike length, and up to 350 m in the down-dip. Grades vary up to hundreds of grams per tonne Au but are more typically <30 g/t. Silver grades are in the order of hundreds to thousands of grams per tonne.

The principal mineralized veins are Al Este, Bonanza, Cerro Martillo/Dorada, Dominador, El Valle/Discovery Wash, Fortuna, Martillo Flats, Pampa Campamento, Playa, Providencia, Quebrada Colorada, Quebrada Orito, and Vista Norte.

Gold and silver are associated with a variety of quartz vein textures and grain sizes. Vein textures often have crustiform textures, although the highest grade gold-silver mineralisation is reported to be associated with massive banded quartz-adularia. Chalcedonic to coarse-grained quartz is found in banded, saccharoidal, comb and bladed carbonate replacement vein textures. Hydrothermal brecciation is common, having prepared the brittle rhyolites for vein emplacement, resulting in zones that are locally up to 22 m in true width. The mineralised veins are believed to have formed from periodically boiling low salinity (< 2 wt% NaCl) fluids at between <200 to 255° C.

Gold and silver mineralisation consists of disseminated electrum, native gold, native silver, silver sulphosalts, and silver halides occurring in a gangue of predominantly quartz, adularia, carbonate, and clay. Electrum is the most common form of precious metals in the deposit and occurs as micron- to millimetre-size subrounded and irregular grains. Two phases of electrum are present: a primary phase, which contains ~55 to 65% Au, and a secondary phase, which has resulted from supergene processes that have remobilised silver, and which typically consists of over 95% Au. Sulphide minerals are relatively rare, and this may be due to oxidation, or to an initial low overall abundance, but where present are in order of abundance pyrite, galena, sphalerite, chalcocite and covellite. Abundant Fe- and Mn-oxyhydroxides are common with only trace occurrences of relict sulphides.

Gangue minerals occur as fracture and breccia-filling and replacement quartz, adularia, carbonates, and clay minerals. Gangue minerals occur as open space filling as well as replacements of primary host rock mineral phases.

Alteration, which produces the assemblage above, is dominated by quartz ±adularia replacement or flooding within or near the veins systems, grading outwards into quartz-sericite/illite ± adularia. Argillisation is locally developed with in rhyolite, although argillic and sericitic alteration is best developed in the ash flow tuffs and dacite flows of the dome complex.

Dating of adularia from the veins suggests that mineralisation occurred at ~53 to 52 Ma (Early Eocene). Two mineralisation and alteration events have been recorded from fluid inclusion studies. The principal event was due circulation of neutral, reduced fluids, resulting in replacement of host rock phenocrysts and groundmass by quartz, adularia, albite, carbonate, clays, calcite and chlorite, as well as quartz-adularia flooding and breccia-filling in the vicinity of the veins. A second, more widespread, alteration process was derived from acidic, oxidised hydrothermal solutions, resulting in the formation of lithocaps of quartz-alunite alteration and quartz-alunite breccia-filling, with minor copper and silver, and little or no gold.

The oxide zone which extends 250 to 280 m below the current surface comprises largely silver halides, native gold and silver, and electrum with varying fineness. Base metals are rare in both the oxide and primary ore, although trace sphalerite, galena and chalcopyrite are found in the early banded veins in the unoxidised ore.

During 2000 production totalled 9 t (0.289 Moz) Au and 124 t (4 Moz) Ag at an average gold recovery of 94% and cash cost of $US 48 (total production cost of $US 96) per oz.

Reserves and resources at 31 December, 2000 (Meridion Gold) were:
    Proven + probable reserve -4.7 Mt @ 10.1 g/t Au, 186 g/t Ag for 47 t (1.532 Moz) Au.
    Mineral resources total 4.5 Mt @ 8.4 g/t Au. 160 g/t Ag for another 38 t (1.217 Moz) of Au.

Reserves and resources at 31 December, 2010 (Yamana Gold website) were:
    Proven + probable reserve - 8.55 Mt @ 7.29 g/t Au, 188.6 g/t Ag, for 5.47 t Au; plus
    Measured + indicated resource - 2.66 Mt @ 8.85 g/t Au, 236.5 g/t Ag, for 23.6 t Au; plus
    Inferred resource - 4.47 Mt @ 8.12 g/t Au, 258.2 g/t Ag, for 36 t Au.

Reserves and resources at 31 December, 2014 (Yamana Gold website) were:
    Proven + probable reserve - 10.409 Mt @ 5.03 g/t Au.

For detail consult the reference(s) listed below. This summary has been drawn, in large part, from "Collins, S.E., Chester, P.E., Moore, M. and Scott, K.C., 2010 - Technical report on the El Peñón Mine, Northern Chile; an NI 43-101 Report prepared for Yamana Gold Inc. by Scott Wilson Roscoe Postle Associates Inc., 132p."

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 9/3/2015
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.


El Penon

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Anonymous  2001 - El Penon Geology: in   extract from Meridian Gold web site http://www.meridiangold.com/property/elpenon.html    7p
Arancibia G, Matthews S J, Cornejo P, Pérez de Arce C, Zuluaga J I and Kasaneva S,   2006 - 40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile : in    Mineralium Deposita   v41 pp 505-516
Robbins C H, Lorson R C  1998 - Geology and discovery of the El Penon gold-silver deposits, northern Chile: in    http://www.dregs.org/abs_curr.html    1p
Warren I, Archibald D A and Simmons S F,  2008 - Geochronology of Epithermal Au-Ag Mineralization, Magmatic-Hydrothermal Alteration, and Supergene Weathering in the El Penon District, Northern Chile: in    Econ. Geol.   v103 pp 851-864
Warren I, Zuluaga J I, Robbins C H, Wulftange W H, Simmons S F,  2004 - Geology and geochemistry of epithermal Au-Ag mineralization in the El Penon district, 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 113-139


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