El Faldeo


Main commodities: Au Zn Pb Ag
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The El Faldeo epithermal Au-Zn-Pb-Ag deposit is located ~27 km south of Cochrane in the Andean foothills, ~100 km east of the axis of the Patagonian Andes in southern Chile.

For details of the regional geological and metallogenic setting see the Southern Andes and Patagonia record.

The deposit is hosted by a 200 m thick sequence comprising a basal sedimentary breccia overlain by dacitic and rhyolitic tuffs, felsic lavas, hydrothermal breccias and epiclastic rocks that constitute the Ibáñez Formation. This sequence directly overlies a Palaeozoic metamorphic basement of schist, phyllite, quartzite, shale and marble. All of these rocks are intruded by tonalite (dated at ~158 Ma) and granodiorite intrusive units and related subvolcanic porphyries, sills and dykes. The volcano-sedimentary rocks were deposited in a subsiding pull-apart basin with coeval formation of spaces for magmatic emplacement and mineralisation in an extensional tectonic regime during the mid-Jurassic. Tectonic exhumation probably started in Late Miocene time.

The principal structural trends in the district are NW-SE, north-south and ENE-WSW, occurring as steeply dipping faults. The NW-SE faults can be continuously traced over lengths of up to 40 km. Shear-sense indicators suggest dextral strike movements. The north-south and ENE-WSW faults are normal, and most of the dykes and veins were emplaced along them or have the same attitude.

Mineralisation and alteration is mainly hosted by dacitic and rhyolitic tuffs and intrusions, and occurs as early epithermal gold and late mesothermal Zn, developed during four hypogene stages over an area of 12 km2.

The first stage involved propylitic replacement of the primary mineral assemblage and deposition of disseminated pyrite. The second stage produced silicic alteration accompanied by epithermal disseminated and veinlet pyrite, arsenopyrite and gold at 170 to140°C. The gold-rich bodies occur as four 10 to 30 m thick and 150 to 240 m long lenses hosted by silicified beds of felsic tuff. Estimated resources are ~15 t of gold contained in mineralized rocks grading 1.2 g/t Au (i.e., 12.5 Mt of ore).

The third stage of mineralisation overprints the second and has been dated at 140±4 and 142±5 Ma (K-Ar whole rock; Palacios et al., 1996). It coincides with brecciation, stockwork formation and mineral dissemination accompanied by quartz-sericite-calcite alteration and with pyrite, sphalerite, Ag-rich galena and mesothermal (330 to 250°C) gold mineralisation. This stage produced both stratabound and irregular shapes and was responsible for an estimated 8 Mt @ 4% Zn, 2% Pb, 30 g/t Ag, 0.6 g/t Au. Fluid inclusion barometry suggests maximum depths of 100 m and 800 m for the second and third stages respectively (Palacios et al., 1996). The fourth and final stage mainly affected the intrusive rocks and corresponds to an open-space filling event in which quartz, calcite, barite, pyrite and chalcopyrite were deposited.

Supergene alteration partially overprints the hypogenic assemblages, forming kaolinite, jarosite and limonite.

The most recent source geological information used to prepare this summary was dated: 1997.    
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
   Selected References:
Parada, M.A., Palacios, C. and Lahsen, A.,  1997 - Jurassic extensional tectono-magmatism and associated mineralization of the El Faldeo polymetallic district, Chilean Patagonia: geochemical and isotopic evidence of crustal contribution: in    Mineralium Deposita   v.32, pp. 547-554.

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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