El Pachon

San Juan, Argentina

Main commodities: Cu Mo
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The El Pachón porphyry copper-molybdenum deposit is located at an altitude of between 3600 and 4200 m in SW San Juan Province, Argentina, some 2 km from the Chilean border, 5 km SE of the Los Pelambres mine in Chile, and 100 km from the Pacific Ocean coast, but 1300 km from the Atlantic Ocean (#Location: 31° 45' 22"S, 70° 25' 37"W).

For details of the geological setting see the Los Pelambres record.

Mineralisation at El Pachon occurs within an arcuate zone of potassic alteration containing a string of tonalite porphyry dykes, that curves 7 km across the Argentine border from the Los Pelambres porphyry Cu-Mo deposit (Atkinson et al., 1996).

Four paragenetically constrained Re-Os molybdenite dates from El Pachón were between 9.16 and 8.43 Ma, in contrast to the 10.58 to 9.52 Ma ages for Los Pelambres (Bertens et al., 2006). The same authors interpret these data to demonstrate a progressive migration of porphyry intrusion from NW to SE, and that this centre represents a distinct magmatic-hydrothermal system which was emplaced contemporaneously with the uplift and exhumation of the more northwesterly Los Pelambres deposit.

At El Pachón, copper and molybdenum deposition was centred on a quartz diorite porphyry emplaced within a gently dipping sequence of andesitic flows, and is surrounded by potassic alteration. A small plug of dacite porphyry is mapped near the centre of the deposit, partially surrounded by igneous breccia and by vuggy, tourmaline bearing, hydrothermal quartz-magnetite breccia (Atkinson et al., 1996).

Two major stages of sulphide mineralisation contributed to the formation of the orebody. Approximately 70% of the ore is within the andesitic wall rocks. The potassic alteration passes outwards into a propylitic zone, while both zones are overprinted by a sericitic phase (Atkinson et al., 1996).

Hypogene mineralisation comprises chalcopyrite, pyrite, molybdenite and traces of bornite. An immature, relatively small supergene enrichment blanket has been developed in the upper parts of the orebody, underlain by hypogene mineralisation with intact anhydrite fracture filling. Chalcocite and minor covellite partially replace the hypogene chalcopyrite ore in the supergene blanket (Atkinson et al., 1996).

The mineral resource of the un-mined deposit in December 2014 (Glencore, 2015) at a 0.2% Cu cu-off, was:
    Measured + indicated resource - 1.58 Gt @ 0.55% Cu, 2.1 g/t Ag, 0.012% Mo;
    Inferred resource - 1.70 Gt @ 0.40% Cu, 2 g/t Ag, 0.01% Mo.

The most recent source geological information used to prepare this summary was dated: 2003.    
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 Pachon

  References & Additional Information
   Selected References:
Atkinson W W, Souviron A, Vehre T I, Faunes G. A  1996 - Geology and mineral zoning of the Los Pelambres porphyry copper deposit, Chile: in Camus F, Sillitoe R H, Petersen R, (Eds) 1996 Andean Copper Deposits: New Discoveries, mineralization, Styles and Metallogeny Soc. Econ. Geologists   Spec. Publ. No. 5 pp 131-156

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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