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el Bronce |
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Chile |
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Au Ag Cu
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Super Porphyry Cu and Au
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El Bronce is a small epithermal gold-silver vein system in the western foothills of the Andean cordillera in central Chile, 150 km north of Santiago.
The mineralised veins are hosted by early Cretaceous and early to late Cretaceous volcanic rocks which are predominantly andesitic breccias, tuffs and lavas. A sub-circular 14 to 16 km diameter late Cretaceous caldera is located immediately north of the district, while two late Cretaceous intrusive phases cut the volcanic pile, 1). an older 86 Ma quartz monzo-diorite and 2). a younger 80 to 79 Ma dioritic to granodioritic set of dykes and sills and the dioritic-tonolitic ring dyke complex .
The El Bronce structure is part of a radial fault system related to the Cretaceous caldera and contains the bulk of the gold in the district. It comprises a series of extensional faults, dykes & veins in a 3 x 17 Km, NE trending zone between two NNW-SSE faults. This zone contains a first order dilational jog which in turn hosts the main ore vein systems. Each of these systems include 1). hydrothermal breccia ore, 2). massive ore, 3). stockwork zone, and 4). disseminated ore. Ore minerals are dominated by coarsely crystalline pyrite, sphalerite, chalcopyrite, galena, tetrahedrite-tennantite, and minor bornite. The gangue includes quartz, carbonates, barite and chlorite. Five paragentic stages are observed, 1). quartz-pyrite-gold, 2). quartz-pyrite-sphalerite-chalcopyrite-gold, 3). tetrahedrite-tennantite-galena-silver, 4). barite, and 5). carbonate-chlorite-sphalerite.
The deposit contained 25 tonnes of gold, 105 tonnes of silver and 16 000 tonnes of Cu, at grades of 4.8 g/t Au, 20 g/t Ag and 0.3% Cu.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
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.
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Selected References:
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Camus F, Boric R, Skewes M A, Castelli J C, Reichhard E, Mestre A 1991 - Geologic, structural, and fluid inclusion studies of El Bronce epithermal vein system, Petorca, central Chile: in Econ. Geol. v86 pp 1317-1345
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Sillitoe, R.H., 1991 - Gold metallogeny of Chile - an introduction: in Econ. Geol. v.86, pp. 1187-1205.
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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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