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El Arco |
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Baja California Norte, Mexico |
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Cu Au
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Super Porphyry Cu and Au
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The El Arco porphyry copper deposit is located in the far south of Baja California Norte state on the Baja California Peninsular of Mexico (#Location: 28° 1' 58"N, 113° 23' 48"W).
It is the oldest porphyry copper occurrence in Mexico, having been emplaced in association with a Middle Jurassic (165 Ma) dioritic porphyry stock which intrudes lower Cretaceous mafic to intermediate submarine volcanics. It was emplaced within a relatively primitive Jurassic island arc and was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith (107 Ma) which was developed above the easterly plunging subduction zone that was active between the upper Jurassic and the late Oligocene.
The copper mineralisation at El Arco is concentrated in a core of potassic alteration in the dioritic porphyry stock, surrounded by propylitic alteration in andesitic lavas. It has undergone green-schist facies metamorphism, but lacks a phyllic zone. Later, unmineralised andesite dykes cut the orebody (Sillitoe, 1976).
The deposit contains Cu with associated Au, while molybdenum is essentially absent. Chalcopyrite and pyrite mineralisation is present as a vein stockwork, associated with an alteration assemblage of K-feldspar, quartz, albite, chlorite, epidote and calcite. Minor molybdenite and magnetite are also present. This zone is bounded by a low pyrite halo of similar mineralogy, but lacking K-feldspar, and with less quartz.
A cupriferous oxidised capping, characterised by goethite, chrysocolla, malachite, dioptase, wad and pitch limonite, has developed over the high chalcopyrite-low pyrite hypogene mineralisation, with no associated supergene blanket (Sillitoe, 1976).
Published resource estimates include: 660 Mt @ 0.6% Cu, 0.3 g/t Au (Barton et al., 1995) and 600 Mt @ 0.6% Cu (Sillitoe, 1976).
The most recent source geological information used to prepare this decription 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.
El Arco
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Selected References:
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Valencia, V.A., Barra, F., Weber, B., Ruiz, J., Gehrels, G., Chesley, J. and Lopez-Martinez, M., 2006 - Re-Os and U-Pb geochronology of the El Arco porphyry copper deposit, Baja California Mexico: Implications for the Jurassic tectonic setting : in J. of South American Earth Sciences v.22, pp. 39-51.
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Valencia-Moreno, M., Ochoa-Landin, L., Noguez-Alcantara, B., Ruiz, J., and Perez-Segura, E., 2007 - Geological and metallogenetic characteristics of the porphyry copper deposits of Mexico and their situation in the world context: in Alaniz-Alvarez, S.A. and Nieto-Samaniego, A.F., (Eds.), 2007 Geology of Mexico: Cele brating the Centenary of the Geological Society of Mexico: Geological Society of America, Special Paper 422, pp. 433-458.
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Weber, B. and Martinez, M.L., 2006 - Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico): in Mineralium Deposita v.40, pp. 707-725.
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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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