Mineralium Deposita

, 40:707 | Cite as

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

  • Bodo Weber
  • Margarita López Martínez


The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.


Porphyry Geochemistry Radiogenic isotopes Baja California 



The authors are very grateful to R. Martínez-Müller, B. Rascón, and Grupo México Corporation for their logistical support and for assistance with the collection of samples. We wish to thank M.S. Hernández-Bernal, T. Hernández-Treviño, J. Morales-Contreras, G. Solís-Pichardo, and P. Schaaf from the Laboratorio Universitario de Geoquímica Isotópica, Universidad Nacional Autónoma de México, Mexico City, for assistance with sample preparation and data acquisition. Also, we acknowledge V. Pérez-Arroyo, G. Rendón-Marquez, and A.S. Rosas-Montoya for their help with sample preparation at CICESE. We want to thank H. Romero-Espejel for helpful discussions and editing the maps presented in Figs. 2 and 3. Many thanks go to V. Valencia (University of Arizona) for interesting discussions and to B. Lehmann (University of Clausthal) for his valuable review and his comments. This study was financed by grant No 32502-T from the Consejo Nacional de Ciencia y Tecnología and internal project no. 644111 from the Centro de Investigación Científica y de Educación Superior de Ensenada.


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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.Departamento de GeologíaCentro de Investigación Científica y de Educación Superior de Ensenada (CICESE)Ensenada, Baja CaliforniaMéxico
  2. 2.Geology DepartmentCICESESan DiegoUSA

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