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Mineralium Deposita

, Volume 53, Issue 7, pp 947–966 | Cite as

A genetic link between magnetite mineralization and diorite intrusion at the El Romeral iron oxide-apatite deposit, northern Chile

  • Paula A. RojasEmail author
  • Fernando Barra
  • Martin Reich
  • Artur Deditius
  • Adam Simon
  • Francisco Uribe
  • Rurik Romero
  • Mario Rojo
Article

Abstract

El Romeral is one of the largest iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile. The Cerro Principal magnetite ore body at El Romeral comprises massive magnetite intergrown with actinolite, with minor apatite, scapolite, and sulfides (pyrite ± chalcopyrite). Several generations of magnetite were identified by using a combination of optical and electron microscopy techniques. The main mineralization event is represented by zoned magnetite grains with inclusion-rich cores and inclusion-poor rims, which form the massive magnetite ore body. This main magnetite stage was followed by two late hydrothermal events that are represented by magnetite veinlets that crosscut the massive ore body and by disseminated magnetite in the andesite host rock and in the Romeral diorite. The sulfur stable isotope signature of the late hydrothermal sulfides indicates a magmatic origin for sulfur (δ34S between − 0.8 and 2.9‰), in agreement with previous δ34S data reported for other Chilean IOA and iron oxide-copper-gold deposits. New 40Ar/39Ar dating of actinolite associated with the main magnetite ore stage yielded ages of ca. 128 Ma, concordant within error with a U-Pb zircon age for the Romeral diorite (129.0 ± 0.9 Ma; mean square weighted deviation = 1.9, n = 28). The late hydrothermal magnetite-biotite mineralization is constrained at ca. 118 Ma by 40Ar/39Ar dating of secondary biotite. This potassic alteration is about 10 Ma younger than the main mineralization episode, and it may be related to post-mineralization dikes that crosscut and remobilize Fe from the main magnetite ore body. These data reveal a clear genetic association between magnetite ore formation, sulfide mineralization, and the diorite intrusion at El Romeral (at ~ 129 Ma), followed by a late and more restricted stage of hydrothermal alteration associated with the emplacement of post-ore dikes at ca. 118 Ma. Therefore, this new evidence supports a magmatic-hydrothermal model for the formation of IOA deposits in the Chilean Iron Belt, where the magnetite mineralization was sourced from intermediate magmas during the first Andean stage. In contrast, the beginning of the second Andean stage is characterized by shallow subduction and a compressive regime, which is represented in the district by the emplacement of the Punta de Piedra granite-granodiorite batholith (100 Ma) and marks the end of iron oxide-apatite deposit formation in the area.

Keywords

Iron oxide-apatite Sulfur stable isotopes Geochronology Northern Chile 

Notes

Acknowledgements

We acknowledge the reviewers Patrick Williams and Daniel Harlov for their constructive comments that greatly improve the manuscript. Bernd Lehmann is thanked for the editorial handling and for additional comments on the manuscript. We acknowledge the facilities and technical assistance of the Isotopic Geology Unit at the SERNAGEOMIN Laboratory Centre for Ar/Ar dating and Mathieu Leisen from CEGA for U-Pb dating. David Dettmann from the University of Arizona, Tucson, is thanked for the sulfur isotope analyses.

Funding information

This study was funded by FONDECYT grant no. 1140780 to FB. We thank for the additional support from the Millennium Science Initiative (MSI) through Millennium Nucleus for Metal Tracing along Subduction grant NC130065, and the Servicio Nacional de Geología y Minería (SERNAGEOMIN). We appreciate the support and help from the geology team at Compañía Minera del Pacífico (CMP).

Supplementary material

126_2017_777_MOESM1_ESM.pdf (68 kb)
ESM Table 1 (PDF 68 kb)
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ESM Table 2 (PDF 61 kb)
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ESM Table 3 (PDF 77 kb)
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ESM Table 4 (PDF 64 kb)
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ESM Table 5 (PDF 50 kb)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Paula A. Rojas
    • 1
    Email author
  • Fernando Barra
    • 2
  • Martin Reich
    • 2
  • Artur Deditius
    • 3
  • Adam Simon
    • 4
  • Francisco Uribe
    • 5
  • Rurik Romero
    • 2
  • Mario Rojo
    • 6
  1. 1.Department of GeologyUniversidad de ChileSantiagoChile
  2. 2.Department of Geology and Andean Geothermal Center of Excellence (CEGA)Universidad de ChileSantiagoChile
  3. 3.School of Engineering and Information TechnologyMurdoch UniversityMurdochAustralia
  4. 4.Department of Earth and Environmental SciencesUniversity of MichiganAnn ArborUSA
  5. 5.Servicio Nacional de Geología y Minería (SERNAGEOMIN)SantiagoChile
  6. 6.Compañia Minera del Pacífico (CAP)La SerenaChile

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