Chromite and platinum group elements mineralization in the Santa Elena Ultramafic Nappe (Costa Rica): geodynamic implications

Authors

  • F. ZACCARINI Department of Applied Geosciences and Geophysics, University of Leoben. Peter Tunner Str. 5, 8700 Leoben, Austria.
  • G. GARUTI Department of Applied Geosciences and Geophysics, University of Leoben. Peter Tunner Str. 5, 8700 Leoben, Austria.
  • J.A. PROENZA FERNANDEZ Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona (UB)C/ Martí i Franquès s/n, E–08028 Barcelona, Spain.
  • L. CAMPOS Escuela Centroamericana de Geología, University of Costa Rica. San Pedro de Montes de Oca, 240-2060 UCR, San José, Costa Rica
  • O.A.R. THALHAMMER Department of Applied Geosciences and Geophysics, University of Leoben. Peter Tunner Str. 5, 8700 Leoben, Austria.
  • T. AIGLSPERGER Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona (UB)C/ Martí i Franquès s/n, E–08028 Barcelona, Spain.
  • J.F. LEWIS Department of Earth and Environmental Sciences, The George Washington University. 2029 G St. NW, Washington, D.C. 20052, U.S.A

Keywords:

Chromitite, Santa Elena Ultramafic Nappe, Costa Rica, Chromite composition, PGE-PGM, Geodynamic setting

Abstract

Chromitites associated with strongly altered peridotite from six distinct localities in the Santa Elena ultramafic nappe (Costa Rica) have been investigated for the first time. Santa Elena chromitites commonly display a compositional variation from extremely chromiferous (Cr/(Cr+Al)=0.81) to intermediate and aluminous (Cr/(Cr+Al)=0.54). This composition varies along a continuous trend, corresponding to calculated parental liquids which may have been derived from the differentiation of a single batch of boninitic magma with Cr-rich and (Al, Ti)-poor initial composition. Fractional precipitation of chromite probably occurred during differentiation of the boninitic melt and progressive metasomatic reaction with mantle peridotite. The distribution of platinum group elements (PGE) displays the high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio typical of ophiolitic chromitites and, consistently, the platinum group minerals (PGM) encountered are mainly Ru-Os-Ir sulfides and arsenides. Textural relations of most of the platinum group elements suggest crystallization at magmatic temperatures, possibly under relatively high sulfur fugacity as indicated by the apparent lack of primary Os-Ir-Ru alloys. The chemical and mineralogical characteristics of chromitites from the Santa Elena ultramafic nappe have a strong affinity to podiform chromitites in the mantle section of supra-subduction-zone ophiolites. Calculated parental melts of the chromitites are consistent with the differentiation of arc-related magmas, and do not support the oceanic spreading center geodynamic setting previously proposed by some authors

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Published

2011-11-30

Issue

Vol. 9 No. 3-4 (2011): Subduction Zones of the Caribbean: The sedimentary, magmatic, metamorphic and ore-deposit records (UNESCO/IUGS IGCP)

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