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Fuchsite and other Cr-rich phyllosilicates in ultramafic enclaves from the Almadén mercury mining district, Spain

  • D. Morata (a1), P. Higueras (a2), S. Domínguez-Bella (a3), J. Parras (a2), F. Velasco (a4) and P. Aparicio (a5)...

Fuchsite and other Cr-rich phyllosilicates, paragenetic with dolomite, are present in some ultramafic enclaves from the ‘frailesca’ rock (a lapilli- to block-size pyroclastic lithic-tuff), in the Almadén mercury mining district, Spain. Analyses (EMPA and TEM) of fuchsite and Cr-chlorite showed a relatively large range in levels of Cr2O3. Petrographic relationships between these phyllosilicates and primary relics of Cr-spinel crystals, as well as their high Cr content, indicate that these Cr-rich minerals originated from primary chromian spinels through an early hydrothermal alteration stage. The hydrothermal fluids accounting for this early alteration would be of relatively high temperature, high aCO2 and a K, and variable a Na/K. In a later alteration stage, fuchsite was partially or totally replaced by illite and Cr-illite, giving rise to an argillitic alteration.

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