The Almadén mercury metallogenic cluster (Ciudad Real, Spain): alkaline magmatism leading to mineralization processes at an intraplate tectonic setting

Resumo

The Silurian to Devonian Almadén mercury deposits are directly or indirectly related to mafic rocks. The geochemical characteristics of these rocks suggest mantle-derived intraplate magmas, that yielded basalts (s.l.) and olivine diabases of alkaline affinity, and quartz-diabases of transitional to tholeiitic affinity. These rocks may have been generated from a volatile- and incompatible element-rich asthenospheric source of EM-I type. O, C, and S isotopic data provide new insights into the genesis of the Almadén mineralization. Close correspondence between estimated and observed δ18Ofluid values suggest that hydrothermal activity in the Almadén basin was favoured by interaction of basal tic rocks with sea water. The observed δ13C values (-3.6‰ to - 9.0‰), together with the lack of correlation between δ18O and δ13C, favour multiple carbon sources for the hydrothermal carbonates associated with the Almadén deposits, i.e., besides sea water, both mantle and organic sedimentary carbon sources were contributors to these carbonates. This is supported by sulphur isotopic data (-1.6‰ to +9.0‰; cinnabar), which may reflect variable source compositions. Leaching of spilitic sulphur by geothermal waters provides an indirect mechanism for incorporating variable proportions of magmatic and seawater sulphur into the mineralizing fluids, explaining the different isotopic compositions of Hg sulphides. Ar-Ar, Rb/Sr, and 87Sr/88Sr data suggest that hydrothermal activity in the Almadén basin was long-lasting, spanning both the Silurian and Devonian. The huge, unique Almadén Hg deposits would have required exceptional conditions for their genesis. It is proposed that a world-class geochemical anomaly such as Almadén should ultimately reflect the characteristics of the mantle source for the alkali basaltic magmas; indeed, mantle metasomatic activity and the initial low degrees of partial melting inherent to the genesis of the Almadén mafic magmas could have converged to provide an efficient mechanism for Hg pre-enrichment of the basaltic rocks.