Geology and geochemistry of high-grade, volcanic rock-hosted, mercury mineralisation in the Nuevo Entredicho deposit, Almaden district, Spain

The Nuevo Entredicho deposit contains the richest concentration of mercury in the Almaden district, locally grading as much as 45% Hg. This ore deposit is hosted within an alkaline, conically shaped diatreme, about 150 in in diameter, which was subsequently filled with phreatomagmatic breccias. The diatreme cuts an Ordovician to Silurian clastic sedimentary rock sequence that is intercalated with basaltic sills. Structural analysis reveals a complex tectonic history with three main phases of Hercynian deformation. Mineralisation occurs as cinnabar replacements in volcanic tuffs and breccias and as recrystallised veins in tensions cracks associated with pyrophyllite and hydrothermal pyrite, which is strongly enriched in Cu, Pb and Hg. Lead isotopes in pyrite are characterised by high Pb-207/Pb-204 ratios (15.70-15.75), suggesting a contribution of ancient upper continental crust remobilised by Silurian-Devonian volcanism, with no mantle involvement. Sulphur isotopes of epigenetic cinnabar and pyrite range from + 10.3 to + 10.8parts per thousand and from + 10.6 to + 11.9parts per thousand respectively, suggesting a uniform sulphur source or a constant mixing ratio in the ore fluids. These isotopic compositions differ from those measured in the syngenetic deposits of the Almaden district; they suggest a higher temperature of ore formation of about 300 degreesC, and a genesis related to a distinct hydrothermal flow path at the Nuevo Entredicho deposit. Deposition of anomalously high-grade mercury ore at Nuevo Entredicho is related to a combination of (1) an abundance of black shale that provided sulphur and increasingly reducing conditions with high sulphide/sulphate ratios, (2) explosive Silurian-Devonian mafic magmatism that provided an initial source of mercury, (3) tectonic activity that lead to structurally favourable sites for ore deposition, and (4) replacement of secondary, carbonate-rich volcanic rocks.