Dating of alteration episodes related to mercury mineralization in the Almaden district, Spain

The Almaden mercury deposits represent one of the largest geochemical anomalies on Earth, but the mode and timing of their formation remains a matter of controversy. There are two main possibilities: hydrothermal solutions associated with alkali basalt volcanism in the Silurian and Devonian; or regional hydrothermal activity during later Hercynian metamorphism. Although these models can be distinguished by determining the age of ore deposition, no suitable isotopic method had been applied to the problem prior to this study. We report here on vacuum encapsulated and more traditional laser 40Ar/39Ar dating performed on illite concentrates and Cr-rich micas that are intimately associated with mercury mineralization in both Devonian and Silurian host rocks. Illite is associated with a later stage of Hg mineralization (cinnabar + kaolinite + pyrophyllite + chlorite) which locally replaces an earlier episode associated with carbonates and Cr-mica. The dating results from illite separates suggest that the later stages of Hg mineralization or remobilization occurred about 360 Ma, which is about 20 Ma younger than any mercury-hosting rocks in the district and coincides with thr onset of regional deformation during the Hercynian orogeny. Cr-mica ages are variable and range from 365 Ma to 427 Ma. This age range spans the period between the deposition of the Criadero quartzite (the oldest sedimentary unit hosting mercury), and the later episode of mineralization. This suggests that mineralization started at least by the mid to lower Silurian and the younger argon ages for Cr-mica could represent partial to nearly total argon loss, caused by the Hercynian metamorphic event. It appears that both postulated styles of mercury mineralization occurred, separated in time by up to 80 million years.