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: hydrotherm al solutions associated with alkali basalt volcanism in the Silurian a nd Devonian; or regional hydrothermal activity during later Hercynian metamorphism. Although these models can be distinguished by determinin g the age of ore deposition, no suitable isotopic method had been appl ied to the problem prior to this study. We report here on vacuum encap sulated and more traditional laser Ar-40/Ar-39 dating performed on ill ite concentrates and Cr-rich micas that are intimately associated with mercury mineralization in both Devonian and Silurian host rocks. Illi te is associated with a later stage of Hg mineralization (cinnabar + k aolinite + pyrophyllite + chlorite) which locally replaces an earlier episode associated with carbonates and Cr-mica. The dating results fro m 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 ag es are variable and range from 365 Ma to 427 Ma. This age range spans the period between the deposition of the Criadero quartzite (the oldes t sedimentary unit hosting mercury), and the later episode of minerali zation. This suggests that mineralization started at least by the mid to lower Silurian and the younger argon ages for Cr-mica could represe nt partial to nearly total argon loss, caused by the Hercynian metamor phic event. It appears that both postulated styles of mercury minerali zation occurred, separated in time by up to 80 million years.