First lead isotopic data for cinnabar in the Almaden district (Spain): implications for the genesis of the mercury deposits

摘要

The Almaden district constitutes the largest and probably the most intriguing mercury concentration in the world. Two types of mineralization are recognized: 1) stratabound, of Lower Silurian age, well represented by the large Almaden deposit; and 2) fully discordant mercury deposits of minor importance in terms of size, and exemplified by the deposit of Las Cuevas. The latter ones can be found at different positions along the Almaden stratigraphic column. Both types of deposits are always associated with the so-called frailesca rocks (diatremes of alkali basaltic composition). This paper reports the first lead isotope compositions of cinnabar in the district. Whole samples and stepwise leaching cinnabar aliquots display relatively homogeneous isotopic compositions (~(206)Pb/~(204)Pb = 18.112-18.460; ~(2070Pb/~(204)Pb = 15.635-15.705;~(208)Pb/~(204)Pb = 38.531-38.826). Taken together with Jebrak et al.'s (2002) pyrite lead isotope results, the new cinnabar isotopic data define a steep array trend on the ~(207)Pb/~(204)Pb- ~(206)Pb/~(204)Pb diagram, indicating a mixed contribution of lead and probably mercury from different sedimentary sources in the Almaden basin. The Almaden Hg deposits are related to a contemporaneous mafic magmatism that might have provided part of the mercury. Hydrothermal leaching of organic matter from sedimentary rocks and formation of Hg organic complexes enhanced metal solubility, promoting transport from and within the volcanic units.