The recent paper by Concha et al. (1992) on the Ag-base metal mineralizations in the Hiendelaencina District (Central Spain) provides general information about the geological setting together with paragenesis, fluid inclusion and isotopic geochemistry data of this important group of ore deposits. The authors develop a complex metallogenic model based on the assumption that these ore deposits are epithermal and genetically related to the Stephanian-Permian andesitic volcanism found in the area. The aim of this note is to discuss the arguments on which such a model is based.
The authors begin by stating that the relationship of the mineralized veins (La Bodera, Hiendelaencina) and the volcanism (Atienza) is obscure. In spite of this, they build all the model on such an assumption. Basic arguments used in this and related papers (Doblas et al. 1988;Martinez et al. 1988) are the low temperatures of formation, absence of cogenetic granitic rocks, the relationship with alleged extensional tectonics, the spatial association of some barite-rich and sulphide-poor veins with the andesites, and the isotopic and fluid inclusion similarities among the different outcrops.
These Ag-Zn-Pb-barite veins occur only in the easternmost part of the Spanish Central System. However, we think that they share with other Ba-F (Pb-Zn) veins of the Spanish Central System, well known to us, many features in common. We would stress, the association of the metallic minerals with quartz, fluorite and barite; the formation at moderately to low temperatures (< 300 ~ and pressures (< 500 bars, hydrostatic pressure); the evidence of mixing of different types of low to medium saline water-rich fluids involved in their genesis, and the presence of a vertical zonation. Almost all of them are emplaced in metamorphic rocks of Hercynian granitoids without any evidence of synchronous related magmatism.
The authors rely on sulphur stable isotopes to constrain the origin of the fluid arguing first that there is a magrnatic component represented by barites hosted by the Atienza volcanics (534S= +6.19 to +6.68%o) and sphalerites at La Bodera (634= +6.37 to +6.74%0). The similarity of the values found at both places, 10 km distant from each other, does not prove, as the authors state, that the source of the sulphur is the same and of magmatic origin. In geothermal systems, complete oxidation of hypogene sulphur can, in fact, give rise to barites depleted in 34S (Field and Fifarek 1985). In fact, in order to explain the 634S values in barites from Atienza, the authors invoke a high SO2/H2S ratio in the fluid. However, in such volcanic-hosted environments, reaction of the magma-derived gas with ground waters usually gives rise to a strong acid alteration (argillic or advanced argillic) of the host rocks. This is not the case here. As the authors realize, the hydrothermal alteration is weak and local, and of the sericite-adularia type.