A new sulphur isotopic study of some Iberian Pyrite Belt deposits:: evidence of a textural control on sulphur isotope composition

The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured delta(34)S values range from -26 to +10 parts per thousand, mostly for massive and stockwork ores, in agreement with data previously published. In situ laser delta(34)S analyses reveals a dose correlation of delta(34)S with texture. Primary diagenetic textures are dominated by relatively low delta(34)S (-8 parts per thousand to -2 parts per thousand), whereas stockwork feeder textures are: dominated by higher delta(34)S (similar to + 3 parts per thousand to +5 parts per thousand), Intermediate textures (mainly coarse textures in stratiform zones) have intermediate delta(34)S, although they are mostly dominated by the high delta(34)S component. Rare barite has a homogeneous delta(34)S around +18 parts per thousand which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in delta(34)S are caused by variations in the extent of the sulphate reduction, which governs the SO4:H2S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in delta(34)S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and delta(34)S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types.