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 pro vince of global importance. Our study presents 113 new sulphur isotope anal yses from deposits selected to represent the textural spectrum of ores. Mea sured delta(34)S values range from -26 to +10 parts per thousand, mostly fo r 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 d elta(34)S (-8 parts per thousand to -2 parts per thousand), whereas stockwo rk feeder textures are: dominated by higher delta(34)S (similar to + 3 part s per thousand to +5 parts per thousand), Intermediate textures (mainly coa rse textures in stratiform zones) have intermediate delta(34)S, although th ey 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 w ith direct derivation from Lower Carboniferous seawater sulphate. A dual so urce of sulphide sulphur in the IPB deposits has been considered. A hydroth ermal source, derived from reduction of coeval seawater sulphate in the con vective systems, is represented by sulphide in the feeder zones. Here varia tions 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 deriv ed from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical vari ation in delta(34)S and texture from SW (more bacteriogenic and primary tex tures) to NE (more hydrothermal textures and delta(34)S) which reflects a v ariation in the relative input of each source was likely controlled by loca l geological environments. Given that the sulphur isotope characteristics o f the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we sugge st that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types.