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

Abstract 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 34S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser 34S analyses reveals a close correlation of 34S with texture. Primary diagenetic textures are dominated by relatively low 34S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher 34S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate 34S, although they are mostly dominated by the high 34S component. Rare barite has a homogeneous 34S around +18‰, 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 34S 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 34S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and 34S) 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.

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