Recent hydrocarbon exploration in the Solway Basin has revealed the existence of Lower Carboniferous paralic sediments which contain numerous discrete anhydrite beds. Anhydrite S isotope ratios from the Easton-1 Well are compared with new isotopic data from barite mineralization that occurs along the faulted southern margin of the Northumberland-Solway Basin and published data from the Northern Pennine orefield. Similarities between the δ34S of anhydrite and barites from major mines at Haydon Bridge and the Alston district of the Northern Pennine orefield supports a Lower Carboniferous marine sulphate source for mineralization at these sites. Identification of extensive evaporite deposits hi the Solway Basin resolves several longstanding problems associated with mineralization hi the Northern Pennines including the source of barite sulphate and the origin of hypersaline mineralizing fluids.

In an earlier investigation of the S isotope geochemistry of Carboniferous-hosted barite mineralization in and around the Alston district of theNothern Pennine orefield (NPO, Fig. 1a), Solomon et al. 1971 drew attention to the similarity between the δ34S of Lower Carboniferous marine sulphate and the sulphur isotopic composition of barite deposits. A model for barite mineralization was proposed in which deep, circulating, BaCl2 brines mixed with shallow (< 1 km depth), sulphate-rich, connate fluids composed of Lower Carboniferous sea water. Although the model proposed by Solomon et al. 1971 is consistent with aspects of barite mineralization, more recent studies of the evolution of pore fluid geochemistry in sedimentary basins (Land 1987; Hanor 1994a, b) argue against a marine connate origin for the sulphate component

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