Kilometre-scale compartmentalization of fluid sources to a fossil hydrothermal system

The West Cumbria iron ore field exhibits kilometre-scale compartmentalization of fluid sources, as evidenced by sulphur isotope data. Barite accompanying haematite ore from 13 mines in an along-strike width of 5 km has δ34S(VCDT) isotopic compositions ranging from +7.9 ‰ to +23.6 ‰. The large variation is strongly controlled by NNW-SSE faulting, in which the heaviest compositions are to the north and lightest in the south. The heavy and light compositions are comparable with those of Carboniferous and Permian evaporites respectively, both of which occur in West Cumbria and are the assumed primary sources of the barite sulphur, though input from the oxidation of Carboniferous coal pyrite may have provided a minor component. The derivation of sulphur from stratigraphically distinct sources shows that faults isolated fluid flow in compartments on a local scale. Published data for barite from an adjacent fault block in a potential waste disposal site at Sellafield are further distinct, indicative of a primary pyrite source. These data emphasize that fluid sources can be highly compartmentalized in a faulted system and require supporting evidence to confirm if they can be correlated.