A preliminary investigation of carbon and oxygen isotopes in ore-associated carbonates from Bergslagen, south-central Sweden, reveals a large spread in isotopic composition: δ13C ranges from - 7 to + 65%, and δ18O between + 4 and + 26% (rel. SMOW). However, each locality seems to be characterized by a unique isotopic composition. It is suggested that the isotopic composition of the carbonates originally had a uniform marine signature, and that the present variability in isotopic composition is due to secondary alteration effects. The extent of alteration, and consequently the observed isotopic compostion of the carbonates, is a function of the ore-forming activity and the post-depositional history of each district and it is thus suggested that carbon and oxygen isotope studies may be useful tools for sulphide ore prospecting.