The paragenesis of a suite of hydrothermal minerals (sulphides, baryte, rhombohedral carbonates, quartz, sericite, and chlorite) at four mines in this district is discussed. Of particular interest are the conversion of pyrrhotine to marcasite, which is shown to take place by loss of iron, the replacement of blende by chlorite, and the possible formation of primary goethite. The occurrence of gudmundite is recorded for the first time in the British Isles.