Abstract: The Bonser vein, one of the most productive sources of copper in the English Lake District, contains a mineral assemblage comprising quartz, chlorite, calcite, dolomite, stilpnomelane, magnetite, pyrrhotine, pyrite, marcasite, native bismuth, bismuthinite, laitakarite, joseite, arsenopyrite, chalcopyrite, sphalerite, galena, and cosalite. The phases pyrrhotine, sphalerite, arsenopyrite, laitakarite, joseite, and cosalite were analysed by electron microprobe. The occurrence of laitakarite is the first reported in the British Isles. Textures of the ore and gangue minerals show that the vein minerals were deposited successively in open voids. The sequence of deposition was of quartz forming throughout, early chlorite and stilpnomelane with some dolomite and calcite, magnetite as the earliest opaque phase, followed by arsenopyrite, pyrrhotine, sphalerite, chalcopyrite, pyrite, and marcasite; the bismuth- and lead-bearing minerals were the last to form.

The data on the coexisting phases and their compositions have been used to estimate the temperatures of mineralization and the activities of sulphur and oxygen. Magnetite and arsenopyrite appear to have formed at 350–400% with a_S2 reaching 10⁻¹² to 10⁻¹⁴ atm. and a_O2 around 10⁻²⁴ to 10⁻²⁹ atm. The assemblage of quartz, chlorite, stilpnomelane, calcite, dolomite, pyrrhoine, chalcopyrite, sphalerite, and (late) arsenopyrite was probably deposited at ∼ 240°C, with a_S2 decreasing to 10⁻¹⁵ to 10⁻¹⁶ atm and a_O2 to 10^{−38) to 10{su−44} atm. The later minerals probably formed at temperatures as low as 200°C and under similar conditions of sulphur and oxygen activity.