Mineralization in the Bonser vein, Coniston, English Lake District: mineral assemblages, paragenesis, and formation conditions

The assemblage in this vein includes quartz, chlorite, calcite, dolomite, stilpnomelane, magnetite, pyrrhotite, pyrite, marcasite, native bismuth, bismuthinite, laitakarite, joseite, arsenopyrite, chalcopyrite, sphalerite, galena and cosalite; microprobe analyses are reported for pyrrhotite, sphalerite, arsenopyrite, laitakarite, joseite and cosalite. The sequence of deposition has been established; the Bi- and Pb-bearing minerals were the last to form. Magnetite and arsenopyrite appear to have formed at 400o-350oC with aS₂ 10^-12 to 10^-14 atm and aO₂ approx 10^-24 to 10^-29 atm. Quartz, chlorite, calcite, pyrrhotite, chalcopyrite and sphalerite were probably deposited at approx 240oC with aS₂ decreasing 10^-15 to 10^-14 atm and aO₂ 10^-38 to 10^-44 atm, and the later minerals were deposited at 200oC under similar conditions. -R.A.H.