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

Abstract

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, laltakarite, 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 as2 reaching 10 -12 to 10 -1* atm. and a02 around 10 -24 to 10 -29 atm. The assemblage of quartz, chlorite, stiipnomelane, calcite, dolomite, pyrrhoine, chalcopyrite, sphalerite, and (late) arsenopyrite was probably deposited at ~ 240 ~ with as~ decreasing to 10

Eighth supplementary list of British Isles minerals (English)

“…Inclusions (<5 ~tm) in galena at nine localities in the Lake District (Stanley and Vaughan, 1981). Rare isolated granules (-<20 ~tm) in quartz, Wet Swine Gill, Caldbeck Fells, Cumbria (Fortey etal., 1984).…”

“…[Also, as small yellow cubes, from Tynagh mine, Co. Galway, Eire; XRD and qualitative EDS analysis (A. de Haller specimen; P. Perroud, pers. comm., 1989 (Stanley et al, 1991).…”

“…Trigonal From Pengenna mine, St. Kew, Cornwall; Hannaford, near Barnstaple, Devon; and localities in the Lake District, Cumbria (Ixer and Stanley, 1980). Inclusions (<5 ~tm) in galena at nine localities in the Lake District (Stanley and Vaughan, 1981). Rare isolated granules (-<20 ~tm) in quartz, Wet Swine Gill, Caldbeck Fells, Cumbria (Fortey etal., 1984).…”

“…PB4(CO3)C12.H20 Monoclinic A new species, forming aggregates of tiny subparallel elongate colourless crystals, as an alteration product of phosgenite in lead veins at Bounds Cliff, St. Endellion, Cornwall (Stanley et al, 1991).…”

Chalcomenite from Ballybunnion, Co. Kerry, Eire

“…Chalcomenite is new to the British Isles and is the first selenium mineral to be found in Ireland; several selenides have been reported recently from Coniston, Cumbria (Stanley and Vaughan, 1982) and Hope's Nose, Torquay, Devon (Criddle and Stanley, 1986).…”