Copper, lead, zinc and cobalt mineralization in the English Lake District: classification, conditions of formation and genesis.

The two types of mineralization in this area are the chalcopyrite-pyrite-arsenopyrite assemblage of Lower Devonian (Caledonian) age and the galena-sphalerite assemblage of early Carboniferous age. When the former type is associated with granite, more complex assemblages also occur with W or Mo. Using fluid-inclusion and experimental data, it is estimated that the Lower Devonian veins formed at 350o-200oC with typical aS₂ values of 10^-10 to 10^-16 atm and aO₂ = or < 10^-45 atm. The solutions were probably brines with approx 5-10 equivalent wt.% NaCl. The Lower Carboniferous veins were formed at 110o-130oC from highly saline brines (approx 23 equiv. wt.% NaCl). The Lower Devonian veins show a clear relationship with the underlying composite granite batholith; the later galena- sphalerite veins show no such relationship. The Lower Devonian fluids may have come from the Borrowdale volcanic rocks whereas the Lower Carboniferous veins may have involved sea-water in leaching the metals from basement rocks. -R.A.H.