Abstract

The copper, lead, zinc, and minor cobalt mineralization of the English Lake District has been classified on the basis of mineralogy, spatial distribution and geological setting of the veins, and age of mineralization. The two major types of mineralization are: (1) ‘chalcopyrite-pyrite-arsenopyrite’, of Lower Devonian (Caledonian) age, and (2) ‘galena sphalerite’ of early Carboniferous age. When the former is closely associated with granite intrusions, more complex mineral assemblages are found involving tungsten or molybdenum bearing phases. Also distinguished from these two major types are later assemblages which probably result from their alteration. A later (Upper Carboniferous–Permian) baryte mineralization is also present. Minor graphite and stibnite deposits, and the major haematite mineralization found in the area, are incorporated into the classification but are not discussed, since they have been dealt with by other authors.

Using data from fluid inclusions and experimental data on particular mineral assemblages occurring in the major types of mineralization, estimates have been made of the temperatures and compositions of the ore-forming fluids. The Lower Devonian veins formed at temperatures in the range of 350–200°C with typical aS2, values of 10−10–l0−16 atm and aO2 of ~10−25–10−45 atm or even less. The solutions were probably brines of low to moderate salinity (~5–10 equiv. wt% NaCl). The Lower Carboniferous veins formed at temperatures in the region of 110–130°C from highly saline brines (~23 equiv. wt% NaCl).

Theories regarding the origins of the mineralization are also discussed. The distribution of Lower Devonian veins shows a clear relationship to the underlying composite granite batholith; the Lower Carboniferous galena-sphalerite veins show no such relationship, but tensional tectonic activity in the early Carboniferous may have provided a mechanism for the opening of fissures. The source of the mineralizing fluids forming the Lower Devonian veins may have been the Borrowdale Volcanic Group, with contributions from fluids associated with the granites, which are also likely to have provided a source of heat. The high salinities of the fluids producing the Lower Carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks.

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