Skarn-type copper mineralisation in the vicinity of Belstone Consols Mine, Okehampton, Devon

This report summarises geochemical, geophysical and drilling investigations carried out on copper-arsenic-zinc mineralisation around the former Belstone Consols Mine, situated on the northern margin of the Dartmoor Granite and just inside the Dartmoor National Park. Significant concentrations of metalliferous sulphides are restricted to the Meldon Chert Formation of the Lower Carboniferous sequence and predominantly to talc-silicate rock-types within that formation. Although examined only within a limited area around the former mine, similar strata occur as a narrow belt with a strike length of some 22km between Sourton Tors and Drewsteignton. The main drainage crossing this belt is flooded by minerals derived from the granite and stream sediments provide little evidence of the location, nature or richness of any sulphide ores. Soil geochemical surveys, however, do indicate clearly the presence and the general composition of near-surface mineralisation, even when sited on steep valley slopes or over rather narrow ore beds. Definition into discrete sulphide-rich beds appears possible but exact location of the structures is somewhat less certain. Surface geophysical surveys immediately to the west of Belstone Consols Mine detected and traced a number of horizons of contrasting resistivity and chargeability, and have provided a new insight into the geological structure of that area. Most of the geophysical markers do not relate directly to potentially economic mineralisation, although higher chargeability values were observed over the principal mineralised zones revealed by subsequent drilling. Magnetic surveys indicate that pyrrhotite is no more than a minor constituent of the mineralisation in the vicinity of the mine. Drilling proved the presence of significant copper and arsenic mineralisation, surprisingly with little zinc. Although cobalt is not important as an accessory metal, high values of bismuth are quite common. Tin is well developed in most talc-silicate lithologies but is undoubtedly present mainly as replacements in the garnets. Metal values were not as rich as had been hoped but locally did exceed 3%. The mineralisation is wider spread than anticipated and the worked ore beds cannot be identified with certainty. It seems, however, that a previously unknown mineralised horizon can be recognised higher in the Meldon Chert Formation.