Mineral investigations in the Teign Valley, Devon. Part 2 : base metals.

This report describes the search for new base metal reserves in the Teign Valley between Dunsford and Chudleigh, an area worked for lead and zinc ores, with associated silver and copper, in the late 19th century. A programme of geochemical drainage and soil surveys was followed by geophysical surveys and diamond drilling. Chemical analyses were carried out on waters, stream sediments and panned concentrates collected from secondary drainage. The water samples, including effluences from old mine workings, were only rarely anomalous in base metals. However, stream sediment and panned concentrate analyses revealed copper, lead, zinc and arsenic anomalies caused by the Teign Valley lode zone and manganese anomalies which reflected areas of former open-cast mining. One cluster of anomalies suggested possible lead-zinc-copper-arsenic-barium mineralisation to the east of the River Teign. Soil sampling was carried out mainly across interfluve ridges to the west of the river. Interpretation of the soil analyses confirmed the common occurrence of anomalous lead, zinc and copper within the Teign Valley lode zone and indicated that a few parallel mineralised structures may also be present. Some anomalies suggested the presence of disseminated mineralisation within the bedded succession of shales, cherts and tuffs. Induced polarisation (IP) geophysical surveys were carried out in four separate areas containing geochemical anomalies using the dipole-dipole array. Locally, more detailed measurements were made using the gradient array. Anomalies believed to be related to concealed sulphide mineralisation were recorded in all four areas. In the Dunsford area, chargeability anomalies coincident with lead anomalies in soil may be caused by disseminated mineralisation. Near Bridford, anomalies with different characteristics were attributed to disseminated and vein-style mineralisation. The presence of a small high-grade galena vein was suggested by anomalies to the east of the main vein at Wheal Exmouth. Sixteen interlinked traverses north-east of Bovey Tracey defined two significant anomalies compatible with the presence of sulphide mineralisation; soil geochemistry indicated significant lead with copper but only minor zinc enrichment. Four short inclined diamond drillholes were sited north-east of Bovey Tracey, between Lower and Higher Coombe, to investigate the clusters of geochemical anomalies which IP data suggested were caused by sulphide mineralisation. The mineralisation was found to comprise disseminated and thin, discontinuous strata-bound veinlets of sulphides within shales, cherts and tuffs close to the Lower-Upper Carboniferous boundary. Galena and sphalerite with a little chalcopyrite, arsenopyrite and loellingite are associated with pyrite, quartz and siderite. Chemical analysis of drillcore revealed appreciable zinc concentrations in some sections, one containing 2% zinc over 3 m. Lead values are lower, with a maximum of 0.2% over 1 m; several l-3 m lengths containing 0.1% lead are present. Copper concentrations are very variable; the best intersection contained 0.14% over 1 m. Finely disseminated galena and sphalerite have not been reported previously from the Teign Valley and their discovery opens up the potential for this type of deposit concealed within the Carboniferous condensed sequence both here and in other areas of south-west England.