Mineralisation in the Middle Devonian volcanic belt and associated rocks of South Devon

This report describes the results of further exploration within the belt of Middle Devonian volcanic rocks between Plymouth and Totnes in south Devon. Soil sampling was carried out to augment the coverage of the volcanic belt described in Mineral Reconnaissance Programme Report No. 79 (Leake et al., 1985) and to extend it into adjacent sedimentary rocks. The 4815 overburden samples indicate that the area as a whole is highly enriched in antimony and, to a lesser extent, arsenic. No evidence was found of further stratiform exhalative mineralisation in addition to the massive pyrite and ferruginous carbonate at Higher Ludbrook and the baryte at Lower Burraton described in MRP Report No. 79. However, four main areas showing evidence of metal-enriched sedimentary rocks have been outlined. In three of the areas, enrichment in Mn in the soils derived from the sedimentary rocks is accompanied by low amplitude enrichment in Zn and Pb, reaching around 200 ppm Zn and 170 ppm Pb. The fourth area, adjacent to the separate belt of volcanic rocks northwest of Totnes, is more extensive and of higher amplitude (reaching over 700 ppm Zn and 600 ppm Pb). The soil and dAlIhole data indicate that extensive hydrothermal systems were associated with the alkali basaltic volcanism in the area and that submarine hydrothermal activity took place. The close similarity in geology between the area and the Rhenish basin in Germany, which hosts the Meggen SEDEX deposit, indicates that south Devon and east Cornwall remain prospective for submarine exhalative mineralisation. Overburden samples indicate that polymetallic mineralisation occurs within a zone about 3 km long in the west of the area. The zone is enriched in As, Pb, Zn, Mn and Cu and is similar to polymetallic mineralisation carrying gold which occurs further south in Devon, described in Mineral Reconnaissance Programme Report No 121 (Leake et al., 1992). Evidence for further polymetahic mineralisation is present in the northeast of the area but this differs geochemically from the other areas in having a higher proportion of Zn to Pb and in the presence of anomalous concentrations of Sn. Proximity to the Dartmoor granite suggests that thii anomalous zone could be related to the contact aureole of the granite. Two further boreholes were drilled to investigate the source of the zone of anomalous antimony in soil at Ladywell, as the earlier hole described in MRP report No. 79 did not intersect sufficient mineralisation to account for the surface anomaly. One hole intersected a zone of oxidised rock containing 120 ppm Sb over 6.4 m within a wider zone showing lower amplitude enrichment in antimony (75 ppm over 21m) and containing minor amounts of bournonite, tetrahedrite and stibnite. This enrichment in antimony may be primary, in association with one episode of volcanic&y. No evidence of an association of precious metals with this mineralisation was found, though there was some enrichment in mercury (up to 11 ppm). The second hole showed no enrichment in antimony but contained minor amounts of base metal sulphides in association with carbonate veinlets and sections of dark slate enriched in Zn (up to 1600 ppm Zn over 1 m).