Exploration for gold in the Crediton Trough, Devon. Part 2 : detailed surveys

This report describes the results of geochemical sampling to trace the source of three groups of highamplitude gold anomalies in panned drainage sediment within the outcrop of the Permian (New Red Sandstone) sedimentary rocks of the Crediton Trough. The sites for follow-up were chosen after appraisal of regional-scale survey data described in the previous report (Cameron et al., 1994). They comprised the Deckport and Solland areas at the western end of the Crediton Trough, and the Smallbrook area adjacent to the faulted southern margin of the Permian rocks some 20 km further east. This work consisted of more detailed drainage sampling and reconnaissance overburden sampling at all three sites, augmented by detailed overburden sampling and the mechanical excavation of a series of pits and trenches at Smallbrook. Microchemical mapping of gold grams from drainage sediment and from excavations in overburden aided the interpretation of the origin of the gold. At Deckport, where the Bow Breccia (Early Permian) is in faulted contact with the Crackington Formation (Late Carboniferous), follow-up sampling indicated strongly that the major source of gold was the Bow Breccia. Telluride inclusions were more frequent in the gold grains from the southern part of the Permian outcrop, than in gold from most other sites in the Crediton Trough. This may indicate that the source is related to a nearby centre of igneous activity, the root of which may be marked by the lamprophyric dykes and vent agglomerate south and south-west of Hatherleigh. The Solland area is immediately east of the trace of a component of the Sticklepath-Lustleigh Fault. Gold persists in drainage sediment towards the southern, faulted contact of the Bow Breccia with Bude Formation (Late Carboniferous) strata to the south. Overburden sampling across the trace of a fault to the east of Solland, parallel to the Sticklepath-Lustleigh Fault, indicated that the gold was not associated with this fault but occurred in alluvial terrace material derived from further south. However, the analysis of overburden samples indicated high values of uranium associated with this fault. At Smallbrook, where the highest-amplitude drainage enrichments in gold had been found, further sampling showed a sharp cut-off for gold just north of the boundary fault with the Crackington Formation. The gold grains from the Small Brook differ from grains from other locations in the Crediton Trough in being finer grained, generally rounded, not enriched in palladium and with fewer and smaller inclusions. Gold was found physically and by analysis in panned overburden pit samples at several sites to the south-east of the Small Brook, particularly in the residual overburden derived from the Newton St Cyres Breccia (Late Permian), to a maximum of fourteen grains from one site. Trenching and pitting confirmed that the shallow overburden samples closely reflected the weathered bedrock beneath. Gold was found (12-35 ppb Au) in several unpanned CO.5 mm fraction samples, but no highly anomalous levels were detected. Microchemical mapping of a gold grain extracted from Newton St Cyres Breccia showed internal chemical characteristics and inclusions identical to grains from the alluvium of the Small Brook, and may indicate an igneous association. The horizontal and vertical distribution of gold in the overburden and weathered bedrock indicate that it is widely dispersed in the Newton St Cyres Breccia in the form of a fossil placer. The source of the gold is probably the older Permian sequence, within which a rich source of mineralisation may exist to the west of Smallbrook. Excavations in alluvium adjacent to the Small Brook indicate the widespread presence of gold (maximum 1180 ppb Au in panned material) from above 1 .O m. Three grains of gold extracted from core from the faulted contact between Permian and Carboniferous rocks in the Upton Pyne Borehole, 5 km east of the Small Brook, were similar in chemistry, but not in shape, to grains from the Small Brook. This discovery reinforces the potential for gold mineralisation close to the contacts, both faulted and unconformable, of the Permian red-bed sequence in Devon. Further work, including drilling, is recommended to determine the concentration of gold in the basal Permian rocks and to determine the potential and controls exerted by Permian igneous rocks on the mineralisation.