Exploration for gold in the Crediton Trough, Devon. Part 1 - regional surveys

The results of geochemical surveys in the Crediton Trough of Devon, an area of Permian and Carboniferous rocks, north of Dartmoor, are given in two reports. This report (Part 1) describes the broad-scale drainage and lithogeochemical survey carried out mainly in the area from Hatherleigh in the west, to the valley of the River Exe in the east, over the outcrop of Permian red-bed sediments, minor alkaline basalts and lamprophyric lavas and the surrounding Carboniferous sediments. The Permian outliers at Hollacombe (near Holsworthy), Peppercornbe (near Clovelly), and Holcombe Rogus (southwest of Wellington) together with parts of the Permian outcrop of the Tiverton Basin and west of Cullompton were also sampled. In addition, the results of an interpretation of Landsat Thematic Mapper (TM) imagery over the survey area are presented. Part 2 contains details of follow-up overburden surveys at Deckport, Solland and Smallbrook. The area was selected for gold exploration on the basis of the model of precious metal transport developed to account for the widespread gold in south Devon, which suggested that gold mineralisation might be present in the Permian sequence and the contact with underlying Carbotierous rocks. Drainage surveying confirmed the presence of gold for the first time at numerous localities on the Permian outcrop, and subsequent microchemical mapping of grains demonstrated a number of close similarities with gold from south Devon, strongly suggesting a similar origin. The analysis of rock samples from the Crediton Trough also showed gold to be locally enriched, up to 1.8 ppm in alkaline basalts and up to 42 ppb in samples of Permian sedimentary breccias. Extensive manganese and zinc drainage anomalies at the southern boundary of the Crediton Trough can be related to mineralisation within the Permian and Carboniferous, some of which was worked in the vicinity of Newton St. Cyres. Cinnabar was reported for the first time from this area, and detrital tin, copper and lead anomalies, thought to variously reflect ore minerals or contamination, were also recorded by the drainage survey. The distribution of gold anomalies in the drainage samples indicates that the source is probably associated with the early Permian sediments, the boundary faults between the Permian and Carboniferous sequences, and structures in the Permian, especially where they are underlain by volcanic rocks. Further overburden sampling at three sites is reported in Part 2. Satellite imagery interpretation showed that all the gold anomalies lie on or near lineaments, usually a set trending north-east, which may be more closely associated with mineralising pathways than other directions.