Exploration for gold in the South Hams district of Devon

Panned drainage samples collected from the area south of a line between Plymouth and Brixham in Devon (roughly equivalent to the South Hams District) show that gold is widely distributed over much of the area. At 44 out of 450 sites the concentration of gold in the panned concentrate exceeds 0.5 ppm. Drainage gold anomalies are present over the entire Lower Devonian sequence and the Start Complex but are less frequent over the Middle Devonian rocks which occur in the north-west of the area. Other metallic elements determined in the samples suggest that there is no simple pathfinder for gold and the factors influencing its concentration are complex. Many of the grains are very intricate in shape with projections which could not survive if transported in the streams for anything more than trivial distances from source. The internal compositional characteristics of the gold are highly variable and of particular interest. One type of grain contains variable concentrations of palladium, often showing intricate growth zonation. In some areas the gold is enriched in silver rather than palladium and there is a regularity of distribution which suggests zonation of grain type, reflecting differences in source. Other grains, essentially of pure gold, have a more irregular distribution. Follow-up overburden sampling by hand auger was carried out along reconnaissance traverses up to 9 km long and by shorter lines in areas of interest. Pit digging and power augering to a maximum depth of 7.3 m showed, in the area south of Brownstone, near Holbeton, that gold is present in head and weathered bedrock, as well as in near-surface overburden. Anomalies are concentrated along an east-west zone at the bottom of the valley. There is a correlation between gold and cassiterite abundance in the overburden but there are also a few gold-rich samples with little cassiterite. Geophysical surveys in the Brownstone area showed VLF anomalies and a 0.5 mGal positive gravity anomaly coincident with a local galvanic resistivity low and chargeability high. Four holes drilled to test the source of anomalies in the east-west zone of anomalous gold in overburden, intersected mostly black slate, pyritiferous in part, and widespread lensoid vein quartz often with minor carbonate, particularly as euhedral rhombs lining voids. The depth extension of the east-west anomalous zone of gold at surface comprises a zone of intense oxidation alteration, brecciation and carbonate veining. Samples from this altered zone contain minor levels of gold, reaching a maximum of 380 ppb, whereas elsewhere gold concentrations are very low. The fourth hole, drilled about 200 m west of the other three, did not encounter an altered zone but intersected pyritiferous black slate with vein quartz and carbonate containing minor gold (maximum 54 ppb) and widespread minor sphalerite and galena in fracture coatings. At Churchill, near Marlborough, close to the boundary of the Start Complex with Lower Devonian rocks to the north, soil traverses delineate a zone of enrichment in As, Sb, Ba, Mn, Fe and Cu. Pit samples within the zone show isolated gold anomalies (maximum 710 ppb). Rock samples collected from a new road cut in a mixed volcanic-sedimentary sequence show evidence of extensive albitisation, probable potassium feldspar alteration and anomalous concentrations of As and Sb. The geochemistry of the weathered rock is similar in many respects to that associated with veining and alteration in coastal sections near Wadham Rocks which is characterised by potassium feldspar alteration and enrichments in As, Sb, Cu and Au (maximum 400 ppb). Further zones with anomalous concentrations of As and Sb occur throughout the region, some with associated scattered gold enrichments. Two major phases of mineralisation are thought to be responsible for the gold anomalies in drainage and soil. The fast comprises polymetallic mineralisation associated with zones of intense hydrothermal alteration and predates the main deformation of the rocks. In the second phase, which accounts for the vast majority of the gold grains in drainage, saline oxidising solutions carrying precious metals circulated within and beneath the Permo-Triassic red-bed sequence which had been deposited on the eroded Devonian surface. Deposition of gold occurred where conditions became more reducing, particularly within Devonian rocks by reaction with pyritiferous slates and other reactive rocks.