Gold exploration in the Duns area, Southern Uplands, Scotland

Following promising indications during regional geochemical sampling in the Duns area of south-east Scotland, a detailed exploration programme was conducted by the Mineral Reconnaissance Programme (MRP) in or&r to locate and characterise the source of alluvial gold occurrences discovered in the area. A drainage survey, involving the collection of panned concentrates and stream sediments, resulted in the identification of four areas for follow-up investigation. Close-spaced soil sampling carried out over these areas led to the recognition of markedly anomalous gold concentrations (up to 250 ppb), sometimes accompanied by, or closely-associated with enrichment in arsenic. Excavations to bedrock in two of these areas revealed localised gold mineralisation (up to 5 ppm) in hydrothermally altered greywacke, with attendant enrichment in adjacent minor intrusives, which were themselves altered. Lithogeochemical sampling of surface outcrops revealed widespread low-tenor (10 - 100 ppb) gold enrichment in hydrothermally altered Silurian greywackes, siltstones and subordinate porphyritic intrusives. Gold values in excess of 100 ppb are associated with dilatent features in the greywacke country rock such as fracture zones, hydrothermal breccias and localised quartz veining. Mineralisation and alteration of the greywackes is mainly developed adjacent to minor talc-alkaline intrusions of late-Caledonian age. The emplacement of these plutons at a high crustal level is believed to be associated with a previously undetected major east-south-east-trending linear structure, recognised by regional and local aeromagnetic data. Within the same area detailed gravity data revealed the presence of discrete anomalies characteristic of concealed intrusive bodies. Petrological investigations indicate the localised hydrothermal alteration of greywacke samples collected in proximity to minor porphyritic intrusions. Pervasive propyllitic alteration of some of these intrusives is also evident. Relict sulphide mineralisation was revealed in the form of pseudomorphs after pyrite and possibly arsenopyrite, occurring as disseminations and in fracture veinlets. A later phase of mineralisation, associated with fracturing and involving the precipitation of iron oxyhydroxide minerals, suggests the invasion of high-Eh meteoric waters. Full data listings, together with detailed logs of samples, are available from the Mineral Reconnaissance Programme Database, BGS, Keyworth, on request.