Geological Processes of Gold Concentration and Depletion in Caledonian Terrains

Extensive exploration for gold in Scotland, undertaken as part of this research, has succeeded in locating several significant and previously unknown geological gold enrichments. Field and laboratory studies of these localities has identified several key metallogenic processes as responsible for the development of gold mineralisation in Scotland, knowledge of which will be useful to further exploration. These processes have been operative at deep crustal, and medium to shallow crustal levels and in the surface environment. Deep crustal processes comprised the formation of deep crustal gold reservoirs by large scale tectonism involving subduction and orogenesis in a collisional tectonic setting. Tapping of these deep crustal gold reservoirs by uprising magmas of the Silurian to Lower Devonian Appinite and Newer Granite suites provided a means of transportation of gold to mid-crustal levels. Hydrothermalism associated with magma emplacement resulted in gold deposition in breccia-pipes and in zones of enhanced crustal permeability at middle to high crustal levels. Gold deposition occurred as a result of boiling, effervescence and cooling of hydrothermal fluids at these crustal levels. The plutonism and cogenetic volcanism (represented by the ORS lavas) formed very large, very low grade repositories for gold. Subsequent erosion resulted in the unroofing of the concentrated and the large, low grade gold accumulations, exposing them to the supergene environment. Oxidation of sulphides, and leaching and remobilisation of the gold occurred, and gold was reconcentrated close to the water-table on mineralised structures and in deeply weathered regolith materials. Deep weathering under the warm, humid climatic conditions prevalent during Tertiary times is considered the most likely agent of this supergene gold remobilisation. Subsequent reworking of gold enriched regolith materials by modern-day alluvial activity created the alluvial gold concentrations seen today in several parts of Scotland. Scotland therefore shows potential for the development of both bedrock and alluvial gold deposits. None of the deposits discovered thus far has been proven to be commercially viable, but the potential for economic deposits exists in times of enhanced gold prices