Andesitic arcs, epithermal gold and porphyry-type mineralization in the Western Pacific and Eastern Europe

Gold-poor and gold-rich porphyry and epithermal gold-silver deposits of Caenozoic and Cretaceous age in the western Pacific and central and eastern Europe are confined to predominantly andesitic continental margins and island arcs. The arc volcanic rocks overlie a complex non-oceanic basement of stacked nappes, the uppermost of which is often ophiolitic. The emplacement of ophiolite triggered a reversal of tectonic polarity, subduction and arc magmatism. Andesitic volcanism and the formation of a strato-volcano were followed by the ascent of magma and porphyry copper mineralisation. Regional propylitic alteration came from convecting, predominantly meteoric fluids. Magmatic degassing caused advanced argillic alteration. This is commonly unmineralised in perched water tables, but may sometimes host high sulphidation gold or enargite gold at deeper levels within the propylitic zone. The onset of extensional stress after the andesitic volcanism allowed meteoric water to circulate to depths of 5 km or more, with consequent cooling of an ascending magma body at depth and the development of low sulphidation epithermal gold mineralisation near and immediately above the sub-volcanic basement surface. All andesitic arcs are mineralised. Regional controls on their ore potential include the level of erosion and geological features in the basement that focus the ascent of fluids. Differences between the mineralisation of the European and the western Pacific arcs may reflect the presence of older, more granitic basements in many of the European arcs.