Geochemistry of caldera and wallrock alteration associated with volcanogenic sulphide vein mineralization, Snowdonia, North Wales

A major Ordovician submarine caldera occurs near Snowdon in North Wales. Th e main lithologies are bimodal basic and acid, intrusive and extrusive igne ous rocks with volcaniclastic sediments. Minor volcanogenic base-metal sulp hide vein mineralization occurs, dominantly within the caldera. The caldera rocks have undergone extensive hydrothermal alteration, and additional ext reme metasomatism is associated with mineral veins. The geochemistry of the alteration is described, rocks within the caldera being compared with thos e outside as well as with the mineral vein wall-rocks. Major changes in the alkalis, alkaline earth and certain other major and trace elements are obs erved. The base metals were commonly mobile, their movement being controlle d by hydrothermal circulation in the volcanic pile. Some 'immobile' element s have been redistributed by strong alteration close to mineral veins. The alteration is compared in terms of mineralogy and geochemistry with modern geothermal systems, with which similarities are found despite different env ironments and ages.