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SummaryPorphyry copper deposits are large crustal sulphur anomalies containing copper and other metals. Much of the sulphur is oxidized and, along with a major portion of the copper, deposited from fluids evolved from certain calc-alkaline melts of intermediate composition. These melts are emplaced with characteristic textures as dykes and stocks in and above the cupolas of small granitic batholiths. The depth of emplacement is shallow, and the intrusions are comagmatic with overlying andesite—rhyolite volcanics. Intrusion and mineralization are relatively rapid events and occur towards the end of previous episodes of vulcanism and intrusion. Porphyry copper deposits are clustered in both space and time on regional, continental and global scales. More may have formed in the past 65 million years than in earlier geologic periods, especially in certain apparently favourable continental regions. Sources of sulphur, copper and of the associated magmas are unknown. Much additional mapping of continental and oceanic margins is needed to explain the abnormal, or unusual, coincidence of several igneous and tectonic processes probably responsible for the formation of porphyry copper deposits.