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Institute of Geological Sciences, London, England
United Nations Development Programme, Rangoon, Burma
Synopsis: Three main types of destructive plate boundary are recognized — island arc type, continental margin type and collision type — each displaying distinctive mineralization.
The first, characterized by an ensimatic island arc magmatic belt developed within oceanic crust, has associated deposits, which include diorite-type porphyry copper and gold, Kuroko-type zinc-copper-lead sulphides, mercury, native sulphur-pyrite and Besshi-type cupriferous sulphides.
The second, characterized by a continental margin type magmatic belt developed within continental crust, includes associated porphyry copper and molybdenum and, locally, tin-tungsten deposits. Where complete, mineral zonation away from the plate margin into continental crust comprises copper-molybdenum, silver-lead-zinc, tin-tungsten and antimony. A modification of this type of plate margin results from the formation of a marginal basin by rifting and outward migration of incipient continental margin mountain belts bordered on the ocean side by a subduction zone. Granite plutons with associated tin, tungsten and fluorite are emplaced in this setting.
Centres of porphyry copper mineralization may be controlled locally by transform faulting at both types of plate margin.
Tectonic wedges of oceanic crust and upper mantle are scraped up with overlying sediments and emplaced above subduction zones that border magmatic arcs. These wedges may contain mineral deposits previously formed in oceanfloor spreading-related settings, including Cyprus-type massive sulphides, podiform chromite, platinum, magnesite and asbestos.
The collision type of destructive plate boundary is formed during and following the final stage of subduction of ocean floor between two continents, between two island arcs or between a continent and island arc. Slices of tectonically emplaced oceanic crust and upper mantle, forming ophiolites, are obducted or thrust on to the continental crust or inactive island arc on the subducting plate to form elongate belts at the junction of the two plates. Collision belts contain mineral deposits, interpreted as having formed at oceanic ridges and magmatic arcs, and may also include tin-tungsten, iron-titanium ores associated with anorthosites in the lower continental crust, native silver-nickel-cobalt arsenides, gemstones and strata-bound uranium-vanadium-copper mineralization in molasse derived from the associated mountain belts.
