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The Green Tuff belt of Japan and its Kuroko-type exhalative volcanogenic massive sulfide deposits exhibit many unusual features which are best explained if the Green Tuff belt and the deposits formed as the result of an aborted attempt to rift the volcanic chain and open a new marginal sea. This failed rift hypothesis can account for the distribution of mining districts in the Green Tuff belt, the observed extensive and substantial premineralization subsidence and postmineralization uplift in the belt, its volcanic evolution, and other features. The premineralization subsidence results from the same mechanism proposed for the formation of rift valleys at mid-ocean ridges—a dynamic loss of fluid pressure in asthenospheric material that must well up as the lithosphere extends. The Green Tuff belt of Japan is strikingly similar in geology, tectonics, and mineral deposits to Archean greenstone belts, suggesting that Archean belts may be failed rifts. Ore deposits formed in failed rifts have a high probability of permanent incorporation into stable cratons because the deposits are underlain and surrounded by sialic crust and are near sea level. Areas of crustal extension are ideal sites for the kind of very vigorous hydrothermal circulation required to form massive sulfide deposits because they combine unusually high rates of heat input into the crust with a tensional, fracture-dilating stress environment. It is suggested that all exhalative massive sulfide deposits may form in rift settings. Differences between traditional deposit classes reflect mainly the different geologic settings of the rifts. Exploration implications of the failed rift hypothesis are detailed.

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