Abstract

Volcanic-hosted, massive sulfide deposits of Zn-Pb-Cu type were derived either from seawater-dominated, buoyant fluids that built mounds on the seafloor, e.g., the ores of the Hokuroku Basin, Japan, or from saline fluids that reversed buoyancy on mixing with seawater and filled basins on the seafloor, e.g., several ores of the Iberian pyrite belt and the Mount Read province in Tasmania. The Hokuroku ores formed above subduction zones during protracted periods of regional extension, but both the Iberian pyrite belt and the Mount Read province formed under extensional stress during continent-continent (Iberia) or arc-continent (Mount Read) collision and orogenesis. In the Iberian pyrite belt oblique convergence resulted in transcurrent faulting, and this may also be the case for the Mount Read province. Transcurrent faulting may have allowed easy vertical access for the ore-related magmas, some of which were sourced in the asthenosphere, as well as the blueschist and eclogitic facies emplaced in adjacent terrains, and possibly also saline fluids exsolved from the magmas.

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