The Semail ophiolite of Oman contains numerous gossans, minor sulfide showings, and ferromanganoan deposits as well as several massive sulfide deposits, the largest of which (8 x 10 6 metric tons) is the deposit of Lasail. Detailed field mapping of the lavas and shallow intrusions around this deposit in conjunction with petrographic studies of their metamorphic assemblages enables the ore-forming hydrothermal activity to be related to the location and evolution of an underlying magma chamber. It can be demonstrated that the deposit formed in the short time interval between the eruption of a lava unit (the Geotimes unit) at a marginal ocean-basin spreading axis and the eruption of a second lava unit (the Lasail unit) at an off-axis (or arc) seamount. Moreover, it was the seamount, not the spreading axis, magma chamber which drove the ore-forming hydrothermal convection, providing both a near-surface heat source and, by virtue of its intrusive pressure, a localized fracture system on its flanks. Subsequent eruptions from this same magma chamber then provided a protective capping for the deposit. The orientations and distributions of andesitic cone sheets show that the magma chamber lay some 1.5 km from the deposit itself and was in fact a magma cupola developed on the flanks of a much larger magma chamber.Similar, though less detailed, studies of other deposits in Oman indicate that: only very small mound deposits lie within the spreading axis lavas themselves; the majority of deposits lie at the boundary between the spreading axis and seamount lavas; and another group of deposits is related to the magma chambers that fed a third submarine lava unit (the Alley unit), also associated with the construction of the seamounts. These results may provide useful constraints for future mineral exploration programs in the Oman and perhaps also in other ophiolite complexes and on the ocean floor.

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