Abstract

It is difficult to link the formation of volcanic massive sulfide orebodies, which are at or very close to the sea floor on spreading ridges, to their preservation at depth in oceanic crust. The relationship of volcanic massive sulfide orebodies to other features of crustal construction in the Troodos ophiolite (Cyprus) suggests that a mechanism for preservation required burial to an optimum depth interval during the completion of crustal construction. Incomplete burial may result in dissolution on the sea floor or from downward-permeating sea-water. Excessive burial leads to mechanical fragmentation by diking together with the dispersal of metals and sulfur through the action of high-temperature hydrothermal fluids. In a study involving the location of orebodies with respect to contours of increase of dike density below the volcanic surface, it is found that the optimum depth interval for preservation is on average 0.5 + or - 0.2 km (1Sigma ; range 0.2-0.8 km) beneath the volcanic surface, about 0.1 km in thickness, and everywhere located about 0.1 km above the 25 percent dike density surface.

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