Abstract

Copper-iron sulfide mineralizations were found in metabasalts dredged from offsets of the axis of the Mid-Atlantic Ridge in the Romanche and Vema fracture zones. The basalts were probably originally emplaced at ridge axis. The Cu-Fe sulfides form "disseminated"- and "stockwork"-type deposits. Chalcopyrite is the main sulfide phase; pyrite and pyrrhotite are also present. Fe-hydroxides, containing as much as 13 percent Cu, are a common alteration product of the chalcopyrite; atacamite (Cu 2 Cl(OH) 3 ), delafossite (FeCuO 2 ) or tenorite (CuO), and probably mooihoekite (Cu 9 Fe 9 S 16 ) or haycockite (Cu 4 Fe 5 S 8 ) were also observed as secondary phases. These findings are explained by a model of metallogenesis which shows that metal sulfide deposits are formed within the oceanic crust at spreading centers by deposition from hydrothermal systems and that, upon discharge on the sea floor, they give rise to metalliferous sediments. The main source of metals for the hydrothermal systems is the basaltic oceanic crust, though for sulfur reduction of seawater SO 4 (super -2) is also important. Sulfide deposition below the sea floor causes fractionation of metals, particularly of Fe from Mn. These findings support the hypothesis that "massive" metal sulfide deposits are present in the oceanic crust and that "massive" sulfide ores of ophiolite complexes were formed in ancient spreading centers.

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