Abstract

Northern Apennines ophiolite complexes, consisting of peridotite-gabbro-basalt assemblages overlain by chert of Late Jurassic age, are probably fragments of oceanic crust created at a Mesozoic spreading center. Metalliferous sedimentary deposits are found at the base of the chert formation, close to the basalt-chert contact. The main mineral component of these deposits is braunite; the deposits are rich in Mn and contain less than 1 percent Fe; no equivalent Fe-rich sedimentary rocks have been observed. The geochemistry of the minor transition metals, of the rare-earth elements, Ba, U, and Th in these deposits, as well as their stratigraphic position, indicate that they are not similar to “hydrogenous” ferromanganese deposits from modern oceans; they instead show affinities to metalliferous deposits of hydrothermal origin associated with modern spreading centers. Fe-Cu-Zn-sulfide deposits are common in basalt of the Apennine ophiolite and are often close to the metalliferous sedimentary rocks. The metalliferous sedimentary rocks and the metal sulfide mineralizations probably originated as a result of the mobilization of metals from basalt during circulation of thermal waters in the basaltic-gabbroic crust close to a Mesozoic spreading center. Upper mantle volatiles may have provided additional metals to the thermal waters. The sulfide deposits formed during the subbottom convective circulation, whereas the manganiferous sediment originated from precipitation of the metals after discharge of the thermal solutions through the sea floor. Extensive metal fractionation may have occurred during the sub-bottom circulation, especially during deposition of the sulfide phases; the lack of Fe in the metalliferous sedimentary rocks may be due to such fractionation. This model can be applied to associations of sulfide ore and metalliferous sedimentary deposits in other ophiolitic complexes of various ages and to metallogenesis in modern spreading centers.

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