『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 sufide
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 subbottom 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.』
Introduction
Northern Apennine ophiolite complex
Geologic setting of the manganese deposits in the Apennine ophiolite
complexes
Manganese deposits
Mineralogy of the manganese deposits
Chemistry of the manganese deposits
Iron, manganese, and silicon
Minor metals and barium
Uranium and thorium
Rare-earth elements
Formation of the Apennine Mn deposits at a spreading center
Absence of iron-rich sediments in the Apennine deposits
Metal sulfide deposits in the Apennine ophiolites
Summary of a model of metallogenesis for the Apennine ophiolites
Acknowledgments
References cited