Geological setting and structural styles of Volcanic Massive Sulfi de deposits in the northern Apennines (Italy): evidence for seafl oor and sub-seafl oor hydrothermal activity in unconventional ophiolites of the Mesozoic Tethys

Resumen

This paper is an
overview of the geological setting and structural styles of Cu-sulfi
de deposits of the VMS-type, associated with Tethyan ophiolites (the
Ligurides) in the northern Apennines (Italy). The Italian deposits
represent a rare example of VMS associated with both the
mantle-peridotite basement and the overlaying volcanic pile, within
a single ophiolite sequence. This peculiar feature is due to the
particular geodynamic evolution of the Mesozoic Ligurian ocean that
allowed the upper mantle to be exposed on the seafl oor for a long
period before the outfl ow of MORB-type lava and the deposition of
pelagic sediments (cherts, Palombini shales and Calpionella
limestones). Middle to Upper Jurassic uprising of the basaltic magma
initially provided the heath source for convective circulation of
hydrothermal fl uids through the overlaying mantle section, giving
rise to sulfi de deposition in crosscutting stockworkveins and seafl
oor-stratiform ore bodies within the serpentinized mantle peridotite
and the serpentinite breccia formed by submarine erosion of the
upper mantle. The setting and structure of VMS associated with the
volcanic pile indicate that hydrothermal activity continued during
and after the eruption of pillow basalts at the ocean fl oor,
forming stockwork veins and conformable stratabound ore bodies
within the basalt unit. Furthermore, hydrothermal activity formed
seafl oor-stratiform deposits, at the top of the volcanic pile,
covered with a thick horizon of cherts containing exhalative
deposits of Mn.