In southern Aosta Valley, magnetite ore deposits occur within serpentinized ultramafics associated with the Piedmont metaophiolite nappe. The ultramafics mostly consist of lherzolite with minor dunite bodies, and underwent high pressure (eclogite facies) metamorphism, partially re-equilibrated under blueschist to greenschist facies conditions. Within the ultramafics, the ore occurs in form of massive, disseminated, pod-like and stringer bodies, ranging from hundred meters to decameters in size. Microscopic observations and microprobe analyses show the sporadical presence, within the magnetite grains, of relict cores ranging in composition from chromite to Cr-rich magnetite. It is suggested that the magnetite ore bodies derive from old (podiform) chromitite bodies, generated during the emplacement of a refractory mantle portion into an oceanic accretion environment. Transformation of chromite into magnetite occurred during the subsequent subduction that was responsible also for the high pressure metamorphic overprint.
The magnetite ore deposits of the Southern Aosta Valley: chromitite transformed during an alpine metamorphic event
ROSSETTI, Piergiorgio
1994-01-01
Abstract
In southern Aosta Valley, magnetite ore deposits occur within serpentinized ultramafics associated with the Piedmont metaophiolite nappe. The ultramafics mostly consist of lherzolite with minor dunite bodies, and underwent high pressure (eclogite facies) metamorphism, partially re-equilibrated under blueschist to greenschist facies conditions. Within the ultramafics, the ore occurs in form of massive, disseminated, pod-like and stringer bodies, ranging from hundred meters to decameters in size. Microscopic observations and microprobe analyses show the sporadical presence, within the magnetite grains, of relict cores ranging in composition from chromite to Cr-rich magnetite. It is suggested that the magnetite ore bodies derive from old (podiform) chromitite bodies, generated during the emplacement of a refractory mantle portion into an oceanic accretion environment. Transformation of chromite into magnetite occurred during the subsequent subduction that was responsible also for the high pressure metamorphic overprint.File | Dimensione | Formato | |
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Rossetti et al 2009 - Magnetite ores.pdf
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