This work provides the first detailed mineralogical-petrographical characterization of the massive sulphide deposit of Colle della Borra (Val Soana, Western Alps). The massive sulphide deposit is hosted within an ophiolitic sequence composed of calcschists with interlayered metabasites, micaschists, chloriteschists, quartzites and marbles, metamorphosed under eclogitic facies conditions (MODESTI, 2021). In particular, the ore deposit is interposed between Grt-Cld micaschists in the foot-wall and chloriteschists in the hanging-wall. The ore mineral association mainly comprises pyrite, chalcopyrite and pyrrhotite, along with sphalerite; the proportion of the three main ore minerals vary considerably in different specimens. The gangue minerals are dominated chlorite, followed by quartz, albite, carbonate, epidote, white mica and garnet. Thanks to a detailed micro-textural and geochemical study, supported by spot electron microprobe analyses and elemental maps of some trace elements (Co, Ni, As), a primary seafloor, hydrothermal-volcanogenic origin of the mineralization is suggested, which occurred on the Ligurian Piedmont Ocean floor and was followed by subduction and exhumation during the Alpine orogenic cycle. The primary oceanic features were largely obliterated by subsequent metamorphic recrystallization. However, colloform textures and fine-grained aggregates (2-10 µm) were recognized in some pyrites, and are considered as evidence of a primary origin. Further evidence is provided by chemical zoning in a pyrite crystal. In particular, Co zoning allows to clearly identify two generations of pyrite: a primary one (PyI), at the core of the crystal, with concentrations of Co (0.15-0.29 wt%) typical of pyrites from seafloor massive sulfide deposits; a secondary one (PyII), overgrown on the first, with much higher Co concentrations (2.13-2.49 wt%), attributable to metamorphic processes. The textural characters of the two generations of pyrite also reflect the metamorphic history: the anhedral morphology of PyI testifies the corrosion that occurred at some point during the prograde path, while the overgrowth of PyII on PyI, reflects the recrystallization processes that occurred during the subsequent retrograde/exhumation phase.

Caratterizzazione minero-petrografica delle mineralizzazioni a solfuri massivi nella zona piemontese del Colle della Borra (Val Soana, Alpi Occidentali)

Gosio, Francesco
2021/2022

Abstract

This work provides the first detailed mineralogical-petrographical characterization of the massive sulphide deposit of Colle della Borra (Val Soana, Western Alps). The massive sulphide deposit is hosted within an ophiolitic sequence composed of calcschists with interlayered metabasites, micaschists, chloriteschists, quartzites and marbles, metamorphosed under eclogitic facies conditions (MODESTI, 2021). In particular, the ore deposit is interposed between Grt-Cld micaschists in the foot-wall and chloriteschists in the hanging-wall. The ore mineral association mainly comprises pyrite, chalcopyrite and pyrrhotite, along with sphalerite; the proportion of the three main ore minerals vary considerably in different specimens. The gangue minerals are dominated chlorite, followed by quartz, albite, carbonate, epidote, white mica and garnet. Thanks to a detailed micro-textural and geochemical study, supported by spot electron microprobe analyses and elemental maps of some trace elements (Co, Ni, As), a primary seafloor, hydrothermal-volcanogenic origin of the mineralization is suggested, which occurred on the Ligurian Piedmont Ocean floor and was followed by subduction and exhumation during the Alpine orogenic cycle. The primary oceanic features were largely obliterated by subsequent metamorphic recrystallization. However, colloform textures and fine-grained aggregates (2-10 µm) were recognized in some pyrites, and are considered as evidence of a primary origin. Further evidence is provided by chemical zoning in a pyrite crystal. In particular, Co zoning allows to clearly identify two generations of pyrite: a primary one (PyI), at the core of the crystal, with concentrations of Co (0.15-0.29 wt%) typical of pyrites from seafloor massive sulfide deposits; a secondary one (PyII), overgrown on the first, with much higher Co concentrations (2.13-2.49 wt%), attributable to metamorphic processes. The textural characters of the two generations of pyrite also reflect the metamorphic history: the anhedral morphology of PyI testifies the corrosion that occurred at some point during the prograde path, while the overgrowth of PyII on PyI, reflects the recrystallization processes that occurred during the subsequent retrograde/exhumation phase.
2021-07-21
120
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12608/21359