First finding of Cr-rich magnetite from Liconi mine (Cogne) - Geological and mineralogical features
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Data di Pubblicazione:
2009
Citazione:
First finding of Cr-rich magnetite from Liconi mine (Cogne) - Geological and mineralogical features / S. Martin, S. Carbonin, S. Tumiati. ((Intervento presentato al 9. convegno Workshop on Alpine Geological Studies tenutosi a Cogne nel 2009.
Abstract:
Recent analyses of magnetite from the Cogne mining area have shown a relevant Cr
content (Cr2O3 up to ~12 wt%). These Cr-rich magnetites were found in serpentinitic rocks
from the Liconi mine at 2504 m asl (Monte Creya), near Cogne. Liconi is the oldest mining
site in this area and has been exploited since 1400 A.D. The Liconi magnetite-rich
serpentinite is the largest and longest mineralized lens in the Cogne mining area (up to 150 m
in thickness and 300 m in length, Stella, 1916). The mineralized serpentinite is transposed
with metadolomite, marble and calcschists (Elter, 1960; 1971; 1987) at the northern boundary
of the Gran Paradiso and Grivola ophiolite units.
The magnetite-rich serpentinite and the associated rocks suffered eclogite alpine
metamorphism, as indicated by the presence of antigorite in the serpentinite, of lawsonite in
the calcschists, and of other high-T minerals, such as vesuvianite, andradite, grossularite,
clinozoisite and diopside, in magnetite-bearing rodingites. The eclogitic mineral associations
overprinted serpentine, chlorite, zeolites and epidote, which were related to oceanic
metamorphism. Magnetite occurs as: (i) micrometric (3-50 mm), idiomorphic, disseminated,
Cr-bearing crystals in serpentine matrix, with phantoms of primary silicate minerals; (ii)
lenses and bands of Cr-free, very fine aggregates in serpentine matrix; (iii) Cr-free lenses
within calcschists and ophicalcites; (iv) Cr-free elongated crystals in a whitish matrix
composed of Ca-rich rodingite minerals.
The typical Cogne magnetite is Cr-free (Stella, 1921; Compagnoni et al., 1979; Diella
et al., 1994), with Fe2O3 + FeO up to 96-98 wt% and MgO and MnO in small but significant
concentrations. Recent SEM and X-ray powder diffraction analyses carried out at the Val
d’Aosta ARPA laboratory (Martin et al., 2005) confirmed a Cr-free composition for magnetite
in rodingites (e.g. our sample Co14), carbonate rocks and mylonitic serpentinites, whereas
they indicated the presence of Cr in the disseminated magnetite in serpentinites (e.g. our
sample Co15). The Cr-rich disseminated magnetite is associated with Cr-enriched serpentine
minerals and, sometimes, chlorite, and with minor pyrite, pyrrhotite and pentlandite.
Quantitative electron microprobe chemical analyses of the Liconi magnetites were
carried out at the IGG, CNR, Padova. The composition of magnetite in sample Co15 is as
follows: Cr2O3: 0 to ~12 wt%; MnO: 0.2 to 3 wt%; MgO: 1 to 3 wt%; ZnO: 0 to 0.7 wt%.
Cr2O3 shows a strong positive correlation with MnO and ZnO, a weak positive correlation
with MgO, a clear negative correlation with FeO and no evident correlation with TiO2. In
sample Co14 and in the mylonitic serpentinites, the magnetite compositions are Cr-free and
very similar to those reported in the literature for the Cogne ore mineral. The evolution from
Cr-rich to almost pure magnetite involves substitution of Cr by Fe3+, and substitution of Mg
and Mn by Fe2+. Compared with analogous magnetites from the Mt. Avic massif within the
Piemonte ophiolite nappe (Diella et al., 1994; Fontana et al., 2008), the most Cr-poor among
Cogne magnetites show higher MgO (2.5 wt% vs. 0.4 wt%) and lower TiO2 contents (<0.1 wt
% vs. >0.1 wt%).
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
S. Martin, S. Carbonin, S. Tumiati
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