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The Cogne magnetite deposit (Western Alps, Italy): a Jurassic seafloor ultramafic-hosted hydrothermal system?
Full text linkThe Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18 ∙ 10^6 tons at 45-50 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular ores (>1) is observed. Trace element analyses of magnetite from different sites and lithologies by laser-ablation inductively-coupled mass spectrometry indicate that magnetites have typically hydrothermal compositions, characterized by high Mg and Mn (median values up to ~24100 and ~5000 ppm, respectively), and low Cr, Ti and V (median values up to ~30, ~570 and ~60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that has hydrothermal fluid-controlled composition [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni ± Ti ± V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater–peridotite or seawater–Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent
the deep segment of a seafloor, high-temperature (~300–400°C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields
The Cogne magnetite deposit (Western Alps, Italy): a Late Jurassic seafloor ultramafic-hosted hydrothermal system?
Full text linkThe Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18 ∙ 10^6 tons at 45-50 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular ores (>1) is observed. Trace element analyses of magnetite from different sites and lithologies by laser ablation inductively coupled plasma mass spectrometry indicate that magnetites have typically hydrothermal compositions, characterized by high Mg and Mn (median values up to ~24100 and ~5000 ppm, respectively), and low Cr, Ti and V (median values up to ~30, ~570 and ~60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that has hydrothermal fluid-controlled composition [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni ± Ti ± V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater–peridotite or seawater–Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent the deep segment of a seafloor, high-temperature (~300–400°C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields.Il giacimento di magnetite di Cogne (Alpi Occidentali, Italia) è il più grande tra le mineralizzazioni a magnetite prive di apatite e solfuri che sono ospitate nelle serpentiniti appartenenti alle unità ofiolitiche delle Alpi Occidentali. La magnetite di composizione prossima al termine puro, insolita in rocce ultramafiche, e il tonnellaggio significativo del giacimento (18 ∙ 10^6 tonnellate con concentrazioni di Fe del 45-50% in peso), rendono Cogne un interessante caso studio per indagare i processi responsabili della formazione di magnetite nelle ofioliti. La magnetite di Cogne si presenta come minerale nodulare, vene e disseminazioni in serpentiniti derivanti da peridotiti di mantello e in peridotiti impregnate da fuso (troctoliti) totalmente serpentinizzate. Il rapporto Co/Ni aumenta a partire dalle serpentiniti povere in magnetite (0.05) fino al minerale nodulare (>1). L'analisi degli elementi in traccia nella magnetite proveniente da differenti siti e litologie, ottenuta tramite laser ablation inductively coupled plasma mass spectrometry, indica che la magnetite ha una composizione tipicamente idrotermale, caratterizzata da alte concentrazioni di Mg e Mn (valori mediani fino a ~24100 e ~5000 ppm, rispettivamente) e bassi Cr, Ti e V (valori mediani fino a ~30, ~570 e ~60 ppm, rispettivamente). Inoltre, le variazioni nel contenuto di elementi in traccia distinguono la magnetite che ha una composizione controllata dal fluido idrotermale [alti rapporti (Mg, Mn, Co, Zn)/Ni] dalla magnetite la cui composizione risente della geochimica della roccia incassante (alti Ni ± Ti ± V). La datazione radiometrica con il metodo U-Th-Pb dell'uraninite associata alla magnetite vincola l'età della formazione del giacimento al Giurassico superiore (circa 150 Ma), durante uno stadio avanzato dell'apertura della Tetide alpina. La modellazione termodinamica delle interazioni fluido-roccia indica che i fluidi risultanti dalle reazioni acqua marina-peridotite e acqua marina-Fe-gabbro non sono sufficientemente ricchi in Fe per generare il giacimento di Cogne. Ciò suggerisce che processi di frazionamento, come la separazione di fase, furono di cruciale importanza per produrre fluidi idrotermali in grado di precipitare grandi quantità di magnetite in varie tipologie di rocce incassanti ultramafiche. Il contesto oceanico e le somiglianze geochimiche e mineralogiche con alcuni moderni depositi vucanogenici a solfuri massivi di dorsale oceanica ospitati in ultramafiti suggeriscono che la sezione mineralizzata di Cogne possa rappresentare il segmento profondo di un sistema idrotermale di fondale oceanico di alta temperatura (~300-400°C). La presenza di analoghe concentrazioni di magnetite nella litosfera oceanica attuale potrebbe contribuire a spiegare l'esistenza di significative anomalie magnetiche situate in corrispondenza di sistemi idrotermali idrotermali sia attivi che inattivi impostati su rocce ultramafiche
Ancient extractive metallurgy of copper in the Aosta Valley (Western Alps, Italy): new evidence from pre-Roman slags from the Misérègne site
No full textMany metallurgical sites are scattered across the Aosta Valley territory (e.g. Tumiati et al., 2005). The age of some
of them is unknown and often the slag heaps are the sole vestiges of metallurgical activities. Such is the case of
Miséregne (Fénis), where the village is built on an enormous slag deposit.
We performed radiocarbon dating on charcoal fragments included in the slags and obtained ages comprised between
the 4th and 1st century BC. So far, these are the most ancient ages for early mining activity in the Aosta Valley, probably
carried out by the local population of the Salassi.
We studied the slags following a petrologic approach. The occurrence of matte inclusions in the slags indicates that
the processed raw minerals were Cu-Fe sulfides. Relying on morphologic and micro-textural features, we classified the
slags into three categories, i.e., coarse, massive and flat, as suggested by Addis (2013). In all the slag classes the most
abundant phase is olivine. Other common phases are spinel group minerals and sulfides, mainly pyrrhotite and bornite
solid solutions. The olivine crystal shapes allowed us to qualitatively estimate the degree of undercooling (ΔT) and the
rate of cooling of the slags: the coarse slags show a high ΔT; the massive slags seem to record an initial, long-lasting
low-ΔT stage, followed by an increase in the cooling rate; the flat slags underwent a brief low-ΔT stage, followed by a
sudden marked increase in ΔT. Combining this information with the slag bulk chemistry, the sulfide compositions, the
slag morphology and the presence of inclusions of charcoal, quartz and other slag fragments, we hypothesize that the
massive and coarse slags come from the lower and, respectively, the upper portion of the same slag contained in the
furnace, while the flat ones were tapped slags. Minimum furnace working temperatures estimated by means of olivinespinel
geothermometry are in the ranges 932-964°C, 968-1037°C, 1202-1239°C for the coarse, massive and flat slags,
respectively. According to the above observations, the flat slags should record conditions that are the closest to the
actual furnace working temperature
The Lovignanaz Cu-Fe sulphide mine: safeguard of a pre-Roman mining site
No full textThe antique Lovignanaz mine (also known as Molina mine) is located in the western side of the Clavalité valley (1350-1425 m a.s.l.), to the South of the Fénis village, in the southern side of the Aosta Valley (Italy). The mine galleries are developed within chloriteschists, talcschists and metagabbros transposed with calcschists belonging to the Piedmont nappe (Zermatt-Saas zone). The rocks hosting the Fe-Cu sulphide mineralization show blueschist to eclogite facies mineral associations like those studied in Saint-Marcel rocks (Martin et al., 2008, and ref. therein). The mineralization is characterized by disseminated chalcopyrite and pyrite associated with minor ilmenite, rutile, pyrrhotite and magnetite (Fénis. Une communauté au fil de l'histoire, 2000 and ref. therein).
The oldest galleries were dug with techniques described by Agricola in De Re Metallica (1556) and have been attributed to the Romans. These techniques consist in excavation along mineralized layers after “fire-setting”, that is after weakening the rocks with a fire (De Re Metallica, book V).
Other galleries have been excavated with more recent techniques, revealing the use of explosives, and have been attributed to XVIII-XIX centuries mining activity (Gerbore E. E., in: Fénis..., op. cit.).
The remnants of a furnace for rock-roasting have been found along the right side of the Clavalité river (lat. N 45° 41’ 46’’; long. E 7° 29’ 51’’). In the same site, slags with charcoal and red burnt soil, probably related to an old activity, were observed during the field work. Several glassy slags, maybe produced during XVIII-XIX centuries mining activity, were found below a large landslide that partly covers the mine adits.
Near the oldest mine entrances traces of old working instrument have been found.
These mines were also used to extract millstones: few metres from the mine entrances in the Mouilé locality (Fénis..., op. cit.), some of these stones are still unexcavated on the rock walls.
In the lower part of the valley, in correspondence of Miserègne village (Fénis), there is a dump where many large slags with charcoal fragments are piled up. Some of these fragments were analyzed with 14C method and one of them has been dated to IV century b.C.. This age is the oldest found for charcoal associated to slags of the Cu mines in the Aosta Valley. Other slags from the Saint Marcel valley yielded early medieval ages (Tumiati et al., 2005).
The Lovignanaz mining site has been destroyed by landslide events, the last one having occurred in 2000, which partly covered mine entrances and other structures linked to mining activity. The presence of structural lineaments and the practice of “fire-setting” during mining activity may have contributed to trigger the landslide.
References:
AAVV, 2000, Fénis. Une communauté au fil de l'histoire., Musumeci Editore;
Agricola [Bauer G.], 1556, De Re Metallica, ed. 1950 by Hoover H. C. and Hoover L. H., Dover Publications, New York, book V, 118-121;
Casartelli P., 2003, Archeometallurgia nella valle di Saint-Marcel (AO): studio preliminare delle scorie di fusione della miniera di Servette, tesi di laurea, Univ. degli studi dell’Insubria;
Lorenzini C., 1995, Le antiche miniere della Valle d’Aosta, Musumeci Editore, 62-64;
Martin S., Rebay G., Kienast J.-R., Mével C., 2008, An eclogitised oceanic palaeo-hydrothermal field from the St. Marcel valley (Italian Western Alps), Ofioliti, 33 (1), 49-63;
Tumiati S., Casertelli P., Mambretti A., Martin S., 2005, The ancient mine of Servette (Saint-Marcel, Val d’Aosta, Western Italian Alps): a mineralogical, metallurgical and charcoal analysis of furnace slags, Archaeometry 47, 2, 317-34
The siderite mineralizations in the Southern Alps: a signal of the Permo-Triassic rifting?
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Eclogitic metatrondhjemites from metaophiolites of the Western Alps
Full text linkIn the Urtier valley (southern Aosta Valley, Italy), the Piemonte metaophiolites mainly consist of serpentinized peridotites including pods and boudinaged layers of Fe-metagabbro and trondhjemite transposed in the main eclogitic foliation. The contact between serpentinized peridotites and Fe-metagabbro/trondhjemite is locally lined by chloriteschist and rodingite. The high pressure parageneses in the Fe-metagabbro are omphacite-garnet-rutile-glaucophane-phengite, and in the metatrondhjemite plagioclase-quartz-phengite-clinozoisite-epidote-garnet, respectively. Bulk-rock major and trace elements in addition to O isotope analyses were performed in both rock types. Fe-metagabbros are characterized by MgO wt% ranging between 6.11 and 9.63%, EREE= 20-101 ppm, (La/Yb)N = 0.22-0.91; trondhjemites have SiO2 43%, Al2O3 ranging between 21 and 24%, CaO ranging between 17 and 20%, EREE = 172 - 272 ppm, (La/Yb)N ranging between 7.78 and 13.70. The 18O is 5.9 per-mil in a Fe-metagabbro sample and 7.4 per-mil in a trondhjemite sample, suggesting that these rocks have been affected by a weak oceanic low temperature alteration. The high CaO content may indicate a metasomatic process which could have occurred during the oceanic stage or at high pressure conditions
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