The Cogne serpentinite-hosted magnetite ore: a subseafloor...

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015

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The Cogne serpentinite-hosted magnetite ore: a subseafloor hydrothermal deposit?

Ph.D. candidate: Luca Toffolo, II course Tutor: Prof.ssa Silvana Martin,

Co-Tutor: Prof. P. Nimis Cycle: XXIX

Abstract

The Cogne magnetite deposit (NW Italian Alps) is hosted by a serpentinite unit, which is part of the Piemonte ophiolite nappe. Considering the tonnage (1.9 · 106 tons at 37% Fe average grade) and the peculiar chemical composition of magnetite (Cr, Ti, V in traces), that of Cogne seems to be a unique occurrence worldwide. In order to decipher the ore-forming processes, I dedicated the second year of my PhD to the analyses of rocks and minerals, exploring the possibility that Cogne is a fossil, metamorphosed, subseafloor hydrothermal system. This hypothesis is supported by the enrichment of fluid-mobile elements (Co, Mn, Mg, Zn) compared to immobile elements (Ti, Cr, V) in magnetite together with the geochemical fingerprint of associated pure diopside. As next step, I will further investigate the linking with seafloor hydrothermal systems, by performing new analyses (δ56Fe in magnetite and U/Pb dating on uraninite) and thermodynamic modelling. Full Report

1. Introduction The Cogne magnetite deposit (1.9 · 106 tons at 37% Fe average grade) is hosted by a 2.5 km long fully serpentinized peridotite sliver, which is part of the Piemonte ophiolite nappe (Dal Piaz et al., 2010), northwestern Italian Alps. Here, magnetite has compositions close to the pure endmember and is characterized by only trace amounts of Cr, Ti and V. This is an unusual composition for ultramafic-hosted magnetite, and together with its high modal amount, to my knowledge, makes that of Cogne a unique occurrence worldwide. Several authors debated the origin of Cogne magnetite (Stella, 1921; Di Colbertaldo et al., 1967; Compagnoni et al., 1979, 1981; Zucchetti et al., 1988; Diella et al. 1994; Rossetti et al., 2009; Carbonin et al., 2014), however the geological processes that formed it have not been completely understood yet. I dedicated the second year of my PhD to the analyses of rocks and minerals aiming at reconstructing the sequence of the geological events that affected the Cogne serpentinite. Moreover, based on these new data, I explored the possibility that Cogne is a fossil subseafloor hydrothermal field. 2. Materials and methods 68 thin sections from the Cogne mine area were studied petrographically, employing micro-Raman (Chemistry Department, University of Padova) in support of mineral identification; serpentine minerals were identified following the indications of Groppo et al. (2006) and the observations of Carbonin et al. (2014). Selected samples were investigated with scanning electron microscope (SEM; Geosciences Department, University of Padova) and mineral compositions were determined by electron microprobe (EPMA; Geosciences Department, University of Padova). 16 samples, representative of typical lithologies, were analyzed by X-ray fluorescence (XRF; Geosciences Department, University of Padova), determining major, minor and some trace elements. Moreover, trace elements in magnetite and diopside were measured by means of laser ablation inductively coupled mass spectrometer (LA-ICP-MS; Geosciences Department, University of Bremen, Germany); data reduction was performed with GeoPro™ software. 3. Results 3.1 Petrographic features of the ore


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The Cogne magnetite ore outcrops in three sites (Fig. 1), where it shows different structures and mineral assemblages: 1) Liconi mine; 2) Western slope of Montzalet; 3) Larsinaz mine.

Fig. 1 3D model of the Cogne mine area, obtained with ArcGISTM software. Numbers indicate the sampling sites. Lithologies after Dal Piaz et al. (2010).

At site 1, the magnetite ore is in contact with hanging wall magnetite-poor (3-6 vol%) serpentinised harzburgites, characterized by typical lizardite + antigorite ± talc pseudomorphs on former peridotite silicate minerals (i.e. mesh and bastite textures) and relics of accessory Cr-spinel (Cr#: 0.53; Mg#: 0.58; 0.03 wt% TiO2). Based on its structure, magnetite ore can be: - massive (~90 vol%); - “leopard” (~50 vol%), if the rock is speckled with up to 1 cm rounded aggregates of crystals (resembling a nodular chromitite); “pegmatoid”, when the overall structure mimics that of a coarse-grained intrusive rock. Gangue minerals are, in order of decreasing abundance: antigorite (Mg#: 0.95-0.99; 0.07-2.4 wt% Al2O3; Fig. 2B), which always shows indented crystal boundaries with magnetite; olivine (Fo98), that replaces antigorite, seldom as pseudomorphs; lizardite (Mg#: 0.94-0.98; 4.1-6.3 wt% Al2O3; Fig. 2A); brucite (Mg#: 1); clinochlore (Mg#: 0.95; 15.5 wt% Al2O3; Fig. 2C); carbonates (calcite, magnesite, dolomite); Ti-clinohumite. Magnetite is always rich in clinochlore (Mg#: 0.95-97; 10.9-19.1 wt% Al2O3; Fig. 2C) inclusions, sometimes crystallographically oriented, which can be associated with brucite or calcite, rare Lz, very rare uraninite and apatite. Rare lithologies found in the serpentinite-magnetite ore sequence are: tremolitites, nematoblastic rocks composed of tremolite + pyrite + talc ± magnetite; diopsidites, made up of diopside (Mg#: 0.98; Cr2O3 and TiO2 below or near the detection limit of EPMA) + magnetite + antigorite + andradite + calcite + chlorite. In the latter rocks antigorite and andradite replace diopside (+ magnetite?) by a retrograde reaction. In addition to this, hanging wall serpentinites are separated from the calcschists sequence above by a thin rodingite layer (less than 3 m thickness), which can have the following mineral assemblages: diopside + antigorite + chlorite; grossular + diopside + epidote + calcite + pyrite.


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At site 2 (never described before), magnetite occurs associated either with chalcopyrite as stockwork-like veins crosscutting antigorite-serpentinites or with acicular crystals of pure diopside as impregnations (Mt up to 45 vol%; Di 10 vol%) in serpentinite. In the former case the veins, dismembered during alpine orogenesis, are hosted in non-pseudomorphic low-Cr (~80 ppm) antigorite-serpentinite; in the latter situation the serpentinite is pseudormofic after harzburgite and preserves Cr-spinel relics (Cr#: 0.56; Mg#: 0.58; 0.10 wt% TiO2) and bastites included in magnetite regions, meaning that the rock was not fully serpentinized when magnetite formed.

At site 3, magnetite ore can assume massive and “leopard” structures. Gangue minerals are antigorite (Mg#: 0.94-0.97; 0.4-3.4 wt% Al2O3; Fig. 2B), pure diopside (Mg#: 0.97; 0.13 wt% MnO; Cr2O3 and TiO2 below the detection limit of EPMA), minor clinochlore (Mg#: 0.93-0.96 11.7-18.1 wt% Al2O3; Fig. 2C) and lizardite (Mg#: 0.93-0.95; 5.6-7.6 wt% Al2O3; Fig. 2A). Diopside often forms impregnations or veins in antigorite-serpentinite. In diopside-rich regions magnetite is sub-/euhedral, while it is anhedral when the matrix is composed of antigorite. Textural relations indicate that diopside formed during a late stage of magnetite growth. Moreover, diopside crystallized in static conditions since it forms radiating aggregates of crystals. In addition to this, it can form veins (probably pre-orogenic), in which it is associated with lizardite or minor antigorite ± chlorite. As in site 1, here magnetite is rich in clinochlore inclusions (Mg#: 0.93-0.96; 10.6-19.8 wt% Al2O3; Fig. 2C), which can be associated with rare andradite and very rare diopside and uraninite. Another peculiarity of site 3 is the presence of pseudomorphic rocks after a mafic magmatic protolith, likely a troctolite. In these rocks amoeboid cm-sized pseudomorphs after plagioclase composed of Al-rich lizardite (5.6-7.6 wt% Al2O3; Fig. 2A) + clinochlore (Fig. 2C) + euhedral antigorite + euhedral magnetite are intergrowth with coarse-grained interlocking antigorite + anhedral magnetite regions, replacing former olivine. Coronas of fine-grained interlocking antigorite surround the ex-plagioclase areas. Antigorite have similar compositions throughout the rock and is Al-poor (< 2.1 wt% Al2O3; Fig. 2B). Moreover, these rocks show variable amounts of magnetite that is concentrated in lizardite-rich regions. In general, lizardite has > 4 wt% Al2O3 (with maximum values in pseudomorphic serpentinite after troctolite) and Mg# comprised between 0.93 and 0.98, while antigorite has generally < 3.9 wt% Al2O3 and Mg# varying from 0.94 to 0.99 (Fig. 2D). Moreover, Mg# and Al (and Si and Al) in antigorite show an inverse correlation (Fig. 2B) probably due to Tschermak’s substitution (AlAlMg–1Si–1). Clinochlore shows a wide variation in Al2O3 content (10-20 wt%; Fig. 2D) and Mg# ranging from 0.92 to 0.97. Serpentine minerals as well as clinochlore that compose the gangue of the ore do not contain Cr (≤ detection limit of EPMA), in contrast to what is observed in serpentinites after peridotites, where Cr2O3 can be as high as 0.2 wt%, 0.9 wt% and 2.2 wt% for lizardite, antigorite and clinochlore, respectively. All these phases, moreover, have highest Mg# in Site 1 and highest Al content in site 3. Clinochlore included in magnetite in both site 1 and 3 has overlapping compositions. Magnetite and diopside geochemical data will be presented in the following sections.

The observation that lizardite is always Al-rich can be explained as a result of prograde metamorphism: as shown by experimental data of Caruso and Chernosky (1979), Al stabilizes lizardite to high temperatures, therefore Al-poor lizardite could have been completely replaced by antigorite. The presence of neoblastic forsterite is an evidence for high temperature metamorphism as well, and, since Al has an analogue effect on the stability of antigorite to that it has for lizardite (Bromiley and Pawley, 2003), this may explain why forsterite has been observed only in the ore of site 1, where antigorite has lowest Al content. Prograde metamorphism can be also indicated by the antigorite-magnetite indented crystal boundaries. In fact, Debret et al. (2014) described a similar texture in serpentinites from the Western Alps and they explained this as a result of magnetite dissolution accompanying lizardite to antigorite transition, being antigorite Fe-richer then lizardite. However, compared to those samples, at Cogne antigorite has lower FeOtot content. Moreover, antigorite has lower FeOtot than the lizardite it directly replaces. Therefore, another reaction, which imply opposite redox conditions of those needed for magnetite dissolution according to Debret et al. (2014), can account for these observations:


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0.96 Fe2+3Si2O5(OH)4 (lizardite) + 0.05 O2 + 0.08 SiO2 = Fe2+

2.85Si2O5(OH)3.7 (antigorite) + 0.01 Fe3O4 + 0.07 H2O (O’Hanley and Dyar, 1993)

This reaction may also account for the highest Mg# observed in antigorite. The SiO2 source may be internal, i.e. high temperature serpentinization of relic mantle pyroxene, or external, namely hydrothermal fluids. Si-rich hydrothermal fluids can be either produced by dehydration reactions in subduction zones (Manning, 1995) or found in the upwelling/discharge zones of mid-ocean ridge hydrothermal systems (Alt, 1995). For all these reasons, it is difficult to assign to antigorite a precise setting. 3.2 Rock geochemistry Serpentinized peridotites have MgO/SiO2 vs. Al2O3/SiO2 (Fig. 3A), Co/Ni ratio (0.05-0.07; Fig. 3B) and Cr content (1600-2400 ppm) similar to those of abyssal peridotites (Niu, 2004; Paulick et al., 2006; Andreani et al., 2014). In particular, most of the compositions fall in the harzburgite field (Al2O3/SiO2 < 0.04; Andreani et al., 2014); relic spinel composition in those rocks is in agreement with a harzburgitic protolith, even though Takazawa et al. (2007) showed that similar compositions are typical of impregnated dunites, defined by the authors as “dunite with interstitial gabbroic material”. Magnetite ore, as well, has MgO/SiO2 vs. Al2O3/SiO2 similar to abyssal peridotites (Fig. 3A), but Co/Ni greater than 1 (with Coore > Coserpentinised peridotite and Niore << Ni serpentinised peridotite; Fig. 3B) and very low Cr content (< 80 ppm). The high MgO/SiO2 ratio of those rocks is due to both the higher Mg content in magnetite and to the presence of brucite in the gangue and as inclusion in magnetite In contrast to the above trends, diopside-rich metasomatic rocks have low MgO/SiO2 and Al2O3/SiO2 ratios, due to high SiO2 concentration, and generally preserve the geochemical fingerprint of the protolith (e.g. Cr concentration). One sample of diopside-rich magnetite ore from site 3 plots significantly farther from the trends described above because of the presence of high modal chlorite. The geochemical data of diopside will be treated in section 3.4.


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0.920

0.930

0.940

0.950

0.960

0.970

0.980

0.990

1.000

0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450

Mg#

Al (apfu)/7 oxygens

Lizardite

1. Magnetite ore matrix

1. Inclusion in magnetite

2. Pseud. serp. perid.

2. Bastite

2. Vein

3. Pseud. Serp. Perid.

3. Pseud. Serp. Troc.


3. Vein

0.920

0.930

0.940

0.950

0.960

0.970

0.980

0.990

1.000

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 0.180 0.200

Mg#

Al (apfu)/7 oxygens

Antigorite



1. Bastite

2. Bastite

2. Alteration of clinopyroxene

2. Vein



3. Bastite

3. Ex-plagioclase, pseud. serp. troc.

3. Ex-olivine, pseud. serp. troc.

3. Diopsidite

Fig. 2 Serpentine and chlorite compositions calculated from electron microprobe analyses. A) Lizardite; B) Antigorite; C) Chlorite; D) Comparison between the three phases. Numbers in the legends indicate the sampling sites (see Fig. 1). “Pseud. serp. perid./troc.”: pseudomorphic serpentinite after peridotite/troctolite.

0.92

0.925

0.93

0.935

0.94

0.945

0.95

0.955

0.96

0.965

0.97

2 2.5 3 3.5 4 4.5 5

Mg#

Al (apfu)/28 oxygens

Chlorite

1. Inclusion in Mt



3. Bastite

3. Ex-plagioclse, pseud. serp. troc.

3. Alteration of clinopyroxene

0.920

0.930

0.940

0.950

0.960

0.970

0.980

0.990

1.000

0 0.5 1 1.5 2 2.5

Mg#

Al (apfu)/14 oxygens

Lizardite

Antigorite

Chlorite

B

A C

C

D


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9.52

54.72

4.00

33.47

46.66

12.54

5.70

29.57 78.96

85.91

21.60

6.22

44.08

50.97 6.75

27.36

79.08 86.01 Terrestrial array

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

MgO

/SiO

2

Al2O3/SiO2


1. Magnetite ore

1. Diopside-rich rock


2. Magnetite + chalcopyrite ore

2. Diopside-rich Pseud serp. perid.


3. Pseud. serp. troc.

3. Magnetite ore


Magnetite ore - Chanté mine

Serpentinites, Rainbow massif - MAR (Andreani et al., 2014)

Abyssal peridotites, MAR - ODP Leg 209 (Paulick et al., 2006)

Talc alteration, MAR - ODP Leg 209 (Paulick et al., 2006)

Gabbro, MAR - ODP Leg 209 (Paulick et al., 2006)

Abyssal peridotites, Pacific and Indian Ocean Ridge-transform systems (Niu, 2004)

83.50

6.75

50.97

4.00 12.54

46.66 5.70

29.57

78.96

21.60

85.91

86.01

33.47

6.22

70.17

9.52

44.08

27.36

79.08

54.72

Co/Ni = 1

Co/Ni = 0.057

0

50

100

150

200

250

300

350

400

450

500

0 500 1000 1500 2000 2500 3000 3500

Co

(ppm

)

Ni (ppm)


1. Leopard ore

1. Diopdside-rich rock


2. Magnetite + chalcopyrite ore


3. Pseud. serp. troc.

3. Leopard ore


Massive ore; Chanté mine (Valle d'Aosta)

Troctolite, MAR (Casey, 1997)

Troctolite, MAR (Werner, 1997)

Troctolite, SW Ind. Ridge (Holm, 2002)

Serpentinites, Rainbow massif - MAR (Andreani et al., 2014)

Abyssal peridotites, MAR - ODP Leg 209 (Paulick et al., 2006)

Talc alteration, MAR - ODP Leg 209 (Paulick et al., 2006)

Gabbro, MAR - ODP Leg 209 (Paulick et al., 2006)

Diabase, MAR - ODP Leg 209 (Paulick et al., 2006)

Abyssal peridotites, Pacific and Indian Ocean Ridge-transform systems (Niu, 2004)

Fig. 3 Rocks from Cogne (numbers next to the markers indicate Fe2O3tot) compared to a variety of abyssal rocks (references in

the legend) in the planes: A) MgO/SiO2 vs. Al2O3/SiO2; B) Co vs. Ni. Numbers in the legends indicate the sampling sites (see Fig. 1). Data for Cogne rocks are also from Carbonin et al. (2014). “Terrestrial array” after Niu (2004).

A

B


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3.3 Magnetite geochemistry Magnetite is a nearly pure endmember. The main cations substituting for Fe are Mg and Mn, whose maximum values are found in site 1, being 24600 ppm and 4000 ppm, respectively. Concerning the trace elements, Ni, Co, Ti, Zn and Si are generally present in higher concentrations than V, Cr and Ca (Fig. 4A). Highest concentrations of Co are found in the ore of site 1 (~400 ppm), the lowest are in serpentinized peridotite (~80 ppm). Ti and Cr are highest in in serpentinized peridotite (~550 ppm and ~40 ppm, respectively) and lowest in the ore of site 3 (~70 ppm and ~1 ppm, respectively). Ni and V reach maximum values in serpentinized troctolite (~650 ppm and ~70 ppm, respectively) and minimum contents in site 3 ore (~80 ppm and ~8 ppm, respectively). Zn concentration shows little variation: the maximum value is in magnetite + chalcopyrite stockwork (~160 ppm) and the minimum in site 3 ore (~90 ppm). Si and Ca (as well as a fraction of Mg) are probably due to silicate inclusions. Principal component analysis was performed on magnetite compositions (Fig 4B). The principal component 1 (PC1) positively correlates with Mn, Mg, Co and Zn, meaning that these variables vary together; PC2 increases mostly with V; PC3 is negatively correlated to Zn, Ti and Cr. Using this information, it is possible to identify two magnetite endmembers: a) hydrothermal magnetite (Mt II), characterized by highest Co, Mn, Mg, Zn, which can be interpreted as fluid-controlled variables; b) magnetite from serpentinization of mantle minerals (Mt I), in which the concentrations of (Si), Ni, Ti, V and Cr (rock-controlled variables) are highest. Moreover Mt II is characterized by constant Zn/Co ratio (~0.28; similar to the ratio in mantle olivine; cf. De Hoog et al., 2010). This could mean that fluids that precipitated magnetite have leached metals from ultramafic rocks. 3.4 Trace elements in diopside Trace elements in diopside from various samples were compared using CI-chondrite (Palme and O’Neill, 2014) for normalization (Fig. 5). Diopside in diopside-rich rocks has low Sr, low Ti, very low Cr, higher LREE than HREE and a negative Eu anomaly (Fig 5A); in the central part of a diopsidite vein, diopside has an analogous pattern (Fig. 5B), but slightly higher MREE and HREE. On the contrary, mantle clinopyroxene relics show a rather flat pattern (Fig 5A), with orders of magnitude higher Ti and Cr compared to diopside crystals. Diopside from the hanging wall rodingite shows a peculiar pattern, as well (Fig 5A): it is rather flat with positive Sr anomaly and low contents of Ti and Cr. For comparison purpose, during this study diopside from other locations was analysed: - in veins and impregnations in a pyrrhotite + magnetite-bearing serpentinite from Piccola Bellalana mining assay (western border of Mont Avic massif, Valle d’Aosta region); - in the clasts of a serpentinite breccia dredged on the Rainbow massif (Mid-Atlantic Ridge 36°14’ N), an ultrabasic oceanic core complex hosting high temperature hydrothermal vents, sustained by mafic intrusions (Andreani et al., 2014). As shown in Fig. 5B, the trace elements patterns are very similar, meaning that they could have been formed by analogue processes. Diopside veins at Cogne show similar setting and mineralogy to the diopsidites described in Oman (Fig. 5C; Python et al., 2007); however, trace elements pattern are very different, since diopside in the latter situation has lower trace elements content and has positive Sr and Eu anomalies; this pattern is closer to that shown by diopside in the rodingite in the hanging wall of Cogne deposit. Scambelluri et al. (2001) also described hydrothermal diopside occurrence in high-pressure serpentinites at Erro-Tobbio (Western Alps). However, there the mineral has positive Sr anomaly, no Eu anomaly, low CeN and high Cr (Fig. 5C). In the light of these observations together with bulk rock meaurements, at Cogne diopside can have been formed by hydrothermal alteration of low-Al, very low-Cr (~10 ppm) protoliths (dunites?) by Si-Ca-rich fluid, probably derived from leaching of mafic rocks. In this perspective, the slight negative Eu anomaly together with the low Sr content may indicate that plagioclase alteration contribution to fluid composition was little or that Eu and Sr were not fixed in diopside, being efficiently transported in a fluid phase (Eu is mobile in oxidized fluid as Eu3+).


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Fig. 4 A) Average trace element compositions (in ppm) in magnetite from selected samples; data obtained by LA-ICP-MS. B) PCA performed on single-spot concentrations measured by LA-ICP-MS. See text for explanation.

A

B


9

0.0001

0.001

0.01

0.1

1

10

100

Ba La Ce Sr Pr Nd Sm Eu Gd Tb Ti Dy Y Ho Er Tm Yb Lu Cr

Sam

ple/

CI c

hond

rite

C.-g. in Di + Mt + Adr + Atg + Cc rock - Site 1

Rodingite - Site 1

Mantle Cpx - Site 2

Veinlets cutting mantle Cpx - Site 2

F.-g. in Di + Atg + Mt rock - Site 3

C.-g. in Di + Chl + Atg matrix of Mt ore - Site 3

0.0001

0.001

0.01

0.1

1

10

100

Ba La Ce Sr Pr Nd Zr Sm Eu Gd Tb Ti Dy Y Ho Er Tm Yb Lu Cr Mn

Sam

ple/

CI c

hond

rite

Diopside vein - Site 3

Po-Mt bearing serpentinite - Piccola Bellalana, Valle d'Aosta

Serpentinite breccia - Rainbow massif, MAR (36°14' N)

0.0001

0.001

0.01

0.1

1

10

100

Ba La Ce Sr Pr Nd Zr Sm Eu Gd Tb Ti Dy Y Ho Er Tm Yb Lu Cr

Sam

ple/

CI c

hond

rite

Rodingite - Site 1

Diopside vein - Site 3

Diopsidite - Oman (Python et al., 2007)

Vein - Erro-Tobbio serpentinite (Scambelluri et al., 2001)

Fig. 5 Selected diopside trace elements patterns. A) Diopside from the Cogne mine area; f.g. = fine-grained; c.g. = coarse-grained. B, C) Comparison between diopside from different locations. Data for Cogne and Piccola Bellalana obtained by LA-ICP-MS.

A

B

C


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4. General conclusions According to these new observations and data, the Cogne serpentinite record this sequence of events: (i) an early seafloor serpentinization process, marked by replacement of peridotite minerals with lizardite and minor magnetite (Mt I; Cr-bearing if found in ex-peridotites), forming typical mesh textures and bastites (see also Carbonin et al., 2014); (ii) a subsequent hydrothermal metasomatic event, in which a fluid leaching ultramafic and mafic rocks caused the deposition of abundant magnetite (Mt II) and minor chlorite (+ brucite ± calcite, at site 1; + chalcopyrite, at site 2; ± andradite, at site 3), and successively the formation of diopside-rich lithologies, which share their geochemical fingerprint with other ophiolitic (Piccola Bellana mining assay) and subseafloor (Rainbow hydrothermal field) occurrences; (iii) a prograde subduction metamorphism, responsible for the replacement of lizardite by antigorite and the subsequent stabilization of olivine at the expense of antigorite. The above sequence would be consistent with a subseafloor hydrothermal origin of the Cogne deposit during the oceanic stage of the Piemonte Zone. Little is known about the subseafloor part of modern ultramafic-hosted hydrothermal fields, therefore, if the analogy with Cogne will be established, this study may contribute to our understanding of their anatomy and may also account for their positive magnetic anomalies (see Tivey and Dyment, 2010). 5. Research plans Several issues are still open and concern: (i) the origin of magnetite, in particular the source of the fluid and the age of the deposit; (ii) the petrologic meaning of diopside-rich rocks; (iii) the physico-chemical conditions of hydrothermal metasomatism. In order to answer (i), I will measure δ56Fe in magnetite (10 magnetite separates samples have been already sent to IFREMER, Brest, France), being this ratio successfully used for distinguishing between magmatic and hydrothermal magnetite (Knipping et al., 2015), and I will try to apply U-Pb dating technique on uraninite inclusions in magnetite using EPMA. Concerning (ii), I will further investigate diopside-rich rocks by comparing them on experimental data produced by the “Petrology of the ocean crust” research team at the Geosciences Department of the University of Bremen. Finally, I will try to determine (iii) by taking advantage of thermodynamic modelling (using SUPCRT92 and EQ3/6 software packages). References ALT, J.C. 1995. Subseafloor Processes in Mid‐Ocean Ridge Hydrothennal Systems. 85-114. In: Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S. and Thomson, R.E. (ed.). Seafloor Hydrothermal Systems. Physical, Chemical, Biological, and Geological Interactions. American Geophysical Union, Washington DC, USA, pp. 466. ANDREANI, M., ESCARTIN, J., DELACOUR, A., ILDEFONSE, B., GODARD, M., DYMENT, J., FALLICK, A. E. AND FOUQUET, Y. 2014. Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid‐Atlantic Ridge 36° 14′ N). Geochemistry, Geophysics, Geosystems, 15 (9), 3543-3571. BROMILEY, G. D. and PAWLEY, A. R. 2003. The stability of antigorite in the systems MgO-SiO2-H2O (MSH) and MgO-Al2O3-SiO2-H2O (MASH): The effects of Al3+ substitution on high-pressure stability. American Mineralogist, 88 (1), 99-108. CARBONIN, S., MARTIN S., TUMIATI S. and ROSSETTI P. 2014. Magnetite from the Cogne serpentinites (Piemonte ophiolite nappe, Italy). Insights into seafloor fluid–rock interaction. Eur. J. Mineral. CARUSO, L. J. and CHERNOSKY, J. V. 1979. The stability of lizardite. The Canadian Mineralogist, 17 (4), 757-769.


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COMPAGNONI, R., ELTER, G., FIORA, L., NATALE, P. and ZUCCHETTI, S. 1979. Nuove osservazioni sul giacimento di magnetite di Cogne in Valle d’Aosta. Rend. Soc. Min. Petr., 35, 755-766. COMPAGNONI, R., ELTER, G., FIORA, L., NATALE, P. and ZUCCHETTI, S. 1981. Magnetite deposits in serpentinized lherzolites from the ophiolitic belt of the Western Alps, with special reference to the Cogne deposit (Aosta Valley). Proceedings of the Intern. Symp. Mafic Ultramafic Complexes Mafic and Ultramafic Complexes, Athens, 9–11 October 1980, 3, 376-394. DAL PIAZ, G.V., GIANOTTI, F., MONOPOLI, B., PENNACCHIONI, G., TARTAROTTI, P. and SCHIAVO, A. 2010. Note illustrative della carta geologica d’Italia alla scala 1:50.000 Chatillon F. 91. Ispra-Servizio Geologico d’Italia, Treviso, 152 pp. DEBRET, B., ANDREANI, M., MUÑOZ, M., BOLFAN-CASANOVA, N., CARLUT, J., NICOLLET, C., SCHWARTZ, S. and TRCERA, N. 2014. Evolution of Fe redox state in serpentine during subduction. Earth and Planetary Science Letters, 400, 206-218. DE HOOG, J. C. M, GALL, L. and CORNELL, D. H. 2010. Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry. Chemical Geology, 270 (1), 196-215. DI COLBERTALDO, D., DI FURIA, E. and ROSSI, F. 1967. Il giacimento a magnetite di Cogne in Val d’Aosta. Istituto Lombardo, A101, 361-394. DIELLA, V., FERRARIO, A. and ROSSETTI, P. 1994. The magnetite ore deposits of the southern Aosta Valley: chromitite transformed during an Alpine metamorphic event. Ofioliti, 19, 247-256. GROPPO, C., RINAUDO, C., CAIRO, S., GASTALDI, D. and COMPAGNONI, R. 2006. Micro-Raman spectroscopy for a quick and reliable identification of serpentine minerals from ultramafics. European Journal of Mineralogy, 18 (3), 319-329. KNIPPING, J. L., BILENKER, L. D., SIMON, A. C., REICH, M., BARRA, F., DEDITIUS, A. P., LUNDSTROM C., BINDEMAN, I. and MUNIZAGA, R. 2015. Giant Kiruna-type deposits form by efficient flotation of magmatic magnetite suspensions. Geology, G36650-1. MANNING, C. E. 1995. Phase-equilibrium controls on SiO2 metasomatism by aqueous fluid in subduction zones: reaction at constant pressure and temperature. International Geology Review, 37 (12), 1074-1093. NIU, Y. 2004. Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath mid-ocean ridges. Journal of Petrology, 45 (12), 2423-2458. O'HANLEY, D. S. and DYAR, M. D. 1993. The composition of lizardite 1T and the formation of magnetite in serpentinites. American Mineralogist, 78, 391-404. PALME, H. and O'NEILL, H. 2014. Cosmochemical Estimates of Mantle Composition. 1-39. In: Holland, H.D. and Turekian, K.K. (ed.). Treatise on Geochemistry Vol. 3 – The Mantle and the Core, 2nd Edition, Elsevier, Amsterdam, Netherlands, Kidlington, UK, Waltham, MA, USA, 576 pp. PAULICK, H., BACH, W., GODARD, M., DE HOOG, J. C. M., SUHR, G. and HARVEY, J. 2006. Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15 20′ N, ODP Leg 209): implications for fluid/rock interaction in slow spreading environments. Chemical Geology, 234 (3), 179-210. PYTHON, M., CEULENEER, G., ISHIDA, Y., BARRAT, J. A. and ARAI, S. 2007. Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres. Earth and Planetary Science Letters, 255 (3), 289-305. ROSSETTI, P., GATTA, G.D., DIELLA, V., CARBONIN, S., DELLA GIUSTA, A. and FERRARIO, A. 2009. The magnetite ore districts of the southern Aosta Valley (Western Alps, Italy): a mineralogical study of metasomatized chromite ore. Mineral. Mag., 73 (5), 737-751. SCAMBELLURI, M., RAMPONE, E. and PICCARDO, G. B. 2001. Fluid and element cycling in subducted serpentinite: a trace-element study of the Erro-Tobbio high-pressure ultramafites (Western alps, NW Italy). Journal of Petrology, 42 (1), 55-67. TAKAZAWA, E., ABE, N., SEYLER, M. and MEURER, W.P. 2007. Hybridization of dunite and gabbroic materials in Hole 1271B from Mid-Atlantic Ridge 15°N: implications for melt flow and reaction in the upper mantle. In Kelemen, P.B., Kikawa, E. and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 209: College Station, TX (Ocean Drilling Program), 1–23


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TIVEY, M. A. and DYMENT, J. 2010. The magnetic signature of hydrothermal systems in slow spreading environments. Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, 43-66. ZUCCHETTI, S., MASTRANGELO, F., ROSSETTI, P. and SANDRONE, R. 1988. Serpentinization and metamorphism: their relationships with metallogeny in some ophiolitic ultramafics from the Alps. In: Zuffar’ Days Symposium in Honor of Piero Zuffardi. University of Cagliari, Cagliari, October 10-15, 1988, 137-159.


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SUMMARY OF ACTIVITY IN THIS YEAR Courses:

G. GODARD: “Corso sull’utillizzo del software THERMOCALC” (ad personam course), Museo Nazionale dell’Antartide, Siena, March 9 -13 2015 G. GODARD, S. TUMIATI: “Applicazione dell’algebra lineare alla petrologia e alla geologia”, Geosciences Department, University of Padova, April 23-24 2015. W. BACH, S. DAVIES, C. DEVEY, A. FEHR, T. FREUDENTHAL, G. FRÜH-GREEN, C. GARRIDO, M.GODARD, W. HALE, B. ILDEFONSE, M. IVARSSON, J. JAMIESON, F. KLEIN, J. KÖPKE, B. MENEZ, A. MORRIS, S. PETERSEN, V. RATHMEYER, U. RÖHL, V. SCHLINDWEIN, T. SCHROEDER, R. STEIN, D. TEAGLE, R. TRIBUZIO, H. VILLINGER: “ECORD Summer School 2015: Ocean crust processes: magma, faults, fluxes and life”, MARUM, Bremen, Germany, August 31 - September 11 2015. Communications:

TOFFOLO, L., MARTIN, S., and NIMIS, P., 2015. The Cogne serpentinite-hosted magnetite ore: a subseafloor hydrothermal deposit? Talk given to Wolfgang Bach’s research group, University of Bremen, Germany, July 8 2015. TOFFOLO, L., MARTIN, S., and NIMIS, P., 2015. The Cogne serpentinite-hosted magnetite ore: a subseafloor hydrothermal deposit? Talk given in occasion of the ECORD Summer School 2015, MARUM, Bremen, Germany, September 8 2015. Posters: TOFFOLO, L., MARTIN, S., and NIMIS, P., 2015. The Cogne magnetite deposit (NW Italian Alps): is it an ophiolitic analogue of Rainbow? Workshop: Mantle, water and life: the ultramafic-hosted Rainbow hydrothermal field, Lyon, France, June 10-12 2015. TOFFOLO, L., MARTIN, S., and NIMIS, P., 2015. Hydrothermal magnetite ore in seafloor ultramafic rocks? A case from Cogne (NW Italian Alps). 25th Goldschmidt Conference 2015, Prague, Czech Republic, August 16-21 2015. Publications: None. Teaching activities: None. Other: July 1 – October 30 2015: ERASMUS+ Traineeship at University of Bremen; supervisor: Prof. W. Bach, head of the “Petrology of the ocean crust” research group.

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